In our February Problem Clinic there was a question on when it is safe to construct a block wall on a concrete slab foundation. The answer indicated that the determining factor is the compressive strength of the concrete. A reader wrote to point out the fallacy of that contention. The only way that answer would be correct is if the concrete foundation was bearing directly on an infinitely stiff subbase—perhaps on solid bedrock. Otherwise, the determining factor for when it’s safe to build on the footing is much more related to the stiffness of the subbase and the bending strength of the concrete. The weight of the wall may be carried by the concrete’s compressive strength, but it is transferred directly to the subbase. As this happens, bending forces are developed in the concrete, which the tensile strength of the concrete must resist and distribute. The controlling factors are the strength of the subbase and the thickness of the slab, which will determine the bending strength and load distribution within the slab to the subbase. Certainly, compressive strength relates to tensile strength, in an indirect way, but it is not the determining factor.

Question: So what's the answer to the question? How soon can block wall construction begin on the edge of a concrete slab foundation?

Answer: As with most things, there are lots of factors to consider: How rapidly is the concrete in the slab gaining strength (which is related to the mix and to the concrete temperature)? How high and heavy will the block walls be and how quickly will they go up? How deep is the footing around the slab perimeter? What kind of reinforcement is in the slab? A typical safety factor for slab design is 1.7 to 2.0, so it might be reasonable to begin loading a properly designed slab when it has developed half of its tensile strength, which generally happens within a few days. If you’re not sure of the concrete’s strength, and want to make sure you don’t crack the slab, you should wait 5 to 7 days.