DEAR TIM: I’ve got an odd problem. I have concrete spalling on a sidewalk that leads from my concrete driveway to my home. The concrete driveway and sidewalk were poured at the same time. The driveway is in great shape, but the sidewalk has all sorts of places where the top surface of the concrete has crumbled, and I see the rocks below. The contractor has no clue how it happened. What can be done to repair it, short of tearing it all out and starting over? What’s the most common cause of spalling? – Tom D., Yosemite West, California
DEAR TOM: Concrete spalling seems to be a growing epidemic if my email inbox is any sort of trusted indicator. Years ago, reports of these failures were primarily from parts of the USA that received cold and freezing weather. Now I’m getting reports from places that rarely receive freezing temperatures. In almost all cases the root cause of concrete spalling is a workmanship error.
Spalling is usually defined as the disintegration of the upper surface of a concrete slab. In most cases this failure extends down about 1/4 inch or less. When the upper finished surface of the concrete crumbles, you end up seeing the rougher stone aggregate that’s a part of most concrete slabs. This inner core of the concrete rarely crumbles, and the spalling defect is primarily an aesthetic issue.
The most common cause of spalling is errors on the part of contractors. They either add additional water to the concrete as they finish it or they start to finish the concrete before the concrete bleed water leaves the surface of the concrete.
Contractors often use water as a finishing aid in case the concrete starts to harden while they are working on other parts of the slab. The addition of the water softens the upper surface, allowing them to provide an attractive finish, but it compromises the strength of the concrete. You always need plenty of finishers on a job so the slab can be finished without adding any water.
Portland cement is the glue that holds together the sand and stone in concrete. It’s vital to have the correct amount of Portland cement at the surface because this is where weather attacks concrete and where concrete receives abuse from car and truck tires, as well as foot traffic.
Concrete is porous. You can demonstrate this on concrete that has a sandy finish. If the concrete is warm and dry simply drip some water onto the surface. You’ll see it soak into the concrete rapidly and spread sideways. In winter months, water can soak into concrete and freeze on a microscopic level.
Water expands as it freezes and puts lots of pressure on the concrete crystal matrix. Concrete that has a sufficient amount of Portland cement in it, is mixed properly with not too much water, is finished properly and is cured will resist decades of freezing weather. You can even put de-icing salts on concrete with no spalling risk if it’s been installed correctly. I can show you sidewalks in many cities that have had salt on them every winter and there’s no damage.
You can repair concrete slabs that are experiencing spalling, and it’s not too hard to do. The repair can last decades if it’s done correctly.
I’ve repaired spalled concrete slabs by placing a thin overlay of cement stucco over them. This stucco can be as thin as 1/8 inch, but it’s better if you apply at least 1/4 inch.
The stucco is just a mixture of Portland cement and medium sand. You can use different grades of sand: fine, medium or coarse, depending upon the final look you want for the concrete slab. Use fine sand if you want to get an ultra-smooth steel-trowel finish. Use coarse sand if you want an aggressive surface for the best traction.
The key to success when applying this stucco is to make sure you have removed the entire weak, compromised concrete upper layer. Some concrete may be ready to spall, but it’s not yet happened. A powerful pressure washer held close to the concrete surface and at a very low angle does a great job of lifting up these weak areas of concrete. Use a 15-degree tip on the pressure washer wand and hold it just an inch or two from the edge of spalled areas.
Once you have all the damaged concrete out of the way and cleaned up, install the side forms on the sidewalk that will give you the thickness you’ve decided to go with. Try to do this work on cool, overcast days. The worst conditions to work with concrete are sunny, breezy, hot days.
To get the new stucco to bond permanently to the rough, spalled concrete you need to use cement paint. I was taught this magic trick decades ago by true seasoned concrete masons. Cement paint is just Portland cement mixed with water to the consistency of regular paint.
Get the spalled concrete damp, brush on the cement paint and immediately cover the paint before it dries with the cement stucco. The cement stucco mixture should be two parts sand and one part Portland cement for an ultra-strong surface. Add just enough water to make the stucco the consistency of stiff, not runny, applesauce. Don’t add any water as you finish the stucco!
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