Contraction / Control Joints

To control random cracking in concrete slabs contraction/ control joints are placed on it. The fresh concrete mixture is the plastic mass that can be molded into any kind of shape, but the material hardens there is the reduction in volume (shrinkage).  While the concrete is very strong in compression, the tensile strength is 8% to 12% of the compressive strength. The tensile stresses act against the weakest property of the concrete material, this result in cracking of the concrete.

Two Basic Strategies

There are two basic strategies to control the cracking of the concrete. One method is, by providing steel reinforcement in the slab, which helps to hold the random cracks tightly. The most commonly used method to control random cracks in concrete slabs is to place Contraction/ control joints in concrete slabs at predetermined position to create weakened planes where the concrete can crack in straight line. Since the crack take place below the finished concrete surface it will produce aesthetically pleasing appearance.

When the following considerations are addressed, the concrete slabs on ground have consistently performed well. The soil or granular fill supporting the slab should be either undisturbed soil or well compacted. The contraction joints should be placed to produce panels that should be square as possible and should never exceed the length to width ratio of 1½ to 1. Normally joints are spaced at the distances equal to 24 to 30 times the slab thickness. The joint spacing that is greater than 15ft requires the use of load transfer devices.

At the time of placement, contraction joints may be tooled into the concrete surface. Joints may be tooled into the concrete surface immediately with the first pass of the floating operation. The joints that are tooled should be re established with each successive pass of finishing operations. The joints may be sawed into the hardened concrete surface. Any crack that is obtained before the concrete is sawed will provide the sawed joint ineffective and timing is more important. For most concrete mixtures the sawing should be completed within 6 to 18 hours and should never be delayed more than 24 hours. The early entry saws are available, that may allow cutting to begin within a few hours after placement. Contraction/ control joints should be established to a depth of ¼ the slab thickness. For the effective control of random cracking, proper joint spacing and depth are very essential.

Questions:

• What are Contraction/ control joints?
• What are the two basic strategies to control the cracking of concrete?