Q. The specification for our parking structure job says chloride ion content of the concrete should not be more than 0.15 percent by weight of the cement. We also had to submit certification of chloride ion content of the admixture, the coarse aggregate, and the fine aggregate. We have obtained the necessary certifications for the components of the mix and have a concrete specimen ready for testing. But WHEN should the test be done? We need to start placing concrete as soon as possible.

A. Building Code Requirements for Reinforced Concrete" (ACI 318-83) sets limits on water-soluble chloride ion content as a precaution against possible corrosion of reinforcing steel. The 0.15 percent limit called for by your specification applies to reinforced concrete exposed to chlorides during its service life. Limits for concrete under other conditions are shown in the table.

MAXIMUM WATER-SOLUBLE CHLORIDE ION
CONTENT FOR CORROSION PROTECTION

as required by "Building Code Requirements
for Reinforced Concrete" (ACI 318-83)
Type of memberMaximum water-soluble
chloride ion in concrete
Percent by weight of cement
Prestressed concrete0.06
Reinforced concrete
exposed to chlorides
in service
0.15
Reinforced concrete
dry or protected
from moisture
1.00
Other reinforced concrete0.30


ACI 318 sets these limits for concrete at an age of 28 days but does not specify the time of test. We believe you can safely test before 28 days because research indicates that the soluble chloride content of a mix diminishes rapidly in the first 10 days. The soluble chlorides begin to combine with various ingredients of the concrete and become insoluble. If you can meet the specified limit at less than 28 days, the concrete will still be satisfactory (and maybe even better) at 28 days. Concrete Construction reported some background information on this in a special issue on the chloride question, October 1982, page 803.