--- title: "Pavement design factors; IRC rigid design steps" subject: "Transportation Engineering II" chapter: 2 key: "semester_7/transportation_engineering_ii:pavement-factors-irc-rigid-steps" source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/pavement-factors-irc-rigid-steps site: BCT Engineering --- # Pavement design factors; IRC rigid design steps Transportation Engineering II · Chapter 2: Highway Pavement - Asked in: 2074 Asoj, 8 marks; 2068 Baisakh, 8 marks (asked 2 times) - Video: [IRC design steps](https://www.youtube.com/watch?v=BdmkfDVewPg) — NPTEL-NOC IITM - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/pavement-factors-irc-rigid-steps - Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-2 ## Question What are factors affecting pavement design? Write down the steps of IRC design guidelines for rigid pavement. ## Short answer Main factors are wheel load, traffic, climate, subgrade strength, materials, environment, failure criteria and reliability. IRC:58 steps: collect data, compute design traffic, assume thickness, find radius of relative stiffness, compute load and warping and frictional stresses, check critical combinations, revise thickness, and design joints and steel. ## Answer #### 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). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. At what condition is subgrade strength (CBR) taken for design? A. Dry condition B. Soaked condition C. Frozen condition D. Optimum moisture unsoaked Answer: B. Strength is taken in the soaked condition, the worst expected. (Key point: Subgrade strength condition) 2. After assuming a trial slab thickness, which step comes next in the IRC rigid design? A. Design the dowel bars B. Compute radius of relative stiffness, then load and warping stresses C. Finish and open to traffic D. Choose the bituminous binder grade Answer: B. With a trial h, l is computed, then load stresses, temperature and frictional stresses are found. (Key point: Step order in rigid design) 3. Which formula is used for the corner load stress? A. Kelly's formula sigma_c = (3P/h^2)[1 - (a sqrt2 / l)^0.6] B. Bradbury's formula C E alpha t / 2 C. sigma = W L f / (2 x 10^4) D. Pickett and Ray chart only Answer: A. Corner load stress uses Kelly's formula; edge load stress uses the Pickett and Ray chart. (Key point: Corner load stress) 4. Each critical stress combination (summer midday, winter midday, night, corner) must: A. Exceed the flexural strength B. Not exceed the permissible flexural stress (modulus of rupture divided by a factor of safety) C. Equal the wheel load D. Be ignored if friction is small Answer: B. The thickness is revised by trial until all combinations are within the permissible flexural stress. (Key point: Critical combinations) 5. What is provided at transverse and longitudinal joints in the last step? A. Tie bars at transverse, dowels at longitudinal B. Dowel bars at transverse joints and tie bars at longitudinal joints C. No steel at any joint D. Dowels at both Answer: B. Dowels transfer load across transverse joints while tie bars hold longitudinal joints together. (Key point: Joints and steel) ## Related in Transportation Engineering II - [Rigid pavement thickness by IRC procedure](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Arigid-thickness-irc-procedure) — Transportation Engineering II, chapter 2 - [Factors affecting pavement design](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Apavement-design-factors) — Transportation Engineering II, chapter 2, asked 3× - [IRC pavement design: 5640 CVPD, CBR 6%](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Airc-design-5640-cvpd-cbr-6) — Transportation Engineering II, chapter 2 - [Temperature stresses in rigid pavement; dowel bar design](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Atemperature-stress-rigid-dowel-bar-procedure) — Transportation Engineering II, chapter 2 - [Plain CC pavement thickness by IRC, 7 m highway](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aplain-cc-pavement-thickness-irc-7m) — Transportation Engineering II, chapter 2 - [Pavement design factors in AASHTO method](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Apavement-design-factors-aashto) — Transportation Engineering II, chapter 2 ## Related in other subjects - [Factors affecting foundation design, improving bearing capacity](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Afactors-design-improve-bc) — Building Technology, chapter 2 - [Factors affecting evapotranspiration](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Afactors-affecting-et) — Engineering Hydrology, chapter 3 - [Soil moisture forms; factors affecting CWR](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asoil-moisture-forms-cwr-factors) — Irrigation and Drainage Engineering, chapter 2 - [High speed circuit design guidelines](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ahigh-speed-guidelines) — Instrumentation II, chapter 7, asked 3×