used to handle bulk cement. Bulk cement is measuredin tons (2,000 lb) and smaller quantities are bagged incloth or paper sacks, each containing 94 pounds ofcement. A 94-pound sack of cement is equal to 1 cubicfoot by loose volume.Cement bags should not be stored on damp floors,but should rest on pallets. The bags should be stackedagainst each other to prevent circulation of air betweenthem, but not stacked against outside walls. If the stacksof cement are to be stored undisturbed for long periods,they should be covered with tarpaulins.Cement bags that have been stacked in storage forlong periods sometimes acquire a hardness calledWAREHOUSEPACK. This can usually be loosened byrolling the sack around. Cement that has lumps or is notfree flowing should not be used.AGGREGATEseparately-the fine aggregates on sieves with openings1/4 inch or smaller and the coarse aggregate on sieveswith square openings from about 1/4 inch and larger.The fine sieves are numbered—the larger the number, thesmaller the sieve opening; for instance, the No. 100sieve has 100 openings per inch, and the No. 4 sieve has4 openings per inch.The grading of both coarse and fine aggregate andthe relative proportions of each in the mix can greatlyaffect the properties of the fresh concrete. Concretemade with coarse sand or not enough sand is hard topump and will be harsh and difficult to trowel. Also,aggregates can segregate or separate from the cementpaste during placement, producing nonuniformconcrete. Air-entraining will help in overcominggrading problems of this kind. Coarse aggregates shouldbe round or subround in shape. This thape allows thecement paste to coat the particles more easily duringmixing.The aggregates used in concrete must be strong,HANDLING AND STORAGE OFdurable, and chemically inert and generally occupy 60AGGREGATESto 75 percent of the concrete mix in volumc (70 to 85percent by weight). Natural aggregate deposits areAggregates containing particles of different sizesexcavated from pits, rivers, lakes, or seabeds. Thesehave a natural tendency to segregate whenever loaded,natural deposits consist of gravel and sand that can betransported, or otherwise disturbed. Aggregates shouldreadily used in concrete after minimal processing.always be handled and stored by a method thatCrushed aggregates are produced by crushing quaryminimizes segregation.rock, boulders, cobbles, or large-size gravel. CrushedStockpiles should not be built up in cone shapes,aggregates are usually washed and graded before beingformed by dropping successive loads at the same spot.used in concrete. The most commonly used aggregatesThis process causes larger aggregate particles toare sand and gravel and when combined with cementproduce a strong, durable mass that is practicallysegregate and roll down the sides, leaving the pile withwithout voids.a large amount of fine aggregate at the top and a largeamount of coarse aggregate at the bottom. A stockpileThe coarse aggregates used in a mix usually consistshould be built up in layers, each made by dumpingof gravel or crushed stone up to 1 1/2 inches in size.successive loads alongside each other.Course aggregates are primarily used as filler. TheseIf aggregate is dropped in a free fall from aaggregates can pass through a 3-inch sieve and areretained on a No. 4 sieve. In massive structures likeclamshell, loader, or a conveyor, some of the finedams, the coarse aggregates may include natural stonesmaterial may be blown aside, causing segregation offines on the lee side of the pile. Clamshells, loaders, andor rocks, ranging up to 6 inches or more in size.conveyors should be discharged in contact with theFine aggregates are those materials that can passpile.through a No. 4 sieve but are retained on a No. 100 sieve.The fine aggregates and sand in concrete are used to fillthe voids between the large aggregates. Care should betaken to prevent dirt and othcr debris from getting mixedinto the sand. The foreign material affects the bondingquality of the sand.The gradation of the aggregate is a major factor inthe workability, water requirements, and strength ofconcrete. Fine and coarse aggregates are usually sievedThe bottom of an overhead charging bin shouldalways slope at least 50 degrees towards the centeroutlet. If the slope is less than 50 degrees, segregationwill occur as the material is discharged. When a bin isbeing charged, the material should be dropped from apoint directly over the outlet. Material dropped in at anangle or discharged against the sides of the bin willsegregate. Since a long drop causes both segregation andthe breakage of aggregate particles, the length of a drop7-2
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