Concrete Slab Cracks
DEAR TIM: I just had a new driveway installed 6 months ago. However, within 3 months, a crack developed in the middle of one of the sections. The crack is getting wider every week. The installer has told me that virtually all concrete slabs crack and that there is nothing he can do to prevent it. Could this really be true? Is there a way to stop the crack from widening? R. A.
DEAR R. A.: I believe that your concrete contractor is only telling you half of the truth. He is, in my opinion, 100 % correct about concrete's desire to crack. However, I disagree with his statement concerning crack prevention.
Concrete is a very unique material. When installed properly, it can be one of the most durable and long lasting products you can use around your home. However, many concrete contractors fail to follow established guidelines with respect to concrete placement. Durable, high strength, and crack resistant concrete does not happen by accident. You must do many things to achieve these qualities.
Concrete can crack for many reasons. Shrinkage is a primary cause of cracking. As concrete hardens and drys it shrinks. This is due to the loss, thru evaporation, of excess mixing water. Thus, in most cases, the wetter or soupier the concrete mix, the greater the shrinkage will be. Concrete slabs can shrink as much as 1/2 inch per 100 feet. The actual amount is 1/16th inch for every ten feet of horizontal distance. This shrinkage causes forces in the concrete which literally pull the slab apart. Cracks are the end result of these forces.
Concrete does not require much water to achieve maximum strength. In fact, a wide majority of concrete used in residential work, in many cases, has too much water. This water is added to make the concrete easier to install. It is a labor saving device. This excess water can not only promote cracking, it can severely weaken the concrete.
Also, rapid drying of the slab will significantly increase the possibility of cracking. The chemical reaction which causes concrete to go from the liquid or plastic state to a solid state requires water. This chemical reaction, or hydration, continues to occur for days and weeks after you pour the concrete. You can make sure that the necessary water is available for this reaction by adequately curing the slab. The use of liquid curing compounds, covering the slab with plastic, wet burlap, and other methods can be used to cure concrete.
Cracking can be minimized by following other guidelines as well. Install the proper strength concrete for your intended use. Concrete is available in many different strengths. The ground upon which the concrete will be placed must be compacted. Never pour concrete on frozen ground. Install adequate control and isolation joints. Control joints occur at regular intervals in the slab and are intended to account for horizontal and vertical movement in slabs. These joints can be formed with a tool or sawcut soon after the slab has hardened. The purpose of these joints is to create a zone of weakness where the forces which are pulling on the slab will relieve themselves. Isolation joints allow a slab to move independently of other fixed or stationary objects.
As a last resort, consider installing reinforcing steel. Reinforcing steel for residential purposes comes in two basic varieties, wire mesh or rigid reinforcing bars (rebars). The use of reinforcing steel can help in the event a crack develops. The steel can often stop the crack from widening and displacing vertically. Reinforcing steel is also quite inexpensive. It is usually very easy to properly install. Steel can significantly enhance the strength and durability of concrete. In addition to all of the other measures taken to prevent concrete from cracking, steel offers a low cost last line of defense.