Pavement engineering materials: Review on the use of warm-mix asphalt

Construction and Building Materials 36 (2012) 1016–1024

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Review

Pavement engineering materials: Review on the use of warm-mix asphalt S.D. Capitão a,⇑, L.G. Picado-Santos b, F. Martinho c,d a

Department of Civil Engineering, ISEC – Polytechnic Institute of Coimbra and IST-CESUR, Rua Pedro Nunes, 3030-199 Coimbra, Portugal Department of Civil Engineering and Architecture, Instituto Superior Técnico – Technical University of Lisbon (IST-CESUR), Av. Rovisco Pais, 1049-001 Lisboa, Portugal c FM Consulting, Tomar, Portugal d IST - Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisboa, Portugal b

h i g h l i g h t s " Although WMA technologies have benefits comparing to HMA they have some drawbacks. " Mix design procedures used for WMA with additives can be similar to those used for HMA. " Although paving conditions are adverse, performance of WMA is generally acceptable. " The construction process is generally easy for WMA produced with additives. " A conclusion about LCCA is not possible yet as long-term performance of WMA is unknown.

a r t i c l e

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Article history: Received 24 February 2012 Received in revised form 15 May 2012 Accepted 4 June 2012 Available online 2 August 2012 Keywords: Additives Asphalt pavements Fatigue resistance Foam bitumen Hot-mix asphalt Life-cycle analysis Mix design Rutting Stiffness modulus Warm-mix asphalt

a b s t r a c t Warm asphalt mixtures have been used worldwide aiming at saving energy and reducing emissions throughout the production process, without decreasing the in-service performance. This has been achieved with wax additives, chemical additives and foaming techniques. Benefits and drawbacks are mentioned in the literature for each process. This paper is a review of the main aspects involved in WMA technology, including constituent materials, mix design and mechanical performance issues, as well as technological specificities. Some discussion associated to life-cycle analysis is also considered. In the view of the literature review, it can be stated that WMA is a very interesting technology, able to contribute to achieve environmental objectives along with acceptable performance. WMA processes themselves require some additional complexity that must be considered by the players involved. Ó 2012 Elsevier Ltd. All rights reserved.

Contents 1. 2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Benefits and drawbacks of WMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Drawbacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Main mechanisms involved in temperature reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Organic additives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Chemical additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Foamed bitumen technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Laboratory design of warm asphalt mixes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Mix design particularities for additive technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. Tel.: +351 239 790 200; fax: +351 239 790 201. E-mail address: [email protected] (S.D. Capitão). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.06.038

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5.

6. 7. 8.

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4.3. Mix design particularities for foamed bitumen technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical performance of warm asphalt mixes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Water sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Stiffness modulus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. Resistance to fatigue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5. Resistance to low temperature fracture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6. Resistance to permanent deformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Guidance on mixing, laying and compaction of warm asphalt mixes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison between life-cycle cost analysis of WMA and HMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction Over the last two decades, the production and appliance of asphalt mixtures have been improving, particularly to achieve economic and environmental objectives. Recently, the improvement has paid more attention to the reduction of energy consumption throughout the process, without changing the in-service mechanical performance of these asphalt mixtures. There is a growing international pressure on the reduction of fossil fuels consumption and the emission of greenhouse effect gases (GHG), such as CO2 (carbon dioxide). Unfortunately, the production of hot-mix asphalt (HMA) for pavements is to blame for a significant percentage of the energy consumption and the release of pollutant gases. This is a consequence of drying and heating mineral aggregates, and bitumen at temperatures above 140 °C. If a significant temperature decrease could be achieved within the production practice of asphalt mixtures, while the workability of the material is adequate and mechanical performance attained is the same as or even better than HMA, the gain for the environment and the society in general would be significant. Therefore, the scientific and technical community have developed a number of new technologies for asphalt materials, generally referred to as warm mix asphalt (WMA), which require lower production temperature. As a result, since these technologies spend less energy than traditional HMA, they significantly contribute to the aforementioned objectives. There is a wide range of usage temperatures within the WMA family. PIARC [1] and EAPA [2], among others, have stated that WMA are generally produced in a temperature range from 100 to 140 °C, while half-warm mix asphalt (HWMA) are fabricated between 70 and 100 °C. The temperature decrease is around 30 °C for the first case and can attain up to 80 °C for the second case. At the present time, there are a number of available technologies to fabricate WMA and there are others in progress. In addition, according to the literature [1–5], the WMA technologies promise several benefits compared to HMA, which can be grouped in three categories: environmental, production and paving, and economic. There are also a few drawbacks to point out to WMA. The main goal of this paper is to make a review on the aspects associated with WMA, involving laboratory mix design, production technologies, laying and compaction procedures. A revision of the mechanical performance of WMA and a balance between the advantages and drawbacks of this type of asphalt mixtures, together with a discussion on WMA life-cycle analysis (LCA) is also carried out. 2. Benefits and drawbacks of WMA 2.1. Benefits Taking into account the lower temperatures applied in WMA technologies, a number of environmental benefits can be expected.

Most of them have been observed directly in the European countries [5,6] and in the USA [4]. A significant reduction on pollutant and GHG emissions has been reported. Depending on the WMA technology used, emissions declared in the literature have some variation. Nevertheless, irrespective of the WMA production process, a significant reduction of emissions is observed. Evaluations carried out in a number of European countries [2,5], for instance, made it clear the decreasing of various emissions throughout the production process in plant, as follows: 30–40% for CO2 (carbon dioxide) and SO2 (sulfur dioxide), 50% for VOC (volatile organic compounds), 10–30% for CO (carbon monoxide), 60–70% for NOx (nitrous oxides), and 25–55% for dust. Reductions from 30% to 50% for asphalt aerosols/fumes and polycyclic aromatic hydrocarbons (PAHs) have also been reported, which have a substantial influence on the exposure of the workers and the surrounding area of construction sites to those products. Since operating temperature and emissions are lower, it is easier for plants to be allowed in the proximity of urban areas. In addition, in cold weather, when applying WMA techniques with hard bitumen, such as higher asphalt recycling rates or high modulus asphalt, the material workability is better as the viscosity of the stiff binder decreases and the drop of temperature with time is less important. This also allows higher haulage distances, reduces the risk of compaction troubles and requires less time to cool the laid material before opening it to traffic or to place the next layer [3,4]. Lowering the production temperature allows reducing the energy consumption up to 35%, or more, depending on the WMA process applied [5] and on how much the temperature is reduced [4]. The associated cost will decrease accordingly. In a scenario of energy price rising, the cost saving can be far more interesting, depending on the amount of temperature reduction. However, a number of WMA processes require initial investment to modify the layout of the plant, whereas some of them require permanent purchase of additives. Therefore, the cost examination must also consider the analysis of this type of issue [3]. Compared to cold asphalt mixtures (CAM), the WMA technology is also beneficial because it does not need curing time before opening up to traffic and does not require a sealing layer as for some of the CAM applications. In addition, the laying and compaction operations, and the coating of aggregates by the binder are better than for CAM, leading to a better in-service material [4]. 2.2. Drawbacks As reported in the literature, the number of testing sites and projects where WMA has been used is already significant. However, there still remain a number of challenges to overcome [3,4]. There are some concerns related to WMA cost in its whole lifecycle, as the technologies available for WMA generally increase the initial production cost [4]. On the one hand, this can be connected to the additional equipment needed for plants, allowing the use of specific technologies or additives. On the other hand, the use of

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additives brings some supplementary cost, which could be only partially compensated by lowering the operating temperature [4,5]. Furthermore, there is some danger that some of the environmental advantages recognized to WMA could be offset by the carbon emissions related to the production of additives [3]. Since there is not enough experience yet on long-term performance of WMA, the technologies widespread can face some struggle, unless the governments establish more severe environmental regulations or WMA grant clear construction and characteristics benefits [7]. Some authors [3,4] refer to concerns related to in-service moisture susceptibility of WMA associated to the reduced optimum binder content derived, for instance, from volumetric mix design procedures. Moreover, the generally good workability of some WMA products, resulting in a lowered void content compared to conventional HMA, together with a less oxidative hardening of the binder throughout the production process, can increase the in-service rutting potential of some WMA products, despite that it can also conduct to an increased durability. 3. Main mechanisms involved in temperature reduction 3.1. Overview Some of the WMA technologies involve a temporary or permanent adjustment of various bitumen properties, such as viscosity, for instance. In a number of technologies the adhesion between binder and aggregate particles is chemically adjusted to improve the way mineral aggregates are coated by bitumen. When surfactants are also included they will act at the microscopic interface of aggregates and bitumen, reducing friction at that interface, allowing lower mixing and compaction temperatures. There are also techniques that introduce water into the process aiming to improve temporarily the workability of the asphalt mixture. Despite the huge number of reported WMA technologies found in the literature, they can be, basically, classified in three main groups [2,3,8]: organic additives, chemical additives and foaming technologies. The main goals of the most part of the processes involved in WMA is lowering production and handling temperatures of the mixture, as well as achieving the same, or even better, in-service performance compared to HMA. The diversity of features related to the known technologies allows adjusting the production process to the available raw materials. Obviously, this requires a careful evaluation of the aspects involved before the application of a specific technology in a real project. 3.2. Organic additives The use of organic additives is accomplished by adding an organic wax to bitumen or blending it to asphalt concrete mixtures, reducing the viscosity of the binder. Since the binder is at a high temperature, this phenomenon is maintained throughout the mixing and compaction procedures. When the asphalt cools, the additive crystallises, forming a lattice structure of microscopic particles, increasing the binder stiffness and its resistance to deformation [3,10]. This type of additives is generally formed by a long chain of hydrocarbon atoms, which is solid at room temperature and has a melting point generally around 100 °C [3,9]. The most common commercial products available, such as SasobitÒ [10,11], are produced from natural gas using the so-called Fisher–Tropsch (FT) process [3,9,10]. Nevertheless, there are a few additional references about the alternative organic additives, such as the following examples: Asphaltan-B, which is a blend of wax obtained by solvent extraction from lignite or brown coal

(Montan wax) and fatty-acid amides [4,5]; Thiopave™, which is a technology that uses a sulphur-enhanced additive patented by Shell [12,13]. Based on the literature review, it can be stated that using organic additives allows a mixing temperature reduction of asphalt about 20–30 °C [3,5], although slightly different values, below and above this range, can also be found [4,9]. 3.3. Chemical additives Different types of chemical additives are reported in the literature within the WMA technology. In a few cases, additives are formed by a package of products such as surfactants, emulsification agents, aggregate coating enhancers and anti-stripping additives. Chemical additives are usually added to the binder during the production process, although there are also techniques in which the package of products is used by means of a bituminous emulsion [3,4]. Rediset™ WMX and CecabaseÒ RT, for instance, are both chemical additives referred to in the literature as product packages formed by surfactant and adhesion agents, among other components. Those types of products chemically enhance active adhesion and improve the wetting of aggregates by bitumen without changing considerably the binder performance [14–16]. The American technology commercially called Evotherm™ is a typical case wherein a package of additives is used in a form of emulsion [17,18]. Since the aggregates are heated before mixing, the water within the emulsion vaporizes during the production process and the binder covers the aggregate particles. Meanwhile, the initial technology has evolved: firstly, to a process in which chemical additives are introduced into the plant’s binder line (Evotherm Dispersed Additive Technology), incorporating much less water than with emulsion; recently, to a third generation process (Evotherm 3G), which is a water-free WMA technique, where the additive is incorporated into bitumen before its deliver to asphalt plants [17]. As stated in the bibliography, chemical additives may reduce the mix and compaction temperatures around 30 °C [2,16,19]. 3.4. Foamed bitumen technologies A large number of foaming methods are referred to in the literature [2,3,5,8], and most of them are proprietary. Bitumen foam is generally obtained by adding a small amount of cold pulverised water into preheated bitumen [20,21]. The water vaporises and the liberated steam is encapsulated within bitumen, resulting in a temporary expansion of its volume together with a reduction of its viscosity [3,20]. The expanded volume gradually decays with time and the bitumen reverts to its original characteristics. This phenomenon promotes a better distribution of the binder within the asphalt mixture [7]. After foamed bitumen is obtained, it can be mixed-together with aggregate at an ambient temperature. Alternatively, aggregate can be previously heated at a moderate temperature (under 100 °C) to improve some properties of asphalt mixtures [21]. Lower production temperatures are allowed as workability of mixture and coating of the aggregate particles are temporarily improved. Despite the main mechanism involved among those techniques basically being the same, some differences can be pointed out in-between them [8,17]. The water required to the foaming process can lead to stripping troubles. In fact, as reported by Van de Ven et al. [20], throughout the mixing action the foamed bitumen collapses to its original state, and the bitumen rather sticks to the fine fractions of aggregates than to coarse particles. Since only a part of bitumen beads connects to high dimension particles, usually it is advisable to use adhesion or coating promoters (chemical additives) to improve

S.D. Capitão et al. / Construction and Building Materials 36 (2012) 1016–1024

the covering of aggregates by bitumen and reduce water sensitivity of the asphalt mixtures [3,20]. Foamability is a term sometimes applied to grade qualitatively the process within a specific technique or related to a bitumen source. The expansion ratio of the binder and its half-life (time in seconds needed to reduce 50% the maximum volume obtained after expansion), just to mention a few, are two important parameters used to evaluate the process [21]. Some authors subcategorise the foam technologies into two groups: water based and water containing [3]. In the first case, the water is introduced into the process by means of a specific equipment to generate foam. In the second case, the blend incorporates a finely crushed synthetic zeolite (a crystalline hydrated aluminium silicate), which contains about 20% of water trapped in its structure. When the product is heated above a determined temperature (85 °C in the case of Aspha-minÒ, for instance [4,22,23]) the water is released as steam, generating foam bitumen [8,22,23]. Processes that add small amounts of water to the mix, such as zeolite or damp aggregates, release a small amount of steam compared to processes that inject water directly into hot bitumen. Therefore, in the first case the expansion of the binder phase is much lower than the achieved for water based processes [8]. Among the different techniques found in the literature to inject water into the process, Low Energy Asphalt (LEA) [4,5,21] and WAM Foam [4,5,21,23] justify additional comments as the used procedures are different in some extent. In the LEA process coarse aggregate is heated to usual temperatures used for HMA and blended to hot bitumen, which generally incorporates a coating and adhesion promoter. In the second stage, the process receives wet fine aggregates not submitted to heating. The moisture is then liberated as steam and leads to the production of foaming bitumen, rapidly encapsulating fine aggregates. The temperature of the mix suddenly reduces at this moment as part of the energy is spent to vaporise the moisture [4,5]. WAM-Foam™ is also a 2-phase mixing technique, wherein two different bitumen grades, a soft bitumen and a hard bitumen are blended with the aggregate. The soft binder is generally around 20–30% of the total binder content. In the first phase, a soft binder is used to completely coat the coarse aggregate. In the second stage, a hard binder is foamed onto the pre-coated aggregate [17]. The reduction on mixing and compaction temperatures achieved can vary to some extent depending on the foaming process used, as reported hereafter: the majority of water based processes can achieve production temperatures 20–30 °C lower than conventional HMA [2,4]; techniques that incorporates zeolite reach reductions around 30 °C [2]; WAM-foam™ producers claim that it can be compacted below 90 °C and mixed in the range 100– 120 °C [4,5]; LEA has been produced in a temperature range of 100–125 °C [4].

4. Laboratory design of warm asphalt mixes 4.1. Overview A significant part of the recent mix design methods carried out in Europe and in the United States of America to design HMA involves a number of steps, aimed to cover the following topics: selection of materials, study of volumetric properties of the mixture and selection of binder content, the evaluation of some additional parameters, such as workability, water sensitivity, coating, compactability, and assessment of mechanical performance of the mixture [3,17,24]. When the asphalt mixtures’ compositions are well-known, the process can be simplified. Based on the literature review, it can be stated that the majority of WMA studies carried out has used mix design processes similar

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to those of HMA. For instance, the Superpave (AASHTO R 35) is commonly applied in the USA [25] whereas the Marshall method (EN 12697-34:2004 + A1 and EN 13108-1:2006), among other European methods, such as the French one [26], were usually carried out in Europe. Asphalt mixtures (HMA or WMA) design based on gyratory compaction, uses some parameters for evaluating the ease of compaction. This is achieved by measuring the number of gyrations needed to reach specific volumetric properties, such as voids content, or by calculating some indexes associated to the compaction energy, which are obtained from the densification curve. These evaluations allow comparing the compaction of one mixture to another and the densification produced by traffic [27]. However, there are a few studies [17,21] where a laboratory foaming plant, such as the Wirtgen WLB10Ò, was used. In those cases, the complexity of the process is higher, requiring the control of various working aspects, which are crucial within the mix design process. The amount of foamant water added, the air flow control and the discharge ratio of bitumen from the nozzle, for instance, are some of the practical concerns pointed out in NCHRP report 691 [17]. Moreover, the specific equipments developed are not widespread in laboratories neither are they able to simulate all types of available foaming technologies [21]. The use of WMA additives is also likely to bring some supplementary problems as the amount of additives to incorporate is very low, increasing the difficulties to disperse them homogeneously within the bitumen or in the mix [17]. If reclaimed asphalt pavement (RAP) or by-products are incorporated into the mix, the complexity of the process is also higher, as problems related to binder absorption and blending of the new and recycled binders can arise [17]. Apart from the aforementioned particular features pointed out, the methodologies used for the selection of aggregates and bitumen, production of specimens and measurement of various properties of compacted mixes are similar to those used for HMA [17]. The draft appendix to AASHTO R 35 – special mixture design considerations and methods for warm mix asphalt (WMA) – published in Appendix A of the NCHRP report 691 [17], indicates a number of procedures to consider when designing WMA in laboratory by the American method. The next subsection highlights some specific features involved in designing WMA reported in the literature. 4.2. Mix design particularities for additive technologies Additives are usually solid at ambient temperature and, hence, they are provided in the form of pastilles or pellets [8,10,15,16]. Some of them are also available as liquids but usually with a shorter period of validity. The intended process of incorporating the additives at real scale production must be as much as possible followed in the mix design methodology [8]. Generally, for mix design purposes, additives are pre-blended a few seconds with the heated bitumen by means of a low-shear mixer apparatus before mixing with aggregates. In some cases they can be added to the mixture in the mixing bowl, just after addition of the bitumen [8]. When a pre-blended form of binder is supplied, it can be used instead of blending the additive in the laboratory. Usually, additive suppliers recommend the amount of product to add to the mixture. Table 1 summarises the recommended quantity of additives found in the literature. For additives that change the binder’s viscosity, such as SasobitÒ, the mixing temperature reduction can be derived from the evolution of the additive’s dynamic viscosity with temperature, as performed by Silva et al. [28]. The chosen temperature is the one that matches the target viscosity (0.2 Pa.s, for instance), in other words the temperature allowing satisfactory workability. If additives used have a small influence on the binder’s viscosity, as it happens usually with chemical additives [14–16,19], the

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Table 1 Recommended amount of some available WMA additives. Additive Organic additives Astec PERÒ

Asphaltan-B SasobitÒ SonneWarmix™

5. Mechanical performance of warm asphalt mixes

Addition rate range

Typical addition rate

5.1. Overview

0.5–0.75% by total weight of RAP (only for high levels of RAP) [8] 2–4% by weight of the total binder [4] 0.8–4% by weight of the total binder [8] 0.5–1.5% by weight of the total binder [8]

–

The literature reports that WMA technologies have been used to produce a variety of asphalt mixtures, such as dense-graded asphalt concrete [15,19,27], polymer-modified asphalt [29], stone mastic asphalt (SMA) [3], crumb rubber modified (CRM) asphalt concrete [29], asphalt concrete incorporating RAP [30], among others. As a result, the mechanical properties of WMA can vary in a large range depending on the specific WMA technique applied as well as the type of material fabricated. As in the case of HMA, water sensitivity, stiffness modulus, resistance to cracking and permanent deformation of WMA varies with the type of mixture as well as with some of the composition’s parameters. Obviously, testing conditions, such as temperature and loading characteristics, also have a significant influence on the observed performance. Therefore, what is stressed in the following points is the general tendency of WMA’s mechanical performance variation as compared to a similar HMA. The following points present a summary concerning the main mechanical properties usually evaluated on WMA and compare the observed mechanical performance with that of control mixtures generally used as reference.

Chemical additives CecabaseÒRT 0.3–0.5% by weight of binder [8,19] Rediset™ WMX 1.5–2.5% by weight of binder [8,16] Evotherm™ About 5% of diluted chemical package by weight of binder [8]

2.5% by weight of binder [5] 1.5% by weight of the total binder [8] 0.75% – maximum recommended for unmodified, virgin mixes [8] – – –

production temperature cannot be viscosity based. In these cases it can be estimated from the progress of specimens’ air voids content during the laboratory compaction procedure. The specimens’ height must be recorded along compaction in order to verify if the target air void content is achieved. Generally, this is possible either for impact (according to EN 12697-30: 2004 + A1:2007) [19] and gyratory compaction procedures (according to AASHTO T 312 or EN 12697-31: 2007) [17] as it happens within the Marshall and Superpave methods, respectively. 4.3. Mix design particularities for foamed bitumen technologies Since the production of water based foamed bitumen in the laboratory is rather difficult, sometimes conventional HMA mix design procedures are used as a support to preliminary mix design of WMA [3]. Obviously, in these cases the laboratory mix design process is not satisfactory as no foamed bitumen is produced. Therefore, pilot WMA produced in plant can be used to compact specimens in order to carry out further evaluation in the laboratory for mix design completion. Zeolite foaming technologies are generally easier to reproduce in the laboratory for mix design purposes because the powder is added as a granular component. In the case of AdveraÒ WMA, for instance, 0.15–0.30% by the total weight of mix is usually preblended with hot binder prior to mixing in a mechanical mixer. Aspha-minÒ, also used as synthetic zeolite, is added at a rate of 0.3% by the total weight of mix. It is usually incorporated in the mixture at the same time as the heated bitumen. Zeolite should not be heated prior to mixing with hot bitumen to avoid releasing the internal moisture earlier than needed [8]. A comprehensive analysis of the features involved within the laboratorial production of foamed bitumen is beyond the scope of this paper. However, this type of study can be found in Jenkins [21] who has studied a significant number of parameters that influence the foaming characteristics and, hence, are crucial to control the mix design process and the production of WMA by foaming technologies. The NCHRP report 691 [17] states that batches of foam bitumen produced in a specific equipment in the laboratory should be larger than needed for individual specimens production. Indeed, to ensure that foamed bitumen has the same characteristics for all samples it is better to split the batch into the needed subsamples for testing purposes.

5.2. Water sensitivity Some asphalt mixtures suffer a substantial reduction of resistance over the years in the presence of water. This phenomenon is known as water sensitivity or water damage. The loss of mechanical performance is due to the failure of the binder-aggregate interface and/or the cohesion within the binder–filler mastic [31]. A deeper discussion on all the phenomena involved can be found in Caro et al. [32]. In general, the material’s degradation happens because there is a lack of electrochemical affinity between the binder and the aggregate surface. In the case of WMA, since the aggregates are not completely dry before contacting with the binder, the moisture left behind during the construction process can also increase the water susceptibility of asphalt mixtures. Therefore, additives such as surfactants act as a bridge between the asphalt binder and the aggregate surface, promoting adhesion and resisting the action of water. This is generally achieved since the additive molecules have polarised extremities whose charges attract the opposite charge of the other material, allowing them to bind to the aggregate surface and also being compatible with the asphalt binder [33]. Adhesion promoters can also coat large amounts of fine aggregates or dusts since they reduce the surface tension at the binder-aggregate interface [34]. Kim et al. [35] concluded that although the field performance data indicated that both WMA and HMA show similar performance, field performance must be careful examined over the inservice life of the pavement. This must be done because moisture damage can occur as a consequence of rutting and/or cracking development. Water sensitivity of WMA is generally assessed by the procedures described in EN 12697-12 and AASHTO T 283. Tensile strength ratio (TSR) is the control parameter measured on compacted specimens, which are divided in two groups. One of them is formed by specimens submitted to a specific water conditioning procedure and the other is the control group formed by dry samples. Amongst the various types of WMA, tensile strength generally increases as the compaction temperature increases. The same happens when the temperature conditioning of the mixes in the laboratory is performed for at least 2 h (short-term aging), allowing a higher absorption and aging of the bitumen [17].

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Within the NCHRP Project 09-43 [17], for instance, it was observed that the TSR was the same or better in 67% of the produced WMA as compared to HMA when the process incorporated antistrip additives, as it happened, for instance, with Evotherm™. On the contrary, when that type of additive was not included, TSR hardly improved and showed even a decreasing in 79% of the mixtures. Results presented by Sanchez-Alonso et al. [27] generally agree with the conclusion of NCHRP. Kavussi and Hashemian [36] also achieved significant improvements in the water sensitivity performance on WMA foam mixes, produced with non-limestone fine aggregates, by adding 2% of hydrated lime powder as anti-striping agent. Moreover, WMA mixes show typically lower tensile strength than the corresponding HMA evaluated as a reference. In the view of these conclusions, it is recommended to regularly evaluate the water sensitivity on WMA, using anti-strip additives when necessary [17] and/or increasing slightly the production temperature [27], despite there is some differences between the minimum specified values for TSR from one country to another. 5.3. Stiffness modulus Generally, stiffness (stress/strain ratio), evaluated according to EN 12697-26, AASHTO TP 79 or AASHTO T 321-03, decreases as manufacturing temperature decreases, although the variation is not proportional [27,37,38]. This happens for HMA and for WMA as well. As for HMA, stiffness modulus decreases as the testing temperature increases, for the same frequency, and increases as loading frequency increases, for the same temperature [39]. Stiffness modulus increases with mixing and compaction temperatures, as coating of coarse aggregates particles and bonds between them improve [38]. Cardone et al. [38] have concluded that synthetic waxes (used as organic additive) tend to increase stiffness (obtained at 20 °C) of traditional dense-graded asphalt for compaction temperatures between 100 and 140 °C. The crystallisation due to wax additives can ensure good stiffness values for WMA dense-graded asphalt, despite that they are produced and compacted at lower temperatures than usually HMA is [38]. Sanchez-Alonso et al. [27] have studied the influence of various additives and drew some different conclusions. Four were chemical additives and had adhesion promoters in their composition (two of them including wax compounds), one organic and one zeolite. For mixtures produced at 140 and 160 °C all the additive mixes showed lower stiffness modulus (obtained at 20 °C) than the reference HMA. The opposite occurred at a production temperature of 120 °C, as all the WMA mixes had higher stiffness moduli than the reference [27]. An important finding to draw out from Sanchez-Alonso et al. results is that lowering manufacturing temperatures from typical values used for HMA to 120 °C, stiffness modulus (obtained at 20 °C) decreased around 50% for HMA and in a range from 10% to 50% for WMA, depending on the additive applied. The study confirmed that mixes with wax additives had consistently higher modulus values than the other types of WMA. Some results [37] have shown that lowering manufacturing temperatures of WMA mixtures lead to significant reduction on the stiffness modulus obtained at high temperatures (45 °C), while the effect is negligible for low test temperatures (4 °C). Jenkins et al. [30] have tested various WMA incorporating 10– 20% of RAP and polymer modified bitumen (some with elastomers and some with plastomers). It has been concluded that flexural stiffness of WMA with no polymer modified bitumen (control mixtures), measured at 25 °C and 10 Hz, have a significant reduction in various cases (40–60%), whereas similar mix compositions incorporating polymer modified bitumen merely showed slight variations for the same testing conditions. This allows declaring that

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elastomers or plastomers can be useful to improve the performance of WMA, if necessary. For SMA produced with WMA additives together with 0.2% by total weight of fibres and about 6.5% of bitumen, Zaumanis [3] concluded that stiffness modulus depends on the type of additive, compaction method and temperature. He stated that lowering the compaction temperature of WMA to at least 125 °C led to a small reduction of stiffness moduli compared to those of HMA. A further temperature reduction is considered to lower significantly the mix’s stiffness. As far as CRM mixes is concerned, Akisetty [40] states that, in general, WMA additives, such as Aspha-minÒ and SasobitÒ, in spite of allowing a reduction of compaction temperatures, have no negative influence on stiffness modulus. 5.4. Resistance to fatigue Fatigue testing can be conducted according to EN 12697-24 or AASHTO T 321-03. This type of tests induces continuous damage on the specimen until failure occurs. This can be observed through a gradual reduction of the material’s initial stiffness. Failure is generally assumed to occur when that reduction is 50%. Alternative approaches based on dissipated energy and continuous damage are sometimes also applied to infer fatigue performance of asphalt mixtures. The parameter e6 – strain level leading to failure at one million loading cycles – is often calculated by regression analysis of testing data (pairs of strain and number of cycles to failure values) to rank the fatigue performance of asphalt mixtures [41]. As reported in the literature [42], it can be stated that WMA usually tend to suffer more fatigue damage at lower strain levels than HMA, as observed in flexural testing carried out at 20 °C and 10 Hz. Moreover, apparently, WMA are less sensitive to the increasing of tensile strain level. Therefore, the use of WMA can be advantageous in heavy duty pavements [21,42]. For SasobitÒ WMA mixes, as reported by Diefenderfer et al. [42], fatigue performance of WMA, based on e6, decreased 22% as compared to that of the reference HMA, when compaction temperature of WMA was 110 °C instead of 150 °C applied to HMA. Nevertheless, the values calculated for e6 were far above 200 lm/m, which clearly represent satisfactory fatigue behaviour. This performance was observed on WMA produced in the laboratory, while for specimens extracted from trial sections the reduction obtained for e6 was less than 10%. The combination of RAP and polymer modified bitumen together with WMA technologies, as studied by Jenkins et al. [30], introduced some differences to the WMA fatigue trends. Therefore, they recommend further research regarding the limits of recycled asphalt content and bitumen chemistry. However, other studies carried out simultaneously in the laboratory and in full-scale [43] have concluded that some WMA foam techniques combined with high RAP don’t have negative effects regarding the strain induced on the material. They specifically state that these WMA materials seem to carry loads more efficiently and thus reduce the in-service strain levels achieved at higher temperatures (e.g. 40 °C). Apparently, the incorporation of SasobitÒ in CRM mixes slightly benefits fatigue performance of the mix whilst the AsphaminÒ reduces somewhat the fatigue life. However, Xiao et al. [44] have concluded that the influence of those WMA additives is not statistically significant on the fatigue performance of CRM warm mixes. 5.5. Resistance to low temperature fracture Low temperature cracking performance is only of more importance in very cold climates. Since WMA mixtures are produced at a relatively low temperature, the bonding at the interface bindercoated aggregate is in question at low in-service temperature. In

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addition, as there is a significant differential thermal contraction within asphalt mixes at very low temperatures, thermal cracking is likely to occur in those circumstances. This promotes the development of micro-cracks, which evolve to larger ones and also allows the development and propagation of other cracks [45]. Because this mechanism is detrimental to the material’s strength and durability the fracture resistance of the material must be evaluated before using WMA in very cold climates. A number of different testing protocols are applied to evaluate low temperature performance of asphalt mixtures. The tensile stress restrained specimen test (TSRST), carried out according to AASTHO TP-10-93, and the Superpave indirect tensile test (IDT) are commonly used in the USA. In Europe the testing protocol being developed is described in EN 12697-46: low temperature cracking and properties by uniaxial tension tests. Several studies found in the bibliography [46,47] have evaluated the low temperature cracking performance of WMA. They have generally concluded that WMA show good performance regarding low temperature cracking, irrespectively the type of technology applied to produce the asphalt mixtures. Obviously, those conclusions were derived from various different parameters which depend on the type of testing protocol used in each study. In the study carried out by Das et al. [46], for instance, Superpave IDT tests were conducted at three different temperatures (0, 10 and 20 °C). The study also included a statistical analysis of multiple fracture parameters based on Superpave IDT results obtained at 20 °C, which showed only a minor negative effect of wax modification (wax F–T and wax Asphaltan-B) as compared to HMA. However at 0 °C they found a positive effect on fracture properties for the wax WMA. Min-Yong et al. [45] have also studied the resistance fracture at low temperature of WMA. The results showed that WMA with wax additives or Evotherm™ at 20 °C exhibit similar or better performance than the HMA control mix. 5.6. Resistance to permanent deformation Permanent deformation performance is crucial in hot climates. It can be evaluated in the laboratory by various testing methods, which use a variety of parameters to rank mixtures [48]. With this purpose, the wheel-tracking test (WTT) is usually carried out in Europe, according to EN 12697-22, despite other WTT procedures which are applied in several other countries (AASHTO TP 63 and AASHTO T 324). The rut depth induced on the material increases as the number of wheel passes raises, while the testing temperature is high, generally around 50 or 60 °C. In European countries permanent deformation potential can also be assessed by the cyclic compression test, according to EN 12697-25. WMA produced with wax additives, such as SasobitÒ, change the properties of the binder at higher in-service temperatures, allowing an improved resistance to rutting [23,28]. However, WMA mixes show generally worse performance than the HMA used as control, particularly when foam technologies are used [49]. In spite of these tendencies which are also observed for CRM mixes with SasobitÒ or AsphaminÒ, the difference between the control CRM mix and the warm CRM mixes is not statistically significant [40]. In addition, WTT carried out on the Hamburg wheel-tracking device (AASHTO T324) wherein specimens are maintained within a water bath during wheel passes reveal that WMA’s resistance to rutting is rather worse than that of control mixtures due to moisture damage. This is likely to occur because there is less stiffening of the binder throughout the mixing and compaction processes. It has been found [37] that the mixture resistance to permanent deformation decreases as production temperature decreases,

despite the variation is not proportional. Rutting potential depends on the amount of additive and the type of WMA technology used. Because of that, some American state agencies follow a specific procedure to derive a minimum production temperature for WMA, aiming to avoid rutting potential problems. 6. Guidance on mixing, laying and compaction of warm asphalt mixes As mentioned above, temperatures of mixing and compaction mainly depend on the WMA technology applied and the type of additive added. Moreover, wax and chemical additives, and zeolite powder can be introduced into the mixing process by a number of procedures [8], which are summarised below. Products added in small amounts into the process and supplied in the form of pellets or pastilles are preferably added by means of a pneumatic feeder, which better controls the quantity added. In the case of drum plants, the pipe should deliver the product close to the point where the binder is introduced. If there is a RAP collar mounted on the plant, it can be used as an access point. In batch plants additives are introduced directly into the mixer. Since the amount of additive is considerably higher in the case of Thiopave™, it is generally used a conveyor belt system directly feeding the mixing drum or the pug mill, for drum plants and batch plants, respectively. If additives are liquid at ambient temperatures, they can be injected from a heated container into the plant’s binder line by means of a dosing pump, or pre-blended into the binder at the terminal or at the refinery instead. For this later process it is advisable that storage tanks provide continuous stirring of the treated bitumen to promote homogeneity. Since zeolite is a powder, it is usually added by means of a compressed air system. In drum plants, to prevent it to get eliminated by the baghouse system, zeolite should be added to the bitumen in a mixing box prior to injecting it into the drum mixer. For batch plants the feeding pipe should be installed as close as possible to the centre of the pug mill. As far as foam bitumen technologies is concerned, mainly in water based processes, conventional plants require additional foaming equipment to produce WMA. Therefore, the production procedure is significantly modified. This also recommends a better implementation of the composition in the asphalt plants working framework. Descriptions of specific details on plants operation are beyond the scope of this paper but they can be found in Prowell et al. [8]. Adequate laying and compaction temperatures selection are crucial to avoid problems. Despite a general temperature reduction is allowed within WMA, for cold-weather conditions, long hauling distances or high RAP, for instance, it is advisable to select a temperature slightly higher. In some cases, the angle of attack of the paver screed, the material flow between equipments or thermal segregation can be negatively influenced [8]. Lowering production and compaction temperatures to a definite limit is generally not detrimental for workability of WMA mixes [23]. In most cases, it has been easier to achieve required densities with WMA comparing to HMA, even at significantly lower temperatures (slightly above 100 °C for a 45% RAP Aspha-min WMA) [8]. Nevertheless, some cases wherein the production temperature was close to its lowest extreme have been reported to need higher compaction energy [23]. 7. Comparison between life-cycle cost analysis of WMA and HMA The life cycle analysis (LCA) of asphalt pavements is usually divided into four phases: production of raw materials, construction,

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maintenance and repair, and demolition (followed by re-construction) or recycling (end-of-life) [50,51]. Santero et al. [50] also have considered the ‘use’ phase (in-service life span). In the case of asphalt mixes, LCA allows analysing and assessing the environmental impact of the material during its entire life cycle, from the production of the raw materials to their end-of-life. The LCA also incorporates some economic aspects involved in the process, such as energy consumption in each life-cycle stage [51]. When some features are similar between the compared materials the evaluation of various LCA components is not necessary [50]. Some authors [50] state that LCA of asphalt pavements has a few unresolved issues which can introduce bias into the conclusions. Despite the uncertainty of the available LCA quantitative models [50,52], this type of study is usually performed for decision-making purposes, amongst different technologies or processes. The comprehensive discussion of problems involving LCA of WMA and HMA are beyond the scope of this paper. Nevertheless, a reflection on some questions involving LCA of WMA and HMA is presented below. Apart from WMA additives, the production process of constituent materials is approximately the same for WMA and HMA. As mentioned before, emissions associated with additives can offset somewhat the general reduction of by-products released. In the protocol elaborated by the Transport Research Laboratory [53], for instance, wherein all GHG are transformed in ‘carbon dioxide equivalent’ (CO2e), FT wax and adhesion agents are represented by 5700 and 1200 kgCO2e/t, respectively, while bitumen is represented by 280 kgCO2e/t. Based on this data, one can conclude that in a typical mixture composition with 5% of binder content, the additives’ contribution to CO2e can attain 50% of that of bitumen. Apparently, the construction stage integrates the most important advantages of WMA compared to HMA. Actually, temperature reduction in WMA leads to significant decreasing on fuel consumption and CO2 emissions. Typical reduction of both is in a range of 10–30%, despite higher values can be found in the literature [5,8]. Although there are not yet too much available data concerning long-term performance of WMA, based on the observations carried out to date there are no reasons to believe that WMA have lower performance than HMA. Therefore, maintenance and repair would be more or less the same in both cases. Identical conclusion can be made regarding deposit or recycling for both types of technology.

8. Summary and conclusions WMA comprise a great number of asphalt mixtures, as a large amount of different mixes (dense and gap graded, SMA, asphalt rubber, RAP, etc) can be fabricated, laid and compacted by using WMA technologies instead of traditional HMA. The required temperature reduction can be achieved through the use of additives (wax and chemical additives) or foaming techniques. Generally speaking, authors claim that WMA have a significant number of advantages comparing to HMA, basically associated with energy saving which lead to a major reduction of GHG emissions and pollutants. Even though some drawbacks have also been pointed out, benefits of WMA in a whole seem to surmount their drawbacks. Three main mechanisms groups were identified to produce WMA mixtures. The level of mixing and compaction temperatures lowering depend on the process applied together with other factors, such as the type of mixture and the paving conditions. Mix design of WMA is generally carried out by procedures used for HMA particularly when temperature reduction is achieved by means of additives, as conventional laboratory procedures seem to be adequate. Conversely, for water based foaming techniques the mix design procedure is far more complex because

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specific laboratory foaming equipment is required, although sometimes trial compositions are evaluated as if they were HMA. As this process is not enough it requires further evaluation during the production process in plant to finish the mix design process. In terms of water sensitivity, WMA show generally good performance if adhesion agents are included in the mix. Actually, since mixing temperatures are sometimes near the lower acceptable limit, damp aggregate remains within the mixture, allowing moisture susceptibility. In these cases, slightly increasing the temperature can also contribute to overcome problems. Stiffness modulus measured at high temperatures (45 °C, for instance) is apparently influenced by some WMA technologies. On the contrary, that influence is almost insignificant for testing at low temperatures. At moderate temperatures (20 °C), moduli can visibly decrease but the variation is not of remarkable importance. Wax WMA show consistently higher stiffness modulus than other WMA. Using polymer modified bitumen tend to improve stiffness performance when needed. Resistance to fatigue cracking of WMA is generally good but somewhat lower than that of HMA use as control, apart from some wax WMA. Nevertheless, for high in-service temperatures WMA tend to suffer lower strain levels under load. Therefore, in these circumstances WMA are less sensitive to fatigue damage than HMA used as reference. Statistical analysis has concluded that fatigue performance of CRM warm mixes asphalt with additives has no significant difference of that of the performance of CRM without additives. Resistance of WMA to cracking at low temperature is apparently generally good but slightly lower than the observed for similar HMA. It appears that permanent deformation behaviour of WMA is apparently very dependent on the production lowering temperature achieved. For high levels of temperature reduction WMA performance is consistently reduced. SasobitÒ WMA seems to be the only one that does not follow the tendency, as the crystallisation of wax improves resistance to deformation at high temperatures. The general tendency observed has been interpreted as a consequence of lower oxidative hardening of binder originated from lowering production temperatures. Therefore, it is recommended that the permanent deformation resistance must be evaluated prior to construction. Foaming technologies usually require important plant modifications as the injection of water into the process need specific additional equipment. In the case of foaming based on zeolite the necessary modifications can be much less important. Although the introduction of additives is preferably made by means of specific pneumatic equipments, it is generally allowed to add them into the process without a defined apparatus. Laying and compaction operations are generally improved, as workability of WMA is adequate and the release of fumes and odours for workers is much lower. Even if paving conditions are challenging WMA is usually a good contribution to help paving operations. However, as stated in the literature the operation and maintenance of plants used for WMA production require additional care to avoid some functioning problems. As far as LCA of WMA compared to that of HMA is concerned, environmental aspects tend to be more favourable for WMA, despite the fact that definitive conclusions are not yet possible as long-term performance of WMA is not completely known.

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