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Ask the Architect

Our concrete is cracking


Q. Our house was lifted, and we’re finally back in again. We started noticing that our new garage floor has cracks, not big, just thin, but going in several directions, mostly in the middle of the floor. The concrete was poured in the summer, in good weather, and the garage is enclosed. We park our cars there. I’m wondering if the ground is soft and the concrete is starting to sag, or maybe they didn’t put the right metal in the slab like it shows in the plans. How can I tell, and is there any way to fix the cracks before they get much worse? They only just started to show.

A.    Your concrete slab, like most, required a mesh of steel that consists of large woven wire crisscrossing at a spacing of every 6 inches apart in both directions. This is a standard practice, so I hope they installed the “welded wire fabric” as a bare minimum. What that metal mesh does is add flexibility to the concrete, which otherwise has no flexure and will begin to show cracking like you described. That isn’t, however, the only way that concrete is subjected to stresses and strains. There are many factors involved in the right job, so even though you could spend a lot of money to X-ray the slab, or cut a section to see if it was reinforced, or have it checked by taking a core sample cut into a cylinder shape and placed in a crushing machine, it could be a combination of issues.

    Concrete is made of materials that need to be mixed in fairly exacting proportions: Portland cement, various types of crushed stone and water. Even that is too simplistic an explanation, because the stone may be rock, sand or gravel, the water must be clean, without minerals and chemicals or salt, and the Portland cement must be extremely dehydrated and made of the right proportions of lime and calcium to create the level of strength required.

    The whole concrete curing process is a chemical reaction. Temperature and moisture have to be handled with some expertise, or the batch is subject to being weak. If the cement was mixed or poured in extremely hot weather last summer, or was placed on top of bare ground that drew moisture down very quickly, the slab will be too dry too fast. The opposite could happen if it was raining, or if water could be seen floating to the top while the cement was being worked into place. If there were no expansion joints tooled into the slab surface, as is often the case with garage floors, then the normal expansion and contraction due to temperature changes will cause random cracking instead of controlled cracking. In other words, all slabs may crack due to temperature changes. I suggest monitoring the cracking and applying a flexible epoxy top coat to hide the cracks and improve the look. Good luck!

© 2020 Monte Leeper. Readers are encouraged to send questions to yourhousedr@aol.com, with “Herald question” in the subject line, or to Herald Homes, 2 Endo Blvd., Garden City, NY 11530, Attn: Monte Leeper,  architect.