Citing a potential to prevent curling and reduce drying shrinkage-induced cracking in slabs by 90 percent or more compared to undosed mixes, Premier Construction Products Group launched a powdered admixture, PREVent-C, at World of Concrete 2012.
The patent-pending technology “reduces both drying and autogenous shrinkage-induced macro and microcracks in concrete,” notes Premier CPG President Claudio Manissero. “PREVent-C facilitates expansion of concrete at the same rate as drying shrinkage during the curing period to reduce the capillary surface tension of pore water. Expansion ceases during hydration of concrete and does not continue once the concrete has hardened.”
The admixture, he adds, also reduces compressive creep and autogenous shrinkage; improves durability; decreases curling and related cracks, along with the potential for water and salt slab infiltration; and, enables a reduction in slab control joints.
PREVent-C suits cast-in-place or precast concrete, grouts, mortars and shotcrete, and is especially effective in conditions where mixes are highly restrained. It is the charter offering of PremierCPG, formed to promote the first construction industry-geared product of West Conshohocken, Pa.-based Premier Magnesia LLC—one of the world’s principal suppliers of high-purity, calcined magnesium compounds.
The inaugural PREVent-C field application saw replacement of inlet slabs on the Glen Elder Dam and Spillway in north-central Kansas; located at the base of the Solomon River, the structure creates Waconda Lake. A 644-ft.-wide spillway, controlled by 12 radial gates, required significant slab work, as concrete in both the inlet apron and pier noses/gate structure had extensive deterioration. Results of core testing revealed freeze-thaw damage largely caused by absorptive aggregate, the attendant cracking then promoting alkali-silica reactivity.
Retrofit consisted of hydrodemolition of deteriorated concrete and placement of new mixes back to the existing lines and grades. Due to uneven deterioration, concrete removal varied from full depth (18 in.) to intermediate levels. Islands of existing, sound concrete were saw-cut at sharp 90-degree angles.
The new concrete was highly restrained with steel rebar, shallow and deep points, and the random corners—magnifying conditions for cracking. Further, the replacement mix design had a high ratio of sand to coarse aggregate, and the largest aggregate available in the area was only ½ in. In response to shrinkage cracking the mix exhibited throughout the first half of the project’s 1,500-yd. total, U.S. Bureau of Reclamation, Materials Engineering and Research Laboratory engineers approved the use of PREVent-C mixes on three large demonstration slabs requiring about 125 yd.
One month after the PREVent-C mix placement, project engineers, along with contractor Norcon Corp. of Schofield, Wis., and local producer Beloit Ready-Mix observed no cracks in two of the demonstration slabs and a 90 percent reduction in the third when compared to earlier replacement slabs.
Premier worked with Norcon and Beloit Ready-Mix to assure PREVent-C compatibility with the existing concrete mix design. Lab testing was conducted at Tourney Consulting Group, which applied ASTM-C157 methods to assess shrinkage and other physical properties. Beloit Ready-Mix President Blaine Englebert attributes the success of the Glen Elder PREVent-C demonstration to strong technical service and field support from Premier CPG and Tourney.
“Given the excessive and complex edge conditions that concentrate stresses, and the cracking that occurred in the earlier placed concrete, we were pleasantly surprised to see the results of the Premier admixture,” notes Norcon Project Manager Justin Penrose. “We feel this product has a bright future in preventing or minimizing shrinkage-related cracking in the concrete industry.”
“Based on our laboratory testing, we felt comfortable that PREVent-C would help reduce shrinkage cracking. However, we were surprised at how well [it] performed by virtually eliminating the shrinkage cracking,” adds Tourney Vice President Glenn Schaeffer. — Premier Construction Products Group, Huntersville, N.C., 800/227-4287, www.premiercpg.com