A common adage is that there are two guarantees with concrete. One, it will get hard and two, it will crack. Cracking is a frequent cause of complaints in the concrete industry. The Concrete Foundations Association has produced a new flyer to help contractors educate their customers about the causes of cracks and when they should be a concern. A more detailed explanation of cracking is presented in this article.
Cracking can be the result of one or a combination of factors such as drying shrinkage, thermal contraction, restraint (external or internal) to shortening, subgrade settlement, and applied loads. Cracking can not be prevented but it can be significantly reduced or controlled when the causes are taken into account and preventative steps are taken.
Another problem associated with cracking is public perception. Cracks can be unsightly but many consumers feel that if a crack develops in their wall or floor that the product has failed. In the case of a wall, if a crack is not structural, is not too wide (the acceptable crack of a crack depends on who you ask and ranges from 1/16” to 1/4”) and is not leaking water, it should be considered acceptable. It is in the best interest of you, the wall contractor, to educate your customers that the wall will crack and when it should be a concern to them.
Cracks that occur before hardening usually are the result of settlement within the concrete mass, or shrinkage of the surface (plastic-shrinkage cracks) caused by loss of water while the concrete is still plastic.
Settlement cracks may develop over embedded items, such as reinforcing steel, or adjacent to forms or hardened concrete as the concrete settles or subsides. Settlement cracking results from insufficient consolidation (vibration), high slumps (overly wet concrete), or a lack of adequate cover over embedded items.
Plastic-shrinkage cracks are most common in slabs and are relatively short cracks that may occur before final finishing on days when wind, a low humidity, and a high temperature occur. Surface moisture evaporates faster than it can be replaced by rising bleed water, causing the surface to shrink more than the interior concrete. As the interior concrete restrains shrinkage of the surface concrete, stresses can develop that exceed the concrete's tensile strength, resulting in surface cracks. Plastic-shrinkage cracks are of varying lengths spaced from a few centimeters (inches) up to 3 m (10 ft) apart and often penetrate to mid-depth of a slab.
Cracks that occur after hardening usually are the result of drying shrinkage, thermal contraction, or subgrade settlement. While drying, hardened concrete will shrink about 1/16 in. in 10 ft of length. One method to accommodate this shrinkage and control the location of cracks is to place construction joints at regular intervals. For example, joints can be constructed to force cracks to occur in places where they are inconspicuous or predictable. Horizontal reinforcement steel can be installed to reduce the number of cracks or prevent those that do occur from opening too wide.
The major factor influencing the drying shrinkage properties of concrete is the total water content of the concrete. As the water content increases, the amount of shrinkage increases proportionally. Large increases in the sand content and significant reductions in the size of the coarse aggregate increase shrinkage because total water is increased and because smaller size coarse aggregates provide less internal restraint to shrinkage. Use of high-shrinkage aggregates and calcium chloride admixtures also increases shrinkage. Within the range of practical concrete mixes – 470 to 750 lb/yd3 (5- to 8-bag mixes) cement content – increases in cement content have little to no effect on shrinkage as long as the water content is not increased significantly.
Concrete has a coefficient of thermal expansion and contraction of about 5.5 x 10-6 per °F. Concrete placed during hot midday temperatures will contract as it cools during the night. A 40°F drop in temperature between day and night-not uncommon in some areas-would cause about 0.03 in. of contraction in a 10-ft length of concrete, sufficient to cause cracking if the concrete is restrained. Thermal expansion can also cause cracking.
Structural cracks in residential foundations usually result from settlement or horizontal loading. Most (but not all) structural cracks resulting from applied loads are nearly horizontal (parallel to the floor) and occur 16” to 48” from the top of the wall. They are much more prevalent concrete block construction. They can be brought about by hydrostatic pressure or heavy equipment next to the foundation.
Diagonal cracks that extend nearly the full height of the wall are often an indication of settlement. In either of the above conditions, an engineer should be consulted. Diagonal cracks emanating from the corner of windows and other openings are called reentrant cracks and are usually the result of stress build-up at the corner. Diagonal reinforcement at the corner of openings can reduce the instance of crack formation and will keep the cracks narrow.
Other procedures which can reduce cracking in concrete include the following practices.
- Minimize the mix water content by maximizing the size and amount of coarse aggregate and by using low-shrinkage aggregate.
- Use the lowest amount of mix water required for workability and placement; do not permit overly wet consistencies.
- Use calcium chloride admixtures only when necessary.
- Prevent rapid loss of surface moisture while the concrete is still plastic through use of spray-applied finishing aids or plastic sheets to avoid plastic-shrinkage cracks (more important in slabs)
- Provide contraction joints at reasonable intervals, 30 times the wall thickness is a good “rule-of-thumb”.
- Prevent extreme changes in temperature after placement and initial cure.
- Properly place and consolidate the concrete.
Cracks can also be caused by freezing and thawing of saturated concrete, alkali- aggregate reactivity, sulfate attack, or corrosion of reinforcing steel. However, cracks from these sources may not appear for years. Proper mix design and selection of suitable concrete materials can significantly reduce or eliminate the formation of cracks and deterioration related to freezing and thawing, alkali-aggregate reactivity, sulfate attack, or steel corrosion.
For more information, refer to Design and Control of Concrete Mixtures, EB001, and Diagnosis and Control of Alkali-Aggregate Reactions in Concrete, IS413.