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A laboratory study on stone matrix asphalt using ground tire rubber
Construction and Building
MATERIALS
Construction and Building Materials 21 (2007) 1027–1033
www.elsevier.com/locate/conbuildmat
A laboratory study on stone matrix asphalt using ground tire rubber Chui-Te Chiu *, Li-Cheng Lu Department of Civil Engineering, Chung Hua University, 707 Sec 2, WuFu Road, Hsin Chu 300, Taiwan, ROC Received 7 January 2006; received in revised form 9 February 2006; accepted 27 February 2006 Available online 2 May 2006
Abstract This research investigated the feasibility using asphalt rubber (AR), produced by blending ground tire rubber (GTR) with an asphalt, as a binder for stone matrix asphalt (SMA). Two different sizes of GTR produced in Taiwan were used. The potential performance of AR–SMA mixtures was also evaluated. The results of this study showed that it was not feasible to produce a suitable SMA mixture using an asphalt rubber made by blending an AC-20 with 30% coarse GTR with a maximum size of 0.85 mm. However, SMA mixtures meeting typical volumetric requirements for SMA could be produced using an asphalt rubber containing 20% of a fine GTR with a maximum size of 0.6 mm. No fiber was needed to prevent drain-down when this asphalt rubber was used. The AR–SMA mixtures were not significantly different from the conventional SMA mixtures in terms of moisture susceptibility from the results of AASHTO T283 tests. The results of the wheel tracking tests at 60 C show that rutting resistance of AR–SMA mixtures was better than that of the conventional SMA mixtures. 2006 Elsevier Ltd. All rights reserved. Keywords: Ground tire rubber; Asphalt rubber; Stone matrix asphalt; Rutting
1. Introduction Based on many research reports and engineering case studies [1–3], it has been shown that the use of SMA on road surfaces can achieve better rut-resistance and durability. The SMA mixtures are designed to have high coarse aggregate content, high asphalt content, and high filler content. For ordinary SMA, the use of regular asphalt cement together with fibrous material as a drainage inhibitor is sufficient. Under high temperatures and heavy loading, a harder asphalt grade will also suffice. A polymer (such as EVA or SBS) modified binder may be used to substitute the fibrous material. It is possible to increase the capability of resistance to permanent deformation at the expense of a higher price and greater instability. The demand for higher pavement quality from users is ever-increasing. The cost of a pavement failure is also mounting higher. Hence, there is
*
Corresponding author. Tel.: +886 35186715; fax: +886 35372188. E-mail address: [email protected] (C.-T. Chiu).
0950-0618/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2006.02.005
a strong desire to have better asphalt mixture from highway agencies. The use of modified asphalt has been formally enlisted in Chinese National Standard (CNS). In recent highway maintenance projects in Taiwan, we also found cases where SMA, porous asphalt were adopting both modified asphalt and fibrous material [4]. The use of polymer modified asphalt demands a high price. Many articles also showed the potential of using asphalt rubber (AR) as a replacement [5]. In the state of Arizona of USA [6], the binder content was as high as 8.5% when used as a friction layer in an open-graded design. Without using any fibrous material, there is never a problem of excessive drain down of binder. Our study also verified this observation. The use of AR provides better drainage suppression than modified asphalt. It also provides better cost benefits due to the avoidance of using fibrous materials. However, there are currently insufficient reports and guidance on the proportioning of the mix ingredients and its possible benefits. This study addresses the issue of mix designs of AR–SMA and the evaluation of the potential performance of AR–SMA mixtures.
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C.-T. Chiu, L.-C. Lu / Construction and Building Materials 21 (2007) 1027–1033
2. Experiments This research was conducted in two stages. First, mix designs of SMA using an asphalt rubber as binder were performed. Then, the AR–SMAs were evaluated for their potential performance. The mix design and the evaluation procedures used are described in this section. 2.1. Mix proportioning for SMA and AR–SMA The ground tire rubber (GTR) was acquired from Taiwan EPAs waste tire grinding facility. Their production process provided GTRs of two different sizes – a coarse GTR and a fine GTR. The coarse GTR passed the US standard sieve #20 (0.85 mm), while the fine GTR passed the #30 sieve (0.60 mm). The particle size distribution according to ASTM D5644 and the chemical compound composition according to ASTM D297 for these two GTRs are listed in Table 1. From Table 1, we can see that
Table 1 Properties of the two ground tire rubber used in this study Item
Denotation of ground tire rubber
Sieve no. (size, mm)
Coarse GTR
Fine GTR
Sieve analysis No. 10 (2.000) No. 16 (1.180) No. 20 (0.850) No. 30 (0.600) No. 40 (0.425) No. 50 (0.300) No. 80 (0.180) No. 100 (0.150) No. 200 (0.075)
100 97.7 62.6 24.3 8.1 3.3 0.9 0.5 0.0
100 100 100 97.7 61.1 33.9 12.5 7.5 0.0
Specific gravity
1.169
1.161
Chemical components Major rubber components Acetone extract (%) Rubber hydrocarbon (%) Carbon black content (%) Ash content (%) Natural rubber content (%)
NR/BR/SBR 11.5 51.5 31.0 6.0 26.0
NR/BR/SBR 11.0 50.5 32.5 6.0 34.0
**Not
available.
the specific gravity and chemical compound composition for these two GTRs are very close to one another. The two GTRs differ from one another mainly in their particle sizes. The mix design procedure for SMA as proposed in NCHRP Report No. 425 [7] was followed in performing the mix designs to be used. Locally available materials that meet the normal SMA specifications were used to produce the reference mix. An AC-20 asphalt and 0.4% mineral fiber were used in this reference mix. Different amounts of coarse and fine GTRs were blended with an AC-20 and tested to determine the proper proportion of GTR to produce asphalt rubber blends that would meet the requirements of ASTM D6114 specifications. It was found that, when the coarse GTR was used, 30% GTR by weight of the asphalt was needed in order to meet ASTM D6114s requirement on apparent viscosity at 175 C, as shown in Table 2. When the fine GTR was used, only 20% GTR was needed. Thus, the blending proportions of 30% and 20% were used for the coarse and the fine GTRs, respectively, to produce the asphalt rubbers used in this study. The asphalt rubbers were used as a binder, in place of the AC-20 of the reference mix, to produce the AR– SMA mixes. Due to the high viscosity of the asphalt rubber, no fiber was used in the AR–SMA mixes. Mix proportions were adjusted based on the volumetric properties of the 50-blow Marshall specimens. If the volumetric properties as required in the SMA specifications were not met, the mineral filler and/or sand composition of the SMA mixes would be adjusted, and new Marshall specimens would be made and tested until all the requirements were met. Fig. 1 shows a flow chart of the steps used to design the AR–SMA mixes to be used in this study. 2.2. Performance tests used We adopted two performance tests in the laboratory. The tests performed were moisture susceptibility and rutting resistance tests. The moisture susceptibility test according to AASHTO T283 procedure was performed
Table 2 Properties of asphalt rubber together with those listed in ASTM D6114 Binder designation
Apparent viscosity, 175 C: cP Penetration, 25 C, 100 g, 5 s: 0.1 mm Penetration, 4 C, 200 g, 60 s: 0.1 mm Softening point: C Resilience, 25 C: % (ASTM D5329) Recovery, 25 C: % (ASTM D6084) Flash point: C Thin-film oven test residue, penetration retention, 4 C: % of original a
Not available.
ASTM D6114
This study
Type I
Type II
Type III
AR (fine)
AR (coarse)
1500–5000 25–75 10 57.2 25 – 232.3 75
1500–5000 25–75 15 54.4 20 – 232.2 75
1500–5000 50–100 25 51.7 10 – 232.2 75
1762 51.7 26.3 56.9 – 57 >232.3 99
2581 –a – – – – – –
C.-T. Chiu, L.-C. Lu / Construction and Building Materials 21 (2007) 1027–1033
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Fig. 1. Flow chart of steps used to design AR–SMA.
on the four SMA mixes, which were compacted to an average air void content of 6.0%. We prepared four Marshall specimens for the dry group and four specimens for the wet group. A tensile strength ratio (TSR) of wet group to dry group, was computed from the results of the indirect
tensile strength test at 25 C. The higher the TSR value, the less the strength should be influenced by the water soaking condition, or the more water-resistant it should be. Normal SMA specification requires a TSR value of 70% or more.
Fig. 2. Sketch of compaction device for preparing samples for wheel tracking test.
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Rutting resistance test was done according to the wheel tracking test outlined in section 3-7-3 of the Japanese ‘‘Road Paving Test Regulation’’ for the four SMA mixes and the dense-graded mix. In this test, the sample was a square slab with a side 30 cm long and 5 cm thick. Fig. 2 shows the compaction device used to prepare the square slab specimen and Fig. 3 shows the sketch of wheel tracking device used in this study. The specified air voids content of the slab specimen was controlled by controlling the amount of mixture in the specified steel mold. The target air voids content for the slab specimens was 4%. The finished slab was allowed to cool at room temperature for 12 h. It was then put into an oven at 60 C for another 5 h before being placed into the wheel tracking device as shown in Fig. 4. The wheel tracking device was maintained at 60 C. The loading wheel was a piece of solid, hard rubber, 20 cm in diameter, with a width of 5 cm. The wheel weighed 70 kgf and traveled back and forth at 21 rounds per minute. This is equivalent to pressing the slab 42 times
Fig. 5. Gradation curves of the three aggregate blends used in this study.
per minute. We then recorded the depth of the track depression (rut depth) at various times. Calculation of the rate of rutting (RR) was based on a time interval between 45 and 60 min as shown in Fig. 5. The RR can be computed as (d2 d1) ‚ 15 with a unit of mm/min. A lower RR means better resistance to permanent deformation. For each of the five kinds of mixtures evaluated, we made three samples and took their averages to represent the rut resistance for each mixture. 3. Experimental results and discussion 3.1. Results from the mix proportioning study
Fig. 3. Sketch of wheel tracking device used in this study.
Fig. 4. Method of determining rate of rutting on a typical plot of rut depth data.
Drainage test using the wire basket method as proposed in the NCHRP Report No. 425 [7], was run on all the mixes evaluated. In this test, the laboratory-prepared loose mix was placed in a wire basket with a plate underneath it and placed in a forced draft oven for 1 h at a pre-selected temperature. At the end of 1 h, the basket containing the sample is removed from the oven along with the plate and the mass of the plate was determined. The amount of increased weight of the plate is the amount of draindown of the mix. As specified in NCHRP [7], the oven temperature for performing the drainage test should be at the mixing temperature and/or the mixing temperature plus 15 C. Thus, an oven temperature of 170 C was used for the two reference SMA mixes, and an oven temperature of 190 C was use for the two AR–SMA mixes. All the SMA mixtures in this study passed the drainage test. The reference mix was designed following the procedure proposed by NCHRP Report No. 425 [7] by using an AC20 asphalt and an aggregate blend made up of a 13 mm nominal maximum size coarse aggregate (80%), sand (11%), and mineral filler (9%). The gradation plot of the aggregate blend used in the reference mix is shown as ‘‘SMA13’’ in Fig. 5. A dosage of 0.4% fiber by weight of the binder was used to prevent drain-down. The reference mix was prepared at different binder contents and 50-blow Marshall specimens were made and evaluated for their volumetric properties. The plots of air voids (Va), voids in mineral aggregate (VMA) and volume of coarse aggregate
C.-T. Chiu, L.-C. Lu / Construction and Building Materials 21 (2007) 1027–1033
(VCA) versus binder content of the reference mix are shown in Fig. 6. It can be seen that the optimum binder content occurred at 6.5% when the air voids is equal to 4%, VMA is equal to 17% and VCA is equal to 36%. An AR–SMA mix was first designed using an asphalt rubber containing 30% coarse GTR and using the same aggregate blend as the reference mix. Similarly, this AR– SMA Mix was prepared at different binder contents and 50-blow Marshall specimens were made and evaluated for their volumetric properties. The plots of Va, VMA and VCA versus binder content of this mix are shown in Fig. 6 as ‘‘#20AR–SMA’’. It can be seen that at all binder contents, the air voids were all much greater than 4%, and the VCA of the mix was greater than the VCA of the dry rodded coarse aggregate, which is also shown on the plot. This indicates that the addition of coarse rubber powder has caused the loss of coarse aggregate structure. The coarse aggregates were pushed apart, resulting in high VCA and air voids. A second AR–SMA mix using the AR containing the coarse GTR was tried out by reducing the amount of mineral filler from 9% to 4.5%. The aggregate blend for this mix contained 84.5% coarse aggregate, 11% sand and 4.5% mineral filler. The plots of Va, VMA and VCA versus binder content of this mix are shown in Fig. 6 as ‘‘#20AR– SMA, half filler’’. It can be seen that at all binder contents, the air voids were still much greater than 4%. A third AR–SMA using the AR with the coarse GTR was experimented by reducing the sand content from 11% to 5.5%. The aggregate blend for this mix contained 85.5% coarse aggregate, 5.5% sand and 9% mineral filler. The plots of Va, VMA and VCA versus binder content
1031
of this mix are shown in Fig. 6 as ‘‘#20AR–SMA, half sand’’. It can be seen that at all binder contents, the air voids were still much greater than 4%. The results from three trial mixes using the AR containing 30% coarse GTR indicated that it was not feasible to produce satisfactory SMA mixes using the coarse GTR. An AR–SMA mix using an AR containing 20% fine GTR was designed using a similar procedure. This mix used the same aggregate blend as the reference mix. The plots of Va, VMA and VCA versus binder content of this mix are shown in Fig. 6 as ‘‘#30AR–SMA’’. It can be seen that the Va and VMA of this mix matched fairly closely to those of the reference mix, which is denoted by SMA13. The optimum binder content occurred at 6.7% when the air voids is equal to 4%. The VMA of this mix at the optimum binder content is 18% which is about 1% higher than that of the reference mix. To evaluate the effects of aggregate source, another aggregate with a nominal maximum aggregate size of 19 mm was used to design another reference SMA mix for this study. The gradation plot of this aggregate is shown in Fig. 5 as ‘‘SMA19’’. The optimum binder content of this mix was 6.4%. An AR–SMA mix using an AR containing 20% fine GTR was also designed satisfactorily. Its optimum binder content was 7.0%. The two designed SMA mixes and the two designed AR–SMA mixes using fine GTR were evaluated for their water susceptibility and rutting performance. A typical dense-graded mix was also produced and evaluated for its rutting resistance for comparison purpose. The composition and volumetric properties of these five mixes evaluated are shown in Table 3.
Fig. 6. Volumetric properties of mixtures obtained in the mix proportioning study.
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Table 3 Asphalt mixtures evaluated for performance in the laboratory Aggregate source
Tai-Nan
Mix designation NMSa (mm) Binder Drainage inhibitor Binder content (%) Va (%) VMA (%) VCA mix
SMA13 13 AC-20 Mineral fiber 6.5 4.1 17.1 36.1
a b
Yi-Lan AR–SMA13 13 AR (Fine) Not used 6.7 4.0 17.9 31.5
SMA19 19 AC-20 Mineral fiber 6.4 4.0 17.2 34.3
Tao-Yan AR–SMA19 19 AR (Fine) Not used 7.0 4.0 17.1 35.2
DGAC13 13 AC-10 Not used 5.2 4.2 14.1 NAb
Nominal maximum size. Not available.
3.2. Moisture susceptibility test results The results of moisture susceptibility are shown in Fig. 7. The indirect tensile strength of all dry groups averaged at around 50 psi and was lower than that for dense graded mixtures, which is usually around 100 psi. This was in agreement with previous findings that the ‘‘unconfined strength test value’’ from a gap graded mixture is lower than that of a dense graded sample [1]. The TSR value, which represents the moisture susceptibility, appears to separate into two groups according to their source of aggregate. The two SMA mixtures made with aggregates from Tai-Nan (identified as SMA13 & AR–SMA13) had TSR values greater than 80%. The other two SMA mixtures made with aggregates from Yi-Lan (identified as SMA19 & AR–SMA19) had TSR values between 70% and 75% and were close to the lower limit required by a typical SMA specification. The AR–SMA mixture using aggregates from Yi-Lan (AR–SMA19) had a higher TSR value than that for its reference SMA mixture (SMA19). However, for the AR–SMA mixtures using aggregates from Tai-Nan, the AR–SMA mixture (AR– SMA13) had a slightly lower TSR value than that for its reference SMA mixture (SMA13). From these results, we can conclude that the use of asphalt rubber does not significantly affect the moisture susceptibility of the SMA mixtures.
Fig. 8. Rate of rutting of different mixes evaluated in wheel tracking test.
3.3. Rutting resistance test results Fig. 8 shows the wheel tracking test results from five types of asphalt mixtures. The dense graded mixture (DAGC13), which was tested for comparison purpose, had a rate of rutting (RR) of 0.123 mm/min. This is very close to the value reported in many prior references. The RR values of the other four mixtures were all lower than that for DAGC13. These results met our expectation for the SMA mixtures. All four RR values were below 0.10 mm/min. From Fig. 8, we can observe distinct differences due to the source of aggregate and whether or not an asphalt rubber was used. The SMA mixtures using aggregates from Yi-Lan had smaller RR values than the ones using aggregates from Tai-Nan. This difference could also due to the difference in the aggregate particle size. Within the same source of aggregate, the AR–SMA mixture showed lower RR values than the conventional SMA mixture. Thus, we can conclude that the use of asphalt rubber can improve the rutting resistance of a SMA. 4. Conclusions
Fig. 7. Moisture susceptibility test results.
The main conclusions from this study are summarized as follows:
C.-T. Chiu, L.-C. Lu / Construction and Building Materials 21 (2007) 1027–1033
(1) In order to meet ASTM D6114s requirement on apparent viscosity at 175 C for asphalt rubber, an AC-20 asphalt needs to be blended with 30% (by weight of the total binder) of a coarse GTR with a maximum size of #20 sieve. To meet the same requirement, an AC-20 needs to be blended with only 20% of a fine GTR with a maximum size of #30 sieve. (2) It was not feasible to produce a suitable SMA mixture using an asphalt rubber made by blending an AC-20 with 30% coarse GTR with a maximum size of #20 sieve. The coarse aggregate structure appeared to be destroyed when this coarse GTR was used. (3) SMA mixtures meeting typical volumetric requirements for SMA could be produced using an asphalt rubber containing 20% of a fine GTR with a maximum size of #30 sieve. No fiber was needed to prevent drain-down when this asphalt rubber was used. No adjustment in aggregate gradation was needed to be made for this mixture. (4) The AR–SMA mixtures were not significantly different from the conventional SMA mixtures in terms of moisture susceptibility from the results of AASHTO T283 tests. (5) The results of the Wheel Tracking Tests at 60 C show that rutting resistance of SMA mixtures were much better than that of a conventional dense-graded mixture. The AR–SMA mixtures had better rutting resistance than the conventional SMA mixtures.
1033
Acknowledgements This research was part of a project (Project No. NSC912211-E-216-015) supported by the National Science Council of Taiwan. Special thanks are due to Jang Jung Paving Company for providing the necessary help in setting up the wheel tracking device. References [1] Heavy duty surfaces: the arguments for SMA, European asphalt pavement association, P.O. Box 175, 3620 AD Breukelen, The Netherlands, 1998. [2] Highway Technet, technology application, stone upon stone put to the test, US department of transportation, federal highway administration, 1998. [3] Brown ER, Haddock JE, Mallick RB, Bukowski J. Performance of stone matrix asphalt (sma) in the US, AAPT vol. 66, 1997. [4] Chui-Te Chiu, Chang-Lin Pan. The efforts of using ground tire rubber on pavements in Taiwan, beneficial use of recycled materials in transportation applications, Arlington (VA), November 13–15, 2001, Air and Waste Management Association, VIP-114, p. 641–50. [5] Leni Figueiredo Mathias Leite, Romulo Santos Constantino, Alexander Vivoni. Rheological studies of asphalt with ground tire rubber. In: Proceedings for asphalt rubber 2000, Vilamoura, Marinotel, Portugal, November 14–17, 2000, p. 421–34. [6] Gene R. Morris, Douglas D Carlson. The apparent unique behavior of thin asphalt rubber hot-mix overlays in Arizona, beneficial use of recycled materials in transportation applications, Arlington (VA), November 13–15, 2001, Air and Waste Management Association, VIP114, p. 651–66. [7] Brown ER, Cooley LA, Designing stone matrix asphalt mixtures for rut-resistance pavement, NCHRP report 425, 1999.
MATERIALS
Construction and Building Materials 21 (2007) 1027–1033
www.elsevier.com/locate/conbuildmat
A laboratory study on stone matrix asphalt using ground tire rubber Chui-Te Chiu *, Li-Cheng Lu Department of Civil Engineering, Chung Hua University, 707 Sec 2, WuFu Road, Hsin Chu 300, Taiwan, ROC Received 7 January 2006; received in revised form 9 February 2006; accepted 27 February 2006 Available online 2 May 2006
Abstract This research investigated the feasibility using asphalt rubber (AR), produced by blending ground tire rubber (GTR) with an asphalt, as a binder for stone matrix asphalt (SMA). Two different sizes of GTR produced in Taiwan were used. The potential performance of AR–SMA mixtures was also evaluated. The results of this study showed that it was not feasible to produce a suitable SMA mixture using an asphalt rubber made by blending an AC-20 with 30% coarse GTR with a maximum size of 0.85 mm. However, SMA mixtures meeting typical volumetric requirements for SMA could be produced using an asphalt rubber containing 20% of a fine GTR with a maximum size of 0.6 mm. No fiber was needed to prevent drain-down when this asphalt rubber was used. The AR–SMA mixtures were not significantly different from the conventional SMA mixtures in terms of moisture susceptibility from the results of AASHTO T283 tests. The results of the wheel tracking tests at 60 C show that rutting resistance of AR–SMA mixtures was better than that of the conventional SMA mixtures. 2006 Elsevier Ltd. All rights reserved. Keywords: Ground tire rubber; Asphalt rubber; Stone matrix asphalt; Rutting
1. Introduction Based on many research reports and engineering case studies [1–3], it has been shown that the use of SMA on road surfaces can achieve better rut-resistance and durability. The SMA mixtures are designed to have high coarse aggregate content, high asphalt content, and high filler content. For ordinary SMA, the use of regular asphalt cement together with fibrous material as a drainage inhibitor is sufficient. Under high temperatures and heavy loading, a harder asphalt grade will also suffice. A polymer (such as EVA or SBS) modified binder may be used to substitute the fibrous material. It is possible to increase the capability of resistance to permanent deformation at the expense of a higher price and greater instability. The demand for higher pavement quality from users is ever-increasing. The cost of a pavement failure is also mounting higher. Hence, there is
*
Corresponding author. Tel.: +886 35186715; fax: +886 35372188. E-mail address: [email protected] (C.-T. Chiu).
0950-0618/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2006.02.005
a strong desire to have better asphalt mixture from highway agencies. The use of modified asphalt has been formally enlisted in Chinese National Standard (CNS). In recent highway maintenance projects in Taiwan, we also found cases where SMA, porous asphalt were adopting both modified asphalt and fibrous material [4]. The use of polymer modified asphalt demands a high price. Many articles also showed the potential of using asphalt rubber (AR) as a replacement [5]. In the state of Arizona of USA [6], the binder content was as high as 8.5% when used as a friction layer in an open-graded design. Without using any fibrous material, there is never a problem of excessive drain down of binder. Our study also verified this observation. The use of AR provides better drainage suppression than modified asphalt. It also provides better cost benefits due to the avoidance of using fibrous materials. However, there are currently insufficient reports and guidance on the proportioning of the mix ingredients and its possible benefits. This study addresses the issue of mix designs of AR–SMA and the evaluation of the potential performance of AR–SMA mixtures.
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C.-T. Chiu, L.-C. Lu / Construction and Building Materials 21 (2007) 1027–1033
2. Experiments This research was conducted in two stages. First, mix designs of SMA using an asphalt rubber as binder were performed. Then, the AR–SMAs were evaluated for their potential performance. The mix design and the evaluation procedures used are described in this section. 2.1. Mix proportioning for SMA and AR–SMA The ground tire rubber (GTR) was acquired from Taiwan EPAs waste tire grinding facility. Their production process provided GTRs of two different sizes – a coarse GTR and a fine GTR. The coarse GTR passed the US standard sieve #20 (0.85 mm), while the fine GTR passed the #30 sieve (0.60 mm). The particle size distribution according to ASTM D5644 and the chemical compound composition according to ASTM D297 for these two GTRs are listed in Table 1. From Table 1, we can see that
Table 1 Properties of the two ground tire rubber used in this study Item
Denotation of ground tire rubber
Sieve no. (size, mm)
Coarse GTR
Fine GTR
Sieve analysis No. 10 (2.000) No. 16 (1.180) No. 20 (0.850) No. 30 (0.600) No. 40 (0.425) No. 50 (0.300) No. 80 (0.180) No. 100 (0.150) No. 200 (0.075)
100 97.7 62.6 24.3 8.1 3.3 0.9 0.5 0.0
100 100 100 97.7 61.1 33.9 12.5 7.5 0.0
Specific gravity
1.169
1.161
Chemical components Major rubber components Acetone extract (%) Rubber hydrocarbon (%) Carbon black content (%) Ash content (%) Natural rubber content (%)
NR/BR/SBR 11.5 51.5 31.0 6.0 26.0
NR/BR/SBR 11.0 50.5 32.5 6.0 34.0
**Not
available.
the specific gravity and chemical compound composition for these two GTRs are very close to one another. The two GTRs differ from one another mainly in their particle sizes. The mix design procedure for SMA as proposed in NCHRP Report No. 425 [7] was followed in performing the mix designs to be used. Locally available materials that meet the normal SMA specifications were used to produce the reference mix. An AC-20 asphalt and 0.4% mineral fiber were used in this reference mix. Different amounts of coarse and fine GTRs were blended with an AC-20 and tested to determine the proper proportion of GTR to produce asphalt rubber blends that would meet the requirements of ASTM D6114 specifications. It was found that, when the coarse GTR was used, 30% GTR by weight of the asphalt was needed in order to meet ASTM D6114s requirement on apparent viscosity at 175 C, as shown in Table 2. When the fine GTR was used, only 20% GTR was needed. Thus, the blending proportions of 30% and 20% were used for the coarse and the fine GTRs, respectively, to produce the asphalt rubbers used in this study. The asphalt rubbers were used as a binder, in place of the AC-20 of the reference mix, to produce the AR– SMA mixes. Due to the high viscosity of the asphalt rubber, no fiber was used in the AR–SMA mixes. Mix proportions were adjusted based on the volumetric properties of the 50-blow Marshall specimens. If the volumetric properties as required in the SMA specifications were not met, the mineral filler and/or sand composition of the SMA mixes would be adjusted, and new Marshall specimens would be made and tested until all the requirements were met. Fig. 1 shows a flow chart of the steps used to design the AR–SMA mixes to be used in this study. 2.2. Performance tests used We adopted two performance tests in the laboratory. The tests performed were moisture susceptibility and rutting resistance tests. The moisture susceptibility test according to AASHTO T283 procedure was performed
Table 2 Properties of asphalt rubber together with those listed in ASTM D6114 Binder designation
Apparent viscosity, 175 C: cP Penetration, 25 C, 100 g, 5 s: 0.1 mm Penetration, 4 C, 200 g, 60 s: 0.1 mm Softening point: C Resilience, 25 C: % (ASTM D5329) Recovery, 25 C: % (ASTM D6084) Flash point: C Thin-film oven test residue, penetration retention, 4 C: % of original a
Not available.
ASTM D6114
This study
Type I
Type II
Type III
AR (fine)
AR (coarse)
1500–5000 25–75 10 57.2 25 – 232.3 75
1500–5000 25–75 15 54.4 20 – 232.2 75
1500–5000 50–100 25 51.7 10 – 232.2 75
1762 51.7 26.3 56.9 – 57 >232.3 99
2581 –a – – – – – –
C.-T. Chiu, L.-C. Lu / Construction and Building Materials 21 (2007) 1027–1033
1029
Fig. 1. Flow chart of steps used to design AR–SMA.
on the four SMA mixes, which were compacted to an average air void content of 6.0%. We prepared four Marshall specimens for the dry group and four specimens for the wet group. A tensile strength ratio (TSR) of wet group to dry group, was computed from the results of the indirect
tensile strength test at 25 C. The higher the TSR value, the less the strength should be influenced by the water soaking condition, or the more water-resistant it should be. Normal SMA specification requires a TSR value of 70% or more.
Fig. 2. Sketch of compaction device for preparing samples for wheel tracking test.
1030
C.-T. Chiu, L.-C. Lu / Construction and Building Materials 21 (2007) 1027–1033
Rutting resistance test was done according to the wheel tracking test outlined in section 3-7-3 of the Japanese ‘‘Road Paving Test Regulation’’ for the four SMA mixes and the dense-graded mix. In this test, the sample was a square slab with a side 30 cm long and 5 cm thick. Fig. 2 shows the compaction device used to prepare the square slab specimen and Fig. 3 shows the sketch of wheel tracking device used in this study. The specified air voids content of the slab specimen was controlled by controlling the amount of mixture in the specified steel mold. The target air voids content for the slab specimens was 4%. The finished slab was allowed to cool at room temperature for 12 h. It was then put into an oven at 60 C for another 5 h before being placed into the wheel tracking device as shown in Fig. 4. The wheel tracking device was maintained at 60 C. The loading wheel was a piece of solid, hard rubber, 20 cm in diameter, with a width of 5 cm. The wheel weighed 70 kgf and traveled back and forth at 21 rounds per minute. This is equivalent to pressing the slab 42 times
Fig. 5. Gradation curves of the three aggregate blends used in this study.
per minute. We then recorded the depth of the track depression (rut depth) at various times. Calculation of the rate of rutting (RR) was based on a time interval between 45 and 60 min as shown in Fig. 5. The RR can be computed as (d2 d1) ‚ 15 with a unit of mm/min. A lower RR means better resistance to permanent deformation. For each of the five kinds of mixtures evaluated, we made three samples and took their averages to represent the rut resistance for each mixture. 3. Experimental results and discussion 3.1. Results from the mix proportioning study
Fig. 3. Sketch of wheel tracking device used in this study.
Fig. 4. Method of determining rate of rutting on a typical plot of rut depth data.
Drainage test using the wire basket method as proposed in the NCHRP Report No. 425 [7], was run on all the mixes evaluated. In this test, the laboratory-prepared loose mix was placed in a wire basket with a plate underneath it and placed in a forced draft oven for 1 h at a pre-selected temperature. At the end of 1 h, the basket containing the sample is removed from the oven along with the plate and the mass of the plate was determined. The amount of increased weight of the plate is the amount of draindown of the mix. As specified in NCHRP [7], the oven temperature for performing the drainage test should be at the mixing temperature and/or the mixing temperature plus 15 C. Thus, an oven temperature of 170 C was used for the two reference SMA mixes, and an oven temperature of 190 C was use for the two AR–SMA mixes. All the SMA mixtures in this study passed the drainage test. The reference mix was designed following the procedure proposed by NCHRP Report No. 425 [7] by using an AC20 asphalt and an aggregate blend made up of a 13 mm nominal maximum size coarse aggregate (80%), sand (11%), and mineral filler (9%). The gradation plot of the aggregate blend used in the reference mix is shown as ‘‘SMA13’’ in Fig. 5. A dosage of 0.4% fiber by weight of the binder was used to prevent drain-down. The reference mix was prepared at different binder contents and 50-blow Marshall specimens were made and evaluated for their volumetric properties. The plots of air voids (Va), voids in mineral aggregate (VMA) and volume of coarse aggregate
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(VCA) versus binder content of the reference mix are shown in Fig. 6. It can be seen that the optimum binder content occurred at 6.5% when the air voids is equal to 4%, VMA is equal to 17% and VCA is equal to 36%. An AR–SMA mix was first designed using an asphalt rubber containing 30% coarse GTR and using the same aggregate blend as the reference mix. Similarly, this AR– SMA Mix was prepared at different binder contents and 50-blow Marshall specimens were made and evaluated for their volumetric properties. The plots of Va, VMA and VCA versus binder content of this mix are shown in Fig. 6 as ‘‘#20AR–SMA’’. It can be seen that at all binder contents, the air voids were all much greater than 4%, and the VCA of the mix was greater than the VCA of the dry rodded coarse aggregate, which is also shown on the plot. This indicates that the addition of coarse rubber powder has caused the loss of coarse aggregate structure. The coarse aggregates were pushed apart, resulting in high VCA and air voids. A second AR–SMA mix using the AR containing the coarse GTR was tried out by reducing the amount of mineral filler from 9% to 4.5%. The aggregate blend for this mix contained 84.5% coarse aggregate, 11% sand and 4.5% mineral filler. The plots of Va, VMA and VCA versus binder content of this mix are shown in Fig. 6 as ‘‘#20AR– SMA, half filler’’. It can be seen that at all binder contents, the air voids were still much greater than 4%. A third AR–SMA using the AR with the coarse GTR was experimented by reducing the sand content from 11% to 5.5%. The aggregate blend for this mix contained 85.5% coarse aggregate, 5.5% sand and 9% mineral filler. The plots of Va, VMA and VCA versus binder content
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of this mix are shown in Fig. 6 as ‘‘#20AR–SMA, half sand’’. It can be seen that at all binder contents, the air voids were still much greater than 4%. The results from three trial mixes using the AR containing 30% coarse GTR indicated that it was not feasible to produce satisfactory SMA mixes using the coarse GTR. An AR–SMA mix using an AR containing 20% fine GTR was designed using a similar procedure. This mix used the same aggregate blend as the reference mix. The plots of Va, VMA and VCA versus binder content of this mix are shown in Fig. 6 as ‘‘#30AR–SMA’’. It can be seen that the Va and VMA of this mix matched fairly closely to those of the reference mix, which is denoted by SMA13. The optimum binder content occurred at 6.7% when the air voids is equal to 4%. The VMA of this mix at the optimum binder content is 18% which is about 1% higher than that of the reference mix. To evaluate the effects of aggregate source, another aggregate with a nominal maximum aggregate size of 19 mm was used to design another reference SMA mix for this study. The gradation plot of this aggregate is shown in Fig. 5 as ‘‘SMA19’’. The optimum binder content of this mix was 6.4%. An AR–SMA mix using an AR containing 20% fine GTR was also designed satisfactorily. Its optimum binder content was 7.0%. The two designed SMA mixes and the two designed AR–SMA mixes using fine GTR were evaluated for their water susceptibility and rutting performance. A typical dense-graded mix was also produced and evaluated for its rutting resistance for comparison purpose. The composition and volumetric properties of these five mixes evaluated are shown in Table 3.
Fig. 6. Volumetric properties of mixtures obtained in the mix proportioning study.
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Table 3 Asphalt mixtures evaluated for performance in the laboratory Aggregate source
Tai-Nan
Mix designation NMSa (mm) Binder Drainage inhibitor Binder content (%) Va (%) VMA (%) VCA mix
SMA13 13 AC-20 Mineral fiber 6.5 4.1 17.1 36.1
a b
Yi-Lan AR–SMA13 13 AR (Fine) Not used 6.7 4.0 17.9 31.5
SMA19 19 AC-20 Mineral fiber 6.4 4.0 17.2 34.3
Tao-Yan AR–SMA19 19 AR (Fine) Not used 7.0 4.0 17.1 35.2
DGAC13 13 AC-10 Not used 5.2 4.2 14.1 NAb
Nominal maximum size. Not available.
3.2. Moisture susceptibility test results The results of moisture susceptibility are shown in Fig. 7. The indirect tensile strength of all dry groups averaged at around 50 psi and was lower than that for dense graded mixtures, which is usually around 100 psi. This was in agreement with previous findings that the ‘‘unconfined strength test value’’ from a gap graded mixture is lower than that of a dense graded sample [1]. The TSR value, which represents the moisture susceptibility, appears to separate into two groups according to their source of aggregate. The two SMA mixtures made with aggregates from Tai-Nan (identified as SMA13 & AR–SMA13) had TSR values greater than 80%. The other two SMA mixtures made with aggregates from Yi-Lan (identified as SMA19 & AR–SMA19) had TSR values between 70% and 75% and were close to the lower limit required by a typical SMA specification. The AR–SMA mixture using aggregates from Yi-Lan (AR–SMA19) had a higher TSR value than that for its reference SMA mixture (SMA19). However, for the AR–SMA mixtures using aggregates from Tai-Nan, the AR–SMA mixture (AR– SMA13) had a slightly lower TSR value than that for its reference SMA mixture (SMA13). From these results, we can conclude that the use of asphalt rubber does not significantly affect the moisture susceptibility of the SMA mixtures.
Fig. 8. Rate of rutting of different mixes evaluated in wheel tracking test.
3.3. Rutting resistance test results Fig. 8 shows the wheel tracking test results from five types of asphalt mixtures. The dense graded mixture (DAGC13), which was tested for comparison purpose, had a rate of rutting (RR) of 0.123 mm/min. This is very close to the value reported in many prior references. The RR values of the other four mixtures were all lower than that for DAGC13. These results met our expectation for the SMA mixtures. All four RR values were below 0.10 mm/min. From Fig. 8, we can observe distinct differences due to the source of aggregate and whether or not an asphalt rubber was used. The SMA mixtures using aggregates from Yi-Lan had smaller RR values than the ones using aggregates from Tai-Nan. This difference could also due to the difference in the aggregate particle size. Within the same source of aggregate, the AR–SMA mixture showed lower RR values than the conventional SMA mixture. Thus, we can conclude that the use of asphalt rubber can improve the rutting resistance of a SMA. 4. Conclusions
Fig. 7. Moisture susceptibility test results.
The main conclusions from this study are summarized as follows:
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(1) In order to meet ASTM D6114s requirement on apparent viscosity at 175 C for asphalt rubber, an AC-20 asphalt needs to be blended with 30% (by weight of the total binder) of a coarse GTR with a maximum size of #20 sieve. To meet the same requirement, an AC-20 needs to be blended with only 20% of a fine GTR with a maximum size of #30 sieve. (2) It was not feasible to produce a suitable SMA mixture using an asphalt rubber made by blending an AC-20 with 30% coarse GTR with a maximum size of #20 sieve. The coarse aggregate structure appeared to be destroyed when this coarse GTR was used. (3) SMA mixtures meeting typical volumetric requirements for SMA could be produced using an asphalt rubber containing 20% of a fine GTR with a maximum size of #30 sieve. No fiber was needed to prevent drain-down when this asphalt rubber was used. No adjustment in aggregate gradation was needed to be made for this mixture. (4) The AR–SMA mixtures were not significantly different from the conventional SMA mixtures in terms of moisture susceptibility from the results of AASHTO T283 tests. (5) The results of the Wheel Tracking Tests at 60 C show that rutting resistance of SMA mixtures were much better than that of a conventional dense-graded mixture. The AR–SMA mixtures had better rutting resistance than the conventional SMA mixtures.
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Acknowledgements This research was part of a project (Project No. NSC912211-E-216-015) supported by the National Science Council of Taiwan. Special thanks are due to Jang Jung Paving Company for providing the necessary help in setting up the wheel tracking device. References [1] Heavy duty surfaces: the arguments for SMA, European asphalt pavement association, P.O. Box 175, 3620 AD Breukelen, The Netherlands, 1998. [2] Highway Technet, technology application, stone upon stone put to the test, US department of transportation, federal highway administration, 1998. [3] Brown ER, Haddock JE, Mallick RB, Bukowski J. Performance of stone matrix asphalt (sma) in the US, AAPT vol. 66, 1997. [4] Chui-Te Chiu, Chang-Lin Pan. The efforts of using ground tire rubber on pavements in Taiwan, beneficial use of recycled materials in transportation applications, Arlington (VA), November 13–15, 2001, Air and Waste Management Association, VIP-114, p. 641–50. [5] Leni Figueiredo Mathias Leite, Romulo Santos Constantino, Alexander Vivoni. Rheological studies of asphalt with ground tire rubber. In: Proceedings for asphalt rubber 2000, Vilamoura, Marinotel, Portugal, November 14–17, 2000, p. 421–34. [6] Gene R. Morris, Douglas D Carlson. The apparent unique behavior of thin asphalt rubber hot-mix overlays in Arizona, beneficial use of recycled materials in transportation applications, Arlington (VA), November 13–15, 2001, Air and Waste Management Association, VIP114, p. 651–66. [7] Brown ER, Cooley LA, Designing stone matrix asphalt mixtures for rut-resistance pavement, NCHRP report 425, 1999.