Main sources of these aggregate are natural deposits ofsand ,and gravel or obtained from quarries by cuttingrocks .the rocks are generally of three types i.e.Igneous,Sedimentsary and Metamorphic rocks. Theaggregates obtained from Igneous rocks match therequirements to a great extent. Also sedimentary rocksprovide good quality of aggregates but metamorphicrocks are not used for aggregates because of formationof thin structure which are not desirable.
These aggregates are generallyobtained from natural aggregatesby performing certain chemicalreactions , applying force,heating the natural aggregatesetc. For example -broken bricks,fly ash, blast- furnace slag etc.
Fine aggregates are those aggregates whichcan pass through 4.75mm IS-sieve. Forexample – Silts, clay, Loam etc. Fineaggregates are natural sand , crushed stonesand and crushed gravel stone . Fineaggregates contribute towards reducing thenumber of voids, increases the workability,increases the volume, reduces the cost andproper density is provided.
These are the aggregates which are theresidue of the 4.75 mm IS sieve andpassed through 75 mm IS sieve .Coarseconsists of crushed gravel, uncrushedgravel and partially crushed gravel. Theseaggregates are formed by naturaldisintegration of rocks or by artificialcrushing of rocks or gravel.
All-in-aggregates are combinemixture of fine aggregates andcoarse aggregates .It containsvarious fractions of fine andcoarse aggregates. For Highquality concrete these type ofaggregates are used.
In rounded aggregates less amount ofcement is required for preparing concrete of good quality. There are 33% to 35% voids in rounded aggregates .Due to smooth and rounded structure there is poor locking among or between theaggregates and thus reducing strength of structure but they provide high workability.
These aggregates vary in surface as comparedto rounded aggregates. They are not perfect ally round . They have 35 to 37%of voids. Because of more surface area they required more cemented paste. The interlocking provided by these aggregate is not up to the required level but better than rounded aggregates.
These aggregates have angular structure i.e. having shape and well defined edges, and arerough particles. The voids percentage lies in the range of 37 to 40 %.Due to rough and angularstructure the interlocking is quite good and also required more cement paste for a workable concrete with high strength. Crushed is an example of angular aggregate.
The flaky aggregates are those in which theleast dimension of the aggregates is less than 3/5 the times the mean dimensions. Mean dimensions the average size of sieves fromwhich the aggregates passes or on which they are retained. The percentage of voids are high for these aggregates. Elongated can be define as those aggregates which has its length or greatest dimension , greater than 9/5 of its mean dimension .
• Normal weight aggregates(Sands ,gravel , crushes rocks etc.)•Heavy weight aggregates(Magnetite, scrap iron etc.)•Light weight aggregates(naturally manufactured )
Specific GravityAbsolute: the ratio of the weight of the solid to the weight of an equal volume of water (both at a stated temperature) • refers to volume of the material excluding all poresApparent: ratio of the weight of the aggregate (dried in an oven at 212- 230ºF for 24 hours) to the weight of water occupying a volume equal to that of the solid including the impermeable pores • volume of solid includes impermeable pores (but not capillary pores) Used for calculating yield of concrete or the quantity of aggregate required for a given volume of concrete.
Particle Shape and Surface Texture • Rough textured, angular, elongated particles require more water to produce workable concrete than do smooth, rounded, compact aggregates. • Aggregates should be relatively free of flat and elongated particles (limit to 15% by weight of total aggregate). • Important for coarse and crushed fine aggregate - these require an increase inmixing water and may affectthe strength of the concrete, if cement water ratio is not maintained.
Shrinkage of Aggregates:Large Shrinkage = fine grained sandstones, slate, basalt, trap rock, clay- containingLow Shrinkage = quartz, limestone, granite, feldspar
• Flakiness test• Abrasion test• Crushing test• Impact test