Crack Control by Construction Methods

Careful construction will minimize cracking. Some of these methods as per ACI 224 R 01 are listed below. 

Selecting Concrete aggregates

The aggregates should be selected to make concrete of high-strain capacity. It is important that fine and coarse aggregates be clean and free of unnecessary fine material, particularly clays. The fine aggregate should have a sand equivalent value greater than 80% and should be verified frequently. The sand should have sufficient time in storage for the moisture content to stabilize at a level of less than 7% on an oven-dry basis.

Use Shrinkage-compensating cement

Shrinkage-compensating cement can be used to compensate for the shrinkage in restrained elements. The principal property of these cement is that the expansion induced in the concrete while setting and hardening is designed to offset the normal drying shrinkage. With correct usage (particularly with early and ample water curing required for maximum expansion), the distance between joints can sometimes be tripled without increasing the level of shrinkage cracking. Details on the types and the correct usage of shrinkage-compensating cement are given in Section 3.6 and ACI 223.

Proper Handling and Batching of Concrete

Aggregates should be handled so as to avoid contamination, segregation, and breakage. Handling and batching is best done by finish screening and rinsing coarse aggregate into their various sizes and placing them in the appropriate bins at the batch plant. Every effort should be made to uniformly batch and mix the concrete so that there will be a minimum of variation in slump and workability, which invariably lead to demands for a greater margin of workability.

Use of Cold Concrete

In mass concrete structures, reducing water and cement contents to a practical minimum and using cold concrete will reduce temperature differentials that cause cracking. Less mixing water reduces drying shrinkage.

In warm weather, cold concrete reduces slump loss, increases pumpability, and improves the response to vibration. Chipped ice can be substituted for all or a part of the batched mixing water. 

In cold weather, concrete is naturally cold, and every effort should be made to use it as cold as possible without inviting damage from freezing. It is pointless to try protecting surfaces, edges, and corners by placing needlessly warm concrete in cold weather. These vulnerable parts require protection by insulation or protective enclosures (ACI 306R).

Revibration

When done as late as the formed concrete will respond to the vibrator, revibration can eliminate cracks and checks where something rigidly fixed in the placement prevents a part of the concrete from settling with the rest of it. Settlement cracks are most apparent in the upper part of wall and column placements where revibration can be readily used. Deep revibration corrects cracks caused by differential settlement around blockout and window forms and where slabs and walls are placed monolithically (ACI 309R).

Proper Finishing

Proper flatwork finishing can make a difference in many types of cracking (See also ACI 302.1R). Low-slump concrete should be used. More than a 75 mm slump is rarely necessary, except in hot weather, where both slump and moisture are lost quite rapidly. Finishing should not be done in the presence of surface water. Precautions should be taken to prevent plastic shrinkage. Any required marking and grooving should be carefully cut to the specified depth. Proper curing should be promptly conducted.

Proper Curing and Protection of Concrete

Concrete should be brought to a level of adequate strength and protected from low temperatures and dry conditions that would otherwise cause cracking. The curing and protection should not be discontinued abruptly. If the new concrete is given a few days to gradually dry or cool, creep can reduce the possibility of cracking when the curing and protection are fully discontinued. Subsequent application of a curing compound after initial curing will slow the drying action.

Other Miscellaneous methods

 Other miscellaneous methods that could be adopted to reduce cracking are listed below:

• Reinforcement and embedments should be firmly positioned with the designated thickness of cover to prevent corrosion, expansion, and cracking;
• Concrete should not be placed against hot reinforcement or forms;
• Formwork support should be both strong and stiff enough to be free of early failures and distortion causing cracking;
• Subgrade and other supports should not settle unevenly, which may lead to cracking due to overstress in the structure;
• Calcium chloride should not be used if steel reinforcement is present. If acceleration in setting or strength gain is needed, additional cement, hot water, or a non-chloride accelerator should be used;
• Special care is needed in handling precast units to prevent overstress due to handling. Pickup points and rigging should be considered;
• Avoid using unvented salamanders in cold weather or gasoline-operated equipment where ventilation is not adequate. They will increase the danger of carbonation, causing shrinkage and surface cracking;
• Contraction joints should not be omitted and grooves should be of sufficient depth and well within the maximum permitted spacing. In hot weather or arid environments, contraction joints should be installed in the fresh concrete as inserts or saw-cut when the concrete is hard enough so that it is not damaged by the blade;
• Reactive elements of coarse aggregate should be neutralized through the use of low-alkali cement, suitable pozzolans, or both. The use of certain cherts and similar expansive aggregates can cause cracks and popouts; and
• Correct amounts of entrained air should be used and monitored to prevent cracking due to freezing and thawing and exposure to deicing salts.

Source: ACI 224R-01 Control of Cracking in Concrete Structures