Green pavements: reuse of plastic waste in asphalt mixtures

Nowadays, the disposal of plastic waste is an issue of major concern worldwide because of its considerable volume and growth. An option to tackle this problem is to recycle this waste. This alternative reduces the quantity of net discards, conserves both material and energy and provides a comparatively simple way to make a substantial reduction in the overall volume of solid waste. The purpose of this study is to investigate an environmentally friendly approach about the influence of recycling different percentages of urban and rural plastic waste by adding them in a dry process on an asphalt mixture, through a comparative laboratory study. The resulting mixtures are analyzed considering their volumetric parameters and the values from diverse laboratory mechanical tests. The performance tests include, Marshall stability, Marshall quotient, indirect tensile strength, fracture energy, resilient modulus, permanent deformation and creep compliance, which were carried out on unmodified and modified hot asphalt mixtures. The obtained results are presented and discussed in this paper showing that the reuse of recycled plastics in asphalt mixtures is a viable alternative that contributes to the reduction of plastic wastes as well as the protection of the environment.     

Estimating the moisture damage of asphalt mixture modified with nano zinc oxide

One of the main distresses of hot mix asphalt (HMA) is moisture damage. The most common method for decreasing this type of distress is using antistrip additives. In this study, the effect of nanoparticles was evaluated as an antistrip agent on the moisture damage of HMA. Two types of aggregates were evaluated in this study with different sensitivities against moisture damage (limestone and granite aggregate) and the asphalt binder with 60/70 penetration grade and nano zinc oxide (ZnO) in two different percentages by weight of the asphalt binder. The tests employed to evaluate the effects of modifying asphalt binder by nanomaterials on the moisture damage of asphalt mixture were surface free energy (SFE) and AASHTO T283. The results showed that the ratio of wet/dry values of indirect tensile strength for the mixtures containing nano ZnO for two types of aggregate were higher than the control mixtures. In addition, the results of the SFE method showed that adding nano ZnO increased the total SFE of the asphalt binder, which led to better coating of the aggregate with asphalt binder. Nano ZnO decreased the acid to base ratio of SFE of asphalt binder, while it led to improving adhesion between the asphalt binder and acidic aggregate that are prone to moisture damage.     

Reconsideration of the fatigue tests for asphalt mixtures and binders containing high percentage RAP

When applying reclaimed asphalt technology in a flexible pavement project, most performance concerns are related to low temperature and fatigue cracking since the stiffness of the HMA mixture could dramatically increase through adding a high percentage of reclaimed asphalt pavement (RAP) material. The purpose of this study is to evaluate asphalt mixtures with high RAP contents, prepared using two RAP addition methods, for their performance based on fatigue-cracking resistance rather than relying on volumetric properties. Asphalt mixture samples were prepared with three RAP binder content replacement percentages (30, 40 and 50%) using two preparation methods: the as-is RAP gradation (traditional method) and the splitting of the RAP gradation into coarse and fine fractions (fractionated method). Asphalt mixture beam fatigue and binder fatigue time-sweep tests were performed. Beam fatigue samples also underwent freeze–thaw cycling for freeze–thaw damage evaluation. Rather than basing the performance based solely on S–N_f curves to illustrate the fatigue performance, the beam fatigue test data was analysed through a dissipated energy approach. Faster fatigue degradation was observed for the 40% RAP binder and beam mixture when subjected to repeated loading. From a morphology aspect, this can be explained by the binder’s phase separation and physical hardening effects.     

Laboratory investigation of utilizing high percentage of RAP in rubberized asphalt mixture

The utilization of crumb rubber and reclaimed asphalt pavement (RAP) has proven to be economical, environmentally sound and effective in increasing the performance properties of the asphalt mixtures. The objective of this research was to investigate the laboratory engineering behavior characteristics of the rubberized asphalt binders and mixtures made with PG 64-22 and a softer binder (PG 52-28) containing a high percentage of RAP (30%). Some of the testing used for this research included viscosity, dynamic shear rheometer (DSR), bending beam rheometer (BBR), indirect tensile strength (ITS), resilient modulus, and fatigue life evaluations. The experimental design included the use of two aggregate and RAP sources, two virgin binder grades (PG 64-22 and PG 52-28), two types of crumb rubber (ambient and cryogenic), and four rubber contents (0%, 5%, 10%, and 15%). The results indicated that: (1) the crumb rubber improved the aging resistance of the aged binder and prolonged the fatigue life of the mixtures containing 0% RAP, in addition, results indicated a decrease of ITS and resilient modulus values was found as the rubber content increased, regardless of rubber type; (2) the utilization of softer binder decreased the influence of aged binder and decreased the resilient modulus values of the mixtures. In most cases, regardless of rubber types, the rubberized mixtures containing 30% RAP made with PG 52-28 binder did not show a significant increase in fatigue life with those made with PG 64-22 binder.     

Physical and environmental properties of asphalt mixtures containing incinerator bottom ash

This paper presents parts of the results from a research project sponsored by Taiwan Environmental Protection Administration (TEPA), investigating both the physical and environmental properties of asphalt mixtures using different amount of incinerator bottom ash (IBA) as fine aggregate substitution. The Marshall mix design method was used to determine the design asphalt content and evaluate the potential performance of these IBA-asphalt mixtures. Water sensitivity and wheel track rutting were also performed on these mixtures. Leachates, from both laboratory and outdoor leaching tests, were performed to measure the concentration of selected heavy metals and the level of daphnia toxicity. While with adequate Marshall stability, the IBA-asphalt mixtures were shown to have excessively high Marshall flow and excessively low VMA (voids in the mineral aggregate). The results of the wheel tracking tests also indicated that the IBA-asphalt mixtures had low rutting resistance. The results of the water sensitivity test according to procedure of AASHTO T283 method showed that the IBA-asphalt mixtures had a higher tensile strength ratio (TSR) as compared with the conventional asphalt mixtures. Considering the environmental aspects, outdoor leaching tests showed that IBA had a high level of daphnia toxicity. From an ecological perspective, IBA could be identified as hazardous waste in Taiwan. However, after being mixed with asphalt binder, the concentration of heavy metals and the levels of daphnia toxicity were significantly reduced. The leachates of 10-day flat plate leaching tests on Marshall specimens containing IBA indicated that the heavy metal were undetectable and the daphnia toxicity was ineffective.     

Estimation of optimum binder content of recycled asphalt incorporating a wax warm additive using response surface method

This paper presents a new approach to estimate the optimum binder content (OBC) of recycled asphalts (RAs) incorporating a warm mix additive based on the interaction effects of compaction temperature, RA content and binder content using volumetric and strength characterisation. The experimental design was developed using response surface method (RSM) based on central composite design for various compaction temperatures (130–160 °C), RA contents (30–50%) and binder contents (4.9–6.0%). Laboratory tests were performed and analysed to meet the desired volumetric and strength properties according to the Malaysian specifications for the design of dense asphalt mixtures. Statistical analysis and mathematical models proposed by RSM were used to determine the OBC. The results showed that compaction temperature is the most significant factor in determining the OBC. There are minimum differences in the OBC variation of samples incorporating different dosage of RA. The developed model can be used for quick estimation of OBC for various levels of compaction temperature and RA content.     

Potential and limits of FTIR methods for reclaimed asphalt characterisation

In recent years, as a result of both environmental policy and industrial economic strategy, the amount of asphalt pavement that has been recycled has risen dramatically. As a result of this, several international research projects have shown the need for improvements in reclaimed asphalt (RA) characterisation to support these higher recycling rates. During the life of a pavement, as well as the structural damage induced by traffic, the intrinsic properties of asphaltic materials are affected through the oxidative ageing of the bituminous binder. The molecular changes associated with this oxidation can be monitored via variations in Fourier transform infrared (FTIR) spectra. Therefore, if the relationships between binder properties, such as penetration, softening point and complex shear modulus and the spectra parameters can be established, the characterisation of RA produced by the milling of old pavements can be greatly improved. However, the interpretation of oxidation parameters from these spectra is not straightforward, and many different techniques are used to calculate them in the asphalt community. This paper gives a brief overview of asphalt oxidation and the FTIR principle. It then presents the results from four different calculation methods for the oxidation parameters applied to a set of more than 100 spectra obtained during the RILEM TC-ATB-TG5 round robin test. From this study, it is obvious that the conclusions of the analysis performed on the same set of raw spectra can differ considerably according to the FTIR oxidation index calculation method adopted. Using the RILEM database, the potential links between a given ageing index and the physical properties are then evaluated. As a result of this study it can be concluded that ageing comparison through FTIR should be considered as relative and restricted for use in measurements at different ageing steps on the same type of initial mixture. For FTIR spectra to be comparable between different laboratories, harmonisation of the data acquisition procedures is needed. Also the different calculation methods of the oxidation indicators seem to reflect different physical properties. So a first step toward harmonisation would be to agree on the relevant physical properties actually needed for efficient RA characterisation. This could then lead to a common FTIR index calculation method, which would reflect the properties of interest. For this, additional research is needed to improve knowledge of the relationship between the chemical oxidative process of a binder and the evolution of its physical properties.     

Dynamic mechanical characterization of asphalt concrete mixes with modified asphalt binders

The primary objective of this work is to characterize and compare the dynamic mechanical behavior of asphalt concrete mixes with styrene butadiene styrene (SBS) polymer and crumb rubber modified asphalt binders with the behavior of mixes with unmodified viscosity grade asphalt binders. Asphalt binders are characterized for their physical and rheological properties. Simple performance tests like dynamic modulus, dynamic and static creep tests are carried out at varying temperatures and time. Dynamic modulus master curves constructed using numerical optimization technique is used to explain the time and temperature dependency of modified and unmodified asphalt binder mixes. Creep parameters estimated through regression analysis explained the permanent deformation characteristics of asphalt concrete mixes. From the dynamic mechanical characterization studies, it is found that asphalt concrete mixes with SBS polymer modified asphalt binder showed significantly higher values of dynamic modulus and reduced rate of deformation at higher temperatures when compared to asphalt concrete mixes with crumb rubber and unmodified asphalt binders. From the concept of energy dissipation, it is found that SBS polymer modification substantially reduces the energy loss at higher temperatures. Multi-factorial analysis of variance carried out using generalized liner model showed that temperature, frequency and asphalt binder type significant influences the mechanical response of asphalt concrete mixes. The mechanical response of SBS polymer modified asphalt binders are significantly correlated with the rutting resistance of asphalt concrete mixes.     

Investigating the effect of nanoparticles on the rutting behaviour of hot-mix asphalt

Rutting is considered as one of the major damages in asphalt mixtures. In this study, different types of nanoparticles such as TiO₂, Al₂O₃, Fe₂O₃ and ZnO in different percentages were added to the base asphalt binder in order to decrease the rutting potential of hot-mix asphalt (HMA). In the first step, asphalt binder tests for characteristics such as penetration grade, ductility, softening point and viscosity were performed on the asphalt binder modified by the nanoparticles. Then, after preparing HMA samples, the static creep test was done at two stress levels at a specific temperature. Results of this study showed that using the nanoparticles improved the behavioural properties of the asphalt binder and decreased rutting in asphalt mix samples. Furthermore, scanning electron microscope images taken from the asphalt binder samples modified by the nanoparticles demonstrated that these nanoparticles were properly distributed in the asphalt binder space and had a positive effect on the rutting performance of the asphalt mixes.     

Predicting the tensile relaxation modulus of asphalt mixes based on the mix design and environmental factors

The main objective of this study was to predict the tensile relaxation modulus of asphalt mixes, without having to perform the common relaxation modulus tests, by developing a predictive model based on the mix characteristics, ageing condition, temperature and loading time. To this end, cylindrical asphalt mixture specimens containing crushed stone aggregates with 60/70 penetration asphalt binder were fabricated using two aggregate gradations, two binder contents, two air void levels and three ageing conditions with four replicates. Uniaxial tensile relaxation modulus tests were conducted on the specimens at four temperatures using the trapezoidal loading pattern at a low level of strain. Tensile relaxation modulus master curves of all the experimental combinations were constructed by the sigmoidal model. Statistical analysis of variance and regression analysis was performed on the test data and a predictive model was developed. Finally, the predictive model was verified using a group of measured values other than those used for the development of the model, and it was found that the predicted values correlated well with the measured ones.     

Linking asphalt binder fatigue to asphalt mixture fatigue performance using viscoelastic continuum damage modeling

Fatigue cracking is a major form of distress in asphalt pavements. Asphalt binder is the weakest asphalt concrete constituent and, thus, plays a critical role in determining the fatigue resistance of pavements. Therefore, the ability to characterize and model the inherent fatigue performance of an asphalt binder is a necessary first step to design mixtures and pavements that are not susceptible to premature fatigue failure. The simplified viscoelastic continuum damage (S-VECD) model has been used successfully by researchers to predict the damage evolution in asphalt mixtures for various traffic and climatic conditions using limited uniaxial test data. In this study, the S-VECD model, developed for asphalt mixtures, is adapted for asphalt binders tested under cyclic torsion in a dynamic shear rheometer. Derivation of the model framework is presented. The model is verified by producing damage characteristic curves that are both temperature- and loading history-independent based on time sweep tests, given that the effects of plasticity and adhesion loss on the material behavior are minimal. The applicability of the S-VECD model to the accelerated loading that is inherent of the linear amplitude sweep test is demonstrated, which reveals reasonable performance predictions, but with some loss in accuracy compared to time sweep tests due to the confounding effects of nonlinearity imposed by the high strain amplitudes included in the test. The asphalt binder S-VECD model is validated through comparisons to asphalt mixture S-VECD model results derived from cyclic direct tension tests and Accelerated Loading Facility performance tests. The results demonstrate good agreement between the asphalt binder and mixture test results and pavement performance, indicating that the developed model framework is able to capture the asphalt binder’s contribution to mixture fatigue and pavement fatigue cracking performance.     

Laboratory characterisation of asphalt mixes containing RAP and RAS

Due to its economic and environmental benefits, using reclaimed asphalt pavement (RAP) and reclaimed asphalt shingles (RAS) in new hot-mix asphalt (HMA) has become an integral part of today's asphalt industry. The advantages of using RAP and RAS in HMA are not limited to economic and environmental benefits, and may result in improving a number of mix performance characteristics including rutting and resistance to moisture-induced damage. Despite aforementioned benefits, concerns over premature pavement distresses resulting from using RAP and RAS limit their usage in HMA. Furthermore, because of the lack of mechanistic performance data, use of new mixes containing RAP and RAS remains limited. Therefore, the present study was undertaken to investigate the effects of using different amounts of RAP and RAS on laboratory performance of HMA, and to generate valuable input design parameters for implementation of the mechanistic-empirical pavement design guide (M-EPDG), using local materials. Four types of base course mixes containing 0% RAP, 25% RAP, 40% RAP and 20% RAP+5% RAS, and three types of surface course mixes containing 0% RAP, 25% RAP and 20% RAP+5% RAS were tested. Laboratory tests were conducted to evaluate stiffness, low-temperature cracking, fatigue life, rut and moisture-induced damage potential of the mixes. It was found that dynamic modulus and creep compliance of the asphalt mixes increase and decrease, respectively, with an increase in the amount of RAP and/or RAS used in the mix. Fatigue life was found to increase with increasing RAP content up to 25%, and to decrease when the RAP and/or RAS content exceeded 25%, or when RAS was used in the mix. It should be noted that this conclusion was drawn based on a 15% increment in RAP content. Hamburg wheel tracking (HWT) test results showed increased resistance to rutting and moisture-induced damage, with an increase in the amount of RAP and/or RAS. However, the tensile strength ratio test results were not confirmed by HWT. The findings of this study are expected to be helpful in understanding the effects of using different amounts of RAP and RAS on the performance of asphalt mixes produced using local materials. Furthermore, valuable design input parameters, developed in this study for new mixes containing RAP and RAS, may be used for calibration of the M-EPDG input parameters, with local materials.     

The characterisation of foamed asphalt cement using a rotational viscometer

Warm mix asphalt using foaming technology is a widely used alternative to traditional Hot Mix Asphalt in the USA. However, there has been relatively limited research exploring the behaviour of the foamed asphalt cement, especially using traditional asphalt cement testing equipment. This research used the rotational viscometer to develop four new metrics that quantify the behaviour of foamed asphalt cement. These four metrics showed that increasing the foaming temperature increased the observed viscosity, but the initial observed viscosity decreased with lower asphalt binder grades. However, the point at which the observed viscosity crossed the actual viscosity increased with lower asphalt binder grades. Overall, the Wirtgen foamer had higher observed viscosity vs. the PTI foamer and provided a more robust foaming material. However, it is recommended that the AccuFoamer also be explored in future research, along with comparing laboratory produced foamed asphalt cement with field produced foamed asphalt cement.     

Comparing various fitting models to construct the tensile relaxation modulus master curve of asphalt mixes

This study was to compare the relative ability of seven common fitting models, i.e. Pure Power Law (PPL), Generalized Power Law (GPL), Modified Power Law (MPL), Modified Power Law Series (MPLS), Standard Sigmoid (SS), Generalized Logistic Sigmoid (GLS) and Prony Series (PS), to construct the tensile relaxation modulus master curve of dense graded asphalt mixes. To this end, cylindrical asphalt mixture specimens containing crushed stone aggregates with 60/70 penetration asphalt binder were fabricated using two aggregate gradations, two binder contents, two air void levels and three ageing conditions with three replicates. Direct tension relaxation modulus tests were conducted on the specimens at four different temperatures using the trapezoidal loading pattern at a low level of input strain. Tensile relaxation modulus master curves were constructed using all the fitting models together with the numerical shifting technique. Finally, both the graphical and statistical comparisons were made among the fitting models, and the best one was found to be PS, followed by MPLS, GLS, MPL, SS, GPL and PPL.     

Interaction mechanism of cement and asphalt emulsion in asphalt emulsion mixtures

The interaction characteristics of cement asphalt composite mastic (CAM) and performance properties of cement asphalt emulsion mixtures (CAEM) were evaluated in this work using chemical and mechanical test methods to investigate the effect of the presence of cement on asphalt emulsion mixtures (AEM). The chemical composition of the CAM was obtained through use of X-ray diffraction, Fourier-transform infrared spectroscopy, and environmental scanning electron microscopy (ESEM) as a means to describe the interactions between the cement and asphalt in the composite materials. Test results demonstrated that cement can hydrate with the water phase of the asphalt emulsion. Asphalt droplets can simultaneously enclose cement particles and delay the hydration reaction process of cement. The interaction mechanism of cement particles or hydration products and residual asphalt is a physical compound process. The influence of these findings on asphalt emulsion mixture design and performance properties was assessed using varying mix design components and conducting laboratory-based mechanical test methods for rutting resistance and moisture susceptibility. Mix design components varied including added water content, emulsion content, and cement dosage levels. The optimum fluids content was determined based on the dry indirect tensile strength. It was found that the cement content significantly impacts the optimum fluids content for both added water and emulsion. Furthermore, the presence of cement improves the dry tensile strength, rutting resistance, and moisture susceptibility. Based on microstructural analysis of CAM and CAEM, the mechanism by which cement improves the performance of AEM is attributed to the ability of hydration products to increase both the stiffness of the asphalt binder and the adhesion at the mastic–aggregate interface. In practical applications, this study recommends a mix design method for cement-modified asphalt emulsion mixes (CAEM) based on selection of optimum cement and emulsion contents using indirect tensile strength and verification of the design through evaluation of the moisture susceptibility and rutting resistance of the CAEM mix. Threshold values of CAEM mix mechanical properties to determine the quality of the design are proposed.     

Engineering characterization of recycled asphalt concrete and aged bitumen mixed recycling agent

A recycling agent is commonly used to restore the aged bitumen to a condition that resembles that of the virgin bitumen. Three reclaimed asphalt pavement (RAP) stockpiles were sampled, and the aged binders recovered from RAP binders were mixed with recycling agents at ten levels to produce bitumen blends. The blends using virgin bitumen as the softening agent exhibited a significantly different rheological behavior from ones using the rejuvenating agent. The addition of a recycling agent could shift up or down the master curve of the blend vertically, depending on the engineering properties of the recycling agent. A normalized viscosity ratio (NVR) model was used to characterize the rheological properties of aged bitumen mixed with softening and rejuvenating agents. An interaction parameter was introduced into the model to consider the physico-chemical reaction between aged bitumen and recycling agent. This mixing rule was compared to the method specified in the blending chart by the Asphalt Institute (AI). The blending chart was shown to be applicable to determine the amount of the softening agent required to meet the target viscosity. The NVR model appeared to be a better tool for the rejuvenating agent to predict the viscosity of a recovered bitumen blend than the AI chart.     

Evaluation of low-temperature performance of asphalt paving mixtures

Low temperature cracking is the major damage in asphalt pavement, and many test methods have been used to evaluate the anti-cracking property of asphalt mixture. This paper evaluates the low temperature performance of asphalt mixture using four tests namely: beam bending test, indirect tensile test, contraction coefficient test and thermal stress restrained specimen test. Five types of asphalt mixtures namely: A, B, C, D and E were evaluated. Results show that compared with the thermal stress restrained specimen test, beam bending test, indirect tensile test and contraction coefficient test are not appropriate for the evaluation of asphalt mixtures low temperature performance. Moreover, results of gray relational analysis demonstrate that the bending strain energy density is significantly correlated to fracture temperature. It is reasonable to adopt the critical values of bending strain energy density to evaluate the low temperature performance of asphalt mixture in the absence of fracture temperature.     

多聚磷酸与SBS复合改性沥青的改性机制

为研究多聚磷酸(PPA)与苯乙烯-丁二烯-苯乙烯嵌段共聚物(SBS)复合改性沥青的微观结构和改性机制,对不同掺量(质量分数)的PPA和SBS复合改性沥青样品分别进行了四组分试验、红外光谱试验、荧光显微试验和差示扫描量热试验。结果表明:随着PPA掺量的增加,沥青质含量增多,胶团之间的作用力增强,促使沥青由溶胶结构转变为溶-凝胶结构,提高沥青的黏度;在SBS改性沥青中加入PPA,可增强SBS之间的交联作用及SBS与沥青之间的接枝作用,加强SBS改性沥青的空间网状结构,促使SBS更好地相容于沥青中,改善其高温储存稳定性,并促使SBS分散为细小颗粒,增强溶胀作用,利于SBS发挥改性效果;在低SBS掺量改性沥青中加入PPA,形成的网络结构要优于高SBS掺量单独改性;加入PPA对沥青的玻璃化转变温度没有明显影响,表明PPA对SBS改性沥青的低温性能影响较小。     

Use of waste high density polyethylene as bitumen modifier in asphalt concrete mix

The purpose of this study is to investigate the possibility of using various plastic wastes containing High Density Polyethylene as polymer additives to asphalt concrete. It was investigated that the influence of HDPE-modified binder obtained by various mixing time, mixing temperature and HDPE content on the Marshall Stability, flow and Marshall Quotient (Stability to flow ratio). The binders used in Hot Mix Asphalt (HMA) were prepared by mixing the HDPE in 4–6% and 8% (by the weight of optimum bitumen content) and AC-20 at temperatures of 145–155 and 165 °C and 5–15 and 30 min of mixing time. HDPE-modified asphalt concrete results in a considerable increase in the Marshall Stability (strength) value and a Marshall Quotient value (resistance to deformation). Four percent HDPE, 165°C of mixing temperature and 30 min of mixing time were determined as optimum conditions for Marshall Stability, flow and Marshall Quotient (MQ). MQ increased 50% compared to control mix. It can be said that waste HDPE-modified bituminous binders provide better resistance against permanent deformations due to their high stability and high Marshall Quotient and it contributes to recirculation of plastic wastes as well as to protection of the environment.