The Use of Lightweight Fines for the Internal Curing of Concrete
By George C. Hoff, P.E., DEng.
President, Hoff Consulting LLC
Introduction
The benefits of using lightweight aggregates in concrete to help reduce cracking in slabs and bridge decks has been intuitively known for decades by the lightweight aggregate industry but the reasons as to why this occurred were not extensively examined and the benefits were not widely promoted. It was believed, and correctly so, that the lower modulus of the LWA and the improved transition zone around the LWA particles due to their generally vesicular surface, helped reduce stress concentrations between the paste and the aggregate and those reductions subsequently reduced the amount of early-age cracking in the concrete. In the 1980's, the production of high-strength concrete (HSC) became more common and, to accomplish it, came the use of higher cement contents, supplementary cementing materials such as silica fume, fly ash and blast furnace slag cement, and lower water-binder ratios as a result of the extensive use of superplastizers. The term "high-performance" concrete (HPC) also emerged with a focus on providing special properties of concrete above what would normally be expected from concrete produced for general use. Most of the HPC was directed at improved durability. The durability improvements came by reducing or eliminating the transport mechanisms of the environment into the concrete and this generally followed the same modifications of the mixture proportions that occurred for HSC.
With the mixture proportion changes for HSC/HPC, came a shrinkage problem. Concrete shrinkage, over time, induces cracking that can severely reduce the life expectancy of concrete. Long-term drying shrinkage has typically been what is addressed in the literature and what is considered in structural design. Lately, much more attention has been paid to early-age shrinkage, as it can be responsible for cracking when the concrete has not gained significant strength to withstand internal stresses. The components of both early-age and long-term shrinkage are drying, autogenous, and thermal shrinkage, with carbonation shrinkage also contributing to the overall long-term shrinkage (1).
The use of HSC/HPC in pavements is further complicated by the "fast track" approach to paving where the concrete gets only minimal moist curing. ACI 308-92 (2) notes that for moist curing pavements and other slabs on the ground at temperatures above 40oF (5oC), "the recommended minimum period of maintenance of moisture and temperature for all procedures is 7 days or the time necessary to attain 70% of the specified compressive or flexural strength, whichever period is less." This curing period may have to be extended when fly ash is included as part of the binder because of its slower initial strength development. Also, when silica fume and other pozzolans are used, the water demand is much greater. For structures and buildings, ACI 308-92 (2) notes that the use of liquid membrane-curing compound not be authorized "when the concrete has a water-cement ratio of 0.4 or less by weight." This implies that even by not allowing the moisture in the concrete to leave using the membrane, initially there is probably insufficient moisture in the concrete for complete hydration to occur and that moist curing procedures should be followed.
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