Introduction.
The properties to be considered while selecting aggregate for concrete are strength, particle shape, specific gravity, bulk density, voids, porosity, moisture content and bulking.
1. Strength.
The strength should be at least equal to that of the concrete. Rocks commonly used as aggregates have a compressive strength much higher than the usual range of concrete strength. The test conducted for
strength evaluation are crushing test, impact-test and ten per cent fines test. Of these the first one is the most reliable. Generally the specifications prescribe 45 per cent for aggregate used for concrete other than wearing surface and 30 per cent for concrete for wearing surfaces, such as runways, roads etc. limit for the crushing value.
The toughness of aggregate is measured by impact test. The impact value should not exceed 30 per cent for wearing surface and 45 per cent for remaining concretes. Hardness of aggregate is tested by abrasion test.
The abrasion value is restricted to 30 per cent for wearing surfaces and 50 per cent for concrete for other purposes.
modulus of elasticity of concrete is approximately equal to the weighted average of the moduli of the cement paste and the aggregate, as such the modulus of the coarse aggregate has an important influence on the stiffness of concrete.
A high value reduces the dimensional changes due to creep and shrinkage of cement paste, but at the cost of higher internal stresses.
In concrete that is to be subjected to wide variations of temperature and humidity, internal cracking is reduced by the use of a more compressible aggregate, but in practice this effect is rarely of sufficient importance to determine the choice of aggregate.
3. Bond strength.
Due to difference between the coefficients of thermal expansion of paste and aggregate and to the shrinkage of cement paste during hardening, concrete is in a state of internal stress even if no external forces are present. It is reported that the stresses are likely to be greatest at the paste-aggregate interfaces where minute cracks exist, even in concrete that has never been loaded.
Under increasing external load, these cracks spread along the interfaces before extending into the paste or aggregate particles. The strength of the bond between aggregate and cement paste thus has an important influence on the strength of concrete. There is no standard test for bond but it is known that the rougher the surface texture of the particles, the better the bond.
The role of particle shape is less well understood; the greater specific surface of angular particles should enable greater adhesive force to be developed, but the angular shape probably causes more severe concentrations of internal stress.
4.Shape and texture.
The shape influences the properties of fresh concrete more than when it has hardened. Rounded aggregate are highly workable but yield low strength concrete. Same is the case with irregular shaped aggregate.
Flaky aggregate require more cement paste, produce maximum voids and are not desirable. Angular shape is the best. Crushed and uncrushed aggregates generally give essentially the same strength for the same cement content.
The shape and surface texure of fine aggregate govern its void ratio and significantly affect the water requirement.
5. Specific gravity.
The specific gravity of most of the natural aggregates lies between 2.6-2.7.
The specific gravity and porosity of aggregates greatly influence the strength and absorption of concrete. Specific gravity of aggregates generally is indicative of its quality.
A low specific gravity may indicate high porosity and therefore poor durability and low strength. The concrete density will greatly depend on specific gravity.
6. Bulk density .
The bulk density of aggregate depends upon their packing, the particles shape and size, the grading and the moisture content. For coarse aggregate a higher bulk density is an indication of fewer voids to be filled by sand and cement.
7. Void ratio.
The void ratio is calculated as
Void ratio =1-(bulk density/apparent specific gravity. )
If the void in the concrete is more the strength will be less.
8. Porosity
The entrapped air bubbles in the rocks during their formation lead to minute holes or cavities known as pores. The porosity of rocks is generally less than 20 per cent; the concrete becomes permeable and ultimately affects the bond between aggregate and cement paste, resistance to freezing and thawing of concrete and resistance to abrasion of aggregate. The porous aggregate absorb more moisture, resulting in loss of workability of concrete at a much faster rate.
9. Moisture Content.
The surface moisture expressed as a percentage of the weight of the saturated surface dry aggregate is known as moisture content. A high moisture content increases the effective water/cement ratio to an appreciable extent and may render the concrete weak.
10. Bulking.
The increase in the volume of a given mass of fine aggregate caused by the presence of water is known as bulking. The water forms a film over the fine aggregate particles, exerts force of surface tension and pushes them apart increasing the volume.
The extent of bulking depends upon the percentage of moisture present in the sand and its fineness. With ordinary sand bulking varies from 15-30 percent. It increases with moisture content up to a certain point (4-6%), reaches maximum, the film of water on the sand surface breaks, and then it starts decreasing.
In preparing concrete mixes if sand is measured by volume and no allowance is made for bulking, the moist sand will occupy considerably larger volume than that prepared by the dry sand and consequently the mix will be richer. This will cause, less quantity of concrete per bag of cement.
For example, if the bulking of sand is 10% and if mix ratio is 1:2:4, the actual volume of sand used will be 1/1.1 × 2 =1.82 instead of 2 per unit volume of cement. The mixproportion then would be 1:1.82:4 rather than 1: 2: 4. Which indicates lesser production ofconcrete.
Also, there will be chances of segregation, honeycombing and reduced yield ofconcrete.
Bulking of sand can be determined, in field, by filling a container of known volume (A) withdamp sand in the manner in which the mixer hopper will be filled.
The height of sand in thecontainer is measured. The sand is then taken out of container carefully, ensuring no sand islost during this transaction.
The sand is then either dried and filled back into the gauge box, orthe container is filled with water and the damp sand is poured in to displace the water.Whichever method is adopted, the new depth of aggregate in the container gives the unbulkedvolume (B).Then percentage bulking expressed as a percentage of the dry volume =
A-B/B=100
11. Fineness Modulus.
It is a numerical index of fineness, giving some idea about the mean sizeof the particles in the aggregates.
The fineness modulus (F.M.) varies between 2.0 and 3.5 forfine aggregate, between 5.5 and 8.0 for coarse aggregate, and from 3.5 to 6.5 for all-in aggregate.Aggregate, whose F.M. is required, is placed on a standard set of sieves (80, 63, 40, 20, 12.5, 10,4.75, 2.36, 1.18 mm and 600, 300, 150 m) and the set vibrated.
The material retained on eachsieve after sieving represent the fraction of aggregate coarser than the sieve in question butfiner than the sieve above. The sum of the cumulative percentages retained on the sievesdivided by 100 gives the F.M. A fineness modulus of 3.0 can be interpreted to mean that thethird sieve i.e., 600 m is the average size.
The test procedure is given IS: 2386 (Part I).The object of finding F.M. is to grade the given aggregate for the required strength andworkability of concrete mix with minimum cement.
Higher F.M. aggregate result in harshconcrete mixes and lower F.M. result in uneconomical concrete mixes.
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