Failed Concrete Can Often Be Fixed

Q DEAR TIM: Some of the concrete walkways in our association's 100-acre island development are disintegrating. The surface is crumbling. Other adjacent concrete, poured at the same time, is in great shape. What is the problem, and can a thin concrete overlay be added?

-- Roland M., Boothbay Harbor, Maine

ADEAR ROLAND: Your association was probably the victim of poor workmanship. Spalling of a concrete walkway can almost always be traced to issues of mixing, placing, finishing and curing the concrete. Because you are on a small island, I will assume that all the concrete is made on site in small batches, not delivered in large ready-mixed trucks that come over on a ferry.

The good news is that you are going to be able to install a concrete overlay that will perform well for the next 50 years, if not longer. Even better, if you and the other residents are willing to do a little bit of labor, this is a do-it-yourself task that many homeowners can complete with professional results.

Let's talk first about why the original concrete failed, so you can avoid the same problem with the overlay project. Because you have concrete in great shape next to concrete that has flaked or chipped, my instincts tell me that the failure is due to a finishing error. The installer probably troweled water into the surface of the concrete to make the finishing go faster with less effort. He could have also troweled it too soon, while the bleed water was on the surface.

The spalling happened because there simply was not enough cement paste at or near the surface of the concrete slabs. The cement in concrete is the glue that binds the sand and stone together. If you add too much water to concrete, you dilute the cement in the mixture. When this happens, concrete is susceptible to spalling from freeze-thaw cycles. I have to believe that you get cold weather with snow and rain in the winter. These are ideal conditions for freeze-thaw failure.

To repair your damaged sidewalks, you need a few materials and tools. The concrete overlay mixture is made from sand, very small stones, Portland cement and fresh water. Look closely at the surface of the existing sidewalks that are in good condition. See if the sand particles in the concrete match the sand on your beaches. I would be surprised if the contractor didn't use the abundant sand you have around you. If the concrete was ready-mixed and brought from the mainland, then visit the concrete plant and find out where they got their sand. You want to use the same sand if possible to get a close match on the finish.

Stones used in concrete impart great strength. But the stones must be sized properly. The diameter of any stone should not be more than one-third the thickness of the overlay. This means that if you are pouring a thin concrete overlay that is one-half-inch thick, the largest stone in the mix should not be much larger than one-eighth inch in diameter. That is the size of grains of coarse sand.

I would use one measure of Portland cement for each 2.5 measures of sand. If you plan to do a thicker concrete overlay, then use three measures of gravel, two measures of sand, and two measures of Portland cement.

The key to ensuring a permanent bond between the existing slab and the new mixture is to sweep all loose material away from the old slab and to make sure it is clean and slightly damp. You should also apply a coat of cement paint immediately before you pour the overlay. Cement paint is made by mixing fresh water with Portland cement. Blend it to the consistency of regular paint and brush it on the damp slab.

I recommend that you practice the technique on a remote sidewalk section. You can also practice in a garage using precast concrete pads that you can buy at a home center.

Try to work on overcast days when the temperature is cooler. Sunny, hot and windy days are the worst conditions in which to pour concrete or work with concrete overlay materials. After you pour the material onto the slabs, use a straightedge to remove excess material. Let the mixture start to set until your thumb can depress it only an eighth of an inch. When this happens, it is time to use a magnesium or wood float to create a sandy top finish on the overlay. You can also use a broom to create a broom finish.

Immediately after the concrete is finished, apply a spray-on curing compound. The compound prevents the water from leaving the overlay mixture too rapidly. Water is needed to allow the concrete to achieve its full strength.

Tim Carter can be contacted via his Web site,http://www.askthebuilder.com/printer_Submit_Question.shtml.

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