Longlasting concrete jobs are a balance of controlling the environment and the mixture as much as possible, proper joint applications, and a solid handle on timing. Knowing how to do a great job from start to finish is the key to making any final product one that makes clients happy for a long time to come.

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.

In addition to the ingredients mentioned above for standard concrete, several admixtures are available to help adapt the concrete to specific weather conditions and to specific other considerations.

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.

The first three days of the curing process is the most critical The film 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 done correctly. 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 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.

It is also necessary to install joints in the concrete slabs. There are several kinds of joints that should be installed: isolation joint (also known as an expansion joint), a contraction joint, and a construction joint. An isolation joint separates the slab from other fixed objects, such as the foundation wall of the house, another adjacent slab, a steel or masonry column, etc. These 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 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.

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 20x20 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

Bad Jointing

Good Jointing

Better Jointing

Proper concrete construction, applications, and treatments form a durable product. While it takes time to familiarize people with the appropriate steps it is an important education on what it takes to do such a vital job well.

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.