I understand that the products of reinforcing steel corrosion occupy a greater volume than the original steel. This volume increase exerts tensile stresses in the concrete, causing it to spall. We're considering using galvanized reinforcing steel in which a zinc coating acts as a sacrificial anode, protecting the steel. However, I've heard that zinc also expands when it corrodes. Wouldn't this cause the same problem as steel? Also, what would happen if, rather than using galvanized rebar, you simply added finely divided zinc powder to the mix? Would the rebar be protected?
Depending on the oxidation state, the corrosion products of steel can occupy more than 6 times the volume of the original steel. Also, the corrosion products of steel are highly insoluble in concrete. On the other hand, the usual zinc corrosion product, zinc oxide, occupies only 11_2 times the volume of zinc. In addition, the zinc corrosion products are more soluble in the concrete environment, so they may diffuse some distance from the metal-concrete interface. Therefore, zinc corrosion induces much lower tensile stresses in concrete than steel corrosion. Adding zinc powder to the concrete mix would not provide cathodic protection to the rebar. There must be direct electrical contact between the zinc and steel to provide an avenue for electrons to flow from the anode (zinc) to the cathode (steel).