# Irc method of design of flexible pavement 1

Assistant Professor em KG Reddy College of Engineering and Technology
28 de Feb de 2018
1 de 22

### Irc method of design of flexible pavement 1

• 1. DESIGN OF FLEXIBLE PAVEMENT G . R AV I K U M A R
• 2. IRC METHOD Design approach: The objective of this method of design is, to design the pavement structure to control rut. The design of pavement involves in two steps, 1. Provision of thickness of pavement 2. Deciding the Design life/ Life time
• 3. THICKNESS OF PAVEMENT The thickness of highway pavement (H), is the combination of three layers. Layer-1: Contains surface course and binding layer. Take thickness as h1 and modulus of elasticity as E1. Layer-2: Contains granular course or drainage layer. Take thickness as h2 and modulus of elasticity as E2. Layer-3: Contains prepared subgrade soil. Depth is not specified and take modulus of elasticity as E3. The total thickness of flexible pavement (H) = h1+h2+h3
• 4. THICKNESS OF PAVEMENT The total thickness of highway pavement, H is decided based on, • CBR Value of subgrade soil • Design traffic
• 5. CBR VALUE • The CBR test should be carried out at appropriate moisture content. • The CBR test should be conducted on a number of test specimens, and the average of at least 3 consistent values taken as the design CBR value. • A weaker soil subgrade with lower CBR value, requires a flexible pavement of higher thickness.
• 6. DESIGN TRAFFIC/ TRAFFIC LOADS • The design traffic is a function of initial traffic of different classes of heavy vehicles, their axle loads, growth rate, design period and lane distribution factor. • The study involved in the following factors: • Magnitude of wheel loads • Wheel load repetitions • Equivalent Wheel Load Factors (EWLF) • Cumulative Standard Axle (CSA) values
• 7. MAGNITUDE OF WHEEL LOADS • If the magnitude of wheel load will be high, then the thickness of the pavement should be high. • So, while designing the pavement, it is necessary to consider various wheel load factors, such as  Maximum wheel load  Contact pressure  Wheel load configuration such as dual or multiple wheel load assembly  The repetition of these loads during the design life of pavement • It is also essential to estimate the total traffic volume consisting of all the categories of vehicles expected to flow on the road.
• 8. MAGNITUDE OF WHEEL LOADS Maximum Wheel Load: • Generally the wheel load is assumed to be distributed over a circular area. But by measurement of the imprints of tyres with different load and inflation pressures. • Three terms in use with reference to tyre pressure are:  Tyre pressure  Contact pressure • Generally these terms should mean same thing. The contact pressure is found to be more than the tyre pressure when tyre pressure is less than 7 kg/cm2
• 9. MAGNITUDE OF WHEEL LOAD Contact Pressure: Contact Pressure, p = 𝐿𝑜𝑎𝑑 𝑜𝑓 𝑤ℎ𝑒𝑒𝑙 𝐶𝑜𝑛𝑡𝑎𝑐𝑡 𝐴𝑟𝑒𝑎 = 𝑃 𝐴 • The concept of contact pressure is important for the analysis of stresses and the stress distribution within the pavement. • If the loaded are by wheel is assumed to be circular in shape, then the load P = 𝐴𝑝 = 𝜋𝑎2 𝑝
• 10. MAGNITUDE OF WHEEL LOAD Wheel Load Configurations: The wheel load configurations are important to know the way in which the loads of a vehicle are applied on the pavement surface.
• 11. MAGNITUDE OF WHEEL LOAD (IRC: 37-2001) • For highways maximum legal axle load as specified by IRC is 8170kg with a maximum equivalent single wheel load of 4085kg. • The design load axle load of two axle heavy commercial vehicles by IRC taken as 1020kg and the design wheel load on each dual wheel assembly is taken as 5100kg. • The maximum total legal load on the tandem axles of HCR is 19000kg and thus the legal load on each axle is 9500kg.
• 12. WHEEL LOAD REPETITIONS Effect of Repeated Application of Wheel Loads: • The effect of load repetitions during the design life of flexible pavement are to be taken in to account. • Higher number of load repetitions during the design life of the pavement will require higher thickness of flexible pavement. • The deformation of pavement of subgrade due to single application of wheel load may be small. • But due to repeated application of the heavy loads, there would be increased magnitude of both plastic and elastic deformations.
• 13. WHEEL LOAD REPETITIONS Effect of Number of Repetitions of Different Magnitudes of Loads: • Traffic composition in India is of mixed type and it is essential to evaluate the effects of number of repetitions of different magnitudes of loads. • It essential to convert the various wheel loads to one single standard wheel load for the structural design of flexible pavement. • For this purpose, it is required to carry out the traffic surveys. • From this objective, the concept EWLF (Equivalent Wheel Load Factors) has been developed.
• 14. EQUIVALENT WHEEL LOAD FACTORS If the pavement structure fails with N1 number of repetitions of P1 kg load and similarly if N2 number of repetitions of P2 kg load can also cause failure of the same pavement structure, then P1N1 and P2N2 are considered equivalent. • The concept is developed by American ‘Association of State Highways Officials’ (AASHO). • Mc1eod had given a procedure for evolving equivalent load factors for designing flexible pavements. • AASHO conducted a survey and then derived AASHO Road Test equations, were widely accepted for the determination of Equivalent Wheel Load Factors.
• 15. Typical outcome results of these studies are represented in following table: Wheel Load, Kg Repetitions to Failure, Number Equivalent to 2268 kg Equivalent Wheel Load factor (EWLF) 2268 1,05,000 1.0 1 2722 50,000 2.0 2 3175 22,500 4.7 4 3629 13,000 8.2 8 4082 6,500 16.3 16 4536 3,300 32.0 32 4990 1,700 62.0 64 5443 1,000 105.0 128