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CIP 5—Plastic Shrinkage Cracking

Sat, 29 Aug 2026 01:23:46 GMT

WHAT is Plastic Shrinkage Cracking

Plastic shrinkage cracks appear in the surface of fresh concrete soon after it is placed and while it is still plastic. These cracks appear mostly on horizontal surfaces. They are usually parallel to each other on the order of 1 to 3 feet apart, relatively shallow, and generally do not intersect the perimeter of the slab. Plastic shrinkage cracking is more likely to occur when high evaporation rates cause the concrete surface to dry out before it has set.

Plastic shrinkage cracks are unsightly but rarely impair the strength of concrete floors and pavements. These cracks, however, can permit the ingress of other aggressive chemicals that impact durability and as weak points for the initiation of later age cracking due to other reasons.

HOW to Minimize Plastic Shrinkage Cracking

The use of vapor retarders under a slab on grade can increase the risk of plastic shrinkage cracking. However, it may be necessary for interior slabs that will have a floor covering at any point during its service life. See CIP 29.

A

In the very hot and dry periods, use fog sprays to discharge a fine mist upwind and into the air above the concrete. Fog sprays reduce the rate of evaporation from the concrete surface and should be continued until suitable curing materials can be applied.

If concrete is to be placed on a dry absorptive subgrade in hot and dry weather, dampen the subgrade but not to a point that there is freestanding water prior to placement. The formwork and reinforcement should also be dampened.

When adverse conditions exist, erect temporary windbreaks to reduce the wind velocity over the surface of the concrete and, if possible, provide sunshades to control the surface temperature of the slab. If conditions are critical, schedule placement to begin in the later afternoon or early evening. However, in very hot conditions, early morning placement can afford better control on concrete temperatures.

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 moisture-retaining coverings, such as 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 an evaporation retardant on the surface behind the screeding operation and following floating or troweling, as needed, until curing is started.

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Hot Weather Concreting, ACI 305R, American Concrete Institute, Farmington Hills, MI., www.concrete.org
Guide for Concrete Floor and Slab Construction, ACI 302.1R, American Concrete Institute, Farmington Hills, MI.
Standard Practice for Curing Concrete, ACI 308, American Concrete Institute, Farmington Hills, MI.
Concrete Slab Surface Defects: Causes, Prevention, Repair, IS177, Portland Cement Association, Skokie, IL, www.cement.org
Bruce A. Suprenant, Curing During the Pour, Concrete Construction, June 1997.
Eugene Goeb, Common Field Problems, Concrete Construction, October 1985.
Vapor Retarders Under Slabs on Grade, CIP 29, NRMCA, Alexandria, VA, www.nrmca.org

References

F

Start curing the concrete as soon as possible. Spray the surface with liquid membrane curing compound or cover the surface with wet burlap and keep it continuously moist for a minimum of 3 days.

Follow These Rules to Minimize Plastic Shrinkage Cracking

Dampen the subgrade and forms when conditions for high evaporation rates exist.
Prevent excessive surface moisture evaporation by providing fog sprays and erecting windbreaks.
Cover concrete with wet burlap or polyethylene sheets between finishing operations.
Use cooler concrete in hot weather and avoid high concrete temperatures in cold weather.
Cure properly as soon as finishing has been completed.

Attempts to eliminate plastic shrinkage cracking by modifying the concrete mixture composition to affect bleeding characteristics have not been found to be consistently effective. To reduce the potential for plastic shrinkage cracking, it is important to recognize ahead of time, before placement, when weather conditions conducive to plastic shrinkage cracking will exist. Precautions can then be taken to minimize its occurrence.

WHY Do Plastic Shrinkage Cracks Occur

The development of these cracks can be minimized if appropriate measures are taken prior to and during placing and finishing concrete.

Note: Plastic shrinkage cracks should be distinguished from other early or pre-hardening cracks caused by settlement of the concrete around reinforcing bars, formwork movement, early age thermal cracking, or differential settlement at a change from a thin to a deep section of concrete.

Plastic shrinkage cracks are caused by a rapid loss of water from the surface of concrete before it has set. The critical condition exists when the rate of evaporation of surface moisture exceeds the rate at which rising bleed water can replace it. Water receding below the concrete surface forms menisci 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 to resist the tensile forces, cracks do not form. If the surface dries very rapidly, the concrete may still be plastic, and cracks do not develop at that time; but plastic shrinkage cracks will surely form as soon as the concrete starts to stiffen. Synthetic fiber reinforcement incorporated in the concrete mixture can help resist the tension when concrete is very weak. Conditions that cause high evaporation rates from the concrete surface, and thereby increase the possibility of plastic shrinkage cracking, include:

Wind velocity in excess of 5 mph
Low relative humidity
High ambient and/or concrete temperatures

Small changes in any one of these factors can change the rate of evaporation of water from the concrete surface. ACI 305R provides a chart to estimate the rate of evaporation and indicates when special precautions might be required. However, the chart isn’t infallible because many factors other than rate of evaporation are involved.

Concrete mixtures with an inherent reduced rate of bleeding or quantity of bleed water are susceptible to plastic shrinkage cracking even when evaporation rates are low. Factors that reduce the rate or quantity of bleeding include high cementitious materials content, high fines content, reduced water content, entrained air, high concrete temperature, and thinner sections. Concrete containing silica fume requires particular attention to avoid surface drying during placement due to its very low rate of bleeding.

Any factor which delays setting increases the possibility of plastic shrinkage cracking. Delayed setting can result from a combination of one or more of the following: cool weather, cool subgrades, high water contents, lower cement contents, retarders, some water reducers, and supplementary cementitious materials.

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Accelerate the setting time of concrete and avoid large temperature differences between concrete and ambient air temperatures.

Consider using synthetic fibers (ASTM C1116) to minimize plastic shrinkage cracking.

H

If plastic shrinkage cracks should appear during final finishing, the finisher may be able to close them by refinishing. However, when this occurs precautions, as discussed above, should be taken to avoid further cracking.

CIP 6—Joints in Concrete Slabs on Grade

Sat, 29 Aug 2026 01:23:29 GMT

WHAT are Joints

Concrete expands and shrinks with changes in moisture and temperature. The overall tendency is to shrink and this can cause cracking at an early age. Irregular cracks are unsightly and difficult to maintain but generally do not affect the integrity of concrete. Joints are simply pre-planned cracks. Joints in concrete slabs can be created by forming, tooling, sawing, and placement of joint formers.

Types of joints include:

a. Contraction joints are intended to create weakened planes in the concrete and control the location where cracks, resulting from dimensional changes, will occur.

b. Isolation or expansion joints separate or isolate slabs from other parts of the structure, such as walls, footings, or columns; and driveways and patios from sidewalks, garage

HOW to Construct Joints

Raveling during saw cutting is affected by the strength of the concrete and aggregate characteristics. If the joint edges ravel during sawing, it must be delayed. However, if delayed too long, sawing can become difficult and uncontrolled cracking may occur.

A

All panels should be square or nearly so. The length should not exceed 1.5 times the width. Avoid L-shaped panels.

For contraction joints, the joint groove should have a minimum depth of ¼ the thickness of the slab, but not less than 1 inch [25 mm]. Timing of jointing operations depends on the method used:

Preformed plastic or hard board joint strips are inserted into the concrete surface to the required depth before finishing.
Tooled joints must be run early in the finishing process and rerun later to ensure groove bond has not occurred.
Early-entry dry-cut joints are generally run 1 to 4 hours after completion of finishing, depending on the concrete’s setting characteristics. These joints are typically not as deep as those obtained by the conventional saw-cut process, but should be a minimum of 1 inch [25 mm] in depth.
Conventional saw-cut joints should be run within 4 to 12 hours after the concrete has been finished.

The maximum joint spacing should be 24 to 36 times the thickness of the slab. For example, the joint spacing for a 4-inch [100 mm] thick slab should be about 10 feet [3 m]. It is further recommended that joint spacing be limited to a maximum of 15 feet [4.5 m].

Use premolded joint filler such as asphalt-impregnated fiber sheeting, compressible foam strips, or similar materials for isolation joints to separate slabs from building walls or footings. At least 2 inches [50 mm] of sand over the top of a footing will also prevent bond to the footing.

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Joints in Concrete Construction, ACI 224.3R, American Concrete Institute, Farmington Hills, MI.
Guide for Concrete Floor and Slab Construction, ACI 302.1R, American Concrete Institute, Farmington Hills, MI.
Slabs on Grade, ACI Concrete Craftsman Series CCS-1, American Concrete Institute, Farmington Hills, Ml.
Joint Planning Primer, Concrete Construction, August 1997.
Bruce A. Suprenant, Sawcutting Joints in Concrete, Concrete Construction, January 1995.

References

F

To isolate columns from slabs, form circular or square openings, which will not be filled until after the floor has hardened. Slab contraction joints should intersect at the openings for columns. If square openings are used around columns, the square should be turned at 45 degrees so the contraction joints intersect at the diagonals of the square.

Follow These Rules for Proper Jointing

Plan exact location of all joints, including timing of contraction joint sawing before construction.
Provide isolation joints between slabs and columns, walls and footings, and at junctions of driveways with walks, curbs or other obstructions.
Provide contraction joints and joint filling materials as outlined in specifications.

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:

WHY are Joints Constructed

slabs, stairs, light-poles and other points of restraint. They permit independent vertical and horizontal movement between adjoining parts of the structure and help minimize cracking when such movements are restrained.

c. Construction joints are surfaces where two successive placements of concrete meet. They are typically placed at the end of a days work but may be required when concrete placement is stopped for a period such that the previously placed concrete has set and hardened. In slabs they may be designed to permit movement and/or to transfer load. The location of construction joints should be planned. It may be desirable to achieve bond and continue reinforcement through a construction joint for load transfer

Cracks in concrete cannot be prevented entirely, but they can be controlled and minimized by properly designed joints. Concrete cracks because:

a. Concrete is weak in tension and, therefore, if its natural tendency to shrink is restrained, tensile stresses that exceed its tensile strength can develop, resulting in cracking.

b. 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 from the tensile stresses, are easy to maintain and are less objectionable than uncontrolled or irregular cracks.

G

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 sometimes used for interior slabs, however, a beveled 1 by 2 inch [25 by 50 mm] strip, nailed to bulkheads or form boards, can be used in slabs that are at least 5 inches [125 mm] thick to form a key which will resist vertical loads and movements. Keyed joints are not recommended for industrial floors. Metal dowels should be used in slabs or pavements 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.

If the slab contains wire mesh, cut out alternate wires, or preferably discontinue the mesh, across contraction joints. Note that wire mesh will not prevent cracking. Mesh tends to keep the cracks and joints from opening up.

H

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.

I

CIP 7 - Cracks in Residential Basement Walls

Sat, 29 Aug 2026 01:23:09 GMT

WHAT Types of Cracks May Occur?

Cast-in-place concrete basements provide durable, high quality living space. Cracking of concrete is a natural occurrence and at times can be undesirable. Most common causes of cracking include:

WHY do Basement Cracks Occur?

Causes, Evaluation and Repair of Cracks, ACI 224.1R, American Concrete Institute, Farmington Hills, MI. www.concrete.org
Code Requirements for Residential Concrete, ACI 332-14, American Concrete Institute, Farmington Hills, MI.
International Residential Code, International Code Council, Washington, DC, www.iccsafe.org
Residential Concrete, National Association of Home Builders,Washington, DC, www.nahb.org
Residential Construction Performance Guidelines, National Association of Home Builders, Washington, DC.
Casting Residential Foundation Walls in Cold Weather, Concrete Foundations Association, Mount Vernon, IA, www.cfawalls.org .
Backfilling Foundation Walls, Concrete Foundations Association, Mount Vernon, IA.
Cracking in Foundation Walls, Concrete Foundations Association, Mount Vernon, IA.

References

Some cracking is normal in concrete basement walls. Volume changes and other movements at an early age result in different types of cracks, as dis-cussed earlier. These cracks can grow if the walls are not properly designed, due to the continued hor-izontal pressures applied by soils, water and tem-perature. Cracking can be minimized and problems prevented if the design and construction practices that follow are implemented.

Most builders or third party providers offer limited warranties for basements. A typical warranty will require repair only when cracks leak, have measura-ble vertical displacement, or if the crack width ex-ceeds 1/8-in. (3 mm). The National Association of Home Builders (NAHB) requires repair or correc-tive action when cracks in basement walls cause leaks into the basement.

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Temperature and drying shrinkage cracks. Gener-ally, newly placed concrete is at its largest volume. As concrete hardens it dries and starts to shrink. Temperature variations cause concrete to expand and contract. When these volume changes are re-strained, cracking results.

Re-entrant corner cracking occurs diagonally from the corners of windows, doors or openings in the concrete walls. These cracks result from shrinkage.

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Pour lines are visible demarcations between place-ment of two concrete loads, typically due to a de-lay in placing between the loads and if proper con-solidation was not performed to homogenize the two portions across the separation. Pour lines are often perceived as cracks. In extreme cases they may perform as cracks if the first placement has partially hardened before the second placement. This is often referred to as a cold joint.

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Vertical form lines occur between form panels and can sometimes cause weak zones due to the use of form ties that support two layers of formwork dur-ing concrete placement. Cracks may initiate at form lines.

Restraint cracks may form in some portions of walls where contact with footings restrains the shrinkage of the concrete wall.

G

Settlement cracks occur from non-uniform support of footings or occasionally from expansive soils.

Crazing and surface cracking may occur due to a lack of adequate curing and protection if construc-tion is during extreme cold or hot weather.

F

Structural cracks may occur during backfilling if concrete strength is not adequate or the walls are not adequately supported as the design intends. This is most likely to occur when heavy equipment gets too close to the walls during the backfill pro-cess or when pressure due to backfill material ex-ceed that anticipated in the design, for example with liquefied soils.

H

Placement and curing . Place concrete in a continuous operation to avoid cold joints and segregation. Adding excess water to concrete to facilitate placement will increase segregation, cause honeycombing or excessive cracking, and reduce strength. Consider placement points no greater 20 or 30 feet around the perimeter of the wall to minimize segregation. Properly designed higher slump concretes with admixtures will flow horizontally for long distances and placement points can be spread out. Curing should begin after placement. Forms should be left in place 5 to 7 days or at least until concrete attains adequate strength to support itself. Forms removed too early can result in premature drying and may cause cracking. In cold weather, forms should be insulated with blankets or other materials to retain heat. During hot dry weather, forms should be covered with wet burlap to retain moisture. Liquid membrane-forming curing compounds can be sprayed at the required coverage after forms are removed to prevent excessive drying.

Joints . Some cracks in basement walls can be controlled to occur in properly located formed joints.

Backfilling and final grading . Backfilling should be done carefully to avoid damaging the walls. Brace the walls, if possible, or backfill after first floor or other structural systems are in place. Finish grade to slope ½ to 1-in. per foot (40 to 80-mm per m) for at least 8 to 10 feet (2.5 to 3 m) to drain water away from the foundation. Considering settlement, maintain this final grade to prevent water from standing along the foundation and exceeding the designed wall pressure.

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HOW to Design & Construct Quality Basements?

Site conditions and excavation . Soil type and conditions should be properly assessed for appropriate design and construction of foundations specific to the building site. The excavation should be at least to the level of the bottom of the basement slab and can be to the bottom of the footing. 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 or well compacted fill. Uniform soil bearing capacity should be ensured or the design should accommodate any variation.

Concrete . Use concrete with adequate strength in accordance with the Code and project specifications. Excess water should not be added to concrete in the truck mixer. Water-reducing admixtures can be used to increase flow. Air-entrained concrete should be used for walls exposed to moisture and freezing temperatures.

A

Cast-in-place concrete basement walls are the strongest and most effective foundation for a residence. However, climate conditions, unusual or unforeseen loads, material quality and workmanship may impact the quality of the finished basement. Proper design and construction is important. The following steps should be followed:

Formwork and reinforcement . Formwork must be installed and braced to withstand the pressure of the fresh and flowing concrete. Reinforcement is used to control crack width. Wall thickness and reinforcement should be provided in accordance with International Residential Code (IRC), ACI 332, or locally adopted Code.

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Crack repair . Cracking is not necessarily a sign of poor materials or workmanship or a structural problem with the concrete wall. If repair is necessary, epoxy injection, dry-packing, or routing and sealing techniques can be used to repair and stabilize cracks. Before repairing leaking cracks, the drainage around the structure should be checked and corrected if necessary. Details of these and other repair methods are provided in Ref. 1. Seek professional advice to evaluate and repair active cracks that are widening with time.

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Waterproofing and drainage . Waterproof membranes are best applied to the exterior of foundation walls. These are spray-applied, painted, or mechanically fastened sheet systems. Positive side waterproofing (exterior) is generally better than negative side (inside) to keep water from leaking through cracks. Drainage systems should be designed to remove excessive soil moisture along the basement wall. Provide foundation drainage by installing drain tiles or plastic pipes around the exterior of the footing and properly connect them to a removal system or drain to daylight. Surface and roof drainage should direct water away from the residence. Water should be drained to lower elevations suitable to receive storm water run off.

CIP 8—Discrepancies in Yield

Sat, 29 Aug 2026 01:22:22 GMT

WHAT is Concrete Yield

Concrete yield is defined as the volume of freshly mixed concrete produced from a known quantity of concrete materials. Ready mixed concrete is sold on the basis of the volume of fresh, unhardened concrete in cubic yards (yd ³ ) or cubic meters (m ³ ) as discharged from a truck mixer. Most materials are batched by weight so it is important to determine the volume of concrete produced from the actual batched quantities of materials. The density of the fresh concrete is used to establish this relationship.

The basis for calculating the volume or yield is described in ASTM C94, Specification 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 materials by the density (unit weight) of the concrete determined in accordance with ASTM C138. ASTM C94 requires that the yield should be determined from the average of three determinations of representative samples, each from three different delivery vehicles. It should thereby be recognized that the volume of concrete

supplied is not determined from the calculated dimensions of the constructed member.

WHY do Yield Problems Occur

ASTM C94 and C138, ASTM Book of Standards, Volume 04.02, ASTM International, West Conshohocken, PA. www.astm.org
Ready Mixed Concrete, Gaynor, R.D. & Lobo, C.L., NRMCA Publication 186, NRMCA, Alexandria, VA, www.nrmca.org .
Users Guide to ASTM C94, Daniel, D.G. & Lobo, C.L., NRMCA Publication 2PMNL49, 2nd edition, NRMCA, Alexandria, VA, www.nrmca.org .
Improving Concrete Quality, Obla, K.H., CRC Press, available from NRMCA, Alexandria, VA.
No Minus Tolerance on Yield, Malisch, W. R. and Suprenant, B. A., Concrete Producer, May 1998.
Causes for Variation in Concrete Yield, Suprenant, B. A., The Concrete Journal, March 1994.

References

For slabs on grade accurately finish and compact the subgrade to the proper elevation.

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 contingencies. Repetitive operations and slip form placement permit more accurate estimates of the amount of concrete that will be needed. Sporadic operations involving a alternating placement in slabs, footings, walls, and as incidental fill around pipes will require a bigger allowance for contingencies.

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HOW to Prevent Yield Discrepancies

Check concrete yield by measuring concrete density in accordance with ASTM C138 early in the job. Repeat these tests if a problem arises. Ensure that the scale is accurate and placed on a level surface, that the volume density measure is accurately determined, that a flat plate is used for strike off, and that the outside of the measure is cleaned prior to weighing. Concrete yield in ft ³ (m ³ ) is total batch weight in pounds (kg) divided by density in lb/ft ³ (kg/m ³ ). The total batch weight is the sum of the weights of all materials from the batch ticket.

Construct forms with adequate bracing and shoring to minimize deflection and bulging when concrete is placed. This is important for elevated slabs.

A

To prevent or minimize concrete yield problems:

Measure formwork accurately. Order sufficient

Example —Materials batched for 8 yd ³ / 6 m ³ load.

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B

Yield discrepancies can be real or perceived. Real discrepancies result when quantity of materials batched do not produce the intended yield. This can be evaluated by density measurements. If yield determined from density measurement indicates a discrepancy, it should be corrected by the concrete

producer. Perceived discrepancies, typically under-yield, is when the concrete ordered does not fill the forms due to contingencies discussed below:

A note in ASTM C94 states: It should be understood that the volume of hardened concrete may be, or appears 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 are the responsibility of the producer.

The volume of hardened concrete in place may be about 2 percent less than its volume in a freshly mixed state due to reduction in air content, settlement and bleeding, decrease in volume of the cementitious paste, and drying shrinkage due to loss of moisture.

Deflection or distortion of the forms resulting from pressure exerted by the concrete.

B

A

Miscalculation of form volume or slab thickness when the actual dimensions exceed the assumed dimensions used in estimates, even by a small amount. For example, a 1/8-inch (3-mm) difference in a 4-inch (100-mm) thick slab would mean a shortage of 3 percent or 1 yd ³ in a 32-yd ³ (1 m ³ in a 32-m ³ ) order.

Placement on irregular subgrade or granular fill and settlement prior to or during placement.

C

Over the course of a large job, the small amountsof concrete returned each day or used in mud sills or incidental footings can add up to be perceived as a deficiency in yield.

D

An over-yield can be an indication of a problem if the excess concrete is caused by too much air, water, or aggregate, or if the forms have not been properly filled.

Differences between actual and target weights and air content in concrete, within the permitted tolerances, will result in discrepancies in yield.

quantity of concrete to complete the job and reevaluate the amount required towards the end of the placement. Provide this estimate to the concrete producer so that the order for the last 2 or 3 loads can be adjusted to provide the required quantity of concrete. This can prevent waiting for a short load after the plant has closed or the concrete trucks have been scheduled for other jobs. Disposal of returned concrete has environmental and economic consequences to the concrete producer and the purchaser.

Measure forms and accurately estimate the volume of concrete needed.
Estimate potential waste, additional thickness, form deflection and other contingencies—order more than the original volume estimate by at least 4 to 10 percent
Towards the end of the placement reevaluate required volume and inform the concrete producer with adequate time to adjust subsequent loads. Avoid short loads to complete a placement.
To check yield, measure the density of representative samples from three separate loads—yield is the total batch weight for each load divided by the average density.

Follow These Rules to Avoid Yield Discrepancies

CIP 9 - Low Concrete Cylinder Strength

Sat, 29 Aug 2026 01:21:31 GMT

WHAT Constitutes Low Cylinder Strength

Strength test results of concrete cylinders are used as the basis of acceptance of ready mixed concrete when a strength requirement is specified. Cylinders are molded from a sample of fresh concrete, cured in standard conditions and tested at an age indicated in the specification, usually at 28 days. For strength test results to be reliable, procedures must be in accordance with ASTM standards. The average strength of a set of two 6×12 in. (150×300 mm) or three 4×8 in. (100×200 mm) cylinders made from the same concrete sample constitutes one test result. In some cases cylinders are tested at 7 days to get an early indication of the potential strength, but these test results are not to be used to determine the acceptability of the concrete.

ASTM Standards C31, C39, C172, C1077, ASTM Book of Standards, Volume 04.02, American Society for Testing and Materials, West Conshohocken, PA. www.astm.org
Specification for Structural Concrete, ACI 301, American Concrete Institute, Farmington Hills, MI, www.concrete.org
Building Code Requirements for Reinforced Concrete, ACI 318, American Concrete Institute, Farmington Hills, MI.
In-Place Concrete Strength Evaluation-A Recommended Practice. NRMCA Publication 133, NRMCA, Alexandria, VA., www.nrmca.org
Effect of Curing Condition on Compressive Strength of Concrete Test Specimens, NRMCA Publication 53, NRMCA Alexandria, VA.
Review of Variables that Influence Measured Concrete Compressive Strength, David N. Richardson, NRMCA Publication 179, NRMCA, Alexandria, VA.
Low Strength Tests? Maybe Not!, E.O. Goeb, Concrete Products, December 1992.
Why Low Cylinder Tests in Hot Weather? E.O. Goeb, Concrete Construction, Jan. 1986.
CIP 10 – Strength of In-Place Concrete; CIP 34—Making Concrete Cylinders in the Field, CIP Series, NRMCA, Alexandria, VA

References

HOW to Reduce Low Strength Tests?

Ensure that sample of concrete at the jobsite is obtained in accordance with ASTM C172
Ensure that the cylinders are made and cured in accordance with the standard curing requirements in ASTM C31.
Ensure that cylinders are handled with care at the jobsite and during transportation.
Ensure the cylinders are tested in the laboratory in accordance with ASTM C39.

The major reasons for low compressive strength tests are:

1. Improper cylinder handling, curing and testing - this is typically the reason in most cases;

2. The addition of excessive water to the concrete mixture at the jobsite due to delays in placement or requests for a higher slump to facilitate placement and finishing;

Cylinders used for acceptance of concrete should not be confused with field-cured cylinders. Tests of field-cured cylinders are used to evaluate whether the in-place concrete has been properly cured and protected, to estimate the early-age strength in the structure to strip forms or for post-tensioning, and to continue construction activity.

The ACI Building Code, ACI 318, and the Specification for Structural Concrete, ACI 301, recognize that when mixtures are proportioned to meet the requirements of the standards, strength test results will fail to comply with acceptance criteria about one time in 100 tests due to normal variability.

The strength acceptance criteria used are:

a. The average of three consecutive tests equals or exceeds the specified strength, and

b. No single test is lower than the specified strength by more than 500 psi (3.5 MPa) when the specified strength is less than or equal to 5000 psi (35 MPa), or

No single test is lower than (0.9 × specified strength) when the specified strength is greater than 5000 psi (35 MPa)

An example of these strength acceptance criteria is provided in the table. If the strength test results fail either condition (a) or (b), steps must be taken to increase the strength of the concrete. If the results fail condition (b), an investigation should be made to ensure structural adequacy of that portion of the structure.

3. High air content in the concrete (and test specimens); and

4. Errors in production and unanticipated factors during delivery

When low compressive strength test results are reported:

1. Collect all test reports and analyze the results before taking action. Labs should not provide reports of only failing tests.

2. Look at the pattern and numbers of reported strength results.

Considering the sequence of results—is there a violation on compliance with the strength acceptance criteria discussed above
The strength range of two or three cylinders prepared from the same sample should rarely exceed 8.0% or 9.5% of the strength average, respectively.
Do the results indicate that the cylinders are being loaded to complete failure

3. Do the test reports provide any causal reasons?

Review the dates and times of batching, sampling, pick up from jobsite and delivery to the lab
Review concrete and ambient temperatures, number of days cylinders were left in the field, procedures used for initial curing in the field, duration of transportation, and subsequent curing in the lab
Review the slump, air content, and density, if measured
Review for any reported cylinder defects.

It is important that procedures are conducted in accordance with ASTM standards. Almost all deficiencies in handling and testing cylinders will result in a lower measured strength. All violations add up to cause significant reductions in measured strength. The more significant factors are improperly finished surfaces, initial curing over 80° F (27°C); frozen cylinders; extra days in the field; damage during transportation; delay in stripping molds and curing at the lab; improper capping; and insufficient care in breaking cylinders.

The laboratory should be held responsible for deficiencies in its procedures. Field-testing technicians and laboratory personnel should be certified; construction workers untrained in concrete testing must not make and handle cylinders. ACI 318 requires laboratories to conform to ASTM C1077 for their quality system and personnel qualifications. Laboratories should be inspected by the Cement and Concrete Reference Laboratory (CCRL) laboratory inspection or an equivalent program. Field testing personnel must have a current ACI Grade I Field Testing Technician certification, or equivalent. Laboratory personnel should have the ACI Grade I and II Laboratory Testing Technician and/or the ACI Strength Testing Certification, or equivalent.

If the deficiency justifies investigation, first verify testing accuracy and then compare the structural requirements with the measured strength. If testing is deficient or if strength is greater than actually needed in that portion of the structure, there is little point in investigating the in-place strength. However, if procedures conform to the standards and the strength as specified is required for the structural capacity of the member in question, further investigation of the in

-place concrete may be required. (CIP 10).

CIP 10 - Strength of In-Place Concrete

Sat, 29 Aug 2026 01:21:14 GMT

WHAT is the Strength of In-Place Concrete?

Concrete structures are designed to carry dead and live loads during construction and in service. Samples of concrete are obtained during construction and standard ASTM procedures are used to measure the potential strength of the concrete as delivered. Cylin-ders are molded and cured at 60 to 80°F (17 to 27°C) for one day and then moist cured in the laboratory until broken in compression, normally at an age of 7 and 28 days or at an alternative specified age.

The in-place strength of concrete will not be equiva-lent to that measured on standard cylinders and will generally be lower. Job practices for handling, plac-ing, consolidation, and curing concrete in structures are relied upon to provide an adequate percentage of that potential strength, measured on cylinders, in the structure. Structural design principles recognize this and

ACI 318, Building Code Requirements for Structural Concrete, ACI, Farmington Hills, MI, www.concrete.org .
ACI 228.1R, In-Place Methods to Estimate Concrete Strength, ACI, Farmington Hills, MI.
ACI 214.4R, Guide for Obtaining Cores and Interpreting Compressive Strength Results, ACI, Farmington Hills, MI.
ASTM C 31, C 39, C 42, C 805, C 803, C 873, C 900, ASTM Book of Standards, Vol. 04.02, American Society for Testing and Materials, West Conshohocken, PA
Guide to Nondestructive Testing of Concrete, G.I. Crawford, Report FHWA-SA-97-105, Sept. 1997, Federal Highway Administration, Washington, DC.
CIP 9, 39, Concrete in Practice Series, NRMCA, Alexandria, VA, www.nrmca.org.
TIP 11, Testing Concrete Cores, Technology in Practice Series, NRMCA, Alexandria, VA,
In-Place Strength Evaluation - A Recommended Practice, NRMCA Publication 133, NRMCA, Alexandria, VA.
Understanding Concrete Core Testing, Bruce A. Suprenant, NRMCA Publication 185, NRMCA, Alexandria, VA.

References

WHY Measure In-Place Strength?

Tests of in-place concrete may be needed when standard cylinder strengths are low and do not comply with strength acceptance criteria outlined in ACI 318. 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, or the specified strength is really needed. (See CIP 9 and TIP 11) In many cases, the concrete can be accepted for the intended use without in-place strength testing.

There are many other situations that may require the investigation of in-place strength. These include: 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 structural adequacy, additional curing of the structure may provide the necessary strength. This is particularly possible with concrete containing fly ash, slag cement or some blended cements.

the ACI Building Code, ACI 318, has a process of assuring the structural safety during construction. In cold weather a slower rate of in-place strength gain can be expected.

Means of measuring, estimating, or comparing the strength of in-place concrete include: rebound ham-mer, penetration probe, pullouts, maturity, cast-in-place cylinders, tests of drilled cores, and load tests of the structural member or system.

Cores drilled from the structure is one of the methods of evaluating whether the structural capacity of a con-crete member is adequate. Drilled cores generally test lower than standard-cured cylinders. The ACI Build-ing Code (ACI 318) recognizes that concrete con-struction can be considered structurally adequate if the average of three cores from a region represented by non-compliant concrete strength tests is equal to or exceeds 85 percent of specified strength, ƒ´ _c with no single core less than 75 percent of ƒ´ _c . Measured core strengths are not corrected for age. TIP 11 discusses core testing for acceptance of concrete. ACI 214.4R provides detailed guidance on core testing, evaluating existing structure capacity using in-place strengths, and determining an equivalent ƒ´ _c value for evaluat-ing the structural capacity of an existing structure. The latter process should not be used to determine the acceptability of concrete furnished to a project.

HOW to Investigate In-Place Strength?

C

Probe Penetration Resistance, ASTM C803 - Probes or pins driven into concrete can be used to study relative strength of in-place concrete:

The length to diameter ratio (L/D) should be around 2, but try to obtain cores with L/D of at least 1½;

Store the cores in sealed watertight bags or containers and transport to the laboratory. Test the cores in accordance with ASTM C42. Saw or grind core ends within 2 days after drilling. Keep cores in a sealed condition for at least 5 days after last wetted. Review the requirements for conditioning cores in current versions of ACI 318 and ASTM C42.

D

Obtain cores with a minimum diameter of 3.7 in. (85 mm) or at least twice the nominal maximum aggregate size. Smaller diameter cores are permitted when it is not feasible to obtain the required size;

E

Test a minimum of 3 cores for each location in the structure represented by low strength tests;

Avoid drilling cores from the top layers of columns, slabs, walls, or footings, which will be 10 to 20 percent weaker than cores from the mid or lower portions; and

If only one set of cylinders is low, often the question can be settled by comparing rebound hammer or penetration probe results on concrete in areas represented by acceptable 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 C42 - If core drilling is necessary observe the following:

B

A

Probes not firmly embedded in the concrete should be rejected;

D

C

Probes should be spaced at least 7 in. apart and not be close to the edge of the concrete;

Accurate measurement of the exposed length of the probe is required;

B

A

Different size probes or pins, or a change in driving force may be necessary for large differences in strength or concrete density;

E

Develop a strength calibration curve for the materials and conditions under investigation; and

Surface conditions, moisture conditions, and

aggregate characteristics can affect the results.

F

Rebound Hammer, ASTM C805 - This method is also used to evaluate the relative in-place strength:

A

Wet all surfaces for several hours or overnight because drying affects rebound number;

Don’t compare readings on concrete cast against different form materials, concrete of varying moisture content, readings from different impact directions, on members of different mass, or results using different hammers;

B

D

C

Don’t grind off the surface unless it is soft, finished or textured;

Test structural slabs from the bottom; and

D

E

Do not test frozen concrete.

Surface conditions, moisture conditions, and aggregate characteristics can affect the results

Maturity, ASTM C1074 – If concrete maturity is used to estimate the in-place strength please refer to CIP 39

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 C873 and C900.

CIP 4 - Cracking Concrete Surfaces

Fri, 28 Aug 2026 23:41:28 GMT

WHAT are Some Forms of Cracks?

Concrete, like other construction materials, contracts and expands with changes in moisture and temperature, and deflects depending on load and support conditions. Cracks can occur when provisions

to accommodate these movements are not made in design and construction. Some forms of common cracks are:

Omission of isolation and contraction joints and improper jointing practices.

A

B

Improper subgrade preparation.

C

The use of high slump concrete or excessive addition of water on the job.

Improper finishing.

D

E

Rapid loss of moisture from newly placed concrete in dry conditions

HOW to Prevent or Minimize Cracking?

Curing. Curing is an important step to ensure durable crack-resistant concrete. 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.

A

Concrete. In general, use concrete with a moderate slump (not to exceed 5 inches [125 mm]). Higher slump can be used provided the mixture is designed to produce the required strength without excessive bleeding and/or segregation. This is generally accomplished by using water-reducing admixtures. Use air-entrained concrete for outdoor slabs exposed to freezing weather (See CIP 2). Concrete mixtures can be designed for reduced shrinkage to minimize cracking.

Finishing. Initial screeding must be promptly followed by bull floating. DO NOT perform subsequent finishing operations with water present on the surface or before the concrete has completed bleeding. Do not overwork or over-finish the surface. For better traction on exterior surfaces use a broom finish. When ambient conditions are conducive to a high evaporation rate, use means to avoid rapid drying and associated plastic shrinkage cracking by using wind breaks, fog sprays, and covering the concrete with wet burlap or polyethylene sheets between finishing operations.

Subgrade and Formwork. All topsoil and soft spots should be removed. 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. In winter, remove snow and ice prior to placing concrete and do not place concrete on a frozen subgrade. Smooth, level and uniformly compacted subgrades help prevent cracking. All formwork must be constructed and braced so that it can withstand the pressure of the concrete without movement. Vapor retarders directly under a concrete slab increase bleeding and greatly increase the potential for cracking, especially with high-slump concrete. When it is required to place concrete directly on polyethylene vapor retarders (CIP 29) take special care to ensure that finishing operations are performed after all bleed water has dissipated from the surface. In dry conditions lightly dampen subgrade, formwork and reinforcement immediately prior to concrete placement.

Joints. Anticipated volume changes due to temperature and/or moisture should be accommodated by contraction joints saw cut or tooled at the proper time with a depth of about ¼ to ⅓ the thickness of the slab, and with a spacing between 24 to 36 times the slab thickness. A maximum 15 feet spacing for contraction joints is often recommended. Panels between joints should be square and the length should not exceed about 1.5 times the width. Isolation joints to the full thickness of the slab should be provided whenever restriction to freedom of either vertical or horizontal movement is anticipated—such as where floors meet walls, columns, or footings. See CIP 6 for information on joints.

E

D

B

C

Control of Cracking in Concrete Structures, ACI 224R, American Concrete Institute, Farmington Hills, MI.
Guide for Concrete Floor and Slab Construction, ACI 302.1R, American Concrete Institute, Farmington Hills, MI.
Concrete Slab Surface Defects: Causes, Prevention, Repair, IS177, Portland Cement Association, Skokie, IL.
Grant T. Halvorson, Troubleshooting Concrete Cracking During Construction, Concrete Construction, October 1993.
Cracks in Concrete: Causes, Prevention, Repair, A collection of articles from Concrete Construction Magazine, June 1973.

References

WHY Do Concrete Surfaces Crack ?

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

Fig. A: Plastic shrinkage cracks (CIP 5)

Fig. B: Cracks due to improper jointing (CIP 6)

Fig. C: Cracks due to continuous external restraint Example: Cast-in-place wall restrained along bottom edge of footing

Fig. D: Cracks due to lack of isolation joints(CIP6 ) Fig. E: D-Cracks from freezing and thawing

Fig, F: Craze Cracks (See CIP 3)

Fig. G: Settlement cracks

F

Reinforcement. Wire mesh and reinforcement in slabs cannot prevent cracking. When placed at the proper location, reinforcement can reduce crack width. Providing sufficient concrete cover (at least 2 inches [50 mm]) to keep salt and moisture from contacting the steel should prevent cracks in reinforced concrete caused by expansion of rust on reinforcing steel.

Follow These Rules to Minimize Cracking

Design the members to handle all anticipated loads.
Provide proper contraction and isolation joints.
In slab on grade work, prepare a stable uniformly compacted subgrade.
Place and finish according to recommended and established practices.
Protect and cure the concrete properly.

Most random cracks that appear at an early age, although unsightly, rarely affect the structural integrity or the service life of concrete. Two exceptions are:

D-cracks, which occur due to freeze-thaw deterioration of some types of porous aggregate in concrete. These cracks initiate at joints at the bottom of exterior slabs and typically appear at later ages.

· Cracking due to alkali aggregate reactions will lead to long term structural damage (CIP 43).

Inadequate or no curing.

F

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

CIP 1—Dusting Concrete Surfaces

Fri, 28 Aug 2026 23:26:16 GMT

WHAT is Dusting

Formation of loose powder resulting from disintegra-tion of surface of hardened concrete is called dusting or chalking. The characteristics of such surfaces are:

a. They powder under any kind of traffic

b. They can be easily scratched with a nail or even by sweeping.

Poor finishing practices such as broadcasting dry cement to speed up finishing or sprinkling water to the surface while finishing

B

C

Floating and/or troweling operations following the condensation of moisture from warm humid air on cold concrete. In cold weather 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.

D

Inadequate ventilation in enclosed spaces. 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.

Insufficient curing. This omission often results in a soft surface skin, which will easily dust under foot traffic.

E

F

Inadequate protection of freshly placed concrete from rain, snow or drying winds. Allowing the concrete surface to freeze will weaken the surface and result in dusting.

HOW to Prevent Dusting

Do not place concrete directly on polyethylene vapor retarders or non-absorptive subgrades as this can contribute to problems such as dusting, scaling, and cracking. Place 3 to 4 inches [75 to 100 mm] of a trimable, compactible fill, such as a crusher-run material, over vapor retarders or non-absorptive subgrade prior to concrete placement. When high evaporation rates exist, lightly dampen absorptive subgrades just prior to concrete placement, ensuring that water does not pond or collect on the subgrade surface. However, it may essential to place concrete directly on polyethylene vapor retarders for interior slabs that can receive floor coverings at any point in its service life (CIP 29). For such cases take special care to ensure that finishing operations are performed after all bleed water has dissipated from the surface

A

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, or by accelerating the setting time.

DO NOT perform any finishing operations with water present on the surface or while the concrete continues to bleed. Initial screeding must be promptly followed by bull floating. Delaying bull floating operations can cause bleed water to be worked into surface layer. Do not use a jitterbug, as it tends to bring excess mortar to the surface. DO NOT add water to the surface to facilitate finishing operations.

Concrete with the lowest water content with an adequate slump for placing and finishing will result in a strong, durable, and wear-resistant surface. In general, use concrete with a moderate slump not exceeding 5 inches [125 mm]. Concrete with a higher slump may be used provided the mixture is designed to produce the required strength without excessive bleeding and/or segregation. Water-reducing admixtures are typically used to increase slump while maintaining a low water content in the mixture. This is particularly important in cold weather when delayed set results in prolonged bleeding.

Provide proper curing by using liquid membrane curing compound or by covering the surface with water, wet burlap, or other curing materials as soon as possible after finishing to retain moisture in the slab. It is important to protect concrete from the environment at early ages.

E

D

B

C

Guide for Concrete Floor and Slab Construction, ACI 302.1R. American Concrete Institute, Farmington Hills, MI.
Slabs on Grade, Concrete Craftsman Series CCS-1, American Concrete Institute, Farmington Hills, MI.
Concrete Slab Surface Defects: Causes, Prevention, Repair, IS177, Portland Cement Association, Skokie, IL
The Effect of Various Surface Treatments, Using Zinc and Magnesium Fluosilicate Crystals on Abrasion Re-sistance of Concrete Surfaces, Concrete Laboratory Report No. C-819, U.S. Bureau of Reclamation.
Residential Concrete, National Association of Home Builders, Washington, DC.
Trouble Shooting Guide for Concrete Dusting, Concrete Construction, April 1996.

References

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 or before the concrete has finished bleeding. Working this bleed water back into the top ¼-inch [6 mm] of the slab produces a very high water-cement ratio and, therefore, a low strength surface layer.

A

F

Placing concrete in cold weather requires concrete temperatures exceeding 50°F [10°C] as well as an accelerating admixture.

Follow These Rules to Prevent Dusting

Use moderate slump concrete not exceeding 5 inches [125 mm].
Do not start finishing operations while the concrete is bleeding.
Do not broadcast cement or sprinkle water on concrete prior to or during finishing operations.
Ensure that there is adequate venting of exhaust gases from gas-fired heaters in enclosed spaces.
Use adequate curing measures to retain moisture in concrete for the first 3 to 7 days and protect it from the environment, especially freezing conditions.

CIP 2 - Scaling Concrete Surfaces

Fri, 28 Aug 2026 23:11:49 GMT

WHAT is Scaling

Scaling is local flaking or peeling of a finished surface of hardened concrete as a result of exposure to cycles of freezing and thawing. 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 ⅛ to ⅜ inch [3 to 10 mm] 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 CIP. Often this is due to the early use of a steel trowel, over-finishing or finishing while bleed water is on the surface.

WHY Do Concrete Surfaces Scale

Concrete slabs and surfaces of other members that are saturated with water and exposed to cycles of freezing and thawing are susceptible to scaling. When concrete is saturated with water and temperature approaches freezing, water expands as it forms ice and this causes stresses within concrete. As the number of cycles of freezing and thawing increases, the potential for scaling increases. Deicing chemicals exacerbate this by increasing the saturation of concrete at the surface and the number of freezing and thawing cycles. Air entrained concrete contains millions of small air bubbles that accommodate the expanding water and ice and prevent the stress buildup.

Most scaling is caused by:

The use of non-air-entrained concrete or too little entrained air, especially at the surface.

A

B

Using concrete that has a low strength that allows

permeation to water.

C

Using the improper concrete mixture or mixture proportions for the application.

Application of excessive amounts of deicing chemicals, especially on newly installed concrete that tends to be saturated and of lower strength.

D

Insufficient curing resulting in a weak concrete surface.

E

Improper finishing procedures of concrete slabs.

F

HOW to Prevent Scaling

The potential for scaling in concrete slabs can be reduced by using good quality dense concrete with entrained air, following good practice for installing and curing, and by minimizing the use of deicing chemicals.

For concrete that will be continuously moist, exposed to freezing temperatures and will be subject to the use of deicing chemicals, the following recommendations should be followed:

For most slab construction, place concrete at a slump in the range of 3 to 5 inches [75 to 125 mm]. Do not add excessive water at the jobsite. High slump obtained by adding water increased the potential for segregation and excessive bleeding and can result in weak mortar layer at the surface. Water reducing admixtures can provide improved workability and retain good concrete quality.

A

Concrete should be air-entrained. The recommended total air content for concrete containing ¾-inch [19 mm] or 1-inch 25 mm] coarse aggregate is 6 percent.

The quantity of supplementary cementitious materials (SCM) should not exceed one of the following: 25% fly ash, 50% slag cement or 10% silica fume, expressed as percent by weight of the cementitious materials. SCMs are beneficial to concrete, however, at higher quantities change the rate of setting, bleeding, and strength gain. These impact the process of finishing. With appropriate modifications of the finishing procedures, it is possible to use higher quantities of SCMs, but these need to be evaluated.

For exterior slabs, order concrete with specified strength of 4000 psi [28 MPa], consistent with the requirements of ACI 332, Code for Residential Concrete. For concrete that will not be continuously moist or where deicing chemicals will not be applied, the specified strength should be 3500 psi [24 MPa].

Placing and finishing procedures can reduce the entrained air content in concrete, making it more susceptible to scaling.

E

D

B

C

Guide to Durable Concrete, ACI 201.2R, American Concrete Institute, Farmington Hills, MI.
Scale-Resistant Concrete Pavements, IS117.02P, Portland Cement Association, Skokie, IL.
Protective Coatings to Prevent Deterioration of Concrete by Deicing Chemicals, National Cooperative Highway Research Program Report No. 16.
Code Requirements for Residential Concrete, ACI 332, American Concrete Institute, Farmington Hills, MI.
Residential Concrete, National Association of Home Builders , Washington, DC.
Slabs on Grade, Concrete Craftsman Series CCS-1, American Concrete Institute, Farmington Hills, MI.
Eugene Goeb, Deicer Scaling: An Unnecessary Problem, Concrete Products, February 1994.
Concrete in Practice Series, CIP 5, 11, 14, NRMCA, Alexandria, VA, www.nrmca.org .

References

Do not use a jitterbug or vibrating screed with high slump concrete as it increases segregation and result in a weak mortar layer at the surface.

F

G

Do not overwork the surface of concrete. Excessive finishing reduces entrained air in the surface layer. For most exterior surfaces a broom finish is adequate.

Do not perform finishing operations with bleed water present on the surface. Bull floating must promptly follow initial screeding. Delay subsequent finishing until bleed water has risen and dissipated from the surface. This is critical when placing air-entrained concrete in dry and windy conditions where the surface may appear to be dry while concrete is continuing to bleed. The use of fog sprays or evaporation retardants are recommended in these conditions. See CIP 14 for finishing concrete

H

Provide proper curing by using pigmented liquid membrane curing compound or by covering the surface of newly placed slab with wet burlap and plastic sheets. Proper curing involves maintaining concrete at adequate temperature and moisture for optimum performance.

J

I

Protect concrete from the harsh winter environment. Apply a commercially available silane or siloxane-based breathable concrete sealer or water repellent specifically designed for use on concrete slabs. Follow the manufacturer’s recommendations. The concrete should be reasonably dry prior to the application of a sealer. Late summer with a few dry days preceding application is an ideal time .

Be cautious about placing exterior concrete in late fall, winter or early spring when conditions are such that it will be exposed to freezing temperatures shortly after placement while concrete is still saturated.

K

Avoid using deicing chemicals on newly placed concrete, if possible. Use clean sand for traction. When used, deicing chemicals should be applied in moderate amounts. Excessive applications increases potential for scaling. When conditions permit, hose off accumulation of salt deposited by cars on driveways and garage slabs. Deicing chemicals composed of calcium chloride and sodium chloride (rock salt) are considered acceptable for concrete. Never use ammonium sulfate or ammonium nitrate or magnesium-based salts as a deicer; these are chemically aggressive and destroy concrete surfaces. Magnesium-based salts are used for pre-snow deicing of roads and can be tracked by cars and accumulate on concrete surfaces. Poor drainage causing salt solutions to accumulate on concrete surfaces increases the severity of the exposure and may cause scaling.

L

Minor scaling is a cosmetic issue and may not need to be repaired. On the other hand repairing concrete slabs with excessive and progressing scaling may not be feasible.

It is possible to repair light to moderately scaled surfaces. The repaired surface will only be as strong as the base sur-face to which it is bonded. The surface should be prepared to remove the unsound surface and should be free of dirt, oil or paint. The surface receiving the repair must be sound. To accomplish this, use a hammer and chisel, sandblasting, high-pressure washer, or jack hammer. The clean, rough, textured surface can be repaired with thin bonded resurfac-ing such as:

A

Portland cement concrete resurfacing

Polymer-modified cementitious-based repair mortar

B

HOW to Repair Scaled Surfaces

C

Latex modified concrete resurfacing

Repair material will not match the color and characteristics of the original concrete.

For concrete that will be exposed to severe freezing and thawing conditions order good quality air-entrained concrete with a strength of 4000 psi [28 MPa]
Do not add excessive water and place concrete at a slump of 3 to 5 inches [75 to 125 mm].
Finish concrete after bleed water has dissipated and avoid using steel trowels when finishing.
Properly cure the concrete
Consider sealing the surface with a commercial breathable sealer.
Avoid the use of deicing chemicals in the first winter and subsequently use them in moderate amounts.

Follow These Rules to Prevent Scaling

CIP 3 - Crazing Concrete Surfaces

Fri, 28 Aug 2026 20:44:43 GMT

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 that is rich in paste or mortar. These cracks are rarely more than ⅛ inch [3 mm] deep and are more noticeable on steel-troweled surfaces. The irregular hexagonal areas between cracks are typically no more than 1½ inch [40 mm] across and may be as small as ½ or ⅜ inch [12 or 20 mm] 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. Craze cracking is more visible when the surface is drying after it has been wet.

Crazing cracks are sometimes referred to as shallow map or pattern cracking. They do not affect the structural integrity of concrete.

Craze cracks are generally not a precursor to future deterioration, durability, or wear resistance, especially in interior slabs. Crazed surfaces can be unsightly and cracks can become more obvious as dirt gets embedded in them.

WHY Do Concrete Surfaces Craze

Concrete surface crazing usually occurs if one or more good concre te practices are not followed during installation.

Placing a concrete mixture with a higher slump by addition of excessive water. This causes concrete to segregate and create a paste or mortar rich layer at the surface with finishing operations. It also delays setting and increases bleeding.

A

B

Placement methods, especially on higher slump concrete, that depresses coarse aggregate below the surface and results in an excessive concentration of paste or fines at the surface. These can include the use of a jitterbug, vibratory screeds, or excessive or improper floating with an inclined blade that creates too much pressure on the surface.

C

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 bull float or darby with water on the surface or while the concrete continues to bleed will produce a high w/cm ratio, weak surface layer which will be susceptible to crazing, dusting, and other surface defects.

Sprinkling cement on the surface to dry up bleed water on concrete surfaces is a frequent cause of crazing. This concentrates fines on the surface. Spraying water on the concrete surface to facilitate finishing operations will result in a weak surface susceptible to crazing or dusting.

D

Lack of or delayed curing will result in crazing and other surface defects. Curing should begin immediately after final finishing. Intermittent wetting and drying the surface will increase the potential for craze cracking

E

Premature finishing of concrete when the surface appears to be dry should be avoided. Conditions that increase evaporation from the surface include low humidity, high temperature, direct sunlight, or high wind velocity. Delayed setting of the underlying concrete due to temperature differences between the surface and the base can exacerbate surface drying. Mixtures with a w/cm below 0.45 and containing supplementary cementitious materials will have lower bleeding.

F

G

Occasionally carbonation of the surface soon after finishing results in crazing as it causes shrinkage of the surface layer. Carbonation is a chemical reaction of concrete with between carbon dioxide and can result if unvented heaters are used. This will result in a softer surface that will dust.

HOW to Prevent Crazing

Concrete surface crazing usually occurs if one or more good concrete practices are not followed during installation.

Start curing the concrete as soon finishing is completed, particularly after hard troweling. At this stage the concrete surface should not be allowed to rapidly dry. Keep the surface wet by either ponding with water, covering it with damp burlap and keeping it continuously moist for a minimum of 3 days, or spraying the surface with a liquid-membrane curing compound. Avoid alternate wetting and drying of concrete surfaces at an early age. Avoid curing with water that is more than 20° F [10°C] cooler than concrete.

A

Place concrete on 3 to 4 inches [75 to 100 mm] compactible granular fill to absorb some water from concrete and reduce bleeding. Moisten the subgrade only if conditions exist for a high evaporation rate. For interior slabs placed on a vapor retarder, avoid adding water to the concrete to increase slump.

AVOID the use of jitterbugs and vibrating screeds, especially with concrete with slump greater than 3 in. [75 mm]. DO NOT overwork or over-finish concrete with bull floats or other finishing tools while concrete is still plastic. This results in excessive mortar at the surface. DO NOT perform any finishing operation while bleed water is present on the surface or before the bleeding process is completed. NEVER sprinkle or trowel dry cement or a mixture of cement and fine sand on the surface of the plastic concrete to absorb bleed water. DO NOT sprinkle water on the slab to facilitate finishing. Remove bleed water by dragging a garden hose across the surface. AVOID premature floating and troweling the surface.

Place concrete at a moderate slump (3 to 5 inches [75 to 125 mm]). Higher slump (up to 6 or 7 inches [150 to 175 mm]) can be used if it is achieved by using mid-range or high-range water-reducing admixtures. These mixtures will be less susceptible to segregation and will bleed less.

If concrete has low bleeding characteristics or if evaporation rate is high, protect the surface from drying by using evaporation retarders or other means to reduce the drying of the surface. These precautions will be necessary with slower setting concrete or if concrete is placed on cold subgrade that can cause differential setting.

E

D

B

C

Guide for Concrete Floor and Slab Construction , ACI 302.1R, American Concrete Institute, Farmington Hills, MI. www.concrete.org
Guide to Residential Concrete Construction , ACI 332.1R, American Concrete Institute, Farmington Hills, MI. www.concrete.org
Concrete Slab Surface Defects: Causes, Prevention, Repair, IS 177T, Portland Cement Association, www.cement.org
Ward Malisch, Avoiding Common Outdoor Flatwork Problems, Concrete Construction, July 1990.