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Resources Ready-Mix




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  • Admixtures
    What are Admixtures?

    Admixtures are natural or manufactured chemicals which are added to the concrete before or during mixing. The most often used admixtures are air-entraining agents, water reducers, retarders and accelerators.

    Why Use Admixtures?

    Admixtures are used to give special properties to fresh or hardened concrete. Admixtures may enhance the durability, workability or strength characteristics of a given concrete mixture. Admixtures are used to overcome difficult construction situations such as hot or cold weather placements, pumping requirements, early strength requirements or very low water-cement ratio specifications.

    How to Use Admixtures

    Consult Cadman Lab Systems about which admixtures may be appropriate for your application. Admixtures should be evaluated for compatibility with cement(s), construction practices, job specifications, and economic advantage before being used.

    Follow this Guide to Use Admixtures

    AIR-ENTRAINGING AGENTS – are liquid chemicals added during mixing to produce microscopic air bubbles in concrete. These bubbles improve the concrete’s durability and increase its resistance to damage from freezing and thawing and deicing salts. Air entraining admixtures improve workability and may reduce bleeding and segregation. For exterior flatwork (parking lots, driveways, sidewalks, pool decks, patios) that is subject to freezing and thawing weather cycles, or in areas where deicer salts are used, specify an air content of 5 to 7%. Air-entrainment is not necessary for interior structural concrete since it is not subject to freezing and thawing. In high cement content concretes adding air will reduce strength by about 5% for each 1% of air added; but in low cement content concretes adding air has less effect and may increase strength slightly.

    WATER-REDUCERS – are used for two different purposes: (1) to lower the water content and increase the strength; (2) to obtain higher slump using the same water content. Water-reducers will generally reduce the required water content for a given slump by about 10%. This increases strength or allows the cement content to be reduced and maintain the same water-cement ratio. Water-reducers are used to increase slump for pumping concrete and are used in hot weather to offset the increased water demand. Water-reducers may aggravate slump loss problems. Water-reducers tend to retard concrete and sometimes have accelerators blended in to offset retardation. Water-reducers are Type A Chemical Admixtures in ASTM C 494.2.

    RETARDERS – are chemicals which delay the initial set of concrete by an hour or more. Retarders are often used in hot weather to counter the rapid setting caused by high temperatures. For large jobs, or in hot weather, specify concrete with retarder to allow more time for placing and finishing. Most retarders also act as water reducers. Retarders are covered by ASTM C 494.2 Types B and D.

    ACCELERATORS – reduce the initial set time of concrete. Liquid accelerators are added to the concrete at the plant. Accelerators are recommended in cold weather to get high-early strength. Accelerators do not act as antifreeze; rather, they speed up the strength gain and make the concrete stronger to resist damage from freezing. Accelerators are sometimes used to allow finishing operations to begin early. Calcium chloride is the most commonly used accelerator, although non-chloride (non-corrosive) accelerators are available. Calcium chloride is specified at not more than 2% by the weight of the cement. Pre-stressed concrete and concrete with embedded aluminum or galvanized metal should not contain any calcium chloride because of the potential for corrosion. See NRMCA Publication No. 173.3. Accelerators are covered by ASTM C 494.2 Types C and E.

    HIGH RANGE WATER-REDUCERS (HRWR) – are a special class of after-reducers. Often called superplasticizers, HRWR’s reduce the water content of a given concrete from 12 to 25%, which increases strength. HRWR’s can also greatly increase the slump to produce “flowing” concrete. For example, adding a normal dosage of HRWR to a concrete with a slump of 3 to 4 inches will produce a concrete with a slump of about 8 inches. Within 30 to 60 minutes the concrete will return to its original slump. HRWR’s are covered by ASTM Specification C 494.2 Types F and G, and C 10175 Types 1 and 2.

    References
    • ASTM C 260, “Standard Specification for Air-Entraining Admixtures for Concrete,” American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103.
    • ASTM C 494 “Standard Specification for Chemical Admixtures for Concrete.”
    • “Understanding Chloride Percentages,” NRMCA Publication No. 173.
    • “Superplasticizers in Ready Mixed Concrete,” NRMCA Publication No. 158.
    • ASTM C 1017 “Standard Specification for Chemical Admixtures for Use in Producing Flowing Concrete.”

  • Air Content Loss in Pump
    What is Air Loss in Pumping?

    Increasingly, specifiers are testing concrete at the discharge end of concrete pumps and, in some cases, finding air contents much lower than that in samples tested at the truck chute. It is normal to find 0.5 to 1.0 percent less at the pump discharge. However, when the new 5” line, long boom pumps have the boom in an orientation with a long, near vertical downward section of pipe, the air content at the discharge may be less than half of that of the concrete going into the pup hopper. When the boom is upward or horizontal, except for a 12 ft. section of rubber hose, there generally is no significant loss of air. There is some controversy over how frequently air loss is a problem in pumped concrete. Certainly is doesn’t occur every time, or even most times.However, it does occur often enough to be considered seriously until better solutions are developed.

    Why is Air Lost?

    There are several mechanisms involved, but air loss will occur if the weight of concrete in a vertical or near vertical downward pipe is sufficient to overcome frictional resistance and let a slug of concrete slide down the pipe. One part of the theory is that when the concrete slides down the pipe, it develops a vacuum which greatly expands the air bubbles; and when they hit an elbow in the boom or a horizontal surface, the bubbles collapse. You can demonstrate the effect of the impact by dropping concrete 15 or 20 ft. into a tray. Naturally, the transition from several hundred psi of pressure in the line to a near vacuum condition may make matters worse. Most field experience suggests that air loss is greatest with high cement content, flowable concrete mixes which slide down easier; however, air loss has also been experienced with 5 ½ sacks concrete of moderate slump.

    How to Prevent Air Loss

    Keep concrete from sliding down the line under its own weight. Where possible, avoid vertical or steep downward boom sections. Be cautious with high slumps, particularly with high cement content mixes and mixes containing silica fume. Steady, moderately rapid pumping may help somewhat to minimize air loss, but will not solve most problems.

    • Try inserting four 90 degree elbows just before the rubber hose. (Do not do this unless pipe clamps are designed to comply with all safety requirements.) This helps but won’t be a perfect solution.
    • Use a slide gate at the end of the rubber hose to restrict discharge and provide resistance.
    • Use of a 6 ft. diameter loop in the rubber hose with an extra section of rubber hose is reported to be a better solution than (a) or (b).
    • Lay 10 or 20 ft. of hose horizontally on deck pours. This doesn’t work in columns or walls and requires labor to handle the extra hose.
    • Reduce the rubber hose size from 5 to 4 in. A transition pipe may be needed to avoid blockages.

    Precautions
    • Before the pour, plan alternative pump locations and decide what will be done if air loss occurs. Be prepared to test for air content frequently.
    • Sampling from the end of a pump line can be very difficult. Wear proper personal protective equipment. Never sample the initial concrete through the pump line.
    • Sample the first load on the job after pumping 3 or 4 cubic yards. Temper it to the maximum permissible slump. Swing the boom over near the pump to get the maximum length of vertical downward pipe and drop the sample in a wheel barrow. If air is lost, take precautions and sample at the point of placement.
    • If air loss occurs, do not try to solve the problem by increasing the air content delivered to the pump beyond the upper specification limit. High air content concrete with low strength could, or almost surely will be placed in the structure if boom angles are reduced or somewhat lower slump concrete is pumped.

    References
    • Gaynor, R.D., “Summer Problem Solving,” Concrete Products, June, 1991, p. 11.
    • Gaynor, R.D., “Current Research at NRMCA,” Concrete Products, April, 1992, pp. 6-7.
    • Hoppe, Julian J., “Air Loss in Free-Falling Concrete” Queries on Concrete, Concrete International, June 1992, p. 79.
    • Gorsha, Russel P., “Air Loss in Free-Falling Concrete,” Queries on Concrete, Concrete International, August 1992, p. 71.
    • “Effects of Pumping Air Entrained Concrete,” Washington Aggregates and Concrete Association, March 20, 1991, 12 pp.
    • Dyer, R.M., “An Investigation of Concrete Pumping Pressure and the Effects on the Air Void System of Concrete,” Master’s Thesis, Department of Civil Engineering, University of Washington, Seattle, Washington, 1991.
    • Personal correspondence with authors and photographs, July 13, 1992 (copies available on request).
    • Yingling, James; G.M. Mullings; and R.D. Gaynor, “Loss of Air Content in Pumped Concrete,” Concrete International, Volume 14, Number 10, October 1992, pp. 57-61. NRMCA Publication No. 180.

  • Cold Weather Concreting
    What is cold weather?
    Cold weather is defined as a period when the average daily temperature falls below 40°F [4°C] for more than three successive days. These conditions warrant special precautions when placing, finishing, curing and protecting concrete against the effects of cold weather. Since weather conditions can change rapidly in the winter months, good concrete practices and proper planning are critical.

    ASK CADMAN ABOUT WINTER HEAT!

    DON'T LET FREEZING TEMPERATURES SCARE YOU. Concrete can be placed safely without damage throughout the winter months as long as certain precautions are taken. Cadman supplies “Winter Heat” - a cost effective system that guarantees your concrete will arrive at the ideal temperature during colder weather.

    Winter Heat Includes:

    • A guaranteed minimum temperature of 60 degrees when your truck arrives at the job site.
    • Consistent temperatures that provide more reliable finishing and placement operations in adverse conditions.
    • Meets ACI job requirements upon arrival at your site.

    We do our part to offer you the best results. Depending on temperature, additional measures may also include: Wind blocks, insulated blankets (before and after pouring), accelerators and higher cement mixes. Ask your sales representative for ideas to best fit your current conditions.

    How to place concrete in cold weather?

    Recommended concrete temperatures at the time of placement are shown below. The ready mixed concrete producer can control concrete temperature by heating the mixing water and/or the aggregates and furnish concrete in accordance with these guidelines.

    Cold weather concrete temperature should not exceed these recommended temperatures by more than 20°F [10°C]. Concrete at a higher temperature requires more mixing water, has a higher rate of slump loss, and is more susceptible to cracking. Placing concrete in cold weather provides the opportunity for better quality, as cooler initial concrete temperature will typically result in higher ultimate strength. Slower setting time and strength gain of concrete during cold weather typically delays finishing operations and form removal.

    Chemical admixtures and other modifications to the concrete mixture can accelerate the rate of setting and strength gain. Accelerating chemical admixtures, conforming to ASTM C 494—Types C (accelerating) and E (water-reducing and accelerating), are commonly used in the winter time. Calcium chloride is a common and effective accelerating admixture, but should not exceed a maximum dosage of 2% by weight of cement. Non-chloride, non-corrosive accelerators should be used for prestressed concrete or when corrosion of steel reinforcement or metal in contact with concrete is a concern. Accelerating admixtures do not prevent concrete from freezing and their use does not preclude the requirements for concrete temperature and appropriate curing and protection from freezing.

    Accelerating the rate of set and strength gain can also be accomplished by increasing the amount of portland cement or by using a Type III cement (high early strength). The relative percentage of fly ash or ground slag in the cementitious material component may be reduced in cold weather but this may not be possible if the mixture has been specifically designed for durability. The appropriate decision should afford an economically viable solution with the least impact on the ultimate concrete properties. Concrete should be placed at the lowest practical slump as this reduces bleeding and setting time. Adding 1 to 2 gallons of water per cubic yard [5 to 10 L/m3] will delay set time by ½ to 2 hours. Retarded set times will prolong the duration of bleeding. Do not start finishing operations while the concrete continues to bleed as this will result in a weak surface.

    Adequate preparations should be made prior to concrete placement. Snow, ice and frost should be removed and the temperature of surfaces and metallic embedments in contact with concrete should be above freezing. This might require insulating or heating subgrades and contact surfaces prior to placement. Materials and equipment should be in place to protect concrete, both during and after placement, from early age freezing and to retain the heat generated by cement hydration. Insulated blankets and tarps, as well as straw covered with plastic sheets, are commonly used measures. Enclosures and insulated forms may be needed for additional protection depending on ambient conditions. Corners and edges are most susceptible to heat loss and need particular attention. Fossil-fueled heaters in enclosed spaces should be vented for safety reasons and to prevent carbonation of newly placed concrete surfaces, which causes dusting. The concrete surface should not be allowed to dry out while it is plastic as this causes plastic shrinkage cracks. Subsequently, concrete should be adequately cured. Water curing is not recommended when freezing temperatures are imminent. Use membrane-forming curing compounds or impervious paper and plastic sheets for concrete slabs.

    Forming materials, except for metals, serve to maintain and evenly distribute heat, thereby providing adequate protection in moderately cold weather. With extremely cold temperatures, insulating blankets or insulated forms should be used, especially for thin sections. Forms should not be stripped for 1 to 7 days depending on the setting characteristics, ambient conditions and anticipated loading on the structure. Field-cured cylinders or nondestructive methods should be used to estimate in-place concrete strength prior to stripping forms or applying loads. Field-cured cylinders should not be used for quality assurance.

    Special care should be taken with concrete test specimens used for acceptance of concrete. Cylinders should be stored in insulated boxes, which may need temperature controls, to insure that they are cured at 60°F to 80°F [16°C to 27°C] for the first 24 to 48 hours. A minimum/maximum thermometer should be placed in the curing box to maintain a temperature record.

    Cold Weather Concreting Guidelines
    • Use air-entrained concrete when exposure to moisture and freezing and thawing conditions are expected.
    • Keep surfaces in contact with concrete free of ice and snow and at a temperature above freezing prior to placement.
    • Place and maintain concrete at the recommended temperature.
    • Place concrete at the lowest practical slump.
    • Protect plastic concrete from freezing or drying.
    • Protect concrete from early-age freezing and thawing cycles until it has attained adequate strength.
    • Limit rapid temperature changes when protective measures are remove
    References
    • Cold Weather Concreting, ACI 306R, American Concrete Institute, Farmington Hills, MI.
    • Design and Control of Concrete Mixtures, Portland Cement Association, Skokie, IL.
    • ASTM C94 Standard Specification for Ready Mixed Concrete, ASTM, West Conshohocken, PA.
    • ASTM C 31 Making and Curing Concrete Test Specimens in the Field, ASTM, West Conshohocken, PA.
    • Cold Weather Ready Mixed Concrete, NRMCA Pub 130, NRMCA, Silver Spring, MD.
    • Cold-Weather Finishing, Concrete Construction, November 1993
  • Concrete Blisters
    What Are Blisters?
    Blisters are hollow, low-profile bumps on the concrete surface typically from the size of a dime up to an inch, but occasionally even 2 or 3 inches in diameter. A dense troweled skin of mortar about 1/8 inch thick covers and underlying void which moves around under the surface during troweling.

    The void forms under a dense surface skin by one of two phenomenons. Some believe that incidental air voids rise in sticky concretes and are trapped under a dense surface skin produced by troweling. Others believe that bleed water rises and collects to form a void under this skin. That water is reabsorbed into the underlying concrete leaving a layer of irregular void space under the surface which is then consolidated by troweling to form a round blister which moves during subsequent troweling. Frequently, the blister is lined with a faint layer of “washed” sand.

    In poorly lighted areas, small blisters may be difficult to see during finishing and may not be detected until they break under traffic.

    Why Do Blisters Form?

    Blisters form when the fresh concrete surface is sealed by troweling while the underlying concrete is plastic and bleeding or able to release air. The small round blisters form fairly late in the finishing process, after floating and after the first troweling.

    Moderately rapid evaporation of bleed water makes the surface ready to be troweled while the underlying concrete is still bleeding or still plastic and releasing air. Evaporation from the surface is increased by wind, low relative humidity or a warm concrete surface. If evaporation is too rapid, the slab will be affected to a depth of an inch or more and blisters will be prevented – but plastic shrinkage cracks may develop!

    Entrained air is often involved since it reduces the rate of bleeding and supplies the fat necessary to produce the dense impermeable surface layer. A cool subgrade will delay set in the bottom and make the top set first.

    Blisters are more likely to form if:

    • The subgrade is cool and the concrete in the bottom sets slowly.
    • Entrained air is used or is higher than normal so that the surface is ready to finish earlier.
    • A dry shake is used, particularly over air entrained concrete.
    • The concrete is sticky from higher cement content or excessive fine sand. Lean mixes bleed rapidly for a shorter period, have higher total bleeding and tend to delay finishing.
    • The slab is thick.
    • The slab is on polyethylene and the slump is less than 3 or 4 inches.
    • Excessive use of a jitterbug or a vibrating screed which works up a thick mortar layer on top.

    How to Prevent Blisters

    The finisher should be wary of a concrete surface that appears to be ready to trowel before it would normally be expected to be. Emphasis in finishing should be on placing, straight edging and floating the concrete as rapidly as possible and without working up an excessive layer of fat. After these operations are completed, further finishing should be delayed as long as possible and the surface covered with polyethylene or otherwise protected from evaporation. In initial floating the float blades should be flat to avoid densifying the surface too early. Use of an accelerator or heated concrete often prevents blisters in cool weather.

    If blister are forming, try to either flatten the trowel blades or tear the surface with a wood float and delay finishing as long as possible. Any steps than can be taken to slow evaporation should help.

    Follow These Rules to Avoid Blisters

    • Do not seal surface before air or bleed water from below have escaped.
    • Avoid dry shakes on air-entrained concrete.
    • Use heated or accelerated concrete to promote even setting throughout the depth of the slab.
    • Do not place slabs directly on polyethylene sheeting.
    References
    • Guide for Concrete Floor and Slab Construction,” ACI 302.1R, ACI Manual of Concrete Practice, American Concrete Institute, P.O. Box 19150 Redford Station, Detroit, MI 48219.
    • Carl O. Peterson, “Concrete Surface Blistering-Causes and Cures,” Concrete Construction, September 1970, p. 317. Concrete Construction publications, Inc., 426 S. Westgate, Addison, IL 60101.
    • “Finishing,” Concrete Construction, August 1976, p. 369.
    • J.C. Yeager, “Finishing Problems and Surface Defects in Flatwork,” Concrete Construction, April 1979, p. 247-258.
    • Problems and Practices, ACI Journal, December 1995, p. 492.

  • Concrete Maintenance
    Properly Maintained Concrete

    Proper maintenance and care for your concrete will ensure you get the best looking concrete for its maximum life. Concrete should be regularly sealed with a concrete sealer when the previous seal has worn down. Sealers can last anywhere from one to five years, if your contractor applied a seal, he should explain to you how long the seal will last. Seals are available in both matte and high gloss finishes, and are recommended to be applied by contractors but can be applied by homeowners with proper care. Sealed concrete can be slippery, if concerned, it is recommended you use (or ask your contractor to use) a texturing agent such as Increte System's Shur-Grip.

    Poorly Maintained Concrete

    Concrete that has been poorly cared for can be cleaned with a variety of cleaning products, or even colored with Increte Systems Stain-Sealer, which works like a concrete paint to both color and seal your concrete. Chips and divots can be patched with a concrete patcher. In extreme cases a concrete overlay can be applied, although it is recommended that only trained professionals do this. A concrete overlay is a new surface for your pre-existing concrete, and can be finished like normal concrete.

  • Corrosion of Steel In Concrete
    What is Corrosion of Steel?

    ASTM terminology (G 15) defines corrosion as “the chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties.” For steel embedded in concrete, corrosion results in the formation of rust which has two to four times the volume of the original steel and none of the good mechanical properties. Corrosion also produces pits or holes in the surface of reinforcing steel, reducing strength capacity as a result of the reduced cross-sectional area.

    Why is Corrosion of Steel a Concern?

    Reinforced concrete uses steel to provide the tensile properties that are needed in structural concrete. It prevents the failure of concrete structures which are subjected to tensile and flexural stresses due to traffic, winds, dead loads, and thermal cycling. However, when reinforcement corrodes, the formation of rust leads to a loss of bond between the steel and the concrete and subsequently delamination and spalling. If left unchecked, the integrity of the structure can be affected. Reduction in the cross sectional area of steel reduces its strength capacity. This is especially detrimental to the performance of tensioned strands in pre-stressed concrete.

    Why Does Steel in Concrete Corrode?

    Steel in concrete is usually in a non-corroding, passive condition. However, steel reinforced concrete is often used in severe environments where sea water or deicing salts are present. When chloride moves into the concrete, it disrupts the passive layer protecting the steel, causing it to rust and pit.

    Carbonation of concrete is another cause of steel corrosion. When concrete carbonates to the level of the steel rebar the normally alkaline environment, which protects steel from corrosion, is replaced by a more neutral environment. Under these conditions the steel is not passive and rapid corrosion begins. The rate of corrosion due to carbonated concrete cover is slower than chloride-induced corrosion.

    Occasionally, a lack of oxygen surrounding the steel rebar will cause the metal to dissolve, leaving a low pH liquid.

    How to Prevent Corrosion.

    Quality Concrete and Concrete Practices The first defense against corrosion of steel in concrete is quality concrete and sufficient concrete cover over the reinforcing bars. Quality concrete has a water-to-cementitious material ratio (w/c) that is low enough to slow down the penetration of chloride salts and the development of carbonation. The w/c ratio should be less than 0.50 to slow the rate of carbonation and less than 0.40 to minimize chloride penetration. Concretes with low w/c ratios can be produced by (1) increasing the cement content; (2) reducing the water content by using water reducers and superplasticizers; or (3) by using larger amounts of fly ash, slag, or other cementitious materials. Additionally, the use of concrete ingredients containing chlorides should be limited. The AI 318 Building Code provides limits on the maximum amount of soluble chlorides in the concrete mix.

    Another ingredient for good quality concrete is air entrainment. It is necessary to protect the concrete from freezing and thawing damage. Air entrainment also reduces bleeding and the corresponding increased permeability due to the bleed channels. Spalling and scaling can accelerate corrosion damage of the embedded reinforcing bars. Proper scheduling of finishing operations is needed to ensure that the concrete does not scale, spall, or crack excessively.

    The correct amount of steel will help keep cracks tight. ACI 224 helps the design engineer to minimize the formation of cracks that could be detrimental to embedded steel. In general, the maximum allowable crack widths are 0.007 inch in deicing salt environments and 0.006 inch in marine environments.

    Adequate cover over reinforcing steel is also an important factor. Chloride penetration and carbonation will occur in the outer surface of even low permeability concretes. Increasing the cover will delay the onset of corrosion. For example, the time for chloride ions to reach a steel rebar at 2 inches from the surface is four times that with a 1 inch cover. ACI 318 recommends a minimum of 1.5 inches of cover for most structures, and increases it to 2 inches of cover for protection from deicing salts. ACI 357 recommends 2.5 inches of minimum cover in marine environments. Larger aggregates require more cover. For aggregates greater than ¾ inch, a rule of thumb is to add to the nominal maximum aggregate size ¾ inch of cover for deicing salt exposure, or 1 – ¾ inch of cover for marine exposure. For example, concrete with 1 inch aggregate in a marine exposure should have a 2 – ¾ inch minimum cover.

    The concrete must be adequately consolidated and cured. Moist curing for a minimum of seven days to 70°F is needed for concrete with a 0.40 w/c ratio, whereas six months is needed for a 0.60 w/c ratio. Numerous studies show that concrete porosity is reduced significantly with increased curing times and, correspondingly, corrosion resistance is improved.

    Modified Concretes and Corrosion Protection Systems – Increased corrosion resistance can also come about by the use of concrete additives. Silica fume, fly ash, and blast-furnace slag reduce the permeability of the concrete to the penetration of chloride ions. Corrosion inhibitors, such as calcium nitrite, act to prevent corrosion in the presence of chloride ions. In all cases, they are added to quality concrete at w/c less than or equal to 0.45.

    Water repellents may reduce the ingress of moisture and chlorides to a limited extent. However, ACI 222 indicates that these are not effective in providing long-term protection. Since good quality concrete already has a low permeability, the additional benefits of water repellents are not as significant.

    Other protection techniques include protective membranes, cathodic protection, epoxy-coated reinforcing bars, and concrete sealers (if reapplied every four to five years).

    How To Limit Corrosion
    • Use good quality concrete air-entrained with a w/c of 0.40 or less.
    • Use a minimum concrete cover of 1.5 inches and at least 0.75 inch larger than the nominal maximum size of the coarse aggregate.
    • Increase the minimum cover to 2 inches for deicing salt exposure and to 2.5 inches for marine exposure.
    • Ensure that the concrete is adequately cured.
    • Use fly ash, blast-furnace, slag, or silica fume and/or a proven corrosion inhibitor.
    References
    • “Building Code Requirements for Reinforced Concrete,” ACI 318, ACI Manual of Concrete Practice, Part 3 American Concrete Institute, Detroit, Mi.
    • “Corrosion of Metals in Concrete,” ACI 222R, ACI Manual of Concrete Practice, Part 1.
    • “Control of Cracking in Concrete Structures,” ACI 224R, ACI Manual of Concrete Practice, Part 3.
    • “Design and Construction of Fixed Offshore Concrete Structures,” ACI 357R, ACI Manual of Concrete Practice, Part 4.
    • Perenchio, W.F., “Corrosion of Reinforcing Steel,” ASTM STP 169C, 1994, pp. 164-172.
    • Whiting, D., ed., Paul Klieger Symposium on Performance of Concrete, ACI SP-122, 1990, 499 pp.
    • Berke, N.S., Pfeifer, D.W., and Weil, T.G., “Protection Against Chloride Induced Corrosion,” Concrete International, Vol. 10, No. 12, 1988, pp. 44-55.

  • Cracking Basement Wall
    What Types of Cracks May Occur?

    Cast-in place concrete basements provide durable, high quality extra living space. At times when proper construction practices are not used undesirable cracks occur, such as:

    • Temperature and drying shrinkage cracks. With few exceptions, newly placed concrete has the largest volume that it will ever have. This shrinkage tendency is increased by drying and/or a drop in temperature and can lead to random cracking if steps are not taken to control the location of the cracks by providing control joints.
    • Settlement cracks. These occur from non-uniform support of footings or occasionally from expansive soils.
    • Other structural cracks. In basements these cracks generally occur during backtilling, particularly when heavy equipment gets too close to the walls.
    • Cracks due to lack of joints or improper jointing practices.
    Why Do Basement Cracks Occur?

    In concrete basement walls some cracking is normal. The “Home-Owners Warranty” (HOW Program) requires repair only when cracks leak or exceed the following:

    Crack Width Vertical Displacement
    Basement Walls 1/8" -
    Basement Floors 3/16" 1/8"
    Garage Slabs & Patios 1/4" 1/8"

    Most cracks normally occur because one or more of the following rules of “good concrete practice” were not followed:

    • Providing uniform soil support.
    • Using moderate slump concrete and avoiding addition of water to the concrete mixture on the job.
    • Observing proper concrete placement practices.
    • Providing control joints every 20 to 30 feet.
    • Backfilling carefully and, if possible, waiting until the first floor is in place in cold weather. (Concrete gains strength at a slower rate in cold weather.)
    How to Construct Quality Basements

    Since the performance of concrete basements is affected by climate conditions, unusual loads, materials quality and workmanship, care should always be exercised in their design and construction.
    The following steps should be followed:

    Site conditions and excavation. Soil investigation should be thorough enough to insure design and construction of foundations suited to the building site. The excavation should be to the level of the bottom of the footing. The soil or granular fill beneath the entire area of the basement should be well compacted by rolling, vibrating or tamping. Footings must bear on undisturbed soil.

    Formwork and reinforcement. All formwork must be constructed and braced so that it can withstand the pressure of the concrete. Reinforcement is effective in controlling shrinkage cracks and is especially beneficial where uneven side pressures against the walls may be expected. Observe state and local guidelines for wall thickness and reinforcement if needed.

    Joints. Shrinkage and temperature cracking of basement walls can be controlled by means of properly located and formed joints. As a rule of thumb, in 8 feel high and 8 inch thick walls, vertical control joints should be provided at a spacing of about 30 times the wall thickness. These wall joints can be formed by nailing a ¾ inch thick strip of wood, beveled from ¾” to ½” in width, to the inside of both interior and exterior wall forms. After the removal, the grooves should be caulked with good quality joint filler.

    Concrete. In general, use concrete with a moderate slump (up to 5 inches). Avoid retempering. Concrete with a higher slump may be used providing the mixture is specifically designed to produce the required strength without excessive bleeding and/or segregation. In areas where weathering is severe and where the walls may be exposed to moisture and freezing temperatures air entrained concrete should be used.

    Placement and curing. Place concrete in a continuous operation to avoid cold joints. If concrete tends to bleed and segregate slump must be reduced and the concrete placed in the form every 20 to 30 feet around the perimeter of the wall. Higher slump concretes that do not bleed or segregate will flow horizontally for long distances and reduce the number of required points of access to the form. Provide adequate curing and protection to fresh concrete. It should not be allowed to freeze in cold weather. Preventive measures could be taken by completely enclosing the structure with polyethylene sheets and, if necessary, providing heat.

    Waterproofing and drainage. Spray or paint the exterior of walls with damp proofing asphaltic compound. Provide foundation drainage by installing drain tiles or plastic pipes around the exterior of the footing, then covering with clean granular fill to a height of at least 1 foot prior to backfill. Water should be drained to lower elevations suitable to receive storm water run off.

    Backfilling and final grading. Backfilling should be done carefully to avoid damaging the walls. Brace the walls or, if possible, have first floor in place before backfill. To drain the surface water away from the basement finish grade should fall off ½ to 1 inch per foot for at least 8 feet to 10 feet away from the foundation.

    References
    • “Control of Cracking in Concrete Structures,” report by ACI Committee 224.
    • “Manual of Acceptable Practices,” Volume 4, U.S. Department of Housing and Urban Development.
    • “Solid Concrete Basement Walls,” National Ready Mixed Concrete Association. 4. “Guide to Residential Cast-in-Place Concrete Construction,” ACI332R.

  • Cracking Concrete Surfaces
    What are Some Forms of Cracks?

    Concrete, like other construction materials, contracts and expands with changes in moisture content and temperature and deflects depending on load and support conditions. When provisions for these movements are not made in design and construction, then cracks can occur. Some forms of common cracks are:

    Figure A - Plastic Shrink Cracking
    Figure B – Cracks Due to Improper Jointing
    Figure C – Cracks Due to Continuous External Restraint
    Figure D – Basement Floor Cracks
    Figure E – D-Cracks from Freezing and Thawing
    Figure F – Craze Cracks
    Figure G – Settlement Cracks

    Cracks rarely affect structural integrity. Most random individual cracks look bad and although they permit entrance of water they do not lead to progressive deterioration. They are simply unsightly. Closely spaced pattern cracks or D-cracks due to freezing and thawing are an exception and may lead to ultimate deterioration.

    Why Do Concrete Surfaces Crack?

    The majority of concrete cracks usually occur due to improper design and construction practices, such as:

    • Omission of isolation and control joints and improper jointing practices.
    • Improper subgrade preparation.
    • The use of high slump concrete or addition of water on the job.
    • Improper finishing.
    • Inadequate or no curing.
    How to Prevent or Minimize Cracking

    All concrete has a tendency to crack and it is not possible to consistently produce completely crack free concrete. However, cracking can be reduced and controlled if the following basic safeguards are observed:

    Subgrade and Formwork. All top soil and soft spots should be removed. Regardless of its type, the soil beneath the slab should be compacted soil or granular fill, well compacted by rolling, vibrating or tamping. The slab and, therefore, the subgrade should be sloped for proper drainage. Smooth, level subgrades help prevent cracking. All formwork must be constructed and braced so that it can withstand the pressure of the concrete without movement. Polyethylene vapor barriers increase bleeding and greatly increase cracking of high slump concrete. Cover the vapor barrier with 1 to 2 inches of damp sand to reduce bleeding. Immediately prior to concrete placement, dampen the subgrade, formwork, and the reinforcement.

    Concrete. In general, use concrete with a moderate slump (not over 5 inches). Avoid retempering. If higher slump, up to 7 inches, is to be used, proportions will have to be changed and special mixtures developed to avoid excessive bleeding, segregation and low strength. Specify air-entrained concrete for outdoor slabs subjected to freezing weather.

    Finishing. DO NOT perform finishing operations with water present on the surface. Initial screeding must be promptly followed by bullfloating. For better traction on exterior surfaces us a broom finish. If evaporation is excessive reduce it by some means to avoid plastic shrinkage cracking. Cover the concrete with wet burlap or polyethylene sheets in between finishing operations if conditions are severe.

    Curing. Start curing as soon as possible. Spray the surface with liquid membrane curing compound or cover it with damp burlap and keep it moist for at least 3 days. A second application of curing compound the next day is a good quality assurance step.

    Joints. Provisions for contraction or expansion movements due to temperature and/or moisture change should be provided with construction of control joints by sawing, forming or tooling a groove about ¼ the thickness of the slab, no further apart than 30 times the thickness. Often closer spacing of control joints will be necessary to avoid long thin areas. The length of an area should not exceed about 1.5 times the width. Isolation joints should be provided whenever restriction to freedom of either vertical or horizontal movement is anticipated; such as where floors meet walls, column, or footings. These are full-depth joints and are constructed by inserting a barrier of some type to prevent bond between the slab and the other elements.

    Cover Over Reinforcement. Cracks in reinforced concrete caused by expansion of rust on reinforcing steel should be prevented by providing sufficient concrete cover (at least 2 inches) to keep salt and moisture from contacting the steel.

    Follow These Rules to Minimize Cracking
    • Design the members to handle all anticipated loads.
    • Provide proper control and isolation joints.
    • In slab-on-grade work, prepare a stable subgrade.
    • Place and finish according to established rules.
    • Project and cure the concrete properly.
    References
    • ACI Standard Recommended Practice for Concrete Floor and Slab Construction, ACI 302, ACI Manual of Concrete Practice.
    • “Causes of Floor Failures,” by A. T. Hersey, ACI Journal, June 1973.
    • “Cracks in Concrete: Causes, Prevention, Repair,” A collection of articles from Concrete Construction Magazine, June 1973.
    • “Why and How: Joints for Floors on Ground,” Report No. RP026.01B, Portland Cement Association, Skokie, Ill.
  • Cracking Plastic Shrinkage
    What is Plastic Shrinkage Cracking?

    Plastic shrinkage cracks are cracks that appear on the surface of a freshly placed concrete slab during finishing operation or soon after. These cracks are usually parallel to each other on the order of 1 to 3 feet apart, and 1 to 2 inches deep, and rarely do they intersect the perimeter of the slab. Plastic shrinkage cracks rarely impair the strength of concrete floors and pavements, nevertheless, they are unsightly. The development of these cracks can be minimized if appropriate measures are taken prior to and during construction. (Note: Plastic shrinkage cracks should be distinguished from other early or prehardening cracks caused by settlement of the concrete on either side of a reinforcing bar due to bleeding and resistance to settlement on either side of a top reinforcing bar, because of formwork movement, or differential settlement at a change from a thin to a deep section of concrete.)

    Why Do Plastic Shrinkage Cracks Occur?

    Plastic shrinkage cracks occur when the rate of evaporation of surface moisture exceeds the rate at which rising bleed water can replace it and the surface dries. As the bleed water evaporates and recedes below the concrete surface, menisci develop between the fine particles of cement and aggregate causing a tensile force to develop in the surface layers. If the concrete surface has started to set and has developed sufficient tensile strength, cracks do not form. However, if the surface dries before sufficient tensile strength develops, the tensile force in the surface layers will exceed the tensile strength and cracks will develop during the setting process. If the surface dries very rapidly, the concrete may still be plastic, and cracks do not develop at that time; but plastic cracks will surely form as soon as the concrete stiffens a little more. Plastic fibers can help resist the tension when concrete is very weak.

    The rate of evaporation of water is higher when the relative humidity is low, the wind velocity is high, and when the concrete surface is warmer than the surrounding air. ACI 305 (1) provides a chart that can be used to estimate the rate of evaporation and to tell when special precautions need to e taken. However, the chart isn’t infallible because many factors other than rate of evaporation are involved. Increasing the cement content tends to increase plastic cracking. Two factors are involved: reduced bleeding, and the smaller menisci between the fine particles which produce higher tensile forces. Concrete containing silica fume requires very careful attention to rate of evaporation to avoid plastic shrinkage cracking. Increased slump tends to increase plastic cracking. Anything that delays setting tends to increase plastic cracking when the rate of evaporation is high. Examples include: cool weather, cool subgrades, lower cement content, retarders, and most water reducers.

    How to Minimize Plastic Shrinkage Cracks

    Attempts to eliminate plastic shrinkage cracking by increasing the bleeding characteristics of the concrete either by increasing slump or by using different cement or aggregate or by addition of a retarder have not been found to be consistently effective. To reduce plastic shrinkage cracking, it is important to recognize ahead of time, before placement, when weather conditions may occur that are conducive to plastic shrinkage cracking.

    Precautions can then be taken to minimize its occurrence. They are:

    • Have proper manpower, equipment, and supplies on hand so that the concrete can be placed and finished promptly. If delays occur, cover the concrete with wet burlap, polyethylene sheeting or building paper between finishing operations. Some contractors find that plastic shrinkage cracks can be prevented in hot dry climates by spraying a chlorinated rubber curing compound, or monomolecular film, on the surface behind the screeding operation and before floating or troweling.
    • Start curing the concrete as soon as possible. Spray the surface with liquid membrane curing compound or cover the surface with we burlap and keep it continuously moist for a minimum of 3 days.
    • If concrete is to be placed on a dry subgrade or on previously placed concrete, the subgrade or the concrete base should be thoroughly dampened. The formwork and reinforcements should also be dampened.
    • The use of vapor barriers under a slab on grade greatly increases the risk of plastic shrinkage cracking. If a vapor barrier is required, cover it with a 2-inch layer of damp sand.
    • In the very hot and dry periods, use fog sprays. Erect temporary windbreaks to reduce the wind velocity over the surface of the concrete and, if possible, also provide sun shades to control the surface temperature of the slab. If conditions are critical, schedule placement to begin in the later afternoon or early evening.
    • Consider using synthetic fibers (ASTM C 1116) to resist plastic shrinkage cracking.
    • Make the concrete set faster.
    References
    • ACI Standard Recommended Practice for Hot Weather Concreting (ACI 305R), ACI Manual of Concrete Practices, Part 2.
    • “Report on Behavior of Concrete in Hot Climate,” by R. Shalon, RILEM. No. 62, March-April 1978.
    • “Plastic Shrinkage” by W. Lerch, Journal of the American Concrete Institute, volume 28, No. 8, February 1957.
    • “Control of Rapid Drying of Fresh Concrete by Evaporation Control,” by W. A. Cordon and J. D. Thorpe, Journal of the American Concrete Institute, Proceedings Volume 62, No. 8., August 1965.
    • “Cracking of Fresh Concrete as Related to Reinforcement,” by P. D. Cady, et al, Journal of the American Concrete Institute, Proceedings Volume 72, No. 8, August 1975.
  • Crazing Concrete Surfaces
    What is Crazing?

    Crazing is the development of a network of fine random cracks or fissures on the surface of concrete or mortar caused by shrinkage of the surface layer. These cracks are rarely more than 1/8 inch deep and are more noticeable on steel-troweled surfaces. The irregular hexagonal areas enclosed by the cracks are typically no more than 1 ½ inches across and may be as small as ½ or 3/9 inch in unusual instances. Generally, craze cracks develop at an early age and are apparent the day after placement or at least by the end of the first week. Often they are not readily visible until the surface has been wetted and it is beginning to dry out.

    Crazing cracks are sometimes referred to as shallow map or pattern cracking. They do not affect the structural integrity of concrete and rarely do they affect durability or wear resistance. However, crazed surfaces can be unsightly. They are particularly conspicuous and unsightly on concrete which contains calcium chloride.

    Why do Concrete Surfaces Craze?

    Concrete surface crazing usually occurs because one or more of the rules of “good concrete practice” were not followed. The most frequent violations are:

    • Poor or inadequate curing. Intermittent wet curing and drying or even the delayed application of curing will permit rapid drying of the surface and provoke crazing.
    • Too wet a mix, excessive floating, the use of a jitterbug or any other procedures which will depress the coarse aggregate and produce an excessive concentration of cement past and fines at the surface.
    • Finishing while there is bleed water on the surface or the use of a steel trowel at a time when the smooth surface of the trowel brings up too much water and cement fines. Use of a bullfloat or darby while bleed water is on the surface will produce a high water-cement ratio weak surface later which will be susceptible to crazing, dusting and other defects.
    • Sprinkling cement of the surface to dry up the bleed water is frequent cause of crazing surfaces. This concentrates fines on the surface.
    • Occasionally carbonation of the surface causes crazing. Carbonation is a chemical reaction between cement and carbon dioxide or carbon monoxide from unvented heaters. In such instances the surface will be soft and will dust as well.
    How to Prevent Crazing

    To prevent crazing start curing the concrete as soon as possible. The surface should be kept wet by either flooding the surface with water or, covering the surface with damp burlap and keeping it continuously moist for a minimum of 3 days or, spraying the surface with a liquid membrane curing compound. Curing retains the moisture required for proper combination of cement with water. This chemical reaction between cement and water is called hydration.

    Use moderate slump (3 to 5 inches), air entrained concrete. Higher slump (up to 6 or 7 inches) can be used providing the mixture is designed to produce the required strength without excessive bleeding and/or segregation. Air entrainment helps to reduce the rate of bleeding of fresh concrete and thereby reduces the chance of crazing.

    NEVER sprinkle or trowel dry cement or a mixture of cement and fine sand into the surface of the plastic concrete to absorb bleed water. Remove bleed water by dragging a garden hose across the surface. DO NOT perform any finishing operation while bleed water is present on the surface.

    Dampen the subgrade prior to concrete placement to prevent it absorbing too much water from the concrete. If an impervious membrane, such as polyethylene, is required on the subgrade cover it with 1 to 2 inches of damp sand to reduce bleeding.

    References
    • “Guide for Concrete Floor and Slab Construction,” ACI 302.1, Manual of Concrete Practice.
    • “Slab Construction Practices Compared by Wear Tests,” by L. Blake Fentress, ACI Journal, July, 1973.
    • “Concrete Slab Surface Defects: Causes, Prevention Repair,” Portland Cement Association (IS 177 T).
    • “Solutions to the Problems of Scaling, Crazing, Dusting of Concrete Slabs,” Modern Concrete, November, 1963.
  • Curing In-Place Concrete
    What is Curing?

    Curing is the maintaining of a satisfactory moisture content and temperature in concrete. Curing begins after placement and finishing so that the concrete may develop the desired strength and hardness.

    Without an adequate supply of moisture, the Portland cement in the concrete cannot react to form a quality product. Drying may remove the water needed for this chemical reaction called “hydration” and the concrete will be weak. Temperature is an important factor in proper curing, since the rate of hydration is temperature dependent. For exposed concrete, relative humidity and wind conditions are also important; they contribute to the rate of moisture loss from the concrete.

    Why Cure?
    Several important reasons are:

    Predictable strength gain. Laboratory tests show that concrete in a dry environment can lose as much as 50% of its potential strength compared to similar concrete that is moist cured.1 Concrete placed under high temperature conditions will gain early strength quickly but later strengths may be reduced. Concrete placed in cold weather will take longer to gain strength, delaying form removal and subsequent construction.

    Improved durability, especially on non-air entrained concrete slabs that may be subject to freezing conditions during construction. Well cured concrete has better surface hardness and therefore is more watertight.

    Better serviceability and appearance. A concrete slab that has been allowed to dry out too early will have a soft surface with poor resistance to wear and abrasion. Proper curing reduces crazing, dusting, and scaling.

    How to Cure

    Moisture Requirements for Curing – the concrete surface must be kept continuously wet or sealed to prevent evaporation for a period of at least several days after finishing. See the table for examples.

    Systems to keep concrete wet include:

    • Burlap or cotton mats and rugs used with a soaker hose or sprinkler. Car must be taken not to let coverings dry out and adsorb water from the concrete. The edges should be lapped and the materials weighted down so that they are not blown away.
    • Straw that is sprinkled with water regularly. Straw can easily blow away, and if it dries, can catch fire. The layer of straw should be 6 inches thick, and should be covered with a tarp.
    • Sprinkling on a continuous basis is suitable provided the air temperature is well above freezing. The concrete should not be allowed to dry out between soakings, since alternate wetting and drying may damage the concrete.
    • Ponding of water on a slab is an excellent method of curing. The water should not be more than 200°F cooler than the concrete and the dike around the pond must be secure against leaks.
    • Damp earth, sand, or sawdust will cure flatwork, especially floors. There should be no organic or iron staining contaminants in the materials used.

    Sealing materials include:

    Liquid membrane-forming compounds – must conform to ASTM Specifications3 at the rate of application that is specified. Apply to the concrete surface about one hour after finishing. Do not apply to concrete that is still bleeding, or has a visible water sheen on the surface. While a clear liquid may be used, a white pigment will give reflective properties, and allow for inspection of coverage. A single coast may be adequate but where possible a second coat, applied at right angles to the first, is desirable for even coverage. If the concrete will be painted, or covered with vinyl or ceramic tile, then a liquid compound that is non-reactive with the paint or adhesives must be used, or a compound that is easily brushed or washed off. On floors, the surface should be protected from the other trades with scuff-proof paper after the application of the curing compound.

    Plastic sheets – either clear, white (reflective) or pigmented. Plastic should conform to ASTM Stardards5, be at least 4 mils thick, and preferably reinforced with glass fibers. The plastic should be laid in direct contact with the concrete surface as soon as possible without marring the surface. The edges of the sheets should overlap and be fastened with waterproof tape and then weighted down to prevent the wind from getting under the plastic. Plastic will make dark streaks wherever a wrinkle touches the concrete so plastic should not be used on concretes where appearance is important.

    Waterproof paper – used like plastic sheeting, bug does not mar the surface. Should also conform to ASTM Standards.

    References
    • “Effect of Curing Condition on Compressive Strength of Concrete Test Specimens,” NRMCA Publication No. 53.
    • “How to Eliminate Scaling,” Concrete International, February, 1980. American Concrete Institute, Box 19150 Redford Station, Detroit, Michigan 48219.
    • ASTM C 309, “Specification for Liquid Membrane Forming Compounds for Curing Concrete,” American Society for Testing Materials, 1916 Race Street, Philadelphia, PA, 19103.
    • ACI 308, “Standard Practice for Curing Concrete,” ACI Manual of Concrete Practice, Part 2, American Concrete Institute.
    • ASTM C 171, “Specification for Sheet Materials for Curing Concrete,” American Society for Testing Materials.
    • ACI 306, “Cold Weather Concreting,” ACI Manual of Concrete, Part 2, American Concrete Institute.
  • Curling of Concrete Slabs
    What is Curling?

    Curling is the distortion of a slab into a curved shape by upward or downward bending of the edges. This distortion can lift the edges of the slab from the base leaving an unsupported edge or corner which can crack when heavy loads are applied. Sometimes, curling is evident at any early age. In other cases, slabs may curl over an extended period.

    Why do Concrete Slabs Curl?

    Typically, upward curling of the edges of a slab is caused by shrinkage or contraction of the top relative to the bottom. When one surface of the slag changes size more than the other, the slab will warp its edges in the direction of relative shortening. This curling is most noticeable at the sides and corners.

    Changes in slab dimensions which lead to curling are most often related to moisture and temperature gradients in the slab. One primary characteristic of concrete which affects curling is drying shrinkage. The most common occurrence of curling is when the top part of the slab dries and shrinks with respect to the bottom.

    The slab edges curl upward (Figure 1A). Immediate curling of a slab is most likely related to poor curing and rapid surface drying; and anything that increases drying shrinkage, such as an admixture, will tend to increase curling.

    In slabs, bleeding and poor curing both tend to produce surface concrete with higher drying shrinkage potential than the concrete in the bottom of the slab. Bleeding is accentuated in slabs on polyethylene or topping mixtures placed on concrete slabs; and shrinkage differences from top to bottom in these cases are larger than for slabs on an absorptive subgrade.

    Thin slabs and long joint spacing tend to increase curling. For this reason, thin unbonded toppings need to have a fairly close joint spacing.

    In industrial floors, close joint spacing may be undesirable because of the increased number of joints and increased joint maintenance problems. However, this must be balanced against the probability of intermediate random cracks and increased curling at the joints. The other factor that can cause curling is temperature differences between the top and bottom of the slab. The top part of the slab exposed to the sun will expand relative to the cooler bottom causing a downward curling of the edges (Figure 1B). Alternately, during a cold night when the top cools and contracts with respect to a warmer subgrade, the curling due to this temperature differential will add to the upward curling caused by moisture differentials.

    How to Minimize Slab Curling

    The primary factors controlling dimensional changes of concrete which lead to curling are drying shrinkage, construction practices, moist or wet subgrades, and day-might temperature cycles. The following practices will help to minimize the potential for curling:

    • Use the lowest practical slump and avoid adding retempering water, particularly in hot weather.
    • Use the larges practical maximum size aggregate and/or the highest practical coarse aggregate content to minimize drying shrinkage.
    • Take precautions to avoid excessive bleeding. Use a damp, but absorptive, subgrade so that all the bleed water is not forced to the top of the slab.
    • Avoid using polyethylene vapor barriers unless covered with at least two inches of damp sand.
    • Avoid a higher than necessary cement content if the subgrade is wet in service. Dense, impermeable concrete will produce larger top to bottom moisture differentials and curl more. Use of fly ash is preferable to very high cement content, and consideration should be given to specifying strength at 56 to 90 days.
    • Cure the concrete thoroughly, including joints and edges. If membrane curing compounds are used, apply at twice the recommended rate in two applications at right angles to each other.
    • For floor areas where curling tends to be a problem, cure the concrete with a heavy wax floor sealing compound of the type used on terrazzo. (Note: Tile adhesives will not stick to these materials.)
    • Use a joint spacing in feet equal to two times the slab thickness in inches (PCA recommendation for maximum size aggregate less than 3/4 inch).
    • For thin toppings, clean the base slab to ensure bond and consider use of studs and wire around the edges and particularly in the slab corners.
    • Use a thicker slab.
    • The use of properly designed and placed slab reinforcement may help reduce curling.
    References
    • Cement and Concrete Terminology, ACI SP-19, American Concrete Institute (ACI), P.O. Box 19150 Redford Station, Detroit, Michigan 48219.
    • Guide for Concrete Floor and Slab Construction (ACI 302), ACI Manual of Concrete Practice, Part 2.
    • “Shrinkage and Curling of Slabs on Grade, Part 11 Warping and Curling,” R.F. Ytterberg, ACI Concrete International May 1987, pp. 54-61.
    • “Drying Shrinkage of Concrete,” R.C. Meininger, NRMCA Engineering Report No.l RD3 ( A Summary of Joint Research Laboratory Series), June 1966, 22 pp. NRMCA “Concrete in Practice” (CIP) Series.
    • “Slabs on Grade,” ACI Concrete Craftsman Series.
    • Transportation Research Record 1207, Pavement Design, National Research Council, Washington, D.C., 1988, p. 44.
    • “Design and Control of Concrete Mixtures,” Portland Cement Association, Thirteenth Edition, 1988.
  • Discoloration
    What is Discoloration?

    Surface discoloration is the non-uniformity of color or hue on the surface of a single concrete placement. It may take the form of dark blotches or mottled discoloration on flatwork surface, gross color changes in large areas of concrete caused by a change in the concrete mix, or light patches of discoloration caused by efflorescence. In this context, it is not intended to include stains caused by foreign material spilled on a concrete surface.

    Why Does Discoloration Occur?

    Discoloration due to changes in cement or fine aggregate sources in subsequent batches in a placement sequence could occur, but it generally rare and insignificant. Cement that has hydrated to a greater extent will generally be lighter in color. Inconsistent use of admixtures, insufficient mixing time, and improper timing of finishing operations can cause this effect.

    A yellowish to greenish hut may appear on concrete containing ground slag as a cementitious material. This will disappear with time. Concrete containing ground slag does, however, have a generally lighter color. The discoloration of concrete cast in forms or in slabs on grade is usually the result of a change in either the concrete composition or a concrete construction practice. In most studies, no single factor seemed to cause discoloration. Factors found to influence discoloration are: the use of calcium chloride, variation in cement alkali content, delayed hydration of the cement paste, admixtures, hard-troweled surfaces, inadequate or inappropriate curing, concreting practices and finishing procedures that cause surface variation of the water-cement ratio, and changes in the concrete mix.

    How to Prevent Discoloration
    • Minimizing or eliminating the use of high-alkali content cements will reduce the occurrence of discoloration.
    • Calcium chloride in concrete is a primary cause of concrete discoloration. The chances for discoloration are much less if calcium chloride or chloride-bearing chemical admixtures are not used.
    • The type, kind, and condition of formwork can influence surface color. Forms with different rates of absorption will cause surfaces with different shades of color. A change in the type or brand of a form release agent can also change concrete color.
    • Eliminate trowel burning of the concrete. The most common consequence is that metal fragments from the trowel are embedded in the surface of the concrete. Also, concrete which has been hard troweled may have a dark discoloration as a result of densifying the surface which reduces the water-cement ratio. The resulting low water-cement ratio affects the hydration of the cement ferrites which contributes to a darker color. Concrete surfaces that are troweled too early will increase the water-cement ratio at the surface and lighten the color.
    • Concrete which is not properly of uniformly cured may develop discoloration. Uneven curing will affect the degree of hydration of the cement. Curing with polyethylene may also cause discoloration. When the plastic sheeting is in direct contact with the concrete, it will cause streaks. Using an even application of quality spray or curing compound may be the better alternative.
    • The discoloration of a slab may be minimized or prevented by moistening absorptive subgrades, following proper curing procedures, and adding proper protection of the concrete from drying by the wind and sun.
    How to Remove Discoloration

    Certain treatments have been found to be successful in removing or decreasing the surface discoloration of concrete flatwork. Discoloration caused by calcium chloride admixtures and some finishing and curing methods can be reduced by repeated washing with hot water and a scrub brush. The slab should be alternately flushed and brushed, and then dried overnight until the discoloration disappears. If a discoloration persists, a dilute solution (1% concentration) of hydrochloric (meiotic) acid or dilute solutions (3% concentration) of weaker acids like acetic or phosphoric acid may be tried. Prior to using acids, dampen the surface to prevent it from penetrating into the concrete and flush with clean water within 15 minutes of application.

    The use of a 20% to 30% water solution of diammonium citrate (2lbs. in 1 gallon of water) has been found to be very effective treatment, by the PCA, for more severe cases of discoloration.4,5,6 Apply the solution to a dried surface for 15 minutes. A whitish gel that forms should be diluted with water and brushed. Subsequently, the gel should be completely washed off with water. More than one treatment may be required.

    Some types of discoloration, such as trowel burning, may not respond to any treatment. It may be necessary to paint or use another type of coating to eliminate the discoloration. Some types of discoloration may, however, fade with wear and age.

    Precautions

    Chemical methods to remove discoloration may significantly alter the color of concrete surfaces. Inappropriate or improper use of chemicals to remove discoloration may aggravate the situation. A trial treatment on an inconspicuous area is recommended. Acids should be thoroughly flushed from a concrete surface.

    The user of chemicals should refer to a Material Safety Data Sheet (MSDS) or manufacturer guidelines to be aware of toxicity, flammability, and/or health hazards associated with the use of the material. The appropriate safety procedures such as the use of gloves, goggles, respirators, and waterproof clothing are recommended.

    References
    • “Surfaces Discoloration of Flatwork,” Greening, N.R. and Landgren, R., Portland Cement Association Research Department Bulletin RX 203, 1966.
    • “Discoloration of Concrete, Causes and Remedies,” Kosmatka, S.H., Concrete Products, April 1987.
    • “Discoloration of Concrete Flatwork,” Neal, R.E., Lehigh Portland Cement Company, 1977.
    • “Removing Stains and Cleaning Concrete Surfaces,” PCA, 1988.
    • “Discoloration: Myths, Causes and Cures,” Rech, D.P., Owl Rock Products, 1089.
    • “Removing Stains from Mortar and Concrete,” Corps of Engineers Miscellaneous Paper C-6808.
    • “Discolored Concrete Surfaces,” Goeb, Eugene O., Concrete Products, Vol. 96, No. 2, February, 1993.
  • Discrepancies in Yield
    What is Concrete Yield?

    Concrete yield is defined as the volume of freshly mixed concrete from a known quantity of ingredients. Ready mixed concrete is sold on the basis of the volume of fresh, unhardened concrete – usually in cubic yards (yd3).

    The basis for calculating the volume is described in the American Society for Testing and Materials, ASTM Specification C94 for Ready Mixed Concrete. The volume of freshly mixed and unhardened concrete in a given batch is determined by dividing the total weight of the batch by the average weight per cubic foot of the concrete determined in accordance with ASTM C 138. Three unit weight tests must be made, each from a different truck using a 1/2 ft3 container.

    ASTM C94 notes: “It should be understood that the volume of hardened concrete may be, or appear to be, less than expected due to waste and spillage, over excavation, spreading forms, some loss of entrained air, or settlement of wet mixtures, none of which is the responsibility of the producer.”

    Why do Yield Problems Occur?

    Most yield complaints concern an imagined or real deficiency of concrete volume. Apparent under yield develops when insufficient concrete is ordered to fill the forms and to take care of contingencies discussed below. An actual under yield should be corrected using unit weight measurements and yield calculations. An over yield can also be an indication of a problem if the excess concrete is caused by too much air or aggregate, or if the forms have not been properly filled.

    Apparent concrete shortages are sometimes caused by the following:

    • Miscalculation of form volume or slab thickness exceeding the assumed thickness by a fraction of an inch. A 1/8 inch error in a 4 inch slab would mean a shortage of 3 percent or 1 yd3 in a 32 yd3 order.
    • Deflection or distortion of the forms by the pressure of the concrete.
    • Irregular subgrade, placement over granular fill, and settlement of subgrade prior to placement can increase slab thickness.
    • Over the course of a large job, the small amounts of concrete returned each day or used in mud sills or incidental footing can accumulate.
    How to Prevent Yield Discrepancies

    To prevent or minimize concrete yield problems:

    • Check concrete yield by making ASTM C 138 concrete unit weight tests early in the job. Repeat these tests if a problem arises. Be sure that the scale is accurate, that the unit weight bucket is properly calibrated and that a flat plate is used for strike off. Concrete yield volume in cubic fee is total batch weight in pounds divided by unit weight in pounds per cubic foot. The total batch weight is the sum of the weights of all ingredients from the batch ticket. As a rough check the mixer truck can be weighed empty and full. The difference is the total batch weight.
    • Measure formwork accurately. Near the end of large pours, carefully measure the remaining volume so that the amount in the last 2 or 3 trucks can be adjusted to provide the required concrete. This can prevent waiting for an extra ½ yd3 after the plant has closed or the concrete trucks have been scheduled for other jobs.
    • Estimate extra concrete needed for waste and increased placement dimensions over nominal dimensions. Include an allowance of 4 to 10 percent over plan dimensions for waste, over excavation and other causes. Repetitive operations and slip form operations permit more accurate estimates of the amount of concrete that will be needed. On the other hand, sporadic operations involving a combination of concrete uses such as slabs, footings, walls, and as incidental fill around pipes, etc. will require a bigger allowance for contingencies.
    • Construct forms so that they can withstand the pressure of the concrete without deflection or distortion.
    • For slabs on grade, the subgrade should be accurately finished and compacted to the proper elevation.

    Follow These Rules to Avoid Under Yield:

    • Measure volume needed accurately.
    • Estimate waste and potential increases thickness – order more than required by at least 4 to 10 percent.
    • To check yield use the ASTM C 138 unit weight test method on three samples from three different loads – yield is the total batch weight divided by the average unit weight.
    References
    • ASTM C94, Standard Specification for Ready Mixed Concrete, American Society for Testing and Materials, 1916 Race Street, Philadelphia, Pennsylvania 19103.
    • ASTM C 138, Standard Test Method for Unit Weight, Yield and Air Content of Concrete, American Society for Testing and Materials.
    • “An Analysis of Factors Influencing Concrete Pavement Cost,” by Harold J. Haim, Portland Cement Association, 5420 Old Orchard Road, Skokie, Illinois 60077.
  • Dusting Concrete Surfaces
    What is Dusting?

    Chalking or powdering at the surface of a concrete slag is called dusting.

    The characteristics of such surfaces are:
    They powder under any kind of traffic.
    They can be easily scratched with a nail of even by sweeping.

    Why Do Concrete Floors Dust?

    A concrete floor dusts under traffic because the wearing surface is weak. This weakness can be caused by:

    • Any finishing operation performed while bleed water is on the surface. Working this bleed water back into the top ¼ inch of the slab produces a very high water-cement ratio and, therefore, a low strength surface layer.
    • Placement over a non absorptive subgrade or polyethylene. This reduces normal absorption by the subgrade, increases bleeding and, as a result the risk of surface dusting.
    • Insufficient or no curing. This omission often results in a soft surface skin which will easily dust under foot traffic.
    • Floating and/or troweling of condensation moisture from warm humid air on cold concrete. In cold weather the concrete sets slowly, in particular cold concrete in basement floors. If the humidity is relatively high water will condense on the freshly placed concrete which, if troweled into the surface, will cause dusting.
    • Inadequate ventilation in close quarters. Carbon dioxide from open salamanders, gasoline engines or generators, power buggies or mixer engines may cause a chemical reaction known as carbonation which greatly reduces the strength and hardness of the concrete surface.
    • Inadequate protection of freshly placed concrete from rain, snow or drying winds.
    How to Prevent Dusting

    In general, use concrete with a moderate slump (not over 5 inches). However, concrete with a higher slump (up to 6 or 7 inches) may be used providing the mixture is designed to produce the required strength without excessive bleeding and/or segregation. The higher slump levels can be used in hot weather when setting time is reduced and less time is available for bleeding. In cold weather delayed setting will increase bleeding and require use of lower slump. Concrete having a low water-cement ratio and moderate slump helps produce a strong wear resistant surface.

    NEVER sprinkle or trowel dry cement into the surface of plastic concrete to absorb bleed water. Remove bleed water by dragging a garden hose across the surface. Excessive bleeding of concrete can be reduced by using air entrained concrete, by modifying mix proportions and by reducing setting time.

    DO NOT perform any finishing operations with water present on the surface. Bleed water can be worked into surface fines from delayed bullfloating. Initial screeding must be promptly followed by bullfloating. Do not use a jitterbug to bring excess mortar to the surface.

    Avoid direct placement of concrete on polyethylene or non absorptive subgrades. Place 1 to 2 inches of damp sand over polyethylene or non absorptive subgrade prior to concrete placement. On absorptive subgrades dampen the surface just prior to concrete placement.

    Provide proper curing by using liquid membrane curing compound or by covering the surface with wet burlap. Protect young concrete from the environment.

    When placing concrete in cold weather use warm concrete as well as an accelerator.

    How to Repair Dusting

    To minimize or eliminate dusting, apply a chemical floor hardener such as zinc or magnesium fluorosilicate in compliance with manufacturer’s directions on thoroughly dried concrete. If dusting persists, use hardeners with cementitious properties of their won, such as latex formulations, boiled linseed oil or paint.

    In sever cases; a serviceable floor can be obtained by wet-grinding the top surface, followed by properly bonded placement of a topping course. If this is not practical, installation of a floor covering, such as carpeting or vinyl tile covering is the least expensive solution to severe dusting.

    Follow These Rules to Prevent Dusting
    • Use moderate slump concrete.
    • Finish properly.
    • Cure properly.
    References
    • “Job Conditions Affect Cracking and Strength of Concrete in Place,” by Richard H. Campbell et al, ACI Journal, January 1976.
    • “Guide for Concrete Floor and Slab Construction,” ACI 302.1 R.
    • “Causes of Floor Failures,” by A.T. Hersey, ACI Journal, June 1973.
    • “Slab Construction Practices Compared by Wear Test,” by Blake Fentress, ACI Journal, July 1973
    • “Cement Mason’s Manual for Residential Construction,” Portland Cement Association.
    • “The Effect of Various Surface Treatments, Using Zinc and Magnesium Fluorosilicate Crystals on Abrasion Resistance of Concrete Surfaces,” Concrete Laboratory Report No. C-819, U.S, Bureau of Reclamation.
  • Finishing Concrete Flastwork
    What is Finishing?

    Finishing is the operation of consolidating, leveling and creating a concrete surface of a desired texture and hardness. The finish can be strictly functional or decorative.

    Why Finish Concrete?

    Finishing makes concrete attractive and serviceable. The final texture, hardness, and joint pattern on slabs, floors, sidewalks, patios, and driveways depend on the concrete’s end use. Warehouse or industrial floors usually need to be level and smooth, while other interior floors that are covered with carpet do not have to be as exact. Exterior slabs must be sloped to carry away water and must provide a texture which will not be slippery when wet.

    How to Finish Concrete?

    The finishing operation should be carefully planned. Skill, knowledge and experience are required to deal with a variety of concrete mixtures and field conditions. Having the proper manpower and equipment available, and timing the operations properly for existing conditions, is critical. A slope of 1/8 in. per foot is necessary to avoid low spots and to drain water away from buildings.

    Delays after the concrete arrives create problems in finishing and can reduce final quality. Complete the excavation, compaction, form work and placement of mesh and rebar ahead of time.

    Guidelines for placing & consolidating concrete:
    • A successful job depends on selecting the correct concrete mix for the job. Consult Cadman Lab Systems.
    • If possible place concrete directly from the truck chute or use wheelbarrows, buggies or pumps to avoid excessively wet, high slump concrete. Start at the far end and work to the near end. On a slope use stiffer concrete and work up the slope.
    • Spread the concrete using a short-handled, square-ended shovel, a concrete rake, or a come along. Do not use a garden rake since it will cause segregation.
    • Tamp the concrete with a spade or 2 by 4 along the edges of the forms to release air voids and consolidate the concrete.
    • Use a lumber or metal straightedge (called a screed) to strike off the concrete and level it. Rest the screed on edge on the top of the forms, tilt it forward and draw it across the concrete with a sawing motion. Keep a little concrete in front of the screed to fill in any low spots. (Do not use a jitterbug or vibrating screed to work up an excessive layer of mortar on the surface.)
    Follow these Rules to Finish Concrete

    FLOAT the concrete as soon as it has been struck off. A float is a wood or metal tool used to further level the concrete surface and to embed the large aggregates. On small jobs a float is hand held; on larger jobs a long-handled bull float may be used. One or tow passes should be enough to smooth and level the surface without sealing the concrete. Floating must end before visible bleed water rises to the surface.

    WAIT for the concrete to stop “bleeding”. Bleeding occurs as the solids in the concrete settle. All other finishing operations MUST WAIT until the concrete has stopped bleeding and the water sheen has left the surface. Any finishing operations done while the concrete is still bleeding WILL RESULT in later problems such as dusting, scaling, crazing and blisters. The waiting period depends on: the amounts of water cement and chemical admixtures in the concrete; and the weather.

    EDGE the concrete all the way around. Spade the concrete next to the form gently with a small mason’s trowel and then use the edging tool to give the concrete rounded edges.

    JOINT the concrete by grooving it. The jointer should have a blade one-fourth the depth of the slab (1 in. deep joints on a 4 in. slab). Use a straight piece of lumber as a guide. A shallow-bit groover should only be used for decorative grooves. See pages 11 and 12 for joint spacing.

    TROWEL the concrete according to its end use. For sidewalks, patios and driveways, troweling may not be required. Repeated passes with a steel trowel will produce a smooth floor that will be slippery when wet. For a smooth floor make successive passes with a smaller steel trowel and increased pressure. Excessive troweling may create dark “trowel burns.” Tilting the trowel will cause an undesirable “chatter” texture.

    TEXTURE the concrete surface after floating (for sidewalks, patios or driveways) or after troweling (for interior flatwork) with a coarse or fine push-broom to give a non-slip surface. For information about architectural surface finishes such as exposed aggregate, dry shake color, integral color, and stamped or patterned concrete.

    NEVER sprinkle water or cement on concrete while finishing it. This may cause dusting or scaling.

    CURE the concrete as soon as all finishing is completed and the water sheen has left the surface.

    References
    • Concrete in Practice (CIP) Series. Available from: National Ready Mixed Concrete Association, 900 Spring Street, Silver Spring, Maryland 20910.
    • “Cement Mason’s Guide,” Publication No. PA 122.02H, Portland Cement Association, 5420 Old Orchard Road, Skokie, Illinois 60077.
    • “Residential Concrete,” National Association of Home Builders, 15th & “M” Sts., N.W., Washington, D.C. 20005.
    • “Concrete Craftsman Series-Slabs on Grade,” American Concrete Institute, P.O. Box 19150 Redford Station, Detroit, Michigan 48219.
    • ACI 302, “Guide for Concrete Floor and Slab Construction,” ACI Manual of Concrete Practice, Part 2, American Concrete Institute.
    • “Finishing and Related Problems,” Concrete Construction Magazine, 426 S. Westgate, Addison, Illinois 60101.
    • “Handling, Placing, Finishing and Curing Concrete,” Eugene D. Hill, Jr., NRMCA Publication No. 177.
  • Flexural Strength of Concrete
    What is Flexural Strength?

    It is the ability of a beam or slab to resist failure in bending. It is measured by loading un-reinforced 6x6 inch concrete beams with a span three times the depth (usually 18 in.). The flexural strength is expressed as “Modulus of Rupture” (MR) in psi.

    Flexural MR is about 12 to 20 percent of compressive strength. However, the best correlation for specific materials is obtained by laboratory tests.

    Why Test Flexural Strength?

    Designers of pavements use a theory based on flexural strength. Therefore, laboratory mix design based on flexure may be required, or a cement content may be selected from past experience to yield the needed design MR. Some also use MR for field control and acceptance of pavements. Very few use flexural testing for structural concrete. Agencies not using flexural strength for field control generally find the use of compressive strength convenient and reliable to judge the quality of the concrete as delivered.

    How to Use Flexural Strength

    Beam specimens must be properly made in the field. Pavement concretes are stiff (1/2 to 2 ½ inch slump). Consolidate by vibration in accordance with ASTM C 31 and tap side to release bubbles. For higher slump, after rodding, tap the molds to release bubbles and spade along the sides to consolidate. Never allow the beam surfaces to dry at any time. Immerse in saturated lime water for at least 20 hours before testing.

    Specifications and investigation of apparent low strengths should take into account the higher variability of flexural strength results. Standard deviation for projects with good control range from about 40 to 80 psi. Values over 100 psi indicate testing problems, and there is high likelihood that testing problems, or moisture differences within a beam, will cause low strength.

    Where a correlation between flexural and compressive strength has been established, core strengths by ASTM C 42 can be used for compressive strength to check it against the desired value using the ACI 318 85 percent criteria. It is impractical to saw beams from a slab for flexural testing. Sawing beams will greatly reduce measured flexural strength and should not be done. Some use has been made of measuring indirect tensile strength of cores by ASTM C 496, but experience is lacking on how to apply the data.

    Another procedure for in-place strength investigation uses compressive strength of cores calibrated by comparison with acceptable placements on either side of the concrete in question:

    What are the Problems with Flexure?

    Flexural tests are extremely sensitive to specimen preparation, handling, and curing procedure. Beam specimens are very heavy, and allowing a beam to dry will yield lower strengths. Beams must be cured in a standard manner, and tested while wet.2A short period of drying can produce a sharp drop in flexural strength.

    Many state highway agencies have used flexural strength but are now changing to compressive strength for job control on concrete paving. Cylinder strengths are also used for concrete structures.

    “The data points to a need for a review of current testing procedures. They suggest also that, while the flexural strength test is a useful tool in research and in a laboratory evaluation of concrete ingredients and proportions, it is too sensitive to testing variations to be usable as a basis for the acceptance or rejections of concrete in the field.”3 The CSI Spec-Data Sheet by NRMCA, the Municipal Concrete Pavement Manual by ACPA, ACI 325, and ACI 330 on Concrete Pavements, all point to the use of compressive strength as more convenient and reliable. The Pennsylvania DOT uses compressive strength of cylinders; 3750 psi is specified with 3000 psi for opening a pavement to traffic.

    The concrete industry and inspection agencies are much more familiar with traditional cylinder compression tests for control and acceptance of concrete. Flexure can be used for design purposed, but the corresponding compressive strength should be used to order and accept the concrete. Any time trial batches are made, both flexural and compressive tests should be made so that a correlation can be developed for field control.

    References
    • “How Should Strength be Measured for Concrete Paving?” by Richard C. Meininger, NRMCA TIL 420, and “Data Summary,” NRMCA TIL 451.
    • “Significance of Tests and Properties of Concrete and Concrete-Making Materials,” Chapter 12 on Strength, ASTM STP 169B.
    • “Studies of Flexural Strength of Concrete, Part 3, Effects of Variations in Testing Procedures,” by Stanton Walker and D.L. Bloem, NRMCA Publication No. 75 (ASTM Proceedings, Volume 57, 1957).
    • “Variation of Laboratory Concrete Flexural Strength Tests,” by W. Charles Greer, Jr., ASTM, Cement, Concrete and Aggregates, Winter, 1983.
    • “Concrete Mixture Evaluation and Acceptance for Air Field Pavements” by Richard O. Meininger and Norman R. Nelson, ASCE Air Field Pavement Conference, September, 1991. NRMCA Publication No. 178.
  • Flowable Fill Materials
    What is Flowable Fill?

    A low strength material mixed to a wet, flowable slurry used as an economical fill or backfill material placed by pouring it into the cavity to be filled. Slumps measured in the ordinary way are generally 8 in. or higher. It is self-leveling with a consistency similar to pancake batter; it can be placed with minimal effort and no vibrations or tamping. It hardens and develops strength.

    ACI Committee 229 calls it “Controlled Low Strength Material” (CLSM); it is not considered concrete. Other names used for this material are flowable mortar or lean-mix backfill. If it is anticipated or specified that the flowable lean-mix backfill may be excavated at some point in the future the strength must be much lower than the 1200 psi which ACI uses as the upper limit for CLSM. The late-age strength of removable CLSM materials should be in the range of 30 to 150 psi as measured by compressive strength in cylinders.

    Why is CLSM Used?

    Flowable CLSM mixtures are an economical alternative due to the saving of labor and time over placing and compacting soil or granular materials.

    Uses of Flowable Fill Include:

    • BACKFILL (Sewer Trenches, Utility Trenches, Bridge Abutments, Conduit Trenches, Pile Excavations, and Retaining Walls)
    • STRUCTURAL FILL (Foundation Sub base, Sub footing, Floor Slab Base, and Pipe Bedding)
    • OTHER USES (Abandoned Underground Storage Tanks, Wells, Abandoned Utility Company Vaults, Voids Under Pavement, Sewers and Manholes, and to contend with Muddy Conditions)
    How is Flowable Fill Ordered?

    Ask for it by intended use and indicate whether it is required to be easily removed later. Ready mixed concrete producers generally have developed proportions for flowable CLSM products that make best use of economical aggregates and/or fly ash.

    Strength (for later removability) At least 20 psi is needed at 3 days and 30 psi at 28 days (ASTM C 403 Penetration Resistance Numbers of 500 to 1500) should be obtained to assure required bearing capacity in-place as fill. However, later age strength must be limited to assure convenient removal with power equipment.

    Strength (when it can or must be higher) When higher strength structural fills are required or can be tolerated because removal is not required use higher cement and/or fly ash contents.

    Testing Flowable CLSM Mixtures
    • Sample and remix sample (ASTM C 172)
    • Compressive Strength. Use 6 x 12 in. plastic cylinder molds, fill to overflowing and then taps sides lightly. Cure cylinders in the molds (covered) until time of testing (or at least 14 days). Strip carefully using a knife to cut plastic mold off. The process of capping with sulfur compounds can break these low strength materials. Neoprene caps have been used, some do not cap, but high strength gypsum plasters seem to work best.
    • Slump testing is not recommended since a very wet consistency is required for the proper self-leveling consistency. ASTM C 939 for flow of grout can be used by wet screening to remove coarse particles. An efflux time of 10 to 26 seconds through a special flow cone with a ½ in. discharge tube has been used.
    • Until weight and yield (ASTM C 138) by normal procedures.
    • Air content by pressure meter (ASTM C 231). (if air entrainment is being used)
    • Penetration resistance tests such as ASTM C 4003 may be useful in judging the setting and strength development up to a penetration resistance number of 4000 (roughly 100 psi compressive cylinder strength).
    • Density tests are not required since it becomes rigid after hardening.

    Setting and Early Strength may be important where equipment, traffic, or construction loads must be carried. Judge setting by scraping off loose accumulations of water and fines on top and see how much force is necessary to cause an indentation in the material. ASTM C 403 penetration can be run to estimate bearing strength.

    Density in place is usually in the 115 to 145 lb. / cu.ft. range, higher than that obtained from most compacted soils or aggregates. If lightweight fills are needed or if greater thermal insulation is needed, high air-entrainment, foam materials, and/or lightweight aggregates can be used.

    Flowability of CLSM is important, so the mixture will flow into place and consolidate due to its fluidity without vibration or puddling action.

    Durability. CLSM fill materials are not designed to resist freezing and thawing, abrasive or erosive actions, or aggressive chemicals. If this is required, use a high quality concrete. Fill materials are usually buried in the ground or otherwise confined. If CLSM deteriorates in place it will continue to act as a granular fill.

    How is Flowable Fill Delivered and Placed?

    CLSM is delivered in ready mix concrete trucks and placed easily by chute in a flowable condition directly into the cavity to be filled or into a pump for final placement. Keep the drum agitating. For efficient pumping some granular material is needed in the mixture.

    CAUTIONS

    Fluidized CLSM is a heavy material and during placement (prior to setting) will exert a high fluid pressure against any forms, embankment, or wall used to contain the fill.

    Placement of Flowable Fill around and under tanks, pipes, or large containers such as swimming pools can cause the container to float or shift.

    References
    • “Fly Ash Design Manual for Road and Site Applications,” Volume 2: Slurried Placement, by GAI Consultants, Inc., for the Electric Power Research Institute, Palo Alto, California.
    • NRMCA Promotion Pointer No. 273, “Flowable Fill, A New Produce = A New Market,” July, 1985.
    • Iowa Department of Transportation Supplemental Specifications for Flowable Mortar, January 19, 1988.
    • Specification for Lean Mix Backfill, prepared by Scientific Service, Inc., Under HUD Contract, Reprinted by NAA and NRMCA, August, 1984.
  • Grout
    What is Grout?

    ACI1 defines grout as “a mixture of cementitious material and water, with or without aggregate, proportioned to produce a pourable consistency without segregation of constituents.”

    The terms grout and mortar are frequently used interchangeably but there are clear distinctions. Grout need not contain aggregate wheas mortar contains fine aggregate. Grout is supplied in a pourable consistency whereas mortar is not. Grout fills space whereas mortar bonds elements together, as in masonry construction.

    Grout is often identfified by its application. Some examples are: Bonded prestressed tendon grout, auger cast pile grout, masonry grout, and preplaced aggregate grout. Controlled low strength material (flowable fill) is a type of grout.

    Why is Grout Used?

    Grout is used to fill space or cavities and provide continuity between building elements. In some applications, grout will act in a structural capacity. In projects where small quantities of grout are required, it is proportioned and mixed on site. The ready mixed concrete producer is generally called upon when large quantities are needed.

    How to Specify Grout

    ASTM C 476 for masonry grout dictates proportions by loose volumes and is convenient for small quantities of grout mixed on site. These grout mixtures have high cement contents and tend to produce much higher strengths4 than specified in ACI 5305 or Model Codes.

    When grout is ordered from a ready mixed concrete producer, the specifications should be based on consistency and compressive strength. Converting loose volume proportions into batch weights per cubic yard is subject to errors and can lead to controversies on the job.

    Specifications should address the addition of any required admixtures for grout. Conditions of delivery, such as temperature, time limits, and policies on job site addition of water, should be specified. Testing frequency and methods of acceptance must be covered in specifications.

    How to Test Grout

    The consistency of grout affects its strength and other properties. It is critical that grout consistency permit the complete filling of void space without segregation of ingredients. Consistency of masonry grout may be measured with a slump cone (ASTM C 143), and slumps of 8-11 in. are suggested. This is particularly applicable for grouts containing ½ in. or smaller coarse aggregate.

    For grouts without aggregate, or only fine aggregate passing a No. 8 sieve, consistency is best determined with a flow cone (ASTM C 939). For flow values exceeding 35 seconds, use the flow table in ASTM C 109, so modified to use 5 drops in 3 seconds.

    Masonry grout (“blockfill”) for strength tests specimens should be cast in molds formed by masonry units having the same absorption characteristics and moisture content as the units used in construction (ASTM C 1019). Never use non absorbent cube or cylinder molds for this purpose.

    Strength of other types of grout is determined using 2 in. cubes per ASTM C 942. Method C 942 allows for field preparation, recognizes fluid consistency, and also affords a means for determining compressive strength of grouts that contain expansive agents or grout fluidizers. This is extremely important since “expansive” grouts can lose substantial compressive strengths if cubes are not confied. However, cyliderical specimens (6 x 12 in or 4 x 8 in.) may give more reliable results for grouts containing coarse aggregate.

    Special application grouts often require modification of standard test procedures. All such modifications should be noted in the specifications and discussed prior to the start of the job.

    References
    • “Cement and Concrete Terminology,” AGI Committee 116R, ACI Manual of Concrete Practice, Part 1.
    • Cementitious Grouts and Grouting, S.H. Kosmatka, Portland Cement Association, 1990.
    • ASTM C 476, “Standard Specification for Grout for Masonry,” Annual Book of ASTM Standards, Vol. 04.05.
    • Hedstrom, E.G., and Hogan, M.B., “The Properties of Masonry Grout in Concrete Masonry,” Masonry: Components to Assemblages, ASTM STP 1063, ed. John H. Matthys, 1990, pp. 47-62.
    • “Building Code Requirments for Masonry Structures (ACI 530-88/ASCE 5-88) and Specifications for Masonry Structures (ACI 530.1-88/ASCE 6-88),” ACI-ASCE Standards, American Concrete Institute/American Society of Civil Engineers, 1988.
    • ASTM C 143, “Test Method for Slump of Hydraulic Cement Concrete,” Annual Book of ASTM Standards, Vol. 04.02.
    • ASTM 0 939, “Test Method for Flow of Grout for Preplaced- Aggregate Concrete (Flow Gone Method),” Annual Book of ASTM Standards, Vol. 04.02.
    • ASTM C 1019, “Standard Method of Sampling and Testing Grout,” Annual Book of ASTM Standards, Vol. 04.05.
    • ASTM C 942, “Standard Test Method for Complressive Strengths of Grouts for Preplaced-Aggregate Concrete in the Laboratory,” Annual Book of ASTM Standards, Vol. 04.02.
    • ASTM C 109, “Standard Test Method for Compressive Strength of Hydraulic Cement Mortars,” Annual Book of ASTM Standards, Vol. 04.01.
  • Hot Weather Concreting
    What is Hot Weather?

    Hot weather may be defined as any period of high temperature in which special precautions need to be taken to ensure proper handling, placing, finishing and curing of concrete. Hot weather problems are most frequently encountered in the summer, but the associated climatic factors of high winds and dry air can occur at any time, especially in arid or tropical climates. Hot weather conditions can produce a rapid rater of evaporation of moisture from the surface of the concrete, and accelerated setting time, among other problems. Generally high relative humidity tends to reduce the effects of high temperature.

    Why Consider Hot Weather?

    It is important that hot weather be taken into account when planning concrete projects because of the potential effects on fresh and recently placed concrete. High temperatures alone cause increased water demand, which in turn will raise the water-cement ratio and yield lower potential strength. Higher temperatures tend to accelerate slump loss and can cause loss of entrained air. Temperature also has a major effect on the setting time of concrete; concrete placed under high temperatures will set quicker and can therefore require more rapid finishing. Concrete that is cured at high temperatures early will not be as strong at 28 days as the same concrete cured at more moderate (70°F) temperatures.

    High temperatures, high wind velocity, and low relative humidity can affect fresh concrete in tow important ways; the high rate of evaporation may induce early plastic shrinkage or drying shrinkage cracking, and the evaporation rate can remove surface water necessary for hydration unless proper curing methods are employed. Thermal cracking may result from rapid drops in the temperature of the concrete, such as when concrete slabs or walls are placed on a hot day followed by a cool night. High temperature also accelerates cement hydration and contributes to the potential for cracking in massive concrete structures.

    How to Concrete in Hot Weather

    The key to successful hot weather concreting is (1) recognition of the factors that affect concrete and (2) planning to minimize their effects. Use proven, local recommendations for adjusting concrete proportions, such as use of water reducing, set retarding admixtures. Perhaps a moderate heat of hydration cement (ASTM Type II – moderate heat)2 or pozzolanic admixture (fly ash) can reduce the effects of high temperatures.

    Advance timing and scheduling to avoid delays in delivery, placing and finishing is a must; trucks should be able to discharge immediately and adequate personnel should be available to place and handle the concrete. When possible, deliveries should be scheduled to avoid the hottest part of the day.

    In the case of extreme temperature conditions or with mass concrete, the concrete temperature can be lowered by using chilled water or ice as part of the mixing water.

    Other measures such as sprinkling and shading the aggregate prior to mixing can be used to help lower the temperature of the concrete. If low humidity and high winds are predicted, then windbreaks, sunscreens or mist fogging may be needed to avoid plastic shrinkage cracking in slabs.

    Follow These Rules for Hot Weather Concrete
    • Concrete mixture designs may include: set retarders and water reducers,4 the lowest practical cement factor. Modify mixtures as appropriate – retarders, moderate heat of hydration cement, 2 Pozzolanic admixtures or other proven local solutions.
    • Adequate manpower to quickly place, finish and cure the concrete.
    • Limit the addition of water at the job site – add water only on arrival at the job site to adjust the slump. Later additions should be avoided; in no instance should they exceed 2 or 2 ½ gallons per cubic yard. Never add water to concrete that is more than 11/2 hours old.
    • Slabs on grade should not be placed on polyethylene sheeting – if a vapor barrier is required, then a bed of damp sand should be placed over it.
    • Finish as soon as the sheen has left the surface, start curing as soon as finishing is completed. Continue curing for at least 3 days: cover to prevent evaporation or use a liquid membrane curing compound, or cure slabs with water. (See pages 21-22). The addition of white pigment to membrane curing compounds will help by reflecting heat away from the concrete surface.
    • Moisten the subgrade, forms and reinforcement prior to placement. However, avoid standing water.
    • Protect field test cylinders by shading and preventing evaporation. Field curing boxes with ice or refrigeration may be used to ensure required 60°– 80°F for cylinders.6 (See page 17)
    • Do not use accelerators!
    References
    • ACI 305, “Hot Weather Concreting,” ACI Manual of Concrete Practice, Part 2. American Concrete Institute, P.O. Box 19150, Detroit, Michigan 48219.
    • ASTM C 150, “Standard Specification for Portland Cement,” American Society for Testing Materials, 1916 Race Street, Philadelphia, Pennsylvania 19103.
    • “Cooling Ready Mixed Concrete,” NRMCA Publication No. 106.
    • ASTM C 494, “Chemical Admixtures for Concrete.”
    • ASTM C 618, “Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Portland Cement Concrete.”
    • ASTM C 31, “Making and Curing Concrete Test Specimens in the Field.”
  • Joints in Concrete Slabs
    What Are Joints?

    Although concrete expands and contracts with changes in moisture and temperature the general overall tendency is to shrink and, therefore, crack. Irregular cracks are unsightly and difficult to maintain. Joints are simply pre-planned cracks.

    Some forms of joints are:
    • Control (contraction) joint – These joints are constructed to create planes of weakness so that cracks will occur at the desired location.
    • Isolation (expansion) joints – They separate or isolate slabs from other parts of the structure such as walls, footings, or columns, and driveways and patios from sidewalks, garage slabs, stairs, light poles and other obstructions. They permit movement of the slab and help minimize cracking caused when such movements are restrained.
    • Construction joints – These are joints that are placed at the end of a day’s work. In slabs they may be designed to permit movement and/or to transfer load. Often in reinforced concrete a conscious effort is made to clean the joint and bond the next day’s work.
    Concrete Joints
    Why Are Joints Constructed?

    Concert cracks cannot be prevented entirely, but they can be controlled and minimized by properly designed joints, because:

    • Concrete is weak in tension and, therefore, if its natural tendency to shrink is restrained, tensile stressed develop and cracks are likely to occur.
    • At early ages, before the concrete dries out, most cracking is caused by temperature changes or by the slight contraction that takes place as the concrete sets and hardens. Later as the concrete dries it will shrink further and either additional cracks may form or preexisting cracks may become wider.
    • Joints provide relief for the tensile stresses and are less objectionable than random cracks.
    How to Construct Joints

    Joints must be carefully designed and properly constructed if uncontrolled cracking of concrete flatwork is to be avoided. The following recommended practices should be observed:

    • The maximum joint spacing in feet should not exceed 2.5 times the thickness in inches. For example in an 8 in. slab the joints should be no further apart than 20 feet.
    • All panels should be square or nearly so. The length should not exceed 1.5 times the width. L-shaped panels should be avoided.
    • The joint groove should have a depth of ¼ the thickness of the slab, but not less than one inch. Tooled joints must be run early in the finishing process and rerun later to assure groove bond has not occurred.
    • Control joints can be tooled during finishing or sawed with a carborundum blade at an early age. Sawed joints may not be practical if the concrete is made with hard aggregate such as quartz gravel or trap rock. Sawing is easier if coarse aggregates contain materials such as limestone or sandstone. If the joint edges ravel during sawing it must be delayed, but if sawing is delayed too long it may become difficult. With abrasive saw blades’ sawing is often done at an age of one day or even earlier.
    • Premolded joint filler, building paper or polyethylene should be used to isolate slabs from building walls or footings. At least two inches of sand over the top of a footing will also prevent bond to the footing.
    • To isolate columns from slabs, form circular or square openings which will not be filled until after the floor has hardened. Slab control joints should intersect at the openings for columns. If square openings are used around columns the square should be turned at 45 degrees to have the control joints intersect at the diagonals of the square.
    • If the slab contains wire mesh cut out alternate wires across control joints. Note that wire mesh will not prevent cracking. Mesh tends to keep the cracks and joints tightly closed.
    • Construction joints key the two edges of the slab together either to provide transfer of loads or to help prevent curling or warping of the two adjacent edges. Galvanized metal keys are preferred for interior slabs, however, a beveled 1 by 2 inch strip, nailed to bulkheads or form boards, can be used in slabs that are at least 5 inches thick to form a key which will resist vertical loads and movements. Metal dowels can also be used in slabs that will carry heavy loads. Dowels must be carefully lined up and parallel or they may induce restraint and cause random cracking at the end of the dowel.
    • Joints in industrial floors subject to heavy traffic require special attention to avoid spalling of joint edges. Such joints should be filled with a material capable of supporting joint edges. Manufacturer’s recommendations and performance records should be checked before use.
    Follow These Rules for Proper Jointing
    • Plan exact location of all joints before construction
    • Provide isolation joints between slabs and columns, walls and footing, and at junctions of driveways with walks, curbs or other obstructions.
    • Provide control joints and joint filling materials as outlined in specifications.
    References
    • ACI 302.1, “Guide for Concrete Floor and Slab Construction,” ACI Manual of Concrete Practice.
    • “Slabs on Grade,” ACI Concrete Craftsman Series, American Concrete Institute, Detroit, Mi.
    • “Cracks in Concrete: Causes, Prevention, Repair,” a collection of articles from Concrete Construction Magazine, June, 1973.
  • Low Concrete Strength
    What Constitutes Low Cylinder Strength?

    Cylinders are molded from a sample of fresh concrete. Procedures must be in accordance with ASTM standars.1 The average strength of the set of 2 or 3 cylinders, broken at 28 days, constitutes one “test.” Additional cylinders are often made for 7 day tests or to be field cured to check early strength for form stripping.

    Under ACI Standards2, concrete is acceptable if no one “test” is lower than specified by more than 500 psi and the average of three consecutive “tests” equals at least the specified strength. If an average of three “tests” in a row dips below the specified strength, steps must be taken to increase the strength of the concrete. If a “test” falls more than 500 psi below the specified strength there may be more serious problems. An investigation would be made to ensure structural adequacy; and, again, steps taken to increase the strength level.

    Why Are Compressive Tests Low?

    Two major reasons are: (a) improper handling and testing – found to contribute in the majority of low strength investigation, and (b) reduced concrete quality due to an error in production, or the addition of too much water to the concrete on the job due to delays in placement or requests for wet concrete. High air content, for example, can be cause of low strength.

    Collect all test reports and analyze results before taking action. Look at the pattern of strength results. Does the sequence actually violate the specification? Do the test reports give any clue to the cause? Look at the slump, air content, concrete and ambient temperatures, numbers of day’s cylinders were left in the field, and any reported cylinder defects.

    If deficiency justifies investigation, first verify testing accuracy and then compare the structural requirements with the measured strength.3 If testing is deficient or if strength is greater than that actually needed, there is little point in investigating the in place strength. However, if procedures conform to the standards and the specified strength is required for the member in question, further investigation of the in-place concrete may be required. (See page 19-20 on “Strength of In-Place Concrete.”)

    Have ASTM testing procedure been followed? Minor discrepancies in curing cylinders in mild weather will probably not affect strength much, but if major violations are discovered large reductions in strength can occur.4 Almost all deficiencies in handling and testing cylinders will lower strength. A number of violations may combine to cause significant reductions, such as: extra days in the field; curing over 80°F; frozen cylinders; impact during transportation; delay in curing at the lab; improper caps; and insufficient care in breaking cylinders.

    The laboratory should be held responsible for deficiencies in its procedures. Use of qualified lab personnel is essential; untrained construction workers must not make and handle cylinders. All labs should meet ASTM C 1077 criteria for laboratories testing concrete and concrete aggregates and be CCRL inspected.5 Personnel testing concrete should be qualified by the ACI certification program or equivalent.

    How to Make Standard Cylinder Test

    It is essential that testing personnel be trained in the proper application of the ASTM Standards for strength tests of field-made, laboratory-cured cylinders:

    • Sample concrete failing from chute in two increments, in the middle part of the load, after some has been discharged.
    • Transport sample to the location of curing for the first day.
    • Remix the sample to ensure homogeneity.
    • Use molds conforming to standards.
    • Rod concrete in three layers and tap sides of the mold to close rod holes.
    • Finish tops smooth and level to allow thin caps.
    • If necessary, move cylinders immediately after molding; support the bottom.
    • Cure cylinders in the field at 60° to 80°F.
    • Protect from loss of moisture.
    • Transport day-old cylinders to the laboratory; handle gently or cure in accordance with C 31 at the job.
    • Demold and promptly place in moist curing at 73+/- 3F.
    • Maintain water on cylinder surfaces at all times
    • Caps on cylinders must be flat and less than 3/16 inch thick.
    • Use minimum 5000 psi capping material.
    • Wait at least 2 hours for sulfur caps to harden
    • Use calibrated testing machine.
    • Measure cylinder diameter and check cap plainness.
    • Center cylinder and use proper loading rate.
    • Observe break pattern (vertical cracks through the cap indicate improper load distribution).

    Test reports must be promptly distributed to the concrete producer, as well as the contractor and engineer. This is essential to the timely solution of problems.

    References
    • ASTM Standards C 31, C 39, C 172, C 470, C 617, and C 1077. American Society for Testing and Materials, 1916 Race St., Philadelphia, PA. 19103
    • “Building Code Requirements for Reinforced Concrete.” ACI 318. American Concrete Institute, P.O. Box 19150, Detroit, Mich. 48219.
    • “In-Place Concrete Strength Evaluation-A Recommended Practice.” NRMCA Publication No 133.
    • “Effect of Curing Condition on Compressive Strength of Concrete Test Specimens.” NRMCA Publication No. 53.
    • Cement and Concrete Research Laboratory (CCRL). National Institute of Standards and Technology, Gaithersburg, MD 20899.
    • “Review of Variables that Influence Measured Concrete Compressive Strength,” David N. Richardson, Journal of Materials in Civil Engineering, 1991. NRMCA Publication No. 179.
  • Radon Resistant Buildings
    What is Radon?

    Radon is a colorless, odorless, radioactive gas which occurs naturally in soils in amounts dependent upon the geology of the location. The rate of movement of radon through the soil is depended primarily upon soil permeability and degree of saturation, and differences in air pressure within the soil. Soil gas enters buildings through cracks or openings in the foundation, slab, or basement walls when the air pressure in the building is less than that of the soil.

    Radon gas decays to other radioactive elements in the uranium series. Called “radon progeny,” they exist as solid particles rather than as a gas.

    Why be Concerned About Radon Levels in Buildings?

    The concern is due to an association with the development of lung cancer. Radon progeny can become attached to dust particles in the air. If inhaled, they can lodge in the lung. Energy emitted during radioactive decay while in the lung can cause tissue damage which has been linked to lung cancer.

    The level of health risk associated with radon is related to the concentration of radon in the air and the time a person is exposed to that air. The U.S. Environmental Protection Agency (EPA) has developed a risk profile for radon exposure at various concentrations, and established an action level concentration above which efforts should be made to reduce radon levels.1 It is prudent to take measures during construction which will reduce the amount of radon entering a building.

    How to Construct Radon Resistant Concrete Buildings

    Solid concrete is an excellent material for use in constructing radon resistant buildings. It is an effective barrier to soil gas penetration if cracks and openings are sealed.

    Solid concrete slabs and basement walls are commonly used in residential buildings. Buildings resistant to radon may be easily constructed with concrete. In concrete construction, the critical factor is to eliminate all entry routes through which gases can flow from the soil into the building.

    Follow these guidelines to reduce radon entry:
    • Design to minimize utility openings. Sump openings should be sealed and vented outdoors.
    • Minimize random cracking by using control and isolation joints in walls and floors. Planned joints can then be easily sealed.5 If done properly, any cracks will occur at the joints and can be easily sealed.
    • Monolithic slab foundations are an effective way to minimize radon entry. For slab-on-grade homes in warm climates, pour foundation and slab as a single monolithic unit.
    • Use materials which will minimize concrete shrinkage and cracking (larger aggregate sizes and proper water-cementitious ratio).
    • When using polyethylene film beneath the slab, place a layer of sand over the polyethylene.
    • Remove grade stakes after striking off the slab. (If left, they can provide entryways through the slab.)
    • Construct the joints to facilitate caulking.
    • Cure the concrete adequately.
    • Caulk and seal all joints and openings in the walls or floor. (If cracks occur, they should be widened, and then caulked and sealed.)2,3 The construction of radon resistant buildings requires adhering to accepted construction practices with attention to a few additional details. In instances where high radon levels are expected, installation of a sub-slab ventilation system incorporating an open-graded aggregate base beneath the slab may be warranted during construction. These systems provide a positive means of evacuating soil gas from beneath the slab, diverting it directly to the outside.
    References
    • “A Citizen’s Guide to Radon – What It Is and What To Do About It,” U.S. Environmental Protection Agency, OPA-86-004, 1986, 13 pp. Available from state radiation protection offices or EPA regional offices.
    • “Radon Reduction in New Construction-An Interim Guide,” U.S. Environmental Protection Agency, OPA-87-009, 1987, 7 pp. Available from EPA, (513) 569-7771.
    • “Radon Reduction Techniques for Detached Houses Technical Guide,” U.S. Environmental Protection Agency, EPA/625/5-861019, 1986, 50 pp. Available from EPA Center for Environmental Research Information, (513) 569-7562.
    • “Production of Radon-Resistant Slab-on-Grade Foundations,” Florida Institute of Phosphate Research, Bartown, Florida, 1987, 9 pp.
    • “Guide to Residential Cast-in-Place Concrete Construction,” ACI 332R-84, American Concrete Institute, Detroit, Michigan, 1984, 36 pp., (313) 532-2600.
    • “Production of Radon-Resistant Foundations,” A.G. Scott and W.O. Findlay, American ATCON, Inc., Wilmington, Delaware, 1987, 54 pp. Available from NTIS, Alexandria, Virginia, PB89-116149/WBT, (703) 487-4650.
    • Technical information on radon-resistant construction is available from the National Association of Home Builders, National Research Center, Radon Research Program, (301) 249-4000.
  • Scaling Concrete Surfaces
    What is Scaling?

    When concrete scales from freezing and thawing the finished surface flakes or peels off. Generally it starts as localized small patches which later may merge and extend to expose large areas. Light scaling does not expose the coarse aggregate. Moderate scaling exposes the aggregate and may involve loss of up to 1/8 to 3/8 inch of the surface mortar. In severe scaling more surface has been lost and the aggregate is clearly exposed and stands out.

    (Note-Occasionally concrete peels or scales in the absence of freezing and thawing. This type of scaling is not covered in this section. Often this is due to the early use of a steel trowel – see reference 6 – or finishing while bleed water is on the surface.)

    Why Do Concrete Surfaces Scale?

    Concrete slabs exposed to freezing and thawing in the presence of moisture and/or deicing salts are susceptible to scaling. Most scaling is caused by:

    • The use of non-air-entrained concrete or too little entrained air. Adequate air entrainment is necessary for protection against freezing and thawing damage. However, even air entrained concrete will scale if other precautions are not observed.
    • Application of calcium or sodium chloride deicing salts. If other salts such as ammonium sulfate or ammonium nitrate are used they can cause scaling as well as inducing severe chemical attack of the concrete surface.
    • Any finishing operation performed while bleed water is on the surface. If bleed water is worked back into the top ¼ inch of the slab a very high water-cement ratio and, therefore, a low strength top surface later is produced.
    • Insufficient or no curing. This omission often results in a weak surface skin which will scale if it is exposed to freezing and thawing in the presence of moisture and deicing salts.
    How to Prevent Scaling

    To prevent scaling the use of air-entrained concrete is a must. Severe exposures require air contents of 6 to 7 percent in freshly mixed concrete made with 3/4 inch or 1 inch aggregate. In moderate exposures where deicing salts will not be used 4 to 6 percent air will be sufficient. Air-entrained concrete having a low water-cement ratio and moderate slump (up to 5 inches) helps produce a strong wear resistant surface.

    DO NOT use deicing salts, such as calcium or sodium chloride, on new or recently placed concrete. Use clean sand for traction. Never use ammonium sulfate or ammonium nitrate as a deicer, these are chemically aggressive and destroy concrete surfaces. Poor drainage which permits water or salt and water to stand on the surface for extended periods of time greatly increases the severity of the exposure and causes scaling. (This is often noticed in gutters and sidewalks where the snow from plowing keeps the surface wet for long periods of time.) Light applications of salts can be more damaging than heavy applications; even salts carried on cars may cause severe scaling of newly placed driveways.

    Provide proper curing by using liquid membrane curing compound or by covering the surface of freshly placed slab with we burlap. Curing insures proper combination of cement with water known as hydration which allows the concrete to achieve its highest potential strength.

    DO NOT perform any finishing operations with water present on the surface. Initial screeding must be promptly followed by bull-floating. (Do not use a jitterbug or vibrating screed to work up an excessive layer of mortar on the surface.)

    Protect concrete from the harsh winter environment. It is important to protect the young concrete from becoming saturated with water prior to freeze and thaw cycles of the winter months. Seal the surface with 50/50 mixture of boiled linseed oil and mineral spirits or other surface sealer specifically designed for use on slabs on grade. The concrete should be reasonably dry prior to the application of a sealer. Late summer is the ideal time for surface treatment. The sealer can be sprayed on or brushed on the surface of the concrete. CAUTION: Linseed oil will darken the color of the concrete & car should be taken to apply it uniformly.

    How to Repair Scaled Surfaces

    The repaired surface will only be as strong as the base surface to which it is bonded. Therefore, the surface to be repaired should be free of dirt, oil or paint and most importantly it must be sound. To accomplish the use a hammer and chisel, sandblasting or jack hammer to remove all weak or unsound material. The clean, rough, textured surface is then ready for a thin bonded resurfacing such as:

    • Portland cement concrete resurfacing.
    • Latex modified concrete resurfacing.
    Follow These Rules to Prevent Scaling
    • For moderate to severe exposures, use air-entrained concrete of medium slump (3-5 in.) & cure properly.
    • If late fall placement cannot be avoided in moderate to severe climates:
      Do not use deicers for first winter.
      Seal surface with boiled linseed oil.
    • Use correct timing for all finished operations.
    • Select the proper mix to match placing conditions. Specify air-entrained concrete. Use an accelerator and lower slump in cold weather.
    References
    • “Guide to Durable Concrete,” ACI 201.2, ACI Manual of Concrete Practice.
    • “Scaled Concrete,” by Fred F. Bartel, Tews Lime and Cement Company, NRMCA.
    • “Problems of Ice Removal from Pavements,” by William E. Dickinson, Calcium Chloride Institute, NRMCA Publication No. 98.
    • “Protective Coatings to Prevent Deterioration of Concrete by Deicing Chemicals,” National Cooperative Highway Research Program Report No. 16.
    • “Guide for Concrete Floor and Slab Construction,” ACI 302.1, Manual of Concrete Practice.
    • “An Unusual Case of Surface Deterioration on a Concrete Brick Deck,” by John Ryell, ACI Journal, April 1965.
  • Strength of In-Place Concrete
    What is the Strength of In-Place Concrete?

    Drilled cores test lower than properly made and tested standard molded 6 in. x 12 in. cylinders.1 This applies to all formed structural concrete. Exceptions may occur for cores from concrete cast against an absorptive subgrades or cores from lean, low strength mass concrete.

    Means of measuring or comparing the strength of in-place concrete include: rebound hammer, penetration probe, pullouts, cast-in-place cylinders, tests of drilled cores, and load tests of the structural element.

    The standard ASTM test procedure evaluates the strength potential of the concrete.2 Cylinders are molded and cured at 0° to 80°F for one day and then moist cured in the laboratory until broken in compression, normally at 7 and 28 days age. Job practices for handling, placing, compaction, and curing of job concrete are relied upon to provide an adequate percentage of that potential strength in the structure. The ACI Building Code recognizes that under current design practices, concrete construction can be considered structurally adequate if cores average at least 85 percent of specified strength with none below 75 percent.

    Why Measure In-Place Strength?

    Tests of in-place concrete may be needed when standard cylinder strengths are low; however, do not investigate in-place without first checking to be sure that: the concrete strengths actually failed to meet the specification provisions; low strengths are not attributable to faulty testing practices; and the specified strength is really needed. (See page 17-18 on “Low Concrete Cylinder Strength.”) In many cases, the concrete can be accepted for the intended use without in-place strength testing.

    There are many other situations which may require the investigation of in-place strength, including: shore and form removal, post-tensioning, or early load application; investigation of damage due to freezing, fire, or adverse curing exposure; evaluation of older structures; and when a lower strength concrete is placed in a member by mistake. When cores or other in place tests fail to assure 85 percent of the design strength, additional curing of the structure may provide the necessary strength. This is particularly possible with concretes containing slow strength-gaining cement, fly ash, or slag.

    How to Investigate In-Place Strength

    If only one set of cylinders is low often the question can be settled by comparing rebound hammer or probe results on concrete from areas with good cylinder results. Where the possibility of low strength is such that large portions need to be investigated a well organized study will be needed. Establish a grid and obtain systematic readings including good and questionable areas. Tabulate the hammer or probe readings. If areas appear to be low, drill cores from both low and high areas. If the cores confirm the hammer or probe results, the need for extensive core tests is greatly reduced.

    Core Strength, ASTM Method C 42 – if core drilling is necessary observe these precautions: (a) test 3 cores, (b) use 3 ½ in. minimum diameter and larger cores for over 1 in. aggregate, (c) try to obtain a length at least 1 ½ time the diameter, (d) trim to remove steel if the 1 ½ L/D ratio can be maintained, (e) trim ends square with an automatic feed diamond saw, (f) keep cap thickness under 1/8 in., (g) use high strength capping material, (h) check planeness of caps and bearing blocks, (i) do not drill cores from the top layers of columns, slabs, walls, or footings. They will be 10 to 20 percent weaker than cores from the mid or lower portions, and (j) test cores after drying for 7 days if the structure is dry in service; otherwise soak cores 40 hours prior to testing.

    Probe Penetration Resistance, ASTM Method C 803 – Probes driven into concrete can be used to study variations in concrete quality: (a) different size probes or a change in driving force may be necessary for large differences in strength or unit weight, (b) accurate measurement of the exposed length of the probe is required, (c) probes should be spaced at least 7 inches apart and not be close to the edge of the concrete, (d) probes not firmly embedded in the concrete should be rejected and, (e) develop a strength calibration curve for the materials and conditions under investigation.

    Rebound Hammer, ASTM Method C 805 – Observe these precautions: (a) wet all surfaces for several hours or overnight because drying affects rebound number, (b) don’t compare readings on concrete cast against different form materials or concrete of varying moisture content or readings from different impact directions or on members of different mass, or results using different hammers, (c) don’t grind unless the surface is soft, finished or textured, (d) test structural slabs from the bottom, and (e) don’t test frozen concrete.

    Advance Planning – When it is known in advance that in-place testing is required, such as for shore and form removal, other methods can be considered such as: cast-in-place, push-out cylinders and pullout strength measuring techniques covered by ASTM Methods C 873 and C 900.

    References
    • ACI Bibliography 13 on Core Tests. American Concrete Institute, P.O. Box 19150, Detroit, Mich. 48219.
    • ASTM C 31, “Making and Curing Test Specimens in the Field” and C 39, “Compressive Strength of Cylindrical Concrete Specimens,” American Society for Testing and Materials. 1916 Race Street, Philadelphia, PA. 19103.
    • ACTM C 805, “Rebound Number of Hardened Concrete.”
    • ACTM C 803, “Penetration Resistance of Hardened Concrete.”
    • ACTM C 900, “Pullout Strength of Hardened Concrete.”
    • ACTM C 873, “Strength of Cast-in-Place Cylinders.”
    • ACTM C 42, “Obtaining and Testing Drilled Cores and Sawed Beams of Concrete.”
    • “Building Code Requirements for Reinforced Concrete,” ACI 318.
    • “In-Place Strength Evaluation-A Recommended Practice,” NRMCA Publication No. 133.

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