Fly Ash for Improved Workability ofStructural Concrete Made with Crushed Sandas Fine Aggregate

摘要

Use of crushed sand as fine aggregate in structural concrete is becoming morernand more common as the sources of natural sand are depleted. Theirrnreplacement with crushed quarry sand may have economic and environmentalrnbenefits but also a detrimental effect on some properties of the fresh and/orrnhardened concrete.rnThe particle shape of crushed sand is more angular with a rougher surfacerntexture, and usually flakier and more elongated, than that of natural sand.rnMoreover, crushed sand, unless suitably treated, contains much more finesrn(particles smaller than 75μm). As a result the workability of the fresh concrete isrnimpaired with the corresponding need for a higher water requirement, which inrnturn makes for lower concrete strength and higher drying shrinkage. By contrast,rnthe fly ash particle has a spherical shape and a smooth surface. Thus, arncombination of fly ash and crushed sand yields a far superior concrete mix thanrncrushed sand alone and obviates the disadvantage of partial or total replacementrnof natural sand with crushed sand.rnThe above was confirmed experimentally, with the reservation that with a largerrndosage of high-range water reducer admixture, superplasticizer is required tornavoid increase of the water requirement. The drying shrinkage of concretes withrncrushed sand increased with increase in the water requirement, but remainedrnabout the same when the increase in water requirement was offset by arnsuperplasticizer. Fly ash also increased the drying shrinkage. In view of therndiversity of each of the concrete ingredients versus each type of sand, cementrncontent, and chemical and mineral admixtures - the optimal dosage has to bernexamined in each case.