Aggregates for Concrete

1 The importance of using the right type and quality ofaggregates cannot be overemphasized. The fine andcoarse Aggregates generally occupy 60% to 75% of theconcrete volume (70% to 85% by mass) and strongly influ-ence the Concrete s freshly mixed and hardened proper-ties, mixture proportions, and economy. fine Aggregates (Fig. 5-1) generally consist of natural sand or crushedstone with most particles smaller than 5 mm ( in.). coarse Aggregates (Fig. 5-2) consist of one or a com-CHAPTER 5 Aggregates for Concretebination of gravels or crushed stone with particlespredominantly larger than 5 mm ( in.)

2 And generallybetween mm and mm (3 8in. and 11 2in.). Somenatural aggregate deposits, called pit-run gravel, consistof gravel and sand that can be readily used in concreteafter minimal processing. Natural gravel and sand areusually dug or dredged from a pit, river, lake, or stone is produced by crushing quarry rock, boul-ders, cobbles, or large-size gravel. Crushed air-cooledblast-furnace slag is also used as fine or coarse Aggregates are usually washed and graded at thepit or plant.

3 Some variation in the type, quality, cleanli-ness, grading, moisture content, and other properties isexpected. Close to half of the coarse Aggregates used inportland cement Concrete in North America are gravels;most of the remainder are crushed occurring Concrete Aggregates are a mixtureof rocks and minerals (see Table 5-1). A mineral is a natu-rally occurring solid substance with an orderly internalstructure and a chemical composition that ranges withinnarrow limits.

4 Rocks, which are classified as igneous, sedi-mentary, or metamorphic, depending on origin, are gener-ally composed of several minerals. For example, granitecontains quartz, feldspar, mica, and a few other minerals;most limestones consist of calcite, dolomite, and minoramounts of quartz, feldspar, and clay. Weathering anderosion of rocks produce particles of stone, gravel, sand,silt, and Concrete , or crushed waste Concrete , is afeasible source of Aggregates and an economic reality,especially where good Aggregates are scarce.

5 Conven-tional stone crushing equipment can be used, and newequipment has been developed to reduce noise and must conform to certain standards foroptimum engineering use: they must be clean, hard,strong, durable particles free of absorbed chemicals, coat-ings of clay, and other fine materials in amounts that couldaffect hydration and bond of the cement paste. Aggregateparticles that are friable or capable of being split are unde-sirable. Aggregates containing any appreciable amountsof shale or other shaly rocks, soft and porous materials,Fig.

6 5-1. Closeup of fine aggregate (sand). (69792)Fig. 5-2. coarse aggregate . Rounded gravel (left) andcrushed stone (right). (69791)79 HOMEPAGE should be avoided; certain types of chert should be espe-cially avoided since they have low resistance to weather-ing and can cause surface defects such as of the constituents of an aggregatecannot alone provide a basis for predicting the behavior ofaggregates in service. Visual inspection will often discloseweaknesses in coarse Aggregates . Service records areinvaluable in evaluating Aggregates .

7 In the absence of aperformance record, the Aggregates should be testedbefore they are used in Concrete . The most commonly usedaggregates sand, gravel, crushed stone, and air-cooledblast-furnace slag produce freshly mixed normal-weightconcrete with a density (unit weight) of 2200 to 2400kg/m3(140 to 150 lb/ft3). Aggregates of expanded shale,clay, slate, and slag (Fig. 5-3) are used to produce struc-tural lightweight Concrete with a freshly mixed densityranging from about 1350 to 1850 kg/m3(90 to 120 lb/ft3).

8 Other lightweight materials such as pumice, scoria,perlite, vermiculite, and diatomite are used to produceinsulating lightweight concretes ranging in density fromabout 250 to 1450 kg/m3(15 to 90 lb/ft3). Heavyweightmaterials such as barite, limonite, magnetite, ilmenite,hematite, iron, and steel punchings or shot are used toproduce heavyweight Concrete and radiation-shieldingconcrete (ASTM C 637 and C 638). Only normal-weightaggregates are discussed in this chapter.

9 See Chapter 18for special types of Aggregates and Aggregates should meet the require-ments of ASTM C 33 or AASHTO M 6/M 80. These speci-fications limit the permissible amounts of deleterioussubstances and provide requirements for aggregate char-acteristics. Compliance is determined by using one ormore of the several standard tests cited in the followingsections and tables. However, the fact that aggregatessatisfy ASTM C 33 or AASHTO M 6/M 80 requirementsdoes not necessarily assure defect-free adequate consolidation of Concrete , the desirableamount of air, water, cement, and fine aggregate (that is,the mortar fraction) should be about 50% to 65% byabsolute volume (45% to 60% by mass).

10 Rounded aggre-gate, such as gravel, requires slightly lower values, whilecrushed aggregate requires slightly higher values. Fineaggregate content is usually 35% to 45% by mass orvolume of the total aggregate OF AGGREGATESThe important characteristics of Aggregates for Concrete arelisted in Table 5-2 and most are discussed in the followingsection:GradingGrading is the particle-size distribution of an aggregate asdetermined by a sieve analysis (ASTM C 136 or AASHTO80 Design and Control of Concrete Mixtures EB001 Table 5-1.)