Although some of these low strengths may be due to poor cylinder preparation or handling, back up tests and cores have confirmed that many of the compressive strengths are below design strength and projects are being delayed while sufficient strengths are obtained. In most of these cases, strengths are below those obtained from laboratory mix designs and from earlier field results using the same cement factor and aggregates. Factors that may contribute to low compressive strengths may include but are not limited to high air content, flocculation of air bubbles at the aggregate-mortar interface, change in air entrainment admixture, change in cement chemistry, hot cement and aggregate, inadequate mixing, sand/rock ratio, temperature, and dirty aggregate.
Preliminary work has been done by SDDOT and SDSM&T to learn the extent of the problem. Scanning Electron Microscopy has revealed flocculation of air bubbles at the aggregate-mortar interface in two separate samples. Because of the work already underway, it is desirable for SDDOT and SDSM&T to continue the research in a cooperative effort. This may be the best way to get results prior to the 1998 construction season.
Findings: The researchers found the source for the low strengths to be poor aggregate-paste bonds associated with air void clusters and poorly formed cement paste in the interfacial region adjacent to the aggregate. An interaction between the “synthetic” air entraining admixtures, used as substitutes for vinsol resin, and low alkali cements was directly tied to the problem with high summertime temperatures also contributing to the problem. The "synthetics" seem to form thinner-walled air bubbles than vinsol resin, which can lead to significant reductions in strength.
As a result of these findings, the Department now allows only vinsol resins for air entraining agents. Also, the maximum air content allowed was reduced from 8.0% to 7.5%. The technical panel recommended that current procedures for concrete plant inspection should be modified to provide a requirement for a minimum level of inspection.