Cracks at Concrete Slabs from Shrinkage & Thermal Effects
It is concrete’s nature to crack. This fact arises from at least two primary characteristics of concrete. The first of these is concrete’s tendency to shrink. The amount of concrete shrinkage is a function of numerous factors including the gradation and quality of aggregates, water and paste content, admixtures and cement used in the mix, and ambient humidity and temperature conditions. A second characteristic that contributes to concrete cracking is that while concrete is strong in compression, it is weak in tension. Therefore, anything that induces tensile forces within a concrete slab can contribute to its cracking.
Two primary causes of such tensile forces within a concrete slab on grade is concrete shrinkage and thermal movements. Concrete drying shrinkage is the reduction in volume as moisture leaves the hardened concrete. Factors influencing shrinkage include water content, cement type, and curing conditions. Potential problems with these are discussed below. Thermal movements induced by the natural expansion and contraction of concrete as it warms and cools can also contribute. The tensile forces and thus cracking usually occurs because of movement restraint. Different areas of a slab are restrained with respect to one another by base friction and structural members such as footings or columns. As the concrete shrinks, the restraints induce tensile forces within the slab which are relieved when the slab cracks.
Poor construction materials or methods may also contribute to concrete shrinkage and cracking. These include:
– Low strength mix design or too much water added to the concrete mix
– Improperly placed reinforcement steel
– No contraction joints or joints spaced too far apa
– Contraction joints not tooled, formed, or sawn deep enough to induce cracking at the joint
– Lack of proper curing, particularly in hot, dry or windy conditions
– Missing isolation joints around columns, walls or other adjacent structures
For workability during initial placement, concrete is mixed with more water than needed for hydration, or the chemical process that results in hardening. This “water of convenience” evaporates or otherwise leaves the concrete, causing drying shrinkage. Some amount of drying shrinkage is common to all new concrete, and can generally be reduced by reducing the amount of water used in the concrete mix.
Steel reinforcement or welded wire fabric (WWF) is often used in concrete slabs on grade. This reinforcement does not necessarily prevent cracks, but is intended to limit crack width. Typically, WWF should be placed in the middle or upper middle of the slab. This is often not achieved in practice due to a number of reasons. One of these is the curvature of rolled mesh which results in waves which in turn causes varying depths of the mesh within the slab. Even if the mesh is straight and initially placed properly, foot traffic during placement and the concrete itself have the potential to push the mesh toward the bottom of the slab where it cannot properly serve its purpose.
A lack of proper contraction joints can also contribute to cracking. The American Cement Association, formerly known as the Portland Cement Association, recommends that contraction joints be provided at intervals of 30 times the slab thickness. For example, a four inch thick concrete slab on grade would require contraction joints spaced at 120 inches, or 10 feet intervals. Contraction joints weaken the slab at linear joints. This results in cracks occuring along pre-chosen, straight joints, rather than in a more random and unsightly manner.
Proper curing is an important requirement with new concrete. The hydration process is most rapid during the first several days and weeks after concrete placement, but may go on for decades. During the first several days, it is important that water is available to the cement, particularly near the drying surface, for the hydration process. It is also important to maintain a uniform and appropriate curing temperature. Proper curing assures that new concrete is provided enough water and maintained at an appropriate temperature for proper initial hydration. This is accomplished using sprayed curing agents, plastic sheets, wet mats or sprinkling.
In summary, shrinkage cracking cannot be totally prevented. However, with proper materials and construction techniques, cracking can be controlled and limited.
Plastic shrinkage cracking is different, and is defined by the American Concrete Institute as “cracking that occurs in the surface of fresh concrete soon after it is placed and while it is still plastic.” This type of cracking can occur from improper curing when evaporation overcomes the rate of bleed water rising to the surface of the new concrete slab. The surface dries out, shrinks, and cracks due to movement restraint from the deeper concrete beneath the surface. Plastic shrinkage cracks may be wide, but are usually shallow and do not affect the entire depth of slab. They are often parallel with each other, and spaced at approximately one to three feet intervals.
The following photograph shows a crack in a concrete slab on grade due to shrinkage and thermal movement. Note the worn edges and no visible or measurable displacement across the crack:
The next photograph shows a relatively large shrinkage crack which effectively split a large garage floor slab in half. The slab had no contraction joints, and cracked down the approximate middle. There was no visible or measurable displacement across the crack, and no indications of structural distress at the walls or ceilings in the vicinity of the crack. The relatively large width of the crack resulted from the relatively large are of the slab, which increased the overall shrinkage. The lack of additional cracks indicated that the concrete mix and placement were well done, besides not installing contraction joints:
ADDITIONAL RESOURCES:
ACI 224R – Control of Cracking in Concrete Structures
American Cement Association – Concrete Slab Surface Defects: Causes, Prevention, Repair