How to Pour Concrete That Lasts
DEAR TIM: I know you’ve answered others about how to restore damaged concrete. That’s not my issue. I must pour a new sidewalk, driveway and patio and I want it to last. I don’t want it to spall. I don’t want it to crack if possible, and I want it to be maintenance free so it looks good the rest of my life. Is this possible? If so, how is it done as most concrete I see today falls apart in ten years or less? Rhonda G., Rochester, NY
DEAR RHONDA: Here’s the answer you’re looking for: Yes! Yes, it’s possible to pour exterior concrete flatwork such as sidewalks, driveways, patios and steps that will last and last. I agree with you that expert concrete finishing appears to be a dying art.
What if I told you that I could take you to an abandoned village in central New Hampshire where the winters are brutal and show you sections of sidewalk that were installed in the 1930’s before World War II? What would you say if I told you these concrete sidewalks look as if they were install several years ago? There’s no spalling, no cracking, no deterioration whatsoever.
It gets better. I can take you to other concrete flatwork in Cincinnati, Ohio, that’s also been exposed to salt, foot traffic and more and it’s also been down for decades and decades. It looks superb too. It’s not hard to create concrete that lasts. Here’s how I would do it.
The first thing you need to realize is I could write a book about the process. Many have done this before me. Some of the books are very technical. I don’t have that luxury in this very limited space, so I’m going to hit on the high points.
The ground upon which the concrete is poured needs to be solid and compacted. While concrete is very strong when it’s compressed, it only has one tenth the strength when it’s stretched or bent. If you drive over a concrete slab that’s got soft ground under it, the concrete wants to bend and stretch. This is called tension. Tension causes cracks.
You can strengthen concrete to resist tension cracks by adding reinforcing steel. Heavy mesh can be put in the slab as well as one-half-inch steel bars. If the concrete does develope a hairline crack, the crack should not open with the steel in place. Compacting the soil under the slab before the pour also prevents tension cracking.
Basic concrete contains just four ingredients: Stones, sand, water and Portland cement. The Portland cement is the glue that holds together the stones and sand. The water activates the cement powder causing billions of microscopic crystals to form that lock the stones and sand together.
The more cement you add to the mix, the stronger the concrete to a point. The modern standard for exterior slabs in cold climates is a six-bag mix that is rated at 4,000 pounds per square inch of compressive strength.
I don’t have the ability to measure the strength of the old concrete in the abandoned village or back on sidewalks I know of in Cincinnati, but I’m willing to bet the compressive strength of those is probably in excess of 6,000 or more pounds. You just have to add more cement to the mix to get higher compressive strengths.
This added cement helps to resist fracturing by ice crystals too. Concrete absorbs water and when that water freezes, it expands creating tension within the slab on a microscopic level. The more cement you have, the better the concrete can fight the forces of the ice. Adding air into the mix, air entrainment, also helps provide places for the ice to expand into without damaging the concrete.
I maintain that most of the surface failure of concrete, the spalling you mention, is a workmanship error. Many concrete finishers I’ve talked with do not have a solid background in the chemistry of concrete and are unaware that while water is needed to mix concrete, water is the enemy when finishing concrete.
If you add water to concrete to make it easier to pour, you dilute the amount of cement in the mix. This makes it weaker. If you add water to concrete as you finish it because the top layer is getting hard too fast, you dilute the amount of cement in the top layer making it very weak and unable to resist ice and other wear.
If you trowel in the bleed water that often appears as you place and finish concrete, this natural water in the slab will also weaken the top of the slab. Bleed water happens because water is the lightest of the ingredients in concrete. If left alone, the bleed water soaks back into the slab or it evaporates.
You also need to cure concrete so the water that was used to mix it doesn’t evaporate. The water is needed for days and weeks as the chemical reaction of hydration continues to add more and more strength to the concrete long after the finisher goes home to eat dinner. The old timers covered slabs with burlap and kept it wet for days and days to ensure the concrete would cure. You can use spray-on liquids or cover the concrete with plastic.