Concrete Surface-Defects:  Slab Defects Part One


Concrete by it's very nature is strong, durable, predictable and performs well.  That's why it has been used throughout history to construct building that have withstood the test of time. With that said, from time to time concrete defects present themselves for various reasons.  This article will provide an overview of some of the concrete defects that can occur and what can be done to correct them.  


For the sake of this article let's use the following "working definition" for concrete defects: a blemish which appears on the surface of a concrete slab, wall, or deck. A blemish can be in the form of one of the following: blisters, cracking, crazing, curling, delanimantion, discoloration, dusting, efflorescence, low spots, sand boils, popouts, scaling, or spalling. These concrete defects can be minimized and in many situations be prevented.  


The appearing of blisters on a concrete slab during the finishing phase can be frustrating.  So why do they occur?  1. excess amount of entrapped air 2. insufficient vibration during compaction that does not adequately release entrapped air and 3. Finishing when the concrete is still soupy.  To avoid blisters the following should be taken into consideration.  First, use appropriate cement content in the proper range second, don't use concrete with a high slump, third avoid placing a slab directly on polyethylene film or other vapor barriers, fourth avoid overworking the concrete, fifth avoid using air contents over 3% for interior slabs sixth don't attempt to seal too soon, and finally, reduce evaporation over the slab by using a fog spray. 


Another concrete defect is unexpected cracking. Cracking can result from drying shrinkage, thermal contraction, restraint to shortening, subgrade settlement, and various applied loading. Cracking can be significantly reduced when preventative steps are taken. Here are some steps that can help reduce concrete cracking.  


1. Properly prepare the subgrade.  This would include uniform support and proper subbase material with adequate moisture content. 2. Minimize the mix water content by maximizing the size amount of the coarse aggregate and use low-shrinkage aggregate. 3. Use the lowest amount of mix water required by workability. 4. Avoid admixtures containing calcium chloride. 5. Provide isolation joints to prevent restraint from adjoining elements of the structure. 6. Provide contraction joints at reasonable intervals, 30 times the slab thickness. This is not an exhaustive list of steps, but these can help reduce concrete cracking.  


There are many other types of concrete defects which were listed above that will be addressed in future articles.  Hopefully you have found this helpful. Remember concrete is strong and performs well. 


BFI Staff Writer -