4Aggregates are inert materialswhich are mixed with bindingmaterial such as cement or limefor manufacturing of mortar orconcrete. Aggregates are usedas filler in mortar and concreteand also to reduce their cost.
Depending upon the size oftheir particles aggregates areclassified as:(1) Fine aggregates(2) Coarse aggregates
7(1) Fine Aggregates:Aggregates whose particles passthrough 4.75 mm IS sieve are termedas fine aggregates. Most commonlyused fine aggregates are sand (pit orquarry sand, river sand and sea sand)and crushed stone in powdered form,how ever some times sukhi and ash orcinder are also used.
8(a) Sand:It consists of small angular orrounded grains of silica dependingupon the source from which it isobtained. It is classified as:
9It is found as deposited in soil and is to beexcavated out. Its grains are generallysharp or angular. It should be free fromorganic matter and clay. It is usuallyconsidered to be the best fine aggregatefor use in mortar and concrete.(i) Pit or quarry sand:
10(ii) River Sand:It is obtained from the banks andbeds of rivers. It may be fine orcoarse. Fine sand obtained frombeds and banks of rivers is oftenfound mixed with silt and clay so itshould be washed before use. Butcoarse sand is generally clean andexcellent for use especially forplastering.
11(iii) Sea Sand:It consists of fine rounded grains ofbrown colour and it is collected fromsea shores or sea beaches. Sea sandusually contains salts and while usingthat in mortar, etc, causesdisintegration of the work in which it isused. In R.C.C work these salts willattack reinforcement if salt content ishigh. These salts may causeefflorescence. It should be usedlocally after thorough washing.
12(b) Crushed stone:It is obtained by crushing the wastestones of quarries to the particularsize of sand. Sand obtained from bycrushing a good quality stone isexcellent fine aggregate.Mortar made with this sand is usuallyused in ashlar work (good quality ofwork).
13Aggregates whose particles donot pass through 4.75 mm IS aretermed as coarse aggregates.Most commonly used coarseaggregates are crushed stone,gravel; broken pieces of burntbricks,etc.(2) Coarse Aggregates:
14It is an excellent coarse aggregateand is obtained by crushing granite,sand stone or grained lime stoneand all types of stones. Crushedstones are used for the constructionof roads and railway tracks, etc.(a) Crushed stone:
15(b) Gravel:It is an other very good coarseaggregate. It is obtained fromriver beds, quarries and seashores. The gravel obtained fromsea shores should be wellwashed with fresh water beforeuse in order to remove theimpurities which may be clay,salts ,silt,etc. It is commonly usedin the preparation of concrete.
16(c) Broken pieces of bricks:It is also a good artificial source of coarseaggregates. It is obtained by breaking wellburnt bricks. It is generally used in limeconcrete at places where aggregates fromnatural sources are either not available orare expensive. It can be used at placeswhere low strength is required. It should bewatered well before using it in thepreparation of concrete. It is commonlyused for mass concrete in foundations andunder floors.
17GOOD QUALITYIES OF AN IDEALAGGREGATE:An ideal aggregate used for themanufacturing of concrete andmortar, should meet the followingrequirements.(1) It should consist of natural stones,gravels and sand or in variouscombinations of these materials.(2) It should be hard, strong anddurable.
18(3) It should be dense, clear andfree from any coating.(4) It should be free from injuriousvegetable matters.(5) It should not contain flaky(angular) and elongated pieces.(6) It should not contain anymaterial liable to attack steelreinforcement in case of reinforcedconcrete.
19Important characteristics ofaggregates which influence theproperties of resulting concrete mixare discussed as under:CHARACTERISTICS OFAGGREGATES:
20Aggregate containing theconstituents which generally reactwith alkalies in cement causeexcessive expansion, cracking ofconcrete mix, should never be used.Suitability of aggregates should bejudged either by studying its servicehistory or by laboratory tests.1.Composition:
21The size and shape of theaggregate particles mainly influencethe quantity of cement required in aconcrete mix and ultimatelyeconomy of the concrete. For thepreparation of economical concrete,one should use largest coarseaggregates feasible for thestructure.2. Size and shape:
22Type of structure Max. size of aggregate1. Mass concrete work 40 mmi.e. dams, retaining walls,piers and abutments, etc.2. R.C.C work 20 mmi.e. beams, columns, etc3. Flooring 10 mmIt may be clearly noted that the size andshape of the aggregate particles influence theproperties of freshly mixed concrete more ascompared to those of hardened concrete.
23There are so many tests which areto be performed to check the qualityof aggregates but some importanttests are discussed below.QUALITY TESTS OFAGGREGATES:
24The aggregate crushing value givesa relative measure of resistance ofan aggregate to crushing under agradually applied compressive load.The aggregate crushing strengthvalue is useful factor to know thebehaviour of aggregates whensubjected to wear.1. Crushing Test of Aggregate:
25The aggregate impact value gives arelative measure of the resistanceof an aggregate to sudden shock orimpact. The impact value is sometimes used as an alternative to itscrushing value.2. Impact Value Test:
26The aggregate abrasionvalue gives a relativemeasure of resistance of anaggregate to wear when it isrotated in a cylinder alongwith some abrasive charge.3. Abrasion Value:
27In determination of the proportionsof the particles with in certainranges in an aggregate byseparation on various sieves ofdifferent size openings, may bedefined as sieve analysis.SIEVE ANALYSIS:
28FINENESS MODULUS (F.M):The sum of cumulative percentage ofresidues retained on each of the Indianstandard sieves(80mm,40mm,20mm,10mm,4.75mm,2.36mm,1.18mm,600 microns,300microns and150 microns each succeeding sieve hashalf the aperture of the previous one),divided by the 100,is known as “Finenessmodulus” of the aggregates. The finenessmodulus of an aggregate is roughlyproportional to the average size of particlesof the aggregates.
29(OR)Index Number expressing the relativesizes of both coarse and fine aggregatesis called “Fineness Modulus”.Sand Fineness ModulusFine 2.2 to 2.6Medium 2.6 to 2.9Coarse 2.9 to 3.2Note:It is recommended that the finenessmodulus of sand should not be less than2.5 and should not be more than 3.0