From time to time we answer reader’s questions.
Zafra: We’re researching the possibility of acquiring cheap recycled concrete rather than having to pay for gravel for our foundation, but we’re concerned because we’ve read that concrete can wick water upwards to the earth walls.
Owen: Good question. Recycled concrete will work fine if you break it up into rock-sized pieces no larger than about 6″. Mix in plenty of small pieces (or some gravel) so the rubble trench is full of tightly compacted rubble with few air spaces. Compact the rubble with a tamper to reduce settling. Gravel bags are placed on top of the rubble trench.
You can also use concrete rubble in lower courses of ‘gravel bags’ or ‘rubble bags’ if it’s broken into small pieces. Use double bags – one inside the other – for added strength. Add a capillary break (moisture proof barrier) between the rubble bags and earthbags if you’re in a rainy climate or in doubt. (See drawing above.) A few dollars worth of plastic or tarpaper would prevent wicking of moisture up into the earthbags.