How Long Can Concrete Last?
DEAR TIM: I’m about to have a new concrete driveway installed as well as some sidewalks. I asked the bidding contractors how long I might expect the concrete to last. The range given was from twenty to thirty years. That seems like a long time but I thought it could last much longer. How long can concrete last in your opinion? What can be done to extend the useful life of concrete no matter if it’s flatwork, steps or foundation walls? Tom B., Rockdale, IL
You may wonder the same thing about concrete especially if you’ve been the victim of this man-made artificial rock that’s been installed by a non-professional. Your new concrete might have cracked, the top surface spalled off, or any other number of defects could have been part of your bad luck.
What is concrete's life expectancy?
I’d like to share some experiences with you to give you an idea of what’s possible with concrete life expectancy. The first one happens to involve railroads. I was a conductor on a local scenic train for two years and I’ve always been attracted to railroads and how they were built.
I started to notice the concrete abutments and bridge supports at my last home in Cincinnati years ago. One day I happened to see on one abutment a cast date in the concrete. It was 1919! The concrete looked to be in fantastic shape with no cracks, no spalling, and nothing missing. It was dirty of course but otherwise looked almost new. I’ve since looked at other railroad bridge abutments seeing the same old concrete in great shape.
I then remembered taking walks around the older suburb of Pleasant Ridge in Cincinnati that was next to the small village where I lived. There were countless houses built upon slight rises from the street. Many had a set of concrete steps that had a stucco coating over them. These steps were in perfect condition and most, no doubt, were installed in the early 1900s. Stuccoing steps is almost a lost art by the way.
Just a month ago, I drove by the first house I rehabbed in Cincinnati back in 1975. In the fall of that year, I had to install a set of concrete steps from the sidewalk up to the wooden steps leading to the house. I didn’t do everything exactly as I would do today, but the concrete steps looked as good as the day I installed them twenty-four years ago with many a year of harsh winters and no doubt rock salt cast upon the treads for safety! I feel they’ll probably last at least another fifty, or more, years.
While I was back in Cincinnati, I was walking to lunch to meet an attorney friend in Hyde Park. I saw an old sidewalk that was crack-free next to some brand new concrete with a broom finish. The old sidewalk had long since lost its top coating of sand and cement and you could see many of the stones in the concrete. While it may not have looked great, it was still solid and usable. My guess is this sidewalk was installed long before 1950, but that’s just a guess.
What is concrete made of?
It’s important to realize that basic concrete has just four ingredients: sand, stones, Portland cement, and water. The Portland cement is what holds the sand and stones together for years and years assuming you do lots of things right when you mix, pour, finish, and cure your man-made artificial rock.
The more cement you add to your mix the stronger the concrete is going to be. The minimum strength for exterior concrete that most experts recommend is a 4,000 pounds-per-square-inch compressive-strength blend. This is referred to as a six-bag mix per cubic yard of concrete in many areas. The standard bag of cement weighs 94 pounds so you’re talking about having 564 pounds of Portland cement in each cubic yard.
Realize this is a MINIMUM recommendation. Nothing is stopping you from putting in seven or even eight bags per cubic yard. I just checked my local big box store and the retail cost for a bag of cement is $13.75. Would you pay that small additional amount per cubic yard of your new concrete to extend its life by decades? Of course, you would!
But it’s not just a matter of adding more cement. There are so many other things you need to do to get the concrete to last 100 years or more. I’ve gone into great detail about these things in many past columns on my AsktheBuilder.com website. I urge you to read all of them.
How much water do you add to mix concrete?
Keep the amount of mix water used to mix concrete to a minimum. You do need to add enough water to get the mix to be plastic so you can work it, but not so fluid that it sloshes around inside the forms like watery vegetable soup.
Water is the lightest of the ingredients in the mix and when you’re finishing it, clear water can appear upon the surface as you wait for the concrete to get hard enough to finish. Pros call this bleed water.
NEVER trowel this bleed water into the surface of the concrete. It often evaporates or you can pull it off using a rubber hose you drag across the wet concrete. Troweling the bleed water into the concrete dilutes the amount of cement paste near the surface. You don’t want to dilute the cement paste at the surface!
Corrosion-resistant reinforcing steel, concrete thickness, curing, and solid sub-grade under the new concrete are all very important too. I discuss all this and more in many of my past concrete installation columns. Please check them out!