Dear Tim,
We are having a driveway built. Our concrete contractor says decomposed granite is superior to crushed rock as a base under concrete. Our local paving products dealer say "absolutely not. Decomposed granite is not recommended as a base under concrete because it is difficult to achieve adequate and uniform compaction of the granite." What do you recommend?
Cheryl W.
Grants Pass, Oregon
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Cheryl,
I have not had the pleasure of working with decomposed granite. It is not available in the Midwest. But I am a geologist and I know about installing rock under concrete. Here we use a product called pea gravel. It is small rounded rock that really does resemble frozen peas. Because of its round shape it moves around as you walk on it. It can be compacted with a vibratory plate compactor and makes an *excellent* base for poured concrete.
My impression of the small granite pieces is that they have slightly angular edges which would make compaction even better as the pieces would interlock as they do with true crushed gravel. Not only that, granite is one of the most durable rocks on the planet because of its high quartz content.
There is nothing like compacted rock under concrete. It is an ideal base material.
Tim Carter
www.askthebuilder.com
W3ATB