Overlay design under IRC 115 follows a six-step workflow from FWD field data to overlay-thickness recommendation: (1) deflection-basin capture using a Falling Weight Deflectometer with 7-9 geophones, (2) temperature normalisation of bituminous-layer moduli to 35°C reference, (3) back-calculation of layer moduli using software such as ELMOD or BAKFAA, (4) structural number (effective SN_eff) computation from back-calculated moduli, (5) projected traffic loading in million standard axles (MSA) for the design life, and (6) overlay-thickness derivation using the IRC 115 design equations or the Asphalt Institute equation as alternate. The output is a chainage-wise overlay-thickness recommendation with material-specification and rehabilitation-priority ranking.
Step 1 — Field Deflection-Basin Capture
FWD test points are typically spaced at 250 m on project-level surveys and 500 m on network-level work. At each test point, the FWD applies a calibrated impulse load (typically 40 kN, simulating a heavy commercial vehicle wheel) through a circular load plate of 300 mm diameter. Seven to nine geophones positioned at radial distances of 0, 200, 300, 450, 600, 900, 1200, 1500, and 1800 mm from the load centre measure the resulting deflection basin.
Pavement temperature is measured at each test point using infrared probes for surface temperature and embedded probes (or computed from time-of-day correlation) for in-depth temperature. Air temperature, time of day, and date are recorded on each test record for subsequent normalisation.
Step 2 — Temperature Normalisation
Bituminous-layer modulus varies by an order of magnitude between cold (10°C) and hot (50°C) pavement conditions. IRC 115 specifies normalisation to a reference temperature of 35°C using the temperature-correction equations in the standard. Without normalisation, deflection values measured at different times of day across the same project produce non-comparable layer moduli — and overlay-design recommendations become time-of-survey artefacts rather than structural-condition indicators.
Step 3 — Back-Calculation of Layer Moduli
Back-calculation is the iterative process of deriving the elastic moduli of each pavement layer (bituminous course, granular base, sub-base, subgrade) from the measured deflection basin. The mathematical model is a multi-layer elastic system; common software implementations are ELMOD (Dynatest), BAKFAA (FAA), MODULUS (Texas DOT), and EVERCALC (Washington State DOT).
The back-calculation algorithm seeds initial layer-modulus estimates (typically from default IRC 115 ranges by layer type), computes the predicted deflection basin using elastic-layer theory, compares against the measured deflection basin, and iterates until the residual error converges below a tolerance threshold (typically 2-3% RMS error).
| Layer | Typical Modulus Range (MPa) | Notes |
|---|---|---|
| Bituminous Concrete (BC) | 1,500 – 7,000 | Highly temperature-sensitive; normalised to 35°C |
| Dense Bituminous Macadam (DBM) | 1,200 – 5,000 | Lower modulus than BC at same temperature |
| Wet Mix Macadam (WMM) | 200 – 600 | Granular base layer; modulus depends on moisture and grading |
| Granular Sub-Base (GSB) | 150 – 400 | Sub-base layer; primarily drainage and load-distribution function |
| Subgrade | 30 – 150 | Soil-type and moisture dependent; correlates with CBR (E ≈ 10 × CBR for fine-grained soils) |
Step 4 — Structural Number (SN_eff) Computation
From back-calculated layer moduli and known layer thicknesses (from as-built records, GPR surveys, or extracted cores), the effective Structural Number (SN_eff) is computed using the AASHTO formulation adopted in IRC 115:
SN_eff = 0.0045 × h_BC × (E_BC)^(1/3) + 0.0042 × h_GB × (E_GB)^(1/3) + 0.0040 × h_GSB × (E_GSB)^(1/3)
where h_i is the thickness of layer i in mm and E_i is the back-calculated elastic modulus in MPa. The computation is performed for each test point along the project corridor, producing a chainage-wise SN_eff profile.
Step 5 — Projected Traffic Loading
Design traffic for the rehabilitation period is computed from current Average Annual Daily Traffic (AADT) data, traffic-growth-rate projections per IRC SP 84, vehicle-damage-factor application per IRC 37, and lane-distribution factor for multi-lane carriageways. The output is the projected cumulative axle load in million standard axles (MSA) over the design life — typically 10, 15, or 20 years for highway overlay rehabilitation.
Step 6 — Overlay-Thickness Derivation
Overlay thickness is computed from the difference between required Structural Number for projected traffic (SN_required) and the existing effective Structural Number (SN_eff):
h_overlay = (SN_required − SN_eff) / a_overlay
where a_overlay is the structural-layer coefficient of the overlay material (typically 0.40-0.45 for Bituminous Concrete and 0.35-0.40 for Dense Bituminous Macadam under IRC 115). The minimum overlay thickness is constrained to the construction-feasibility minimum of 30-50 mm depending on the overlay material specification.
| Chainage Range | SN_eff | SN_required (15 yr, 50 MSA) | Overlay Thickness |
|---|---|---|---|
| 0+000 – 1+000 | 3.5 | 5.2 | 85 mm BC |
| 1+000 – 2+000 | 3.2 | 5.2 | 100 mm BC |
| 2+000 – 3+000 | 4.0 | 5.2 | 60 mm BC (min thickness applied) |
| 3+000 – 4+000 | 2.8 | 5.2 | 120 mm DBM + 40 mm BC (composite) |
| 4+000 – 5+000 | 3.7 | 5.2 | 75 mm BC |
IRC 115 Deliverable Format for NHAI / DPR Submission
An NHAI-accepted IRC 115 overlay-design report under MoRTH guidelines typically includes: (a) executive summary with overlay-thickness recommendation per chainage segment, (b) FWD field methodology and equipment specification, (c) raw deflection-basin tabulation, (d) temperature normalisation appendix, (e) back-calculated layer-moduli per test point with software output, (f) structural-number computation with intermediate steps, (g) traffic-loading projection with growth-rate justification, (h) overlay-thickness recommendation table with material specification, (i) GIS map overlay showing thickness contours, and (j) NABL accreditation certificate.
Common Back-Calculation Mistakes
- Skipping temperature normalisation — produces back-calculated bituminous moduli that are time-of-day artefacts rather than structural-condition indicators
- Using default layer thicknesses without verification — back-calculation residuals dominate where actual thickness differs significantly from assumed thickness; verify with GPR or cores on representative sections
- Accepting back-calculation results with high RMS error — RMS error above 5% indicates either incorrect layer-system assumption or sensor malfunction; re-test or re-model
- Ignoring non-linearity at high deflections — subgrade modulus drops at higher load levels for some soil types; use multi-load drop sequence to characterise non-linearity
- Pairing FWD-only back-calculation with no NSV surface-condition input — sections with severe surface distress (alligator cracking, deep ruts) require surface-treatment in addition to overlay; FWD alone misses this design dimension
Related Reading
- FWD vs Benkelman Beam — Pavement Evaluation Methods Compared
- NSV vs FWD vs Benkelman Beam — Three-Way Comparison
- FWD Test Cost in India — Per-Test-Point Pricing Guide
- FWD Testing Service — engagement details and quotes
Need an IRC 115 overlay-design FWD survey with full back-calculation deliverable? NKMPV is NABL-accredited (TC-14144 under ISO/IEC 17025:2017) for FWD testing across India and provides ELMOD-based back-calculation with overlay-thickness output. Visit the FWD Testing service page or call +91-82953-60108 with your project corridor length, design traffic, and existing pavement structure for a project-specific quote.