Quang Tuan Nguyen

Nonlinearity, Heating, Fatigue and Thixotropy during Cyclic Loading of Asphalt Mixtures

ABSTRACT This paper describes investigation into the change in complex modulus (norm and phase angle) during cyclic tests on bituminous mixtures. The change can be explained by four phenomena: nonlinearity, heating, thixotropy and fatigue. An experimental campaign has been performed at ENTPE laboratory in order to identify and quantify the first three phenomena. The analysis of the results reveals that the two reversible effects (heating and thixotropy) are very important and can not be ignored when interpreting fatigue tests. Nonlinearity, for the rather small strain level amplitudes considered in our experimental campaign (up to 122 μm/m), is also shown to be reversible. Heating is due to the viscous dissipated energy that heats the specimen. At the beginning of the test, temperature increase in the sample is shown to be proportional with the total dissipated energy. Thixotropy effect is quantified. In addition, it is shown that thixotropy effect can be modeled using an equivalent temperature increase.

Time Temperature Superposition Principle Validation for Bituminous Mixes in the Linear and Nonlinear Domains

AbstractThe mechanical behavior of bituminous mixes is very complex. It is also characterized by great thermal sensitivity. In the small strain domain (for strain amplitudes below approximately 10−4), it can be considered to be linear, therefore the linear viscoelasticity theory can be applied. In addition, the time temperature superposition principle (TTSP) may be assumed, with a good approximation. This has been widely verified regarding the unidimensional behavior of bituminous mixes in the viscoelastic linear domain. This paper provides an overview of TTSP for bituminous materials, and three-dimensional (3D) behavior and nonlinear domain are also considered. The 3D behavior is characterized by measurements of the complex modulus and complex Poisson’s ratio. For both parameters, master curves are plotted to check the validity of TTSP in 3D. Some experiments in the nonlinear domain (considering strains up to some percentages and some crack propagation tests) are also conducted in equivalent conditions o...

Determination of thermal properties of asphalt mixtures as another output from cyclic tension-compression test

This paper describes investigation into the change of temperature of asphalt mixtures during cyclic tests on cylindrical specimens. Tension-compression tests were performed at different strain amplitudes (from 55 μm/m to 133 μm/m). Temperature is measured at the surface and inside the specimen. From the analysis of the experimental results, the thermal conductivity (k) and the convection heat transfer coefficient (h) were determined during the steady temperature phase. The specific heat coefficient (C) of the material was determined during the transient temperature phase. This parameter is also indentified using a finite element simulation of the heat process. A thermal conductivity of 1.35 W/(m.°C) and a specific heat of 820 J/(kg.°C) were found for the asphalt mixtures used in this research. These values are quite realistic when compared to values proposed in the literature that are obtained using the traditional thermo-physical measurements. Furthermore, the thermal parameters obtained from FP2C, a device used for the identification of thermal properties of materials, confirm the results.

Prediction of Linear Viscoelastic Behaviour of Asphalt Mixes from Binder Properties and Reversal

The linear viscoelastic (LVE) behaviour of bituminous materials (observed in the small strain domain) is considered for road design. The objective of this paper is to investigate the links between the LVE properties of binders and asphalts mixes. Complex modulus tests were carried out to determine the LVE behaviours of a bituminous binder and two mixes produced with this bitumen. Dynamic Shear Rheometer (DSR) tests and tension-compression tests (using a Metravib device) were performed on the bitumen. Complex modulus E* and complex Poisson’s ratio ν * of mixes which characterize the (LVE) properties in 3 Dimension (3Dim) were measured using tension-compression tests. The experimental results show the dependence between mixes and associated bitumen behaviours. The prediction of LVE behaviour of bituminous mixes from bitumen properties was carried out using the SHStS (Shift-Homothety-Shift and time-Shift) transformation developed at ENTPE.

Effect of Fatigue Cyclic Loading on the Linear Viscoelastic Properties of Bituminous Mixtures

AbstractThis paper focuses on the three-dimensional (3D) linear viscoelastic (LVE) behavior of bituminous mixtures and the damage occurring after a great number of cyclic loadings. Two complex modulus tests were carried out before and after a fatigue test. The complex modulus E* and complex Poisson’s ratio ν* were measured for large ranges of temperatures (from −25 to 30°C) and frequencies (from 0.03 to 10Hz). First, it is shown that the time temperature superposition principle (TTSP) is verified and applicable in the 3D case. Then, the change of the complex modulus created by fatigue test was analyzed. Lastly, a model with a continuum spectrum called two springs, two parabolic elements, one dashpot (2S2P1D), developed at the Ecole Nationale des Travaux Publics de l’Etat (ENTPE), was used to simulate the 3D linear viscoelastic behavior of the tested bituminous mixture before and after fatigue damage.

Three-Dimensional Characterisation of Linear Viscoelastic Properties of Bituminous Mixtures

This chapter focuses on the three-dimensional linear viscoelastic (3D-LVE) behaviour of bituminous mixtures and in particular on the measurement and modelling of the complex Young’s modulus and Poisson’s ratio (PR). In the first part of the chapter, the LVE definition of PR is reviewed and experimental measurements of the LVE PR carried out over the last 40 years are summarised. The second part of the chapter is devoted to the description of a RILEM round robin test (RRT) organized by Task Group 3 (TG3) “Mechanical testing of bituminous mixtures” of RILEM TC 237-SIB. Within the RRT uniaxial cyclic (sinusoidal tension/compression or haversine compression) tests at different temperatures and frequencies were carried out on cylindrical specimens cored from laboratory compacted slabs. Two types of bituminous mixtures, GB3 (continuously graded) and GB5® (gap-graded), were analysed. Five laboratories participated in the RRT, each laboratory measured axial and transverse (or diametral) strains using different sensors and configurations. In particular, transverse strain was measured along two orthogonal directions in order to evaluate the effect of compaction-induced anisotropy on PR. Results confirmed that PR of bituminous mixtures is a complex function of temperature and frequency and that the time-temperature superposition principle can be applied (for absolute value and phase angle). For the studied mixtures the norm of the complex PR ranged between 0.22 (low temperatures/high frequencies) and 0.60 (high temperatures/low frequencies) whereas the phase angle was less than 6°. A small difference (less than 0.05) was found between measurements carried out in two orthogonal directions. This small difference is probably related to measurement accuracy and not to the anisotropic behaviour of the material. Comparison of data between the different laboratories, which could not be performed at exactly the same temperatures and frequencies, was performed using a common reference given by the 3-dimensional formulation of the 2S2P1D linear viscoelastic model. This model provides a good simulation of experimental data. Based on close results from all participating laboratories, it is possible to conclude that cyclic uniaxial test could be a good candidate to become a standard test for evaluating the 3D-LVE behaviour of bituminous mixtures.

Nonlinearity of bituminous materials for small amplitude cyclic loadings

This paper focuses on the influence of the cyclic loading amplitudes on the behaviour of bitumen and bituminous mixtures. The present work considers only rather small strain amplitude (lower than 100 µm/m for mixtures and 10% for bitumen). Complex modulus tests on mixtures were conducted at four levels of strain amplitude for a large range of temperatures and frequencies. Tension–compression tests at the temperatures lower than 20°C and shear tests at temperatures higher than 20°C were performed to measure the complex modulus of bitumen which is used to produce the mixtures. Moreover, individual strain tests, strain sweep tests and fatigue tests at 10, 30 and 50°C were carried out to study the influence of the strain amplitude and cyclic effects on the complex modulus of bitumen. The obtained results indicated that the nonlinearity (effect of strain amplitude) on both bitumen and bituminous mixtures respects the Time Temperature Superposition Principle (TTSP). However, the temperature–frequency has inverse effects on the nonlinear behaviour of bitumen and bituminous mixtures. The cyclic effect was shown to be not negligible when analysing the nonlinearity phenomenon.

Fatigue Cracking in Bituminous Mixture Using Four Point Bending Test

This paper describes investigation into cracking in bituminous mixture using the four point bending notched fracture (FPBNF) test, which has been developed at the University of Lyon/ Ecole Nationale de Travaux Publics de l’Etat (ENTPE). A special loading path is applied on the notched beam specimen at a constant temperature of - 4.5°C. A monotonic loading was first applied until the peak load and after unloading, many loading/unloading cycles at small amplitude were carried out until the final failure of specimen. Deflection of the beam and crack mouth opening displacement (CMOD) are measured. Crack length is determined experimentally using crack propagation gauges. It is also obtained with an improved method, called Displacement Ratio Crack length (DRCL) method, developed at ENTPE laboratory, which allows back calculating the crack length. This method is based on the relation between two experimental displacement measurements: the crack mouth opening displacement (CMOD) and the deflection of the beam. The results obtained from this method are discussed and compared with the crack length measured with crack propagation gauges. During the test, the fracture behaviour is investigated. The crack propagation is studied as a function of loading/unloading cycle number. The stress intensity factor is evaluated. Two different domains of crack evolution are distinguished: the first domain where pre-existing crack progressively re-opens, the second domain where crack propagates. The Paris fatigue law could be applied in the domain where crack propagates.