Expansive Cracking
Common in large structures, expansive or “thermal” cracking is an issue. Temperature differences between concrete surfaces and interior can cause frequent expansion and contraction of those areas which will result in cracking from the stresses. Huge temperature differences can cause some pretty significant cracks, so contractors prepare for this by the use of internal reinforcements and strategic placement of expansion joints to allow some movement. When necessary, cracks of this type can usually be repaired with a specially designed repair product like epoxy. Similar are the cracks that occur in climates with freeze-thaw cycles. When the ground surface freezes, it expands and can cause heaving of whatever is on top of it, usually your driveway, sidewalk, etc. Again, in addition to proper use of interior reinforcements and strategic joints, contractors also have to prepare the surface on which the concrete is going to be poured to best alleviate the effects of heaving. This may include but is not limited to several inches of packed stone.

Shrinkage Cracking
Extremely common in most concrete is “shrinkage” cracking which occurs during the curing (drying) process. Cement uses water as part of a chemical reaction causing it to harden. Excess water actually dries and like many items, concrete can shrink as the excess moisture disappears. Since the surface usually dries faster, it shrinks more than the rest. The shrinking can cause stress, with enough stress, cracks can appear. These tend to be purely aesthetic and many contractors will assure customers that they will do all they can to prevent them; if they do happen, do not worry about serious problems. One form of crack prevention comes in the form of control joints. They serve two purposes. They provide start and stop points for the shrinking and provide a slightly weaker point in the concrete which will induce cracks as needed from expansive stresses. Other prevention measures are careful control of the water content of the concrete mix. Excess water is bad, but often happens because it makes the concrete easier to work with. Dry concrete just does not go down well, so water content often ends up somewhere in the middle. This excess water can also contribute to “spalling” which is the small divots and flaking you find on a concrete surface. In addition to water, simply making sure the concrete mix is designed for the job is also important. Subtle differences in the mix design will allow it to be used more easily for the job at hand such as the inclusion of air, set retarders, accelerators, fibers, etc.

Regardless of preventive measures, Mother Nature is a tough opponent and will often come out winning. As a customer, you may now understand why even the most experienced contractor avoids warranties against cracking. As a contractor, making that guarantee is sort of like a die hard Cubs fan guaranteeing a World Series victory. Despite all the off-season preparation, come fall you are going to be let down.

Remember, if both parties involved have a good understanding of the nature of concrete and why the unexpected may occur, it will help alleviate some of the stress if it becomes an issue.

Article by Don Ciesielski and courtesy of Contractor Source.