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Rebar in concrete foundation walls is among the more misunderstood aspects of light construction. Most builders have a pretty good feel for the strength of framed walls and floors, but their intuition deserts them when it comes to the foundation. In consequence, there is a lot of folklore about what works and what doesn't. Let's start with fundamentals. The strength of concrete is in compression — it easily resists direct crushing loads. However, this marvelous quality is offset by an extremely low strength in tension, which gives concrete a tendency to crack in service. Concrete, depending on the mix, will take a 2,500- to 8,000-pound-persquare- inch load in compression. But in tension, concrete will resist only about 250 to 350