analysis of digestion time and the crumb rubber percentage in dry-process crumb rubber modified hot...

Construction and Building Materials 25 (2011) 2323–2334

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Construction and Building Materials

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Analysis of digestion time and the crumb rubber percentage in dry-processcrumb rubber modified hot bituminous mixes

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 16 July 2010Received in revised form 31 October 2010Accepted 13 November 2010Available online 28 December 2010

Keywords:Crumb rubberBituminous mixesDry process

0950-0618/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2010.11.029

⇑ Corresponding author. Tel.: +34 958249445; fax:E-mail addresses: [email protected] (F. Moreno), mc

a b s t r a c t

The use of crumb rubber modifier (CRM) in bituminous mixes made by the dry process is not as widelyused as the wet process. Nonetheless, this process has advantages, such as the potential to consume lar-ger quantities of crumb rubber, thus resulting in greater savings in energy and natural resources. Thisresearch study contributes to the further development and evolution of the dry process through the anal-ysis of the effect of the digestion time (the contact time between the crumb rubber and the bitumen) andthe quantity of crumb rubber on the mix design properties. The results of the study showed that thedigestion time had no influence on the selection of the optimal binder content or on the compactionof the mixture. In contrast, the digestion time was found to have an impact on the mechanical perfor-mance of the mix. In this respect, an increase in the quantity of crumb rubber contributed to a corre-sponding increase in the amount of bitumen needed, and also caused the mix to become less compact.This study showed that a crumb rubber percentage of less than 1% of the total weight of the mix and adigestion time of 90 min produced the best results.

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1. Introduction

Over the years, the use of crumb rubber modifier (CRM) fromscrap tires in hot bituminous mixes has become a frequent practicein road construction [1–7]. When crumb rubber interacts with thebitumen used as binder, it modifies the rheological properties ofthe resulting mix (i.e. lower susceptibility to temperature and bet-ter elastic performance) and increases its viscosity, thus allowingits incorporation in greater amounts. All of these factors are condu-cive in improving the mechanical performance of the mix. Morespecifically, there is greater resistance to aging, thanks to the in-crease in the thickness of the rubber binder film that covers theaggregate. The mix performance has also been found to respondbetter to fatigue-related phenomena such as cracking and plasticdeformations [8,9].

The most significant advantages of the use of CRM include thefollowing:

– Improved mechanical performance of asphalt paving mixes.– Lower pavement costs for road conservation and mainte-

nance [10–12], as reflected in greater savings in energy andnatural resources. This process contributes to sustainable

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development, and is more environmentally friendly becauseit also involves the valorization of waste materials as well asa reduction in their volume at landfills.

– More safety guarantees due to better long-term color con-trast for pavement markings because carbon black in therubber acts as a pigment that keeps the pavement blackerfor a longer time [9,13,14].

– Reduction of the noise level of the road surface course [15–17].

The processes used to incorporate crumb rubber in asphalt pav-ing mixes are the following:

(a) Wet process, in which the crumb rubber is added to hot bitu-men. The mixture is mechanically agitated until there is aninteraction between the bitumen and crumb rubber, priorto mixing it with the aggregates. It is then added to themix as modified binder.

(b) Dry process, in which the ground crumb rubber is added tothe aggregate as another ingredient in the mix, prior to theaddition of the bitumen. The bitumen is then modified whenit comes in contact with the rubber.

Both of these processes began to be applied with increasingfrequency towards the middle of the 1960s. This is reflected in

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1 T2: 200 6MIDp < 800. MIDp: Mean intensity of heavy vehicles (heavy vehicles/day).

2324 F. Moreno et al. / Construction and Building Materials 25 (2011) 2323–2334

numerous projects, experiments, and field studies in many coun-tries [9,13,18–27]. Of the two processes, the wet process is morepopular, and has been more frequently used. In contrast, the drymethod is somewhat less popular because it produced poorerresults, especially in the early years (e.g. poor reproducibilityand the premature failure of road surfacing) [13,28–30]. Theselimitations generated a certain lack of confidence in the tech-nique, which meant that the dry process has developed moreslowly and has not been studied in as much depth as the wetprocess.

The main problem with CRM mixes, made by the dry processas well as the wet process, is their lack of cohesion, which isprimarily due to a poor interaction between the crumb rubberand bitumen. This causes lower resistance to moisture, detach-ment of aggregates, and a reduction in the bearing capacity ofthe pavement. The main disadvantage of the incorporation ofcrumb rubber in bituminous mixes is its instability when it isblended with the bitumen. This instability is strongly condi-tioned by the properties of both components (e.g. particle sizeof the crumb rubber, method of obtaining the crumb rubber,percentage of the crumb rubber added to the mix, compositionof the crumb rubber, degree of penetration and softening pointof the bitumen, composition of the bitumen, etc.), as well as bythe characteristics of the mix (mixing time and manner, tem-perature, etc.). Consequently, the manufacture of CRM mixeshas a much higher number of variables, and also means thatthe sensitivity of the results is greater when the process isapplied.

Regarding the wet process, there are many studies and experi-ments that show the impact of the previously mentioned set ofvariables, and which have resulted in a set of reference values per-taining to the optimal characteristics of the crumb rubber (e.g. size,composition, type, etc.), temperature and time of mix, and thecharacteristics of the bitumen [31–33].

In recent years, with a view to fomenting the use of the dry pro-cess, research studies have also been carried out in an effort todetermine the influence of such variables on the characteristicsof dry-method bituminous mixes. Consequently, there are nowstudies on the influence of the digestion time (time period neces-sary for the crumb rubber to interact with the bitumen and obtainoptimal properties) [34–36]. Other research has focused on themanufacturing time [37] as well as the size and quantity of thecrumb rubber added [38,39].

In the same line as these studies, this research also ana-lyzed the digestion time and the quantity of crumb rubberadded to the mix and their potential effect on the propertiesof the mix design. The results obtained underlined the impor-tance of these variables when calculating the optimal contentof bitumen as well as their impact on the most critical prop-erties of the mix. They provided further knowledge of thedry process, regarding the optimal digestion time and thequantity of crumb rubber added to the mix, which will en-able it to be applied more effectively and with more safetyguarantees.

For this purpose, the Marshall test results (values of density,stability, aggregate air voids, mix air voids, and deformation)were analyzed for 10 discontinuous bituminous mixes to be usedon road surface courses. The mixes had the same mineral compo-sition, but had different crumb rubber percentages (0.5%, 1%, and1.5% of the total weight of the mix) and digestion times (45, 90and 120 min). In order to compare the results obtained withthese mixes and to analyze the improvements resulting fromthe use of crumb rubber as a modifier in conventional mixes, areference mix was used. This reference mix had the same min-eral skeleton, but contained bitumen modified with high-perfor-mance polymers.

2. Methodology

2.1. Materials

The reference mix in this study was BBTM 11A, a discontinuous mix for roadsurface courses [40]. This is a hot mix with a maximum aggregate size of8–12 mm from which the 2–4 mm fraction was eliminated. It has a good mineralskeleton and strong cohesion since it is made from modified bitumen. It is generallyspread in a thin layer (2–3.5 cm), and is used for the surface rehabilitation of dete-riorated pavements as well as for newly constructed roads. It has an excellent sur-face macrotexture, thanks to its discontinuous grain size.

As is well-known, aggregates play an important role in the mechanical perfor-mance of the mix. For this reason, the grain size skeleton was mostly composedof 6/12 mm coarse aggregate (65–80% of the total) to provide the mix with bear-ing capacity. The rest of it was composed of 0/3 fraction fine aggregate (20–35%of the total), which along with the bitumen and the filler (7–10%) made up themortar that made the mix cohesive and provided it with resistance to tangentialstresses.

The aggregates selected for the mix design were ophite for the coarsefraction and limestone for the fine fraction with the characteristics shown inTable 1.

The characteristics of the aggregate and filler, as well as of the mixes them-selves, were in accordance with the Spanish Technical NLT Standards [41], RoadTests of the Centro de Estudio de Carreteras [Road Study Center], and the Spanishstandards UNE-EN [42] of the Spanish Standards and Certification Association (AE-NOR). The limits used were in consonance with the requirements for vehicle trafficsuperior to T2.1

As can be observed, both types of aggregate fulfill the requirements of the Span-ish regulations for the manufacture of BBTM 11A bituminous mixes. The materialused as filler was CEM II/B-L 32.5 N (UNE-EN 197-1) cement of the characteristicsshown in Table 2.

To compare the characteristics of dry-process CRM mixes with mixes madewith high-performance bitumen, two types of bitumen were used. Thus, the refer-ence mix, which did not contain crumb rubber, used bitumen modified with BM3cpolymers, whereas the design of the dry process mix formula contained conven-tional B 50/70 bitumen. It was hoped that the results would reflect an enhancementof the conventional bitumen that would make it comparable to the modified bitu-men. Table 3 lists the properties of both types of bitumen.

The crumb rubber particles used in the mixes had a size of 0–0.6 mm with thecharacteristics listed in Table 4.

2.2. Experimental design

To analyze the influence of digestion time and crumb rubber content on themix design, as well as the impact of the crumb rubber on the properties of dry-method bituminous mixes, 10 BBTM 11A mixes were studied. These mixes hadthe same mineral skeleton, but differed in the quantity of crumb rubber addedto the mix (0%, 0.5%, 1% and 1.5%), the digestion time before compaction (45,90 and 120 min), and the type of bitumen used (BM3c and B 50/70), as shownin Table 5.

During the manufacture of the CRM mixes, the temperature of the mix was in-creased to 180 �C (10 �C more than that of the reference mix) so as to facilitate theinteraction between the rubber and the bitumen, thus improving the cohesion ofthe mastic. This process was carefully monitored since the rubber particles couldburn at temperatures higher than 190�. Burning the rubber had to be avoided sincethis would have increased its stiffness and fragility instead of augmenting its elas-ticity, which was the objective. Moreover, temperatures higher than 195� wouldhave led to an aging of the bitumen, which would have had a negative impact onthe performance of the mix.

The manufacturing process first involved a 10 s agitation of the natural aggre-gates in the mineral skeleton in order to better homogenize them. The crumb rub-ber was then added to the aggregates, and mixed with them for a period of 20 s inorder 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 filler. This was followed by a final 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 160–165 �C.

The mixes were designed, according to the results of the Marshall test (NLT-159). The Marshall test consisted in the evaluation of three sets of specimens, witha diameter of 101.6 mm and length of 63.5 mm, for each of the mix formulas stud-ied. Each set of cylinders had the same grain size (the size specified in each for-mula). The only thing that varied was the bitumen percentage of the weight ofthe aggregate (4%, 4.5% and 5%).

Table 1Characteristics of the aggregates used in the mixes.

Test Results

PG-3 limitations Coarse aggregate(6/12 mm)

Fine aggregate(0/5 mm)

Ophite Limestone

Particle granulometry (UNE-EN 933-1) Sieve % of materialpassing

% of materialpassing

% of materialpassing

11.2 – 94 1008 – 60 1004 – 0 852 – 0 560.5 – 0 180.063 – 0 0

Sand equivalent (UNE-EN 933-8) >50 – 78Percent of fractured face (or coarse aggregate angularity) (UNE-EN 933-5) 100% 100% –Flakiness index (UNE-EN 933-3) 620% 20 –Resistance to fragmentation of the coarse aggregate (Los Angeles machine

test) (UNE-EN1097-2)620 11 –

Resistance of coarse aggregate to polishing (accelerated polishingcoefficient (APC)) according to annex D of UNE 146130

P0.50 0.52 –

Cleaning of coarse aggregate (organic impurity content) according toannex C of UNE 146130

<0.5% 0.04% –

Relative density and absorption (NLT-153) Apparent relative density – 3.26 g/cm3 2.74 g/cm3

Apparent relative density on asaturated surface-dry basis

– 3.18 g/cm3 2.70 g/cm3

Real relative density – 3.15 g/cm3 2.68 g/cm3

Absorption coefficient – 1.00% 0.84%

Table 2Characteristics of the cement filler.

Cement filler

Particle grain size (UNE-EN 933-1) Sieves (mm) 11.2 8 4 2 0.5 0.063% material passing 100 100 100 100 100 97

Apparent density (NLT-176) 0.7 g/cm3

Table 3Properties of BM3c and B 50/70 bitumen.

Bitumen BM3c B 50/70

Penetration (UNE-EN 1426) 54 mm 68 mmSoftening point (UNE-EN 1427) 68.1 �C 48.1 �CFragility temperature (Fraas method) (UNE-EN 12593) �17 �C �12 �CElastic recovery at 25 �C (NLT 329) 73% –

Table 4Properties and composition of the crumb rubber used.

Properties

Density 1.15 g/cm3

Color BlackParticle morphology IrregularWater content <0.75%Textile fiber content <0.5% in weightMetal content <0.1% of the rubber weight

Grain sizeSieve (mm) 0.6 0.5 0.25 0.125 0.063% material passing 100 74 19 2 0

Min. (%) Max. (%)

CompositionAcetone extract 7.5 17.5Natural rubber (NR) 21.0 42.0Polymers (NR/SBR) 50.0 55.0Sulfur – 5.0Carbon black 20.0 38.0Ash – 18.5

Table 5Mix characteristics.

Mix Crumb rubber (%)(oftotal mix)

Digestion time(min)

Bitumen

Reference mixBBTM 11A

0 0 BM3c

BBTM 11A 0.5-45 0.5 45 B 50/70BBTM 11A 0.5-90 0.5 90 B 50/70BBTM 11A 0.5-120 0.5 120 B 50/70BBTM 11A 1-45 1 45 B 50/70BBTM 11A 1-90 1 90 B 50/70BBTM 11A 1-120 1 120 B 50/70BBTM 11A 1.5-45 1.5 45 B 50/70BBTM 11A 1.5-90 1.5 90 B 50/70BBTM 11A 1.5-120 1.5 120 B 50/70

Table 6Grain size of the mixes.

Sieves(mm)

BBTM 11Agrain size

BBTM 11A/BBTMA11A + 0.5 CRM

BBTM11A + 1%CRM

BBTM11A + 1.5CRM

11.2 90–100 95 95 958 62–82 72 72 724 28–38 33 33 332 25–35 30 30 300.5 12–22 17 18 180.063 7–9 8 10 11.5

F. Moreno et al. / Construction and Building Materials 25 (2011) 2323–2334 2325

Fig. 1. Grain size curves.

Fig. 2. Marshal test graphs for the mixes with 0.5% crumb rubber.



For each percentage, the mean value of the following parameters was calcu-lated: air voids in aggregates (Ava) in%; air voids in the mix (Avm) in%; apparentdensity (Ad) in g/cm3, deformation (D) in mm, and stability (S) in kN. The crite-rion for selecting the optimal bitumen content was based on the PG-3 regula-tions for this type of mix, consisting in the manufacturing of test cylinders,compacted with 50 blows on each side by a Marshall hammer. Each specimenwas required to have a stability greater than 7.5 kN and air voids in the mixesgreater than 4%.

Furthermore, by varying the digestion times as well as the amount of crumbrubber added to the mix, it was possible to analyze how these variables affectedthe characteristics of the mixes as well as their mechanical performance. TheMarshall test values for stability and deformation provide a good way of empiricallymeasuring the capacity of the mixes to withstand vehicle traffic loads and thestresses that occur under tolerable deformations. This mechanical strength is thesum of that produced by the internal friction of the aggregates and the cohesionprovided by the hydrocarbon binder (which should interact with the crumb rubber,and thus attain values similar to those of high-performance bitumen). The stabilityof a mix must be greater when the loads to be withstood are greater. More stabilityis also necessary when the asphalt pavement layer is closer to the road surface.Since the mixes in this study were to be used in the road surface course, the recom-mended values were higher than 7.5 kN.

Fig. 3. Marshall test graphs for the

3. Analysis of results

It was first necessary to create the mineral skeleton of each ofthe mixes. To avoid any type of variation in the results becauseof air voids in the mineral skeleton, the mixes were batched gravi-metrically so that the size of all of the particles was within thegrain size envelopes established in the regulations [40]. This madeit possible to minimize the effects of excessively large or smallgrain sizes. Furthermore, for amounts of crumb rubber greater than0.5%, it was necessary to eliminate part of the fine aggregate of thismineral skeleton [13] so as to leave space for the crumb rubberparticles (Table 6). The grain size distribution was thus performedby volume (instead of by weight), due to the differences in densitybetween the two components (natural aggregates, 2.95 g/cm3;crumb rubber, 1.15 g/cm3).

As can be observed, when the amount of crumb rubber added tothe mix increased, this modified the mineral skeleton of the mix

mixes with 1% crumb rubber.


(see Fig. 1). The crumb rubber particles had to occupy the air voidsleft by the aggregate in the mix without causing problems in thegrain size. As reflected in the graph, the percentage of 0.5 mmand 0.063 mm fractions that passed increased with the contentof the rubber. This means that there was less aggregate of that sizein the mix since it had been replaced by the rubber particles. Theresulting mineral skeleton was at the center of the grain size enve-lope, the same as the skeleton of the reference mix. On the otherhand, the filler fraction remained constant. It was not necessaryto replace it since the rubber particles were larger than the fillerparticles.

Once the grain sizes were established, the mix formulas werethen designed. The results of the Marshall tests are shown in Figs.2–7.

Based on these results and the criteria in the technical specifica-tions of the Spanish highway regulations [40], the optimal bitumencontent (of the total weight of the mixture) added to each mix isshown in Table 7.

Fig. 4. Marshall test graphs for the

The test results reflect the variations suffered by the mixparameters, based on the digestion time during the tests. Despitethese variations, it was found that the digestion time had littleor no influence on the optimal bitumen content in the mix de-sign. Accordingly, the curves in Figs. 2–4 (depending on thedigestion time during the test) are grouped closer together thanthe curves in Figs. 5–7 (depending on the crumb rubber added tothe mix).

The results obtained showed that when crumb rubber wasincorporated with the dry method, the physical characteristics ofthe mixes (i.e. density and air voids) were scarcely affected bythe digestion process of the rubber. For this reason, the curves ob-tained are very similar. This seems to indicate that the workabilityof the mix, measured in terms of its compactability (density and airvoids, though with the same mineral skeleton) does not depend onthe digestion time in the manufacture of the mix. Thus, this studyshowed that the digestion time did not improve the compactabilityof dry-method CRM mixes.

mixes with 1.5% crumb rubber.


On the other hand, the reaction time of the rubber and bitumendid affect the mix properties, and this caused the mechanical per-formance of the mix to be slightly affected by the digestion time.The variation in the digestion time reflects an evolution in themechanical performance of the mix, depending on the degree ofbinder modification. In certain cases, the properties of the modifiedbinder approximated those of the high-performance binder modi-fied with polymers. The results showed that the time of rubber–bitumen interaction enhanced the performance of the bitumen.This initially improved the properties of the mix, but when thelength of digestion time exceeded a certain threshold, the resultwas just the opposite and mix properties were impoverished. Theoptimal digestion time was found to be 90 min since the mixesmanufactured at that time period generally showed better stabilityand deformation values.

When the percentage of crumb rubber added to the mix wasanalyzed, it was found that it had a significant effect on the selec-tion of the optimal binder content. As the crumb rubber content in

Fig. 5. Marshall test graphs for mixes

the mix increased, generally speaking, the optimal bitumen con-tent also increased. Therefore, a greater quantity of binder wasnecessary in order to guarantee an adequate blend with the rubberparticles.

As can be observed in Figs. 5–7, an increase in the CRM percent-age of the mix entailed a reduction in the density of the mix as wellas in air voids. This was due to the resiliency of the crumb rubber,irrespective of the digestion time, which made the mix less com-pact. Therefore, a high crumb rubber content in the mineral skele-ton of the mix caused a rebound effect, which became greater asthe content of these particles increased, thus reducing the compac-tion of the mixes. The incorporation of small quantities of CRM(0.5% of the total weight of the mix), produced a correspondingreduction in air voids in respect to the reference mix because therubber particles occupied the air voids in the mineral skeleton. De-spite this fact, for greater CRM contents (1% and 1.5% of the totalweight of the mix), there was a significant decrease in the degreeof compaction of the mix. Beside a reduction in its density, this

with a digestion time of 45 min.

Fig. 6. Marshall test graph for mixes with a digestion time of 90 min.


caused an increase in the air voids, which was greater than that inthe reference mix.

This phenomenon seems to be one of those that are possiblyresponsible for the greater susceptibility to moisture that charac-terizes CRM mixes [38] since a decrease in the compaction of themixture and an increase in air voids can lead to higher moisturecontent. CRM mixes have thus been found to be more susceptibleto moisture than conventional mixes.

Furthermore, irrespective of the digestion time and the crumbrubber content, the incorporation of crumb rubber in dry-processbituminous mixes, generally improved the performance of themix against deformations, thanks to the elasticity of the rubberparticles. Consequently, the results obtained showed how themixes were more resistant to deformations than the referencemix with high-performance bitumen modified with polymers.

Finally, after the selection of the optimal bitumen content, theanalysis of the results obtained for each value of the two variables(amount of crumb rubber added to the mix and digestion time

used in its manufacture) indicates that the air voids and densitygraphs are less sensitive to variations in digestion time thoughsomewhat more sensitive to variations in the crumb rubber con-tent. In contrast, the deformation and stability graphs show agreater sensitivity to both these variables (Figs. 8–12). Based onthese results, the optimal values of these variables are a digestiontime of approximately 90 min and a crumb rubber percentage ofless than 1% of the total weight of the mix.

4. Conclusions

This research study analyzed the impact of the digestion timeand the percentage of crumb rubber in the design formula ofdry-method CRM hot bituminous mixes. For this purpose, theMarshall test was performed on various discontinuous mixes forroad surface courses. These mixes had an identical mineral skele-ton, and were manufactured with conventional bitumen. They only

Fig. 7. Marshall test graphs for mixes with a digestion time of 120 min.

Table 7Optimal bitumen content of the mixes.

Mix Optimal bitumen content (%)

Reference BBTM 11A 4.75BBTM 11A 0.5-45 4.75BBTM 11A 0.5-90 4.75BBTM 11A 0.5-120 4.5BBTM 11A 1-45 5BBTM 11A 1-90 5BBTM 11A 1-120 5BBTM 11A 1.5-45 5BBTM 11A 1.5-90 4.8BBTM 11A 1.5-120 4.8


differed in their content of crumb rubber and the digestion timeused in the manufacturing process. These mixes were comparedwith a reference mix, which had the same mineral skeleton, buthad been manufactured with high-performance bitumen, modified

with BM3c polymers. The principal conclusions that can be derivedfrom this research are the following:

– Digestion time does not significantly affect the choice of theoptimal bitumen content to be added to the mix. The character-istics of the mixes in this study were only very slightly influ-enced by variations in digestion time. The results show thatthe air voids and densities of the mixes, as reflected in theMarshall test values, are very similar, irrespective of the diges-tion time.

– Based on the results of this study, it was found that the diges-tion time does not have any impact on the compaction of dry-process CRM mixes.

– In contrast, the mechanical performance of the mixes was affectedby variations inthe digestiontime. It was found that the interactiontime between rubber particles and bitumen had a certain influenceon the final properties of the mixes. According to the resultsobtained, the optimal digestion time was found to be 90 min.

Fig. 8. Densities, depending on the optimal bitumen content.

Fig. 9. Stability, depending on the optimal bitumen content.

Fig. 10. Deformation, depending on the optimal bitumen content.


Fig. 11. Air voids in the mix, depending on the optimal bitumen content.

Fig. 12. Air voids in aggregate, depending on the optimal bitumen content.


– An increase in the percentage of crumb rubber in the mix causedit to become less dense and increased its air void content. Thiswas due to the resilient properties of the rubber that produceda rebound effect in the mineral skeleton, thus causing the mixto become less compact. These results indicate that difficulty ofmix compaction is directly associated with the quantity of crumbrubber added and not with the digestion time used in the manu-facturing process. Consequently, the maximum crumb rubberpercentage to be added is 1% of the total weight of the mix.

– This increase in air voids and reduction in density, resultingfrom the incorporation of larger quantities of crumb rubber,meant that a larger amount of binder must be added in orderto guarantee that mix has good cohesion.

– The addition of crumb rubber from scrap tires improved theperformance of bituminous mixes, and helped them to resistdeformations. In this respect, the results obtained for dry-process CRM mixes were better than those obtained for the ref-erence mix which used high-performance bitumen.

– Despite the insights provided by the results of this research, it iscrucial to further analyze the mechanical performance of dry-process CRM mixes in terms of these variables, and test theirresponse to moisture (moisture sensitivity test UNE-EN12697-12) and to plastic deformation (wheel-tracking testUNE-EN 12697-22).

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analysis of digestion time and the crumb rubber percentage in dry-process crumb rubber modified hot...

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