Highway and Specialized Testing

Flexible Pavement Design

IRC 37-compliant pavement thickness design using IITPAVE analysis

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IRC 37:2018 IRC SP 72 IS 2720 Part 16
Flexible pavement design determines the required thickness of each pavement layer — from subgrade to bituminous surfacing — to safely carry the anticipated traffic over the design period. In India, IRC 37:2018 is the governing guideline, using subgrade CBR and cumulative traffic (in million standard axles, MSA) as the primary design inputs.

What Is Flexible Pavement Design?

A flexible pavement consists of multiple layers: the subgrade (natural soil), granular sub-base (GSB), wet mix macadam (WMM) or unbound granular base, cement-treated base (CTB) where required, dense bituminous macadam (DBM), and bituminous concrete (BC) wearing course. Each layer must be thick enough to distribute traffic loads so that the stress at any layer does not exceed its safe limit. IRC 37:2018 uses a mechanistic-empirical approach. The designer inputs the subgrade CBR value, cumulative traffic in MSA (derived from traffic survey data and axle load measurements), and selects a design period (typically 10, 15, or 20 years). The IITPAVE software then analyses the pavement structure for critical strains — horizontal tensile strain at the bottom of the bituminous layer and vertical compressive strain on top of the subgrade — and iterates layer thicknesses until both criteria are satisfied. NKMPV provides complete flexible pavement design services, from subgrade investigation and traffic analysis to final layer composition. Our designs are accepted by NHAI, state PWDs, and highway consultants. We also perform FWD-based structural evaluation for overlay design of existing pavements. Our team serves projects across Haryana, Punjab, Himachal Pradesh, and Rajasthan.

Design Parameters & Input Requirements

The following parameters form the basis of flexible pavement design per IRC 37:2018. Each parameter must be determined through field investigation or laboratory testing before the design can be executed.

Parameter Value / Range Unit Standard
Subgrade CBR (Soaked) >= 8% for NH/SH (minimum per MoRTH) % IS 2720 Part 16 / IRC 37 Cl. 5
Cumulative Traffic (Design MSA) 1-150 MSA (typical range for NH/SH) MSA IRC 37:2018 Cl. 4
Design Period 10, 15, or 20 years years IRC 37:2018 Cl. 3.2
Traffic Growth Rate 5-7.5% per annum (typical) % IRC 37:2018 Cl. 4.2
Vehicle Damage Factor (VDF) Determined from axle load survey dimensionless IRC 37:2018 Cl. 4.3
Reliability Factor 80-95% depending on road category % IRC 37:2018 Table 3
Horizontal Tensile Strain (Bituminous Layer) Must not exceed fatigue life criterion microstrain IRC 37:2018 Cl. 7.2
Vertical Compressive Strain (Subgrade Top) Must not exceed rutting criterion microstrain IRC 37:2018 Cl. 7.3
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Applicable Indian Standards

IRC 37:2018

Guidelines for Design of Flexible Pavements (Fourth Revision)

IRC SP 72

Guidelines for the Design of Flexible Pavements for Low Volume Rural Roads

IS 2720 Part 16

Methods of Test for Soils — Laboratory Determination of CBR

IRC SP 106

Guidelines for Traffic Data Collection on National Highways

MoRTH 5th Revision

Specifications for Road and Bridge Works — Section 400 (Sub-base, Base) and Section 500 (Bituminous Courses)

Software & Equipment Used

IITPAVE Software

IIT Kharagpur / IRC licensed version

Multi-layer elastic analysis for flexible pavement — computes critical strains at all layer interfaces

Calibrated

CBR Testing Machine

AIMIL AIM-304-1

50 kN capacity with motorized loading frame for subgrade CBR determination

Calibrated

Automatic Traffic Counter Classifier (ATCC)

Portable pneumatic tube / video-based ATCC

24/7 classified traffic volume counting for 7-day minimum duration per IRC SP 106

Calibrated

Weigh-in-Motion (WIM) / Portable Weigh Pads

Portable axle load weighing system

Axle-by-axle load measurement up to 30 tonnes per axle for VDF calculation

Calibrated

Falling Weight Deflectometer (FWD)

Trailer-mounted impulse loading device

40-120 kN impulse load with 7-9 deflection sensors for backcalculation of existing layer moduli

Calibrated

Design Process

1

Subgrade Investigation

7-10 days (including CBR soaking)

Representative soil samples are collected from the proposed road alignment at every 500-metre interval and at subgrade depth. Laboratory CBR tests (soaked, 4-day immersion) are performed per IS 2720 Part 16 at optimum moisture content. The design CBR is determined as the value at which 90% of the tested values are equal to or greater than the design value, as specified in IRC 37:2018 Clause 5.

2

Traffic Analysis & MSA Calculation

10-14 days (field survey + analysis)

Classified traffic volume data from a 7-day Automatic Traffic Counter Classifier (ATCC) survey is combined with axle load data from a 48-72 hour weigh-in-motion survey. The Vehicle Damage Factor (VDF) is calculated for each vehicle category. Cumulative traffic in Million Standard Axles (MSA) is computed using the formula: MSA = 365 x A x D x F x N, where A is commercial vehicles per day, D is lane distribution factor, F is VDF, and N is the growth factor for the design period.

3

Pavement Composition Selection

1-2 days

Based on the design MSA, the appropriate pavement layer composition is selected from IRC 37:2018 catalogue tables. For traffic above 2 MSA, the standard composition includes granular sub-base (GSB), wet mix macadam (WMM) or cement-treated base (CTB), dense bituminous macadam (DBM), and bituminous concrete (BC). For high-traffic corridors (above 30 MSA), a cement-treated sub-base (CTSB) may be included for additional structural capacity.

4

IITPAVE Structural Analysis

1-2 days

The trial pavement section is modelled in IITPAVE software with elastic modulus values for each layer. The software computes horizontal tensile strain at the bottom of the bituminous layer (fatigue criterion) and vertical compressive strain on top of the subgrade (rutting criterion). Layer thicknesses are iterated until both strain values fall within the permissible limits specified in IRC 37:2018 for the design reliability level.

5

Sensitivity Check & Optimisation

1-2 days

Multiple design alternatives are analysed to optimise cost. For example, increasing the CBR through subgrade stabilisation may reduce overall crust thickness. Substituting WMM with CTB can reduce granular layer thickness. Each alternative is checked against IRC 37 strain criteria using IITPAVE. The most economical section satisfying all structural requirements is recommended.

6

Design Report Preparation & Delivery

3-5 days

The final design report includes subgrade CBR data, traffic analysis with MSA calculation, IITPAVE output sheets showing critical strains, recommended pavement composition with layer-by-layer thickness, material specifications for each layer per MoRTH Section 400/500, and a cross-section drawing. The NABL-accredited report is delivered in hard copy and digital format for submission to NHAI, state PWDs, or project consultants.

Where Flexible Pavement Design Is Used

Flexible pavement design per IRC 37 is mandatory for all new national and state highway construction in India. NHAI requires a detailed pavement design report as part of the Detailed Project Report (DPR) for every highway project. The same methodology applies to state PWD roads, urban arterials, and industrial access roads where bituminous pavements are proposed. For existing roads requiring strengthening, FWD backcalculation provides in-situ layer moduli that feed into overlay thickness design. The subgrade CBR test is the foundation of every flexible pavement design. NKMPV also provides bituminous mix design services to ensure the designed layers meet MoRTH material specifications for DBM, BC, and SMA.
National highway new construction pavement design per IRC 37:2018 State highway and major district road pavement crust design Urban arterial and ring road pavement design for municipal corporations Industrial estate and port access road design for heavy vehicle loading Overlay thickness design for existing flexible pavements using FWD data Rural road pavement design per IRC SP 72 for PMGSY projects Airport taxiway and apron flexible pavement design Toll plaza approach road design for concentrated heavy vehicle loading

Detailed Information

Flexible pavement design is a crucial process in highway and road construction to ensure long-term performance, safety, and cost efficiency. At NKMPV Research and Development LLP, we provide professional flexible pavement design services strictly as per IRC 37 guidelines, considering traffic loading, subgrade CBR, climatic conditions, and material properties. Our pavement designs deliver optimized layer thickness and reliable structural performance for highways, urban roads, and infrastructure projects across India.

What Is Flexible Pavement Design?

Flexible pavement design involves determining the appropriate thickness and composition of pavement layers such as subgrade, granular sub-base (GSB), wet mix macadam (WMM), bituminous base, and wearing course. These layers work together to distribute traffic loads safely to the subgrade while accommodating repeated loading and environmental effects.

IRC 37 Guidelines for Flexible Pavement Design

IRC 37 provides standardized procedures for designing flexible pavements in India based on:

  • Design traffic in million standard axles (MSA)

  • Subgrade strength using CBR values

  • Pavement materials and layer properties

  • Environmental and drainage conditions

Our designs fully comply with the latest IRC 37 recommendations used for NHAI, state highways, and urban road projects.

Key Inputs Required

To achieve accurate and reliable pavement design, we consider:

  • Traffic survey and axle load data

  • Subgrade CBR test results

  • GSB and WMM material properties

  • Bituminous mix characteristics

  • Climatic and drainage conditions

Flexible Pavement Design Methodology

Our pavement design process includes:

  • Evaluation of traffic loading (MSA calculation)

  • Analysis of subgrade strength using laboratory and field data

  • Selection of pavement composition as per IRC 37 design charts

  • Calculation of layer thickness for durability and economy

  • Design documentation suitable for DPR and approval submissions

Applications of Flexible Pavement Design

Flexible pavement design is widely used for:

  • National and state highways

  • Urban and rural roads

  • Industrial roads and access roads

  • Airport service roads

  • Heavy traffic corridors

Why Choose NKMPV for Flexible Pavement Design?

  • NABL-accredited testing support

  • IRC & MoRTH compliant design methodology

  • Integration with traffic survey and material testing

  • Experienced pavement and geotechnical engineers

  • Reliable designs for long-term performance

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Conclusion

Accurate pavement design is essential for building durable and economical road infrastructure. With NKMPV’s IRC 37 compliant flexible pavement services, project owners and consultants receive technically sound, field-validated designs supported by comprehensive testing and traffic analysis. Our approach ensures safe load distribution, reduced maintenance, and extended pavement life.

Why Choose NKMPV for Flexible Pavement Design?

NABL Accredited Testing & Design

Our CBR test results and pavement design reports carry NABL accreditation (ISO/IEC 17025:2017). The complete package — from subgrade investigation to final design — is accepted by NHAI, state PWDs, and project consultants without additional verification.

End-to-End Design Capability

We perform every step in-house: CBR testing, traffic surveys (ATCC), axle load measurements, IITPAVE analysis, and design report preparation. No sub-contracting of critical inputs means consistent quality and faster delivery.

IITPAVE Mechanistic-Empirical Analysis

We use the IRC-recommended IITPAVE software for multi-layer elastic analysis, ensuring every design is checked against fatigue and rutting strain criteria — not just catalogue thickness tables. This results in optimised, cost-effective pavement sections.

FWD-Based Overlay Design

For existing roads, we use Falling Weight Deflectometer data to backcalculate in-situ layer moduli and design overlay thicknesses that account for the structural contribution of existing pavement layers — avoiding over-design and unnecessary cost.

Regional Highway Experience

Our team has designed pavement sections for national highways, state highways, and urban roads across Haryana, Punjab, and Himachal Pradesh. We understand the local subgrade conditions, traffic patterns, and climate factors specific to North India.

Frequently Asked Questions

IRC 37:2018 (Guidelines for Design of Flexible Pavements, Fourth Revision) is the governing Indian standard for designing bituminous pavement layer thicknesses. It uses a mechanistic-empirical approach based on IITPAVE software to compute critical strains and determine safe layer thicknesses for a given subgrade CBR and cumulative traffic (MSA). All national and state highway projects in India must follow this guideline.
The two primary inputs are: (1) Subgrade CBR — determined through laboratory soaked CBR testing per IS 2720 Part 16, and (2) Cumulative traffic in MSA — computed from classified traffic count data, axle load survey, vehicle damage factor, growth rate, and design period. Additional inputs include the design reliability level, lane distribution factor, and material properties for each proposed pavement layer.
MSA stands for Million Standard Axles — it represents the cumulative number of standard 80-kN single-axle loads expected over the design period. The formula is: MSA = 365 x A x D x F x N, where A is the initial daily commercial traffic, D is the lane distribution factor, F is the vehicle damage factor (from axle load data), and N is the cumulative growth factor. For a 20-year design period with 7.5% growth, a road with 2,000 commercial vehicles per day can generate 50-100 MSA.
For a national highway with 20-50 MSA, a typical IRC 37:2018 composition is: granular sub-base (GSB) 200 mm + wet mix macadam (WMM) or cement-treated base (CTB) 250 mm + dense bituminous macadam (DBM) 120-140 mm + bituminous concrete (BC) 40-50 mm. The total crust thickness ranges from 600-800 mm depending on subgrade CBR and traffic. For very high traffic (above 100 MSA), a cement-treated sub-base (CTSB) layer is added.
A complete design — including subgrade CBR testing (7-10 days with soaking), traffic survey (7 days minimum), axle load survey (2-3 days), data analysis, IITPAVE modelling, and report preparation — typically takes 3-4 weeks. If CBR and traffic data are already available from the client, the design analysis and report can be delivered in 5-7 working days.
Flexible pavements use bituminous layers (DBM, BC) over granular bases and are designed per IRC 37:2018. They distribute loads through a graded layer system. Rigid pavements use a Portland cement concrete (PCC) slab and are designed per IRC 58:2015. Rigid pavements rely on slab stiffness to distribute loads and are checked for flexural stress rather than strain. The choice depends on traffic type, initial cost vs life-cycle cost, and maintenance capability.

Related Testing Services

California Bearing Ratio Test (CBR - Soaked & Unsoaked)

Laboratory CBR testing for subgrade strength evaluation per IS 2720 Part 16.

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Falling Weight Deflectometer (FWD) Test

Pavement structural evaluation by deflection measurement per ASTM D4694 and IRC 115.

Learn More →

Traffic Survey Services (ATCC)

Classified traffic volume count and axle load data collection per IRC SP 72/106.

Learn More →

Axle Load Test Services for Highways

Vehicle-by-vehicle axle load measurement for pavement design inputs per IRC SP 72/37.

Learn More →

Bituminous Mix Design (BM, DBM, BC, SMA)

Marshall stability, flow, and volumetric analysis for hot mix asphalt per MS-2/MoRTH 500.

Learn More →

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