What Causes Concrete To Crack
Concrete cracks and that is a fact. While often misunderstood, cracks in concrete do not always indicate a flaw or need for correction. In some instances, cracks do not negatively affect the concrete. When, where, and why concrete cracks varies, and it is important to know which type of crack calls for a repair. If you better understand concrete it will put your mind at ease concerning cracks. The ACI, American Concrete Institute, states that “Even with the best floor designs and proper construction, it is unrealistic to expect crack-free and curl-free floors. Consequently, every owner should be advised by both the designer and contractor that it is normal to expect some amount of cracking and curling on every project, and that such an occurrence does not necessarily reflect adversely on either the adequacy of the floor’s design or the quality of its construction (Ytter-berg 1987; Campbell et al. 1976).”
Cracks form in the concrete due to tension or movement. Concrete is an extremely durable material. However, flexibility is not one of its strong suits, so cracks occur as the concrete experiences too much tension or movement. If you can identify the type of crack, you can take the proper steps for your concrete. There is a chance that a crack does not require any attention unless aesthetics are a concern.
Types of cracks:
Plastic Shrinkage Crack – This type of crack occurs during the setting process. After the concrete has been poured, gravity kicks in. The heavier particles sink to the bottom and the lighter particles and water rise to the top. This process is called bleeding and must be monitored closely as the weather and temperature can have a negative effect. If the water evaporates from the top layer too fast, it will start to shrink. The opposing forces between the dry surface layer and damp bottom layer of concrete will cause small cracks (usually 1-2 mm wide). Plastic shrinkage cracking weakens concrete. It depends on the magnitude of the cracks if it needs to be replaced or not. There are a lot of options to fix or even hide these cracks. It would be important to get an expert’s advice on this.
Plastic Settlement Crack – This type of crack also occurs during the setting process. After the slab has been poured, the heavier sediment falls and the lighter sediment and water rise to the top. The problem arises when reinforcing elements (rebar, welded wire mesh, post tensioning tendons, etc.) placed in the slab are not positioned so as to achieve adequate clear space between the elements, and the surface of the slab. If the reinforcing elements are too close to the top of the slab, the cement will pull apart and crack right above the reinforcements because the weakest concrete is near the surface. These cracks weaken the concrete as well. Reinforcements are placed in the concrete to compliment the concrete’s strength. The purpose of reinforcement is not to prevent cracking but to support aggregate interlock and to prevent cracks from widening.
Cracks Due to Loading – Some cracks occur due to the overloading of weight placed on top of the concrete. In other words, the concrete’s weight limit cannot be breached or it is likely to crack. Concrete is very strong but each concrete slab, driveway, sidewalk, patio, etc. does have a limit to what it can hold. You know the limit by knowing the PSI (pounds per square inch) of the concrete. If it is 5000 PSI concrete, it would take 5001 pounds per square inch to squash the concrete. This is harder than it sounds. For instance, the normal driveway will never get to the state of being overloaded. However, having a dumpster or other heavy equipment parked on the driveway is likely to cause cracks due to the weight. Another reason cracks form due to loading is because the ground or subgrade underneath the concrete is not able to hold the weight. Although the concrete may be strong enough, the ground/subgrade may shift due to the weight and cause cracking. The ground is more likely to move when the soil is soft from rain, snow, or groundwater. These cracks are a little more complicated to repair but it can be done.
Can Cracks be prevented?
Because cracks are inevitable, a concrete contractor’s only choice is to try and control or prevent the cracking from happening at random locations. There are a few types of joints and methods that can be used, many of which you run across daily. For instance, the lines seen in sidewalks are joints placed to control the cracking.
Control Joints – This is the most popular method for controlling cracking. These joints appear as neatly crafted lines on the surface but they perform a much deeper purpose. By placing these joints at specific locations, the concrete cracks in a straight line below the surface, keeping the surface of the concrete solid and smooth in appearance. The depth of the joint has to be ¼ of the slab thickness. Control joints allow for the concrete to experience movement without ruining the aesthetics. Essentially, control joints control where the concrete cracks instead of allowing random cracking.
Expansion Joints – This type of joint allows concrete to expand/shrink depending on the temperatures. Heat causes concrete to expand, while cooler temperatures cause concrete to shrink. The difference between control joints and expansion joints is that control joints attempt to control where cracks form and expansion joints attempted to prevent cracks altogether. Expansion joints are also sometimes filled in with a flexible material. The expansion joints penetrate the full depth of the slab because they are designed to allow movement. Then, it is filled with some sort of flexible material.
Isolation Joints – Isolation joints are essentially expansion joints in specific locations. They isolate loading to avoid cracking. Isolation joints are installed to allow vertical and horizontal movement at the meeting place of concrete and a wall, pipe, column, etc. Concrete does not move or expand the same as a wall, pipe, column, etc. so there needs to be a joint at this meeting place to allow for movement.
Reinforcement (rebar, fiber, welded wire mesh, etc.) - As mentioned previously, reinforcements help maintain concrete’s strength and give it support for when cracks form. It keeps the aggregate interlocked, and the reinforcements kick in when it does crack because they help hold the cracked concrete together.
Conclusion
Cracks are inevitable but not all cracks affect the integrity of the concrete. When cracks start to appear it is important to figure out what caused the crack, what type of crack it is, and to give it a little time to see if it progresses. An active crack is a crack that is progressing as time goes on. This type of crack needs to be handled ASAP to avoid large repairs in the future. Dormant cracks are the opposite of active cracks and are not progressing. When in doubt, call a concrete contractor and have them examine what is going on and heed their expert advice.