Question: We're on a job that requires the microsilica concrete to be less than 600 coulombs chloride permeability at 28 days for laboratory-cured specimens and at 80 days for field-cured core samples, as measured in accordance with AASHTO T 277-83. There's a pay reduction of almost $5 a square foot if this isn't met. We've seen specifications before for laboratory specimens, but never for core specimens. How will core specimens affect rapid chloride permeability results?
Answer: It has been our experience to see two coulomb values one specified for lab-cured concrete and another for field-cured concrete. For a 600 coulomb lab value, some engineers use a 1,000 coulomb field value. One problem with requiring the same value for both specimens is maturity of the field concrete. The specifications relate the curing of the field concrete with the laboratory concrete by waiting an additional 52 days. In cold weather, however, 52 days might not be the appropriate waiting time. The maturity method can be used to indicate when the field concrete equals the same maturity as the lab concrete. The maturity concept, however, assumes equal moisture content. Depending on the field curing procedure, it's unlikely to provide the same moisture conditions as an ASTM-specified curing room. Consolidation also is an important issue. Field consolidation rarely will be equal to or better than the consolidation of a cylinder made in accordance with ASTM. Research by the Portland Cement Association indicates that a reduction in consolidation from 100% to 90% increases the amount of chloride ions passed by 100%. Another problem with requirements to check chloride permeability of field specimens at 80 days is a possible delay in acceptance and payment for a concrete pour. To help ensure timely payment, work with the project engineer to obtain an appropriate chloride permeability for a field-cured core at 28 days. This value should reflect the differences in field versus laboratory concrete.