to deterioration and structural failure of the concrete.Alkalis, acids, and sulfates in the water tend to react withthe chemicals in the cement. The result is inadequatecementing and weakened concrete. Water must be freeof these chemicals to be used in concrete mixing.SEAWATER.— The salts in seawater are normallythought of as being corrosive; however, seawater is usedsometimes in concrete mixing with satisfactory results.A loss of 10 to 20 percent in compressive strength canbe expected when the same amount of seawater as freshwater is used. That can be compensated somewhat byreducing the water-cement ratio.AggregatesThe aggregates normally used for concrete arenatural deposits of sand and gravel, where available. Insome localities, the deposits are hard to obtain and largerocks must be crushed to form the aggregate. Crushedaggregate usually costs more to produce and will requiremore cement paste because of its shape. More care mustbe used in handling crushed aggregate to prevent poormixtures and improper dispersion of the sizes throughthe finished concrete. At times, artificial aggregates,such as blast-furnace slag or specially burned clay, areused.TYPES OF AGGREGATE.— Aggregates aredivided into two types as follows:. FINE AGGREGATE. “Fine aggregate” isdefined as material that will pass a No. 4 sieve and will,for the most part, be retained on a No. 200 sieve. Forincreased workability and for economy as reflected byuse of less cement, the fine aggregate should have arounded shape. The purpose of the fine aggregate is tofill the voids in the coarse aggregate and to act as aworkability agent.. COARSE AGGREGATE. Coarse aggregate is amaterial that will pass the 3-inch screen and will beretained on the No. 4 sieve. As with fine aggregate, forincreased workability and economy as reflected by theuse of less cement, the coarse aggregate should have arounded shape. Even though the definition seems tolimit the size of coarse aggregate, other considerationsmust be accounted for.When properly proportioned and mixed withcement, these two groups yield an almost voidless stonethat is strong and durable. In strength and durability,aggregate must be equal to or better than the hardenedcement to withstand the designed loads and the effectsof weathering.It can be readily seen that the coarser the aggregate,the more economical the mix. Larger pieces offer lesssurface area of the particles than an equivalent volumeof small pieces. Use of the largest permissible maximumsize of coarse aggregate permits a reduction in cementand water requirements.One restriction usually assigned to coarse aggregateis its maximum size. Larger pieces can interlock andform arches or obstructions within a concrete form. Thatallows the area below to become a void, or at best, tobecome filled with finer particles of sand and cementonly. That results in either a weakened area or acement-sand concentration that does not leave theproper proportion to coat the rest of the aggregate. Themaximum size of coarse aggregate must be no largerthan the sizes given in table 13-1. The capacity of themixing equipment may also limit the maximumaggregate size.GRADATION.— Gradation of aggregate refers tothe amount of each size of particle used in the mix. Toolarge a proportion of coarse aggregate leaves voids thatrequire more cement paste to fill. That affects theTable 13-1.—Maximum Recommended Size of Coarse Aggregate13-21
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