Construction and Building Materials 73 (2014) 247–254

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Construction and Building Materials

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Mechanical performance of dry process fine crumb rubber asphaltmixtures placed on the Portuguese road network

http://dx.doi.org/10.1016/j.conbuildmat.2014.09.1100950-0618/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Department of Civil Engineering, Instituto Politécnicode Coimbra, Instituto Superior de Engenharia de Coimbra, Rua Pedro Nunes, 3030-199 Coimbra, Portugal. Tel.: +351 239 790 200; fax: +351 239 790 201.

E-mail address: capitao@isec.pt (S.D. Capitão).

J.L. Feiteira Dias a, L.G. Picado-Santos b, S.D. Capitão c,d,⇑a Inovroute, Engenharia e ambiente, Urbanização das Mélias, lote 31, Alagoas – Santa Joana, 3810-010 Aveiro, Portugalb DECivil, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugalc Department of Civil Engineering, Instituto Politécnico de Coimbra, Instituto Superior de Engenharia de Coimbra, Rua Pedro Nunes, 3030-199 Coimbra, Portugald CESUR, Instituto Superior Técnico, Universidade de Lisboa, Portugal

h i g h l i g h t s

� ARdry with fine granulate rubber is an improved pavement material.� Stiffness of ARdry is less sensitive to high temperatures than that of a conventional mix.� Mixing above 175 �C seems to impair the rubber contribution to mechanical performance.� Permanent deformation resistance confirms the low temperature susceptibility of ARdry.� Fatigue performance and rut resistance of ARdry and ARwet are at the same level.

a r t i c l e i n f o

Article history:Received 31 May 2014Received in revised form 24 September2014Accepted 25 September 2014

Keywords:Asphalt rubberAsphalt pavementsFatigue resistanceHot-mix asphaltPerformance propertiesResistance to permanent deformationStiffness modulus

a b s t r a c t

This paper evaluates the mechanical response of two gap-graded asphalt rubber mixtures manufacturedby the dry process (ARdry). The observed behaviour is compared with that of a similar gap-graded mixturewithout rubber granulate, used as reference. The laboratory results are also compared with analogousasphalt rubber mixes produced elsewhere by the wet process (ARwet). The blends were produced in aplant and laid in trial sections and on the Portuguese road EN 370, in order to collect representative spec-imens for laboratory testing. The mechanical evaluation of the mixes was carried out by repetitive four-point bending tests and wheel-tracking tests. The laboratory results and the behaviour observed on theEN 370 allow us to conclude that mechanical performance of the tested ARdry is better than that mea-sured for the reference blend, and is at the same level of performance as ARwet, provided that a propermixture design and some construction directives are used.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Recycled tyre crumb rubber (CR) is a material produced fromend-of-life tyres (ELTs) suitable to be applied as a bitumen modi-fier in bituminous mixtures. This granulate is obtained afterremoval of reinforcing wires and textile fibres from ELTs [1]. Sincethe use of granulated tyre rubber in the manufacture of bituminousmixtures helps to avoid the inappropriate disposal of tyres, theincorporation of this by-product in asphalt is undeniably beneficialto the environment and the society. The best-known CR production

methods are ambient grinding and cryogenic grinding. A detaileddescription of these and other methods can be found in the reviewpublished by Presti [1]. It must be emphasized that rubber granu-lates ready-to-use, and produced by the aforementioned processes,are available on the market.

Two methods are generally followed to incorporate CR particlesinto bituminous mixtures, commonly designated as the ‘‘wet pro-cess’’ and the ‘‘dry process’’. The first one generally involves theintroduction of fine CR (particles with a size lower than 2.36 mm[1]) into hot bitumen (around 180 �C), allowing interactionbetween the two materials by permanent agitation within a tank.After that, the resulting improved binder is added to the aggregateblend to produce asphalt rubber (ARwet). Traditionally, the dry pro-cess (ARdry) consists in adding CR, as a mixture component, atambient temperature into a blend of heated aggregates prior to

248 J.L. Feiteira Dias et al. / Construction and Building Materials 73 (2014) 247–254

introducing bitumen into the process. As mentioned by Rahmanet al. [2], in the dry process the rubber particles, which are added,are coarser (0.4–10 mm) and it is normally assumed that the CR ispart of the aggregate. Reaction between bitumen and CR is consid-ered negligible because the mixtures are fabricated without anysignificant interaction time between bitumen and CR. In this pro-cess, some fractions of the aggregate blend are generally substi-tuted by CR particles of similar sizes [2,3]. Since performance ofARdry tends to demonstrate some variability and in some situationspoor results [2–5], there is a certain lack of confidence in the dryprocess.

This study concerns the use of a modified dry process, in whichthe CR used is much finer than usual (0–0.6 mm nominal size as itcommonly happens in the wet process in Portugal). The interactionbetween the bitumen and the rubber occurs during the time thatthese components come into contact (at least 90 min in this pro-ject). As the rubber particles are very fine, the interaction betweenthe rubber and the binder occurs more quickly allowing a certainmodification of the heated bitumen. Previous studies carried outin this technique by other authors [6–8] have emphasized labora-torial work. The present study complements the information avail-able. Firstly, by including mechanical properties of specimenstaken from a pavement built in a road segment of the Portugueseroad 370 (EN 370) 14.5 km in length, between Portalegre and Avis;secondly, by providing information on the observed behaviour ofthe pavement after five years of service.

The laboratory evaluation of mechanical performance of thestudied ARdry is made in terms of stiffness, based on EN 12697-26 [9], resistance to fatigue, according to EN 12697-24 [10], andresistance to permanent deformation by using wheel-trackingequipment operated under the standard NLT-173 [11]. In addition,a pavement inspection was carried out after five years of service.

2. Materials

2.1. Aggregates and rubber granulate

Two types of aggregates and limestone filler were used in the study. These con-stituents allowed the production of three different compositions, whose gradingcurves were established from three different aggregate fractions of rhyolite (mix-tures TB0 and TB3) and granodiorite (mixture TA), as well as a limestone filler.For each different mixture studied, the aggregate blend was determined based ona target grading Portuguese envelope for gap-graded aggregate mixtures for surfacelayers. Table 1 shows the grading curves of aggregate blends and crumb rubber,Table 2 summarises other aggregate properties determined according to the Portu-guese specifications, and Table 3 presents some physical properties of the rubber.The CR producer receives tyres (car and truck tyres) from an organization licensedby the government to manage end-of-life tyres in Portugal.

The filler used was produced from crushed limestone rocks, with 95.5% of par-ticles smaller than 0.125 mm and 78.6% of particles smaller than 0.063 mm. Theapplied filler met the common Portuguese requirements.

Table 1Grading curves of the aggregate blends and rubber granulate (percentage by weight of m

Sieves (mm) TB0 [reference mix] TB3 [1.5% of crumb r

20 100 10014 88.1 82.410 73.5 66.58 61.2 55.14 30.3 27.52 18.1 17.90.5 9.9 10.11.18 – –0.6 – –0.3 – –0.125 6.0 6.40.075 – –0.063 4.1 4.4

a Ambient grinding CR.

2.2. Binders

The AR mixes were produced with conventional pen 35/50 paving bitumen.After the production and laying of each of the studied blends, the binder was recov-ered and its properties were determined [13]. Table 4 shows the properties of theoriginal bitumen before and after recovery, as well as the properties of the rub-ber-binder after recovery. The two-phase procedure consisted in separating the bin-der from the aggregate by centrifugation with toluene, followed by the separationof the binder from solvent in a rotary evaporator.

It must be emphasized that rubber particles tend to separate from bitumenthroughout centrifugation. This suggests that the interaction between both compo-nents is predominantly physical. Nevertheless, as proved also by several authors[2,6,7], the final bitumen becomes harder because rubber absorbs some light frac-tions of it during the time they remain in contact at high temperature conditions.The results show that all the recovered binders were substantially harder thanthe virgin bitumens.

2.3. Compositions of blends and manufacture temperatures

Table 5 summarises some relevant information about the studied blends. Forthese types of asphalt rubber gap-graded mixtures the Portuguese specificationindicates that the binder content should be adjusted after construction of a trial sec-tion, which is used to evaluate the handling and construction conditions in the field.The minimum binder content should be between 8% and 9% (by weight of the totalmixture). The gap-graded mixture used as reference has a typical binder content of5%. Note that this offers the opportunity to discover if incorporating rubber granu-late by a modified dry process into the mix, as well as higher binder content, couldbe significantly beneficial for the mechanical properties of the obtained mixture.

As in the wet process, there was no substitution of fine aggregate fractions byCR particles of similar sizes because the resulting asphalt mixture is practicallyidentical.

The interaction time between virgin bitumen and rubber granulate was 90 and140 min for blends TB3 and TA, respectively. As demonstrated elsewhere [6], a min-imum interaction time of 90 min is recommended for the method of productionapplied in this project. The interaction occurred during mixing at the plant andthroughout haulage time from plant to test site.

The selected mixing temperature of the blends TB0 and TA was in the typicalrange generally used for hot mix asphalt (around 165 �C). It must be emphasizedthat temperatures applied in these cases were lower than those typically used forthe wet process (around 180 �C or higher), as reported by several authors [1,14].Therefore, the blend TB3, also produced by the dry process, was mixed at a veryhigh temperature with the goal of evaluating the effect on the mixture perfor-mance. In fact, strong hardening of binder and degradation of rubber granulate isexpected to occur at these very high temperatures.

The mixing procedure carried out at the batch plant consisted in allowing a per-iod of 15 s pre-mixing of the rubber granulate with the aggregate. Afterwards, thevirgin bitumen was added to the mixer.

3. Results of laboratorial performance evaluation anddiscussion

3.1. Production of specimens for testing

Prismatic beams were cut from the slabs taken from the trialsections constructed at the test site on the Portuguese EN 370. Pris-matic specimens of 420 � 60 � 60 mm3 were submitted to four-

aterial passing).

ubber] TA [1.9% of crumb rubber] Crumb rubbera

100 –97 –81 –69 –36 –18 10010 –– 100– 98.1– 26.46 –– 0.34 –

Table 2Characteristics of aggregates.

Properties Standard Rhyolite aggregates (TB0 & TB3) Granodiorite aggregates (TA)

10–16 mm 4–12 mm 0–4 mm 6–14 mm 4–6 mm 0–6 mm

Sand equivalent (%) EN 933-8 – – 49 – – 60Methylene blue (g/kg) EN 933-9 – – 7.8 – – 1.7Flakiness index (%) EN 933-3 12 28 – 9 – –Shape index (%) EN 933-4 14 25 – 10 – –Micro-deval (%) EN 1097-1 8 – – 10 – –Los angeles (%) EN 1097-2 14 – – 18 – –Density after drying (mg/m3) EN 1097-6 2.64 2.66 2.62 2.63 2.59 2.59Water absorption (%) EN 1097-6 0.34 0.67 1.65 0.8 0.95 1.15

Table 3Physical properties of crumb rubber [12].

Properties Standard @ Room temperature @ 170 �C @ 210 �C

Indentation hardness (ShA) ISO 7619-1 58 61 65Abrasion resistance (mm3) ISO 4649 132 193 177Density (mg/m3) ISO 2781 1.15 1.21 1.23Tear strength (N/mm) ISO 34-1 46 42.3 40.9Tensile stress–strain (MPa) ISO 37 10.1 8.1 7.3Compression set (%) ISO 815-1 41.7 37.8 43.0

Table 4Characteristics of binders (virgin and recovered from specimens).

Properties Standard Pure bitumen Recovered rubber-binder

Virgin Recovered (TB0) TB3 TA

TB0 & TB3 TA

Penetration @ 25 �C (0.1 mm) EN 1426 40 38 28 27 29Softening point (�C) EN 1427 54 55.3 67.8 71.2 64Resilience (%) ASTM D 5329 11 9 25 29 28Brookfield viscositya (cP) EN 13302 88 102 312 381 298

a Tests carried out at 175 �C.

Table 5Composition of bituminous mixtures and temperatures of manufacture.

Mixtures Aggregate temperature (�C) Bitumen temperature (�C) Mixtures temperature (�C) Binder content (%) Rubber content by weight of bitumen (%)

TB0 165–170 150–155 160–165 5.0 0.0TB3 210 190–195 9.0 17.0TA 165–170 155–160 165–170 8.6 21.8

Table 6Volumetric properties of the blends submitted to mechanical evaluation.

J.L. Feiteira Dias et al. / Construction and Building Materials 73 (2014) 247–254 249

point bending testing for stiffness and fatigue resistance evalua-tion. For permanent deformation resistance evaluation purposesin wheel tracking tests, slab specimens of 300 � 300 � 50 mm3

were also cut from the trial sections. Table 6 summarises some vol-umetric properties of the blends submitted to mechanicalevaluation.

Although the volumetric properties of the three blends are notequal, they meet the requirements of Portuguese specificationsand, therefore, they represent typical compositions of gap-gradedasphalt mixtures used for pavement rehabilitation in Portugal.Since the modified dry process applied in this project is not com-monly used worldwide, this paper presents the mechanical proper-ties with the aim of spreading information about the expectedbehaviour of asphalt mixtures produced by this technique.

Mixtures Density (kg/m3)a Porosity (%)b VMA (%) VFB (%)

TB0 2290 6.0 17.0 64.7TB3 2250 4.3 23.8 81.9TA 2270 7.4 25.7 71.2

a Bulk density � saturated surface dry (SSD).b Maximum density measured by the volumetric procedure.

3.2. Stiffness modulus

The testing equipment used to determine stiffness modulus andphase angle of the studied mixtures was a repetitive four-pointbending testing machine [15]. The testing machine has a clamping

device that allows free rotation and translation at the reactionpoints. For temperature control of the specimens, the loadingframe is placed in a climatic chamber allowing temperaturesbetween 0 and 60 �C, with an accuracy of ±1 �C.

Prismatic beams (three replicates per mix) were tested usingrepetitive four-point bending tests, carried out under controlledstrain conditions (strain level of 100 lm/m), at three temperatures(20, 30 and 40 �C). A sinusoidal wave loading was applied at threefrequencies (10, 5 and 1 Hz). Table 7 displays the obtained resultsof the stiffness modulus and the associated coefficient of variation(CV).

33%

63%

76% 71%

53%

37%

0%

20%

40%

60%

80%

100%

TB0 TB3 TA TB0 TB3 TA

30ºC 30ºC 30ºC 40ºC 40ºC 40ºC

% o

f s�

ffne

ss o

btai

ned

at 2

0o C

Designa�on of the mixtures and test temperatures

10 Hz 5 Hz 1 Hz Average

Fig. 2. Stiffness values of TB0, TB3 and TA at 30 and 40 �C in percentage of thestiffness values measured at 20 �C.

250 J.L. Feiteira Dias et al. / Construction and Building Materials 73 (2014) 247–254

At 20 �C all the mixtures show similar results for the stiffness.As expected, stiffness modulus increased with the frequency.Although all the mixtures deliver lower stiffness modulus whenthe testing temperature increases, the variation observed for eachone was quite different (Fig. 1).

Comparing stiffness moduli of blend TB3 (mixed above 190 �C)with those of the reference mixture, TB0, produced without crumbrubber, it can be observed that the first one generally shows aslightly lower stiffness modulus at 20 and 30 �C. The oppositeoccurs at 40 �C. Similarly, stiffness modulus results of the two ARdry

studied, TA and TB3, show higher values for the first one at 20 and30 �C, and the contrary at 40 �C.

Furthermore, as Fig. 2 shows, for the mixture without rubber,TB0, the stiffness values measured at 30 and 40 �C achieved, onaverage, 76% and 33%, respectively, of the stiffness observed at20 �C. In the case of ARdry TB3, in which the binder has a consider-able level of aging caused by a high mixing temperature, stiffnessvalues were reduced to 63% and 53% for the same temperatureconditions. These results suggest that TB3 is less sensitive to veryhigh testing temperatures than the mixture without rubber. Thetendency is not the same for the asphalt rubber blend TA, to whichthe same testing temperature variations reduced the stiffness val-ues to 71% at 30 �C and 37% at 40 �C. Although more research isneeded to identify a clear trend in results, the hardening of binderassociated to a very high mixing temperature (for TB3) and thepresence of rubber granulate are likely to explain the observedresults.

Fig. 3 allows additional analysis concerning reduction of stiff-ness moduli obtained for the frequency of 10 Hz, which is usedto illustrate the effect of high temperatures on the stiffness of

Table 7Stiffness moduli, Sm (MPa) and CV (%) of TB0, TB3 and TA.

Temperature (�C) Mixtures Sm@10 Hz CV

20 TB0 2493 4.4TB3 2290 4.3TA 2532 4.6

30 TB0 1866 5.4TB3 1584 5.2TA 1672 7.0

40 TB0 780 12.6TB3 1223 4.2TA 855 6.3

0

500

1000

1500

2000

2500

3000

0 5 10

Stiff

ness

Mod

ulus

(MPa

)

Frequency (Hz)

TB0 (20ºC) TB3 (20ºC)TB0 (30ºC) TB3 (30ºC)TB0 (40ºC) TB3 (40ºC)

Fig. 1. Effect of frequency and test temperature

the studied blends. The conventional mixture TB0 suffers a reduc-tion of 63 MPa/�C in the range of 20–30 �C. This rate rises to109 MPa/�C in the range of 30–40 �C. On the contrary, the ARdry

TB3 and TA show a slightly higher stiffness rate reduction in thefirst temperature range, whereas that rate decreases for the rangeof 30–40 �C. This is much more evident for the blend TB3, whichhas a rate of only 36 MPa/�C, whereas for TA it is 82 MPa/�C. Noticethat to produce TB3, the aggregate was heated at 210 �C to allowmixing between 190 and 195 �C [12]. These temperatures areabove the level at which rubber starts to incinerate (175 �C). As

Sm@5 Hz CV Sm@1 Hz CV

2333 6.0 1803 2.92235 6.0 1975 9.72423 6.0 2000 3.4

1802 5.1 1362 3.11196 3.3 1313 5.11665 6.8 1565 4.7

699 8.0 667 8.01150 4.2 1078 4.0767 3.0 934 3.7

0

500

1000

1500

2000

2500

3000

0 5 10

Stiff

ness

Mod

ulus

(MPa

)

Frequency (Hz)

TA (20ºC) TB3 (20ºC)TA (30ºC) TB3 (30ºC)TA (40ºC) TB3 (40ºC)

on stiffness modulus of TB0, TB3 and TA.

63

109

0

20

40

60

80

100

120

[20-30ºC] [30-40ºC]Ra

te o

f s�

ffne

ss r

educ

�on

w

ith

tem

p. (M

Pa/

o C)

Test temperature ranges

TB0 TB3 TA

Fig. 3. Rate of stiffness decay of TB0, TB3 and TA for the ranges of 20–30 �C and 30–40 �C.

100

1000

10,000 100,000 1,000,000 10,000,000

Tens

ile s

trai

n (

m/m

)

Fatigue life (number of load cycles)

TB0 (20ºC) TB3 (20ºC) TA (20ºC)TB0 (30ºC) TB3 (30ºC) TA (30ºC)

μ

Fig. 4. Fatigue laws at 20 and 30 �C derived from four-point bending tests byregression analysis.

J.L. Feiteira Dias et al. / Construction and Building Materials 73 (2014) 247–254 251

reported in Table 3, in this case the bitumen suffered considerablehardening during the production of the mixture, and could haveinduced some degradation effect on the rubber that explains thebehaviour observed for higher temperatures.

3.3. Resistance to fatigue

The type of test carried out in this study to evaluate the fatiguebehaviour was a four-point bending test analogous to the onedescribed for stiffness. The chosen fatigue resistance criterionwas 50% loss of the initial stiffness modulus. Since the in-servicetemperatures normally calculated for Portugal vary between about20 and 30 �C [16], fatigue tests were performed at 20 and 30 �C.The loading form was a sine wave with a frequency of 10 Hz,applied for three different strain levels (300, 500 and 700 lm/m).For each testing temperature, the fatigue performance of each mix-ture was obtained from nine specimens, three per strain level.

Table 8 summarises the fatigue laws (e = a � Nb) and the deter-mination coefficient (R2) obtained from prismatic beams extractedfrom the trial sections. For comparison, the e6 parameter (strainwhich induces specimen decay after 1 million load cycles) was alsocalculated from the fatigue laws obtained by regression analysis.Fig. 4 illustrates the fatigue laws obtained by regression analysis.

The values derived for e6 reveal that TB3 and TA, which incorpo-rate crumb rubber, have a much better fatigue resistance than thereference mixture, TB0, for both testing temperatures. Fig. 4 showsthat the fatigue resistance observed is better at 30 �C than at 20 �C.From the ARdry submitted to testing, TA shows a better perfor-mance than TB3.

On the one hand, the observed tendency occurred because theARdry has a higher binder content than TB0, as indicated in Table 3.On the other hand, the rubber tends to reduce damage induced ineach loading cycle on the material, allowing more load cycles untilfatigue life threshold is attained.

Table 8Fatigue laws (e = a � Nb) parameters, R2 and e6.

Temperature (�C) Parameters TB0 TB3 TA

20 a 11563 8013.6 6475.8b –0.302 �0.243 �0.198R2 99.08 99.2 99.9e6 (lm/m) 178 296 421

30 a 11148 9430.3 7026.4b –0.297 �0.247 �0.201R2 99.9 83.7 99.9e6 (lm/m) 184 310 438

Although TB3 has a higher binder content than TA, two factorscan explain the better fatigue performance of the latter. Firstly, TAincorporates 21.8% of rubber content by weight of bitumen andTB3 has only 17%. Secondly, as referred to above, TB3 was mixedat very high temperature (above 190 �C), causing incineration ofsome rubber components. This produced a considerable hardeningof bitumen and might have induced some performance degrada-tion on the rubber.

3.4. Resistance to rutting

The evaluation of the permanent deformation resistance of thestudied blends was performed by wheel-tracking (WT) tests.According to the standard NLT-173 [11], the wheel of the WTpasses over the specimen for 120 min (approximately 48 passes/min), applying a contact stress of 900 kPa at a temperature of60 �C. However, the representative temperature attained insidesurface layers of the Portuguese pavements is 50 �C [16,17], andthe European standard EN 12697-22 [18], indicates a contact stressof 700 kPa. Therefore, the evaluation of the permanent deforma-tion resistance of the studied blends was carried out for the twogroups of the testing parameters, although the conditions of NLT-173 are more demanding than those of EN 12697-22. Fig. 5 sum-marises the time-deformation curves (rut depth) obtained fromWT tests.

According to the pass criteria indicated in NLT-173 [11] for WTtests, a determined mix composition is accepted if the average rateat which the rut depth increases between the 105 and 120th min-ute of testing, Rrd-105/120, is less than or equal to 15 � 10�3 mm/min(for the most demanding in-service conditions assumed). Table 9shows the results obtained for Rrd-105/120 as well as for the totalrut depth in the wheel path, after 120 min (Rd-120). The resultsobtained for both parameters used to evaluate permanent defor-mation resistance of the studied blends reveal that ARdry performsbetter than the reference mix TB0 produced without rubbergranulate.

Analysing specifically the results obtained from tests carriedout at 60 �C, it is apparent that ARdry passes the permanent defor-mation criteria indicated in NLT-173 (Rrd-105/120 = 15 � 10�3 mm/min), while TB0, produced without crumb rubber, fails. Pass/fail

0

2

4

6

8

10

12

0 15 30 45 60 75 90 105 120

Ru

dept

h (m

m)

Time (min)

TB0 (50ºC)TB3 (50ºC)TA (50ºC)

0

2

4

6

8

10

12

0 15 30 45 60 75 90 105 120

Ru

dept

h (m

m)

Time (min)

TB0 (60ºC)TB3 (60ºC)TA (60ºC)

Contact stress: 700 kPa

Contact stress: 900 kPa

Fig. 5. Wheel-tracking test curves obtained at 50 and 60 �C, and for the two levels of contact stress.

Table 9Rate of rut depth (Rrd-105/120) and total rut depth after 120 min (Rd-120).

Temperature (�C) Contact stress (kPa) Mixtures Rrd-105/120 (10�3 mm/min) Rd-120 (mm) Ranking

50 700 TB0 8.2 3.05 WorstTB3 1.8 0.87 BestTA 4.9 1.62 Intermediate

60 900 TB0 29.3 10.83 WorstTB3 8.7 3.68 BestTA 11.7 5.64 Intermediate

0

1000

2000

3000

4000

5000

S�ff

ness

mod

ulus

(MPa

)

Test temperature

TB3

TA

Oliveira et al. [19]

Moreno et al. [8]

Fontes et al. [20]

Antunes et al. [21]

Fig. 6. Comparison between stiffness moduli measured for ARdry and ARwet for thesame testing conditions.

0

50

100

150

200

250

300

350

400

450

500

20ºCTest temperature

TB3

TA

Oliveira et al. [19]

Fontes et al. [20]

Antunes et al. [21]

6(m

m/m

)ε

Fig. 7. Fatigue performance measured at 20 �C for ARdry and ARwet for the sametesting conditions.

0

1

2

3

4

5

6

02468

101214161820

TB3 TA Antunes et al. [21]

R d- 1

20 (m

m)

Rdr

- 105

/120

(10-

3 /min

)

Rrd-105/120 Rd-120

Fig. 8. Permanent deformation performance measured on WT tests at 60 �C forARdry and ARwet.

252 J.L. Feiteira Dias et al. / Construction and Building Materials 73 (2014) 247–254

criteria, although established for Spanish conditions, are reason-able approximations for Portugal as climatic conditions are similar.

Of the two ARdry, TB3 reveals the better resistance to rutting,probably because bitumen suffered considerable hardening duringthe production related to a very high mixing temperature (above190 �C). In addition, the crumb rubber increases binder viscosity,which has a favourable contribution to rutting resistance of themixtures.

The obtained results suggest that incorporating fine crumb rub-ber in a conventional asphalt mixture by the modified dry processcan improve the permanent deformation resistance of the mixture.This seems to be particularly useful for locations where in-servicepavement temperatures are very high.

3.5. Global analysis of performance results

Based on the results, it can be stated that mechanical perfor-mance of the studied ARdry, by applying the modified procedure

proposed in this study, is generally better than that of the referencegap-graded mixture without rubber granulate.

Another useful analysis that underlines the satisfactorymechanical properties of ARdry studied is the comparison of theresults with others obtained elsewhere for mixes produced by

Table 10General characteristics of ARwet whose results were compiled from the bibliography.

Study Mix type/mixing temperature (�C) Binder type/content (%) CR% of binder weight; CR type; particles max. size) Porosity (%)

Oliveira et al. [19] Dense AC/160 Pen 55–70/4.5 21; cryo. g.; 6 mm 4.3Moreno et al. [8] BBTM 11A/180 Pen 50–70/4.75 20; N/A; 6 mm N/AFontes et al. [20] Gap graded/N/A Pen 35–50/8.5 20; amb. g.; 6 mm 6.0Antunes et al. [21] Gap graded/180 Pen 50–70/7.0 18; cryo. g.; 6 mm N/A

Fig. 9. Views of the gap-graded ARdry laid as pavement surface layer on the EN 370 pavement near Avis, 5 years after construction.

J.L. Feiteira Dias et al. / Construction and Building Materials 73 (2014) 247–254 253

the wet process (ARwet). A summary of the compiled results fromother authors is presented in Figs. 6–8 and Table 10. Althoughthe ARwet presented have different compositions and are quite var-iable in their nature, they may represent typical characteristics ofARwet properties produced in road technology. Therefore, theycan be used to a general comparison of mechanical propertiesbetween typical ARwet and the modified ARdry studied in this pro-ject and laid on the Portuguese EN 370.

In respect of stiffness modulus, ARdry tested in this studyrevealed slightly lower values than the typical values of ARwet

referred to in the bibliography. However, this does not seem tobe a problem as the resistance to fatigue cracking (Fig. 7) as wellas the resistance to permanent deformation of ARdry (Fig. 8) aregenerally similar to those of ARwet compiled from the bibliographyas examples.

4. Brief description of the experimental road work

Taking all the mechanical results into consideration, the mix-ture TA revealed better performance than TB3. Therefore, the firstone was selected to the experimental road work. The mixture TAwas laid as a rehabilitation surface layer (0.04 m thickness) on apavement of the Portuguese National Road EN 370 between sta-tions 21 + 050 (near Avis) and 35 + 584 (near Portalegre).

The experimental road work is about 14500 m in length.The rehabilitated pavement was opened to traffic in May 2009.

Since then, about 1 million standard axle loads have passed overeach lane of the road. Concerning air temperatures, the pavementis located in the most demanding Portuguese region in terms ofhigh temperatures. To give an indication of the climatic conditionsof the region, according to previous studies carried out in Portugal[16,17], it can be stated that about 10% of the observed hourlypavements temperatures at 2.5 cm depth attain values over45 �C, in a typical month of July. At the surface, temperature canachieve 80 �C.

Although climatic conditions are demanding in the region, thisis not a heavy duty pavement in respect of traffic loading. As illus-trated in the pictures in Fig. 9 there is no sign of structural andfunctional pathologies so far.

5. Conclusions

The research study presented in this paper focuses on themechanical characterisation of specimens of typical Portuguesegap-graded ARdry mixtures, comparing the observed mechanicalparameters with those of a similar mixture without rubber granu-late. The specimens were collected from real trial sections. One outof the two studied ARdry was laid on the Portuguese road network(EN 370). The mixture applied (TA) incorporated about 20% (of thetotal binder weight) of fine crumb rubber with 0.6 mm of maxi-mum size. Comparing the obtained results with those measuredfor a reference mix manufactured with pen 35/50 bitumen as wellas those compiled from the bibliography for ARwet, the followingconclusions can be drawn:

� The ARdry gap-graded manufactured with fine granulate rubberproduced from end-of-life tyres are materials with improvedproperties for road pavements as compared to similar mixtureswithout incorporation of rubber granulate.� Although crumb rubber is added to the aggregate for ARdry pro-

duction purposes, rubber-binder TB3 reveals higher viscosityafter recovery than the pure bitumen TB0 used as base binderbecause, during the interaction time, there is some migrationof light fractions of bitumen to the rubber.� The stiffness moduli measured for ARdry suggest that these

mixes are less sensitive to high temperatures (above 30 �C) thanthe reference mix produced with straight run bitumen.� ARdry exhibits the same typical tendency of ARwet having an

improved fatigue cracking performance, which is related tothe high binder content of the mixes as well as the presenceof the rubber.� Using mixing temperatures in plant above 190 �C during pro-

duction of ARdry seems to impair the rubber contribution to per-formance (reduction of stiffness at intermediate temperaturesand less fatigue resistance).� A low temperature susceptibility of ARdry (TB3 and TA) is also

observed in respect of permanent deformation resistance com-pared to the reference mix manufactured without rubber(TB0).

254 J.L. Feiteira Dias et al. / Construction and Building Materials 73 (2014) 247–254

� Although ARdry show stiffness moduli slightly lower in compar-ison to values measured for ARwet described in Table 10, fatigueperformance and resistance to permanent deformation are atthe same level.

Finally, based on the obtained results for the mechanical prop-erties of ARdry and because of its production simplicity comparedto the wet process, it can be stated that the proposed modifieddry process is a very attractive technology. The observed behaviourof the ARdry gap-graded on the Portuguese EN 370 after five yearsof service shows a satisfactory performance, both in regards to thestructural performance and the functional performance.

A study concerning cost published elsewhere [12] suggests costsaving from 6.8–12%, depending on the type of compositionadopted. The next step should be the development of standards,which could lead the industry and road agencies to a broader appli-cation of the technique in road pavement works.

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