Several mechanical properties of aggregate are of interest for the manufacture of concrete, specially high strength concrete subjected to high wear. Some of them are discussed here in brief: 1. Toughness 2. Hardness 3. Specific Gravity 4. Porosity of Aggregate 5. Bulking of Sand.
Property # 1. Toughness:
It is defined as the resistance of aggregate to failure by impact. The impact value of bulk aggregate can be determined as per I.S. 2386, 1963.
The test procedure is as follows:
The aggregate shall be taken as in the case of crushing strength value test i.e., the aggregate should pass through 12.5 mm I.S. sieve and retained on 10 mm I.S. sieve. It should be oven dried at 100°C to 110°C for four hours and then air cooled before test.
Now the prepared aggregate is filled upto 1/3rd height of the cylindrical cup of the equipment. The diameter and depth of the cup are 102 mm and 50 mm respectively. After filling the cup upto 1/3rd of its height, the aggregate is tamped with 25 strokes of the rounded end of the tamping rod.
After this operation the cup shall be further filled upto 2/3rd of its height and a further tamping of 25 strokes given. The cup finally shall be filled to over flowing and tamped with 25 strokes and surplus aggregate removed and the weight of aggregate noted. The value of weight will be useful to repeat the experiment.
Now the hammer of the equipment weighting 14.0 kg or 13.5 kg is raised till its lower face is 380 mm above the upper surface of the aggregate and., allowed to fall freely on the aggregate and the process is repeated for 15 times.
The crushed aggregate is now removed from the cup and sieved through 2.36 mm I.S. sieve. The fraction passing through the sieve is weighed accurately.
Let the weight of oven dry sample in the cup = W kg.
Weight of aggregate passing 2.36 mm sieve = W1 kg.
Then impact value = [(W1/W) x 100]
This value should not be more 30% for aggregate to be used in concrete for wearing surfaces as road and 45% for other type of concrete.
Property # 2. Hardness:
It is defined as the resistance to wear by abrasion, and the aggregate abrasion value is defined as the percentage loss in weight on abrasion.
For testing hardness of aggregate following three methods can be used:
(a) Deval Attrition test.
(b) Dorry abrasion test.
(c) Los Angeles test.
(a) Deval Attrition Test:
This test has been covered by IS 2386 Part (IV)-1963. In this test particles of known weight are subjected to wear in an iron cylinder rotated 10,000 (ten thousand) times at the rate of 30 to 33 revolutions per minute. After the specified revolution of the cylinder the material is taken out and sieved on 1.7 mm sieve and the percentage of material finer than 1.7mm is determined. This percentage is taken as the attrition value of the aggregate. The attrition value of about 7 to 8 usually is considered as permissible.
(b) Dorry Abrasion Test:
This test has not been covered by Indian standard specifications. In this test a cylindrical specimen having its diameter and height of 25 cm is subjected to abrasion against a rotating metal disk sprinkled with quartz sand. The loss in weight of the cylinder after 1000 (one thousand) revolutions is determined.
Then the hardness of rock sample is expressed by an empirical relation as follows:
Hardness or sample = 20 – Loss in weight in grams/3
For good rock this value should not be less the 17. The rock having this value of 14 is considered poor.
(c) Los-Angeles Test:
This test has been covered by IS 2386 (Part-IV) 1963. In this test aggregate of the specified grading is placed in a cylindrical drum of inside length and diameter of 500 mm and 700 mm respectively. This cylinder is mounted horizontally on stub shafts. For abrasive charge, steel balls or cast iron balls of approximately 48 mm diameter and each weighting 390 grams to 445 gram are used. The numbers of balls used vary from 6 to 12 depending upon the grading of the aggregate. For 10 mm size aggregate 6 balls are used and for aggregates bigger than 20 mm size usually 12 balls are used.
For the conduct of test, the sample and the abrasive charge are placed in the Los-Angeles testing machine and it is rotated at a speed of 20 to 33 revolutions per minute. For aggregates upto 40 mm size the machine is rotated for 500 revolutions and for bigger size aggregate 1000 revolutions. The charge is taken out from the machine and sieved on 1.7 mm sieve.
Let the weight of oven dry sample put in the drum = W Kg.
Weight of aggregate passing through 1.7 sieve = W1 Kg.
Then abrasion value = [(W1/W) x 100]
The abrasion value should not be more than 30% for wearing surfaces and not more than 50% for concrete used for other than wearing surface. The results of Los Angeles test show good correlation not only the actual wear of aggregate when used in concrete, but also with the compression and flexural strength of concrete made with the given aggregate.
Table 4.8 gives an idea of toughness, hardness, crushing strength etc. of different rocks.
Property # 3. Specific Gravity:
The specific gravity of a substance is the ratio of the weight of unit volume of the substance to the unit volume of water at the stated temp. In concrete making, aggregates generally contain pores both permeable and impermeable hence the term specific gravity has to be defined carefully. Actually there are several types of specific gravity. In concrete technology specific gravity is used for the calculation of quantities of ingredients. Usually the specific gravity of most aggregates varies between 2.6 and 2.8.
Specific gravity of certain materials as per concrete hand book CA-1 Bombay may be assumed as shown in Table 4.9.
Method of Determination of Specific Gravity of Aggregate:
IS-2386-Part-III-1963 describes various procedures to find out the specific gravity of aggregates of different sizes. Here the method applicable to aggregates larger than 10 mm in size has been described as follows ―
A sample of aggregate not less than 2 kg in weight is taken and washed thoroughly to remove dust, and silt particles etc. The washed sample is placed in a wire basket and immersed in distilled water at a temperature of 27 ± 5°C.
Immediately after immersion, the entrapped air is removed from the sample by lifting the basket containing sample 25 mm above the bottom of the jar or tank and allow it drop 25 times at the rate of 1 mm per sec. During this operation, care should be taken that basket and aggregate remain fully immersed in water. After this, the sample is kept in water for about 24 ± ½ hour.
After this period the basket and aggregate is given a jerk to remove the air etc. and weighed in water at the temperature of 27 ± 5°C. Let the weight of basket and aggregate be A1. The basket and sample of aggregate is removed from the water and allowed to drain for a few minutes. Then the aggregate is taken out from the basket and placed on a dry cloth and dried further. The empty basket is again immersed in water and weighed in water after giving 25 jolts. Let this weight be A2.
The aggregate is surface dried in shade for not more than 10 minutes and the aggregate is weighed in air. Let this weight be B. Now the aggregate is oven dried for 24 ± ½ hour at a temperature of 100 to 110°C. It is then cooled in air tight container and weighed. Let this weight be C.
Weight of sample in water = (A1 – A2) = A
Weight of saturated surface dry in air sample = B
Weight of oven dry sample = C
(a) Then specific gravity = [C/(B – A)]
(b) Apparent specific gravity = [C/(C – A)]
(c) Water absorption = 100 (B – C)
(d) Bulk density = Net weight of the aggregate in kg./capacity or the container in litres
Find the value of- (i) Specific gravity, (ii) Apparent specific gravity, (iii) Apparent particle density, (iv) Bulk particle density.
(i) Mass of oven dry sample C = 480 gram
(ii) Mass of saturated surface dried sample in air B = 490 gram
(iii) Weight of vessel with water = 1400 gram
(iv) Weight of vessel + water + sample = 1695 gram.
(i) Specific gravity = [mass of oven dry sample/(mass or saturated surface sample – sample weight in water)]
= [C/(B – A)] = [480/(490-295)]
= 480/195 = 2.50
(ii) Apparent specific gravity = [C/(C – A)] = [480/(480 – 295)]
= 480/185 = 2.59
(iii) Apparent particle density = 1000 x Apparent specific gravity = 2.59 x 1000
= 2590 kg/m3
(iv) Bulk Particle density = Bulk specific gravity x 1000
= 2.59 x 1000 = 2500 kg/m3
Absolute Specific Gravity:
It can be defined as the ratio of the weight of the solid, referred to vacuum, to the weight of an equal volume of gas free distilled water both taken at the standard or a stated temperature, usually it is not required in concrete technology. Actually the absolute specific gravity and particle density refer to the volume of solid material excluding all pores, while apparent specific gravity and apparent particle density refer to the value of solid material including impermeable pores, but not the capillary pores. In concrete technology apparent specific gravity is required.
Apparent Specific Gravity:
It can be defined as the ratio of the weight of the aggregate dried in an oven at 100°C to 110°C for 24 hours to the weight of water occupying a volume equal to that of the solid including the impermeable pores. This can be determined by using pycno-meter for solids less than 10 mm in size i.e., sand.
Bulk Specific Gravity:
It can be defined as the ratio of the weight in air of a given volume of material (including both permeable and impermeable voids) at the standard temperature to the weight in air of an equal volume of distilled water at the same standard temperature (20°C). The specific gravity of a material multiplied by the unit weight of water gives the weight of 1 cubic metre of that substance. Some times this weight is known as solid unit weight. The weight of a given quantity of particles divided by the solid unit weight gives the solid volume of the particles.
Solid vol. in m3 = 3 wt. of substance in kg/specific gravity x 1000
The weight of aggregate that would fill a container of unit volume is known as bulk density of aggregate. Its value for different materials as per concrete hand book CIA Bombay is shown in Table 4.10.
With respect to a mass of aggregate, the term voids refers to the space between the aggregate particles. Numerically this voids space is the difference between the gross volume of aggregate mass and the space occupied by the particles alone. The knowledge of voids of coarse and fine aggregate is useful in the mix design of concrete.
Percentage voids = [(Gs – g)/Gs] x 100
where Gs = specific gravity of aggregate and g is bulk density in kg/litre.
The weight of a unit volume of aggregate is called as unit weight. For a given specific gravity, greater the unit weight, the smaller the percentage of voids and better the gradation of the particles, which affects the strength of concrete to a great extent.
Property # 4. Porosity and Absorption of Water by Aggregate:
All aggregates, particles have pores with in their body. The characteristics of these pores are very important in the study of the properties of aggregate. The porosity, permeability, and absorption of aggregates influence the resistance of concrete to freezing and thawing, bond strength between aggregate and cement paste, resistance to abrasion of concrete etc.
The size of pores in the aggregate varies over a wide range, some being very large, which could be seen even with naked eye. The smallest pore of aggregate is generally larger than the gel pores in the cement paste, pores smaller than 4 microns are of special interest as they are believed to affect the durability of aggregates subjected to alternate freezing and thawing. Some of the pores are wholly within the body of the aggregate particles and some of them are open upto the surface of the particle.
The cement paste due to its viscosity cannot penetrate to a great depth into the pores except the largest of the aggregate pores. Therefore, for the purpose of calculating the aggregate content in concrete, the gross volume of the aggregate particles is considered solid. However water can enter these pores, the amount and rate of penetration depends upon the size, continuity and total volume of pores.
When all the pores in the aggregate are full with water, then the aggregate is said to be saturated and surface dry. If this aggregate is allowed to stand in the laboratory, some of the moisture will evaporate and the aggregate will be known as air dry aggregate. If aggregate is dried in oven and no moisture is left in it, then it is known as bone dry aggregate. Thus the ratio of the increase in weight to the dry weight of the sample, expressed as a percentage is known as absorption.
The knowledge of absorption of aggregate is important in adjusting water-cement ratio of the concrete. If water available in the aggregate is such that it contributes some water to the dilution of cement paste, in that case the water-cement ratio will be more than the required and the strength will go down.
On the other hand, if the aggregate is so dry that it will absorb some of the mixing water, in that case the mix will have lower water-cement ratio and the mix may become unworkable. Hence, while deciding the water-cement ratio, it is assumed that the aggregate is in saturated but surface dry condition, i.e. neither it will add water to cement paste, nor it will absorb water from the mix.
It has been observed that absorption of water by dry aggregate slows down due to the coating of particles with cement paste. The water absorption by aggregate should be determined for 10 to 30 minutes instead of total water absorption. The value of absorption of water may be taken as follows as recommended by concrete hand book CAI Bombay in Table 4.11.
While using aggregate in the concrete, water on the surface of the aggregate should be taken into account, as it will contribute to the water in the mix and will affect the water-cement ratio of the mix, causing lower strength of the concrete. It is difficult to measure surface water of the aggregate. Therefore its value may be assumed according to I.S. 456, 1964 given in Table 4.12.
Property # 5. Bulking of Sand:
The moisture present in fine aggregate causes increase in its volume, known as bulking of sand. The moisture in the fine aggregate develops a film of moisture around the particles of sand and due to surface tension pushes apart the sand particles, occupying greater volume. The bulking of the sand affects the mix proportion, if mix is designed by volume batching. Bulking results in smaller weight of sand occupying the fixed volume of the measuring box, and the mix becomes deficient in sand and the resulting concrete becomes honeycombed and its yield is also reduced.
The extent of bulking depends upon the percentage of moisture present in sand and its fineness. The increase in volume relative to that occupied by a saturated and surface dry sand increases with an increase in the moisture content of the sand upto a value of 5 to 8%, causing bulking ranging from 20 to 40% as shown in Table 4.13. Fig. 4.8 and Table 4.13 shows bulking of sand with various moisture contents as suggested by concrete hand book CAI, Bombay.
As the moisture content increases, the film of water formed around the sand particles merge and the water moves into the voids between the particles so that the total volume of sand decreases, till the sand is fully saturated. The volume of fully saturated sand is same as that of the dry sand for the same method of filling the container.
Determination of Bulking of Sand:
Since the volume of saturated sand is same as that of dry sand, the most convenient way of determining bulking of sand is by measuring the decrease in volume of the given sand on saturation. For the measurement of bulking of sand, usually a container of known volume, a 30 cm long steel rule, and a 6 mm iron rod is required.
Put sufficient quantity of sand loosely into the container, till it is about two-thirds full. Level off the top of the sand with steel rule, and push this rule at the middle of the surface to the bottom of the container and measure its height. Let the height be h cm.
Now empty this sand into another container. While emptying, care should be taken that no sand particles are lost. Take about 1/3rd to half-full the first container with water and add about half the sand to it and rod it with 6 mm diameter steel rod. The sand should be rodded till the air bubbles cease to come out. At this stage the volume of sand is minimum. At this stage add the remaining sand and rod it also till air bubbles cease to come out. Smooth and level the top surface of the saturated sand and measure its height by pushing the steel rule at the middle of the surface to the bottom of the container. Let this height be h1 cm.
Then % bulking = [(h1/h1) x 100]
Effect of Bulking of Sand:
For volume batching, bulking has to be allowed for by increasing the total volume of sand used, otherwise the mix will be deficient in sand and segregation of the mix may take place. Also the resulting concrete will be honeycombed and its yield will be reduced, raising its cost of production. The volume to be increased can be calculated either by knowing this percentage of bulking as shown above or by bulking factor.
Vm = vol. of moist sand
Vs = vol. of saturated sand
then bulking = [(Vm – Vs)/Vs]
and bulking factor = 1 + [(Vm – Vs)/Vs] = Vm/Vs
Hence to know the total volume of sand to be used can be calculated by multiplying the vol. Vs by the bulking factor. The value of bulking factor can be determined by the curves of Fig. 4.9. Fig. 4.9 gives bulking factor against moisture content upto 20% for three types of sands.