Replanting concrete pads

How to correct shifting from freezing and thawing

By Charles LongCharles Long


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Any concrete pad — like a step or deck support — can shift if water sits beneath it through the freeze/thaw cycle. Such pads normally sit on a 15–20 cm base of crushed stone. The stone allows drainage and, unlike soil, doesn’t subside if properly compacted. When you see signs of shifting or subsidence in the spring, prop up the deck (or whatever) and pull the pad aside. Dig a small test hole beside the base to see how deep the crushed stone extends.

If the base is inadequate, excavate an area twice the size of the pad and 20 cm deep. Fill it with clean crushed rock, tamping every five-centimetre layer with a plate compactor, sledge, or even the butt of a heavy timber. Level the top layer, compact it, and add one to two centimetres of stone dust or coarse sand. Then re-set the pad by wriggling or tapping it into the stone dust.

If your test hole fills with standing water from the saturated ground, then the spring heaves will recur unless the area is drained. For a single pad of minor importance, just top up the base and re-set the pad for another year. But where the pads bear structural supports — under the deck or even the cottage — cut a ditch alongside the affected pads. The bottom of the ditch should be at least as deep as the crushed stone under the pads, and should be sloped away from the cottage; if the building is in the way, go around it, not under it. If you don’t like the look of a ditch, lay perforated pipe along the bottom and cover it with crushed stone, filter cloth, and soil.

This article was originally published on September 17, 2009

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