Several of the complaints received by the Building Department have to do with defective concrete installations. Cleveland is a difficult region in which to install concrete, with its wide temperature swings and its propensity for rain. Nevertheless, concrete is one of the most versatile building materials available, and it is possible to have a nice driveway, patio or sidewalk that is free from cracking and other types of defects if you know a little bit about what to ask your contractor for. Just as with any other type of purchase, the more you know about the process and the product, the better chance you have of getting what you pay for. Here is a mini-course in purchasing poured concrete.
Many people use the words concrete and cement interchangeably, but they are not the same. Concrete is a mix of various materials, just as in making a cake. Concrete is a combination of course aggregate (stone or gravel), fine aggregate (sand), clean water, and (Portland) cement, which is the glue that holds the aggregate together. Each ingredient is added in exactly the correct amount, just as in making a cake. Too much of any ingredient, including water, will spoil the mix and cause the batch to be inferior at best or completely ruin it at worst.
In Bedford, we require the thickness of most concrete slabs to be a minimum of 4-inches in thickness. The apron of a residential property, which is the portion of driveway between the street and the public sidewalk, is required to be 6-inches in thickness, along with the portion of public sidewalk that crosses the driveway. A commercial apron and the related portion of sidewalk is required to be 8-inches in thickness because of the added weight of the vehicles that use commercial aprons. The thickness of all slabs should be uniform. It is not good for a slab to vary in thickness, even if it gets thicker than the recommendation, because the different stresses in the varying thicknesses will cause the concrete to behave differently, ultimately causing cracking. Sections of concrete should also be free of angles and L-shapes. Shapes that require the pour to go around corners or be irregularly shaped should have joints in the concrete pours controlling the stresses and the cracking at the corners and in other strategic locations.
In addition to the ingredients mentioned above for standard concrete, several admixtures are available to help adapt the concrete to special weather conditions and to specific other conditions. One of the most common admixtures is one called “air entrainment”. This is an ingredient that causes thousands of tiny air bubbles to form in the concrete, and it helps the moisture that makes its way into exterior concrete during winter month’s freeze and thaw without cracking the concrete in the process. Water increases in size approximately 9% when it freezes, and the stress on the concrete that results from this increase in mass must be controlled if the concrete is to perform as expected. Adequate air entrainment is required by the building code in exterior porches and steps, and it is highly recommended in all other types of exterior concrete installations, such as sidewalks and drives. Other admixtures are available for other uses. As mentioned above, ingredients, including water, must be added to concrete in specific amounts. Too much water will cause excessive shrinkage during the drying process, with the resultant excessive cracking. When a very fluid mixture is desired, however, because of the need to fill small voids or to properly surround reinforcing steel, an admixture called “super plasticizer” can be added to make the mixture fluid without the deleterious effects of adding more water. Other admixtures are available to make concrete set up more quickly, set up more slowly than normal, or to accomplish other desired purposes.
There are two critical aspects of poured concrete used in sidewalks and driveways – the surface durability and the control of random cracking. To control the durability of the surface and minimize cracking, the amount of water in the concrete must be minimized. Concrete is usually delivered to the jobsite in the mixer with the proper amount of water already in the concrete. Many installers want to add water in order to increase the workability of the material, but this is strongly discouraged. Adding water at the