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Determination of CBR & Road Aggregates | Transportation Engineering - Civil Engineering (CE) PDF Download

Determination of CBR
The CBR test is conducted to determine the pavement thickness required on a given soil as there is a correlation between the CBR of soil and pavement thickness.

Lab Procedure
Determination of CBR & Road Aggregates | Transportation Engineering - Civil Engineering (CE)
           Fig: CBR Test Set up

 Specifications

1. Loading machine is a compression machine which operates at the rate of 1.25mm/minute. If such a machine is not available then a calibrated hydraulic pressure with proving ring to measure load can be used. The capacity of the proving ring is 1000kg. Its plunger has a diameter of 50mm.
2. The cylindrical mould has 150mm diameter and 175 mm height and a detachable perforated base.
3. Compaction rammer has 2.6kg weight and fall of 310mm.
4. Spacer disc has 14mm diameter and 47.7mm thickness.
5. Annular weight has 5kg weight and slotted weight has 2.5kg weight.

Procedure
1. Take 5kg of clean and dry soil and add the water equal to given moisture content. Mix it thoroughly either with hands or a towel and separate the sample into three parts.
2. Grease all the internal parts of the dase, cylindrical mould, collar and spacer disc. Keep the spacer disc on base so that its hole is at bottom. Then fix the cylindrical mould and collar to the stand.
3. Put the soil sample into the cylindrical mould in 3 layers compacting each layer 56 times with the light compaction hammer.
4. Remove the collar , cut off excess soil above the level of cylindrical mould with a sharp cutting edge.
5. Reverse the mould, take out the spacer disc. To simulate the field condition of the over lying pavement, keep the annular weight of 5kg as surcharge weight at the top of the sample.
6. Soak the specimen in water for 4 days.
7. After the soaking period, keep the mould with sample and weights on the machine.
8. Bring the plunger to be in contact with the surface of soil and apply a seating load of 4kg.
9. Set the proving ring to zero. Fix a dial gauge to measure the penetration and set it to zero.
10. Now allow the plunger to penetrate the specimen, record the proving ring readings for 0.5, 1.0, 1.5, ………..12.5mm penetration in the dial gauge.
11. Convert the proving ring reading into unit pressure from the given CBR chart (or) multiply the proving ring division by the P.R. Constant to get the load in kg. Divide this load by the area of the plunger in Sq.cm to get the unit pressure in kg/Sq.cm

CBR (in %) =Determination of CBR & Road Aggregates | Transportation Engineering - Civil Engineering (CE)


Determination of Modulus of Subgrade reaction of soil/Plate Load Test 
In this test, the strength of soil is determined in the form of modulus of subgrade reaction (K), which is extensively used in the design of rigid pavement. It is a field test.
Determination of CBR & Road Aggregates | Transportation Engineering - Civil Engineering (CE)
                 Figure: Plate load test

Procedure
1. Clean the ground surface at which the test is to be done.
2. Keep the standard test plate of 750mm size on the ground.
3. Above this keep some stacking plates on which keep the hydraulic jack.
4. At the top keep the reaction beam.
5. Apply a seating load of 0.75N/mm2 for a few minutes and release.
6. Now apply a load increment sufficient to cause a settlement of 0.75mm. Keep the load until the rate of settlement becomes less than 0.025mm/min.
7. Record the average settlement using a set of dial gauges placed on the stacking plates.
8. Now increase the load to cause a further settlement of 0.75mm and repeat the procedure.
9. Repeat the test for a few more loads.
10. Now plot the settlement values along X-axis and corresponding Bearing pressure values along Y-axis.
11. From this plot find the Bearing pressure corresponding to an average settlement of Δ=0.025cm (0.25mm).

Determination of CBR & Road Aggregates | Transportation Engineering - Civil Engineering (CE)

Road Aggregates
Overview 
Aggregate is a collective term for the mineral materials such as sand, gravel, and crushed stone that are used with a binding medium (such as water, bitumen, Portland cement, lime, etc.) to form compound materials (such as bituminous concrete and Portland cement concrete). By volume, aggregate generally accounts for 92 to 96 percent of Bituminous concrete and about 70 to 80 percent of Portland cement concrete. Aggregate is also used for base and sub-base courses for both flexible and rigid pavements. Aggregates can either be natural or manufactured.
Desirable properties of road aggregates
1. Strength
2. Hardness
3. Toughness
4. Durability
5. Shape of aggregates
6. Adhesion with bitumen
Strength The aggregates used in top layers are subjected to
(i) Stress action due to traffic wheel load,
(ii) Wear and tear,
(iii) crushing.

Hardness 
The aggregates used in the surface course are subjected to constant rubbing or abrasion due to moving traffic. The aggregates should be hard enough to resist the abrasive action caused by the movements of traffic. The abrasive action is severe when steel tyred vehicles moves over the aggregates exposed at the top surface.

Toughness
Resistance of the aggregates to impact is termed as toughness. Aggregates used in the pavement should be able to resist the effect caused by the jumping of the steel tyred wheels from one particle to another at different levels causes severe impact on the aggregates.

Shape of aggregates 
Aggregates which happen to fall in a particular size range may have rounded cubical, angular, flaky or elongated particles. It is evident that the aky and elongated particles will have less strength and durability when compared with cubical, angular or rounded particles of the same aggregate. Hence too flaky and too much elongated aggregates should be avoided as far as possible.

Adhesion with bitumen 
The aggregates used in bituminous pavements should have less a nity with water when compared with bituminous materials, otherwise the bituminous coating on the aggregate will be stripped o in presence of water.

Durability 
The property of aggregates to withstand adverse action of weather is called soundness. The aggregates are subjected to the physical and chemical action of rain and bottom water, impurities there-in and that of atmosphere, hence it is desirable that the road aggregates used in the construction should be sound enough to withstand the weathering action

Freedom from deleterious particles 
Specifications for aggregates used in bituminous mixes usually require the aggregates to be clean, tough and durable in nature and free from excess amount of at or elongated pieces, dust, clay balls and other objectionable material. Similarly aggregates used in Portland cement concrete mixes must be clean and free from deleterious substances such as clay lumps, chert, silt and other organic impurities.

Overview 
In order to decide the suitability of the aggregate for use in pavement construction, following tests are carried out:
1.Crushing test
2.Abrasion test
3.Impact test
4.Soundness test
5.Shape test
6.Specific gravity and water absorption test
7.Bitumen adhesion test

Aggregate Crushing Value 
Aim: To determine the crushing value of the given sample of aggregate.
Apparatus
a) Steel cylinder with open ends and a square plate
b) Plunger with piston
c) Cylindrical measure
d) Weighing balance
e) IS sieves (12.5 mm, 10.0mm & 2.36 mm)
f) Compression testing machine
g) Steel Tamping Rod.
 

Theory
The Principal mechanical properties required in road stones are (i) Satisfactory resistance to crushing under the roller during construction and (ii) adequate resistance to surface abrasion under traffic. Also stresses under rigid tyre rims of heavily loaded animal drawn vehicles are high enough to consider the crushing strength of road aggregate as an essential requirement in India.
Crushing strength of road aggregate may be determined either on aggregate or on cylindrical specimens cut out of rocks. These two tests are quite different is not only the approach but also is the expression of the results.
Aggregate used in road construction, should be strong enough to resist crushing under traffic wheel loads. If the aggregate are weak, the stability of the pavement stretches is likely to be adversely affected, the strength of coarse aggregate is assessed by aggregate crushing test. The aggregate crushing value provides a relative measure of  resistance to crushing under gradually applied compressive load. To achieve a high quality of pavement, aggregate possessing low aggregate value should be preferred.
Determination of CBR & Road Aggregates | Transportation Engineering - Civil Engineering (CE)

Procedure
1. Select clean and dry aggregate passing through IS 12.5 mm and retained on IS 10.0 mm sieve.
2. Weight the empty cylindrical measure. Let the weight be „a‟ g
3. Fill the aggregate in the cylindrical measure in three layers, tamping each layer 25 times with the rounded end of the tamping rod. Weigh the cylindrical measure with aggregate. Let the weight be „b‟ grams. Thus the weight of aggregate = W1 g
4. Transfer the aggregate into the steel cylinder again in three layers tamping each layer 25 times
5. Place the plunger in the steel cylinder such that the piston rests horizontally over the aggregate surface.
6. Keep the assembly of steel cylinder with plunger in the compression testing machine.
7. Set the pointer to read zero and apply the compressive load of 40 tonnes.
8. Stop the machine. Take out the assembly.
9. Sieve the crushed material on IS 2.36 mm sieve and find he weight of material passing this sieve. Let the weight be W2g.
10. Then Aggregate crushing value=W2 / W1*100 %

Tabular Column
Determination of CBR & Road Aggregates | Transportation Engineering - Civil Engineering (CE)

Result 
The aggregate crushing value of the given aggregate Sample is ______
Desirable value 
Strong aggregates give low aggregate crushing value. IRC and ISI have specified that the aggregate crushing value of the coarse aggregate used for cement concrete pavement at surface should not exceed 30 percent. For aggregates used for concrete other than for wearing surfaces, the aggregates crushing value shall not exceed 45 percent, according to the ISS. However aggregate crushing values have not been specified by the IRC for coarse aggregates to be used in bituminous pavement construction methods.
Los Angeles Abrasion Test 
Aim: To determine the abrasion of the given aggregate sample.
Apparatus: Los Angles Abrasion Machine Steel balls-11no. Weighing balance IS Sieves: 20, 12.5, 10, 1.7mm.

Theory
Due to the movement of traffic, the road stones used in the surfacing course are subjected to wearing action at the top. Resistance to wear or hardness is hence an essential property for road aggregates especially when used in wearing course. Thus road stones should be hard enough to resist the abrasion due to the traffic.
Determination of CBR & Road Aggregates | Transportation Engineering - Civil Engineering (CE)
Procedure 
1) Take the clean and dried aggregates in an oven at 105-110° C.
2) Sieve the given aggregates in sieve size 20-12.5mm and weigh that aggregate in 2.5kg.
3) Again sieve the aggregate in sieve size is 12.5-10mm and take that aggregates in 2.5 k. i.e., W1 gm (2.5+2.5=5kg)
4) Pour the given taking aggregates into the los angles abrasion machine.
5) Put the steel balls into the abrasion machine after pouring the aggregates.
6) Start the machine and rotating the drum for 100 revolutions and stop the machine.
7) After stopping the machine, take out the aggregates and sieve the aggregates in 1.7mm sieve size and take the retained aggregates and note down its weight i.e, W2 gm.
8) Then, Los Angles Abrasion value= (W1-W2/W1)X100 %

Tabular Column
Determination of CBR & Road Aggregates | Transportation Engineering - Civil Engineering (CE)

Result 
The Los Angles Abrasion Value of given aggregates is ______% at ______revolution.
Desirable value 
The maximum allowable Los Angeles abrasion values of aggregates as specified by IRC for different methods of construction are given in below table.
Determination of CBR & Road Aggregates | Transportation Engineering - Civil Engineering (CE)
The Los Angeles abrasion value of good aggregates acceptable for cement concrete, bituminous concrete and other high quality pavement materials should be less than 30%. Values up to 50% are allowed in base courses like water bound and bituminous macadam.

The document Determination of CBR & Road Aggregates | Transportation Engineering - Civil Engineering (CE) is a part of the Civil Engineering (CE) Course Transportation Engineering.
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FAQs on Determination of CBR & Road Aggregates - Transportation Engineering - Civil Engineering (CE)

1. What is CBR in civil engineering?
Ans. CBR stands for California Bearing Ratio, which is a measure used in civil engineering to evaluate the strength and bearing capacity of road subgrades and base courses. It is determined by comparing the pressure required to penetrate a soil sample with the pressure required for a standard material to achieve the same penetration.
2. How is CBR determined for road aggregates?
Ans. CBR for road aggregates is determined by conducting a laboratory test called the California Bearing Ratio test. In this test, a soil sample is compacted into a mold and subjected to a load through a plunger. The penetration resistance is then measured and compared to the penetration of a standard material. The ratio of the two penetrations gives the CBR value.
3. Why is CBR important in road construction?
Ans. CBR is important in road construction as it provides a measure of the subgrade or base course material's strength and ability to withstand loads. It helps in designing and selecting suitable materials for road construction, determining pavement thickness, and assessing the potential for subgrade failure and deformation under traffic loads.
4. What are road aggregates in civil engineering?
Ans. Road aggregates, also known as construction aggregates, are materials used in the construction of roads, highways, and pavements. These aggregates include crushed stone, gravel, sand, and recycled materials like asphalt and concrete. They provide strength, stability, and durability to the road surface and help in distributing loads.
5. How are road aggregates tested in civil engineering?
Ans. Road aggregates are tested in civil engineering through various tests, including the sieve analysis test, specific gravity and water absorption test, aggregate crushing value test, and impact value test. These tests assess the physical and mechanical properties of the aggregates, such as particle size distribution, strength, hardness, and resistance to wear and impact.
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