Ramon Bonaquist

Asphalt Mix Master Curve Construction Using Sigmoidal Fitting Function with Non-Linear Least Squares Optimization

This paper presents a new simplified method of constructing a master curve of asphalt mix using test data covering a large range of temperatures from –18°C to 55°C. It utilizes sigmoidal fitting function and compressive cyclic (complex) modulus test data obtained at matrix combination of different frequencies and test temperatures. In the master curve construction, the time temperature superposition was modeled two different ways. First, using known time-temperature superposition equations, and second, shilling test data experimentally, i.e., not assuming any functional form for the time-temperature relationship. The master curve construction was done using an Excel spreadsheet with the solver function, which is a tool for performing optimization with non-linear least squares regression technique. The analysis of over sixty mixtures indicated that the experimental approach agreed the best with the Arrhenius shifting equation, correlation coefficient R 2 being 0.922. Also, the experimental shifting was most flexible, producing the best fit among the studied shifting equations due to the fact that it has the most degree of freedom.

Application of digital image correlation method to mechanical testing of asphalt-aggregate mixtures

The digital image correlation (DIC) method, a noncontact, full-field displacement measurement technique, has been applied to mechanical testing of asphalt concrete. A single couple charged device camera acquires images of an area of interest from a specimen in the undeformed and deformed states. These images are correlated to determine deformations, and advanced mathematical procedures are applied to these deformations to calculate strains. To verify the DIC measurements, vertical displacements for the middle and bottom sections of a specimen subjected to monotonic tension are compared with conventional linear variable differential transformer measurements. A series of DIC images captured during the monotonic and cyclic tests visualizes the evolution of the failure zone (i.e., the fracture process zone) at the crack tip. Also, it is demonstrated that the full-field measurement and post-processing nature of DIC allows a more accurate determination of the stress-strain behavior of the fracture process zone. The applicability of this method to a cylindrical specimen with a curved surface is also investigated by testing a 75-mm-diameter cylindrical specimen. Finally, the DIC method is extended to cyclic testing of asphalt mixtures with the aid of a synchronized image acquisition technique.

Plasticity Modeling Applied to the Permanent Deformation Response of Granular Materials in Flexible Pavement Systems

Mechanistic design methods for flexible pavements are usually based on limiting only two critical pavement responses: tensile strain at the bottom of the asphalt layer for fatigue damage and vertical compressive strain at the top of the subgrade for overall pavement rutting. Rutting in granular base and subbase layers is assumed to be small for all cases. Research was conducted to develop a fundamental method for incorporating rutting of granular base and subbase layers in flexible pavement thickness design. The research concentrated on investigating the potential for using constitutive relationships based on the flow theory of plasticity to limit permanent deformations in granular layers of pavement systems. The flow theory of plasticity is the most popular of the three plasticity theories that have been applied to soils and granular materials. The total strain is viewed as the sum of the reversible elastic component and the irreversible plastic component. A yield function is introduced to differentiate between elastic and plastic behavior. This yield function is a function of the stress state and may change on loading and unloading to describe cyclic hardening behavior. To use a flow theory model to predict the response of a granular pavement layer requires relatively complex finite-element analysis and sophisticated laboratory testing that may not be warranted for normal pavement design. A method is presented for using the yield surfaces from a flow theory model as a design criterion for limiting permanent deformations in granular layers. Only the results of triaxial strength tests are needed to implement the method.

Analysis of Pavement Rutting Data from FHWA Pavement Testing Facility Superpave Validation Study

Since 1986, FHWA has been performing accelerated pavement tests at its Pavement Testing Facility (PTF) located on the grounds of the Turner-Fairbank Highway Research Center. At this laboratory, FHWA uses two accelerated loading facility pavement testing machines to simulate the effects of heavy vehicle loading on full-scale test pavements. In 1992, FHWA, with help from Strategic Highway Research Program staff and contractors, started an experiment to validate selected aspects of the Superpave binder specification using accelerated pavement tests. Twelve test lanes with 48 individual test sites were constructed at the PTF in 1993. The results of accelerated pavement tests on these pavements will be used to validate the Superpave binder parameters for rutting and fatigue cracking.