Investigation of Poor Compressive Strength and Performance of A45 Structural Concrete Mixes

In the last few years, a higher than usual number of concrete mixes with below specification compressive strengths have been observed by SDDOT. The 2020 and 2021 construction seasons saw dramatic increases in instances of failing strengths, particularly among A45 (4500 psi) structural concrete mixes. Failures missing the 4500-psi requirement by over 500 psi, which under specification, fall under “remove and replace” criteria, have become more common. All these failing mixes, on top of their negative impacts on concrete performance and project costs, have introduced increased risk for SDDOT, contractors, and concrete producers in future projects. Contractors and producers have reported that maintaining and achieving quality concrete performance has become increasingly difficult due to the prescriptive nature of the current A45 specification. The exact cause of these instances of low strength and poor performance is currently unknown. It could be the result of materials issues related to the cement, fly ash, admixtures, aggregates, or even incompatibility between some of these materials. Current mix designs could be outdated and may not reflect the materials used in practice. Outside of the mix design, the batching, delivery, placement, and curing methods currently specified could also be contributing to the low strengths observed. Testing procedures and handling of cylinders also impact strength development. Without investigating the cause of these low strengths and reducing the rate at which they occur, concrete mixes used by SDDOT will continue to have these costly problems. Objectives are as follows: (1) Determine the factors contributing to the significant increase in instances of below-specification strength A45 structural concrete observed in recent construction years. (2) Recommend changes to current practice (handling, placement, testing, mix design) to reduce future instances of below-specification strength A45 structural concrete following placement in the field.