2008 Issue

www.utahengineerscouncil.org 48 L IGHTWEIGHTAGGREGATEBATCHEDATAHIGHDEGREEOFABSORBED WATER MAY BE SUBSTITUTED FOR NORMALWEIGHT AGGREGATES TOPROVIDE“INTERNALCURING” INCONCRETECONTAININGAHIGH VOLUME OF CEMENTITIOUS MATERIALS. HIGH CEMENTITIOUS concretes are vulnerable to self-desiccation and early-age cracking, and benefit significantly from slowly released internal moisture. Field experience has shown that high strength concrete is not necessarily high performance concrete and that high performance concrete need not necessarily be high strength. A frequent, unintended consequence of high strength concrete is early-age cracking. Blending lightweight aggregate containing absorbed water is significantly helpful for concretes made with a low ratio of water-to-cementitious material or concretes containing high volumes of supplementary cementitious materials that are sensitive to curing procedures. This process is often referred to as water entrainment. Time dependent improvement in the quality of concrete contain- ing prewet lightweight aggregates is greater than with normalweight aggregate. The reason is better hydration of the cementitious materials provided by the moisture available from the slowly released reservoir of absorbed water within the pores of the lightweight aggregate. The fact that absorbed moisture in the lightweight aggregate is available for internal curing has been known for more than four decades. The first documentation of improved long term strength gains made possible by the use of saturated normalweight aggregates, was reported in 1957 by Paul Klieger, who, in addition, commented in detail on the role of ab- sorbed water in lightweight aggregates for extended internal curing. In his 1965 report, “Concrete Strength Measurement – Cores vs. Cylinders,” presented to the National Sand and Gravel Association and the National Ready Mix Concrete Association, Bloem states, “Mea- sured strength for lightweight concrete cylinders was not reduced by simulated field curing methods employed. This would tend to support the suggestion that the high absorption of lightweight aggregate may have the beneficial effect of supplying curing water internally.” This was confirmed by Campbell and Tobin in their comprehensive program which compared strengths of cores taken from field cured exposed slabs with test results obtained from laboratory specimens cured strictly in accordance withASTMC 31. Their test confirmed that the availability of absorbedmoisture in lightweight aggregate produced a more forgiving concrete that was less sensitive to poor field curing conditions. Furthermore, addressing the long term service performance of lightweight concrete, Holm cited the improved integrity of the contact zone between the lightweight aggregate and the cement matrix. The improved quality was attributed to internal curing, and better cement hydration and pozzolanic activity at the interface, and reduction in stress concentrations resulting from elastic compatibility of the concrete constituents. The benefits of internal curing go far beyond any improvements in long-term strength gain, which from some combinations of materials may beminimal or non-existent. The principal contribution of internal curing results in the reduction of permeability that develops from a significant extension in the time of curing. Powers showed that extending the time of curing increased the volume of cementitious products formed which caused the capillaries to become segmented and discontinuous. It appears that in 1991, Philleo was the first to recognize the po- tential benefits to high performance normalweight concrete possible with the addition of lightweight aggregate containing high volumes of absorbed moisture. Reduced sensitivity to poor curing conditions in concretes containing an adequate volume of prewet lightweight ag- gregate has also been reported. Since 1995, a large number of papers addressing the role of water entrainment’s influence on internal curing and autogenous shrinkage have been published of which Bentz, et al, is typical. The benefits of internal curing are increasingly important when supplementary cementitious materials, (silica fume, fly ash, metokaolin, calcined shales, clays and slates, as well as the fines of lightweight aggre- gate) are included in the mixture. It is well known that the pozzolanic reaction of finely divided alumina silicates with calciumhydroxide liber- ated as cement hydrates is contingent upon the availability of moisture. Additionally, internal curing provided by absorbed water minimizes the “plastic” (early) shrinkage due to rapid drying of concretes exposed to unfavorable drying conditions. For more information contact Darren Medeiros at UTELITE COR- PORATION (www.utelite.com ); 801-467-2800; Cell: 801-243-7473 or ucorp5@qwest.net. UTELITE CORPORATION has been supplying high quality lightweight expanded shale aggregates to theWestern United States for more than 45 years and is a producing member of the Expanded Shale, Clay, and Slate Institute (ESCSI). Utelite aggregates conform to ASTM C 330 Specification Standards. Internal Curing Using Expanded Shale, Clay and Slate Lightweight Aggregates The text is from ASTM 169D Significance of Test and Properties of Concrete and Concrete Making Materials, Chapter 46 – “Lightweight Concrete and Aggregate,” American Society of Testing Materials, West Conshohocken, PA 2006.