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IRC 115 : 2014
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Guidelines for Structural Evaluation and Strengthening of Flexible Road Pavements Using Falling Weight Deflectometer (FWD) Technique

International Comparison — Coming Soon
CurrentEssentialGuidelinesTransportation · Pavement and Road Materials
OverviewValues13InternationalTablesFAQ15Related

Overview

IRC 115:2014 is the Indian Standard (IRC) for guidelines for structural evaluation and strengthening of flexible road pavements using falling weight deflectometer (fwd) technique. IRC 115:2014 is the modern Indian standard for Falling Weight Deflectometer (FWD) based pavement evaluation — the state-of-art technique for structural assessment of flexible pavements and overlay design. FWD simulates moving truck wheel impact (vs BBD's static loading in IRC 81), providing richer and more reliable data. FWD is trailer-mounted, drops a 150-300 kg falling weight on a 150 mm radius load plate producing 40-200 kN impulse; 7-9 geophones measure deflections at 0, 300, 600, 900, 1200, 1500, 1800 mm from plate. The resulting 'deflection bowl' shape allows back-calculation of individual layer moduli (bituminous, base, sub-base, sub-grade). Combined with mechanistic-empirical overlay design per IRC 37, FWD gives comprehensive pavement rehabilitation guidance. Key advantages over BBD: faster (5-min per test vs 30-min), richer data (multi-distance deflections vs single rebound), automated data collection, no dependency on dedicated truck. Amendment No. 1 (2020) added FWD data integration with Pavement Management Systems (PMS). Amendment No. 2 (2023) aligned with Bharatmala NH rehabilitation programme. Equipment cost ₹50 lakh-1 crore vs ₹50k for BBD — justified on high-value corridors (NH, expressways). FWD is now standard for NHAI rehabilitation work; state PWDs increasingly adopting.

Specifies methodology for structural evaluation of existing flexible pavements using Falling Weight Deflectometer (FWD), back-calculation of pavement layer moduli, and design of overlay/strengthening — modern equivalent to IRC 81 (Benkelman Beam).

Status
Current
Usage level
Essential
Domain
Transportation — Pavement and Road Materials
Type
Guidelines
Amendments
Amendment No. 1 (2020) — FWD data integration with Pavement Management Systems (PMS); Amendment No. 2 (2023) — Bharatmala NH rehabilitation programme alignment
Typically used with
IRC 37IRC 81IRC 82
Also on InfraLens for IRC 115
13Key values5Tables15FAQs
Practical Notes
! FWD is the modern standard for pavement evaluation. NHAI uses FWD on all NH rehabilitation projects. State PWDs increasingly adopting.
! FWD vs BBD: FWD 10× faster, more data-rich, better for layer analysis. BBD still used on state projects due to cost. FWD worth for NH/expressway.
! Equipment cost ₹50 lakh-1 crore (Dynatest, JILS, KUAB brands). Plus software ₹2-10 lakh. Operator training essential. Total investment ₹1-2 crore for FWD capability.
! Test productivity: 80-150 tests per day (5-minute per test + travel). Compare BBD 10-30 tests per day. Massive productivity gain.
! Back-calculation software: ELMOD (Dynatest), BAKFAA, EVERCALC (free). Mechanistic-empirical analysis of deflection bowl → layer moduli. Skilled operator essential.
! Layer moduli interpretation: bituminous 2000-8000 MPa (stiffness of asphalt mix); base 200-600 MPa (granular or bound); sub-base 100-300 MPa; sub-grade 40-100 MPa. Values below typical → structural issue.
! Temperature correction: bituminous layer stiffness temperature-dependent. FWD measurement standardized to 35°C (Indian reference). Winter testing temperature correction +20-50% to reference.
! Deflection bowl shape: flat bowl → strong pavement; peaked bowl → weak pavement. Shape analysis identifies which layer is weak.
! Surface Curvature Index (SCI) = d_0 - d_300: indicates bituminous layer stiffness. If SCI > 200 microns, bituminous layer weak.
! Base Damage Index (BDI) = d_300 - d_600: indicates base course condition. If BDI > 150, base weak.
! Sub-base Damage Index (SDI) = d_600 - d_900: sub-base condition.
! Sub-grade Damage Index (SDG) = d_900: sub-grade strength.
! Overlay design using FWD data: mechanistic-empirical method per IRC 37 combines layer moduli + traffic + environment for overlay thickness. More accurate than BBD-based design.
! Pavement Management System (PMS) integration (Amendment No. 1, 2020): FWD data fed into GIS-based PMS. Network-wide condition assessment, automated maintenance scheduling. State PWDs adopting.
! Bharatmala alignment (Amendment No. 2, 2023): NH rehabilitation programme requires FWD evaluation for all candidate corridors. 60,000+ km of NH evaluation ongoing.
! Quality control: calibration of load cell + geophones before each corridor; repeatability check (3 drops per location, variance < 5%); temperature recording at each test.
! Data QC: outlier removal (physically unreasonable deflections), temperature correction, statistical summary per corridor. Cleanup essential before back-calculation.
! Cost of FWD survey: ₹8-15 per metre of pavement. For 10 km test: ₹80k-1.5 lakh. Plus ₹1-3 lakh for back-calculation and analysis. Total ₹2-5 lakh per 10 km corridor.
! Overlay design + material specification: FWD evaluation provides input to IRC 37 design. Output: overlay thickness, material selection, construction specifications.
! Limitations: (1) high equipment cost, (2) skilled operator need, (3) back-calculation can give non-unique results if layer properties similar, (4) deflection bowl interpretation requires experience.
! For urban roads: FWD testing requires lane closure during peak hours may be difficult. Off-peak testing or alternate lanes.
FWDfalling weight deflectometerpavement evaluationoverlay designlayer moduliIRC

International Equivalents

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We're adding equivalent international standards for this code.

Key Values13

Quick Reference Values
falling weight kg150-300
load plate radius mm150
impulse load kN40-200
standard load kN40
impulse duration ms30
geophone count7-9
geophone distances mm0, 300, 600, 900, 1200, 1500, 1800
test duration min5
bituminous modulus MPa2000-8000
base modulus MPa200-600
subgrade modulus MPa40-100
test frequency per 100m1
temperature standard C35
Key Formulas
Burmister model for layered elastic analysis (back-calculation): solve boundary value problem for layered half-space with elastic moduli as unknowns
Surface Curvature Index (SCI): SCI = d_0 - d_300 (difference between plate and 300 mm deflection); indicates bituminous layer stiffness
Base Damage Index (BDI): BDI = d_300 - d_600; indicates base course condition

Tables & Referenced Sections

Key Tables
Table 2.1 — FWD equipment specifications
Table 5.1 — Deflection bowl geometry
Table 7.1 — Typical layer moduli values
Table 9.1 — Test frequency guidelines
Table 10.1 — Temperature correction factors
Key Clauses
Cl. 2 — FWD apparatus: trailer-mounted impact-load device; falling weight (150-300 kg) drops onto load plate producing 40-200 kN impulse; 7-9 geophones measure deflection at plate + radial distances
Cl. 3 — FWD vs BBD (IRC 81): FWD simulates moving truck wheel (impulse loading); BBD uses static truck loading. FWD faster (5-minute per test), more data-rich, less labour-intensive
Cl. 4 — Standard test loading: 40 kN (equivalent to standard axle / 2) on 150 mm radius plate; impulse duration 30 ms; typical deflection response 100-800 microns
Cl. 5 — Deflection bowl: deflections at 0, 300, 600, 900, 1200, 1500, 1800 mm from load plate; shape of bowl indicates pavement strength layer-by-layer
Cl. 6 — Back-calculation: from deflection bowl, compute elastic moduli of pavement layers (bituminous surface, base, sub-base, sub-grade). Specialized software (ELMOD, BAKFAA, EVERCALC)
Cl. 7 — Layer moduli typical values: bituminous surface 2000-8000 MPa, base (DBM/WBM) 200-600 MPa, sub-base 100-300 MPa, sub-grade 40-100 MPa
Cl. 8 — Overlay design: based on back-calculated layer moduli, design overlay thickness to restore pavement to design strength. Mechanistic-empirical method per IRC 37
Cl. 9 — Test frequency: every 100 m on test corridor; minimum 20 tests per corridor for statistical significance; critical locations (dips, cracks) additional
Cl. 10 — Temperature correction: bituminous layer stiffness temperature-dependent; standardize to 35°C pavement temperature (standard for India)
Cl. 11 — Advantages vs BBD: (1) faster survey, (2) multi-distance deflections (layer analysis), (3) automated data collection, (4) independent of truck availability
Cl. 12 — Limitations: higher equipment cost (₹50 lakh-1 crore vs ₹50k for BBD); skilled operator required; software licensing cost
Cl. 13 — Applications: (a) pavement condition survey for rehabilitation, (b) layer stiffness identification for specific material issues, (c) long-term performance monitoring
Cl. 14 — FWD data QC: quality checks for each test (load stability, deflection reasonableness, repeatability); data cleaning before back-calculation
Cl. 15 — Integration with PMS: FWD data fed into Pavement Management System for network-wide condition assessment and budget allocation

Related Resources on InfraLens

Cross-Referenced Codes
IRC 37:2018Guidelines for the Design of Flexible Pavemen...
→
IRC 81:1997Guidelines for Strengthening of Flexible Road...
→
IRC 82:2015Code of Practice for Maintenance of Bituminou...
→

Frequently Asked Questions15

What is FWD (Falling Weight Deflectometer)?+
Per Clause 2: trailer-mounted apparatus that drops a falling weight (150-300 kg) on a load plate producing 40-200 kN impulse. Geophones measure pavement deflection at plate + radial distances. Simulates moving truck wheel for pavement evaluation.
How is FWD different from BBD (IRC 81)?+
Per Clause 3: FWD uses impulse loading (simulates moving truck), BBD uses static loading. FWD is 10× faster (5-min per test vs 30-min), provides multi-distance deflections (vs single rebound), more data-rich, and automated. BBD simpler and cheaper equipment.
What is the standard test load?+
Per Clause 4: 40 kN impulse (equivalent to standard axle / 2) on 150 mm radius load plate. Impulse duration 30 ms. Standard for Indian pavement evaluation.
What is the deflection bowl?+
Per Clause 5: the set of deflection measurements at 0, 300, 600, 900, 1200, 1500, 1800 mm from load plate. Shape of the bowl (flat vs peaked) indicates pavement structural condition layer-by-layer.
How is FWD data analyzed?+
Per Clause 6: back-calculation — from deflection bowl, compute elastic moduli of pavement layers. Software: ELMOD, BAKFAA, EVERCALC. Mechanistic-empirical approach. Skilled operator essential.
What are typical layer moduli values?+
Per Clause 7: bituminous surface 2000-8000 MPa, base (DBM/WBM) 200-600 MPa, sub-base 100-300 MPa, sub-grade 40-100 MPa. Values below typical range → structural issue in that layer.
How often should FWD tests be done?+
Per Clause 9: every 100 m on test corridor; minimum 20 tests per corridor for statistical significance. Critical locations (dips, cracks, transverse joints) get additional tests.
What is temperature correction?+
Per Clause 10: bituminous layer stiffness is temperature-dependent. FWD measurements standardized to 35°C (Indian reference pavement temperature). Winter testing correction +20-50% to reference.
What is SCI (Surface Curvature Index)?+
Per Clause 5: SCI = d_0 - d_300 (deflection at plate minus deflection at 300 mm). Indicates bituminous layer stiffness — high SCI means bituminous weak.
What advantages does FWD offer over BBD?+
Per Clause 11: (1) 10× faster survey (5-min vs 30-min per test), (2) multi-distance deflections enable layer analysis (vs BBD single value), (3) automated data collection, (4) independent of truck availability, (5) better-suited for high-traffic corridors.
What is the cost of FWD equipment?+
Per Clause 12: ₹50 lakh-1 crore (Dynatest, JILS, KUAB). Plus software ₹2-10 lakh. Total investment ₹1-2 crore. Justified on NH/expressway where BBD becomes limiting factor.
When should I use FWD vs BBD?+
FWD for high-value corridors (NH, expressway, major state highways) where data richness justifies equipment cost. BBD for state highway and district road projects where cost is critical. NHAI uses FWD exclusively.
Does FWD data integrate with Pavement Management Systems?+
Per Amendment No. 1 (2020): yes — FWD data fed into GIS-based PMS for network-wide condition assessment, automated maintenance scheduling. Standard practice for modern road agencies.
How does FWD apply to Bharatmala?+
Per Amendment No. 2 (2023): Bharatmala NH rehabilitation programme requires FWD evaluation for all candidate corridors. 60,000+ km of NH being evaluated via FWD for targeted rehabilitation priorities.
What overlay design method uses FWD data?+
Per Clause 8: mechanistic-empirical method per IRC 37. Uses back-calculated layer moduli + design traffic + environment. More accurate than BBD-based empirical design. Overlay thickness, material specification output.

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