Amara Loulizi

MEASUREMENT OF VERTICAL COMPRESSIVE STRESS PULSE IN FLEXIBLE PAVEMENTS: REPRESENTATION FOR DYNAMIC LOADING TESTS

Testing at Virginia Smart Road allowed determination of the vertical compressive stress pulse induced by a moving truck and by falling weight deflectometer (FWD) loading at different locations beneath the pavement surface. Testing was performed on 12 different flexible pavement sections. Stress and temperature were measured using pressure cells and thermocouples, respectively, that had been installed during construction of the road. Target testing speeds were 8 km/h, 24 km/h, 40 km/h, and 72 km/h. The considered depths below the pavement surface were 40 mm, 190 mm, 267 mm, 419 mm, and 597 mm. A haversine or normalized bellshape equation was found to be a good representation of the measured normalized vertical compressive stress pulse for a moving vehicle. Haversine duration times varied from 0.02 s for a vehicle speed of 70 km/h at a depth of 40 mm to 1 s for a vehicle speed of 10 km/h at a depth of 597 mm. For the FWD loading, a haversine with a duration of 0.03 s was found to approximate the induced stress pulse at any depth below the pavement surface. Currently, laboratory dynamic testing on hot-mix asphalt (HMA) specimens is performed using a haversine wave at loading duration of 0.1 s. Because HMA is a viscoelastic material, the loading time affects its properties and, therefore, it is recommended that the loading time of HMA dynamic tests be reduced to 0.03 s to better match loading times obtained from moving trucks at average speed and from FWD testing.

Successful Application of Ground-Penetrating Radar for Quality Assurance-Quality Control of New Pavements

The successful application of ground-penetrating radar (GPR) as a quality assurance–quality control tool to measure the layer thicknesses of newly built pavement systems is described. A study was conducted on a newly built test section of Route 288 located near Richmond, Virginia. The test section is a three-lane, 370-m-long flexible pavement system composed of a granular base layer and three different hot-mix asphalt (HMA) lifts. GPR surveys were conducted on each lift of the HMA layers after they were constructed. To estimate the layer thicknesses, GPR data were analyzed by using simplified equations in the time domain. The accuracies of the GPR system results were checked by comparing the thicknesses predicted with the GPR to the thicknesses measured directly from a large number of cores taken from the different HMA lifts. This comparison revealed a mean thickness error of 2.9% for HMA layers ranging in thickness from 100 mm (4 in.) to 250 mm (10 in.). This error is similar to the one obtained from the direct measurement of core thickness.

APPROACH TO DETERMINING IN SITU DIELECTRIC CONSTANT OF PAVEMENTS: DEVELOPMENT AND IMPLEMENTATION AT INTERSTATE 81 IN VIRGINIA

A major problem in using ground penetrating radar (GPR) for estimating pavement layer thickness is assuming the dielectric properties of that layer. Pavement dielectric properties may vary significantly due to aggregate type, moisture presence, and other conditions. Therefore, uncertainties in the dielectric constant, which may vary from 3 to 15, will result in misleading thickness determination. Obtaining cores for calibration may reduce the error, but the variation in the dielectric constant along the roadway often leads to errors in the thickness determination. A method was developed to determine the dielectric constant, and therefore the thickness, of the hot-mix asphalt (HMA) layer of a pavement using GPR. Because of the different compositions and ages of the layers forming HMA in older pavements, dielectric constant estimation based on the surface reflection may not be accurate and may lead to wrong thickness estimations. The developed method uses a modified common midpoint technique (usually used in seismic testing) to estimate the dielectric constant, based on the reflections from a common point at the bottom of the layer. Data were collected from a 27-km portion of Interstate 81 and processed with this technique. Comparison between the thickness estimated by this method and that measured on cores extracted from the highway revealed a mean error of 6.8%.

Comparing Resilient Modulus and Dynamic Modulus of Hot-Mix Asphalt as Material Properties for Flexible Pavement Design

With the current trend toward developing mechanistic flexible pavement design and the need for more reliable design procedures, accurate characterization of hot-mix asphalt (HMA) properties is needed. Resilient and dynamic modulus tests were performed at five temperatures on two typical mixes used in the Commonwealth of Virginia to compare the test results. The dynamic modulus was measured at six frequencies at each of the testing temperatures, and the resilient modulus test was performed at one loading time. The study found that the size of the specimen statistically affected the measured resilient modulus value. Resilient modulus values obtained in the 100-mm-diameter specimens were higher than those obtained in the 150-mm-diameter specimens at all testing temperatures. No statistical differences were observed in the resilient modulus of the two mixes. However, statistical differences were found in the dynamic modulus of the two mixes. A strong relation between the dynamic modulus test performed at 5 Hz...

DATA COLLECTION AND MANAGEMENT OF THE INSTRUMENTED SMART ROAD FLEXIBLE PAVEMENT SECTIONS

The flexible pavement research facility at the Virginia Smart Road consists of 12 different designs. All sections are closely monitored through a complex array of sensors located beneath the roadway embedded during construction. The environmental sensors include thermocouples for temperature measurements, time domain reflectometry probes to measure moisture content in the base layers, and resistivity probes to measure frost penetration. The dynamic sensors include pressure cells and strain gauges to measure stresses and strains, respectively, induced at different layers from truck loading. Environmental data are collected daily every 15 min for temperature, every hour for moisture, and every 6 h for frost penetration. Truck testing is performed every week with different loading configurations. The loading variables include three load levels, three wheel inflation pressures, and four different speeds. Data are managed by saving environmental data from different instruments separately using date and section number. Truck loading data are saved by test type (based on loading configuration, inflation pressure, and speed), date of test, and section number. A database is being generated for all 12 sections to study the effect of all tested variables on the different flexible pavement designs. The performance of the used instruments and collected data are presented, and the techniques used to manage the overwhelming data are discussed. In addition, based on instrumentation responses, a preliminary discussion of the load distribution in a tested pavement system, the effect of speed on pavement stress and strain responses, and the effectiveness of drainage layer are discussed.

Optimization of tack coat application rate for geocomposite membrane on bridge decks

One of the critical components of the U.S. civil infrastructure, bridges, has rapidly deteriorated in the past two decades and is in need of maintenance and rehabilitation. Geosynthetics may have the potential to provide a long-term solution to some of the problems that are present in these bridges, mainly, chloride intrusion into bridge decks. When installed properly, geosynthetics can act as both a moisture barrier and a stress absorption layer. However, the tack coat application rate is critical, as an excessive amount can cause eventual slippage, whereas too little may result in debonding. A new geocomposite membrane that comprises a low-modulus polyvinyl chloride layer sandwiched between two layers of nonwoven geotextile has recently been introduced for use in highway systems for water impermeation and strain energy absorption. A laboratory testing program was conducted to determine the optimum asphalt binder tack coat application rate that needs to be applied in the field. To accomplish this, a fixture was designed to allow the application of cyclic shear loading at the geocomposite membrane interface when used as an interlayer simulating a concrete bridge deck overlaid with the geocomposite membrane and a hot-mix asphalt (HMA) overlay. The study concluded that 1.75 kg of PG 64-22 binder per m2 is an optimum value to achieve excellent bonding and minimum slippage potential. For the upper surface in contact with a wearing surface mix, a tack coat application rate of 1.5 kg/m2 may be used. When the geocomposite membrane was included between concrete and HMA, failure occurred after a much larger number of applied loading cycles than the number of loading cycles to failure when the geocomposite was absent. In addition, the slope of shear stress versus the number of loading cycles at failure was much greater when the geocomposite was absent.

Pavement response to dual tires and new wide-base tires at same tire pressure

Although concern was raised about the introduction of radial tires due to their higher inflation pressure compared with that of bias tires, radial tires have been proven to reduce the strain at the bottom of the hot-mix asphalt (HMA) layer. However, conventional wide-base single tires have been shown to be more damaging to pavement than dual tires. The damage mainly depends on the tire tread width and inflation pressure. It has been suggested that wide-base tires may produce damage equivalent to that of dual tires if the maximum load per tire is limited to 11.6 kg/mm of tire tread width. Recent advances in tire design and material have led to the design of a new wide-base tire that is wider and flatter in the crown area to provide a uniform contact stress distribution. It operates at an inflation pressure of 690 kPa for 151-kN tandem axle load. An experimental program studied the effects of the newly developed wide-base tire on a flexible pavement section at the Virginia Smart Road under different loading and environmental conditions. Testing results have shown that the newly developed wide-base tires induce approximately the same horizontal tensile strains under the HMA layer as do equivalent dual tires. Hence, the fatigue damage expected from these newly developed wide-base tires is the same as that produced by dual tires. However, the vertical compressive stresses induced by the wide-base tire are greater on the upper HMA layers of the pavement. The difference in stresses diminishes with depth and becomes negligible at the bottom of the subbase layer.

Evaluation of Geosynthetics Used as Separators

Geosynthetics have been used in pavement systems for several purposes, including reinforcement, layer separation, drainage, and moisture barriers. For the layer-separation application, the geosynthetic material is used to prevent soil fines from migrating into the base-course layer as well as stones from this layer from penetrating into the subgrade. This material migration would affect the drainage capability as well as the structural capacity of the pavement. However, such an effect is very hard to detect since soil pumping will occur under the pavement surface, and therefore a comparison of the performance of different types of geosynthetic separators is almost impossible. A 3-year project to study the in situ behavior of geosynthetically stabilized flexible pavements in Bedford County, Virginia, ended recently. Results from ground-penetrating radar surveys and materials excavation are presented. Ground-penetrating radar surveys, falling-weight deflectometer results, rutting measurements, and ground-truth excavation indicated that the separation provided by geotextiles was important in reducing base-course contamination by subgrade soil. Such a reduction will significantly reduce the resilient modulus of the base-course layer. In addition, service-life predictions of evaluated sections were conducted based on the traffic applied and rutting distress. Geosynthetics improved secondary-road pavement performance; geotextiles increased service life more than geogrids, due to their separation function.

Modification of bending beam rheometer specimen for low-temperature evaluation of bituminous crack sealants

It is difficult to evaluate effectively the low-temperature stiffness of bituminous hot-poured crack sealants with existing test methods. The standard bending beam rheometer (BBR) was found to be inappropriate for testing soft bituminous-based hot-poured crack sealant, even at a temperature of-40°C. To address this issue, the moment of inertia of the tested beam was increased by doubling its thickness (from 6.35 mm to 12.7 mm). For the new beam dimensions, only 4% of the beam center deflection is due to shear, a value deemed acceptable for sealant evaluation and comparison. On this basis, the BBR stiffness of hot-poured sealants was obtained at several discrete temperatures between-10°C and -40°C to assess the repeatability of the method for the evaluation of the low-temperature stiffness of bituminous sealants. Ten different sealants were tested at -40°C; three of these 10 were further tested at –35°C, –30°C, –28°C, –25°C, and –20°C; and the three “hard” sealants were tested at –10°C. A minimum of three replicates were used. The coefficient of variation on the measured stiffness after 60 s of loading was always lower than 18%, with almost 75% of the measurements having a coefficient of variation less than 10%. A pairwise comparison showed that the modified BBR could be used to classify sealant products according to their measured stiffness. As to the effect of temperature, it was found that the stiffness varies exponentially with temperature in the range of –40°C to –20°C. A statistical analysis of the results indicated that the modified BBR method could be used to classify sealants based on low-temperature stiffness.

Laboratory Investigation of Cement-Treated Reclaimed Asphalt Pavement Material

AbstractCold in-place recycling is considered as a pavement rehabilitation technique that is more environmentally friendly than the common practice of simply constructing a new hot-mix asphalt (HMA) overlay on the existing wearing surface. This paper describes a laboratory experimental study for the formulation and mechanical characterization of cement-treated reclaimed asphalt pavement (RAP) material to be used as a base layer during the rehabilitation of pavements. RAP content was varied between 0 and 100% in order to study the effects of this parameter on the mechanical properties. Portland cement content was kept constant at 6% by weight of dry constituents. Results have shown that an increase in RAP content results in a decrease in compressive strength, flexural strength, indirect tensile strength, and elastic modulus. However, for any RAP content of 60% or less, acceptable mechanical properties for a pavement base layer were achieved. Unrestrained shrinkage testing showed statistically insignificant...