Concrete Terminology
Many people use the words concrete and cement interchangeably, but they are not the same. Concrete is a mix of various materials with cement being one of its components. 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.


Importance of Curing
Curing of concrete is always necessary for a quality installation. It can never be omitted. 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 curing compound or membrane in a way that provides a uniform film over the wet concrete without marring the texture. The material must be applied at the rate recommended by its manufacturer but generally, it is about one gallon per 150 square feet. That means that if the driveway is 10 feet wide, every 15 feet of it will require one gallon of curing compound.

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 job site. 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.
Concrete Durability
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 job site 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 job site will almost always decrease its quality 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.

Jointing of Concrete
Many shrinkage cracks in concrete slabs occur simply because the joints are too far apart. Depending on the geometry of the concrete placement, joint spacing will vary, but should not exceed 20×20 feet. Contraction joints should 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. 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. By then it may have already developed minute shrinkage cracks so controlling their location will be difficult.
Recommended Joint Spacing for Crack Control |
|
Thickness (inches) |
Spacing (Feet) |
4 |
8-10 |
5 |
10-12 |
6 |
12-15 |
7 |
14-17 |
8 |
16-20 |



Southeast Engineers are proud to offer our clients onsite assessments to diagnose problems, make cost-effective repairs and provide expert advice. Consultations start at $500. Call us at (225) 295-1880 or visit us online before you get started on your spring projects.