Louay N. Mohammad

Louisiana experience with crumb rubber-modified hot-mix asphalt pavement

A comparative study of laboratory and field performance of several applications of crumb rubber-modified (CRM) hot-mix asphalt in Louisiana is presented. Eight CRM asphalt pavement sections were constructed by eight different CRM processes or applications. These eight CRM sections were built at five state highway projects. A control section with conventional asphalt mixture was constructed at each project to compare with the performance of pavement sections built with CRM asphalt mixtures. To evaluate the mixture characteristics of the CRM and conventional mixes, laboratory tests of Marshall stability and flow, indirect tensile strength and strain, and indirect tensile resilient modulus were conducted on field compacted Marshall specimens. Comparisons of the field performances of the pavements were achieved through roadway core air void analysis, rut-depth measurement, international roughness index, pavement structure numbers measured through the Dynaflect (dynamic deflection determination) system, and visual inspections of cracks. The results indicated that the conventional mixtures exhibited higher laboratory strength characteristics than the CRM mixtures. The pavement sections constructed with CRM asphalt mixtures showed overall better performance indices (rut depth, fatigue cracks, and international roughness index numbers) than the corresponding control sections after 5 to 7 years of traffic.

Fracture Resistance Characterization of Superpave Mixtures Using the Semi-Circular Bending Test

The fracture resistance of asphalt mixture is an important property directly related to pavement distresses, such as cracking. This paper reports the investigation of a newly-developed semicircular bending (SCB) test as a candidate test for the fracture resistance characterization of asphalt mixtures. Thirteen Superpave mixtures, designed with four different binder types (AC-30, PAC-40, PG70-22M, and PG76-22M) and four different compaction levels (Ndesign = 75, 97, 109, and 125), were considered in this study. The SCB tests were conducted at 25°C using a three-point bending fixture in a MTS testing system. The fracture resistance was analyzed based on an elasto-plastic fracture mechanics concept of critical strain energy release rate, also called the critical value of J-integral (JC). Preliminary results indicate that the JC values were fairly sensitive to changes in binder type and nominal maximum aggregate size (NMAS) used in Superpave mixtures. This study suggests that the SCB test could be a valuable correlative tool in the evaluation of fracture resistance of asphalt mixtures.

Influence of Asphalt Tack Coat Materials on Interface Shear Strength

Asphalt tack coat is a light application of asphalt, usually asphalt diluted with water. It is used to ensure a bond between the surface being paved and the overlying course. Normally, hot asphalt cements, emulsified asphalts, or cutback asphalts are used as tack coats. The objective of this study was to evaluate the practice of using tack coats through controlled laboratory simple shear tests and determine the optimum application rate. The influence of tack coat types, application rates, and test temperatures on the interface shear strength was examined. Four emulsions (CRS 2P, SS-1, CSS-1, and SS-1h) and two asphalt binders (PG 64-22 and PG 76-22M) were selected as tack coat materials. The residual application rates considered were 0.00 (0.00), 0.09 (0.02), 0.23 (0.05), 0.45 (0.1), and 0.9 (0.2) L/m2 (gal/yd2). A simple shear test was performed to determine the shear strength at the interface at two test temperatures, 25°C (77°F) and 55°C (131°F). The results indicated that CRS-2P emulsion was the best tack coat type and 0.09 L/m2 (0.02 gal/yd2) was the optimum application rate at which a maximum interface shear strength was measured for both test temperatures.

Analytical modeling and experimental study of tensile strength of asphalt concrete composite at low temperatures

In this study, analytical modeling of the tensile strength of hot-mix asphalt (HMA) mixtures at low temperatures was developed. To do this, HMA mixtures were treated as a two-phase composite material with aggregates (coarse and fine) dispersed in an asphalt mastic matrix. A two-phase composite model, which was similar to Papanicolaou and Bakoss [J. Reinforced Plast. Compos. 11 (1992) 104] model with a particle embedded in an infinite matrix, was proposed. Unlike Papanicolaou and Bakoss model, an axial stress was introduced to the fiber end to consider the load transferred from the asphalt mastic the aggregate. Efforts were also made to consider the effect of aggregate gradation, asphalt mastic degradation, and interfacial damage between the aggregates and asphalt mastic matrix on the tensile strength of the HMA mixtures. Experimental investigations were conducted to validate the developed theoretical relations. A reasonable agreement was found between the predicted tensile strength and the experimental results at low temperatures. Parameters affecting the tensile strength of asphalt mixtures were discussed based on the calculated results.

LOUISIANA EXPERIENCE WITH FOAMED RECYCLED ASPHALT PAVEMENT BASE MATERIALS

Utilization of existing recyclable materials has always been key to more efficient and economical highway construction. Use of the foamed-asphalt (FA) technique to stabilize recycled asphalt pavement (RAP) is one strategy for an efficient use of salvaged construction materials. The main objective of this study is to investigate the potential use of FA-treated RAP as a base course material in lieu of a crushed-limestone base beneath a concrete pavement layer. Test sections were constructed at US-190 near Baton Rouge, Louisiana, and used for field evaluation of the FA RAP base. The laboratory mixture design of the FA RAP, the construction of the experimental base section, and the field evaluation of the stiffness of the FA RAP base layers using different in situ testing devices are presented. Preliminary results of both laboratory and field tests showed that the FA-treated RAP mixtures are very promising and can be used as an alternative to the traditional limestone base beneath a concrete pavement layer.

Micromechanics Study on Top-Down Cracking

Top-down cracking is a type of cracking that rivals the severity and prevalence of reflective cracking. It significantly reduces the pavements quality service life. Yet the nature of top-down cracking has not been completely understood. Recent studies of the causes of top-down cracking have focused on identifying the mechanisms that induce tensile stresses at the surface by applying different combinations of surface tractions and the finite element method. Asphalt concrete is treated as a uniform linear elastic material. A new and different approach is presented for investigating the causes of top-down cracking by means of micromechanics. In this approach, asphalt concrete is viewed as a bonded granular material, and the microstructure, including aggregate particle configuration and mastic stiffness, is considered. Theories that predict the existence of tensile stress under compressive loading were reviewed. Both qualitative and quantitative experimental methods were developed to observe the location of top-down cracking and to measure the tensile strains in the pavement. The experimental results indicate the following: (a) top-down cracking may initiate not only at the pavement surface but also at some distance down from the surface; (b) both tensile-type and shear-type cracking could initiate top-down cracking; and (c) top-down cracking may most likely initiate when the mastic is weaker or the pavement temperature is higher. Therefore, a mix sensitive to rutting may also be sensitive to cracking.

Evaluation of Environmental Effectiveness of Titanium Dioxide Photocatalyst Coating for Concrete Pavement

Self-cleaning, air-purifying concrete pavement is a rapidly emerging technology that can be constructed with air-cleaning agents with a super-hydrophilic photocatalyst such as titanium dioxide (TiO2). Although this technology has the potential to support an environmentally friendly road infrastructure, several design and operational parameters may affect its effectiveness and need to be evaluated. The objective of this study was to evaluate the environmental and mix design parameters that may affect the effectiveness of the environmental performance of TiO2 coating. An experimental program was conducted: the effects of relative humidity level, flow rate of pollutants, and mix design parameters, including contents of TiO2 and aggregate sizes, were investigated. The environmental efficiency of the samples to remove nitrogen oxides from the atmosphere was measured by using a newly developed laboratory setup. Results of the experimental program showed that the mix designs without fines achieved the highest photodegradation rates. In addition, the increase from 3% to 5% TiO2 resulted in little improvement in the nitrogen oxide removal efficiency, which decreased with the increase in the humidity level and the pollutant flow rate.

Permanent Deformation Analysis of Hot-Mix Asphalt Mixtures with Simple Performance Tests and 2002 Mechanistic-Empirical Pavement Design Software

A complex laboratory study in characterization of permanent deformation resistance of hot-mix asphalt (HMA) mixtures is presented. Six plant-produced HMA mixtures were selected for this study. The main objective was to characterize the permanent deformation characteristics of HMA mixtures based on four laboratory tests, namely, the dynamic modulus |E*|, flow number, frequency sweep at constant height (FSCH), and Hamburg-type loaded wheel-tracking tests. The secondary objective was to evaluate the sensitivity of the dynamic modulus |E*|-test results in pavement rutting performance prediction with the 2002 mechanistic-empirical (M-E) pavement design software. Test results indicate that the |E*|-test was sensitive to the nominal maximum aggregate size in an HMA mixture. Larger aggregates combined with aged materials tend to have high |E*|-values at high temperatures. However, both the |E*|- and FSCH tests could not correctly rank the permanent deformation characteristics for the six HMA mixtures considered i...

Modeling and evaluation of the cracking resistance of asphalt mixtures using the semi-circular bending test at intermediate temperatures

The semi-circular bend (SCB) test configuration has been favored by many researchers due to the ease of sample preparation, including cores removed from the field and the quick and simple testing procedure. It offers the potential of assessing the cracking resistance of asphalt mixes in the laboratory in the design phase as well as in QA (quality assurance) testing activities. The objective of this study was to conduct a comprehensive evaluation of the SCB test and to utilize this test to evaluate a number of asphalt mixtures against cracking failure. Results of the experimental program were used to validate a three-dimensional (3D) finite element (FE) model, which was used to interpret and to analyze the failure mechanisms in the SCB test. Results of the experimental program showed that the SCB test results successfully predicted the fracture performance of the evaluated mixes and was able to differentiate between them in terms of cracking resistance. Mixtures prepared with polymer-modified binders were the best performers in this test against fracture. Results of the SCB test were in agreement with the DCSE (Dissipated Creep Strain Energy) test and identified the mixtures with high RAP content and the one prepared with unmodified binder as possible poor cracking performers in the field. The SCB test process as well as the propagation of damage were successfully simulated using 3D FE and cohesive elements. The presented modeling approach was in good agreement with measured test results for all mixtures. Based on the results of the FE model, damage that propagates in the vicinity of the notch is mainly caused by a combination of vertical and horizontal stresses in the specimen. The effect of shear was negligible in progressing damage in the specimen.

Engineering Behavior of Lime-Treated Louisiana Subgrade Soil

Lime stabilization is often used to treat subgrade soils when they are soft and cohesive in nature. A study was conducted to investigate the engineering behavior, including the resilient and strength behaviors, of a lime-treated subgrade soil. The lime treatment procedure was adapted from the specifications of the Louisiana Department of Transportation and Development. Silty clay, a soil often found in Louisiana subgrades, is used as a base soil. A summary of various engineering properties of a lime-treated soil from resilient modulus, unconfined compression strength, and California bearing ratio (CBR) tests conducted at five moisture content and dry density levels is provided. Tests were also performed on the raw soil without lime treatment, and these results were compared with those of tests with the lime-treated soil. The comparisons indicate that the present lime treatment method results in an increase in strength and resilient modulus properties and a decrease in plasticity characteristics and plastic strains. A regression model with three constants was used to analyze the resilient modulus test results. The model constants are presented as functions of soil properties. Resilient modulus correlations that use either CBR or unconfined compression strength, moisture content, dry density, degree of compaction, and stresses as dependent attributes are developed.