How to deal with unwanted problems in the sidewalk

The Associated Press
Friday October 13, 2000

Sidewalks take a beating from the very beginning. 

While a house is under construction, a new sidewalk often floats high on a bed of fill-sand, which eventually erodes from the sides and is swallowed by utility trenches. Then, before the expectant family moves in, their new sidewalk will bear weight of delivery trucks, landscape tractors, etc., usually before the concrete is fully cured. Over the years, the soil beneath the sidewalk heaves and settles. It all adds up to cracks and other problems. 

To deal with cracks, the goal is to seal out water because wet soil beneath a sidewalk only exacerbates the problems of frost heave and settling. 

Narrow cracks less than a half inch should be caulked shut with a liquid polymer crack filler. One such product is Sure-Fix Pour-n-Patch, made by W.R. Bonsal Co., Charlotte, N.C. 

Most concrete crack fillers will not cure properly when applied more than one-half inch deep. Deeper cracks require packing the joint with some form of backing, such as foam-plastic backing rod, or pouring the filler in two or more applications, each three-quarters of an inch deep. 

Highlights of the crack repair process begin with first brushing away debris from the cracks and then pouring in liquid-polymer crack filler to a level just below the surface. On edge cracks or breaks, undercut the edge of the concrete with a cold chisel to increase the bonding surface. Install a form board against the side of the slab. Then cover the board with masking tape to keep the patch from sticking to the board. 

Several conditions will cause the portland-rich surface of a sidewalk to pop loose. The problem may be caused by a poor concrete mix, an aggregate that was not sufficiently clean or the failure to protect the surface from certain extremes in weather. 

Of course, cold weather can damage a fresh concrete surface, because the surface is the most susceptible to frost. More often, however, the problem is caused by heat. When concrete is poured on hot days, the surface dehydrates faster than the concrete below it, causing the slab to weaken and separate just below the surface. Fresh concrete should always be covered on hot days. 

Larger damaged concrete areas and surface pops require patching. Standard masonry mortar has little bonding strength when applied in thin layers.