J. Qiu

Turning Back Time: Rheological and Microstructural Assessment of Rejuvenated Bitumen

Countermeasures to the aging of bituminous asphalt binders is a highly important topic for service-life extension of asphalt in the field and for recycling old pavements into new structures with similar functional requirements as the original structure. Countermeasures are usually achieved by applying additives that restore the adhesive and mechanical properties of the original bituminous binder. The additives are commonly termed (asphalt) rejuvenators. This study examined the performance of two very distinct rejuvenating agents. The effectiveness of rejuvenators is usually measured by comparing the penetration and softening point of the rejuvenator-aged bitumen blend with reference values of the virgin binder. The study used a dynamic shear rheometer to evaluate the rejuvenating capabilities of the two additives. The microstructures of the virgin binder and the rejuvenated blends were obtained by atomic force microscopy. Subsequently, the rheological results were related to the microstructure morphologies. From the rheological experiments, both rejuvenators exhibited the desired softening and property-restoring performance. However, there was a strong difference in the amount of rejuvenator needed to achieve complete rejuvenation. By correlating rheology to the microstructural observations, the effects of the rejuvenators were found to be distinct at microscopic length scales: rejuvenation was achieved by distinct chemophysical mechanisms. One of the rejuvenators restored the virgin microstructure, whereas the other rejuvenator generated a new morphology. Thus, the study demonstrated that by combining rheological and microstructural techniques, the mechanism and performance of rejuvenation can be understood. This finding may help guide future designs and optimization of asphalt-rejuvenating agents.

Investigating the Self Healing Capability of Bituminous Binders

ABSTRACT In the Netherlands, the loss of stone at the surface of porous asphalt wearing courses (ravelling) results in a limited service life. Ravelling is caused by repeated traffic fatigue loadings and climatic influences. Bituminous materials exhibit a healing capability which is helpful to extend the service life. However, aging is expected to limit this positive effect. Therefore, upgrading the self-healing capability of the bitumen in time is of major importance. Inspired from mimicking nature, several chemical treatments can be used to improve the self healing capability of asphalt mixtures when the binder starts to harden due to aging, or when micro cracks have initiated. Possibilities of self-healing improvement are discussed in this paper. Preliminary results of measurements of the self healing capability of bituminous binders are also presented.

Crack-Healing Investigation in Bituminous Materials

Self-healing is one of the great potential processes for service-life extension of asphalt pavements. However, the underlying mechanism is not explained, and a proper way to measure it is not mentioned. It is necessary to understand the self-healing mechanism and to develop a setup to measure it. In this paper, the self-healing process was hypothesized as the reverse process of cracking. Three new test methods were developed to mimic the self-healing process of cracks in bituminous materials (including bitumen, mastics, and asphalt mixtures) in the laboratory. The results indicate that the proposed crack self-healing assessment methods are capable of ranking the self-healing capability of bituminous materials. The crack self-healing process contains two important phases, namely, crack closure and strength gain. The occurrence of these phases is dependent on healing time and temperature.

Self-healing characteristics of bituminous mastics using a modified direct tension test

Cracking is one of the main distresses responsible for the service life reduction of asphalt pavement. On the contrary, self-healing is a process that reverses to cracking and increases the service life. Understanding of the cracking and healing behavior of bituminous materials is very important for service life predictions. Instead of a complex and time-consuming fatigue test, a modified direct tension test with a loading–healing–reloading procedure was developed in this article to characterize the cracking and healing behavior of bituminous mastics. A displacement-controlled loading was applied to obtain damaged specimens with different crack sizes at various postpeak elongations. After unloading and healing, the reloading was applied to quantify the healing behavior under different conditions. The healing behavior is very dependent on rest periods, crack phases, and material types. A clear difference in self-healing property between a polymer-modified bituminous mastic and a conventional penetration grade bituminous mastic was observed for different phases of crack. As a result, the modified direct tension test is believed to be an effective tool for characterizing the self-healing capability of bituminous mastics.

Evaluating Laboratory Compaction of Asphalt Mixtures Using the Shear Box Compactor

Laboratory produced test specimens are usually obtained with devices like Marshall compaction, gyratory compaction, or roller compaction. However, with these methods it is difficult to control the sample-to-sample variation of the final density of the test specimens, which can strongly influence the results of performance testing. It is very important to have a repeatable and efficient production method of test specimens available in the laboratory. The shear box compactor was recently developed to simulate field compaction with a constant compressive force and a cyclic shear force with constant maximum shear angle applied to the asphalt mixture. The shear box compactor produces asphalt blocks with a size of 450 mm in length, 150 mm in width, and 145–185 mm in height. Test specimens like beams or cylinders can be obtained from the block for laboratory performance testing. In this paper, the compaction results with the shear box compactor are reported for asphalt mixtures with different gradations and binder types. Asphalt specimens with different mixture compositions, shapes, sizes, and sampling positions were investigated by volumetric properties. Finite element modeling was introduced to obtain more understanding of the compacting process of the shear box compactor. The results indicate that the decreasing of voids content of asphalt mixtures during compaction process is dependent on the gradation than the binder type. The asphalt mixture specimens obtained from the same asphalt mixture block has a variation in voids content of less than 1 %. Test specimens obtained from the upper part of the asphalt block are more compacted than specimens from the lower part. And the specimens obtained close to the side of the block are less compacted due to lack of shear stress. As a result, the shear box compactor provides a reliable means of sample preparation, making it very suitable for producing specimens with constant volumetric properties.

Development of Autonomous Setup for Evaluating Self-Healing Capability of Asphalt Mixtures

Asphalt mixtures have self-healing capabilities, yet most self-healing investigations are carried out with complex and time-consuming fatigue tests. To investigate the self-healing capability in a simple, efficient manner, a beam-on-elastic-foundation (BOEF) test setup was proposed. With a notched asphalt concrete beam fully glued to a low modulus rubber foundation, the BOEF setup was able to control the self-healing process autonomously, including crack closure and healing at different rest periods and temperatures. A load crack opening displacement curve was used for characterizing the self-healing capability of the monotonic response with a loading–healing–reloading procedure. In addition, small numbers of dynamic loads were also applied to the BOEF setup under different cracking and healing conditions. An apparent modulus of the BOEF asphalt beam was obtained on the basis of the dynamic response of the load crack opening displacement. The results indicate that the self-healing capability obtained in the monotonic tests depends on the healing time and healing temperature. The healing temperature has the larger effect of the two. Most of the apparent modulus of the BOEF asphalt beam recovers at the beginning of the self-healing process. The influences of healing time and temperature are limited. As a result, the BOEF setup is proved as a potential tool for the self-healing investigation of asphalt mixtures.

Cracking and Healing Modelling of Asphalt Mixtures

Self healing behaviour of asphalt mixture has been known for many years. This unique behaviour can help asphalt concrete to recover its strength after damage. Healing can also extend the service life of asphalt pavements. A beam on elastic foundation test setup (BOEF) was developed to investigate the self healing behaviour of asphalt mixture in a controlled and effective way. Within this setup, a notched asphalt beam was glued on a low modulus rubber foundation, and a symmetric monotonic load was applied with loading-unloading-healing-reloading cycles. The experimental results indicate the existence of the healing behaviour. Increasing reloading curves are observed for increasing healing time and healing temperatures. In order to further understand the cracking and healing phenomenon, a finite element simulation was carried out with a smeared crack cohesive zone model (CZM). By defining both properties of the bulk asphalt mixture and the cohesive zone, the global load-crack opening displacement (COD) response during cracking and healing process was simulated successfully. The model is also capable to separate the two healing phases including crack closure and strength gain. It is also shown that further implementation of the cracking and healing modelling of asphalt mixtures is important for durable asphalt pavement.

Comparison of Two Beam Fatigue Tests

The objective of this study is to compare the fatigue behavior in two beam fatigue tests: the Four Point Bending Test (FPBT) and the Beam on Elastic Foundation (BOEF) test. It is believed that the BOEF test simulates the fatigue behavior of asphalt mixtures in a more realistic way. The beam specimens used for both the FPBT and BOEF tests are made of gravel asphalt concrete (GAC). Finite element simulations of the BOEF test are carried out to understand better the behavior of the GAC beam. The results show that in the BOEF tests, cracks will develop from the middle bottom of beam while the FPBTs at low temperature show no crack. This finding further supports the idea that improved fatigue tests are needed which better represent the cracking behavior of asphalt pavements. However, the BOEF test also shows the development of an accumulated strain along with the cyclic strain. This shows that further investigation is needed to determine the cause of fatigue cracks; are they due to the cyclic strain or due to the accumulated strain? Another important finding is that at a temperature of 5°C and a frequency of 8 Hz, the fatigue life obtained with the BOEF test is 20 times higher (on average) than FPBT fatigue life.