Concrete Guide For Consumers

 

More than three quarters 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 months 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 jobsite will almost always decrease the quality of the material, but when it is absolutely necessary to add water, it must never be added in an amount that exceeds 1 gallon per cubic yard of concrete. The other aspect of controlling the quality of the installation is the curing process.

Curing of concrete is always necessary for a quality installation. It can never be omitted. It must be done on every installation. Curing is the process of keeping the concrete properly moist while the material is developing the strength necessary for it to perform as needed for a long period of time. Concrete must be cured immediately after it is poured – not the next day or the next week. Concrete can be cured in a variety of ways, but it is usually accomplished by spraying a uniform application of curing compound or membrane in such a manner as to provide a continuous uniform film over the wet concrete without marring the texture of the material. The material must be applied at the rate recommended by the manufacturer for the particular material, but usually about one gallon per 150 square feet. That means that if the driveway is 10-feet wide, each 15-feet of driveway will require one gallon of curing compound. The first three days of the curing process is the most critical, and the film that is developed by the installation of the curing material will help hold the moisture in the concrete for a period of more than a week, during which time the concrete will develop about 75% of its ultimate strength. Properly cured concrete will develop 100% of its designed strength in 28 days, and then it will continue to grow stronger until it has developed as much as 125% of its designed strength if properly cured. Again, the concrete curing material must be applied immediately after the concrete has been finished, even if the concrete has not yet taken its initial set and has become hard enough to walk on. In unusually sunny, hot or windy weather the speed with which the curing compound is applied becomes even more critical because of the probability of premature drying of the surface of the material. The materials and the equipment with which to apply the material should be prepared and ready for the application before the concrete even arrives on the jobsite. Some curing compounds, while basically being a clear liquid, are tinted with a white pigment so that the coverage can be seen and controlled more easily.

It is also necessary to install joints in the concrete slabs. There are several kinds of joints that should be installed. An isolation joint, also called an expansion joint, separates the slab from other fixed objects, such as the foundation wall of the house, another adjacent slab, a steel or masonry column, or some other such object. Preformed strips of material called expansion joint can be placed prior to the pour and used in these locations to separate the new concrete slab from the object being abutted. A contraction joint is used in predetermined locations to control cracking by the installation of man-made “weak points”. Contraction joints can be tooled joints installed in the wet concrete, but are almost always saw-cut after the concrete has taken its initial set. By placing joints or saw-cuts in the concrete that are approximately 1/3 the depth of the concrete in strategic locations, the concrete is encouraged to crack in the bottom of the pre-determined joints and not randomly where the cracks will be aesthetically objectionable. The last type of joint is called a construction joint, and it is simply installed at the end of a single day’s extent of the work.

It is helpful to expound a little bit on the locations for contraction joints in a concrete slab. Many shrinkage cracks in concrete slabs occur simply because the joints are too far apart. No section of concrete slab should be larger than 10-feet by 10-feet square. If a driveway or patio is wider than 10-feet, a joint should be placed down through the center of the drive. Joints crossing the driveway should then be placed in locations that keep the sections roughly square. If the joint down the center divides the driveway into two 6-foot-wide halves, then the cross joints should be placed every 6-feet also. Contraction joints should also be placed in locations that are odd-shaped or have angles in which cracks may tend to develop. Concrete begins to shrink immediately as it is drying. Therefore, the contraction joints should be placed as soon as the concrete is ready for them. Tooled joints are placed while the concrete is still workable. Saw-cut joints should be placed as soon as the concrete is hard enough for the saw blade to operate without breaking off the edges of the cut. Ideally, the saw cut should be placed within 4 to 8 hours after the concrete is placed. That might amount to doing the work the evening after the material has been installed. Those contractors that wait until the next day or for several days after the pour to install contraction joints have waited too long. The concrete may have already developed minute shrinkage cracks by that time, and controlling their location will be difficult.

There are many other factors that can affect the quality of a concrete installation, such as the proper installation and removal of forms and the skill with which the wet concrete is finished, but you will have to rely on the experience and skill of your contractor for these other factors. Don’t be afraid to ask for references to verify experience, and don’t choose a contractor that has been in business for only a very short time. Remember also to have a written agreement for your purchase, so that everyone agrees on what is going to be done, and when everyone agrees to do it. Remember that most disputes stem from the owner of the property simply having a different idea of what was purchased than the contractor has of what he sold.

To summarize, try to avoid extreme weather conditions when placing concrete. Avoid installing concrete in the winter weather if at all possible, and try also to avoid extremely hot weather installations – warmer than the mid-80’s or 90 degrees. The concrete should be installed at the jobsite with the same amount of water that it was delivered with. If water has to be added at the jobsite as a last resort, no more than 1 gallon per cubic yard of concrete may be added. Curing is possibly the most important aspect of the installation, and it must be installed immediately after the concrete is finished. If the entire pour is going to take several hours or all day, the first concrete placed and finished should be cured while the remainder is still being placed. If the moisture evaporates out of the surface of the pour before the curing material is installed, you have lost the battle. Materials used for the curing process should be out and ready for installation before the placement begins. Curing compounds should be placed at the coverage recommended by the manufacturer. To further insure a good cure, you can also use a soaker hose to keep the concrete moist for a period of time of up to a couple of days after the installation. The whole idea of curing is to keep the moisture in the concrete while it is developing strength. The locations of the joints should be pre-determined before the installation begins. Joints should be installed with a chalk line or straight edge to keep the joints straight. Joints that will be saw-cut should be installed within 4 to 8 hours of the installation – not the next day or in subsequent days. Hiring an experienced contractor who agrees to follow these guidelines should result in a quality installation that performs well for years to come.