International Journal of Scientific & Engineering Research Volume 4, Issue 2, February-2013 1 ISSN 2229-5518 IJSER © 2013 http://www.ijser.org Field Measurement of Vertical Strain in Asphalt Concrete Md Rashadul Islam 1 , and Rafiqul A. Tarefder 2 Abstract— Recently developed Mechanistic-Empirical Pavement Design Guide (MEPDG) determines the probable total rutting by summing up the deformations of all layers of the trial pavement. If all distresses outcomes including rutting are within the specification the trial section is de- signed for construction. Therefore, vertical deformation (or strain) needs to be measured to validate the MEPDG for local conditions. However, no direct procedure for measuring the vertical strain in flexible pavement is available to this date. Traditionally, Earth Pressure Cell (EPC) is used to measure the vertical stresses at different layers of pavement. Then, vertical strain is calculated using the measured stress and the stiffness of the corresponding material. The present study describes a procedure to measure the vertical strain of asphalt concrete based on field instrumen- tation. The vertical strain is measured using the Vertical Asphalt Strain Gauge (VASG) in an instrumented pavement section on Interstate 40 (I- 40) in the state of New Mexico, USA and compares the results with numerical model developed using ABAQUS. The numerical model is validat- ed with stress responses measured with installed four EPCs. The stiffness input of the numerical model is obtained from Falling Weight Deflec- tometer (FWD) test. Promising agreement is observed between the strains measured in the field and determined from the numerical model. Therefore, the VASG can be considered an effective sensor to measure the vertical strain of asphalt concrete in flexible pavement. Index Terms— Asphalt concrete, Vertical stain, Vertical stress, Vertical asphalt strain gauge, Earth pressure cell, Falling weight deflectometer test, Numerical analysis, Validation ——————————  —————————— 1 INTRODUCTION utting is one of the major distress outcomes of Mechanis- tic-Empirical Pavement Design Guide (MEPDG). It is de- termined by summing up of permanent deformations of all layers along the wheel path [1]. Therefore, measuring vertical strain is so important for accurate rut prediction. Eq. 1 is used to calculate the permanent deformation in asphalt concrete: ∆ ௣( ுெ஺) = ߝ ௣( ுெ஺) ℎ ுெ஺ = ߚ ଵ௥  ௭ ߝ ௥( ுெ஺) 10 ௞ భೝ  ௞ మೝ ఉ మೝ  ௞ యೝ ఉ యೝ (1) where ∆ ௣( ுெ஺) = Accumulated plastic vertical deformation in Hot Mix Asphalt (HMA) (in.), ߝ ௣( ுெ஺) = Accumulated plastic vertical strain in HMA layer, ߝ ௥( ுெ஺) = Elastic strain calculated by structural response model at mid-depth of each sub layer, ℎ ுெ஺ = Thickness of HMA layer, (in.), n= Number of axle-load repetitions, T= Pavement temperature (⁰F),  ௭ = Depth confining factor, function of HMA total depth and the depth concerned, ߚ ଵ௥,ଶ௥,ଷ௥ = Local field calibration constants,  ଵ௥,ଶ௥,ଷ௥ = Global field calibration constants, Therefore, ߝ ௥( ுெ஺) is the only parameter needed to be meas- ured to determine the plastic deformation. Accurate meas- urement of vertical strain is necessary for probable rut estima- tion. Measuring vertical deformation or strain of asphalt con- crete is much cumbrous. Several researchers measured the vertical deformation or vertical stress, not the vertical strain. Scullion et al. [2] used Multi Depth Deflectometer (MDD) to measure the vertical deflection under Falling Weight Deflec- tometer (FWD) and vehicle loads. A 38 mm diameter (1.5") and 2.1 m (7.0 ft.) deep borehole was prepared in the pave- ment to install the sensors. The researchers measured the rela- tive and the total permanent deformation. However, determin- ing actual deformation from this data is near impossible. Schaevitz HCD-500 Linear Variable Displacement Trans- ducer (LVDT) was used to measure the vertical deformations of the asphalt and concrete and subgrade base in Minnesota Road Research Project [3]. A PVC pipe of 2.4 m (8 ft.) long and 112 mm (4.5") diameter was inserted into the pavement in which a reference rod of 3.6 m (12 ft.) long and 25 mm (1.0 in.) diameter was inserted into the pipe. The pipe was mounted to 25 mm (1.0") below the pavement surfaces. The LVDT was placed at a certain depth on the reference cap. The process is really cumbersome and the inserted 112 mm pipe also deterio- rates the quality of pavement and the hole may act as stress focal point. This installation procedure was not followed in any future studies. National Center for Asphalt Technology (NCAT) performed several instrumentation projects in 2003 and 2006 and studied the vehicle-pavement interaction and effects on pavement deterioration [4], [5]. Vertical stress and horizontal strain of asphalt concrete were measured in these studies with Geokon 3500 Earth Pressure Cells (EPCs) and Construction Technology Laboratories Inc. (CTL) supplied Horizontal Asphalt Strain Gauges (HASGs). Twelve flexible pavements were instru- mented in Virginia Smart Road project [6], [7]. The researchers installed Dynatest Strain Gauges to measure horizontal strain in asphalt concrete and Geokon 3900 EPC to measure vertical R ———————————————— 1 Ph.D. Student, Dept. of Civil Engineering, University of New Mexico, MSC 01 1070, 1 University of New Mexico, Albuquerque, NM 87106; USA. PH (505) 363-6902; email: mdislam@unm.edu. 2 Associate Professor, Dept. of Civil Engineering, University of New Mexico. MSC 01 1070, 1 University of New Mexico, Albuquerque, NM 87106; USA.