Q.: Is it permissible to use concrete with 1% to 2% calcium chloride on an elevated slab that's to be placed on galvanized metal decking? The building is in the Midwest and won't be exposed to any moisture other than the humidity in the air.

A.: You should avoid using calcium-chloride admixtures in concrete to be placed on galvanized metal deck forms because of the danger of corrosion. This could lead to failure of the metal decking, which provides tensile reinforcement for the slab.

ACI 318-95, "Building Code Requirements fo Structural Concrete," section 3.6.3, states: "Calcium chloride or admixtures containing chloride from other than impurities from admixture ingredients shall not be used in prestressed concrete, in concrete containing embedded aluminum, or in concrete cast against stay-in-place galvanized steel forms." The commentary for this section says: "Serious corrosion of galvanized steel sheet and galvanized stay-in-place forms occurs, especially in humid environments or where drying is inhibited by the thickness of the concrete or by impermeable coverings."