A: These are good questions, especially if you’ve ever had problems with concrete being installed by a non-professional. If improperly installed, new concrete can crack, flake, or fail in a number of other ways.
I would like to share some experiences with you to give you an idea of what is possible with concrete life expectancy. The first concerns railways. I was a conductor on a local scenic train for two years, and I’ve always been interested in railroads and their construction.
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Years ago I noticed the concrete piers and bridge supports near my last home in Cincinnati. One day I happened to see a date on an abutment in the concrete. It was 1919! The concrete appeared to be in fantastic shape, with no cracks, no chipping and nothing. It was dirty, of course, but otherwise looked almost new. I’ve looked at other railway bridge piers since then and seen the same thing: old concrete in great shape.
I also took walks around the older suburb of Pleasant Ridge in Cincinnati. There were numerous houses built on slight inclines from the street. Many had a series of stucco-coated concrete steps. These steps were in perfect working order and most were undoubtedly installed in the early 1900s. Incidentally, stucco steps are almost a lost art.
A month ago I drove past the first house I renovated in Cincinnati. I finished the job in the fall of 1975. I had to install a series of concrete steps from the sidewalk to the wooden steps that led to the house. The concrete steps look as good as the day I installed them 44 years ago, despite many severe winters and all of the rock salt thrown on the steps for safety reasons! They will likely last at least another 50 years.
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While in Cincinnati, I went to lunch to meet a friend in Hyde Park. I saw an old sidewalk that was crack free next to brand new concrete with a broom. The old pavement had long since lost its layer of sand and cement, and you could see many of the stones in the concrete. While it might not have looked particularly good, it was solid and usable nonetheless. I suspect this sidewalk was installed well before 1950.
It is important to know that base concrete contains only four components: sand, stones, portland cement, and water. Portland cement will hold sand and stones together for years provided you are doing many things right when you mix, pour, finish, and cure it.
The more cement you add to your mix, the stronger the concrete will be. The minimum strength recommended by most experts for exterior concrete is a mixture of 4,000 pounds per square inch of compressive strength. This is known as the six-bag mix per cubic meter of concrete. The standard bag of cement weighs 94 pounds, so you’re talking about 564 pounds of Portland cement in each cubic meter.
Note that this is a minimum recommendation. Nothing prevents you from putting in seven or even eight bags per cubic meter. I just checked my local large store and the retail cost of a bag of cement is $ 13.75. Would you pay that little extra amount per cubic meter of your new concrete to add decades to its life? Of course you would!
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However, making good concrete isn’t just about adding more cement. You need to keep the amount of water used to mix concrete to a minimum. You need to add enough water to make the mixture plastic so that you can process it, but not so runny that it sloshes around in the molds like watery vegetable soup.
Water is the lightest ingredient in the mix. When you finish the concrete, clear water may appear on the surface while you wait for the concrete to harden enough to finish. Professionals call this blood water.
Never ladle this blood into the surface of the concrete. It often evaporates, or you can pull it off with a rubber hose that you pull over the wet concrete. Troweling the vent water into the concrete will dilute the amount of cement paste near the surface. You don’t want to dilute the cement paste on the surface!
Corrosion-resistant reinforcing steel, concrete thickness, hardening and a solid base under the new concrete are also very important. I have covered these things at length in many of the previous columns on my AsktheBuilder.com website. I urge you to read them all.
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