In addition to the petro-logical (rock structure etc.) character of aggregate, its external characteristics also are of importance which have marked influence on the properties of concrete.
Following characteristics are of importance:
1. Shape and texture of particle.
2. Crushing strength of aggregate.
1. Shape of Particles:
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The shape of the particles depends upon the source of aggregate.
The different shapes of particles are discussed below:
i. Rounded ― Fully water worn or completely shaped by attrition. Example ― River or sea shore gravel, desert, sea shore and windblown sand.
ii. Irregular ― Naturally irregular, or partly shaped by attrition and having rounded edges. Example― Pit sand and gravel or drug flints, cuboid rocks.
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iii. Angular ― Possessing well defined edges formed at the intersection of roughly plane faces. Example ― Crushed rocks of all types, talus crushed slag etc.
iv. Flaky ― Materials of which the thickness is small relative to other two dimensions usually they are angular. Example ― Laminated rocks as slate.
v. Elongated ― Material usually angular in which the length is considerably larger than other two dimensions.
vi. Flaky and Elongated ― Material having the length considerably larger than the width and the width considerably larger than the thickness.
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Angularity:
It can be expressed qualitatively by a figure called angularity number as suggested by F.A.
Shergold:
It is based on the percentage voids in the aggregate after compaction as per IS 2386-1963 part-1. This test gives a value called as angularity number.
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The percentage voids are calculated as follows:
A single sized aggregate is filled in a metallic cylinder of three litre capacity. The aggregate is compacted in a standard manner and percentage voids calculated.
The voids can be found out by filling water in the cylinder till the water level reaches upto the brim or top edge of the cylinder or by knowing the specific gravity and bulk density of the aggregate. If the calculated value of voids is 33%, it is assumed that angularity is zero and solid volume of aggregate is 67% and the angularity is zero. If the voids are 44% then it is assumed that angularity number is 11 and the solid volume of aggregate is 56%.
An aggregate having angularity number between zero to 11 is considered suitable for making concrete. Zero angularity represents the most suitable rounded aggregate and angularity number 11 indicates the most angular aggregate that can be tolerated for making concrete not unduly harsh and uneconomical. Murdok has suggested the following relation for expressing the shape of the aggregate. This relation is called as angularity index fA.
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... Angularity index fA = 3 . fH/20 + 1.0 where fH is angularity number.
The shape of aggregate affects the workability of the concrete to a great extent, hence its strength and durability. Shape affects compressive strength by 22% and flexure strength by 30%.
Flaky Particle:
A particle is said to be flaky, if its least dimension (thickness) is less than 0.6 times the mean sieve size. The mean size is determined by passing and retained of particle on the sieve. If a particle passes through a 25 mm sieve and retained on a 20 mm sieve, then mean size is [(25 + 20)/2] = 22.5 mm.
Thus if the least dimension of any particle is 22.5 x 0.6 = 13.50 mm, then it is known as flaky particle. They orient in one particular plane forming water and air voids underneath. Thus flaky particles in the concrete should not be more than 10 to 15% of the weight of the coarse aggregate.
Elongated Particles:
If the largest dimension (length) of a particle is 1.8 times the mean sieve size, then this particle is called as elongated particle. In any concrete the quantity of flaky and elongated particles should not be much as they affect the workability of concrete.
As the surface area of such particles is greater, they need more water for wetting their surface, leaving smaller quantity of water for workability and produce porous concrete and thus the strength and durability of concrete is reduced. Therefore the quantity of such particles combined should not be more than 35% to 40% of the weight of coarse aggregate. Thus as per I.S. 2386 part I, the dimensions of the coarse aggregate particles should be as shown in Table 4.5.
Surface Texture:
The classification of surface texture is based on the degree to which the particle surfaces are polished or dull, smooth or rough etc. Surface texture depends on the hardness, grain-size, and pore characteristics of the parent rock. Visual estimation of roughness is quite reliable. Though there is no recognised method of measuring the surface roughness, but surface textures given below are good indications.
i. Glassy ― Conchoidal fracture (sea shell). Example ― Black flint vitreous slag.
ii. Smooth ― Water worn or smooth due to fracture of laminated or fine grained rock. Example ― Gravel, chert, slate, marble, and some rhyolites.
iii. Granular ― Fracture showing more or less uniform rounded grain. Example ― Sand stone, oolite.
iv. Rough ― Rough fracture of fine or medium grained rock containing no easily visible crystalline constituents. Example ― Basalt, felsite, lime stone.
v. Crystalline ― Containing easily visible crystalline constituents. Example ― Granite, gabbro, gneiss, dolerite, lime stone.
vi. Honey Combed ― With visibly pores and cavities. Example ― Brick, pumice foamed slag, clinker, expanded clay.
The shape and texture of aggregate influence the concrete strength considerably. The flexure strength is more affected than the compressive strength. These factors are more significant in the case of high strength concrete. Surface texture affects compressive strength by 44% and flexure strength by 26%.
Bond Strength of Aggregate:
The resistance developed to shear the aggregate particles from the hardened cement paste is called bond strength of aggregate. The bond between the cement paste and aggregate is an important factor in the strength of concrete, specially the flexural strength. Bond is partly due to the interlocking of the aggregate and the paste owing to the roughness of the surface of the aggregate particles. Thus rougher the surface as that of a crushed stone, better the bond. Secondly bond is also affected by other physical and chemical properties of aggregate.
The determination of bond strength of aggregate is difficult and no accepted test exists. It is judged by the surface of a crushed concrete. If in a crushed concrete specimen, larger numbers of aggregate particles are seen pulled out from their sockets and some of them seen fractured or broken right through, then the bond is called good. Greater number of crushed aggregate particle suggests weaker aggregate.
Usually an aggregate used in cement concrete is considered at least 10 times stronger than hardened cement paste. Hence the strength of concrete depends upon its bond strength. The bond strength depends upon the strength of cement paste and on the properties of aggregate surface as well. Bond strength is found to increase with the age of concrete.
2. Crushing Strength of Aggregate:
The compressive strength of concrete cannot exceed that of the aggregate used therein. Actually it is not easy to determine the crushing strength of individual particles. It is the bulk aggregate crushing strength which is determined. As stated above, usually aggregate is considered ten times stronger than crushing strength of concrete, but some particles break also and influence its strength. Therefore aggregate to be used in cement concrete should not be weaker than the strength of hardened cement paste.
Determination of Crushing Strength of Aggregate:
Material for the standard crushing test should pass through 12.5 mm I.S. sieve and retained on 10 mm I.S. sieve. The aggregate shall be thoroughly sieved on the above sieves before test. It should be surface dried at the time of test. If it is to be dried by heating, the period of drying should not be more than four hours. The aggregate should be heated to bone dry at 100°C to 110°C for four hours and then cooled to room temperature before use.
Apparatus:
(a) Apparatus used for standard test consists of a 15 cm diameter open ended steel cylinder of height 13.0 to 14 cm; with plunger and base plate.
(b) A 60 cm long steel rod of 1.6 cm diameter and rounded at one end.
Test Procedure:
The cylinder of the test apparatus is filled upto 1/3rd of its height with the prepared aggregate and tamped with 25 strokes by the tamping rod. In this way the next 1/3rd height of cylinder is filled and tamped again by 25 strokes with the tamping rod. Finally the cylinder is filled to its full height and subjected to 25 strokes by the tamping rod.
Now the plunger is inserted in such a way that it rests horizontally on this surface. Care is taken that plunger does not jam in the cylinder. The whole assembly is now placed in a compression testing machine. The load should be applied at a uniform rate in such a way that the total 40 tonne load is applied in 10 minutes or rate of loading should be 22.7 kg/cm2 per minute.
Now the load is released and the whole aggregate in the cylinder is sieved on a 2.26 mm I.S. sieve for the standard test and fraction passing through the sieve weighed. The ratio of the passing fraction should not be more than 45% for all types of concrete and 30% for wearing surfaces.
In case standard size aggregate i.e., aggregate particles passing 12.5 mm sieve and retained on 10 mm sieve is not available, then available aggregate upto 25 mm size can be tested. In this case material should be sieved on sieves shown in Table 4.7. For measuring the sample, cylinder of 11.5 cm diameter and 18 cm height may be used.
Details of aggregate crushing test for nonstandard sizes of aggregates are shown in Table 4.7.
The crushing strength test discussed above has been found insensitive to the variation in strength of weaker aggregates. Therefore when such aggregates are tested by this method, they do not give true picture, because they get crushed before the full load of 40 tonne has been applied and get compacted causing the plunger to jam. Thus the amount of crushing during later stages of the test is reduced. To overcome this difficulty a ten per cent fine value test has been developed. This test is a measure of resistance of an aggregate to crushing.
Resistance to Crushing:
For this test also standard crushing apparatus is used. In this case a load which could produce 10% fines from the standard aggregate i.e., aggregate passing 12.5 mm sieve and retained on 10 mm sieve is used. This is achieved by applying a progressively increasing load on the plunger so that this load could cause the following penetration of the plunger in 10 minutes.
They are:
(a) 15.0 mm for rounded or partially rounded aggregates.
(b) 20.0 mm for normal crushed aggregate.
(c) 24.0 mm for honeycombed aggregate.
After reaching the required maximum penetration, the load is released and the whole material in the cylinder is sieved on 2.36 mm I.S. sieve. The percentage of passing material through the sieve should be between 7.5% and 12.5%. If the above percentage is not met, the test is repeated. The value of load to give the 10% fines can be obtained as follows.
If y is the actual percentage of fines due to a maximum load of x tonnes, then load required to give 10% fines is given by the relation [14x/(y + 4)] tonnes.
The ten percent fine value test shows a fairly good correlation with the standard crushing value test for strong aggregates, while for weaker aggregates the ten percent fines value test is more sensitive and gives a truer picture of differences between more or less weak aggregates. For this reason, this test is usually used for light weight aggregate, but there is no simple relation between the test result and the upper limit of strength of concrete made with the given aggregate.