Quantcast TYPES  OF  AGGREGATE

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to  deterioration  and  structural  failure  of  the  concrete. Alkalis, acids, and sulfates in the water tend to react with the  chemicals  in  the  cement.  The  result  is  inadequate cementing and weakened concrete. Water must be free of these chemicals to be used in concrete mixing. SEAWATER.—  The salts in seawater are normally thought of as being corrosive; however, seawater is used sometimes in concrete mixing with satisfactory results. A loss of 10 to 20 percent in compressive strength can be expected when the same amount of seawater as fresh water is used. That can be compensated somewhat by reducing  the  water-cement  ratio. Aggregates The  aggregates  normally  used  for  concrete  are natural deposits of sand and gravel, where available. In some localities, the deposits are hard to obtain and large rocks must be crushed to form the aggregate. Crushed aggregate usually costs more to produce and will require more cement paste because of its shape. More care must be used in handling crushed aggregate to prevent poor mixtures and improper dispersion of the sizes through the finished concrete. At times, artificial aggregates, such as blast-furnace slag or specially burned clay, are used. TYPES  OF  AGGREGATE.—  Aggregates   are divided into two types as follows: .   FINE   AGGREGATE.   “Fine   aggregate”   is defined as material that will pass a No. 4 sieve and will, for the most part, be retained on a No. 200 sieve. For increased  workability  and  for  economy  as  reflected  by use of less cement, the fine aggregate should have a rounded shape. The purpose of the fine aggregate is to fill the voids in the coarse aggregate and to act as a workability  agent. . COARSE AGGREGATE. Coarse aggregate is a material that will pass the 3-inch screen and will be retained on the No. 4 sieve. As with fine aggregate, for increased workability and economy as reflected by the use of less cement, the coarse aggregate should have a rounded shape. Even though the definition seems to limit the size of coarse aggregate, other considerations must be accounted for. When  properly  proportioned  and  mixed  with cement, these two groups yield an almost voidless stone that is strong and durable. In strength and durability, aggregate must be equal to or better than the hardened cement to withstand the designed loads and the effects of  weathering. It can be readily seen that the coarser the aggregate, the  more  economical  the  mix.  Larger  pieces  offer  less surface area of the particles than an equivalent volume of small pieces. Use of the largest permissible maximum size of coarse aggregate permits a reduction in cement and water requirements. One restriction usually assigned to coarse aggregate is its maximum size. Larger pieces can interlock and form arches or obstructions within a concrete form. That allows the area below to become a void, or at best, to become filled with finer particles of sand and cement only.  That  results  in  either  a  weakened  area  or  a cement-sand  concentration  that  does  not  leave  the proper proportion to coat the rest of the aggregate. The maximum size of coarse aggregate must be no larger than the sizes given in table 13-1. The capacity of the mixing   equipment   may   also   limit   the   maximum aggregate  size. GRADATION.— Gradation of aggregate refers to the amount of each size of particle used in the mix. Too large  a  proportion  of  coarse  aggregate  leaves  voids  that require  more  cement  paste  to  fill.  That  affects  the Table 13-1.—Maximum Recommended Size of Coarse Aggregate 13-21



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