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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.