Pavement design factors; IRC rigid design steps

Factors affecting pavement design

The main factors that control the design of a pavement are:

1. Design wheel load - magnitude of the load, tyre (contact) pressure, contact area, axle configuration (single, tandem, tridem) and repetitions. The legal and standard axle loads (80 kN) are used to convert mixed traffic to ESAL/msa.
2. Traffic - initial commercial vehicles per day (CVPD, vehicles above 3 t laden weight), growth rate, design life, lane and directional distribution, and the vehicle damage factor (VDF).
3. Climate - rainfall and drainage (moisture changes the subgrade strength), temperature (stiffness of bituminous layers; warping and expansion in concrete slabs), frost and freeze-thaw in the hills.
4. Subgrade soil - strength (CBR, modulus of subgrade reaction k or resilient modulus M_R), volume change, drainage and the water-table depth. Strength is taken in the soaked condition.
5. Pavement component materials - strength, modulus, Poisson's ratio, fatigue and rutting resistance of the bituminous mix and granular layers, and the flexural strength and elastic modulus of concrete.
6. Environment and local factors - ground water, terrain, availability of materials, construction quality and maintenance level.
7. Failure criteria and reliability - allowable serviceability loss (PSI), fatigue cracking, rutting limit (20 mm) and the reliability chosen.

Steps of IRC design of rigid pavement

The design follows IRC:58 (Guidelines for the Design of Plain Jointed Rigid Pavements for Highways), based on the Westergaard/Bradbury/Pickett-Ray approach.

1. Collect data - design wheel load (5100 kg for highways, 4100 kg for small roads), tyre pressure, radius of contact area, flexural strength (modulus of rupture), E, Poisson's ratio and thermal coefficient of concrete; modulus of subgrade reaction k (on top of the sub-base); traffic (CVPD), growth rate and design life.
2. Design traffic - find the commercial vehicles at the end of construction, classify the traffic (A to G) and note the thickness adjustment.
3. Assume a trial slab thickness h and the joint spacing (transverse L, longitudinal width B).
4. Radius of relative stiffness $l=\left[\dfrac{Eh^3}{12(1-\mu^2)k}\right]^{1/4}$.
5. Load stresses - edge load stress (IRC chart based on the modified Westergaard equation) and corner load stress (Westergaard's formula as modified by Kelly, $\sigma_c=\frac{3P}{h^2}\left[1-\left(\frac{a\sqrt2}{l}\right)^{1.2}\right]$).
6. Temperature (warping) stresses - from the temperature differential between top and bottom: Bradbury's formula $\sigma_{te}=\dfrac{C\,E\,\alpha\,t}{2}$ with coefficient C from L/l and B/l; corner warping stress $\sigma_{tc}=\dfrac{E\alpha t}{3(1-\mu)}\sqrt{a/l}$.
7. Frictional stress $\sigma_f=\dfrac{WLf}{2\times10^4}$ (W in kg/m3, L in m).
8. Check critical combinations - (a) summer midday, edge: $\sigma_e+\sigma_{te}-\sigma_f$; (b) winter midday, edge: $\sigma_e+\sigma_{te}+\sigma_f$ (with the winter $\Delta T$); (c) night, corner: $\sigma_c+\sigma_{tc}$. Each must not exceed the permissible flexural stress (modulus of rupture divided by a factor of safety, or checked by the fatigue stress ratio in IRC:58-2002/2015).
9. Revise the thickness by trial until the stresses are safe; apply the traffic correction (A to G) to the thickness.
10. Joints and steel - design contraction, expansion and longitudinal joints, dowel bars at transverse joints and tie bars at longitudinal joints (IRC:58 / IRC:15).