Source: Ready Mixed Concrete Research & Education Foundation, Alexandria, Va.
The latest RMC Research & Education Foundation report, “A Concrete Strength Model to Optimize Concrete Mixtures,” and companion Excel tool equip producers and practitioners to pinpoint appropriate binder dosages and limit potential overdesign as measured in 28-day concrete specimen compressive strengths exceeding psi targets.
Report author and Florida consultant Edwin J. Dunstan Jr. refined his model based on observations from years of portland cement and fly ash research. He evaluated it by subjecting 27 mixtures to strength prediction testing at the National Ready Mixed Concrete Association Research Laboratory in College Park, Md. Dunstan cites limitations of equations or laws that have informed cement and concrete strength prediction or modeling for more than a century. Most notably, he examines the water-cement ratio principle Duff Abrams advanced in 1918 from his Chicago lab, plus the respective 1892 and 1926 work of French and German contemporaries R. Feret and J. Bolomey.
In light of differences in modern concrete mix designs and materials versus those of 100 years ago, Dunstan stresses “Hydration is key.” The Feret, Abrams and Bolomey models, he contends, “Do not include maturity that accounts for the increase in strength due to the hydration of cement and supplementary cementitious materials. [They] also assume that the degree of hydration of cement at 28 days is the same for all water-to-cement ratios.” He questions the degrees of cement or binder hydration in two sample mixtures, each with 242 lbs. of water, but one with 500 lbs., the other 600 lbs. of cement—equating to 0.48 and 0.40 water-cement ratios. “Is the degree of hydration of cement in these two mixtures the same?” Dunstan observes.
The relationship between w/c ratio and hydration is the “key” he uses to produce a workable concrete strength model. It explains how a) admixtures influence the hydration of cement; b) water reducing admixtures increase strength; c) the filler effect (nucleation) increases the amount of cement hydration; and, d) changes in temperature affect concrete strength. Dunstan’s methodology requires sequential trial batch testing to determine parameters appropriate to the concrete mix design materials being used. Once parameters are established, strength of other mixtures within the tested constraints can be predicted, leading to material and cost savings for each yard of concrete.
“This model will allow producers to improve their bottom line while also ensuring quality and sustainability,” notes RMC Foundation Chairman Rodney Grogan (Dunn Investment Co./MMC Materials).
“The concrete strength model is another tool in the ready mixed concrete producer’s toolbox,” adds Foundation Executive Director Julie Garbini. “The Foundation’s mission is to improve quality, sustainability and professionalism in the industry and this new, free deliverable meets all those marks.”