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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<ul><li><p>esti</p><p>Keywords:Crumb rubberHot bituminous mixesDry process</p><p>omgree w</p><p>time and percentage of crumb rubber on the mechanical performance of bituminous mixes. This research</p><p>ironmthe imnduciv</p><p>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</p><p>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.</p><p>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</p><p> Corresponding author. Tel.: +34 958249445; fax: +34 958246138.</p><p>Construction and Building Materials 26 (2012) 466474</p><p>Contents lists availab</p><p>B</p><p>evE-mail addresses: fmoreno@ugr.es (F. Moreno), mcrubio@ugr.es (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</p><p>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</p><p>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.</p><p> 2011 Elsevier Ltd. All rights reserved.</p><p>ental sustainability inplementation of newe to the more effective</p><p>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</p><p>F. Moreno, M.C. Rubio , M.J. Martinez-EchevarriaConstruction Engineering Laboratory of the University of Granada, Granada, Spain</p><p>a r t i c l e i n f o</p><p>Article history:Received 30 December 2010Received in revised form 14 June 2011Accepted 18 June 2011</p><p>a b s t r a c t</p><p>The use of crumb rubber frhas become a technique ofto bituminous mixes are th</p><p>Construction and</p><p>journal homepage: www.elsll rights reserved.s crumb rubber modied hoton time and crumb rubber percentage</p><p>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</p><p>le at ScienceDirect</p><p>uilding Materials</p><p>ier .com/locate /conbui ldmat</p></li><li><p>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].</p><p>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.</p><p>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.</p><p>2. Methodology</p><p>2.1. Materials</p><p>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.</p><p>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.</p><p>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 &lt; 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, thereference mix, which did not contain crumb rubber, used BM3c bitumen modiedwith polymers, whereas the dry-process mix formula contained conventional B 50/70 bitumen. It was hoped that the results would reect an enhancement of the con-ventional bitumen that would make it comparable to the modied bitumen. Table 3lists the properties of both types of bitumen.</p><p>The crumb rubber particles used in the mixes had a size of 00.6 mm with thecharacteristics listed in Table 4.</p><p>2.2. Experimental design</p><p>The objective of our study was to analyze the inuence of the digestion timeand percentage of crumb rubber on the mechanical performance of asphalt mixes.Since this was a continuation of a prior study within the context of the same project[31], the experiments were performed on the same mixes used previously. Conse-quently, the 10 BBTM 11A mixes studied had an identical mineral skeleton, exceptfor the nest aggregate fraction of the same size as the crumb rubber, which,depending on the amount of crumb rubber added, was modied so that it couldbe accommodated in the skeleton without causing anomalies (see Table 5). Themixes only varied in their percentage of crumb rubber (0%, 0.5%, 1%, and 1.5% ofthe total weight); the digestion time used before compaction (45, 90, and120 min); and the type of bitumen used (BM3c and B 50/70). The bitumen contentof the mixes was based on our previous study, which had specied optimalamounts of bitumen, according to the Marshall test results (Table 6) [31].</p><p>During the manufacture of the CRM mixes, the temperature of the mix wasincreased to 180 C (10 C more than that of the reference mix) so as to facilitatethe interaction between the rubber and the bitumen, thus improving the cohesionof the mastic. The manufacturing process rst involved a 10 s agitation of the nat-ural aggregates in the mineral skeleton in order to better homogenize them. Thecrumb rubber was then added to the aggregates, and mixed with them for a periodof 20 s to ensure a homogeneous dispersion of the particles throughout the mix.When the bitumen was added, there was a 2 min agitation period until it was thor-oughly blended with the aggregates and crumb rubber. The last ingredient addedwas the ller. This was followed by a nal agitation lasting 3 min, which allowedthe formation of the mastic to provide cohesion. The digestion process of the mixoccurred in an oven at a compaction temperature of 160165 C.</p><p>In order to analyze the inuence of the digestion time and the amount of crumbrubber added to the mixes on their mechanical performance, two determining fac-tors were selected, which were considered crucial for the quality of the mixes: (i)moisture sensitivity; and (ii) resistance to plastic deformations. The response ofthe mixes to moisture was tested by the moisture sensitivity test (UNE-EN12697-12), and their response to rutting (plastic deformations) was tested by thewheel tracking test (UNE-EN 12697-22).</p><p>The moisture sensitivity test involved the manufacture of six test specimenswith a diameter of 101.6 mm and a thickness of 60 mm, compacted with 50 blowson each side by a Marshall hammer. The specimens were subsequently divided intotwo sets of three specimens: a dry set and a wet set. The set of dry specimens wasstored at room temperature in the laboratory (20 5 C), whereas vacuum wasapplied to the wet set for 30 5 min until a pressure of 6.7 0.3 kPa was obtained.The specimens were then immersed in water at a temperature of 40 C over a per-iod of 72 h.</p><p>The next step was to perform an indirect traction resistance test on each of thecylinders (in both the dry set and the wet set). This was done at a temperature of15 C, and after a previous period of adjustment of 120 min to this temperature.The results of the experiment are expressed in terms of the retained strength ofthe test specimens after dividing the strengths of the wet specimens into thestrengths of the dry specimens (ITSR, %). The limit established by the Spanish PG-3regulations for this type of mix [32] stipulates that the retained strength should behigher than 90%.</p><p>The wheel tracking test involved the manufacture of two prismatic test speci-mens of 408 mm 256 mm. Compaction was carried out by a roller compactorwith a smooth steel roller to a thickness of 60 mm and a minimum density of98% of the Marshall density. Two days after their compaction, both specimens wereallowed to adjust to a temperature of 60 1 C, and then were tested at that tem-perature. The test itself involved the application of a load on the test specimen bymeans of the repeated passes of a loaded wheel. The load applied was 700 N andthe number of passes was 10,000. The frequency of the device was 26.5 load cyclesper minute. In each of the wheel passes, the resulting deformation on the test cyl-inder was measured. The objective of the test was to determine the wheel trackingslope (WTS, mm/103 load cycles) measured in the last 5000 load cycles. The limitestablished for this test by the Spanish PG-3 regulations for this type of mix is adeformation slope of less than 0.10 mm/103 load cycles.</p><p>3. Analysis of results</p><p>ing Materials 26 (2012) 466474 467After the manufacture of the mixes with the optimal content ofbitumen, a study was m...</p></li></ul>

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