Ask the Architect

How solid is our garage floor?

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Q. I’m wondering if our garage floor, which was completely removed and replaced with a new concrete floor, is strong enough for our cars and my pickup truck. We did it to get rid of the old floor, which was cracked and had sunk. Our mason told us he does this all the time, but it just seems like the floor is too thin and could break. Is there any way to know if it’s strong enough? It’s 4 inches thick, very flat and they put wires in, not thick rods, that crisscross in a grid. Also, the wires were rusted when they went in and I wonder how long they could last, especially because we live in a development along one of the canals on the South Shore, and after what happened in Surfside, Fla., we aren’t as confident.

A. Living near a shoreline, with dynamic high- and low-tide water movement, means taking greater precautions to make construction last longer and perform better. First, there’s the base, way below the floor slab, which must allow water to run through it to avoid collapse. The base is made of compacted stones that allow water to flow between so they don’t form an obstruction, working kind of like a filter.
I generally design such a base with landscape filter cloth beneath it, to keep individual stones from sinking when the natural underground current of tide water rises and withdraws, pulling the ground material with it as it flows in and out. I rarely, if ever, see this done, and can assure you that if I’m not standing over the work when it’s being carried out, workers will ignore this “crazy” design, hence a reason for failure.
Everyone is an expert, as I am sure you’ve come to know, but ignoring ground and soil dynamics makes it a gamble as to what you’re going to get. It isn’t reassuring when someone says they do this all the time, especially if they can’t give technical reasons for what they do. Floor slabs, just like beams, should be designed for the concentration of the loads they will carry, not just general practice, especially because the steel reinforcing is part of the formula. If the concrete or the reinforcing is undersized for the load, it will, in correct terminology, yield, or give way, not necessarily completely fail. The coating and placement of steel is extremely important.
Except for major roadway and transit construction, I rarely see it done to legal standards. Most often, uncoated, rusted steel is placed randomly or tied in ways that defy the reason it was prescribed. If uncoated steel is too close to a surface, it corrodes faster as salt water gets sucked like a sponge into the concrete, allowing steel to wet and dry in a cycle guaranteed to rust. You’re right to be concerned. Good luck!

© 2021 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.

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