That sinking feeling

That sinking feeling
Date:1 October 2010

One end of my concrete driveway near the garage has dipped below the rest of the driveway, and a large pool of water forms there when it rains. What do I do? Can I cut out and remove that part and pour a new piece of concrete without damaging the rest of the driveway?

Yes, a knowledgeable concrete contractor can certainly cut out the piece of concrete and install a new slab without damaging surrounding concrete. The job can get tricky, however, if the sunken section is positioned so that there’s not enough room for a masonry saw at one or more edges. That can happen in corners where a slab meets a foundation wall or where it meets another slab. Also, the colour of the replacement piece won’t match that of the surrounding concrete, although this can be mitigated by using concrete stain on the entire driveway.

An alternative to cutting and removal is slab jacking (also known as mud jacking). A contractor uses a rock drill or a core drill (a holesaw for masonry) to make some holes in the sunken slab. Then he pumps a material called grout through the hole. This is like concrete, but it lacks coarse aggregate, such as stone or gravel. As this material is forced under the slab, it lifts the slab up. After the slab is lifted, the holes are filled with more grout or a concrete patching compound, or the concrete cores are glued back into place with a specialised adhesive. There’s an obvious green benefit to this procedure. It takes less energy and materials to jack up a slab than to replace it and pulverise the old concrete for use as roadbuilding material. The process generates very little debris to dispose of, an obvious benefit in areas where there is little or no concrete recycling.

It takes a lot of experience to do this work correctly. It’s easy to crack an adjoining slab or unevenly lift a sunken slab – making it worse than it was before. That puts a premium on hiring an experienced contractor.

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