The effect of crumb rubber modifier on the resistance of asphalt mixes to plastic deformation

Materials and Design 47 (2013) 274–280

Contents lists available at SciVerse ScienceDirect

Materials and Design journal homepage: www.elsevier.com/locate/matdes

Technical Report

The effect of crumb rubber modifier on the resistance of asphalt mixes to plastic deformation F. Moreno 1, M. Sol, J. Martín, M. Pérez, M.C. Rubio ⇑ LabIC, Laboratorio de Ingeniería de la Construcción, ETSICCP, Universidad de Granada, C/Severo Ochoa s/n, 18071 Granada, Spain

a r t i c l e

i n f o

Article history: Received 20 October 2012 Accepted 10 December 2012 Available online 20 December 2012

a b s t r a c t This research analyzed the response of bituminous mixes manufactured with rubber to plastic deformation. For this purpose, a set of asphalt mixes containing different percentages of crumb rubber modifier (CRM) added by the dry process as well as the wet process were tested. It also compared the performance of a CRM mix to that of a mix made with high-performance polymer-modified bitumen. The mixes were assessed with the wheel-tracking test and the cyclic triaxial test. Their bearing capacity was also evaluated by determining their stiffness modulus at different temperatures. The results obtained showed that for the dosages and percentages of crumb rubber used, the addition of wet-process and dry-process CRM to asphalt mixes with conventional bitumen increased their resistance to plastic deformation. In fact, the performance of some CRM mixes was superior to that of the mix with high-performance modified bitumen. It also increased their stiffness modulus and creep modulus values and improved their resistance to plastic deformations caused by vehicle traffic loads. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Plastic deformations caused by vehicle traffic loads on pavement surfaces are one of the most problematic types of deterioration in road engineering. Such deformations reduce the service life of the pavement and increase the cost of its rehabilitation and maintenance. Various design factors can be the source of this pathology, such as the mineral skeleton, aggregate properties, filler content, and binder quantity [1–3]. For example, the type of bitumen used in asphalt mixes decisively influences the appearance of plastic deformations, since mixes can be manufactured with conventional bitumen or bitumen modified with elastomeric or thermoplastic polymers. Modified bitumen has evolved considerably over the last few decades, and now has a higher viscosity and lower thermal susceptibility than conventional bitumen. The mixes made with it are thus more resistant to plastic deformations [4]. However, the price of polymer-modified binders is considerably higher than that of conventional bitumen, which could limit its use especially in times of economic and financial crisis [5]. For this reason, it is necessary to seek cost-effective ways to mitigate and prevent rutting in pavement surfaces. In this sense, the use of crumb rubber from end-of-life tires in bituminous mixes is a viable alternative to modified bitumen to improve mix performance and prevent plastic deformations. The addition of crumb rubber modifier (CRM) to asphalt mixes enhances the viscosity ⇑ Corresponding author. Tel.: +34 958249445. 1

E-mail addresses: [email protected] (F. Moreno), [email protected] (M.C. Rubio). Tel.: +34 958249443.

0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.12.022

and elasticity of the bitumen and at the same time, increases their resistance to aging [6,7]. Furthermore, the recycling of crumb rubber contributes to sustainable development (i.e. reduction of a problematic waste that is difficult to cleanly dispose of, revalorization of the used tires, conservation of natural resources, etc.). It has thus become one of the most frequently used by-products in the manufacture of other construction materials, such as concrete [8,9]. It also reduces road maintenance expenses because it helps to extend the useful life of the pavement [10]. In addition, it reduces noise levels [11], contributes to driving safety, and increases the quality of the pavement surface [12,13]. Crumb rubber can be incorporated in bituminous mixes in one of two ways: the dry process or the wet process. In the dry process, the crumb rubber is added to the asphalt plant mixer as though it were another type of aggregate, thus directly modifying the properties of the mix. In contrast, in the wet process, the crumb rubber is added to the bitumen binder. In this way, it first modifies the properties of the bitumen, and when this modified binder is added to the mix, the mix properties are then modified as well [12]. The main difference between these two techniques is that the wet method more effectively modifies the properties of the binder since the crumb rubber particles directly interact with it. This paper evaluates the effect of dry-process as well as wetprocess CRM on the resistance of asphalt mixes to plastic deformations. With this objective, this research study analyzed the mechanical performance of a BBTM 11A asphalt mix to which crumb rubber was added by the dry process as well as the wet process. The study of the dry process involved the use of conventional bitumen with the addition of different percentages of crumb rub-

275

F. Moreno et al. / Materials and Design 47 (2013) 274–280

ber (0.5%, 1.0% and 1.5% of the total mix weight). The study of the wet process used two binders with different quantities of crumb rubber: (i) improved BC 50/70 bitumen with rubber (8.0% crumb rubber of the total binder weight); (ii) high-viscosity rubber-modified bitumen (with 20.0% crumb rubber of the total binder weight). At the same time, the resistance to plastic deformations of the CRM mixes was compared to that of a mix manufactured with high-performance bitumen. Consequently, this study compared the performance of CRM mixes to the performance of a BBTM 11A asphalt mix with BM 3c bitumen modified with thermoplastic elastomers (SBS). The resistance of the mixes to plastic deformations was evaluated with the wheel-tracking test (UNE-EN 12697-22) [14], following the Spanish regulations (PG-3) [15] for testing the mechanical performance of asphalt mixes in pavement surfaces. The second test used was the cyclic triaxial test (UNE-EN 12697-25, Method B) [14], considered by various researchers as an effective method of evaluating the non-recoverable deformations of the mixes [16,17]. On the one hand, this test is able to simulate repeated traffic loads (vertical axial load), and on the other, the confining pressure reproduces the service stresses on the mix and prevents the premature failure of the test specimen [18,19]. Moreover, to analyze the effect of crumb rubber on the bearing capacity of the mixes at high temperatures (a crucial factor in resistance to plastic deformations), the stiffness modulus of the mix was calculated (UNE-EN 12697-26, Annex C) [14] at different temperatures (20 °C, 40 °C and 60 °C).

Table 2 Bitumen characteristics. Bitumen

BM3c

Penetration (UNE-EN 1426) (mm) 54 Softening point (UNE-EN 1427) (°C) 68.1 Fragility temperature (Fraas method) 17 (UNE-EN 12593) (°C) Elastic recovery at 25 °C (NLT 329) (%) 73

HVRMB

BC 50/70 B 50/70

55–70 70.0 15

50–70 53.0 8

68 48.1 12

80

–

–

2.1.2. Filler The filler used was Portland cement, CEM II/B-L 32.5 N (UNE-EN 197-1). Of this filler, 95% had a particle size smaller than 0.063 mm with an apparent density (UNE-EN 1097-3) equal to 0.7 Mg/m3. The filler thus complied with PG-3 [15] requirements (0.5– 0.8 Mg/m3). 2.1.3. Bitumen The asphalt mixes were manufactured with different types of bitumen. The dry-process CRM mixes were manufactured with conventional B 50/70 bitumen. In contrast, the wet-process CRM mixes were manufactured with BC 50/70 rubber-modified bitumen and high-viscosity rubber-modified bitumen (HVRMB). Finally, BM 3c bitumen was used for the mix with high-performance binder. The characteristics of the different types of bitumen are listed in Table 2. 2.1.4. Crumb rubber The crumb rubber used in the dry-process CRM mixes had a particle size of 0.6 mm. The rest of its properties are shown in Table 3.

2. Methodology

2.2. Asphalt mixes proportions

2.1. Materials

With a view to ascertaining the impact of crumb rubber modifier (CRM) on mix resistance to plastic deformations, different quantities of CRM were added to a BBTM 11A (EN-13108-2) mix [14] by the dry process (0.5%, 1.0% and 1.5% of the total mix weight) and the wet process (8.0% and 20.0% crumb rubber of the total binder weight). The reference mix was of similar characteristics as the CRM mixes, but manufactured with BM 3c bitumen modified with thermoplastic elastomers.

2.1.1. Aggregate Two types of aggregate were used in this study: ophite for the coarse fraction of the mineral skeleton (6/12 mm) and limestone for the fine fraction (0/3 mm). Table 1 lists the aggregate properties in compliance with the Spanish PG-3 regulations [15] for BBTM 11A asphalt mixes.

Table 1 Aggregate characteristics. Test

PG-3 Limit

Coarse aggregate (6/12 mm) ophite

Fine aggregate (0/6 mm) limestone

Particle grain size (UNE-EN 933-1)

Sieves (mm) 11.2 8 4 2 0.5 0.063 620 620 100 P50

Material passing (%)

Material passing (%)

86.0 36.0 1.0 0.0 0.0 0.0 13 9 100 52

100.0 100.0 86.0 60.0 25.0 8.7 – – – –

<0.50

0.04

–

>50

–

73

– – – –

2.90 2.81 2.76 1.70

2.97 2.87 2.81 1.90

Shape of coarse aggregate. flakiness index (%) (UNE-EN 933-3) Resistance to fragmentation (Los Angeles machine test, UNE-EN1097-2) Percent of fractured face (%) (or coarse aggregate angularity), (UNE-EN 933-5) Resistance polishing (accelerated polishing coefficient (APC)), (Annex D, UNE 146130) Cleaning of coarse aggregate (organic impurity content) (%) (Annex C, UNE 146130) Sand equivalent (UNE-EN 933-8) Relative density and absorption (UNE-EN 1097-6) Apparent density (g/cm3) Apparent relative density on a saturated surface-dry basis (g/cm3) Density after drying (g/cm3) Water absorption after immersion (%)

276

F. Moreno et al. / Materials and Design 47 (2013) 274–280

Table 3 Properties of the crumb rubber added to the dry-process CRM mixes.

Table 5 Particle-size curves of the mixes (in percentage material passing).

Properties Density (g/cm3) Color Particle morphology Moisture content (%) Textile content (%) Metal content (%) Particle size (mm)

1.15 Black Irregular <0.75 <0.5 <0.1 0.600–0.063

Composition

Min. (%)

Max. (%)

Cetonic extract Natural rubber (NR) Polymers (NR/SBR) Sulfur Carbon black Ash

7.5 21.0 50.0 – 20.0 –

17.5 42.0 55.0 5.0 38.0 18.5

Sieves (mm)

Particle-size envelope (%) BBTM 11A

BBTM 11A (BM 3c, HVRMB, BC 50/70, 0.5% crumb rubber)

BBTMA 11A 1.0% crumb rubber

BBTMA 11A 1.5% crumb rubber

11.2 8 4 2 0.5 0.063

90–100 62–82 28–38 25–35 12–22 7–9

95 72 33 30 17 8

95 72 33 30 18 10

95 72 33 30 18 11

(UNE-EN 12697-26, Annex C) was also calculated for each mix (Table 6) at three different temperatures (20 °C, 40 °C and 60 °C).

3.1. Wheel-tracking test The BBTM 11A asphalt mix in this study had a discontinuous mineral skeleton mainly composed of coarse aggregate (60–65% of the total mix weight), which provided the mix with its bearing capacity. These mixes also contained fine aggregate (20–25% of the total mix weight), which, along with the bitumen, filler, and crumb rubber (except in the reference mix) made up the mortar. This mortar provided the mix with cohesion as well as resistance to tangential tensile strains and stresses. Table 4 shows the composition of the mixes analyzed. The particle size curve of the dry-process CRM mixes manufactured with 1.0% and 1.5% rubber was obtained thanks to the volumetric adjustment of the crumb rubber fractions, which in this case were 0.500 and 0.063 mm. The CRM mix with 0.5% rubber had the same particle size curve as the wet-process CRM mixes and the BM 3c asphalt mix. The low percentage of rubber made it easier to accommodate in the mineral skeleton without causing anomalies in the particle size of the BBTM 11A mix. Table 5 lists the particle size curves of the mixes tested. The optimal bitumen content for each mix was based on the results obtained in the Marshall Test (NLT 159/00) [20]. A bitumen content of 4.75% (of the mix weight) was considered optimal for the mixes made of BM 3c high-performance bitumen, wet-process rubber-modified bitumen (BC 50/70 and HVRMB), and the mix with 0.5% dry-process CRM. In contrast, for the mixes with 1.0% and 1.5% dry-process CRM, the optimal percentage of binder was 5.00% of the mix weight (the value was slightly higher because of the presence of the rubber particles). After this analysis, mix performance was evaluated by subjecting test specimens to the wheel-tracking test and the cyclic triaxial test. The stiffness modulus was also determined by means of the indirect tensile strength test. 3. Results and discussions The wheel-tracking test (UNE-EN 12697-22) and the cyclic triaxial test (UNE-EN 12697-25, Method B) were used to measure plastic deformations. The evolution of the stiffness modulus

The wheel-tracking test evaluated mix resistance to rutting by the repeated passing of a loaded wheel. In this way it was possible to analyze the effect of adding dry-process and wet-process crumb rubber to the mixes and at the same time to compare the results with those obtained for a mix with polymer-modified bitumen. The wheel-tracking test involved the manufacture of two 408 mm  256 mm prismatic specimens with a thickness of 40 mm. These samples were compacted with a device similar to a stainless steel roller to give them a minimum density of 98% of that of test cylinders manufactured according to UNE-EN 1269730 specifications by applying 50 blows on each side. Two days after the compaction process, the test specimens were allowed to adjust to a temperature of 60 ± 1 °C for 4 h, after which, they were tested at that temperature. In the wheel-tracking test, the specimens were subjected to repeated passes of a loaded wheel (10,000 load cycles). The applied load was 700 N and the frequency of the device was 26.5 load cycles per minute. During the test, the deformation depth of the specimen was measured in each cycle in order to calculate the mean wheel-tracking slope (mm for 103 load cycles) between 5000 and 10,000 cycles. Mix performance was reflected in the deformation slope produced by the repeated passing of the loaded wheel (Table 7). The results obtained showed wheel-tracking slope (WTS) values for the different mixes ranging from 0.039 to 0.093 mm/103 load cycles. These values correspond to the dry-process CRM mix with 1.5% crumb rubber and the wet-process CRM mix (BC 50/70), respectively. Despite the fact that the wet-process mix had a WTS value higher than that of the reference mix (0.050 mm/103 load cycles), its deformation resistance values indicated that it was apt for road pavement surfaces. In the wet-process CRM mixes, the effect of the crumb rubber on mix performance was clearly beneficial since the mixes with the highest crumb rubber percentages had the greatest resistance to plastic deformations (Table 7). The resistance to rutting of the HVRMB mix was very similar to that of the BM 3c mix. Nonetheless, it was possible to verify that the incorporation of crumb rub-

Table 4 Mixture proportions of the mixes. Mix BBTM BBTM BBTM BBTM BBTM BBTM

11A 11A 11A 11A 11A 11A

(BM 3c) (BC 50/70, 8.0% crumb rubber/binder) (HVRMB, 20.0% crumb rubber/binder) (dry process 0.5% crumb rubber/mix) (dry process, 1.0% crumb rubber/mix) (dry process, 1.5% crumb rubber/mix)

6/12 Ophite (%)

0/3 Limestone (%)

Filler (%)

Crumb rubber (%)

Bitumen (%)

63.82 63.82 63.82 63.82 63.82 63.82

23.82 23.82 23.82 23.82 22.57 22.07

7.61 7.61 7.61 7.61 7.61 7.61

0.00 0.38 0.95 0.50 1.00 1.50

4.75 4.75 4.75 4.75 5.00 5.00

277

F. Moreno et al. / Materials and Design 47 (2013) 274–280 Table 6 Tests applied. Mix

Tests Wheel-tracking test

Cyclic triaxial test

Stiffness modulus test

No specimens

No specimens

No specimens

Temperature (°C)

2 2 2 2 2 2 2 2 2

20 40 60 20 40 60 20 40 60 20 40 60 20 40 60 20 40 60

Temperature (°C)

Temperature (°C)

BBTM 11A (BM 3c) 2

60

2

40

2

60

2

40

2

60

2

40

2

60

2

40

2

60

2

40

2

60

2

40

BBTM 11A (BC 50/70, 8.0% crumb rubber/binder)

BBTM 11A (HVRMB, 20.0% crumb rubber/binder)

BBTM 11A (dry process 0.5% crumb rubber/mix)

BBTM 11A (dry process, 1.0% crumb rubber/mix)

BBTM 11A (dry process, 1.5% crumb rubber/mix)

Table 7 Deformation slope recorded during the wheel-tracking test. WTS (mm/103 load cycles)

Mix

BBTM BBTM BBTM BBTM BBTM BBTM

11A 11A 11A 11A 11A 11A

(BM 3c) (BC 50/70, 8.0% CR/binder) (HVRMB, 20.0% CR/binder) (dry process, 0.5% CR/mix) (dry process, 1.0% CR/mix) (dry process, 1.5% CR/mix)

Sample 1

Sample 2

Mean value

0.054 0.114 0.049 0.053 0.067 0.041

0.046 0.071 0.053 0.037 0.075 0.037

0.050 0.093 0.051 0.045 0.071 0.039

ber by the dry process was even more effective than by the wet process since some dry CRM mixes were more resistant to rutting and the rubber could be directly added to the mix without previous processing in a refinery. Fig. 1 shows that the mixes with the lowest mean final deformation values were those with the largest percentages of crumb rubber, regardless of whether it was added by the wet process or the dry process. Furthermore, the values obtained for the other CRM mixes are very close to those of the mix of reference. This is evidence that the use of crumb rubber in bituminous mixes is effective and significantly improves mix response to plastic deformation. In the same way as in previous studies [21], the addition of more wet-process crumb rubber made the asphalt mix more resis-

2 2 2 2 2 2 2 2

tant to plastic deformations in the wheel tracking test. Furthermore, it was found that the dry-process CRM mixes were also more resistant when the amount of rubber was increased. In this respect, the lowest mean WTS value (0.039 mm/103 load cycles) was obtained by the 1.5 dry-process CRM mix. 3.2. Cyclic triaxial test The cyclic triaxial compression test is used to evaluate the plastic deformations in asphalt mixes. Accordingly, this test simulates the loads transmitted by the repeated passing of vehicles. At the same time, it also evaluates the service stresses of the mix by means of confining pressure loading. The cyclic triaxial test required two test specimens with smooth flat bottom surfaces parallel to their upper surfaces. An impact compactor (UNE-EN 12697-30) was used to manufacture the specimens, which had a minimum height of 50 mm and a height/diameter ratio of 0.6. The specimens were allowed to adjust to a temperature of 40 °C for 2 h before beginning the test. The test involved the application of a confinement load of 120 kPa and another cyclic sinusoidal out-of-phase axial loading of 300 kPa at a frequency of 3 Hz during 12,000 load cycles. After these cycles, the cumulative permanent deformation in the specimen was calculated as well as its creep modulus, which relates the applied axial stress to the final cumulative plastic deformation in the test specimen.

Fig. 1. Mix final deformation in the wheel-tracking test.

278

F. Moreno et al. / Materials and Design 47 (2013) 274–280

Fig. 2. Results of the cyclic triaxial test.

Fig. 3. Permanent deformation in the wheel-tracking and triaxial tests.

Fig. 2 shows the creep modulus and permanent deformation values obtained in the cyclic triaxial text. The creep modulus values range from 310.9 MPa to168.5 MPa, whereas the mean permanent deformation values range from 1.78% to 0.97%. The highest creep modulus and the lowest plastic deformation corresponded to the1.5% dry-process CRM mix whereas the wet-process CRM mix (BC 50/70) did not respond as well to permanent deformations. Nevertheless, in both the dry-process and wet-process mixes, when more crumb rubber was added, the creep modulus value increased and permanent deformations decreased. In regards to the cyclic triaxial test and the wheel-tracking test (see Fig. 3), the results of both types of CRM mix show the same tendency. The addition of more crumb rubber to the mix (whether by the dry process or the wet process) tended to reduce plastic deformations. The difference in performance between the two mix types was not very significant when a larger amount of rubber was added. Accordingly, when the correct dosage of crumb rubber is used, the addition process does not have a significant impact on mix performance in regards to plastic deformations. A comparison of the results of the CRM mixes with those of the reference mix manufactured with high-performance bitumen reflects that the deformation levels of both CRM mix types were similar to or even lower than those of the reference mix. Thus, in the case of discontinuous mixes, such as those in this research study, the use of 1.5% crumb rubber added by the dry process and 20.0% crumb rubber added to the bitumen by the wet process can guarantee a response to plastic deformations similar to that of a mix manufactured with polymer-modified binder. This is very advantageous since these results show that by using a by-product, a mix with conventional binder can provide the same benefits as a more expensive mix made with high-performance bitumen.

3.3. Indirect tensile strength test The stiffness modulus for different service temperatures (20 °C, 40 °C and 60 °C) was calculated in order to evaluate the effect of wet-process and dry-process CRM on the bearing capacity of the mix. This has a direct impact on mix response to plastic deformations since a mix with a higher stiffness modulus has a greater bearing capacity, and is thus suffers less deformation under traffic loads. In this sense, it is crucial to analyze the effect of temperature variation since the visco-elastic nature of asphalt mixes causes their stiffness modulus to vary considerably. Their response to deformations will also vary accordingly (thus, the necessity of studying the thermal susceptibility of the mix). The stiffness modulus was determined by means of an indirect tensile strength test. The test cylinders, which had a thickness of 30–75 mm and a diameter of 101.6 ± 0.1 mm, were manufactured with an impact compactor (UNE-EN 12697-30). The specimens were stored at a temperature of 5 °C and then allowed to adjust to the test temperature (20 °C, 40 °C and 60 °C). After placing and securing the specimen in a vertical position of one of its diameters, 10 load pulses were then applied so that the device could adjust to the magnitude of the load and its duration. In order to measure the deformation of the diameter, five additional load pulses were applied to measure and record the load variation and deformation in the time period of each pulse. At the same time the surface load factor was also determined. As a final result, the stiffness modulus was obtained for two diameters of the specimen (forming an angle of 90 ± 10°). The data in Fig. 4 show that the CRM mixes (regardless of the process used) generally had a higher stiffness modulus than that of the mix with polymer-modified bitumen. This is proof that the

F. Moreno et al. / Materials and Design 47 (2013) 274–280

279

Fig. 4. Stiffness modulus of the mixes.

Table 8 Decrease in the stiffness modulus with temperature, using 20 °C as the reference value. Mix

BBTM BBTM BBTM BBTM BBTM BBTM

to plastic deformations. Despite the fact that the CRM mixes had similar values, in this case the polymer-modified binder in the reference mix was found to have lower thermal susceptibility, which indicated a more stable performance.

Stiffness modulus (decrease) (%)

11A 11A 11A 11A 11A 11A

(BM 3c) (BC 50/70, 8.0% CR/binder) (HVRMB, 20.0% CR/binder) (dry process, 0.5% CR/mix) (dry process, 1.0% CR/mix) (dry process, 1.5% CR/mix)

40 °C

60 °C

52.9 64.6 59.6 57.9 60.9 66.6

78.2 86.1 84.7 83.2 84.1 87.6

CRM mixes were less susceptible to traffic loads and experienced less deformation. Nevertheless, as the service temperature increased, the differences between the stiffness modulus became smaller. As a result, at higher temperatures, the binder had less of an effect on the bearing capacity and the mineral skeleton (which was the same for all of the mixes) increased in influence. As in previous studies [22], the wet-process CRM mixes had a lower stiffness modulus than the dry-process mixes. Furthermore, it was observed that the impact of the crumb rubber on the stiffness modulus was somewhat lower when the rubber by-product was directly mixed with the binder. Smaller dosages of CRM caused the stiffness modulus to increase, whereas larger dosages had the opposite effect and reduced the modulus value because the mix became more elastic. Nevertheless, the resilience characterizing CRM means that mixes with more rubber by-product had a good recovery from the deformations caused by traffic loads, and thus, were more resistant to plastic deformations. Accordingly, even when the mixes were slightly less stiff (and thus with a lower bearing capacity that made them more susceptible to traffic loads), this was compensated by the fact that mix response was more elastic, which meant that they recovered well from plastic deformations. As can be observed in Table 8, the thermal susceptibility of the mixes varied, depending on the process used to add the crumb rubber. In the case of the dry-process CRM mix, the thermal susceptibility of the mix increased slightly as the crumb rubber content increased. In contrast, the thermal susceptibility of the wet-process mix decreased with the addition of more CRM. This indicates that adding crumb rubber by the wet process has a greater impact on the response of the binder to temperature variation since the binder is the mix component that is most sensitive to this phenomenon. However, the thermal susceptibility of both types of CRM mix was found to be very similar. Consequently, there would be very little difference between the two mixes in regards to their response

4. Conclusions The research study described in this article is an in-depth study of the effect of crumb rubber from end-of-life tires on the response of bituminous mixes to plastic deformations. The analysis focused on the mechanical performance of mixes with different percentages of dry-process and wet-process crumb rubber. These mixes were subjected to the wheel-tracking test, the cyclic triaxial test, and the indirect tensile strength test at different temperatures. The results obtained for the CRM mixes were then compared with those obtained for a mix of the same characteristics that had been manufactured with high-performance polymer-modified bitumen. The following conclusions can be derived from this study:  The results of the plastic deformations recorded in the wheel tracking and the cyclic triaxial tests showed the same tendency when the CRM dosage was increased. In both cases, the addition of CRM improved mix resistance to plastic deformations. Therefore, lower permanent deformation values were recorded in the mixes with higher percentages of crumb rubber, regardless of whether the rubber was added by the dry process (1.5% of the mix weight) or the wet-process (20.0% crumb rubber of the total binder weight).  In both types of CRM mix, the mixes with the largest percentage of crumb rubber had similar or even better creep modulus and WTS values in comparison to those of the mix manufactured with high-performance bitumen.  The addition of CRM by the dry process was found to be more effective than by the wet process. This was reflected in the wheel-tracking test as well as in the cyclic triaxial test. In fact, the dry-process mix with 1.5% crumb rubber had lower permanent deformation values than the reference mix made with polymer-modified bitumen.  At a temperature of 20 °C, the CRM mixes had a higher stiffness modulus than the reference mix, and thus had greater bearing capacity under traffic loads. However, they were also found to have greater thermal susceptibility because when the tests were performed at a higher temperature, the stiffness values were similar for all the mixes analyzed.  The dry-process CRM mixes had a higher stiffness modulus than the wet-process mixes. In both cases, the mixes with the largest dosages of crumb rubber were more elastic than the CRM mixes

280

F. Moreno et al. / Materials and Design 47 (2013) 274–280

with smaller dosages. Because of the resilience of the rubber, they were better able to recover from permanent deformation. Moreover, the stiffness modulus values of mixes with more crumb rubber were higher than those of the reference mix made with high-performance bitumen.  Based on the results obtained, it was found that the addition of a minimum quantity of crumb rubber (1.5% by the dry process and 20.0% by the wet process) guaranteed a better mix response to plastic deformation than the use of a polymer-modified binder. In this regard, the best results were obtained for the dryprocess CRM mix though the other CRM mixes had a resistance to permanent deformations that made them acceptable for use in road pavement surfaces in compliance with Spanish PG-3 regulations [15]. Accordingly, the reuse of rubber from used tires not only improves the environment by reducing deposits at landfills but also gives asphalt mixes added value by enhancing their properties.

References [1] Elliot RP, Ford MC, Ghanim M, Tu YF. Effect of aggregate gradation variation on asphalt concrete mix properties. Transp Res Rec 1991:1317. [2] Harvey J, Monismith CL. Effect of asphalt concrete specimen preparation variables on fatigue and permanent deformation test results using strategic highway research program A-003A proposed testing equipment. Transp Res Rec 1993:1417. [3] Del Río Prat M, Sánchez Alonso E, Vega Zamanillo A, Castro Fresno D. Effects of aggregate shape and size and surfactants on the resilient modulus of bituminous mixes. Can J Civ Eng 2011;38(8):893–9. ISSN 0315-1468, (Canada). [4] Corté JF, Brosseaud Y, Simoncelli JP, Caroff G. Investigation of rutting of asphalt surface layers: influence of binder and axle loading configuration. Transp Res Rec 1994:1436. [5] Potti JJ. Betún modificado con polvo de caucho de neumáticos fuera de uso. Otro tipo de reciclaje. III Congreso Rodoviário Portugués. Lisboa, 2004. [6] Raad L, Saboundjian S, Minassian G. Field aging effects on the fatigue of asphalt concrete and asphalt-rubber concrete. Transp Res Rec. Asphalt Mixtures 2001:1767.

[7] Soon JL, Akisetty C, Amirkhanian SN. The effect of crumb rubber modifier (CRM) on the performance properties of rubberized binders in HMA pavements. Constr Build Mater 2008;22:1368–76. [8] Topçu IB, Bilir T. Experimental investigation of some fresh and hardened properties of rubberized self-compacting concrete. Mater Des 2009;30:3056–65. [9] Turatsinze A, Garros M. On the modulus of elasticity and strain capacity of selfcompacting concrete incorporating rubber aggregates. Resour Conserv Recycl 2008;52:1209–15. [10] Hicks RG, Epps JA. Life cycle cost analysis of asphalt-rubber paving materials. Final Report Volumes I and II to Rubber Pavements Association, Tempe, Arizona; 1999. [11] McNerney M, Landsberger BJ, Turen T, Pandelides A. Comparative field measurements of tyre/pavement noise of selected Texas pavements. Report No. FHWA/TX-7-2957-2. April 2000. [12] CEDEX (Centro de Estudios y Experimentación de Obras Públicas). Manual de empleo de caucho de NFU en mezclas bituminosas. Ministerio de Fomento, Ministerio de Medio Ambiente. Mayo de 2007. [13] State of California Department of Transportation. Asphalt rubber usage guide. Mater Eng Test Services. September 30, 2006. [14] Asociación Española de Normalización y Certificación (AENOR): Manual de normas técnicas UNE-EN. Serie Construcción. Métodos de ensayo para mezclas bituminosas en caliente. Madrid; 2006. [15] Dirección General de Carreteras: Pliego de prescripciones técnicas generales para obras de carreteras y puentes PG-3. Ministerio de Fomento, EdicionesLiteam, Madrid; 2010. [16] Christensen DW., Bonaquist, R., Jack, D. Evaluation of triaxial strength as a simple test for asphalt concrete rut resistance. Final Report. Pennsylvania Transportation Institute, The Pennsylvania State University, University Park, PA; 2002. [17] Huschek S. The deformation behavior of asphaltic concrete under triaxial compression. Proc Assoc Asphalt Paving Technol 1985;54:407–26. [18] Dawn Baumgartner E. Triaxial frequency sweep characterization for dense graded hot mix asphalt concrete mix design. In: Proceedings of the University of Saskatchewan, Saskatoon; 2005. [19] Pellinen TK, Song J, Xiao S. Characterization of hot mix asphalt with varying air voids content using triaxial shear strength test. In: Proceedings of the 8th conference on asphalt pavements for southern, Africa (CAPSA’04); September 2004. [20] Dirección General de Carreteras: Normas NLT. Ensayos de carreteras. Ministerio de Obras Públicas y Transportes (MOPT), 2ª Edición, Madrid; 1992. [21] Fontes L, Trichês G, Pais JC, Pereira P. Evaluating permanent deformation in asphalt rubber mixtures. Constr Build Mater 2010;24:1193–200. [22] Moreno F, Rubio MC. Influence of crumb rubber on the indirect tensile strength and stiffness modulus of hot bituminous mixes. J Mater Civ Eng 2012;23(6):715–24.