READY MIX TECH TOPIC:
BY DANIEL GREEN
When concrete hardens and dries we expect to see a shrinkage amount of about 1/16” for every 10’ in length. We also see thermal movement from the cold of the winter to the heat of the summer of about half that amount. Ideally, these values would indicate since shrinkage is more than thermal movement there should never be a problem like “blow-up”. But, unfortunately we do see them…as an upward dislocation/buckling of the slab…usually located on both sided of a control joint. Concrete usually will “blow-up” during hot weather when it is at its’ largest thermal size.
- After placement the concrete dries and shrinks.
- The concrete cracks at the installed control joints, just as it is intended to do.
- If joints are spaced too far apart, we usually see random cracking as well.
- During the cold weather the joints / cracks are at their widest, as the overall concrete is at its’ smallest thermal size.
- Over the course of time, these joints can get filled with incompressible material such as sand or dirt.
- When the concrete expands during the hot weather, there is insufficient room in the joints to accommodate the lateral expansion.
- The internal forces become too great, the concrete can’t handle the lateral forces, and the slab blow-ups to relieve the forces.
“Blow-up” can be prevented by providing contraction joints at regular spaced intervals. Generally speaking, for a 4” slab thickness, these joints should be spaced at 10’ intervals. The joints should be sealed with an effective flexible sealant which will reject the intrusion of sand, etc. To repair concrete that has “blown-up”, the damaged section must be removed and replaced with new concrete. Install functioning, sealed, expansion joints at the repaired area.