Quality of Concrete Work
Posted by Mark on September 26, 2006
Our driveway and garage was re-concreted a few weeks ago. The contractor already said he will re-do the driveway approach, because he put plastic on the concrete, which left numerous swirls and discoloration. Now the garage has several cracks, and the finish has several big dark discolorations. I’m worried about the cracks appearing and growing so quickly. The contractor said he will make a saw cut in the garage, which should prevent further cracking. He said the cracks are normal and has not offered to re-do the job. But I wonder if the garage needs to be re-done. I question the quality of the work. What do you think?
I understand your concern about the cracks in your garage, however, it is important to find out the reason for the cracks development.
In general it is a natural thing for concrete to crack.
The cracks that appear soon after the concrete is poured are most likely drying shrinkage cracks and are to be expected. If the cracks are, in fact, drying shrinkage there are a few things that can be done to minimize the possibility of cracking.
1) Use of concrete admixtures such as water reducers to lower the water cement ratio or use of shrinkage reducing admixture is recommended.
2) Addition of concrete fibers or reinforcement steel also reduces the possibility of shrinkage.
3) Proper curing is a must although it is very often forgotten.
4) Placement of control joints is always recommended. Control joints can be sealed afterwards with construction sealants to prevent the moisture and water penetration, which can cause problems especially during winter months.
Cracks developed due to drying shrinkage usually pose no structural threat even though they may be unsightly.
Another consideration is the possibility of sub-grade movement. If the cracks continue to get wider there may be movement of the base. If this is the case then saw cutting will do no good at this point. The only recommendation would be sub-grade stabilization. Removing and replacing the concrete is always an option but you may end up with the same cracks if this is not done.
If you decide to re-pour the garage, and you verify that cracks are not caused by sub-grade movements, I would suggest control joints to accommodate the drying shrinkage. It won’t stop the concrete from cracking but the cracks will be straight.
More information regarding the control of cracking in concrete structures can be found at ACI manual section 224R – 01.
