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IRC 37 : 2018Guidelines for the Design of Flexible Pavements

Q: What is MSA in road design?

MSA = Million Standard Axles — the cumulative number of standard (80 kN) axle loads expected over the design period (typically 20 years). A commercial vehicle with heavier axles causes more damage than one standard axle — this is captured by the VDF (Vehicle Damage Factor). Design traffic in MSA = daily commercial traffic × VDF × growth factor × design period.

Q: What is the minimum CBR for subgrade?

Minimum 5% CBR at 97% MDD per IRC 37. If subgrade CBR is less than 5%, the subgrade must be improved — options include: replacement with better soil, mechanical stabilization (addition of granular material), chemical stabilization (lime/cement treatment), or use of geosynthetics. CBR below 2% is very poor and almost always requires treatment.

Q: IRC 37 vs IRC 58 — which to use?

IRC 37 for FLEXIBLE pavements (bituminous surface — DBM + BC). IRC 58 for RIGID pavements (cement concrete — PQC). The choice depends on: traffic volume (rigid preferred for >30 MSA), subgrade condition, initial cost vs lifecycle cost, and maintenance capability. Most NH/SH projects now use flexible pavement per IRC 37.

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AASHTO Guide for Design of Pavement Structures · EN 13108 series · Austroads Guide to Pavement Technology

CurrentEssentialCode of PracticeTransportation · Roads and Pavement

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IRC 37:2018 is the Indian Standard (IRC) for guidelines for the design of flexible pavements. IRC 37 is India's primary code for flexible (bituminous) pavement design — used for every national and state highway project. The 2018 edition adopts a mechanistic-empirical approach replacing the older empirical CBR-based method. Design involves estimating cumulative traffic (MSA), evaluating subgrade (CBR), selecting pavement layers (GSB, WMM, DBM, BC), and verifying against fatigue and rutting criteria. This is the most important IRC code for highway engineers.

Guidelines for structural design of flexible pavements for national highways, state highways, and other major roads using mechanistic-empirical approach. Covers traffic estimation, subgrade evaluation, pavement composition, and thickness design for bituminous pavements.

Quick Reference — IRC 37:2018 Flexible Pavement Design Numbers

Design CBR, MSA, VDF, reliability levels and catalogue thicknesses for flexible pavements.

✓ Verified 2026-04-26

Reference	Value	Clause
Design period — flexible pavement (NH/SH)	15 years (typical); 20 years (expressways)	Cl. 4.2
Subgrade design CBR — minimum	≥ 5% (after 4-day soak)	Cl. 6.4
Subgrade — 90th percentile design value	Used (not average)	Cl. 6.4
Lane distribution factor — single lane each direction	1.0	Cl. 4.5 (Table 4.1)
Lane distribution factor — 2-lane single carriageway	0.75	Cl. 4.5 (Table 4.1)
Lane distribution factor — 4-lane divided (per direction)	0.45 of total	Cl. 4.5 (Table 4.1)
Lane distribution factor — 6-lane divided (per direction)	0.40 of total	Cl. 4.5 (Table 4.1)
Vehicle Damage Factor (VDF) — Initial CV count <150 cvpd	1.5 (rolling), 0.5 (terrain hilly)	Cl. 4.6 (Table 4.2)
VDF — 150-1500 cvpd (plain)	3.5	Cl. 4.6 (Table 4.2)
VDF — >1500 cvpd (plain)	4.5-5.5 typical	Cl. 4.6 (Table 4.2)
Reliability — NH / SH / expressway	90%	Cl. 4.7
Reliability — other roads	80%	Cl. 4.7
Design traffic — MSA range covered	1 to 300 MSA (catalogue)	Cl. 9.0
Allowable rutting — at end of design life	20 mm (90% reliability)	Cl. 5.4
Fatigue cracking limit — % carriageway area	20% (90% reliability)	Cl. 5.4
Bituminous layer modulus — VG-30 at 35 °C	3000 MPa (DBM) / 1700 MPa (BC)	Cl. 7.0
Granular sub-base (GSB) — min CBR	30%	Cl. 6.5
WMM base — modulus	Per Cl. 7.4 (function of subgrade modulus)	Cl. 7.4
Cementitious base (CTSB) — 7-day UCS	1.5-3.0 MPa	Cl. 7.5
Effective subgrade CBR (composite formulae)	Per Annex IX (90th percentile)	Annex IX
Min total bituminous thickness — 30 MSA	120 mm	Cl. 9 (Plate 1-12)
Min total bituminous thickness — 150 MSA	190 mm	Cl. 9 (Plate 1-12)

⚠ IRC 37:2018 (4th Revision) introduced reliability-based design; the IITPAVE software is the official tool. Always corroborate with project-specific traffic survey.

Overview

Status: Current
Usage level: Essential
Domain: Transportation — Roads and Pavement
Type: Code of Practice

International equivalents

AASHTO Guide for Design of Pavement Structures · AASHTO (US)EN 13108 series · CEN (EU)Austroads Guide to Pavement Technology · Austroads (Australia)

Typically used with

IS 73 IS 1203 IS 2720 IS 383 IRC 58 IRC 6

Also on InfraLens for IRC 37

31Key values 11Tables 2Knowledge articles 16FAQs

Practical Notes

! IRC 37:2018 uses mechanistic-empirical approach — software like IITPAVE or KENPAVE is needed for layer analysis.

! For traffic <2 MSA (rural roads), use IRC SP:72 instead of IRC 37.

! CBR of subgrade must be at 97% of MDD (Modified Proctor) — test at optimum moisture content.

! VDF (Vehicle Damage Factor) varies significantly by region — use actual axle load survey data, not assumed values.

! The design catalogue (Table 3) gives ready-to-use pavement compositions — use for preliminary design, verify with mechanistic analysis.

! Drainage layer (OGPC) is now mandatory between DBM and WMM for traffic >20 MSA.

! Temperature correction is critical — resilient modulus of bitumen changes 3-5× between 25°C and 45°C.

! For NH/SH projects, NHAI/MoRTH requires IRC 37 compliance — no alternative code is accepted.

! Always use the latest version of IRC 37 and cross-reference with other relevant IRC codes (e.g., IRC 73 for traffic studies, IRC 58 for rigid pavements if applicable for comparison).

! Thorough subgrade characterization is paramount. Multiple CBR tests at different depths and locations are essential. If CBR is low, consider soil stabilization or a thicker sub-base.

! Traffic estimation must be robust. Account for commercial vehicle composition and future growth conservatively, especially for National Highways carrying significant freight.

! Material quality control for Granular Sub-base (GSB) and Bituminous Macadam (BM) is critical. Ensure specified CBR values are met, and aggregate gradation is within limits to prevent premature failure.

! The mechanistic-empirical design requires accurate material properties. Laboratory testing for Resilient Modulus is preferred over empirical correlations for critical projects.

! Temperature correction for asphalt properties is vital. Use local meteorological data for accurate pavement temperature calculations, as it significantly impacts fatigue life.

! Drainage is often overlooked. Ensure proper camber, side drains, and sub-surface drainage provisions are incorporated as per Cl. 12 to protect the pavement structure from moisture damage.

! For high traffic volume roads, consider using a thicker wearing course or a more durable mix to extend service life and reduce maintenance cycles.

! Layered system analysis (e.g., using software) can provide a more refined design compared to simplified methods, especially for complex layer combinations or varying subgrade conditions.

! The design life of 20 years is a target. Actual performance depends heavily on construction quality, drainage, and adherence to maintenance schedules.

! Shoulder design is equally important. A poorly designed shoulder can lead to pavement edge deterioration, affecting overall structural integrity.

! Always consider the end-of-life scenario during design. Durability, recyclability of materials, and ease of maintenance should be factored in.

! For PMGSY roads, while IRC 37 provides guidelines, specific project requirements and budget constraints might necessitate adaptations, but core principles of sound design must be followed.

! Ensure adequate compaction of all layers. Insufficient compaction is a primary cause of premature rutting and fatigue cracking.

! The concept of Equivalent Axle Load (EAL) is crucial for load analysis and should be correctly calculated based on vehicle classification and axle configurations.

! Regular condition surveys and timely maintenance are essential to achieve the designed service life of flexible pavements.

Frequently referenced clauses

Cl. 3 — Traffic estimation: design traffic in terms of cumulative MSA (Million Standard Axles)

Cl. 4 — Subgrade evaluation: CBR value at 97% dry density

Cl. 5 — Pavement composition: sub-base (GSB), base (WMM/WBM), binder (DBM), surface (BC)

Cl. 6 — Pavement design catalogues for different traffic and subgrade conditions

Cl. 7 — Mechanistic-empirical design: fatigue and rutting criteria

Cl. 8 — Drainage layer requirements

Cl. 9 — Design of bituminous layers: resilient modulus, Poisson's ratio, fatigue life

Cl. 3.1.1 — General

Cl. 4.1 — Traffic Estimation

Cl. 5.1 — Subgrade Evaluation and Classification

Cl. 6.1 — Material Characterisation

Cl. 7.1 — Design Traffic Load

Cl. 8.1 — Pavement Temperature

Cl. 9.1 — Mechanistic Design Approach

Cl. 10.1 — Thickness Design of Flexible Pavements

Cl. 11.1 — Design of Shoulders

Cl. 12.1 — Drainage and Subsurface Water Control

flexible pavementroad designbituminous pavementhighway designpavement thicknessCBRMSAIRC

Engineer's Notes

In Practice — Editorial Commentary

When IRC 37 is your governing code

IRC 37:2018 provides guidelines for the design of flexible pavements — asphalt and bituminous-surfaced roads — across Indian national highways, state highways, and major urban roads.

You reference IRC 37 for: - Pavement design of new roads (asphalt / bituminous surfaced) - Pavement strengthening / overlay design on existing roads - Traffic load assessment for pavement design - Material specification for bituminous layers, granular base, sub-base - Drainage considerations and sub-grade treatment - CBR-based design for low-volume and village roads - Mechanistic-Empirical (M-E) design for major highways

'Flexible' means the pavement bends under load and distributes stress through successive layers — asphalt wearing course, binder course, granular base, granular sub-base, and treated sub-grade. This is distinct from rigid pavement (concrete slab) which is covered by IRC 58:2015.

The 2018 revision adopted the M-E design philosophy: design is now based on stress-strain analysis of each layer using traffic loads and material elastic modulus, rather than purely empirical CBR methods. The CBR method remains permitted for low-volume roads (< 10 MSA traffic).

Pair with: - IRC 58:2015 — rigid pavement design (alternative to flexible) - IRC 6:2017 — bridge / culvert loads (where flexible pavement meets bridge approaches) - MoRTH Specifications — material and construction specifications for flexible pavement layers - IRC SP 72 — guidelines for low-volume rural roads (uses simplified CBR design)

Design parameters — MSA, CBR, and the M-E methodology

Traffic load — Million Standard Axles (MSA): The design life of a pavement is expressed as cumulative standard (8.16 tonne) axle passes. An MSA = 1 million standard axle passes over the design life.

Major highways: 30-150+ MSA design traffic State highways: 10-50 MSA District roads: 2-20 MSA Rural roads: 0.5-5 MSA

MSA is computed from AADT (Average Annual Daily Traffic), vehicle composition, axle load spectrum, and design life per Appendix A of IRC 37.

Sub-grade strength — CBR: California Bearing Ratio (%) — the soaked CBR of the sub-grade material determines the design sub-grade modulus (M_R) per Clause 6.4. - CBR 2-3%: very weak (expansive clay, soft marine clay) — needs soil improvement - CBR 4-7%: typical of most Indian sub-grades (alluvial, sandy silt) - CBR 8-15%: good sub-grade (coarse alluvium, weathered rock) - CBR 20+%: excellent (murram, rocky soils)

IRC 37 requires minimum CBR 4% for highway sub-grade; if natural CBR is lower, use sub-grade improvement (lime/cement stabilisation) or replace top 300-500 mm.

M-E analysis: For design traffic > 10 MSA, IRC 37:2018 mandates mechanistic-empirical analysis using IITPAVE software (developed by IIT Kharagpur, free from MoRTH). Inputs: layer thicknesses, resilient moduli, traffic axle-load spectrum. Outputs: predicted rutting, fatigue cracking life.

Accept design when: - Fatigue (bottom-up cracking in bituminous layer) ≥ design life - Rutting (permanent deformation of surface) ≤ 20 mm over design life

Worked example — NH pavement design for 50 MSA traffic, CBR 5%

Project: 50 km greenfield section of NH with design traffic 50 MSA over 15-year life. Sub-grade natural CBR 5% (improved to 6% with 3% lime stabilisation). Flexible pavement design.

Step 1 — Design traffic: 50 MSA over 15-year life. Use Table 3 of IRC 37 for traffic growth assumptions and axle load distribution per Appendix A.

Step 2 — Design CBR of sub-grade: Stabilised sub-grade CBR = 6% (after lime treatment). Design sub-grade modulus M_R per Eq 3 of Clause 6.4.2: M_R = 10 × CBR¹·⁴ = 10 × 6¹·⁴ = 124 MPa

Step 3 — Layer selection from IRC 37 Plate 1 (M-E design charts): For 50 MSA traffic and sub-grade M_R 124 MPa, Plate 1 suggests:

Bituminous Concrete (BC) wearing course: 40 mm thick, VG-40 grade bitumen
Dense Bituminous Macadam (DBM) binder: 60 mm thick, VG-40
Wet-Mix Macadam (WMM) granular base: 250 mm thick
Granular Sub-Base (GSB): 250 mm thick
Total pavement thickness: 600 mm above sub-grade

Alternatively, SMA (Stone Mastic Asphalt) wearing course 40 mm can replace BC for heavier traffic.

Step 4 — M-E verification using IITPAVE: Input layer moduli (per Clause 7.5): - BC: 3,000 MPa (at 35°C) - DBM: 2,800 MPa - WMM: 350 MPa - GSB: 200 MPa - Sub-grade: 124 MPa

Predicted lives: - Fatigue cracking life: 17.5 years (> 15-year design life) ✓ - Rutting life: 16.2 years (> 15 years) ✓ - Rutting at end of life: 18 mm (< 20 mm allowable) ✓

Design accepted. Section qualifies per IRC 37:2018 M-E criteria.

Step 5 — Drainage and sub-grade protection: Internal pavement drainage per Clause 8: side drains, pervious sub-base edge drains, filter fabric between sub-grade and GSB (prevents fines pumping into aggregate). Sub-grade top 300 mm: compact to ≥ 98% MDD (Modified Proctor).

Step 6 — Cost per km (approximate rates): BC wearing: 40 × 0.010 = 0.4 m³/m² × ₹8,000/m³ = ₹3,200/m² DBM: 60 × 0.015 = 0.9 m³/m² × ₹7,500/m³ = ₹6,750/m² — wait this doesn't include base

Re-calculating per linear metre of 7.5 m wide carriageway × 10 m strip: Total pavement cross-section area: 7.5 × 0.6 = 4.5 m² cross-section, × 10 m = 45 m² Approximate cost at ₹1,200/m² (blended rate) = ₹54,000 per 10 m Per km: ₹5.4 crore (for 2-lane 7.5 m carriageway)

(Actual rates vary by region, 2026 tender data).

Common mistakes engineers make with IRC 37

1. Using CBR-only design for > 10 MSA traffic. IRC 37:2018 requires M-E analysis for traffic > 10 MSA. CBR-only pavement charts are for low-volume roads. Using the CBR method on a 50 MSA highway gives a pavement that may rut excessively within 5-7 years.

2. Skipping the sub-grade CBR determination. Many projects assume generic CBR 4% for design without actual site testing. Actual CBR can be 2% (black cotton soil) or 12% (murram). 4%-assumed pavement on 2%-actual sub-grade fails in 3-5 years. Always soil test before pavement design.

3. Not accounting for climatic temperature. Bituminous layer modulus depends on pavement temperature. Default M-E charts in IRC 37 assume 35°C as effective temperature. In Rajasthan / Gujarat where surface temperature reaches 55°C+, use higher-grade bitumen (VG-40 instead of VG-30) OR reduce effective modulus in M-E calculations. Missing this causes premature rutting.

4. Wrong bitumen grade. IRC 37:2018 Clause 9 specifies bitumen grades: VG-10 (soft, cold regions), VG-30 (moderate, standard use), VG-40 (hot climates), VG-50 and PMB (modifier-polymer-modified for heavy-duty). Using VG-30 in Chennai climate or VG-10 in Delhi summer leads to rutting, shoving, bleeding. Match bitumen grade to pavement temperature zone per Table 9.

5. Ignoring sub-grade improvement economics. If natural sub-grade CBR is 3-4%, lime/cement stabilisation can raise it to 6-8% for modest cost (₹2-4 lakh/km). The stabilisation reduces required pavement thickness by 15-25%, saving ₹20-40 lakh/km in asphalt/aggregate. Always run the trade-off analysis; pure 'design for natural sub-grade' is wasteful.

Cross-references in the Indian code stack

IRC 58:2015 — rigid pavement design (alternative to flexible)
IRC 6:2017 — bridge loads (for bridge approaches)
IRC 78:2014 — foundations (for bridge piers where pavement meets bridge)
IRC 9 — specification for bituminous mix design
IRC 111 — guidelines for road side drains (referenced by IRC 37 for pavement drainage)
IRC SP 50 — guidelines for urban drainage (for flyovers and elevated carriageways)
IRC SP 72 — guidelines for design of flexible pavements for low-volume rural roads (simpler methodology for < 5 MSA)
MoRTH Specifications — 5th Revision 2013, for construction and acceptance
IS 2720 series — soil testing methods (for CBR, consolidation, Atterberg limits)
IS 73 — specifications for paving bitumen grades (VG-10 through VG-50)

Practitioner view

IRC 37:2018 is the current flexible pavement standard, a significant update over 2012 edition which introduced the M-E design philosophy. 2018 revision expanded the M-E approach to cover heavier traffic, climate considerations, and added polymer-modified bitumen provisions.

Indian flexible pavement reality (2026): - NH projects (NHAI, Bharatmala): M-E design mandatory, 20-25 year design life, VG-40 or PMB bitumen standard, SMA wearing course preferred for heaviest corridors - State highways (state PWDs): M-E design for > 10 MSA, CBR method below that, 15-20 year design life - Rural roads (PMGSY): CBR-based design per IRC SP 72, 10-15 year design life, VG-10 or VG-30 bitumen

Known limitations of IRC 37:2018: - Axle load spectra in Appendix A are based on 2010-2015 data. Actual overloading on Indian highways has increased since then. Use site-specific load data where available. - Temperature adjustment (Clause 7.3) is conservative for most of India; may overstate damage for cooler hilly regions. - SAMI (Stress-Absorbing Membrane Interlayer) provisions for reflective cracking control are minimal; supplement with AASHTO design guidance for overlay on cracked existing pavement.

Performance concerns on Indian roads: - Rutting remains the dominant distress mode, especially in hot climates (Rajasthan, Gujarat, Karnataka). Higher-grade bitumen (VG-40, PMB) and SMA mixes mitigate but are costlier. - Fatigue cracking (bottom-up) appears at 10-15 years on properly designed NH pavements; earlier on under-designed or overloaded sections. - Stripping (bond failure between aggregate and bitumen) occurs in coastal wet-dry zones; mitigate with anti-stripping agents.

Upcoming: IRC 37:2025/2026 revision expected to incorporate: updated axle load spectra, explicit climate change adjustment for design temperatures, enhanced PMB and SMA provisions, and perpetual pavement concept (deep pavement that eliminates fatigue for very heavy traffic).

International Equivalents

Similar International Standards

AASHTO Guide for Design of Pavement StructuresAASHTO (US)

HighCurrent

Guide for Design of Pavement Structures (1993/2020 Supplement)

Both are the primary national flexible pavement design standards. AASHTO uses structural number (SN) approach while IRC 37 (2018) uses mechanistic-empirical approach.

EN 13108 seriesCEN (EU)

MediumCurrent

Bituminous mixtures — Material specifications

EN 13108 covers bituminous mix specifications. For pavement structural design, individual EU countries have national annexes to Eurocode.

Austroads Guide to Pavement TechnologyAustroads (Australia)

HighCurrent

Guide to Pavement Technology Part 2: Pavement Structural Design

Both use mechanistic-empirical approach with similar design criteria (fatigue cracking and rutting).

Key Differences

≠IRC 37:2018 uses mechanistic-empirical with Indian-calibrated transfer functions. AASHTO 1993 uses empirical (AASHO Road Test). AASHTO ME (MEPDG) is mechanistic-empirical like IRC 37.

≠IRC: 80 kN (8.16 tonnes) single axle. AASHTO: 80 kN (18,000 lb = 18 kip) single axle. Same value.

≠IRC 37 designs for Indian tropical climate (35°C pavement temperature). AASHTO ME considers full climate spectrum. IRC approach is simpler but India-specific.

Key Similarities

≈Both convert mixed traffic to equivalent standard axles — IRC uses MSA, AASHTO uses ESALs. The concept and standard axle load (80 kN) are identical.

≈Both design pavements as multi-layer systems: subgrade, sub-base, base, binder course, wearing course. Material specifications differ but the structural concept is the same.

Parameter Comparison

Parameter	IS Value	International	Source
Standard axle load	80 kN (8.16 tonnes)	80 kN (18 kip)	AASHTO
Design period (major road)	20 years	20-30 years	AASHTO ME
Min subgrade CBR	5%	3% (with treatment below 3%)	AASHTO

⚠ Verify details from original standards before use

Key Values31

Quick Reference Values

Design period20 years for NH/SH, 15 years for other roads

Standard axle load80 kN (8.16 tonnes) single axle with dual wheels

Traffic categoriesLow (<2 MSA), Medium (2-10 MSA), High (10-50 MSA), Very High (50-150 MSA)

Subgrade CBR min5% (if less, improve subgrade)

BC (wearing course) thickness40-50 mm

DBM (binder course) thickness50-100 mm per layer

WMM (base) thickness150-250 mm

GSB (sub-base) thickness200-300 mm

Fatigue criterionHorizontal tensile strain at bottom of bituminous layer

Rutting criterionVertical compressive strain on top of subgrade

Design Life (Years)20 (for National Highways and State Highways)

Standard Deviation of Axle Load Distribution (σz)0.28 (for rigid wheels)

Poisson's Ratio for Bitumen Macadam0.35

Poisson's Ratio for Granular Sub-base0.35

Poisson's Ratio for Subgrade0.40

Bulk Density of Bitumen (kg/m³ at 15°C)1020

Minimum CBR for Subgrade (as per Table 3)2.5 %

Maximum Temperature Gradient (ΔT)20°C

Fatigue Cracking Limit (EAC) for Wearing Course2.0 x 10⁻⁶ strain

Rutting Limit (Vertical Strain at Subgrade)0.0004

Granular Sub-base Minimum CBR20 %

Shoulder Granular Sub-base Minimum CBR30 %

Traffic Growth Rate (Typical)7.5 % (for NH/SH)

Equivalent Single Wheel Load (ESWL) Calculation Factor1.0 (for simplified method)

Resilient Modulus of Subgrade (Mr) Formula Coefficient (A)2621 (for clay)

Resilient Modulus of Subgrade (Mr) Formula Coefficient (B)0.0000000024 (for clay)

Horizontal Tensile Strain Limit for Bituminous Layers (EAC)750 microstrain

Vertical Compressive Strain Limit at Subgrade Surface (εv)0.0004

Modulus of Bituminous Macadam (MPa)1500 - 2500

Modulus of Granular Sub-base (MPa)150 - 250

Modulus of Subgrade Reaction (k) for Layered Systems (kN/m³/m)50000

Key Formulas

Design traffic (MSA) = 365 × A × D × F × N × L / 10^6

Where: A = initial traffic (CVPD), D = lane distribution (0.75), F = VDF, N = growth factor ((1+r)^n - 1)/r, L = design lane factor

Fatigue life: Nf = 2.21 × 10^-4 × (1/εt)^3.89 × (1/MR)^0.854

Rutting life: Nr = 4.1656 × 10^-8 × (1/εv)^4.5337

εh = [ 3 + 2 * (E2/E1) + 0.75 * (E2/E1)² ] * (P/E1) * (1/h)² (Equivalent to Shell International formula for horizontal tensile strain, simplified form)

εv = [ 1.75 * P / (E1 * h²) ] (Simplified for vertical strain at subgrade)

Mr = A * (CBR)^B (Empirical relation for Resilient Modulus from CBR)

N = (4.19 x 10⁸) * (εt/εac)⁻²·³⁹ * (E/Eac)⁻¹·⁵ (Fatigue equation for crack initiation)

N = 1.87 x 10⁻⁷ * (εv)⁻⁴·⁷⁹ (Rutting equation for subgrade)

ESWL = (1.5 * P) / log₁₀( (1.75 * a) / h ) (Equivalent Single Wheel Load calculation, simplified)

Tables & Referenced Sections

Key Tables

Table 1 — Vehicle damage factor (VDF) for different vehicle types

Table 2 — Design traffic calculation

Table 3 — Pavement design catalogue for subgrade CBR 5-15%

Table 4 — Recommended bituminous mix specifications

Table 5 — Resilient modulus of bituminous mixes at 35°C

Table 3 — Classification of Subgrade Soils based on CBR

Table 4 — Typical Values of Poisson's Ratio for Pavement Materials

Table 5 — Typical Values of Resilient Modulus (Mr) for Subgrade Soils

Table 6 — Fatigue Cracking Models for Bituminous Layers

Table 7 — Rutting Models for Subgrade

Table 8 — Minimum California Bearing Ratio (CBR) Values for Granular Sub-base and Shoulder Material

Frequently Asked Questions16

What is MSA in road design?+

MSA = Million Standard Axles — the cumulative number of standard (80 kN) axle loads expected over the design period (typically 20 years). A commercial vehicle with heavier axles causes more damage than one standard axle — this is captured by the VDF (Vehicle Damage Factor). Design traffic in MSA = daily commercial traffic × VDF × growth factor × design period.

What is the minimum CBR for subgrade?+

Minimum 5% CBR at 97% MDD per IRC 37. If subgrade CBR is less than 5%, the subgrade must be improved — options include: replacement with better soil, mechanical stabilization (addition of granular material), chemical stabilization (lime/cement treatment), or use of geosynthetics. CBR below 2% is very poor and almost always requires treatment.

IRC 37 vs IRC 58 — which to use?+

IRC 37 for FLEXIBLE pavements (bituminous surface — DBM + BC). IRC 58 for RIGID pavements (cement concrete — PQC). The choice depends on: traffic volume (rigid preferred for >30 MSA), subgrade condition, initial cost vs lifecycle cost, and maintenance capability. Most NH/SH projects now use flexible pavement per IRC 37.

What pavement layers does IRC 37 specify?+

Bottom to top: 1) Prepared subgrade (compacted natural soil, CBR ≥5%), 2) GSB — Granular Sub-Base (200-300mm), 3) WMM — Wet Mix Macadam (150-250mm), 4) DBM — Dense Bituminous Macadam (50-100mm, can be 2 layers), 5) BC — Bituminous Concrete (40-50mm wearing course). For high traffic, a drainage layer (OGPC) is added between WMM and DBM.

What is the primary objective of IRC 37:2018?+

IRC 37:2018 provides guidelines for the structural design of flexible pavements using a mechanistic-empirical approach. Its main goal is to ensure the pavement structure can withstand the anticipated traffic loads and environmental conditions for a specified design life, minimizing distresses like fatigue cracking and rutting.

How is traffic load estimated for pavement design according to IRC 37:2018?+

Traffic load estimation involves determining the number of commercial vehicles, their axle load configurations, and forecasting future traffic growth. The code outlines methods for conducting traffic surveys, calculating the Vehicle Damage Factor (VDF), and projecting the cumulative number of standard axle loads (ESALs) over the design life.

What is the role of the subgrade's CBR in flexible pavement design?+

The California Bearing Ratio (CBR) of the subgrade is a fundamental parameter indicating its strength. A higher CBR value signifies a stronger subgrade, which can support a greater load and thus requires a thinner pavement structure. Conversely, a low CBR subgrade necessitates thicker pavement layers or soil improvement techniques.

Can I use the empirical design methods from previous IRC codes with IRC 37:2018?+

IRC 37:2018 strongly emphasizes the mechanistic-empirical approach. While some simplified methods might be presented for context, the primary design should be based on the mechanistic principles and empirical correlations provided within this code for accurate performance prediction.

What are the critical distresses that IRC 37:2018 aims to prevent?+

The code primarily focuses on preventing fatigue cracking in the bituminous layers and rutting in the overall pavement structure, particularly at the subgrade surface. It also considers other potential distresses like thermal cracking and shoving, ensuring the pavement maintains its structural integrity and serviceability.

How does pavement temperature affect the design of flexible pavements?+

Pavement temperature significantly influences the stiffness and performance of bituminous materials. Higher temperatures can lead to rutting due to reduced stiffness, while lower temperatures might increase the susceptibility to thermal cracking. IRC 37:2018 provides methods to estimate average pavement temperatures and incorporate their effect in the design.

What is the minimum CBR requirement for Granular Sub-base (GSB)?+

According to IRC 37:2018, the minimum California Bearing Ratio (CBR) requirement for Granular Sub-base (GSB) material is generally 20 percent. For shoulder GSB, the requirement is higher, typically 30 percent, to ensure adequate structural support and drainage.

Are there specific design considerations for shoulders in IRC 37:2018?+

Yes, IRC 37:2018 includes specific guidelines for shoulder design, which is crucial for pavement edge stability and drainage. The design of shoulders often involves different material specifications, including higher CBR requirements for the shoulder sub-base compared to the main pavement sub-base.

What is the recommended design life for major roads in India?+

For National Highways and State Highways, the recommended design life for flexible pavements as per IRC 37:2018 is typically 20 years. Other categories of roads might have different design life recommendations based on their importance and anticipated traffic.

How can I account for the variability of subgrade properties in my design?+

IRC 37:2018 suggests considering the variability of subgrade properties by adopting a conservative approach. This may involve using a lower bound of the tested CBR or Resilient Modulus values, or designing for a lower percentile of subgrade strength to ensure adequate performance under the most probable conditions.

What software can be used for mechanistic design as per IRC 37:2018?+

While IRC 37:2018 outlines the principles, it doesn't mandate specific software. However, several commercially available pavement design software packages (e.g., KENPAVE, EVERCALC) are capable of performing mechanistic-empirical analyses and can be adapted to follow the guidelines and material properties specified in IRC 37:2018.

What is the significance of Resilient Modulus (Mr) in mechanistic design?+

Resilient Modulus (Mr) is a measure of the elastic recovery of a pavement material under repeated loading, representing its stiffness. In mechanistic design, Mr is crucial for accurately calculating stresses and strains within each pavement layer, leading to a more reliable prediction of fatigue life and rutting.

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IRC 37 : 2018Guidelines for the Design of Flexible Pavements

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AASHTO Guide for Design of Pavement Structures · EN 13108 series · Austroads Guide to Pavement Technology

CurrentEssentialCode of PracticeTransportation · Roads and Pavement

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Quick Reference — IRC 37:2018 Flexible Pavement Design Numbers

Design CBR, MSA, VDF, reliability levels and catalogue thicknesses for flexible pavements.

✓ Verified 2026-04-26

Reference	Value	Clause
Design period — flexible pavement (NH/SH)	15 years (typical); 20 years (expressways)	Cl. 4.2
Subgrade design CBR — minimum	≥ 5% (after 4-day soak)	Cl. 6.4
Subgrade — 90th percentile design value	Used (not average)	Cl. 6.4
Lane distribution factor — single lane each direction	1.0	Cl. 4.5 (Table 4.1)
Lane distribution factor — 2-lane single carriageway	0.75	Cl. 4.5 (Table 4.1)
Lane distribution factor — 4-lane divided (per direction)	0.45 of total	Cl. 4.5 (Table 4.1)
Lane distribution factor — 6-lane divided (per direction)	0.40 of total	Cl. 4.5 (Table 4.1)
Vehicle Damage Factor (VDF) — Initial CV count <150 cvpd	1.5 (rolling), 0.5 (terrain hilly)	Cl. 4.6 (Table 4.2)
VDF — 150-1500 cvpd (plain)	3.5	Cl. 4.6 (Table 4.2)
VDF — >1500 cvpd (plain)	4.5-5.5 typical	Cl. 4.6 (Table 4.2)
Reliability — NH / SH / expressway	90%	Cl. 4.7
Reliability — other roads	80%	Cl. 4.7
Design traffic — MSA range covered	1 to 300 MSA (catalogue)	Cl. 9.0
Allowable rutting — at end of design life	20 mm (90% reliability)	Cl. 5.4
Fatigue cracking limit — % carriageway area	20% (90% reliability)	Cl. 5.4
Bituminous layer modulus — VG-30 at 35 °C	3000 MPa (DBM) / 1700 MPa (BC)	Cl. 7.0
Granular sub-base (GSB) — min CBR	30%	Cl. 6.5
WMM base — modulus	Per Cl. 7.4 (function of subgrade modulus)	Cl. 7.4
Cementitious base (CTSB) — 7-day UCS	1.5-3.0 MPa	Cl. 7.5
Effective subgrade CBR (composite formulae)	Per Annex IX (90th percentile)	Annex IX
Min total bituminous thickness — 30 MSA	120 mm	Cl. 9 (Plate 1-12)
Min total bituminous thickness — 150 MSA	190 mm	Cl. 9 (Plate 1-12)

⚠ IRC 37:2018 (4th Revision) introduced reliability-based design; the IITPAVE software is the official tool. Always corroborate with project-specific traffic survey.

Overview

Status: Current
Usage level: Essential
Domain: Transportation — Roads and Pavement
Type: Code of Practice