FWD VS BBD
Previously Benkleman beam method is used to design overlay of any overloaded road section. But this method had some limitations. Benkleman beam method used to measure of deflection of road under static load condition which is totally opposed to actual dynamic wheel load. It is generally adopted for low specification bituminous layer like BM, premix surfacing etc. so that it would underestimate actual load carrying capacity of DBM or BC when tested for. Bankleman beam method didn’t give us any modulus value which is very required to design a road section in modern concept. So that falling weight method brings more reliable concept for road design as it depends on dynamic load with deflection concept.
Uses of FWD
Falling weight deflectometer (FWD) is a test device used by civil engineers to assess physical properties of road surfaces on motorways, local roads, airport surfaces, port areas and railway tracks. The FWD method is to apply dynamic load to an existing pavement in the form of a standardised fall weight to measure the deflection at different points on the pavement and to simulate deflection in the shell of the wheel under load. This is a device mounted on a trailer that is operated by dropping the weight onto the pavement and measuring the resulting deflection of the pavement. FWD is a device for assessing the stiffness of pavements and road surfaces. Falling weight deflectometer (FWD) is a ROMDAS (road measurement and data acquisition system) used to collect structural and functional conditions in the service of flexible road surfaces at network level.
Falling weight deflection meters are an important research and design tool for imitating heavy traffic and measuring the pavement’s response to the load. Deflection sensors can be used to measure the deformation of the road surface in response to load impulses.
The characteristics of the road surface, including its layer thickness, conditions and temperature fluctuations, influence the predicted load capacity of the road surface as calculated from the layer module. However, the layer modules may not be accurate enough to calculate a measured deflection basin that meets the standard conformity within certain tolerable limits.
In road construction practice, a fall-weight deflectometer (FWD) is recognised as a non-destructive tool for assessing the mechanical properties of each layer of the road system. The calculation of FWD data provides fast information about the in situ stiffness values of individual layers.
Need of back calculation
This chapter briefly explains the methods of data analysis and the software developed for this purpose. FWD data analysis software is provided by FWD vendors, academic institutions and government agencies.
In India KGPBACK is used for interpretation of FWD data which is developed by IIT,Kharagpur. Elastic moduli calculated from KGP application need to be corrected as per IRC 115-2014 and correction may be seasonal correction or pavement temperature correction etc.
Working Principle of FWD
Failing weight deflectometer is consist of circular loading plate with glued rubber pad below of diameter 300 mm to 450 mm. 50 to 350 kg failing weight dropped from a height 100 mm to 600 mm in such a combination to produce 40 Kn load on road surface which is similar to one sided dual wheel set of one axle of 80 Kn. Geophones ( Deflection sensors) are situated at radial distance may in six to nine in numbers.
Positions of deflection sensor are denoted by D1, D2,D3, D4, D5, D6, D7,D8, D9 at radial distance 0mm, 200mm, 300mm, 450mm, 600mm, 900mm, 1200mm, 1500mm, 1800mm for nine number of geophones case.
Pavement temperature is determined by drilling a 40 mm depth hole on road and filled up with glycerol and put thermometer half hourly interval to carryout temperature.
Spacing of test interval of any road section may be varied which is depending upon road surface condition determined in terms of crack detailing of that section. On basis of which road is characterized in three different types namely ‘Poor’, ‘Fair’, ‘Good’.
For detail illustration we consider random FWD data on a single location. These are values at different sensors (Nine numbers of deflection sensors are considered) all the readings are in mm
|Data collected at system|
D1- 0.06013 D2-0.05320 D3-0.04799 D4-0.04289 D5-0.03300 D6-0.02951 D7-0.02306 D8-0.01870 D9-0.01485
Two more things that we need to put in KGPBACK these are layer thickness and ranges of layer moduli.
Layer thickness can be achieved by pit cutting on that above said location and ranges of modulus of different layers are calculated by guidelines given in IRC 115 that is for bituminous layer
• Layer sufficient thick with little cracking- 750Mpa to 3000Mpa
• Distressed Bituminous layer( Poor to Fair characterized) – 400 Mpa to 1500 Mpa
For Subgrade layer it I the range of 5 x CBR to 20 x CBR and for granular it ranges from 100Mpa – 500Mpa.
Say CBR value calculated in our case in terms of laboratory test is 7.0%. Thickness of bituminous layer is 225 mm and thickness of granular is 500 mm.
In our case road is considered as thick bituminous layer with cracks.
So that the ranges are as given below-
1. Bituminous layer = 750-3000 Mpa
2. Granular layer= 100-500 Mpa
3. Subgrade = 5*7 – 20*7 Mpa= 35-140
In our case pavement temperature is 28° C.
Input data in KGPBACK as follows
# INPUT DATA #
No.of Layers = 3
FWD Load (N) = 40000.00
Contact Pressure (MPa) = .56
No.of Deflection points = 9
Deflections measured using FWD (mm) = .06013 .05320 .04799 .04289 .03300 .02951 .02306 .01870 .01485
Radial distances from centre of load(mm) = .0 200.0 300.0 450.0 600.0 900.0 1200.0 1500.0 1800.0
Layer thickness (mm) = 225.00 500.00
Poisson ratio values = .50 .40 .40
Layer Modulus (MPa) Ranges Selected :-
(a) Bituminous Surfacing = 750.0 3000.0
(b) Granular Base = 100.0 500.0
(c) Subgrade = 35.0 140.0
# OUTPUT DATA #
Backcalculated Layer Moduli are:
Surface (MPa) = 2969.2
Base (MPa) = 499.6
Subgrade (MPa) = 139.8
Temperature correction (Lemda)
Temperature correction is only applicable for Bituminous layer and as the FWD test is standardized in 35 degree temperature so that if asphalt temperature is different from 35° C then need to be corrected. For this purpose we use (Reddy,2003) equation given by
ET1 = Lemda x ET2
ET1= Back calculated elastic modulus at T1
ET2= Back calculated elastic modulus at T2 , T1= 35°C = Standard Temperature
For this , Lemda= (1-0.238*In(35))/(1-0.238*In(28)) = 0.74335
Corrected elastic modulus for bituminous layer = Lemda x 2969.2
= 0.74335 x 2969.2 =2207.15 Mpa
Correction of seasonal variation
Worst condition of road is generally considered as monsoon period in India. Granular layer and subgrade are mostly affected by this condition. But it always not feasible to conduct FWD test in monsoon period so that calculated value in winter or in summer season need to corrected.
Based on R-81 Reddy, 2003 research for that four equations (Each two set of winter and summer) is available
Esm= 3.351 x (Esw)-28.9 —————————– 1
Esm= 0.8554 x (Ess)-8.461————————————2
Egm= 0.0003 * Egs^2 + 0.9584* Egs-32.989—————3
Esw=Subgrade modulus in winter (Mpa)
Ess=Subgrade modulus in summer (Mpa)
Egs=Granular layer modulus in summer (Mpa)
Egw=Granular layer modulus in winter (Mpa)
Esm= Elastic modulus of subgrade in monsoon (in Mpa)
Egm= Elastic modulus of granular layer in monsoon (in Mpa)
In our case test had been conducted in summer so that eq no 2 and 3 are required to be used.
Esm= 0.8554 x 139.8 – 8.461= 111.12 Mpa
Egm= 0.0003 x 499.6^2+0.9584 x 493.7 – 32.989=515.05 Mpa
Final determination of subgrade elastic modulus
For design of road subgrade elastic modulus value should be taken as minimum of Esm and subgrade modulus calculated from equation given by IRC 37.
As per code IRC 37
Es= 10*CBR ( CBR <5%)——————-A
Es= 17.6*CBR^.64 (CBR=>5% ——————–AA
In our case it is 7 % CBR >5% so that Eq no AA is applicable.
Es= 17.6*7^.64=61.15 Mpa
Adopted Subgrade modulus = Min (Es, Esm)
= 61.15 Mpa
So that final selected values will be in our case
Bituminous layer elastic modulus= 2207.15 Mpa
Granular layer elastic modulus= 515.05 Mpa
Subgrade elastic modulus = 61.15 Mpa