PERFORMANCE TESTING OF BITUMINOUS MIXES USING FALLING WEIGHT DEFLECTOMETER

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1 ABSTRACT NO. 6 PERFORMANCE TESTING OF BITUMINOUS MIXES USING FALLING WEIGHT DEFLECTOMETER Prof Praveen Kumar Dr G D Ransinchung Lt. Col. Mayank Mehta Nikhil Saboo IIT Roorkee IIT Roorkee IIT Roorkee IIT Roorkee Key Words: Falling Weight Deflectometer (FWD), Non Destructive Testing (NDT), Pavement, Structural Adequacy, Failures ABSTRACT A Falling Weight Deflectometer (FWD) is a Non-Destructive Testing (NDT) Device used by civil engineers to evaluate the physical properties of pavement. The main objective of this study is to carry out a study on performance of bituminous mixes using Falling Weight Deflectometer (FWD). In doing so, emphasis is laid on structural evaluation of flexible pavements to include Strength evaluation of layers of the flexible pavements with different bituminous layers by determining the corresponding E- values of the layers. In the top layer,, SDBC, BC and recycled material has been tried. In Second Layer BM and DGBM has been used. The Base course is of WMM and the Sub-base course consists of GSB. Effect of various parameters on E values have been found in this study. 1. Introduction A Falling Weight Deflectometer (FWD) is a Non-Destructive Testing (NDT) Device used by civil engineers to evaluate the physical properties of pavement. It is designed to impart a load pulse to the pavement surface which simulates the load produced by a rolling vehicle wheel. FWD is a tool used to achieve rapid and repeatable in-situ characterization of the pavement layer stiffness and to evaluate pavement structural condition. It is being widely used in pavement engineering as it plays a crucial role in selecting optimum pavement maintenance and rehabilitation strategies. FWD data is most often used to calculate stiffness-related parameters of a pavement structure. The process of calculating the elastic moduli of individual layers in a multi-layer system (e.g. asphalt concrete on top of a base course on top of the subgrade) based on surface deflections is known as "back calculation". Instead, initial moduli are assumed, surface deflections calculated, and then the moduli are adjusted in an iterative fashion to converge on the measured deflections. 2. About FWD Some of the commercially available FWD models are explained below: 2.1 Dynatest FWD It is available in two models and 8081 which are used for evaluation of road and airport pavements respectively. The system supports from 7 to 15 deflection sensors. Both the models are trailer-mounted and have the capability to apply loads in the ranges of 7 to 120 KN and 30 to 320 KN respectively. ELMOD software is used for backcalculation of pavement layer moduli. 1

2 Fig 1 Dynatest FWD 2.2 KUAB 2m-FWD The KUAB 2m-FWD can be towed by any suitable towing vehicle. KUAB 50, KUAB 150 & KUAB 600 SP are the three most widely used which vary primarily in terms of their loading capacity. The KUAB 50 model is a light and versatile testing system suitable for a broad range of highway, street and parking lot pavements, with a loading range of 12 to 50 kn. The KUAB 2m-FWD Model 150, is capable of generating dynamic load of 12 to 150 kn. Fig 2 KUAB FWD 2.3. JILS FWD JILS-20-FWD is mounted in a two-axle trailer that can be towed by a van or pick-up. The loading capability ranges from 9 kn to 120 kn. The loading plate used is a 300 mm diameter rigid steel disc with an 8 mm thick heavy duty, neoprene pad attached to it for uniform distribution of the applied loading. Up to ten deflection sensors are supported. Fig 3 JILS FWD 2

3 2.4. Carl Bro FWD The Carl Bro trailer-mounted FWD is the PRI The FWD is mounted to the tow vehicle by a double-axle trailer. It is modular in design, i.e., the loading capability and sensor configuration can be varied as per requirement. The mass mechanism generates force magnitudes up to 250 kn. It supports nine deflection sensors, The equipment is available as trailer and vehicle built-in models. Fig 4 Carl Bro FWD 2.5 IITKGP FWD It was the first indigenous FWD model in India by the Transportation Engineering Section of the Department of Civil Engineering, Indian Institute of Technology, Kharagpur, India. This model is mounted in a trailer, which can be towed with the help of a jeep. With this model, it is possible to apply a load of magnitude ranging from 20 kn to 65 kn with a loading time of about 20 to 30 milli-seconds. This loading time is similar to that produced by a vehicle moving at 50 to 60 km/h. Rubber pads of suitable stiffness were used as spring system to obtain these loading times. Six surface deflections can be measured at radial distances of 0, 300, 600, 900, 1200 and 1500mm with the help of geophones. A chain and pulley arrangement is used for lifting and lowering the mass whereas a chuck arrangement is made for holding the mass at any desired height. One load cell and six geophones are used to measure the magnitudes of load and deflections respectively. The load and deflection signals are recorded in the computer with the help of a data acquisition system. 2.6 Geotran FWD Fig 5 IITKGP FWD It has been indigenously developed at Roorkee. Geotran Falling Weight Deflectometer model GTFWD-V is a Vehicle Mounted Falling Weight Deflectometer. It is mounted on a self- toeing Tata 207 vehicle. It supports six deflection sensors. 3

4 3. Experimental Work 3.1 Construction of Test Tracks Fig 6 Geotran FWD The methodology adopted was to construct Test Pavements using different bituminous mixes as per the methods prescribed in MORT&H (2013) & IRC 37 (2001 & 2012) in order to study performance of bituminous mixes by carrying out structural evaluation of these pavements using the deflection data obtained by testing them by FWD. To achieve the above, 4 ft x 8 ft Test Pavement was constructed near the pavement testing lab at Deptt of Civil Engg, IITR. This size was carefully chosen so as to provide realistic test surface for accommodating all the seven FWD sensors and at the same time to be economical in terms of time and effort. For the purpose of the study, the subgrade, sub base and the base layers have been kept same whereas bituminous layers above them have been varied. 200 mm GSB was constructed over the subgrade as a sub base layer. A second layer of 250 mm WMM (laid in two parts 150 & 100 mm thick) was laid as base layer over the GSB. Thereafter, five cases were considered for surface layers using different bituminous mixes. These are as under:- (i) Case 1 : 40 mm BC (Gd 2) 50 mm (ii) Case 2 : 25 mm 50 mm (iii) Case 3 : 20 mm 50 mm (iv) Case 4 : 25 mm 50 mm (v) Case 5 : 20 mm 50 mm FWD testing was done the next day (5 sets of readings) on the completed layers and E-Values were obtained. Fig 7 FWD Testing of Finished SDBC DBM Layers 4

5 Table 1 Results of FWD testing on Test Pavements Case No Layers T (mm) CD (g/cc) E-value (Bitumen Layer) 1 BC (Gd 2) Analysis of Data Collected Using Fwd The analysis of performance of bituminous mixes involves the assessment of the structural condition of the pavement. Assessment of the remaining life is one such method. This can be done by either of the methods as under: (i) By estimating the traffic loads which the pavement was initially designed for and subtracting traffic loads already been carried. (ii) Directly from critical stress or strain levels in the present condition without taking into account the volume of traffic already carried. (iii) By comparing the moduli of the present layers with those which they actually expected to have initially. The layer moduli obtained from different bituminous layers on the Test Pavement backcalculated from FWD deflection data were used to analyse the pavement for critical strains which are indicators of pavement performance in terms of rutting and fatigue cracking. The guidelines as given in IRC : 37, 2012 were used for this purpose. Fatigue model used for 90 % reliability as given in IRC : 37, 2012 is as under:- where, N f = x x [1/Ɛ t ] 3.89 x [1/M R ] (1) N f Ɛ t = Fatigue life in standard axle load repetitions; = Max tensile strain at the bottom of bituminous layer; M R = Resilient modulus of bituminous mix, MPa. 5

6 Rutting model used for 90 % reliability as given in IRC : 37, 2012 is as under:- where, The method adopted is as under; N = 1.41 x 10-8 x [1/Ɛ v ] (2) N = No of cumulative standard axles; Ɛ v = Vertical compressive strain. (i) Measurement of surface deflections using FWD and obtaining the calculated E-Values for different pavement layers. (ii) Backcalculations are done considering pavement to be a three layered system. All bituminous layers to be considered as combined together. Similarly, granular base & sub base layers to be combined (IRC : 115, 2014). (iii) Analysis of Test Pavement using linear elastic theory with backcalculated moduli and layer thicknesses. This includes computation of critical Horizontal Tensile Strain at the bottom fiber of bituminous layer (Ɛ t ) and Vertical Compressive Strain on top of subgrade (Ɛ v ). IITPAVE software was used for the same. (iv) If desired, estimation of remaining life of the pavement using the fatigue in bituminous layer and subgrade rutting performance criteria adopted in IRC : 37, 2012 given by equations 4.3 & Ɛ t & Ɛ v values obtained using IITPAVE software Table.2: Ɛ t & Ɛ v values calculated for Test Pavements using IITPAVE Case No Layers Ɛ t (IITPAVE) Ɛ v (IITPAVE) Acceptable (Yes/No) 1 BC (Gd 2) x x 10-6 Yes x x do x x do- 6

7 x x do x x do- 5. Conclusions Having conducted laboratory tests on materials and having extensively tested the Test Pavements and various other existing pavements, following conclusions can be drawn:- (i) The E-values obtained from FWD testing of Test Pavements of BC & DBM, SDBC & DBM, & DBM, SDBC & BM, & BM are , , , and respectively. (ii) The comparison of FWD obtained E-Values with those calculated from meth-ods as given in IRC : 37 (2012), validates the results obtained through GEOTRAN FWD. (iii) The Test Pavements were found to have Ɛ t & Ɛv values (calculated using E-values from FWD testing) less than the allowable critical strain values. (iv) (v) (vi) It was observed that the E- values increased with the bitumen content of the bituminous layers till the OBC was reached and thereafter reduced with further increase in bitumen content. It was observed that E-values were directly affected by the degree of compaction of the bituminous layer. There was a considerable decrease in the E-values for poorly compacted bituminous layers. It was found that E-value for bituminous layer increased with increase in its thickness. REFERENCES (1) IRC: , Tentative Guidelines For Design of Flexible Pavements. (2) Kumar R.S., Kumar S., Das A., Reddy K.S., Mazumdar M. and Pandey B.B. (2001)., Development of an Impact Apparatus for Evaluation of Elastic Modulus of Pavement Layers. Indian Geotechnical Society Journal, Vol.31, No.3, IGS, New Delhi, pp (3) Mehta Mayank (2014), Study on Performance of Bituminous Mixes using FWD, M Tech. Thesis, IIT Roorkee (4) Reddy M.A., Reddy K.S. and Pandey B.B. (2002). Evaluation of Pavement LayerModuli using Genetic Algorithms. International Journal on Pavement Engineering and Asphalt Technology, pp (5) SHRP. (1993), SHRP s Layer Moduli Backcalculation Procedure. Report No.SHRP-P-655, Strategic Highway Research Program, National Research Council, Washington, DC. 7