Unit-4
Highway Materials
Q1) Explain the desirable properties of aggregate to be used in different types of pavement construction.
A1) Aggregates
Aggregates form the major portion of the pavement structure, bear stresses occurring on the roads, and have to resist wear due to abrasive action of traffic.
Aggregates are also used in flexible as well as in rigid pavements. Therefore, the properties of aggregates are of considerable importance to the highway.
The aggregates are specified based on their grain size, shape, texture, and gradation.
Based on the strength property, the coarse aggregates maybe divided as hard aggregates or soft aggregates (Moorum, kankar, laterite, brick aggregates).
Desirable Properties of Road Aggregates
Strength
The aggregates to be used in road construction, particularly the aggregates used in the wearing course of the pavement should be sufficiently strong/ resistant to crushing to withstand the high stresses induced due to heavy traffic wheel loads.
Hardness
The aggregates used in the surface course are subjected to constant rubbing or abrasion due to moving traffic.
Abrasive action may be increased due to the presence of abrasing material like sand between the tyres of the vehicle and the aggregates exposed to the top surface. Thus, they should be hard enough to resist the wear due to the abrasive action of traffic.
Toughness
Aggregates in the pavement are also subjected to impact due to moving wheel loads. The magnitude of impact increase with the roughness of the road and the speed of the vehicle.
The severe impact is common when heavily loaded steel tyred vehicles move on WBM. The resistance to impact or toughness is thus another desirable property of aggregates.
Durability
The aggregates used in roads are subjected to physical and chemical actions of rains and groundwater, the impurities in them, and that of the atmosphere. Thus the road stones used in the construction should be sound enough to withstand the weathering action.
The property of aggregates to withstand the adverse actions of weather may be called soundness.
Shape of Aggregates
Road aggregates may be rounded, angular, flaky, or elongated.
Flaky and elongated particles have less strength than rounded and cubical particles. Thus, too flaky and too many elongated particles should be avoided.
Q2) What are the different types of bituminous materials used in road construction?
A2)
Bitumen
Bitumen refers to the viscous liquid or solid consisting essentially of hydrocarbons and their derivatives.
Bitumen is soluble in Carbon Disulphide C2S
Substantially non-volatile
Softens when heated
Black or brown.
Petroleum Bitumen: Obtained by refining process of petroleum
Natural Bitumen: Occurring as natural deposits
Straight Run Bitumen: Petroleum bitumen whose viscosity has not been adjusted by blending or by softening with cutbacks or other methods.
Blown Bitumen: Straight run bitumen is further treated by blowing air through it while it is in hot condition.
Composition of Bitumen:
The complex chemical mixture of molecules that are predominantly hydrocarbons.
Carbon | 82-88% |
Hydrogen | 8-11% |
Sulphur | 0-6% |
Oxygen | 0-1.5% |
Nitrogen | 0-1% |
Traces of metal: Vanadium, Nickel, Iron, Calcium, Magnesium
Bitumen constituents are broadly classified as Asphaltenes, Resins, and oils
Q3) What are the various tests carried out on bitumen? Briefly mention the principle and uses of each test.
A3)
The penetration value of bitumen is measured by distance in tenths of mm that a standard needle would penetrate vertically into the bitumen sample under standard conditions of the test.
By this test, we can determine the hardness or softness value of bitumen.
In this test, firstly heat the bitumen above its softening point and pour it into a container of depth attest 15mm.
Bitumen should be stirred wisely to remove air bubbles. Then cool it to room temperature for 90 minutes and then placed it in a water bath for 90 minutes.
Then place the container in the penetration machine adjust the needle to make contact withthe surface of the sample. Make dial reading zero and release the needle for exactly 5 seconds and note down the penetration value of the needle for that 5 seconds.
Just repeat the procedure thrice and note down the average value.
2. Ductility Test
The property of bitumen that allows it to undergo deformation or elongation is called ductility of bitumen.
The ductility of bitumen is measured by the distance in Cm (centimeter), to which the bitumen sample will elongate before breaking when it is pulled by the standard specimen at a specified speed and temperature.
Firstly, the bitumen sample is heated to 75-100°C and melted completely. This is poured into the assembled mold which is placed on a brass plate. To prevent sticking the mold and plate are coated with glycerin and dextrin.
After filling the mold, placed it at room temperature for 30-40 minutes and then placed it in water for 30 minutes.
Then take it out and cut the excess amount of bitumen with the help of a hot knife and level the surface. Then place the whole assembly in a water bath of ductility machine for 85 to 95 minutes. Then detach the brass plate and the hooks of mold are fixed to the machine and operate the machine.
The machine pulls the two clips of the mold horizontally and then bitumen elongates. The distance up to the point of breaking from the starting point is noted as the ductility value of bitumen.
The minimum value should be 75cm.
3. Softening Point Test
The softening point of bitumen indicates the point at which bitumen attains a particular degree of softening under specified conditions of the test.
Take a small amount of bitumen sample and heat it to 75-100oC.
Ring and ball apparatus is used to conduct this test.
Heat the rings and apply glycerin to prevent them from sticking. Fill these rings with bitumen and remove the excess material with a hot sharp knife.
Assemble the apparatus parts, balls are arranged in a guided position that is on the top of the bitumen sample. And fill the beaker with boiled distilled water. Then apply temperature @ 5°C per minute.
At a certain temperature, bitumen softens and the ball slowly moves downwards and touches the bottom plate, this point is noted as a softening point.
4. Specific Gravity Test
The specific gravity of bitumen is the ratio of the mass of a given volume of bitumen to the mass of an equal volume of water at a specified temperature.
Specific gravity is a good indicator of the quality of the binder.
It can be determined by the pycnometer method.
In this method, take a clean and dry specific gravity bottle and take its weight(W1)
In the 2nd case, fill the bottle with distilled water and dip it in a water bath for 30 minutes and note down the weight (W2).
Next, fill half the bottle with a bitumen sample and weigh (W3).
Finally, fill the bottle with half water and half portion with bitumen and weigh (W4). Now we can find out specific gravity from the formulae.
5. Viscosity Test
Viscosity is the property of bitumen that influences the ability of bitumen to spread, penetrate the voids and also coat the aggregates. That is, it influences the fluid property of bitumen.
If the viscosity of bitumen is higher, the compactive effort of bitumen reduces and a heterogeneous mixture arises.
If viscosity is lower, then it will lubricate the aggregate particles.
Viscosity is determined by using a tar viscometer.
The viscosity of bitumen is expressed in seconds is the time required for the 50 ml bitumen sample to pass through the orifice of a cup, under standard conditions of test and at a specified temperature.
6. Flash and Fire Point Test
The Flashpoint of bitumen is defined as the point of the lowest temperature at which bitumen catches vapors of test flame and fires in the form of flash.
The fire point of bitumen is defined as the point of the lowest temperature at which the bitumen ignites and burns at least for 5 seconds under specific conditions of the test.
Flash and fire point test helps to control fire accidents in bitumen coated areas.
By this test, we can decide the bitumen grade with respect to temperature for particular areas of high temperatures.
7. Float Test
A float test is used to determine the consistency of bitumen. But we generally use penetration test and viscosity test to find out the consistency of bitumen except for a certain range of consistencies.
The float test apparatus consists of aluminum float and brass collars.
These collars are filled with melted bitumen sample and cooled to 5°C and then attached into aluminum floats and this assembly is placed in a water bath at a temperature of 50°C.
Note down the time in seconds from the instant the float is put on the water bath until the water breaks the material and enters the float.
8. Water Content Test
When bitumen is heated above the boiling point of water, sometimes foaming of bitumen occurs. To prevent this bitumen should have a minimum water content in it.
Water content in bitumen is determined by the dean and stark method.
In this method, the bitumen sample is kept in 500ml heat-resistant glass container.
The container is heated to just above the boiling point of water.
The evaporated water is condensed and collected. This collected water is expressed in terms of the mass percentage of the sample.
It should not more than 0.2% by weight.
9. Loss on Heating Test
When the bitumen is heated, water content present in the bitumen is evaporated and bitumen becomes brittle which can be damaged easily. So, to know the amount of loss ness we will perform this test.
In this test, take the bitumen sample and note down its weight to 0.01gm accuracy at room temperature.
Then place the sample in the oven and heat it for 5 hours at 163°C.
After that take out the sample and cool it to room temperature and take the weight to 0.01gm accuracy and note down the value. Then for the two values of weight before and after heating, we can compute the loss of mass.
The loss should be less than 5% of total weight otherwise it is not preferred for construction.
Q4) The CBR value of the soil is 5%, calculate the total thickness of a pavement using
(i) Design curve developed by California State Highway Department.
(ii) Design chart recommended by IRC.
(iii) Design formula developed by the US Corps of Engineers.
Assume 4100 kg wheel load or medium-light traffic of 200 commercial vehicles per day for design.
Tyre Pressure = 6 kg/cm2
A4)
(i) Using the design chart of California State Highway Department, the pavement thickness for 4100 kg wheel load and CBR = 5% = 38 cm.
(ii) Using the design chart recommended by IRC for 200 commercial vehicles per day and using curve D and for CBR value = 5% the thickness = 37.5 cm.
(ii) Using the design formula given in equation
t = 
1/2
P = 4100 kg
p = 6 kg/cm2
t = 
1/2
t = 35.5 cm
Q5) Calculate the equivalent C-value of a three-layered pavement section having individual C-values as given below:
Materials | Thickness, cm | C-value |
Bituminous concrete | 10 | 60 |
Cement treated base | 20 | 225 |
Gravel sub-base | 10 | 15 |
A5)
The individual thickness of each layer is converted t their respective gravel equivalent using the following relationship:
= 
1/5
Here,
tg = Gravel thickness
t = Individual thickness
Cg = Cohesiometer value of gravel = 15
C = Respective C-Value
tg = (60/15)1/5 X 10 = 13.2 cm
For base course, tg = (225/15)1/5 X 20 = 34.4 cm
For sub-base course, tg = 10.0 cm
Therefore actual pavement thickness = 10 + 20 = 40 cm
This is to equivalent to gravel thickness = 13.2 + 34.4 + 10 = 57.6 cm
Now, 
= 
1/5
C = (
1/5 X Cg = (
1/5 X 15 = 93
The equivalent C-value of the pavement section is 93.
Q6) Compute the radius of relative stiffness of 15 cm thick cement concrete slab from the following data:
Modulus of elasticity of cement concrete = 2,10,000 kg/cm2
Poisson’s ratio for concrete = 0.13
Modulus of subgrade reaction, K = (i) 3.0 kg/cm3
(ii) 7.5 kg/cm3
A6)
(i) For, K = 3.0
l = 
1/4 = 
1/4 = 67.0 cm
(ii) For, K = 7.5
l = 
1/4 = 
1/4 = 53.3 cm
This indicates that the influence of modulus of subgrade reaction on the slab is relatively small/
The stresses acting on a rigid pavement are;
(i) Wheel load stresses and
(ii) Temperature Stresses.
Q7) Compute the equivalent radius of the resisting section of the 20 cm slab, given that the radius of contact area wheel load is 15 cm.
A7)
h = 15 cm, a = 15
a/h = 15/20 = 0.75, < 1.724
Therefore, b = 
- 0.675h
b = 
- 0.675 X 20 = 14.07 cm
Q8) Calculate the stresses at interior, edge and corner regions of a cement concrete pavement using Wetsergaard’s stress equation. Use the following data:
Wheel load = 5100 kg
Modulus of elasticity of cement concrete, E = 3.0 X 105 kg/cm2
Pavement thickness, h = 18 cm
Poisson’s ratio of concrete, 
= 0.15
Modulus of subgrade reaction, K = 6.0 kg/cm3
Radius of contact area, a = 15 cm
A8)
Radius of relative stiffness (l) is given by
l = 
1/4 = 
1/4 = 70.6 cm
The equivalent of resisting section is given by
a/h = 15/8 = 0.833, < 1.74
b = 
- 0.675h
b = 
- 0.675 X 18 = 14.0 cm
Stress at the interior, (Si)
Si = 
Si = 
= 19.3 kg/cm2
Stress at the Edge, (Se)
Se = 
Se = 
= 28.54 kg/cm2
Stress at the corner, (Sc)
Sc = 
Sc = 
= 24.27 kg/cm2
Q9) Sketch flexible pavement cross-section and show the parts. Enumerate the functions and importance of each component of the pavement.
A9)
A flexible pavement invariably consists of all the courses as shown in Fig 9. Thus, it is a multi-layered system with low flexural strength.
The external loads are largely transmitted to the subgrade through the intervening layers-the base and the sub-base – through interlocking at the grain to grain contacts in the granular structure.
Lateral distribution of the compressive stresses on to a larger area with increasing depth is the basic mechanism of stress transfer.
The thicknesses of the intervening courses are so designed as to keep the stresses transferred to the subgrade soil less than the allowable bearing pressure to ensure that deformations or settlements remain within permissible limits.
The load distribution capacity of each of these layers depends upon the nature of the materials and the mixed design aspects.
The top layer or the surface (or wearing) course, which is in direct contact with the traffic loads has to be necessarily the strongest, while the layers below can be of relatively lower strength.
The surface course, therefore, consists of a mix with a binder material like bitumen and mineral aggregates. The base and sub-base courses consist of granular materials like crushed stone aggregate, gravel, and aggregate-soil mixes.
The base and sub-base courses may consist of more than one layer of slightly different materials and specifications. Another important characteristic of flexible pavement is that the deformations (especially if excessive) of the subgrade are transmitted and reflected on the surface and vice versa; that is why it needs a strong subgrade for successful performance.
The following types of construction have been used in the flexible pavement:
Conventional Flexible Pavements
These are layered systems with high-quality expensive materials are placed in the top where stresses are high, and low-quality cheap materials are placed in lower layers.
Full - Depth Asphalt Pavements
These are constructed by placing bituminous layers directly on the soil sub-grade. This is more suitable when there is high traffic and local materials are not available.
Contained Rock Asphalt Mats
These are constructed by placing dense/open-graded aggregate layers in between two asphalt layers.
Modified dense graded asphalt concrete is placed above the subgrade will significantly reduce the vertical compressive strain on soil sub-grade and protect from surface water.
Q10) Explain the CBR method of pavement design. How is this method useful to determine the thickness of component layers?
A10)
California Bearing Ratio (C.B.R.) Test
The California Bearing Ratio (CBR) test is a measure of the resistance of a material to penetration of a standard plunger under controlled density and moisture conditions.
It was developed by the California Division of Highways as a method of classifying and evaluating soil- subgrade and base course materials for flexible pavements.
CBR test may be conducted in the remolded or undisturbed sample. The test consists of causing a cylindrical plunger of 50mm diameter to penetrate a pavement component material at 1.25mm/minute.
The loads for 2.5mm and 5mm are recorded. This load is expressed as a percentage of standard load value at a respective deformation level to obtain CBR value.
The aim of this test is to the determination of California Bearing Ratio value of the subgrade soil.
The procedure of the California Bearing Ratio Test
Sieve the sample through a 20mm IS sieve. Take 5 kg of the sample of soil specimen. Add water to the soil in the quantity such that optimum moisture content or field moisture content is reached.
Then soil and water are mixed thoroughly. A spacer disc is placed over the base plate at the bottom of mould and a coarse filter paper is placed over the spacer disc.
The prepared soil water mix is divided into five. The mould is cleaned and oil is applied. Then fill one-fifth of the mould with the prepared soil. That layer is compacted by giving 56 evenly distributed blows using a hammer of weight of 4.89kg.
The top layer of the compacted soil is scratched. Againthe second layer is filled and the process is repeated. After 3rd layer, the collar is also attached to the mould, and the process is continued.
After the fifth layer collar is removed and excess soil is struck off. Remove base plate and invert the mould. Then it is clamped to the baseplate.
Surcharge weights of 2.5kg are placed on the top surface of the soil. Mould containing specimen is placed in position on the testing machine.
The penetration plunger is brought in contact with the soil and a load of 4kg (seating load) is applied so that contact between soil and plunger is established. Then dial readings are adjusted to zero.
The load is applied such that the penetration rate is 1.25mm per minute. Load at penetration of 0.5, 1, 1.5, 2, 2.5, 3, 4, 5, 7.5, 10 and 12.5mm are noted.
Two values of CBR will be obtained. If the value of 2.5 mm is greater than that of 5.0 mm penetration, the former is adopted.
If the CBR value obtained from the test at 5.0 mm penetration is higher than that at 2.5 mm, then the test is to be repeated for checking. If the check test again gives similar results, then a higher value obtained at 5.0 mm penetration is reported as the CBR value.
The average CBR value of three test specimens is reported as the CBR value of the sample.
CBR (%) = 
100
CBR (2.5mm) = 
X 100
CBR (5.0mm) = 
X 100
CBR = Higher of CBR2.5 mm/ CBR5.0 mm
Standard Load (2.5 mm Penetration) = 1370 Kg
Standard pressure/stress @ 2.5 mm = 
= 70 Kg/cm2
Standard pressure/stress @ 5.0 mm = 
= 105 Kg/cm2
CBR of a soil sample is the average of CBR of three specimens prepared from the same sample.
Generally, 2.5 mm CBR is more than 5 mm CBR. But if 5 mm CBR is more than 2.5 mm CBR, then the test must be repeated again and again. If the same result comes then a higher value is considered as CBR i.e., CBR @ 5 mm.
The empirical formula for thickness, (T) cm = 
(Applicable for CBR>12%)
