The Theory of Keeping Concrete from Cracking

The theory is concrete cracks because it is not allowed to shrink and expand along with the conditions that surround it.  Keeping this in mind, it is difficult to imagine a concrete slab poured in such a way that allows for the entire foundation to move freely on its own.  In a commercial setting it is possible to design and construct a concrete pad that can, infact, move freely. This type of foundation is known as a floating pad type of foundation. They are constructed in areas of the country where clay is the primary soil of the state. Why? It’s because clay can swell over 60% depending on the conditions around it.  In simple terms – a concrete pad can be literally pushed out of the ground several times a year depending on the seasons. More importantly this is exactly how large structures are built around large bodies of water to keep them from cracking.

How is a floating pad built differently from a regular slab-on-grade type of foundation? Well, as it relates to a basement the foundation is tied to the ground beneath it. Using a fence as an example: The first thing that is done when a fence is constructed is to dig holes in the ground for the posts. Building a slab today, the same type of holes are dug for what is called the piers.  Secondly, once the fence posts are installed and concrete poured around, it provides a stable base for the entire fence. Just like the fence posts, a concrete slab is tied to the piers to keep the entire home stable. This is where the theory is difficult to prove for a regular home or, in our case, a basement.

Concrete is not the only factor when you consider how to keep it from cracking. I just explained how the soil around the concrete is the primary factor in its cracking in a clay soil.  If we eliminate the soil in which a concrete slab is poured, then you would have a new factor to consider, and that would be weather conditions. Hot weather conditions tend to cause concrete to shrink as the moisture dries up within concrete. Cold weather conditions cause concrete to become brittle and separate within its structure. Wet weather conditions cause concrete to swell or expand, and now you should begin to notice a pattern forming. The concrete itself is reacting to the conditions around it so what would happen if it was restricted from moving at all?  Correct, it would crack because when it tries to move, shrink, or swell.It would not be allowed to move.  Holding something still while it is trying to move causes stress and thus makes it crack.

Now that you’re educated enough to understand the primary factors that make concrete crack, can you think of a condition that would allow concrete to move freely? Very few professionals have been able to do so as well. When you consider that concrete is reinforced or embedded with steel, gravel and cement, that is the glue which holds it together. So if concrete moved freely it would separate from the steel and gravel at least. Thus, concrete would have to be poured without any reinforcement and gravel which is the glue that holds it together.

In conclusion: Concrete’s strength comes from its structure, reinforcement, and its ability to react to the conditions around it,thus proving that concrete can’t move freely without separating (cracking) under any conditions. That is why concrete experts focus on the degree of movement or stresses the concrete will be under on a case-by-case basis.

If slabs-on-ground are free to shrink as shown, hypothetically in (a) cracking does not occur. However, in reality slabs are not free from shrinkage because of subbase friction and other restraints. Shrinkage and restraints cause cracking.

If slabs-on-ground are free to shrink as shown, hypothetically in (a) cracking does not occur. However, in reality slabs are not free from shrinkage because of subbase friction and other restraints. Shrinkage and restraints cause cracking

New slabs-on-ground crack when tensile stresses from restrained shrinkage exceed the concrete’s strength. This type of early-age cracking occurs during the first few days of the slab’s life and is caused by early concrete volume changes related to dry shrinkage and thermal contraction. Sawcut contraction joints are the most common method of controlling early-age cracking — when properly installed, cracking should occur in the joints.

Sawcut contraction joints do not prevent cracking but control the location of the cracking. To ensure cracking occurs in the sawcut contraction joints, joints must be installed before shrinkage stresses exceed the tensile strength of the concrete, must be the proper depth and must have the proper spacing. Otherwise, out-of-joint cracking may occur. These unwanted cracks are commonly called “random cracks” or “uncontrolled cracks” and typically create costly repairs and tear-outs for contractors.

To avoid random slab cracking, contractors must understand the mechanisms that cause early-age shrinkage cracking so they can take the necessary actions to avoid these unsightly and costly cracks.

Shrinkage and restraints

Concrete shrinks and expands due to moisture and temperature changes. Because of moisture loss, a 100-foot-long slab-on-ground can shrink from 0.48 to 0.96 inches over several months. If this slab also has a temperature drop of 50° F after casting, then it can shrink another 0.21 to 0.53 inches because of thermal contraction, yielding a total shrinkage of 0.69 to 1.49 inches. Of course, not all this shrinkage will occur during the first couple of days, but sufficient shrinkage does occur to create early-age cracks that will continue to grow in width as the concrete dries.

For a hypothetical slab that is free to shrink as shown in Figure 1, tensile stresses and cracks do not occur. However, tensile stresses and subsequent cracking occurs when concrete shrinkage is restrained by the subbase or other elements that prevent the concrete from freely shrinking. The number and width of the cracks depends primarily on the magnitude of the tensile stresses created by the concrete shrinkage and restraints. Hence, the cracking potential of slabs can be significantly reduced by minimizing the concrete shrinkage and slab restraints or elements that restrain shrinkage.

To reduce concrete dry shrinkage, only use enough water to produce the required workability for placing, consolidating, and finishing concrete. As the water content of the freshly mixed concrete increases, so does the potential for dry shrinkage. Concrete suppliers can minimize the water content by adjusting the cementitious material content and combining aggregate gradation sizes to achieve a uniform aggregate distribution. Also, minimize the addition of water on site when adjusting slumps because additional water increases dry shrinkage of the concrete.

The risk of cracking can be appreciably reduced by reducing the number and severity of restraints. The biggest restraint preventing concrete shrinkage or slab shortening is the subbase. Always place concrete on flat, hard subbases that are free of ruts and holes. If necessary, use a thin layer of fine material to fill in the surface voids of rough subbases so the bottom of slabs are free to slip or move relative to the subbase. Other slab restraints can be eliminated by isolating slabs from footings, walls, columns, and other elements such as drains, manholes, and sumps by inserting preformed joint fillers between slabs and adjacent elements. Do not connect slabs to other elements with steel reinforcement or tie bars.