El barrido de deformaciones como método para evaluar el comportamiento a fatiga de los materiales bituminosos

El comportamiento a fatiga de las mezclas bituminosas es de vital importancia para el diseño de pavimentos. Como componente que cohesiona la mezcla, el ligante hidrocarbonado influye fuertemente en esta propiedad de la mezcla. Por ello es necesario estudiar el comportamiento a fatiga de este material. Clásicamente las propiedades de los ligantes en condiciones de fatiga han sido estudiadas mediante la obtención de la ley de fatiga del material. Para ello son necesarios numerosos ensayos cíclicos a amplitud de deformación o carga constante, denominados barridos de tiempo. Este tipo de ensayos son largos y costosos. Esta tesis doctoral presenta un nuevo procedimiento de ensayo que pretende caracterizar el comportamiento a fatiga de ligantes con menor número de ensayos y de menor duración. El nuevo procedimiento se basa en aplicar ensayos cíclicos de amplitud de deformación ascendente. Este tipo de ensayos son conocidos como barridos de deformaciones y por ello, el nuevo procedimientos se nombró EBADE (Ensayo de BArrido de DEformaciones). Previo a un procedimiento inicial de calibración el EBADE ha sido empleado para caracterizar el comportamiento a fatiga de varios ligantes y se ha demostrado que los resultados obtenidos mediante este ensayo están relacionados con la ley de fatiga del material, cuya obtención implica un consumo de tiempo muy superior. Así mismo, se ha demostrado que la pérdida de módulo experimentada por el ligante durante los ensayos cíclicos es, en su mayor parte, debida a la tixotropía del material y no a un proceso de daño irreversible. Como aportación final, se propone el empleo de dos parámetros obtenidos en los ensayos EBADE, el módulo inicial y la deformación de fallo, como parámetros característicos del comportamiento de los ligantes bituminosos a fatiga, en substitución de las leyes de fatiga obtenidas por medio de los ensayos de barrido de tiempo.The fatigue behavior of bituminous mixtures plays a major role in pavement design. The asphalt binder has a strong influence in that property and because of that it is necessary to study its fatigue behavior. Typically the fatigue properties of binders have been studied by obtaining the fatigue law of the material. This requires several cyclic tests at constant strain or stress amplitude, called time sweep tests. Such tests are long and expensive. This thesis presents a new test procedure that aims to characterize the fatigue behavior of binders with fewer and shorter tests. The new procedure is based on applying increasing strain amplitude cyclic tests. The test was called EBADE, standing for strain sweep tests in Spanish. Prior to an initial calibration procedure, the EBADE has been used to characterize the fatigue behavior of several binders and demonstrated that the results obtained by this test are related to the fatigue law of the material, which calculation involves several time consuming tests. In addition, it has been proven that modulus loss experienced by the binder during cyclic testing is mostly due to the thixotropy of the material and not to a process of irreversible damage. As a final remark, two parameters obtained in the EBADE tests, initial modulus and failure strain, are proposed to characterize the fatigue behavior of asphalt binders, instead of the fatigue laws obtained by means of the time sweep tests

Study of a hot asphalt mixture response based on energy concepts

The main objective of the research reported in this paper is to determine the response of a hot mix asphalt (HMA) in terms of both the tensile strength and energy parameters (based on the assessment of the force-displacement curve) as potential tools for improving the HMA mixture design. The HMAs analyzed were fabricated using a 60-70 penetration asphalt binder, dense-graded aggregate, mineral filler, and different types and contents of mineral filler replacements (i.e., lime, cement, and fly ash). The indirect tensile test was conducted to determine both the HMA tensile strength and force-displacement curve, which allowed for the computation of the HMA toughness as well as the energies involved in the process before and after reaching the tensile strength. Corresponding results suggest that the replacement of mineral filler by cement, lime, and fly ash modified the HMA response in terms of both the tensile strength and energy parameters. In addition, analysis of the energy parameters discussed proved to be useful for determining the optimum mineral filler content of HMA. Consequently, analysis of these energy parameters can benefit the HMA mixture design process. // El objetivo principal de la investigación fue determinar el comportamiento de una mezcla asfáltica en función de la resistencia a la tracción indirecta y parámetros de energía (calculados en función de la curva fuerza-desplazamiento) como herramientas potenciales para mejorar el diseño de mezclas asfálticas. Las mezclas asfálticas analizadas fueron fabricadas con asfalto de penetración 60/70, granulometría cerrada md10 y relleno mineral en diferentes porcentajes y materiales (cal, cemento y ceniza volante). El ensayo de tracción indirecta se utilizó para determinar la resistencia máxima a la tracción y la curva fuerza-desplazamiento, a partir de la cual se calcularon la tenacidad de la mezcla y las energías involucradas en el proceso antes y después de alcanzar la resistencia máxima. Los resultados obtenidos sugieren que reemplazar el relleno mineral por cemento, cal o ceniza volante modifica el comportamiento de la mezcla asfáltica en términos de la resistencia a la tracción y los parámetros de energía. Adicionalmente, el análisis de los parámetros de energía discutidos es útil para establecer el contenido óptimo del relleno mineral. En consecuencia, el análisis de estos parámetros de energía puede beneficiar el proceso de diseño de mezclas asfálticas.Peer ReviewedPostprint (published version

Obtaining the fatigue laws of bituminous mixtures from a strain sweep test: effect of temperature and aging

Fatigue is associated with the deterioration caused by applying repeated loads, and is affected by temperature or aging. Generally, time sweep tests are used to simulate fatigue, in order to obtain the fatigue laws. However, this requires too much time, often preventing its use. A method to estimate the fatigue laws from a strain sweep test is presented. The test was performed on a semi-dense mixture with different types of binder (unconditioned or aged) tested at different temperatures. This test is able to estimate fatigue laws more quickly, allowing the effect of different factors on the mixtures' fatigue life to be studied.Peer ReviewedPostprint (author's final draft

Estimating the fatigue law of asphalt mixtures using a strain sweep test (EBADE test)

Fatigue characterization of asphalt mixtures is a very important issue. However, existing techniques to that end are expensive and time consuming. For that reason the asphalt technicians have been studying several different methods to reduce the testing time needed. Such is the case of the Visco-Elastic Continuum Damage models. In that regard, this paper presents the efforts carried out by the Road Research Laboratory at the UPC-BarcelonaTech to reduce that testing time, by implementing a strain sweep test and approximating the fatigue law of the material from the data obtained. The EBADE test was applied to four mixtures for which the conventional fatigue laws were obtained using time sweep tests. The data collected was used to fit a fatigue model based on two strain values that can be directly computed from the EBADE test, namely Failure strain and No Damage strain. A good correlation was found between the model proposed and the experimental data.Postprint (published version

Effect of filler nature and content on the bituminous mastic behaviour under cyclic loads

The role of the filler in asphalt mixtures is particularly important because of its influence on mastic behaviour. The filler improves the resistance properties of bitumen against the action of traffic loads and temperature. However, the filler can also adversely affect bitumen in mastics excessively brittle and stiff due to inappropriate design. For these reasons, it is interesting to investigate the effect of filler type and content on mastic composition. This paper presents results from a strain sweep test applied to bituminous mastics prepared with different filler types and contents at several temperatures. The obtained stiffness modulus and failure strain results provide information to assess the fatigue behaviour of the analysed mastics.Peer ReviewedPostprint (author's final draft

Complexity of the behaviour of asphalt materials in cyclic testing

This paper compares the results obtained in two types of cyclic tension-compression tests, a time sweep test, constant strain amplitude, and a strain sweep test, increasing strain amplitude every 5000 cycles, called EBADE (standing for the Spanish words for strain sweep test). This comparison has shown that the rapid loss of stiffness during the initial part of cyclic testing is recoverable in bituminous materials. It has been found that reversible phenomena dominate in asphalt binders, while in mixtures are as important as damage. A damage equation has been proposed to describe the evolution of the material distress during the phase II in time sweep tests. In addition, a new methodology to estimate the fatigue law of bituminous mixtures is proposed.Peer ReviewedPostprint (author's final draft

Cyclic testing setups to highlight the importance of heating and other reversible phenomena on asphalt mixtures

This paper highlights the importance of the reversible phenomena in cyclic testing of asphalt mixtures. Several tests were designed to quantify the importance of these phenomena, focusing especially on heating due to viscous dissipation during cycling. Results showed that the decrease in stiffness observed during high frequency cyclic testing, and normally assumed by the conventional fatigue characterization procedures as damage, is in fact caused by reversible phenomena that are characteristic of visco-elasto-plastic materials under these conditions (heating, thixotropy, nonlinearity and healing). By obtaining the thermo-mechanical properties of the study mixture, the importance of heating within all reversible phenomena was studied.Peer ReviewedPostprint (author's final draft

Consideraciones sobre la aplicación de los métodos analíticos en la evaluación del fallo por fatiga de mezclas o pavimentos envejecidos

El comportamiento a fatiga de las capas de mezcla asfáltica del firme ha sido estudiado generalmente a partir de sus leyes de fatiga. Normalmente, éstas son obtenidas en laboratorio a partir de ensayos cíclicos de amplitud de carga o deformación constante, denominados ensayos de barrido de tiempo. Sin embargo, estos ensayos presentan como principal inconveniente la gran cantidad de tiempo que requieren para su ejecución. El ensayo EBADE, Ensayo de BArrido de DEformaciones, se presenta como un método alternativo para estimar las leyes de fatiga de las mezclas, ya que es mucho más rápido de llevar a cabo. Esto permite el estudio del fallo por fatiga de las mezclas bajo distintas condiciones. En este trabajo, usando el ensayo EBADE, se han obtenido las leyes de fatiga de una mezcla de tipo semidensa, fabricada con distintos tipos de ligante, sometida o no a envejecimiento acelerado en laboratorio y a distintas temperaturas. Con estas leyes se ha determinado la vida a fatiga de la mezcla bajo las distintas condiciones consideradas para dos secciones de firme: una de elevado espesor y otra con poco espesor de mezcla bituminosa. Los resultados obtenidos muestran que, para la sección de firme de espesor elevado, la condición más crítica se encuentra en la mezcla no acondicionada ensayada a la temperatura de 20ºC, obteniendo vidas significativamente superiores cuando la mezcla se encuentra envejecida y/o ensayada a temperaturas bajas. Estos resultados no concuerdan con la realidad, lo cual no solo pueden llevar a un mal dimensionamiento del firme, sino que cuestionan la forma actual de evaluar el fallo por fatiga de las mezclas, y por tanto, el dimensionamiento de los pavimentos. Por el contrario, cuando la sección de firme es de poco espesor, su vida a fatiga disminuye considerablemente al encontrarse la mezcla envejecida y/o a bajas temperaturas.Postprint (published version

Analysis of the thixotropic behavior and the deterioration process of bitumen in fatigue tests

The characterization of fatigue damage on bituminous materials under cyclic loading has been classically studied using tests and procedures previously developed for the characterization of metallic materials. However, these materials present important differences in their behavior in cyclic testing. For instance, the significant loss of modulus the bitumen exhibits at early stages of the test or its total recovery when loading is removed.; Comparison between two types of cyclic testing applied to bitumens, time and strain sweep tests has proven that this phenomenon is related with the nonlinear behavior of the bitumen, thixotropy and viscoelasticity, and that the amount of modulus loss during the initial part of cyclic testing is directly related to the strain applied.; Using the framework of the work potential theory, a new expression has been found for the damage law that describes the loss of modulus of bitumens during the linear phase of the fatigue tests. Additionally, a procedure is proposed to estimate the fatigue relation between the strain applied and the number of cycles to failure using only the data obtained in strain sweep tests. These relations fit reasonably well the experimental data obtained in more time consuming time sweep tests.; Applying this estimation procedure implies a great time savings in the characterization of the fatigue behavior of asphalt binders and the determination of their fatigue laws.Peer ReviewedPostprint (author's final draft

DUSST: development of a new stress sweep fatigue test for asphalt mixtures

Fatigue cracking is one of the main damage mechanisms that take place in asphalt pavements. However, its evaluation during the design asphalt mixtures is not usually considered due to the complexity of the tests required, their high costs and the long time required to perform them. As a consequence, several research teams are working on developing new test procedures to reduce the time related to fatigue characterization of asphalt mixtures. In this context, this paper presents the development of a new test procedure consisting of a cyclic uniaxial tension-compression test in which the stress applied increases every 5000 cycles. In this manner, it is possible to evaluate a mixture’s response under cyclic loading at different stress levels in one test. Also, failure of the mixture takes place in a shorter time period than that required by classical time sweep fatigue tests. This paper presents results obtained in the application of this new methodology to a mixture frequently used in Chilean pavement structures (type IV-A-12 by Chilean standards) with three different asphalt binders: a conventional binder (CA-24), a high modulus binder (CA-HM) and a polymer-modified binder (CA-PM). We evaluated three different types of aggregates obtained through two different shredding processes. Results achieved in the development of our Direct Uniaxial Stress Sweep Test (DUSST) show that it is an experimental method with great potential. This test can characterize fatigue behaviors of asphalt mixtures in a shorter time by using important parameters such as initial strain, failure strain, complex modulus and dissipated energy. In addition, we obtained a good sensitivity of the DUSST main parameters to variables evaluated.Peer ReviewedPostprint (author's final draft