Moisture characteristics of mixtures with warm mix asphalt technologies – A review

Construction and Building Materials 142 (2017) 148–161

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Review

Moisture characteristics of mixtures with warm mix asphalt technologies – A review Siyuan Xu a, Feipeng Xiao a,⇑, Serji Amirkhanian a,⇑, Dharamveer Singh b a b

Key Laboratory of Road and Traffic Engineering of Ministry of Education, Tongji University, 4800 Cao’an Highway, Shanghai 201804, China Department of Civil Engineering, Indian Institute of Technology Bombay, Mumbai, Powai, 400076, India

h i g h l i g h t s  Warm mix asphalt has been widely used as an environmentally friendly technology.  Moisture damage is considered as one of the main concerns for WMA mixtures.  The commonly used test methods are tensile strength ratio (TSR) test.  Materials, aggregate and asphalt played important roles in moisture characteristics.  Compacting temperature and warm mix technologies influence the moisture behavior.

a r t i c l e

i n f o

Article history: Received 30 December 2016 Received in revised form 8 March 2017 Accepted 9 March 2017

Keywords: Warm mix asphalt Moisture characteristics Anti-stripping additive Indirect tensile strength

a b s t r a c t Warm mix asphalt (WMA) has been widely used as an environmentally friendly technology. Due to the lower temperature than hot mix asphalt (HMA) and some influences of warm mix technologies, moisture damage is considered as one of the main concerns for WMA mixtures. This review focuses on the influences of various factors on the moisture susceptibility of WMA mixtures. The commonly used test methods are briefly introduced, while tensile strength ratio (TSR) test is the most commonly used method to evaluate the moisture susceptibility. Then the influence of materials and technologies are illustrated. Materials, aggregate and asphalt, play important roles in the moisture characteristics of mixtures. Compacting temperature and other warm mix technologies also significantly influence the moisture characteristics. The purpose of this review is to provide assistance to the project engineer when WMA technologies are employed. Ó 2017 Elsevier Ltd. All rights reserved.

Contents 1. 2.

3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Test methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Traditional technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Advance technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Material influences. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Aggregate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Aggregate type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Aggregate moisture content. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3. Aggregate gradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Asphalt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1. Asphalt grade . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2. Asphalt type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3. Aging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding authors. E-mail addresses: [email protected] (F. Xiao), [email protected] (S. Amirkhanian). http://dx.doi.org/10.1016/j.conbuildmat.2017.03.069 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.

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4.

5.

6.

3.3. Antistripping agent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Influencing technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Compaction temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Foaming water content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Foamed bitumen technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1. Aspha-min. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2. Advera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.3. Water-based technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Organic additives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1. Sasobit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2. Asphaltan B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Chemical additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.1. Evotherm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.2. Rediset. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.3. Cecabase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Recycled materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Recycled asphalt pavement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Steel slag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Recycled asphalt shingles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. Coal ash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction With the development of economy, the transit construction is developing rapidly. Especially, highway pavement construction is developing rapidly as it is one of the most essential communal facilities. And the high-quality highway and pavement structures are subsequently required because of the increases of heavy vehicles and traffic loads. As a commonly used pavement, asphalt pavement needs to enhance its ability to meet the actual loads under various conditions. An example is that the modified asphalt is usually used to promote better adhesions and cohesions of the mixtures to satisfy the requirement of heavy-duty pavements. However, the modified asphalt pavement results in the increases of mixing and compaction temperatures of asphalt mixtures, which has caused the increasing emission of soot and exhaust gas, as well as the energy consumption. While global warming and energy shortage are gaining more and more attention, researchers are trying to find some environmentally friendly materials and technologies. Warm mix asphalt (WMA) is a mixture that can be mixed and compacted under a relatively low temperature (100 °C–120 °C) through some WMA technologies when compared with hot mix asphalt (HMA) at 140 °C–160 °C, while the performance properties of WMA mixtures are not significantly influenced. It has been proved that WMA technologies can significantly reduce energy consumption and emission during the production [1]. A number of additives and foaming technologies have now been used to produce WMA. Due to the variety of WMA technologies, a scientific classification is necessary. Nowadays there are some different classifications of WMA technologies, and one widely used method is based on the classification of additives. According to this method, WMA technologies can be divided into three types: organic additives technologies, chemical additives technologies, and foamed bitumen technologies. Organic additives usually have low melting point generally around 100 °C, and they are adding into the mixture to decrease the viscosity of the binder in the process of mixing and compacting at a relatively low temperature. Organic additives can improve the performances of mixtures, which is attributed to the lattice structure formed by waxes after they cool down. Sasobit, Asphaltan-B are two of the most commonly used organic additives.

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153 153 153 154 155 155 155 155 156 156 157 157 157 157 157 158 158 158 158 159 159 159

One type is called chemical additive, and it also contains several kinds. Chemical additives usually have complicated composition, including surfactants, emulsification agents, aggregate coating promoters and antistripping agents. These additives are generally added to asphalt binders in the process of production. Rediset and Cecabase are two of the popular chemical additives, containing surfactant and adhesion enhancers. These two additives can improve the adhesion between aggregate and binder, and also contribute to the aggregate coating. Some other chemical additives are used in forms of emulsion. Evotherm is a well-known chemical additive from America. The third type is foamed bitumen technologies. Water is added straightly or through wet aggregates, or sometimes through zeolite like Aspha-min and Advera. The bitumen then foams at a high temperature. The foaming process can lead to a temporary increase of the binder volume caused by the water vapor, which can subsequently lead to a temporary drop of the binder viscosity [2]. Moisture damage is one complex but common form of distresses in asphalt pavement. It is exactly failure of adhesion between aggregate and binder or cohesion within the binder, which worsen the performance of mixtures, like strength, stiffness and durability [3,4]. Thus the presence of moisture significantly influences the mixture. It is generally considered that moisture damage results in the premature deterioration of asphalt pavements. Therefore the serviceability of the pavement is seriously harmed due to moisture induced damage. Moreover, some other pavement diseases, such as raveling, rutting, and cracking, would be much more likely to happen [5,6]. Consequently, moisture damage of asphalt pavement has been a hot area of research for many years, because the distress happens frequently and brings some extra costs. The problem needs to be paid a lot of attentions to, which has very important practical significance. Thus moisture damage is typically involved in the mix design in order to avoid it to the greatest extent. So far, a lot of research studies have been done to investigate the moisture damage of asphalt pavement, including the study on the moisture damage of WMA. Researchers were trying to figure out the mechanisms of moisture damage, and then developed various tests and analytical methods to evaluate asphalt mixtures and prevent them from being damaged. Some laboratory tests aim to simulate the actual usage condition, while some aim to character-

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ize the aggregate and binder to assess the susceptibility to moisture damage. These methods have been widely used and proved to be effective [7]. However, the moisture damage of WMA mixtures might be more complicated than HMA mixtures because of WMA technologies. Some different procedures in the production of WMA pavements originate the differences. It is considered that three main factors within WMA mixtures might influence the moisture susceptibility: WMA additives, aggregate conditions, and the foamed water or foaming additives. Numerous research studies are focused on this problem [8]. The objective of this review article is to explore the influences of various asphalt materials and WMA techniques factors on the moisture characteristics of WMA mixtures. The main studied aspects include test methods, virgin materials such as asphalt and aggregate, WMA technologies, recycled materials. 2. Test methods 2.1. Traditional technique It has been for many years that different test methods are being used to assess the moisture susceptibility of asphalt mixtures. There are so many factors that might influence the moisture performance of mixtures and lead to the diversity of test methods. These test methods evaluate the moisture susceptibility of asphalt mixtures through different parameters. The common test methods can be divided into two categories according to the different test objects. One is test on loose mixtures and mixture components in order to assess the stripping potential of mixtures, and another one is test on compacted mixtures in order to assess the moisture susceptibility of mixtures. The summaries of commonly used test methods have been made by some researchers and were presented in their articles [9,10]. This part mainly introduce tensile strength ratio (TSR) test among many conventional tests for its popularity. It is generally agreed that TSR can represent the resistance to moisture damage of asphalt mixtures in most cases and is also the most commonly used parameter. According the AASHTO T283, the modified Lottman test is conducted. The specimens are compacted by Marshal Compactor. Three samples are conditioned under warm water at 60 °C, while the other three samples under dry condition. The indirect tensile strengths (ITS) of all six samples are subsequently measure90d, and then TSR is defined as the ratio of the ITS values of conditioned specimens to unconditioned specimens. This method is considered proper to evaluate the moisture susceptibility of asphalt mixtures, because the process of conditioning could well simulate the actual process of destruction on the mixtures in the field [11]. Moreover, some research studies have done to make a comparison between TSR and other test methods. Rouzbeh et al. used both TSR test and Hamburg Wheel Tracking Test (HWTT) to investigate the moisture susceptibility of WMA mixtures. No correlations were found between two tests according to the research results. In addition, it was found that two tests could lead to different results on the moisture damage potential of mixtures [12]. Similar conclusions were drawn by Ashley et al. [13]. 2.2. Advance technique Energy based methods have been gaining popularity in recent years. These methods are based on the theory of surface free energy, which can be defined as the work that the external force needs to create a new unit surface area of the material like asphalt or aggregate in a vacuum. In addition, the surface free energy of adhesion and cohesion changes when asphalt and aggregates

adhere together. Thus, the surface free energy values of various bitumen binders and aggregates can be measured to evaluate the adhesion and cohesion of mixtures [14–16]. Rouzbeh et al. used surface free energy methods to assess the moisture susceptibility of WMA mixtures and found that surface free energy method is better to evaluate the moisture susceptibility of mixtures than the conventional TSR test [16].Some advanced technologies are being used to assess the adhesion between aggregate and binder of mixtures both in the absence and the presence of moisture. Nanoscale equipment is commonly used. For example, Abdalla et al. used atomic force microscopy (AFM) to investigate the effect of water on the adhesion and cohesion [17]. Kutay et al. used X-ray microtomography to study moisture characteristics of asphalt mixtures [18]. In addition, a new technology, neutron scattering, has been used to investigate the static structure and the microscopic dynamical properties of a matter. Neutron is electrically neutral, and has strong penetration. In addition, neutron is sensitive to light elements and has magnetic moment compared to X-ray. Huang et al. used neutron scattering to detect moisture in asphalt mixtures. The research showed that neutron scattering is an effective tool to investigate the micro-structure of asphalt mixture [19]. 3. Material influences WMA mixtures are composed of asphalt, aggregates, and some other additives. All of these greatly contribute to the mechanical properties of asphalt mixtures. The properties of asphalt mixture mainly depend on the properties of the materials. It has been observed that same WMA technology might result in different influences on various asphalt pavements. Therefore, the effect of materials should be paid enough attention to [20]. In this study, the materials can subsequently be divided into three groups: aggregate, asphalt, and antistripping agent. 3.1. Aggregate Aggregate is the main component of an asphalt mixture, and takes a large proportion (approximately 95% by weight of the mixture). Therefore aggregate plays an important role in determining the performance characteristics of the mixture. 3.1.1. Aggregate type Various types of aggregates have been used in current pavement construction. The differences of the chemical composition between different aggregates result in the differences of the physical and chemical properties of asphalt mixtures. Some researchers have completed the chemical analysis of aggregates. Fig. 1 summaries the chemical compositions of various aggregate, adopted from the published articles from Arabani et al., Punith et al., and Saeid et al. [14,21,22]. As shown in Fig. 1, it can be seen that granite and basalt usually have a large percentage of silicon dioxide (SiO2). On the contrary, limestone, with a large percentage of calcium oxide (CaO), usually has a small percentage of SiO2. This agrees with the general recognition that limestone is an alkaline aggregate and granite is an acidic aggregate. It is generally accepted that the adhesion between asphalt and aggregate is generally dependent on aggregate properties [14,21,22]. Table 1 shows the effect of aggregate on the performance of asphalt mixture and is adopted from the research results completed by Arabani et al. and Burak et al. In these studies, Arabani et al. used the ratio of dynamic modulus test to estimate the moisture sensitivity of WMA mixtures, and higher values of the ratio indicate better resistance to moisture damage. Burak et al. used

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Fig. 1. Chemical compositions of different aggregates.

Table 1 Effect of aggregate type on mix performance. Aggregate type

Limestone Granite Basalt

Ratio of wet/dry values of dynamic modulus at 2000 load cycles (%)

Visual stripping resistance (Nicholson Stripping Test)

Sasobit

Aspha-min

Synthetic zeolite

Organic additives

70%–75% 60%–65% –

75%–80% 70%–75% –

45–50 – 25–30

90–95 – 80–85

Nicholson stripping test to explore the moisture characteristics. Similarly, higher values illustrate the mixtures have better resistances to moisture damage [21,23]. In addition, Table 1 shows that the mixtures made from limestone always show a better resistance to moisture damage than the mixtures made from granite or basalt. The main reason is that the adhesion between asphalt and granite or basalt is weak. It is generally accepted that mixtures made from acidic aggregate always exhibit worse resistance to moisture damage than those made from non-acidic aggregate. On the contrary, the mixture made from alkaline aggregates like limestone shows a relatively good performance [14,22,20]. 3.1.2. Aggregate moisture content The low mixing and compaction temperatures of WMA and the rapid temperature decreasing during the production of foamed WMA mixtures could result in the fact that the moisture might not escape and be trapped inside the mixtures, which means that there is residual moisture in WMA mixtures. In addition, it is generally agreed that water is more likely to adhere to aggregates. Once moist aggregates are used in the production, more moisture is taken into and then be trapped in the mixtures. These trapped moistures could influence coating of asphalt on the surface of the aggregate and thus result in a poor adhesion between aggregates and binders, which eventually lead to various pavement distresses [24–26]. Some researchers have studied on the performance of WMA mixtures involved moist aggregates [27,28]. These articles indi-

cated that specimens containing moist aggregates always showed significantly lower dry and wet ITS values, although these specimens had similar TSR values with those not containing moist aggregates [27,28]. In addition, the results showed that mixtures containing moist aggregates had low resistance to moisture damage [27,28]. The residual moisture could influence the adhesion between asphalt and aggregate, and might result in an inadequate bonding within the binder. Meanwhile, this might eventually lead to moisture damage and subsequently other failures of asphalt pavement. It can be concluded that the use of moist aggregate has detrimental effects, especially the trapped moisture. Therefore, it is important to avoid the existence of moisture in the aggregate, which should be fully dried to ensure the quality of asphalt mixtures [29–31]. 3.1.3. Aggregate gradation Besides the aggregate type and the aggregate moisture, some other factors can also influence the moisture properties of mixtures. For example, the aggregate shape and texture influence the interlock level of asphalt mixtures. It is generally recognized that rough cubical aggregates contribute to better performance than those smooth rounded aggregates due to the high interlock level presented by rough cubical aggregates. However, aggregate gradation is paid more attention to by researchers in practical engineering application, since the aggregate gradation is considered to be closely related to the air void and internal friction of the mixtures [20,32,33]. Furthermore, air void is regarded as a key factor in the moisture damage of asphalt

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mixtures. Several studies have reported the effect of aggregate gradation on mixtures. Fig. 2 is adopted from the research studies completed by Kunnawee et al. [20,32,33]. Fig. 2 shows that mixtures with a relatively fine gradation have a better performance due to the different air void distribution in fine-graded and coarse-graded mixtures. The two mixtures were found to have same air void percentage with different air void distributions. Air voids in mixtures with a relatively fine gradation are supposed to be disconnected and impermeable, while air voids in mixtures with a relatively coarse gradation are connected. The interconnected voids means that water can easily enter the voids and then might be trapped in the voids. Therefore fine-graded mixtures perform better than those coarse-graded mixtures at a relatively low air void percentage [20,32,33]. However, different from Kunnawee et al.’s findings, the results from Ebrahim et al. came to the opposite conclusion. Mixtures with a relatively coarse gradation were found to perform better. This is attributed to larger surface areas of aggregates with fine gradation because larger surface areas mean thinner asphalt film thickness, which means a weak adhesion between asphalt and aggregate [34]. It is still hard to determine the effect of aggregate gradation on the moisture performance of mixtures due to the variety and variability of aggregates. Therefore, it is necessary to pay more attentions to this type of research study in the future. In conclusion, alkaline aggregate is considered to originate strong adhesion and thus exhibit high resistance to moisture damage. Acidic aggregate is observed to influence the adhesion. Generally, limestone is recommended in WMA mixtures, as well as dry stone.

Some studies have proved that mixtures containing high PG binders had better performance after a wet conditioning than those mixtures containing low PG binders, which suggested that higher PG binders were conducive to the resistance to moisture damage of mixtures. It is likely to be because higher PG binders always have higher viscosity and bond strength between aggregate and binder [28,35–37]. However, Kutay et al. indicated a different conclusion. It was observed that moisture in high PG binders was more difficult to dissipate than in low PG binders. In other words, water in high PG binders was more easily to be trapped. Kutay et al. found high PG binders would decrease the resistance to moisture damage of mixtures [18]. 3.2.2. Asphalt type There are some modified asphalts that typically used in order to improve the performance of asphalt and subsequently asphalt mixtures at present. Tire crumb rubber is widely used, as well as some other polymers. Several studies had been conducted to investigate the relationship between the performance of mixtures and asphalt type. Fig. 3 shows the findings of Chamoun et al. [38]. Fig. 3 shows the facts that modified asphalts are proved to be better than unmodified asphalts. Mixtures using modified asphalt have higher TSR values, which means those mixtures have better resistance to moisture damage. It has been proved that modified agents can significantly enhance the performance of an asphalt and then improve the performance of a mixture. In terms of moisture sensitivity, modified agents can increase the adhesion between asphalt and aggregate. Thus mixtures used modified asphalts have less moisture susceptibility and better resistance to moisture damage [39–43].

3.2. Asphalt Asphalt in the mixtures acts as a binder, and ensures that aggregates would not be separated during the traffic loads. Thus, asphalt is the most important part of the mixture. In other words, the properties of asphalt mainly determine the properties of mixtures. 3.2.1. Asphalt grade Asphalt is a complex combination with numerous hydrocarbons and non-hydrocarbons. The variety of the chemical compositions of asphalt results in the variety of its properties, having different performances in the field. Consequently, the performance characteristics of mixtures used different asphalts should be general different. It is generally agreed that mixtures used the asphalt with a high viscosity show better resistance to moisture damage which is commonly considered to be related to the stripping behaviors of mixtures. A high viscosity contributes to a strong adhesion between asphalt and aggregates. Mixtures using asphalt of high viscosity then have less stripping and thus show less moisture susceptibility. granite-fine

granite-coarse

1

3.2.3. Aging The physical and chemical properties of an asphalt change during mixing, transportation, compacting, and the field application. And it can be attributed to traffic load, heat, light, and many other factors. These changes might be detrimental to the performance of asphalt pavement, because the properties of asphalt change due to the aging of asphalt. It is generally accepted that aged asphalt is relatively harder and more embrittling. Therefore, the aging of asphalt can influence the mixtures, and it is necessary to study on it. Fig. 4 shows the findings of Xiao et al. [27]. Fig. 4 shows that TSR values of short-term aged mixtures are higher than unaged mixtures. That is to say, short-term aged mixtures show less moisture susceptibility and better performance. This indicates that short-term aging is be beneficial to the resistance of mixtures to moisture damage. The research results by Li et al. also proved this findings [44]. Short-term aging might help improve the adhesion between asphalt and aggregate. However, apart from short term aging, long-term aging was found to seriously influence the performance of asphalt binder and mixtures. slag-fine 0.91

slag-coarse 0.81

0.81 0.8

Ratio

0.66 0.6

0.76

0.64

0.57 0.36

0.4 0.2 0

Ratio of folow number

Ratio of permanent deformation

Different parameters Fig. 2. Effects of aggregate gradation on mixture performances.

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100

Conditioned ITS (kPa)

Unmodified asphalt Polymer-modified asphalt

TSR (%)

Min. TSR-80% 80 60 40 20 0

Advera

Sasobit

450

Unmodified asphalt Polymer-modified asphalt

300

150

0

Advera

Sasobit

Mix types

Mix types

(a)

(b)

Fig. 3. Effect of asphalt type on mix performance: (a) TSR and (b) Dry ITS.

short-term aged

Unaged 120

Min. TSR-80%

TSR (%)

100 80 60 40 20 0 Aspha-min

Cecabase

Evotherm

Rediset

Sasobit

WMA addives Fig. 4. Effect of short-term aging on the TSR values of different mixtures.

It was found that ITS and TSR values were significantly reduced [45–48]. In conclusion, high-viscosity binder is recommended in WMA mixtures, as well as modified asphalt binder. In addition, shortterm aging is suggested for anti-stripping purpose. 3.3. Antistripping agent It has been found that moisture damage is a common pavement distress due to the adhesion loss between asphalt and aggregate, especially in WMA mixtures. Therefore, a lot of antistripping agents are developed to solve this problem. These antistripping agents can alter the surface characteristics of aggregate. Then the asphalt is more compatible with the aggregates and the coating capability of asphalt is improved. Thus antistripping agents are aiming to improve the adhesion between asphalt and aggregate and subsequently prevent stripping of asphalt pavement from moisture damage. As one of the most commonly used antistripping agent, hydrated lime has been used a lot during last several decades. Amines, diamines, liquid polymers are also often used as liquid antistripping agents. Besides, there are some solid antistripping agents such as cement, fly ash, etc [49,50]. Due to the weak resistance of WMA mixtures to moisture damage, it is critical to add antistripping agent into WMA mixtures. Some research projects have been done to study the effect of antistripping agents on the moisture susceptibility of WMA mixtures [49,51–55]. And the positive effect of antistripping agent on asphalt mixtures has been proved. Aravinda et al. found the addition of antistripping agent can enhance the properties of asphalt [52]. Shafabakhsh et al. found that the use of antistripping agents can decrease the acid-to-base ratio of asphalt so that asphalt is much easier to adhere to the aggregates [49]. The findings by Khodaii et al. proved that hydrated lime can reduce the moisture susceptibility of mixtures. It can be seen from these that antistripping

agents can significantly improve the resistance to moisture damage of asphalt mixtures [54]. Further research had been carried out by Chen et al. to study the effect of size of hydrated lime on how it could improve the resistance to moisture damage. Sub-nano-sized hydrated lime and regular-sized hydrated lime were used to study. According to the research results, hydrated lime with smaller size usually has rougher surface. And it was observed that hydrated lime with smaller size more significantly improves the moisture resistance of the mixture and exhibits better than regular hydrated lime on the moisture susceptibility of asphalt mixtures [56,57]. In summary, antistripping agent is suggested to be added into WMA mixtures to effectively improve the moisture characteristics. 4. Influencing technologies 4.1. Compaction temperature The compaction of asphalt mixture is to decrease its volume and reduce its voids. Compaction makes the binder and aggregate adhere each other and then form a stable pavement structure. Therefore compaction can help a mixture reduce its air void and permeability, and also enhance resistance to distresses like rutting and moisture damage. It is considered that the compaction temperature is a key factor during compaction procedure, because it affects the ease and the final degree of compaction as well as the void ratio of asphalt mixture. It is found that a mixture with a high air void is more susceptible to moisture damage [24,58]. Some research articles reported this point. Fig. 5 shows the findings conducted by Amir et al. [59]. Fig. 5 shows that TSR values and conditioned ITS values of mixtures are decreasing with the decrease of compaction temperature. And the TSR values are obviously lower when the compaction temperature is lower than the boiling point of water. It can be seen that

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100

TSR (%)

80

Non-lime A-foamed

Min. TSR-80%

2%lime A-foamed Non-lime B-foamed

60

2%lime B-foamed

40 20 0

115°C

105°C

90°C

Compaction temperature

(a) Non-lime A-foamed

1,200

Conditioned ITS (kPa)

2%lime A-foamed 1,000

Non-lime B-foamed 2%lime B-foamed

800 600 400 200 0

115°C

105°C

90°C

Compaction temperature

(b) Fig. 5. Effects of compaction temperatures on (a) TSR values, and (b) conditioned ITS of mixtures.

compaction temperature has direct effect on the moisture sensitivity of asphalt mixtures. There might be several reasons. One is that compaction temperature could influence the moisture sensitivity of asphalt mixtures due to aging effect. Second reason is that compaction temperature directly determines the final voids of asphalt mixture, which is a key point to affect moisture characteristics of asphalt mixtures. On the other hand, the moisture in the mixture cannot be volatilized completely when compaction temperature is not high enough, and the resided moisture then weakens the adhesion between asphalt and aggregate. It can be concluded that a sufficiently high temperature is indispensable to achieve high quality of mixtures [28,60–62]. To sum up, an enough high temperature is needed to ensure the complete evaporation of moisture. 4.2. Foaming water content Foamed bitumen technologies are widely used to produce the WMA mixtures in the asphalt industry recently. Some of them straightly inject water into the asphalt binder. The water turns to the steam at a high mixing temperature, and then disperses into asphalt binder, which causes the expansion of the binder. Thus a large volume of asphalt binder is obtained and easily to be blended

with aggregate and then the warm mix is eventually achieved. This kind of technology has gained popularity because it is relatively inexpensive and a one-time mechanical modification [24,28]. It is generally agreed that foaming water content influences the properties of asphalt mixtures by the foaming process of a binder. The foaming water content can determine the expansion ratio of the foamed asphalt. Then the coating behavior of the asphalt on the aggregate is affected. Some researchers have studied on the property. Table 2 shows the findings completed by Ayman et al. [28]. The effect of foaming water content on the moisture sensitivity is not significant according to the results shown in Table 2. Xiao et al. gave the similar conclusions [24]. Yu et al. investigated the impact of foaming water content on the performance-related properties of asphalt mixtures and found that various types of asphalt binders show different sensitivities to foaming water contents [63,64]. Biruk et al. carried out a systematic research on this and found that foaming water content has less a significant effect on the water stability of asphalt mixtures than other factors like compaction temperature. Moreover a higher foaming water content leads to a more stable coating of binder on the aggregate especially at a relatively low temperature or dealing with the large aggregate.

Table 2 Effects of foaming water content on moisture performance of WMA mixtures. Foaming water content (%)

1.8% 2.2% 2.6%

Dry ITS (kPa)

TSR (%)

Gravel

Limestone

Gravel

Limestone

1310–1379 1172–1241 1241–1310

1448–1517 1379–1448 1310–1379

80%-85% 100% 95%-100%

80%-85% 80%-85% 85%-90%

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4.3. Foamed bitumen technologies 4.3.1. Aspha-min Aspha-min is actually a synthetic zeolite. It is a hydrated sodium aluminium silicate, which is hydrothermally crystallized. An 18–22% (by mass) of water is present in the zeolite, although it is always in powder form. This can be attributed to the large vacant spaces in the structure of zeolite, which allows the presence of water. In addition, the internal water is released and turns into the steam at temperatures above 100 °C. Consequently, Aspha-min can be used as a WMA additive [8,23]. Some research projects have been done to evaluate the watercontaining additive. Fig. 6 is adopted from some findings [36,66,67]. According to the results presented in Fig. 6, same conclusion was drawn by different researchers. Mixtures produced with Aspha-min always had lower TSR and TSR values, suggesting this water-containing additive weakens the resistance to the moisture damage of WMA mixtures. There are also some researchers who used surface free energy method to study on Aspha-min [14,21]. Arabani et al. chose limestone and granite to produce the mixtures, and found the addition of Aspha-min decreased the surface free energy of adhesion in mixtures compared to the control mixtures [21]. Saeid et al. used the same aggregate, while hydrated lime was also used to improve the moisture susceptibility. Similar reduction of surface free energy of adhesion was observed [14]. All these suggested that the addition of Aspha-min increases the sensitivity to moisture damage of mixtures. However, Aravinda et al. indicated no

Aspha-min

100

60 40 20 0

Shad et al.

Punith et al.

4.3.2. Advera Advera is another synthetic zeolite similar as Aspha-min. The mechanism behind two zeolites is almost same. Thus it is supposed to show similar effects on the mixtures. Fig. 7 is adopted from some research studies [38,51,68]. It can be noted that Advera is considered to have negative effects on the moisture susceptibility of mixtures, since mixtures containing Advera have lower TSR values than the control mixtures. Zelelew et al. evaluated the performance of plant-produced WMA mixtures containing Advera [69]. Hamburg wheel tracking test was conducted to investigate the moisture susceptibility of mixtures. According to two parameters from this test, the stripping slope and the stripping inflection point, mixtures containing Advera exhibited the lowest resistance to moisture damage [69]. Used surface free energy method, Rouzbeh et al. drew same conclusion in their study [16]. 4.3.3. Water-based technologies These technologies used foamed bitumen to produce the WMA mixture are more direct than used zeolite, since the cold water is generally injected into the hot binder at a high temperature. Some water-based technologies, like Double Barrel Green and LT Asphalt, share similar procedures, injecting cold water into hot bitumen. WAM Foam is a type of different technique, and is considered a two-phase method. A soft binder is used to coat the aggregate firstly. And a hard binder, which has been foamed through injected water, is then mixed together. Another method is to add wet fine aggregates into mixtures. With the volatilization of water in wet fine aggregates, the bitumen is foamed to yield WMA mixtures [1,70]. Due to the direct injection of water, these technologies are considered to be more likely to be prone to moisture damage. Many researchers have investigated the effect of water-based technologies. According to the results, foamed WMA mixtures have

800

Aspha-min

Control

600 400 200 0

Xiao et al.

Shad et al.

Punith et al.

Xiao et al.

Researchers

Researchers

(b)

(a)

Fig. 6. Effect of Aspha-min on the mix performance: TSR (a) and conditioned ITS (b).

Advera

100

60 40 20 0

Zahid et al.

Zahi et al.

Researchers

(a)

Brian et al.

Advera

600

Control

Min. TSR-80% 80

Conditioned ITS (kPa)

TSR (%)

TSR (%)

80

Control

Min. TSR-80%

significant effects of Aspha-min on the surface properties of mixtures were found [52].

Conditioned ITS (kPa)

However, a higher foaming water content can lead to more trapped water and cause an adverse impact on mixtures. Therefore a proper foaming water content and a matched temperature are crucial and should be optimized [65]. In conclusion, separate consideration of foaming water content is unreasonable. It is crucial to set a proper foaming water content and a matched temperature.

Control

400 200 0

Zahid et al.

Zahi et al.

Researchers

(b)

Fig. 7. Effect of Advera on the mix performance: TSR (a) and conditioned ITS (b).

Brian et al.

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Sasobit

Min. TSR-80%

100

Control

TSR (%)

80 60 40 20 0

Zahi et al. Xiao et al. Cao et al. Brian et al. Ebrahim et Punith et Mahmoud Shad et al. Burak et al. al. et al. al.

Researchers

(a) Sasobit

Conditioned ITS (kPa)

1,000

Control

800 600

Not offered

400 200 0

Zahi et al. Xiao et al. Cao et al. Brian et al. Ebrahim et Punith et Mahmoud Shad et al. Burak et al. al. et al. al.

Researchers

(b) Fig. 8. Effect of Sasobit on the mix performance: TSR (a) and conditioned ITS (b).

significantly lower dry and wet ITS values than control HMA mixtures, especially at a low temperature below 100 °C. However, foamed mixtures might have similar TSR values and even higher values [28,59,71]. LEA was studied by Zelelew et al. through hamburg wheel tracking test. The results showed LEA could significantly decrease the resistance to moisture damage of mixtures [69]. Unlike these researchers, some other researchers did not observe any significant differences between foamed mixtures and the control mixtures. As long as the production temperature is high enough to make sure the complete dissipation of moisture, foamed mixtures can exhibit similar performance as the control mixtures [24,45,65]. In conclusion, as water-containing additives, Aspha-min and Advera weaken the resistance to the moisture damage of WMA mixtures. In terms of water-based WMA mixtures, a proper WMA additive content and a matched temperature are crucial to obtain good moisture characteristics. 4.4. Organic additives The organic additive is typically added to asphalt binder to reduce its viscosity and thus decrease mixing and compaction temperatures of the mixtures. After the organic wax cools down, it can form a lattice structure. And it is considered to improve the performance of mixtures. 4.4.1. Sasobit Sasobit (also named FT wax) is a long-chain aliphatic polymethylene hydrocarbon generated from the Fischer-Tropsch process. The synthetic paraffin wax has a melting temperature

around 120 °C. The long carbon chains contribute to a homogeneous solution, which brings a reduction in the viscosity of the binders at the common compaction temperature of asphalt mixture. In addition, Sasobit can form a framework structure inside the mixtures at room temperature, which improves the properties of mixtures [8,23]. The main concern about Sasobit is the potential to influence the adhesion between the binder and the aggregate. Fig. 8 is adopted from some of them. Large differences above 10% between two types of mixtures were observed by Zahi et al. and Xiao et al., indicating the addition of Sasobit significantly improves the sensitivity to moisture damage of mixtures. Slight differences around 5% were observed by more researchers, which suggests that the addition of Sasobit slightly reduces the resistance to moisture damage of mixtures. Meanwhile, increase of the TSR values with the addition of Sasobit were also observed by some researchers. According to the findings from these researchers, Sasobit could improve the moisture susceptibility of mixtures [23,27,32,36,38,43,66,68,72]. Surface free energy methods were used by some researchers to study the influence of Sasobit on the moisture characteristics of WMA mixtures. Some found that Sasobit improved the coating capability of an asphalt and increased the adhesion between the binder and the aggregate, and no significantly detrimental effects were observed [16,52]. According to the results from other researchers, Sasobit caused an increase in acid components, which resulted in a weak adhesion between aggregate and binder [21]. And the results of surface free energy on the adhesion showed that mixtures containing Sasobit are more sensitive to moisture damage [14].

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The chemical additives are always complex mixtures, composed of emulsification agents, surfactants, and polymers. In addition, some other performance promoters are often contained. 4.5.1. Evotherm Evotherm, a form of emulsion, is composed of various additives besides cationic emulsification agents, including antistripping agents, coating promoters, and workability promoters. During the blending procedure, the mixing temperature is considered to accelerate the emulsion breaking and evaporation of water, and the binders then coat the aggregates. The additive decreases the internal friction inside the mixtures at lower temperatures, not influencing the viscosity of the bitumen [2,8]. The study on the effect of Evotherm on moisture susceptibility has been conducted a lot. Fig. 9 shows the results of TSR tests conducted by some researchers [30,43,66,68,74]. It can be noted that two mixtures produced with Evotherm show significantly lower TSR values than the control, while two other tests indicate the opposite trends. The rest one shows the similarity TSR. Zhao et al. used the binders of PG 64-22 to produce the mixtures, along with the limestone and natural sand, and 15% RAP was also used. The mixtures produced with Evotherm and the control were compacted at 125 °C and 150 °C, respectively. Punith et al. used the same binder, while quartz and potassium feldspar were used as aggregate. The compaction temperatures were both 121 °C. They found that the use of Evotherm would decrease the resistance to moisture damage of mixtures. However, Brian et al. and Shad et al. drew different conclusions, while they used similar aggregates and binders with Zhao et al. [66,68,74]. There might be a slight difference in the compaction temperatures. Rouzbeh et al. investigated the effect of Evotherm on the moisture susceptibility of mixtures through energy-based methods [16]. The surface free energy values of the mixtures with virgin binder and with Evotherm were measured. And they found the use of Evotherm will significantly decrease the magnitude of the work of debonding of the binder over the aggregates, which

TSR (%)

100

Evotherm Control

Min. TSR-80%

80 60 40 20 0

Zhao et Punith Brian et Shad et Cao et al. et al. al. al. al.

4.5.2. Rediset Rediset is introduced as a WMA additive from Akzo Nobel, Netherlands, composed of fatty polyamines, polymer, and nonionic components. This additive is easy to dissolve in the binders and can be added into the bitumen or the mixture. Moreover, it can play a role as an antistripping agent. Thus it can improve the resistance to moisture damage of mixtures without other additives [75,76]. However, not every research supports the statement above. Fig. 10 is summarized from the research studies from Xiao et al., Brian et al., and Saeid et al. [27,68,77]. According to the results from Brian et al. and Saeid et al., mixtures containing Rediset had higher TSR values than the control, indicating that those mixtures had better water stability. However, Xiao et al. found the mixtures containing Rediset were prone to moisture damage. Additionally, Xie et al. investigated the effect of Rediset on the moisture susceptibility through laboratory tests. A slightly higher TSR value of mixtures containing Rediset was observed compared to the control mixtures, indicating that Rediset might improve the moisture resistance of mixtures [78]. 4.5.3. Cecabase Cecabase is a chemical additive from CECA, and this technology can reduce the production temperature by 30 °C. The studies on Rediset

100

Control

Min. TSR-80%

80

TSR (%)

4.5. Chemical additives

meant Evotherm was considered to improve the resistance to moisture damage of mixtures [16]. Cao et al. did not find obvious differences between two kinds of mixtures. The TSR values of mixtures produced from Evotherm and hot mix were very close to each other [43]. According to the research results from Peter et al., the result varied with the type of aggregate [71]. While limestone was used, Evotherm was proved to be beneficial. While quartzite was used, it is found that moisture resistance was getting worse. In addition, no significant difference was found when used natural gravel [71].

60 40 20 0

Xiao et al.

Brian et al.

Saeid et al.

Researchers Fig. 10. Effects of Rediset on the moisture performance of WMA mixtures.

Conditioned ITS (kPa)

4.4.2. Asphaltan B Asphaltan B is also named lignite wax, which is generated from lignite. It is mainly fossil fatty acid esters, and is often mixed with amide waxes to enhance its low melting temperature. Prabir et al. investigated the moisture susceptibility of WMA mixtures containing Asphaltan B [73]. The binder with Asphaltan B and virgin binder were compared. The results of energy ratio from the Superpave Indirect Tensile Test indicated that Asphaltan B significantly improves the resistance to moisture damage of mixtures [73]. In summary, organic additives are generally considered to have slightly detrimental effect on the moisture sensitivity of WMA mixtures.

Evotherm Control

800 600

Not offered

400 200 0

Zhao et Punith et Brian et Shad et Cao et al. al. al. al. al.

Researchers

Researchers

(a)

(b)

Fig. 9. Effect of Evotherm on the mix performance: TSR (a) and conditioned ITS (b).

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Cecabase

TSR (%)

120 100

Control

Min. TSR-80%

Table 3 Effects of RAP contents on TSR values of WMA mixtures. RAP content

TSR (%)

80

Zhao et al.

60

Foaming

Evotherm

Sasobit

80%–85% 90%–95% 95%–100% 90%–95% – –

80%–85% 90%–95% – 90%–95% – –

80%–85% – 75%–80% – 70%–75% 65%–70%

40 20 0

Xiao et al.

Mohd et al.

Researchers Fig. 11. Effect of Cecabase on the mix performance.

Cecabase have been conducted by some researchers. Fig. 11 is adopted from the articles by Xiao et al. and Mohd et al. [27,62]. According to the results shown in Fig. 11, similar TSR values of the control mixtures were observed by two researchers, while completely different TSR values of the mixtures containing Cecabase were observed. 1% hydrated lime was used to improve the moisture performance of mixtures. The compaction temperatures of the mixtures containing Cecabase determined by Xiao et al. were 115–121 °C. However, mixtures containing Cecabase were compacted at 130 °C by Mohd et al. Muhammad et al. investigated the moisture susceptibility of mixtures containing Cecabase through contact angle measurements [15]. The surface free energy measures were assessed to explore the moisture susceptibility. It was observed that the addition of Cecabase improved the adhesion of the mixtures, as well as the compatibility ratio. This indicated that Cecabase could improve the resistance to moisture damage of mixtures [15]. It can be seen that WMA technologies have complicated effects on the moisture susceptibility of mixtures. In addition, researchers drew different conclusions from their respective research. One of main reasons is the complication of asphalt mixture. Due to the variety of asphalts and aggregates, the components of mixtures from different research studies are not totally same. In addition, this is also attributed to the uncertainty of manual operation in the lab and field. The other reason comes from the TSR test. Although the TSR test is commonly used to evaluate the moisture susceptibility of asphalt mixtures, it may not be able to fully characterize the properties of asphalt mixtures [12,16]. In conclusion, chemical additives are generally considered to be beneficial in improving moisture performance because of the adhesion promoters. 5. Recycled materials 5.1. Recycled asphalt pavement Recycled asphalt pavement is a material that is obtained from the old asphalt pavement after digging, recycling, and crushing. This material is composed of aged binders and crushed aggregates. The fractionated RAP is added into mixtures as an aggregate and is proved that the addition of RAP has some obvious advantages. On the one hand, RAP is an environmentally friendly material for the reason that it can reduce the consumption of natural aggregates and save energy. On the other hand, the addition of RAP can benefit the asphalt mixtures since it can enhance the resistance to rutting. That is because the aged asphalt remained in RAP is stiff and can provide more resistance to rutting [40,68,79]. However, there are some different perspectives. Table 3 is summarized from the results by Zhao et al. and Fereidoon et al. [74,80]. Table 3 shows the opposite views of the effect of RAP on moisture susceptibility of asphalt mixtures. Zhao et al. found that the use of RAP is good for resistance to moisture damage of mixtures

15% 20% 30% 40% 50% 60%

Fereidoon et al.

[74]. Shu et al. and Brian et al. came to similar conclusions. These researchers illustrates that the addition of RAP is beneficial to the water stability of WMA mixtures, because the aged binders have stronger bonding with the stones than the virgin binders, and the bonding is becoming stronger with the increasing of the RAP contents. Therefore, these researchers indicate used a high proportion of RAP could be an alternative to solve the bad water stability of mixtures [68,74,81]. Fereidoon et al. found that the moisture sensitivity of WMA mixtures is increasing as the content of RAP is increasing [80]. Guo et al. came to similar conclusion. They considered the use of RAP is harmful to the moisture resistance of mixtures, because they thought the addition of RAP could progressively decrease the penetration of the binder, or in other words, to increase the viscosity of the binder [82]. In addition, there are also some researchers found that RAP has slight effect on the moisture susceptibility of asphalt mixtures. Jesse et al. chose Sasobit to study on the problem and Walaa et al. chose a waxed-based WMA additives named as Sonne Warmix. They found that the use of any contents of RAP had no significant effects on the water stability of mixtures [40,83]. In conclusion, the utilization of RAP and WMA technology could be well combined after a comprehensive consideration. 5.2. Steel slag Steel slag is another commonly used recycled material in road construction. Steel slag is a solid waste generated in the process of steel making and is mainly composed of oxides of calcium, iron, silicon, magnesium, and some other elements. Just like RAP, this material is an environmentally friendly material because the utilization of steel slag can save the natural resource and energy. In addition, it has been found that steel slag has obvious difference with natural aggregate. Mahmoud et al. used scanning electron microscope to study on the micro-characteristic of steel slag and indicated that steel slag has higher porosity and rougher surface than natural aggregate. These would result in a higher asphalt-aggregate ratio when used steel slag to produce asphalt mixtures [72]. Some results summarized by Saeid et al. were presented in Fig. 12 [77]. Fig. 12 shows that using steel slag as coarse aggregate contributes to a slight increase of TSR value. This is attributed to the fact that more binder is used when used steel slag because of its higher porosity. In addition, a rougher surface of steel slag leads to a stronger adhesion between binder and aggregate. It can be seen that using coarse steel slag might be beneficial to improve the performance properties of asphalt mixtures [72,77]. It can be concluded that steel slag has positive effect on the moisture characteristics of WMA mixtures. 5.3. Recycled asphalt shingles Recycled asphalt shingles (RAS) are the recycled materials obtained from waste roof shingles. RAS has been commonly used

S. Xu et al. / Construction and Building Materials 142 (2017) 148–161

159

all limestone steel slag as coarse aggregate&limestone as fine aggregate

100

Min. TSR-80%

TSR (%)

80 60 40 20 0 HMA

sasobit

rediset

Mix types Fig. 12. Effects of steel slag on TSR values of mixtures.

in HMA in USA. Some researchers have done some investigations to study its properties [13,29,84]. Ashley et al. used TSR values and stripping inflection point (SIP) from HWTT to investigate the effect of RAS on the moisture susceptibility of mixtures [13]. However, different conclusions were obtained from these two different tests. TSR values showed that the addition of RAS would decrease the resistance to moisture damage of mixtures, while SIP showed that the addition of RAS would increase the resistance to moisture damage. However, Xiao et al. and Punith et al. illustrated no significant impacts of RAS on the moisture susceptibility of mixtures [29,84].It can be concluded that RAS could be used in WMA mixtures.

5.4. Coal ash

lot. Based on these studies, the following conclusions can be drawn:  Alkaline aggregate is considered to originate strong adhesion and thus high resistance to moisture damage. Acidic aggregate is observed to influence the adhesion. Generally, limestone is recommended in WMA mixtures, as well as dry stone.  High-viscosity binder plays a similar role as alkaline aggregate in WMA mixtures. And modified asphalt binder performs better. Short-term aging is suggested while long-term aging not.  As water-containing additives, Aspha-min and Advera weaken the resistance to the moisture damage of WMA mixtures. In terms of water-based WMA mixtures, a proper WMA additive content and a matched temperature are crucial and should be optimized, in order to obtain nice coating of binders on the aggregates and less trapped moisture. Organic additives are generally considered to have slightly detrimental effect on the moisture sensitivity of WMA mixtures. Chemical additives are generally considered to be beneficial because of their adhesion promoters.  Recycled materials show no significant effect on the moisture susceptibility of WMA mixtures, indicating the feasibility of them.

Coal ash, a dark or brown sand-sized material, is the solid waste originated from the combustion of coal. This recycled material has been used in pavement construction for many years. Some investigations have been conducted to study the influence caused by coal ash. It has been proved that a proper addition of coal ash can significantly improve the performances of mixtures. This is attributed to the properties of coal ash, such as high absorptivity and porosity. Moreover, 5%–8% coal ash, by weight of aggregate, can significantly increase the ITS and TSR of mixtures with hydrated lime [85–89]. Punith et al. and Xiao et al. found that a relatively high percentage of coal ash would have a detrimental effect on the performance of mixtures. For example, mixtures containing 10% coal ash have obviously lower ITS and TSR values than mixtures with 5% coal ash [29,84]. In conclusion, the proper content of coal ash is beneficial to WMA mixtures.

Finally, the outlook of research in moisture characteristics of WMA mixtures is given. WMA has been proved to be an environmentally friendly technology, which exhibits satisfactory performance. However, the existing laboratory tests need to be improved and a more suitable method is needed. Commonly used tests, like TSR test, cannot precisely estimate the performance of mixtures. In addition, more filed test results is wanted to improve the WMA technology.

6. Conclusions and outlook

References

Moisture damage is one of the main concerns about WMA mixtures. Currently, there are a lot of test methods to evaluate the moisture susceptibility of mixtures. TSR test, according to AASHTO A283, is the most frequently used method all over the world. In addition, methods based on the surface free energy are gaining popularity. However, these laboratory test methods cannot precisely estimate the performance of mixtures due to the complexity of moisture damage and WMA mixtures. The moisture susceptibility of WMA mixtures is influenced by many factors. The effect of materials on the performance of mixtures, either HMA mixture or WMA mixtures, has been studied a

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