What is a substrate in concrete repair?

The substrate is the underlying surface onto which a repair material, coating, or membrane is applied. In our work, this is usually the original concrete or masonry structure. The success of any repair depends on the quality of the substrate. It must be sound (not crumbling), clean (free of oil/dirt), and have an open texture (profile) to accept the bond. 'Substrate failure' means the old concrete broke, not the repair.