This is essentially a spacer used to lift the rebar matrix off the ground. The reason behind using this is that concrete may flow underneath the rebar. This helps in preventing corrosion of the rebar.
By code, it needs about 2 to 3 inches of coverage from all sides, which becomes an integral part of the poured concrete. It helps in ensuring that there is no part of the rebar sags and helps in minimising the required concrete coverage during the pour.
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