Q: About three years ago in May, I had a concrete slab poured in front of my shop with a broom finish. The following February, I noticed popped spots all over the surface. I contacted the contractor who contacted the ready-mix company. The producer said it was a failed sealer and that there were low spots where water was collecting and freezing, so we replaced the sealer. The following winter, the same thing happened. Finally I had the concrete core sampled and tested and found that 22% of the aggregate was shale. Is this too much? Do I have any way to force the concrete producer to replace the slab?
A: ASTM C33, Concrete Aggregates, is the standard that governs aggregate for concrete, which is what is causing your problem. Because you are in what qualifies as a severe weathering region, C33 limits the percentages of deleterious substances, such as “Clay lumps, friable particles, and chert (less than 2.4 specific gravity)” to 5.0% for “pavements, walks, patios … subject to frequent wetting.” Shale qualifies as an deleterious material. What happens is that the shale absorbs water and when it freezes, it expands and creates the popouts. The good news is that this is typically only a surface phenomenon and doesn’t affect the slab matrix, but it is unsightly. With this evidence, you should talk with the concrete producer, although it will be difficult to pin them down since most residential concrete is not ordered according to a specification, such as ASTM C 94, Ready-Mixed Concrete, which references C33 for aggregate quality. The best they might offer is to provide free concrete if you decide to replace the slab, which will be a small part of the project.As far as a simpler fix, you can try applying a sealer or a topping. Just make sure the concrete gets very dry first. You need to get the shale dry so that it won’t expand when it freezes. Theoretically, the sealer or topping will be impervious and prevent moisture from getting into the shale. The problem is that it only takes a small crack to let ponded water through and as an exterior slab if you seal the surface moisture will wick up into the slab from below.