Michael S Mamlouk

Rational Modeling of Tertiary Flow for Asphalt Mixtures

The objective of this research study was to evaluate several mathematical models to be used in calculating the onset of tertiary flow [referred to as the flow number (FN) parameter] for asphalt mixtures. The FN indicates the onset of shear deformation in asphalt mixtures, which is a significant parameter in evaluating rutting in the field. The FN is obtained from the repeated load permanent deformation (RLPD) laboratory test. Current modeling techniques in determining the FN use a polynomial model fitting approach, which works well for most conventional asphalt mixtures. However, analysis and observations on the use of this polynomial model for rubber-modified asphalt mixtures showed problems in identifying the true FN values. The scope of the work included the collection and analysis of more than 300 RLPD test data files, which comprised more than 40 mixtures, a wide range of test temperatures, and several stress levels. A new comprehensive mathematical model was recommended to accurately determine the FN. The results and analysis were evaluated through manual calculations and found to be accurate, rational, and applicable to all mixture types, whether a tertiary stage was reached or not.

Pavement Preventive Maintenance: Key to Quality Highways

Highway funding is inadequate to maintain current road conditions under traditional maintenance and rehabilitation policies. Through a preventive maintenance program, pavements can be maintained in a cost-effective manner leading to a better pavement quality at lower total costs. Preventive maintenance is a program strategy that arrests light deterioration, retards progressive failures, and reduces the need for routine maintenance activities. The objective of this strategy is to extend the pavement’s functional life by applying treatments before the pavement deteriorates to a condition requiring a corrective treatment, such as a structural overlay. An effective strategy would feature a combination of treatments, such as periodic crack treatment followed by chip sealing. Several highway agencies have used preventive maintenance strategies successfully for both low-volume and high-volume roads. Flexible-pavement preventive maintenance treatments include fog seal, chip seal, slurry seal, microsurfacing, crack treatment, and thin hotmix overlays. The selection of a preventive maintenance treatment should be based on the condition of the existing pavement, traffic volume, and environmental conditions. Other factors include experience, budget constraints, and political reality. The Strategic Highway Research Program demonstrated that preventive maintenance is cost-effective for roads in the National Highway System. FHWA sponsored an implementation project to develop a preventive maintenace workshop and assist states with evaluation of preventive maintenance treatments. The workshop was presented in more than 20 states.

Optimizing Pavement Preservation: An Urgent Demand for Every Highway Agency

Preventive maintenance is a tool that has the potential to both improve quality and reduce expenditures for a pavement network. Preventive maintenance is based on the concept that periodic, inexpensive treatments, such as seals, are more economical than infrequent, high-cost procedures, such as reconstruction. The paper presents a step-by-step procedure for selecting the appropriate preventive maintenance treatment for asphalt pavement and evaluating the optimal timing for that treatment under different pavement, traffic, and climatic conditions. The information presented in this paper is a useful tool for highway engineers and superintendents at various governmental levels throughout the world to develop a preventive maintenance program that would maximize the cost-effectiveness of maintenance treatments. Typical examples of pavement distresses are presented showing appropriate treatments that can be used. A model is presented to provide the basis for the analysis of the cost-effectiveness of a pavement preventive maintenance program.

Mechanistic Roughness Model Based on Vehicle-Pavement Interaction

A mechanistic roughness performance model that takes into account vehicle dynamics was developed for use in flexible pavement design and evaluation. The model was developed in the form of a relation between roughness and number of load repetitions, axle load, and asphalt layer thickness. The model is completely mechanistic and uses vehicle dynamics analysis to estimate the dynamic force profile and finite element structural analysis to estimate the change of pavement surface roughness for each load repetition. The model makes use of the fact that pavement roughness changes the magnitude of the vehicle dynamic forces applied on the pavement and that the dynamic forces change the road roughness. The developed mechanistic roughness performance model can be used to estimate the 80-kN (18-kip) equivalent single-axle load for mixed traffic. The model can also be used to design pavement so that it will last for a certain number of load repetitions before reaching a predetermined roughness level. Performance-based specifications can be developed using the methodology presented in this study. The model has been calibrated and verified with field data elsewhere.

Necessary Assessment of Use of State Pavement Management System Data in Mechanistic–Empirical Pavement Design Guide Calibration Process:

Calibration and validation of the Mechanistic–Empirical Pavement Design Guide (MEPDG) for state department of transportation (DOT) networks requires detailed information for a variety of pavement inputs. The biggest database resource that most states possess is their pavement management system (PMS). It is understandable for any DOT to maximize the use of this resource to the greatest extent possible. However, before any state uses PMS data in MEPDG calibration, it is important that a careful comparison be made of the equality of the measurements, data, and so forth in the PMS database with national Long-Term Pavement Performance (LTPP) values. The purpose of this paper is to review possible differences between several key data found in a state PMS database and the LTPP database. Arizona Department of Transportation (ADOT) PMS data are used for this purpose. The specific variables examined in this paper deal with distress type (rutting, cracking, and roughness) and nondestructive deflection testing (NDT) backcalculated moduli to predict in situ pavement layer properties. It was concluded that significant differences did exist between the ADOT PMS values and LTPP measurements. Differences were found between ADOTs NDT measurements and LTPP data in rut measurements, asphalt cracking, international roughness index, and all layer backcalculated moduli. Consequently, ADOT may incur significant expense to calibrate the MEPDG to Arizona conditions. The paper discusses several possible reasons for differences between the PMS data of a specific agency and those in the LTPP database.

EFFECT OF VEHICLE-PAVEMENT INTERACTION ON PAVEMENT RESPONSE

The structural response of flexible pavements is studied under different dynamic loads and pavement roughness conditions. The factors affecting dynamic load variability are investigated with regard to pavement-vehicle interaction. Furthermore, the study considers the viscoelastic nature of asphalt concrete and the nonlinearity and plasticity of granular and subgrade materials. The Florida COMPAS computer program was used to estimate the dynamic wheel force, and the ABAQUS three-dimensional finite-element program was used to determine the pavement response. The effects of vehicle and pavement characteristics such as vehicle type, vehicle speed, suspension type, level of roughness, pavement stiffness, and layer thickness were studied and statistically analyzed. The walking-beam suspension causes more dynamic load variation than the air-bag and leaf-spring suspension. The dynamic load coefficient for the walking-beam suspension is approximately twice the other suspensions. Vehicle speed is an important factor; the 20 km/hr speed resulted in permanent displacement approximately 10 times the permanent displacement produced by the 130 km/hr speed. The pavement response varies with distance due to roughness. Pavement stiffness and thickness had some effect on pavement response, but truck type and truck suspension type did not have a large effect.

Cracking resistance of asphalt rubber mix versus hot-mix asphalt

ABSTRACT Laboratory beam fatigue tests were performed to evaluate the intermediate- and low-temperature fatigue cracking parameters of typical Asphalt Rubber Mixes (ARM) and hot-mix asphalt (HMA). A notch with known dimensions is cut in the bottom surface of the beam. The beam is subjected to loading-unloading cycles under 3-point bending action. The load applied to the specimen is controlled to obtain a constant rate of crack mouth opening displacement. Such an innovative approach allows us to estimate the rate of crack propagation at any failure level, which cannot be easily measured otherwise. Beam specimens were prepared with different binder contents and tested at two test temperatures of −7 and −1°C. Youngs modulus and non-linear fracture parameters such as critical stress intensity factor, critical crack tip opening displacement and fracture toughness, were obtained at different stages of crack propagation. Statistical analysis shows that ARM has higher fracture toughness and consequently larger resistance to fatigue cracking than HMA. This is particularly beneficial at intermediate and low temperatures, which indicates that AMR can be subjected to a larger number of load repetitions before the development of fatigue cracking as compared to HMA. Another important conclusion is that ARM is less temperature susceptible than HMA. The results also show the ARM has lower modulus values than HMA.

Temperature Gradient and Curling Stresses in Concrete Pavement with and without Open-Graded Friction Course

Curling stresses of concrete pavement can be very damaging, and reducing the temperature swings would be very beneficial. This study includes a field instrumentation effort with pavement temperature sensors to quantify the thermal behavior of concrete pavement with and without an open-graded asphalt rubber friction course. The study shows a nonlinear temperature profile across slab thickness, with a large change in temperature between day and night at the top of the concrete slab, and little change at the bottom of the slab. Adding an open-graded friction course over the concrete pavement reduces the temperature fluctuation between day and night as a result of the aeration effect, which is increased by traffic. A three-dimensional (3D) finite-element analysis with a nonlinear temperature gradient shows that adding the friction course reduces the curling stresses in the summer. Furthermore, since traffic increases the aeration effect, sections without traffic show lower effect of friction course on reducin...

Concept for Mechanistic-Based Performance Model for Flexible Pavements

A concept for a mechanistic-based performance model for flexible pavement was developed that considers the interaction between vehicles and pavement. A dynamic vehicle model was used to estimate the dynamic wheel force, and a three-dimensional finite element nonlinear dynamic pavement model was used to determine the dynamic pavement response. The effect of pavement roughness on vehicle bouncing and the effect of vehicle bouncing on the progression of pavement roughness were investigated under different roughness levels, suspension types, and layer thicknesses. The increase in roughness after each load repetition can be calculated using basic material properties from which the pavement service life can be estimated. The number of equivalent 80-kN single axle load repetitions to failure was estimated under different conditions without the need for empirical observations. It was found that the number of load repetitions to go from one level of present serviceability index (PSI) to the next largely decreases as the PSI level decreases. The air bag suspension results in the longest pavement life, while the walking beam suspension results in the shortest pavement life. The total number of load repetitions to reach failure for thick pavement sections is 14 percent higher than that for medium-thick sections, and 63 percent greater than that for thin sections. The reverse of this analysis can be used to design the pavement section so that it would sustain a certain number of load repetitions before failure using a mechanistic procedure. The proposed concept for a mechanistic-based performance model developed in this study can be refined to increase the mechanistic portion of the model, reduce empirical involvement, and improve computational procedure.

Verification of effectiveness of chip seal as a pavement preventive maintenance treatment using LTPP data

It is hypothesised that maintenance treatments should be applied in the preventive mode before pavements display significant amounts of distress in order to be more cost-effective. The objective of this study was to verify the concept of preventive maintenance by examining the long-term effectiveness of chip seal treatment in four climatic zones in the USA using the long-term pavement performance database. Pavement sections were categorised into smooth, medium and rough pavements, based upon initial condition (IC) as indicated by the international roughness index. Pavement performance of treated and untreated sections was collectively modelled using exponential regression analysis. Effectiveness was evaluated in terms of life extension, relative benefit and benefit–cost ratio. The results showed that preventive maintenance is cost-effective. The life extension, relative benefit and benefit cost ratio were highest for sections whose IC was smooth at the time of treatment. Chip seal treatment effectiveness showed no correlation to climatic conditions or to traffic levels.