The mechanical performance of dry-process crumb rubber modified hot bituminous mixes: The influence of digestion time and crumb rubber percentage

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    Keywords:Crumb rubberHot bituminous mixesDry process

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    time and percentage of crumb rubber on the mechanical performance of bituminous mixes. This research

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    countries generate in huge quantities. For example, 250,000 tonsof scrap tires are produced annually in Spain alone [11]. This tech-nique also has the advantage of improving the performance ofbituminous mixes by doing the following: (i) it reduces the mixssensitivity to temperature; (ii) it improves the elastic performanceof the bitumen; (iii) it reduces the effect of aging; and (iv) itimproves the performance of the mix in response to fatigue-related

    early years [17,2224]. This was largely due to a poor interactionbetween the crumb rubber and the bitumen which caused lowerresistance to moisture, the detachment of aggregates, and a reduc-tion in the bearing capacity of the pavement.

    As a result, there are more studies and experiments of the use ofcrumb rubber in bituminous mixes related to the wet process. Thishas made it possible to establish a set of reference values for differ-ent variables that affect its application (size, type, composition andpercentage of crumb rubber added, mix temperature, mixing time,etc.) with a view to obtaining optimal results. In recent years,however, the dry process has also captured the interest of

    Corresponding author. Tel.: +34 958249445; fax: +34 958246138.

    Construction and Building Materials 26 (2012) 466474

    Contents lists availab


    evE-mail addresses: (F. Moreno), (M.C. Rubio).management of natural, economic, and energy resources. In thissense, road engineering has opted for the application of new tech-niques that contribute to a development that is more environmen-tally friendly (e.g. pavement recycling, use of waste material inbituminous mixes, low-temperature mixes that reduce emissions,etc.) [110]. A technique that has become increasingly useful inrecent years is the use of crumb rubber from scrap tires in bitumi-nous mixes, as a modier of their mechanical properties. This tech-nique permits the valorization of a waste product that developed

    There are two processes by which this waste is applied to bitu-minous mixes: the wet process and the dry process. In the wet pro-cess, the crumb rubber is added directly to the bitumen, and itsproperties are modied. It is then added to the mix as a modiedbinder. In the dry process, the crumb rubber is added directly tothe aggregate as another ingredient in the mix. The bitumen is thenmodied when it comes in contact with the rubber [17]. Neverthe-less, of the two, the dry process is somewhat less popular becauseit initially produced unsatisfactory results, especially during the1. Introduction

    In recent years, policies of envdifferent social contexts has led totechniques and processes that are co0950-0618/$ - see front matter 2011 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2011.06.046project had two phases. The rst phase studied the inuence of these variables on the mix design,whereas the second phase analyzed how the variables affected the mechanical performance of the mixes.This paper presents the results of the second phase which analyzed the mechanical performance of dry-process crumb rubber mixes, more specically, their response to moisture sensitivity and plastic defor-mations. The results obtained indicated that the determining factor in mix performance was the amountof crumb rubber added. In contrast, the inuence of digestion time was found to be negligible. The studyshowed that in reference to moisture sensitivity and plastic deformations, the best performances wereachieved with a digestion time of 45 min and a crumb rubber percentage of 0.5% and 1.0% of the totalweight of the mix.

    2011 Elsevier Ltd. All rights reserved.

    ental sustainability inplementation of newe to the more effective

    phenomena, such as cracking and plastic deformation [12,13]. Fur-thermore, it provides many other benets such as lower roadmaintenance costs [1416], savings in energy consumption andnatural resources, and a higher quality road surface which is con-ducive to better safety conditions [13,1721].Available online 28 July 2011less popular because it initially produced poorer results. The objective of the research presented in thispaper is to improve the application of this technique by analyzing the inuence of the variables digestionThe mechanical performance of dry-procbituminous mixes: The inuence of diges

    F. Moreno, M.C. Rubio , M.J. Martinez-EchevarriaConstruction Engineering Laboratory of the University of Granada, Granada, Spain

    a r t i c l e i n f o

    Article history:Received 30 December 2010Received in revised form 14 June 2011Accepted 18 June 2011

    a b s t r a c t

    The use of crumb rubber frhas become a technique ofto bituminous mixes are th

    Construction and

    journal homepage: www.elsll rights reserved.s crumb rubber modied hoton time and crumb rubber percentage

    scrap tires in hot bituminous mixes in order to improve their performanceat potential in recent years. The two techniques used to add crumb rubberet process and the dry process. Of the two, the dry process is somewhat

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  • F. Moreno et al. / Construction and Buildresearchers since it consumes larger quantities of waste than thewet process. Consequently, research has also been carried out tofoment the use of the dry process and solve the problems thatcan arise in the use of this method [2530].

    The results presented in this paper are part of a larger researchproject. The general objective is to make an in-depth study of thistechnique and contribute to its development and application. Therst phase of the project studied the digestion time (contact timebetween the crumb rubber and bitumen) and the percentage ofcrumb rubber added in relation to their possible inuence on theproperties of the mixes in the design phase [31]. These variableswere analyzed with the purpose of determining the optimal quan-tity of bitumen as well as the principal characteristics of the mix.The results obtained showed that the digestion time did not havea signicant inuence on the determination of the optimal bitu-men content or on the characteristics of the mixes. In fact, increas-ing the amount of crumb rubber in the mix caused its density todecrease, which signied a corresponding increase in air void con-tent as well as the optimal bitumen content.

    As a continuation of the analyses in the rst project phase, anin-depth study was performed of the variables digestion time andpercentage of crumb rubber added, and their possible impact onthe mechanical performance of the mixes (moisture sensitivityand resistance to plastic deformations). The results of this researchare presented in this paper. For this purpose, a study was made ofthe performance of various discontinuous mixes for the road sur-face course (the most important pavement layer). The mixes hadthe same mineral composition, but had different percentages ofcrumb rubber (0.5%, 1%, and 1.5% of the total weight of the mix)as well as different digestion times (45, 90 and 120 min). All ofthem were tested for moisture sensitivity (UNE-EN 12697-12)and resistance to plastic deformations (UNE-EN 12697-22). Thisarticle describes the methodology used in this study. The resultsobtained reected that the addition of crumb rubber signicantlyimproved the performance of mixes and their response to plasticdeformations. In fact for a specic combination of variables (1%crumb rubber, and digestion times of 45 and 120 min), it alsoimproved the response to moisture of a reference mix made withhigh-performance bitumen.

    2. Methodology

    2.1. Materials

    In this study, a BBTM 11A bituminous mix was used (the same mix used in therst project phase) [32]. As a continuation of previous research [31], this secondstudy focused on the digestion time and the percentage of crumb rubber addedby the dry method. The purpose was to analyze the inuence of these variableson the mechanical performance of BBTM 11A bituminous mixes, which are thosegenerally used for the pavement surface course.

    The mix in our study had a discontinuous grain size (from which the 42 mmfraction was eliminated). It had a larger percentage of coarse aggregate which gaveit bearing capacity (approximately 6580% of the total with a maximum size of812 mm). The rest of the mix was composed of ne aggregate (2035% of the to-tal), which along with the bitumen and the ller (710%) were the elements in mor-tar that made the mix cohesive and provided it with resistance to tangentialstresses. The mixes used in our study were spread in thin layers (23.5 mm), andhad an excellent surface macrotexture and good skid resistance.

    The aggregates selected for themix designwere ophite for the coarse fraction andlimestone for the ne fraction with the characteristics shown in Table 1. The charac-teristics of the aggregate and ller, as well as of themixes themselves, were in accor-dance with the Spanish Technical NLT Standards [33], Road Tests of the Centro deEstudio de Carreteras [Road Study Center], and the Spanish standards UNE-EN [34]of the Spanish Standards and Certication Association (AENOR). The limits used com-plied with the requirements for vehicle trafc superior to T2.1 As can be observed,both types of aggregate fulll the requirements of the Spanish regulations for themanufacture of BBTM 11A bituminous mixes. The material used as ller was CEM II/B-L 32.5 N (UNE-EN 197-1) cement of the characteristics shown in Table 2.1 T2: 200 6MIDp < 800. MIDp: mean intensity of heavy vehicles (heavy vehicles/day).To compare the properties of dry-process crumb rubber mixes with high-performance bitumen mixes, two types of bitumen were used. Consequently


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