Flexible pavement design in India is governed by IRC 37:2018 — Guidelines for the Design of Flexible Pavements (4th Revision). This revision adopts a mechanistic-empirical approach replacing the older empirical charts. Understanding this standard is mandatory for anyone involved in highway design in India and is a frequent topic in GATE, ESE, and PSU technical interviews.
Structural Layers of a Flexible Pavement
From top to bottom, a flexible pavement consists of:
| Layer | Material | Function | MORTH Clause |
|---|---|---|---|
| Wearing course | Bituminous Concrete (BC) — IS 73 VG 30/40 | Waterproofing, skid resistance, smoothness | 509 |
| Binder course | Dense Bituminous Macadam (DBM) | Structural layer, distributes loads | 507 |
| Base course | Wet Mix Macadam (WMM) / Dry Lean Concrete (DLC) | Primary load distribution | 406 |
| Sub-base | Granular Sub-Base (GSB) | Drainage, frost protection, load distribution | 401 |
| Subgrade | Natural/improved in-situ soil | Ultimate load bearing | 301 |
IRC 37:2018 Design Inputs
1. Design Traffic (in Million Standard Axles — MSA)
IRC 37:2018 uses the concept of cumulative equivalent standard axle load (CESAL) over the design period:
N = 365 × A × (1+r)^n - 1) / r × D × F
Where:
A = initial traffic in commercial vehicles/day (one direction)
r = annual traffic growth rate (typically 5–7.5% in India)
n = design life (15 years for NH, 10 years for MDR/ODR)
D = lane distribution factor (0.75 for single carriageway 2-lane)
F = vehicle damage factor (VDF — from axle load surveys)Vehicle Damage Factor (VDF):
| Road Type | Terrain | Typical VDF Range |
|---|---|---|
| NH/SH | Plain | 2.5–4.5 |
| NH/SH | Hilly | 1.5–2.5 |
| MDR | Plain | 3.0–5.0 |
| Urban roads | — | 0.5–2.0 |
2. Subgrade CBR
Use soaked CBR at 97% of heavy Proctor MDD. Use P90 (90th percentile) value from multiple tests along alignment. If CBR < 5%, provide minimum 500 mm select fill subgrade improvement.
IRC 37:2018 Mechanistic-Empirical Approach
Unlike the old IRC 37:2001 (which used simple CBR-vs-traffic charts), IRC 37:2018 uses IITPAVE software or design catalogues to limit:
- Horizontal tensile strain (εt) at bottom of bituminous layers → prevents fatigue cracking
- Vertical compressive strain (εv) at top of subgrade → prevents rutting
Allowable strains are tied to number of load repetitions (N) through transfer functions calibrated for Indian conditions.
Design Catalogue Approach (IRC 37:2018 Appendix)
For practical design, IRC 37:2018 provides ready design catalogues. Worked Example:
Given: NH in plain terrain, 4-lane divided carriageway. Initial 2-way traffic = 4000 CVD. Growth rate r = 7.5%. Design life = 15 years. Subgrade CBR = 6%. Use VG 40 bitumen.
Step 1 — Traffic Calculation:
A (1 direction, 2 lanes) = 4000/2 = 2000 CVD
D (2 lanes, single direction) = 0.75
VDF = 3.5 (typical NH plain)
N = 365 × 2000 × [(1.075¹⁵ - 1)/0.075] × 0.75 × 3.5 = ≈ 65 MSA
Step 2 — Design from IRC 37:2018 Catalogue (Table 1, CBR=6, Traffic=50–100 MSA):
| Layer | Material | Thickness (mm) |
|---|---|---|
| Wearing course | BC (VG 40) | 40 |
| Tack coat | SS-1/RS-1 @ 0.25 kg/m² | — |
| Binder course | DBM (VG 40) | 100 |
| Prime coat | MC 30 @ 0.9 kg/m² | — |
| Base course | WMM | 250 |
| Sub-base | GSB (Grading IV) | 200 |
| Total bituminous | 140 mm | |
| Total pavement | 590 mm |
Material Specifications
Bituminous Concrete (BC) — MORTH Clause 509:
- Aggregate: crushed stone, LA abrasion < 30%, soundness (Na₂SO₄) < 12%
- Bitumen: VG 30 (plain terrain), VG 40 (hilly/high traffic), PMB (modified for expressways)
- Grading: BC Grade 1 (19 mm nominal) or Grade 2 (13 mm nominal)
- Marshall stability: > 9 kN, flow 2–4 mm
Dense Bituminous Macadam (DBM) — MORTH Clause 507:
- Grading I (37.5 mm nominal), Grading II (26.5 mm nominal)
- Marshall stability: > 9 kN (VG 30), > 12 kN (VG 40)
- Air voids: 3–5%
Granular Sub-Base (GSB) — MORTH Clause 401:
- CBR (soaked) > 20–30% (depends on grading)
- 5 gradings available; select based on local material
- Compaction: 98% heavy Proctor
Quality Control Tests on Site
| Test | Frequency | Acceptance Criteria |
|---|---|---|
| Marshall test (binder content) | Once per 100 T of mix | Stability > 9 kN, flow 2–4 mm, voids 3–5% |
| Core density (bituminous) | Once per 250 m per lane | ≥ 92% of Marshall density |
| GSB field density | 1 per 500 m² | ≥ 98% heavy Proctor |
| Bitumen extraction test | Once per 500 T | Binder content within ±0.3% of design |
| Roughness (IRC 121) | Every 500 m | IRI < 2.5 m/km (NH) |
Frequently Asked Questions
What is the difference between IRC 37:2001 and IRC 37:2018?
IRC 37:2001 used empirical CBR-vs-traffic thickness charts. IRC 37:2018 uses a mechanistic-empirical approach with IITPAVE software, controlling critical strains at the bottom of bituminous layers and top of subgrade. The 2018 version gives more rational designs, especially for high-traffic roads, and explicitly includes design for cement-treated bases and cold mix technology.
What is the design period for flexible pavements in India?
IRC 37:2018 recommends 15 years design life for NH/SH and 10 years for MDR/ODR. Expressways use 20 years. After the design period, an overlay (strengthening) is applied based on falling weight deflectometer (FWD) measurements and IRC 81 overlay design.
When is Polymer Modified Bitumen (PMB) required?
PMB (IS 15462) is mandatory for expressways and NH with traffic > 150 MSA as per IRC 37:2018, wearing and binder courses. PMB improves rutting resistance at high temperatures (common in Indian summers) and fatigue life under heavy traffic. PMB 40 is the most commonly used grade.