Q. Can you solve a debate I have been having with my contractor about replacing the cracked floor in my garage? The problem is that I want to have steel bars in the slab and make it extra thick, since the existing slab is cracked and damaged from my heavy pickup truck, sometimes filled with heavy construction items. The concrete guy keeps telling me it’s overkill and that just the concrete, 5 inches thick, with a rollout wire mesh, is enough. Can you explain which one I need, and why he keeps telling me that the driveway is concrete, not cement? I want to do this job as soon as it warms up.
A. The first problem to solve is who should be designing the correct slab. Yes, we all know it’s just a slab, but you’ve already seen what happens when someone unfamiliar with the engineering design of even the simplest concrete slab doesn’t apply the correct preventive details. Concrete is very strong in compression, meaning you can press on it to extremes before it even shows small signs of failure. Unfortunately, concrete has no tensile strength. Zero. Zip. This means you can easily pull it apart, or bend it to the point of cracking without much force.
Structural engineers and architects, to a lesser extent, are trained to apply specific formulas that predict the success (and failure) of even a slab of formed concrete. Knowing how a slab fails allows professionals to apply the right reinforcement in the right places. Otherwise you’re just guessing.
There’s a difference in the description of the slab, which is made up of several materials, such as sand, lime, and something called “Portland cement.” Portland cement is a mixture of mined calcium silicates and a lesser amount of calcium aluminates that form a chemical reaction in the presence of water. The chemical reaction causes the mixture to harden and give off heat, called the heat of hydration. It was patented in England in 1824 by bricklayer Joseph Aspdin, and got the name Portland because when it hardened, it appeared to look very similar to the white Portland stone quarried along the Portland Isle coast of Dorset in southern England. Sorry, Oregon, you get no credit here.
When other silicates, such as sand or different sizes of stone, are added, the characteristics change, and the setting times and strength change as well. Ash has been added in some mixes, and glass fibers, another form of silica, have made our latest bridge and roadway construction projects extremely long-lasting and stronger.
Portland cement can be a part of concrete. To determine the reinforcing, the loads need to be applied to formulas, along with the amount of time and other forces, such as expansion and contraction (due to temperature changes), support material strength, shrinkage and internal stress — in other words, it would be irresponsible to throw some guess at you without knowing more. Keep in mind, more concrete is not better. Good luck!
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