STRUCTURAL EVALUATION AND STRENGTHENING OF FLEXIBLE ROAD PAVEMENTS USING FALLING WEIGHT DEFLECTOMETER (FWD) TECHNIQUE-IRC:115-2014

Discussion on

By staff of

Pavement Materials and Geotechnical Division

Evaluating the strength of the existing pavement layers

Estimating the Remaining of life of the existing pavement

Determination of strengthening requirement

Scope

Performance of flexible pavements evaluated by applying loads on the pavements that simulate the traffic loading, recording the response of such loading by measuring the elastic deflection under such loads and analyzing the data considering the influencing factors like subgrade strength, thickness and quality of each pavement layer, drainage conditions and pavement surface temperature.

PRINCIPLE OF PAVEMENT EVALUATION USING FWD

An Impulse loading device in which a transient load is applied to the pavement and the deflected shape of the pavement surface is measured.

Components of FWD:a) Load cells to measure the applied loadb) Loading plate of dia. 300mm to 450mmc) Rubber Pad of 5mm thick glued to the bottom of

Loading plate for uniform distribution of loadd) Falling mass in the range of 50kg to 350kg is

dropped from a height of fall in the range of 100 to 600mm

e) Geophones/Deflection transducers to measure the vertical surface deflection at radial locations.

f) Data Acquisition system

FALLING WEIHT DEFLECTOMETER

A mass of weight is dropped from a pre determined height onto a series of spring/buffers placed on top of loading plate. The corresponding peak load and peak vertical surface deflections at different radial locations are measure and recorded.

Working Principle of FWD

Calibration of the FWD: Static Calibration

The readings of the FWD load cell should be 2 percent of the reference load cell reading. The date of calibration of load cell should not be earlier than one year from the date of testing. Load repeatability

The standard deviation of applied peak load estimated from minimum of 12 load drops should be less than 5% of the mean value of the applied peak load. Absolute calibration of deflection transducers

Deflection transducers should be accurate to 2 percent of reference deflections. The date of calibration of geophones should not be earlier than one year from the date of testing.Deflection repeatability check may also conducted using data collected in load repeatability test.

Classification

Pavement Condition

Good Cracks < 3mm width in less than 5% area of total paved surface and avg. rut depth <10mm

Fair Cracks < 3mm width in 5% to 20% area of total paved surface and avg. rut depth between 10 to 20mm

Poor Cracks > 3.0mm width in 5 to 20% area of toata paved surface or cracking of any type > 20% area of paved surface and avg. rut depth of >20mm

Identification of Uniform sections

Based on the pavement condition survey the uniform sections will be identified in accordance with the table given below

Classification of pavement sections

Note: The length of each uniform section should be minimum 1km except in case of any localized failures.

Estimation of Sample size: N= (z*CV)2/ (ME2) Where, N= sample size z = Normalized normal deviate which can be obtained

from standard statistical tables for a selected confidence level (recommended 90% confidence level)

CV = Coefficient of variation of deflection expressed as percentage (recommended values for ‘good’, ‘fair’ and ‘poor’ sections taken as 15, 30 and 45%)

ME= accept of margin of error (as percentage of mean) , recommended 10%

Therefore minimum sample sizes for ‘good’, ‘fair’ and ‘poor’ sections will be 4,15,33.

Deflection Measurement

Recommended Deflection measurement scheme

Outer Wheel Paths of Outer lanes:

I. For single-lane two-way carriageway – 0.6m from outer edge of outer laneII. For multilane single carriage way - 1.0m from outer edge of outer laneIII. For divided carriageways with two or - 0.75m… more lanes in each direction

Outer Wheel Paths of inner lanes:

I. For multilane single carriage way - 4.0m from outer edge of outer laneII. For divided carriageways with two – 4.2m from outer edge of outer lane lanes in each direction For divided carriageways with three - 4.2m for central lane & lanes in each direction 5.2m for lane adjacent to median

Scheme of Wheel Paths

Section Identity (Chainage) Position of lane in the carriageway (Outer, Inner, etc.) Transverse position of test point in the lane Measurement spacing Time of test Air temperature, pavement temperature at a depth of

40mm Drop number Peak values of load and deflections of each drop of load Whole time history of load and deflections should be stored

for one of the test points of each road sections Deflection transducers configuration Loading plate diameter

Data should be recorded at each test point

Steps for measuring deflection at a test point

Correction for temperature standard temperature 35oC Corrected back calculated modulus ET1 = λ ET2

Where,

λ = temperature correction factor

= 1- 0.238 ln T1/1-0.238 ln T2 Correction for Seasonal Variation

Correction factors for subgrade

Esub_mon = 3.351 * (Esub_win)0.7688 – 28.9

Esub_mon = 0.8554 * (Esub_sum)– 8.461

Correction factors for Granular layer

Egran_mon = - 0.0003*(Egran_sum)2 –0.9584*(Egran_sum)– 32.989

Egran_mon = 10.5523* (Egran_win)0.624 – 113.857

Corrections for deflection data

Bituminous layers – Thick layers without much cracking - 750 MPa to 3000 MPa - Distressed condition (Fair to Poor) - 400 MPa to 1500 MPa Granular Layers - 100 MPa to 500 MPa

Subgrade Modulusa) If No information is available - 20 MPa to 100 MPa

b) If Subgrade CBR is Known - 5*CBR to 20*CBR Mpa

c) IF DCP CBR is Known - 357.87*(DCP)-0.6445

d) From the surface deflections measured at radial distances 1200mm,1500mm and 1800mm

Esubgrade (MPa) = (1-µ2)*P/(3.14*r*w)

Range of Subgrade modulus (MPa)= 1.2*Esubgrade*0.8 - 1.2* Esubgrade*1.2

Ranges of elastic Moduli of different layers

Overlay design using FWD Technique.xlsx

Procedure for overlay design based on FWD Evaluation