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Rubber modified binders as an alternative to cellulose fiber – SBS polymers in Stone Matrix Asphalt
Construction and Building Materials 121 (2016) 727–732
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Rubber modified binders as an alternative to cellulose fiber – SBS polymers in Stone Matrix Asphalt Mario Manosalvas-Paredes a,⇑, Juan Gallego a, Leticia Saiz b, José Ma Bermejo b a b
Department of Civil Engineering: Transportation, E.T.S.I.C.C.P., Technical University of Madrid (UPM), C/Profesor Aranguren, S/N, 28040 Madrid, Spain Recovery Markets Development Technician, SIGNUS Ecovalor, C/Caleruega 102, 5°, 28033 Madrid, Spain
h i g h l i g h t s A SMA control mixture with SBS modified PMB 45/80-65 + cellulose fiber was designed. A second SMA mixture with Rubber (ELT’s) + SBS modified PMB 45/80-65 R was studied. Indirect tensile strength, binder drainage and permanent deformation were tested. The modification with Rubber (ELT’s) + SBS makes it possible to avoid cellulose fiber.
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Article history: Received 13 July 2015 Received in revised form 19 May 2016 Accepted 12 June 2016
Keywords: Stone Mastic/Matrix Asphalt ‘‘SMA” Rubber modifier Polymers Cellulose fiber Binder drainage
a b s t r a c t The introduction of fibers in SMA mixtures to prevent binder drainage complicates work in the mixing plants and generates additional costs. It is therefore interesting to define techniques in order to dispense with the fiber without taking further risks concerning binder drainage during construction or plastic deformation during road life service. In this research paper, two SMA 11 mixtures designed with the same grading curve but with two different binders were studied. The first binder used was modified with elastomeric polymers SBS (styrenebutadiene-styrene) and is termed PMB 45/80-65. The second binder was modified with rubber – end of life tyres (ELT’s) and SBS, termed PMB 45/80-65 R. The study results show that both bituminous mixtures SMA11 –PMB 45/80-65 with fibers and PMB 45/80-65 R without fibers-successfully fulfill the water sensitivity tests, binder drainage and resistance to permanent deformation. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction The SMA mixtures were developed in Germany in the 1960´s under the name of Splittmastixasphalt [1] and are known in Europe as Stone Mastic Asphalt and as Stone Matrix Asphalt in the USA. Initially designed to provide a mixture that offered maximum resistance to studded tyre wear [2], the use nowadays of the SMA mixtures has spread widely because it provides high resistance to plastic deformation under heavy traffic loads with high tyre pressure [3]. The SMA mixtures are based on a simple idea: the need to find a resistant, high quality aggregate that is durable. This is then triturated into a cubic form and subsequently the aggregates are mixed ⇑ Corresponding author. E-mail addresses: [email protected] (M. Manosalvas-Paredes), [email protected] (J. Gallego), [email protected] (L. Saiz), [email protected] (J. Ma Bermejo). http://dx.doi.org/10.1016/j.conbuildmat.2016.06.028 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
with a water resistant mortar. The gap-graded gradation allows for stone-on-stone contact among the coarse aggregate particles and enhances the resistance to permanent deformation [4]. The SMA mixtures were introduced in the USA in 1991 in Georgia, Indiana, Michigan and Wisconsin with very satisfactory results [1,2]. The National Center for Asphalt Technology (NCAT) analyzed 86 SMA projects in 1997 obtaining the following results: more than 90% of the projects had a less than 4.0 mm rutting, approximately 60% of the projects surpassed 6.0% (over the weight of the mixture) of binder content during production. The majority of these projects used added fibers in loose condition and in pellet form. All the projects evaluated used a stabilizing additive or a modified binder in order to prevent binder drainage. The result being a superior performance preventing binder drainage during fabrication while the polymers improve the properties at high and low service temperature [5]. The most frequently used fibers to prevent binder drainage in asphalt binders are mineral fibers (slag wool, rock wool) generally
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added in a 0.4% [6] and cellulose fibers in a 0.3% [7]. The mixtures manufactured with mineral fibers attain a lower void content in mineral aggregate and a lower optimum asphalt content (OAC) compared to those mixtures manufactured with cellulose fibers [1]. The binders used in previous studies have been classified into two groups; pure bitumens [1,6–9] and bitumens modified with SBS, polyolefin and polymers [4,10]. Within the European Union, in the year 2006 the European Committee for Standardization elaborated the regulation EN 13108-5 ‘‘Bituminous mixtures Material specifications. Part 5: Stone Mastic Asphalt”. This guideline rule contemplates the use of both pure bitumens and modified bitumens (EN 14023) as well as the use of additives and in particular organic and inorganic fibers. The introduction of fibers complicates work in the mixing plants as well as producing additional costs. It is therefore interesting to define techniques in order to dispense with the fiber without taking further risks concerning binder drainage during construction or plastic deformation during road life service. This article presents the study of a SMA mixture (EN 13108), manufactured with modified bitumen with elastomer SBS of the type PMB 45/80-65 (EN 14023) and a cellulose fiber stabilizer. The same bituminous mixture was studied with a rubber modifier from end of life tyres (ELT’s) and a SBS known as PMB 45/80-65 R (R stands for rubber). Throughout the study, draft specifications were used for SMA [11] and referred to as SMA-DS from now on. These were drafted by the Spanish Centre for Public Works Studies and Experimentation (CEDEX) within the project for Sustainable Mixtures Environmentally Friendly, as there are no official specifications for SMA mixtures in Spain. 2. Materials 2.1. Aggregates Fig. 1 shows the grading envelope taken from SMA-DS [11] and the laboratory gradation curve selected for the design of the control mixture used in this study. The coarse aggregate corresponds to a porphyry fraction of 5/12 mm and the fine aggregate corresponds to a limestone sand of a 0/3 mm fraction; the mineral filler used was calcium carbonate. Table 1 shows the characteristics of the coarse aggregate and the SMA-DS [11] limits. 2.2. Binders The first binder used in this study was a PMB 45/80-65, a commercial binder used to produce the control mixture. The manufacturer informed that the mixture was modified with SBS but the polymer content used is no longer available. Subsequently, a binder PMB 45/80-65 R was manufactured in the laboratory with crumb rubber – end of life tyres (ELT’s) and SBS polymers. The characteristics of both modifiers are described below: Rubber form ELT’s:
Crushed surfaces (%) Flakiness Index Los Angeles fragmentation (%) Apparent relative density (g/cm3) Absorption coefficient (%) Polished stone value
Regulation
Laboratory UPM
SMA-DS [11]
EN EN EN EN
100.0 6.7 10.5 2.71 0.43
100 620 615 – –
57.5
P56
933-5 933-3 1097-2 1097-6
EN 1097-8
Table 2 Gradation of crumb rubber (UNE 993-2). Sieve (mm) Passing (%)
2.0 100.0
1.5 100.0
1.0 100.0
0.50 94.1
0.250 27.3
0.125 3.7
0.063 0.4
The measurement of the principal components of the rubber was carried out through a thermogravimetric analysis using a Mettler thermobalance TGA/SDTA 851 model, under the following conditions. o o o o
Nitrogen flow = 80 ml/min Oxygen flow = 45 ml/min Test temperature = 30 a 1.000 °C Heating speed = 20 °C/min In Table 3 the results are shown.
Polymers: The polymer used together with the rubber in the binder modification of PMB 45/80-65 R was a SBS polymerized in solution with a radial structure. It is known by the commercial name of Calprene 411. It comes in pellet and powder form and is crumb like and porous. In this study the powder form was used. The composition of the PMB 45/80-65 R is shown in Table 4. This modified binder was manufactured at 185 °C with a lab mixer with a propeller head at 8800 rpm during 60 min. The temperature during the manufacturing process was kept in the range 185 ± 2 °C by using a thermostatic oil bath. The commercial binder PMB 45/80-65 modified with SBS and the laboratory binder PMB 45/80-65 R modified with tyre rubber and SBS polymer offer the characteristics shown in Table 5, according to the Spanish specifications of bitumens (Art. 212 from PG-3) [12] which is based on the European standard EN 14023. It may be observed that both binders comply with the Spanish specifications [12] for this category of binders. It should be stated that in the case of rubber modified bitumens it is common that a fraction of the rubber particles remain unintegrated. At higher temperatures and agitation, these elastic particles decrease in size [13], but a proportion always remains. This would explain some of the characteristics of these binders and the mixtures manufactured with them.
Table 3 Thermogravimetric analysis of crumb rubber.
The gradation of the crumb rubber is showed on Table 2. 100.0 90.0 80.0
Passing (%)
Table 1 Characteristics of the coarse aggregate used in the mixture.
70.0
TGA parameters
Content in the rubber (%)
Plasticizer + additives Polymer Carbon black Ash
4.67 57.41 32.22 6.02
60.0 50.0 40.0
Table 4 Components of PMB 45/80-65 R.
30.0 20.0 10.0 0.0 100
10
1
0.1
Sieve size (mm) Used Gradation
Lower Limit
Fig. 1. Gradation used in this study.
Upper Limit
0.01
Components
Proportions (%)
B 50/70 B 160/220 Crumb Rubber SBS polymer Total
44.4 44.4 8.9 2.3 100.0
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SBS manufactured in the road research laboratory of the Technical University of Madrid (UPM) was designed and tested.
PMB 45/8065
PMB 45/80-65 R
Art. 212 PG-3 [12]
EN 1426 EN 1427
53.0 65.2
51.0 66.3
45–80 P65
EN 13398
91.0
90.0
P80
Regulation
1
Penetration (10 mm) Ring and Ball Temperature (°C) Elastic recovery (%)
Table 6 Characteristics del ARBOCELÒ. Properties
Cellulose fiber
Cellulose content (%) Value of pH, 5 g/100 ml Average length of the fiber (lm) Average width (lm)
80 ± 5 7.5 ± 1.0 1100 45
2.3. Fiber The binder drainage of the asphalt mixture during transportation and laying is one of the most common problems of the SMA mixtures. This study used cellulose fiber ARBOCELÒ in percentages of 0.3% and 0.5% (over the weight of the mixture produced with PMB 45/80-65) added to the bituminous mixture in loose condition by dry process following the manufacturer’s instructions. Characteristics of the cellulose fiber used in the laboratory are shown in Table 6. In order to avoid the use of fibers in SMA mixtures, the authors worked with the hypothesis that the rubber modified binder is less susceptible to drainage than the SBS binder due to the minute devulcanized rubber particles in suspension. For this reason, the fibers were added only to the mixture with SBS modified binder.
3. Methodology In accordance with the draft specifications for SMA (SMA-DS) [11], the design procedure of the bituminous mixture was based on the air void content (EN 12697-8) of compacted samples with 50 blows/side with a Marshall hammer (EN 12697-30) [4,14,15] and posterior checking of the water sensitivity (EN 12697-12), binder drainage (EN 12697-18) and resistance to permanent deformation (EN 12697-22). The air void content is calculated using the bulk density of the test specimens in saturated dry condition (UNE-EN 12697-6) and the maximum density of the mixture is obtained using a pycnometer (UNE-EN 1697-5). The specimens are compacted in order to obtain the bulk density measurement and two determinations are carried out on the maximum density using the pycnometer. The water sensitivity test is carried out on ten cylindrical samples: five test specimens are subjected to a water bath at 40 °C during 72 h and the other five samples are kept dry. The indirect tensile strength ratio (ITSR) is determined as indirect tensile strength ratios (UNE-EN 12697-23) between both groups. In the binder drainage test (Schellenberg method) the remaining residues are determined after pouring out the mixture heated during 1 h at production temperature. Two determinations are carried out in order to obtain the average. Finally the resistance to permanent deformation is carried out on a 4 cm high flat specimen using a repetitive loaded wheel tracking test. Rutting appears in the specimen and the results of the test determine the speed of deformation (Wheel Tracking Slope). The average is calculated using two specimens. In the first instance a SMA 11 mixture was designed and tested using cellulose fiber and a commercial modified binder with SBS, type PMB 45/80-65 and named reference mixture. Subsequently the same SMA 11 with no fiber and PMB 45/80-65 R binder modified with crumb rubber from end of life tyres (ELT’s) and
3.1. Design of SMA 11 mixture with PMB 45/80-65 and 0.3% cellulose fiber Three percentages of binder 5.7%, 5.95% and 6.2% (over mixture) and an initial percentage of cellulose fiber 0.3% (over mixture) added in loose condition by dry way process were tested in order to determine the optimum asphalt content (OAC). Four test specimens were manufactured for each percentage of binder using a Marshall compactor (EN 12697-30). This content of fiber was chosen taking into account the recommendations of the manufacturer: 0.3–0.5%. According to the SMA-DS [11], the binder content should be chosen in order to generate a 4.0–6.0% air void. The volumetric properties were determined for the three contents of PMB 45/8065 (Table 7). A 5.9% of binder content was deduced by interpolation as shown in Fig. 2, in correspondence with 5.0% air voids, at the center of the interval 4.0–6.0%. In Table 8 the values obtained for 5.9% binder content of PMB 45/80-65 and 0.3% cellulose fiber are shown. 3.2. Design of SMA 11 mixture with PMB 45/80-65 and 0.5% cellulose fiber The air voids in the mixture were calculated. Given that the 5.9% of the binder corresponds to an air void of 5.5% in the interval (4–6%) this value was admitted as optimal. Table 8 shows the volumetric characteristics of the mixture. 3.3. Design of the SMA 11 mixture with PMB 45/80-65 R binder (no fiber) To manufacture a mixture with PMB 45/80-65 R a unique content of 5.9% was agreed upon. Four specimens were produced to
Table 7 Volumetric properties for different contents of binder PMB 45/80-65 and 0.3% fiber. Regulation
Air Voids, Vm (%) Voids in mineral aggregate, VMA (%) Voids filled with binder, VFB (%)
EN 12697-8
Binder content
SMA-DS [11]
5.7
5.95
6.2
5.6 19.2
4.7 19.1
4.2 19.3
4–6 P17
70.9
75.3
78.2
683
6.0 5.5
Air Voids (%)
Table 5 Binder properties PMB 45/80-65 and PMB 45/80-65 R.
y = -2.7453x + 21.172 R² = 0.9725
5.0 4.5 4.0 3.5 3.0 5.6
5.7
5.8
5.9
6.0
6.1
6.2
% Binder over the weight of mixture Air Voids
Linear (Air Voids)
Fig. 2. Determination of optimum asphalt content for 5.0% air voids.
6.3
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Table 8 Volumetric properties of the mixtures for 5.9% binder content.
Air Voids, Vm (%) Voids in mineral aggregate, VMA (%) Voids filled with binder, VFB (%)
Regulation
PMB-45/80-65 Cellulose fiber 0.3%
PMB-45/80-65 Cellulose fiber 0.5%
PMB-45/80-65 R Cellulose fiber 0.0%
SMA-DS [11]
EN 12697-8
5.0 19.1 73.6
5.5 19.5 71.8
4.8 19.0 74.8
4–6 P17 683
Table 9 Characterization of the three mixture with 5.9% of binder. Property
Regulation
PMB 45/80-65 Cellulose fiber 0.3%
PMB 45/80-65 Cellulose fiber 0.5%
PMB 45/80-65 R Cellulose fiber 0.0%
SMA-DS [11]
Indirect Tensile Strength Wet, ITSw (kPa) Indirect Tensile Strength Dry, ITSd (kPa) Tensile Strength Ratio, TSR (%) Binder drainage, D (%) Wheel tracking slope, WTSAIR (mm/103cycles)
EN 12697-23
1749.5 1831.0 95.5 0.14 0.084
1629.0 1699.8 95.8 0.05 0.056
1433.0 1478.8 96.9 0.05 0.038
–
determine the air void content. The results are shown in Table 8. No fiber was added to this mixture. Given that the mixture contains a 4.8% air void, within the interval required (4–6%) [11] in close proximity to the 5.0% air void implemented in the reference mixture, a 5.9% PMB 45/80-65 R was also approved to continue with the behaviour tests.
P90 <0.3 <0.07
8.0 7.0
Rut Depth (mm)
EN 12697-12 EN 12697-18 EN 12697-22
6.0 5.0 4.0 3.0 2.0 1.0
4. Mixture behaviour
0.0 0
Once a binder content was determined according to the air void criteria, characterization tests were carried out to evaluate the other properties required in the SMA-DS [11]. The following tests were carried out Water sensitivity (EN 12697-12) Binder drainage, Schellemberg method (EN 12697-18) Wheel Tracking test, permanent deformation (EN 12697-22)
Sample Nº1
8.0 7.0
6.0
6.0
3.0 2.0 1.0
8000
10000
Sample Nº2
Average
With 0.5% fiber, the requirements of SMA-DS [11] for the WTS parameter are met and the binder drainage minimized. The first phase of the study is deemed concluded. The 5.9% PMB 45/80-65 and 0.5% cellulose fiber are thus considered optimum values for the design of the reference mixture. Fig. 5 and Table 9 show the results to resistance to permanent deformation for the mixture without fiber. The permanent deformation of the mixture with PMB 45/80-65 R is even lower than that of the mixture with PMB 45/90-65 and 0.5% fiber. The results obtained with the PMB 45/80-65 R binder satisfy the requirements of SMA-DS [11] in terms of WTS and binder drainage, without the need to incorporate the stabilizing fiber.
7.0
4.0
6000
Fig. 4. Deformation curves, mixtures with 5.9% PMB 45/80-65 and 0.5% cellulose fiber.
8.0
5.0
4000
Number of Cycles
Rut Depth (mm)
Rut Depth (mm)
The indirect tensile strength, water sensitivity, binder drainage and permanent deformation results of the three mixtures under study are shown in Table 9. In Table 9 an infringement is observed in the permanent deformation test, a wheel tracking slope (WTS) of 0.084 mm/103 cycles rather than the 0.070 required in SMA-DS [11]. It therefore becomes necessary to raise the content of stabilizing fiber to 0.5% by weight of the mixture, within the recommended range suggested by the manufacturers of the fiber in order to accomplish the WTS specification as well as to minimize the binder drainage. Figs. 3 and 4 show the results to resistance to permanent deformation for the mixtures with 0.3 and 0.5% of cellulose fiber.
2000
5.0 4.0 3.0 2.0 1.0
0.0 0
2000
4000
6000
8000
10000
Number of Cycles Sample Nº1
Sample Nº2
0.0 0
2000
4000
6000
8000
10000
Number of Cycles Average Sample Nº1
Fig. 3. Deformation curves, mixtures with 5.9% PMB 45/80-65 and 0.3% cellulose fiber.
Sample Nº2
Average
Fig. 5. Deformation curves, mixtures with 5.9% PMB 45/80-65 R (no fiber).
M. Manosalvas-Paredes et al. / Construction and Building Materials 121 (2016) 727–732
Indirect Tensile Strength (kPa)
2000,0 1800,0
1831
1750
1700
1629
1600,0
1479
1433
1400,0 1200,0 1000,0 800,0 600,0 400,0 200,0 0,0
Wet Samples
PMB 45/80-65 with 0.3% cellulose fiber
Dry Samples
PMB 45/80-65 with 0.5% cellulose fiber
PMB 45/80-65 R without cellulose fiber
Fig. 6. Indirect Tensile Strength. Mixtures with 5.9% of binder.
5. Discussion of the results In Fig. 6, the results of the Indirect Tensile Strength have been compiled and show each one of the mixtures studied. The mixture with PMB 45/80-65 and 0.3% cellulose fiber offers the highest tensile strength compared to the other two mixtures both after being dry conditioned or after immersion. Comparing the mixtures PMB 45/80-65 and 0.5% cellulose fiber and PMB 45/80-65 R (no fiber), it is observed that the mixture PMB 45/80-65 and 0.5% cellulose fiber has a superior indirect tensile strength than the PMB 45/80-65 R both dry tested and after immersion. Consistent with the authors’ experience, the bituminous mixtures modified with tyre rubber offer slightly inferior indirect tensile strength compared to those mixtures modified with SBS polymers. This reduced traction resistance could be referred to the presence of elastic rubber particles in the modified binder [13] that have not been devulcanized and which lead to small heterogeneities that may weaken traction resistance. In certain cases this characteristic can be corrected by raising the quantity of rubber modified bitumen although there is a greater risk of plastic deformation. Regarding the fulfillment of the SMA-DS [11] requirements, the three mixtures Indirect Tensile Strength Ratio (ITSR) meet the requirements satisfactorily, in that in each case they are over the 90% stipulated. Fig. 7 compares the deformation curves of the wheel tracking test for each mixture (EN 12697-22). The values of WTS for the mixture with PBM 45/80-65 and 0.3% fiber, with 0.5% fiber and with PMB 45/80-65 R without fiber are 0.084, 0.056 and 0.038 respectively. Therefore the PMB 45/80-65 and 0.3% cellulose fiber mixture surpasses the maximum 0.070 mm/103 cycles allowed in the SMA-DS [11] meanwhile the PMB 45/80-65 and 0.5% cellulose fiber and PMB 45/80-65 R without fiber mixtures meet the specifications satisfactorily. The bituminous mixture PMB 45/80-65 R, offers the best performance regarding permanent deformations with a slope deforma8.0
Rut Depth (mm)
7.0 6.0
731
tion of 0.038 mm/103 cycles, under the 0.070 mm/103 cycles required in SMA-DS [11]. In order to understand this result, it is necessary to remember that plastic deformation tests are carried out at 60 °C. As shown in Table 5 the ring and ball temperature is slightly higher in the case of rubber modified bitumen and SBS which would explain the improved deformation performance of the bituminous mix. Over and above, the persistence of small elastic particles of devulanized rubber [13] can contribute to stabilize the bituminous mix. As far as the binder drainage results (EN 12697-18), the three mixtures met the requirements of the SMADS [11], the mixture manufactured with PMB 45/80-65 R and without fiber offered excellent results (0.05%). As mentioned, this may be explained by the presence of tiny devulcanized rubber particles and at a microscopic level by the higher molecular weight of the composites from the modified rubber. These lend the binder properties that hinder binder drainage. 6. Conclusions In Europe as well as in the United States, it is commonplace to use mineral fibers or cellulose in SMA mixtures so as to avoid binder drainage during transportation and during laying of bituminous mixtures. Nevertheless, the use of fiber results in operative complications in the manufacturing plant as well as increasing costs. The aim of this research was to evaluate the feasibility of designing a SMA mixture with a tyre rubber and SBS modified binder (PMB 45/80-65 R), without having to use the cellulose fiber stabilizers that are commonly used with the SBS modified binder (PMB 45/80-65). In view of the results obtained, it may be said that the option of PMB 45/80-65 R without fiber offers an excellent performance as regards binder drainage and permanent deformation, even better than the SMA with PMB 45/80-65 and cellulose fibers. The persistence of minute devulcanized elastic rubber particles that are suspended in the rubber modified bitumen together with the composites formed during the process, lend a higher consistency at production temperatures and service temperatures. This would explain the binder drainage test results and the permanent deformation resistance results obtained in the research. It was also observed that the water sensitivity is similar in the SMA mixture with PMB 45/80-65 R and the SMA mixture with PMB 45/80-65 and fibers. Nevertheless, the indirect tensile strength is advantageous for SMA with PMB 45/80-65 and cellulose fibers. In this case, the minute rubber particles within the binder PMB 45/80-65 R would also explain this result as they may weaken the tensile strength of the mixture. The above results are important at an industrial level since they appear to indicate that the use of bitumens modified with rubber from end of life tyres and SBS (PMB 45/80-65 R) allow to dispense with the stabilizing fiber habitually used in the bituminous mixtures manufactured with SBS modified binder (PMB 45/80-65), without the risk of plastic deformation or binder drainage.
5.0
References
4.0 3.0 2.0 1.0 0.0 0
2000
4000
6000
8000
10000
Number of Cycles PMB 45/80-65 with 0.3% cellulose fiber (average) PMB 45/80-65 with 0.5% cellulose fiber (average) PMB 45/80-65 R without cellulose fiber (average)
Fig. 7. Comparison of the deformation curves of the mixtures studied.
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Rubber modified binders as an alternative to cellulose fiber – SBS polymers in Stone Matrix Asphalt Mario Manosalvas-Paredes a,⇑, Juan Gallego a, Leticia Saiz b, José Ma Bermejo b a b
Department of Civil Engineering: Transportation, E.T.S.I.C.C.P., Technical University of Madrid (UPM), C/Profesor Aranguren, S/N, 28040 Madrid, Spain Recovery Markets Development Technician, SIGNUS Ecovalor, C/Caleruega 102, 5°, 28033 Madrid, Spain
h i g h l i g h t s A SMA control mixture with SBS modified PMB 45/80-65 + cellulose fiber was designed. A second SMA mixture with Rubber (ELT’s) + SBS modified PMB 45/80-65 R was studied. Indirect tensile strength, binder drainage and permanent deformation were tested. The modification with Rubber (ELT’s) + SBS makes it possible to avoid cellulose fiber.
a r t i c l e
i n f o
Article history: Received 13 July 2015 Received in revised form 19 May 2016 Accepted 12 June 2016
Keywords: Stone Mastic/Matrix Asphalt ‘‘SMA” Rubber modifier Polymers Cellulose fiber Binder drainage
a b s t r a c t The introduction of fibers in SMA mixtures to prevent binder drainage complicates work in the mixing plants and generates additional costs. It is therefore interesting to define techniques in order to dispense with the fiber without taking further risks concerning binder drainage during construction or plastic deformation during road life service. In this research paper, two SMA 11 mixtures designed with the same grading curve but with two different binders were studied. The first binder used was modified with elastomeric polymers SBS (styrenebutadiene-styrene) and is termed PMB 45/80-65. The second binder was modified with rubber – end of life tyres (ELT’s) and SBS, termed PMB 45/80-65 R. The study results show that both bituminous mixtures SMA11 –PMB 45/80-65 with fibers and PMB 45/80-65 R without fibers-successfully fulfill the water sensitivity tests, binder drainage and resistance to permanent deformation. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction The SMA mixtures were developed in Germany in the 1960´s under the name of Splittmastixasphalt [1] and are known in Europe as Stone Mastic Asphalt and as Stone Matrix Asphalt in the USA. Initially designed to provide a mixture that offered maximum resistance to studded tyre wear [2], the use nowadays of the SMA mixtures has spread widely because it provides high resistance to plastic deformation under heavy traffic loads with high tyre pressure [3]. The SMA mixtures are based on a simple idea: the need to find a resistant, high quality aggregate that is durable. This is then triturated into a cubic form and subsequently the aggregates are mixed ⇑ Corresponding author. E-mail addresses: [email protected] (M. Manosalvas-Paredes), [email protected] (J. Gallego), [email protected] (L. Saiz), [email protected] (J. Ma Bermejo). http://dx.doi.org/10.1016/j.conbuildmat.2016.06.028 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
with a water resistant mortar. The gap-graded gradation allows for stone-on-stone contact among the coarse aggregate particles and enhances the resistance to permanent deformation [4]. The SMA mixtures were introduced in the USA in 1991 in Georgia, Indiana, Michigan and Wisconsin with very satisfactory results [1,2]. The National Center for Asphalt Technology (NCAT) analyzed 86 SMA projects in 1997 obtaining the following results: more than 90% of the projects had a less than 4.0 mm rutting, approximately 60% of the projects surpassed 6.0% (over the weight of the mixture) of binder content during production. The majority of these projects used added fibers in loose condition and in pellet form. All the projects evaluated used a stabilizing additive or a modified binder in order to prevent binder drainage. The result being a superior performance preventing binder drainage during fabrication while the polymers improve the properties at high and low service temperature [5]. The most frequently used fibers to prevent binder drainage in asphalt binders are mineral fibers (slag wool, rock wool) generally
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added in a 0.4% [6] and cellulose fibers in a 0.3% [7]. The mixtures manufactured with mineral fibers attain a lower void content in mineral aggregate and a lower optimum asphalt content (OAC) compared to those mixtures manufactured with cellulose fibers [1]. The binders used in previous studies have been classified into two groups; pure bitumens [1,6–9] and bitumens modified with SBS, polyolefin and polymers [4,10]. Within the European Union, in the year 2006 the European Committee for Standardization elaborated the regulation EN 13108-5 ‘‘Bituminous mixtures Material specifications. Part 5: Stone Mastic Asphalt”. This guideline rule contemplates the use of both pure bitumens and modified bitumens (EN 14023) as well as the use of additives and in particular organic and inorganic fibers. The introduction of fibers complicates work in the mixing plants as well as producing additional costs. It is therefore interesting to define techniques in order to dispense with the fiber without taking further risks concerning binder drainage during construction or plastic deformation during road life service. This article presents the study of a SMA mixture (EN 13108), manufactured with modified bitumen with elastomer SBS of the type PMB 45/80-65 (EN 14023) and a cellulose fiber stabilizer. The same bituminous mixture was studied with a rubber modifier from end of life tyres (ELT’s) and a SBS known as PMB 45/80-65 R (R stands for rubber). Throughout the study, draft specifications were used for SMA [11] and referred to as SMA-DS from now on. These were drafted by the Spanish Centre for Public Works Studies and Experimentation (CEDEX) within the project for Sustainable Mixtures Environmentally Friendly, as there are no official specifications for SMA mixtures in Spain. 2. Materials 2.1. Aggregates Fig. 1 shows the grading envelope taken from SMA-DS [11] and the laboratory gradation curve selected for the design of the control mixture used in this study. The coarse aggregate corresponds to a porphyry fraction of 5/12 mm and the fine aggregate corresponds to a limestone sand of a 0/3 mm fraction; the mineral filler used was calcium carbonate. Table 1 shows the characteristics of the coarse aggregate and the SMA-DS [11] limits. 2.2. Binders The first binder used in this study was a PMB 45/80-65, a commercial binder used to produce the control mixture. The manufacturer informed that the mixture was modified with SBS but the polymer content used is no longer available. Subsequently, a binder PMB 45/80-65 R was manufactured in the laboratory with crumb rubber – end of life tyres (ELT’s) and SBS polymers. The characteristics of both modifiers are described below: Rubber form ELT’s:
Crushed surfaces (%) Flakiness Index Los Angeles fragmentation (%) Apparent relative density (g/cm3) Absorption coefficient (%) Polished stone value
Regulation
Laboratory UPM
SMA-DS [11]
EN EN EN EN
100.0 6.7 10.5 2.71 0.43
100 620 615 – –
57.5
P56
933-5 933-3 1097-2 1097-6
EN 1097-8
Table 2 Gradation of crumb rubber (UNE 993-2). Sieve (mm) Passing (%)
2.0 100.0
1.5 100.0
1.0 100.0
0.50 94.1
0.250 27.3
0.125 3.7
0.063 0.4
The measurement of the principal components of the rubber was carried out through a thermogravimetric analysis using a Mettler thermobalance TGA/SDTA 851 model, under the following conditions. o o o o
Nitrogen flow = 80 ml/min Oxygen flow = 45 ml/min Test temperature = 30 a 1.000 °C Heating speed = 20 °C/min In Table 3 the results are shown.
Polymers: The polymer used together with the rubber in the binder modification of PMB 45/80-65 R was a SBS polymerized in solution with a radial structure. It is known by the commercial name of Calprene 411. It comes in pellet and powder form and is crumb like and porous. In this study the powder form was used. The composition of the PMB 45/80-65 R is shown in Table 4. This modified binder was manufactured at 185 °C with a lab mixer with a propeller head at 8800 rpm during 60 min. The temperature during the manufacturing process was kept in the range 185 ± 2 °C by using a thermostatic oil bath. The commercial binder PMB 45/80-65 modified with SBS and the laboratory binder PMB 45/80-65 R modified with tyre rubber and SBS polymer offer the characteristics shown in Table 5, according to the Spanish specifications of bitumens (Art. 212 from PG-3) [12] which is based on the European standard EN 14023. It may be observed that both binders comply with the Spanish specifications [12] for this category of binders. It should be stated that in the case of rubber modified bitumens it is common that a fraction of the rubber particles remain unintegrated. At higher temperatures and agitation, these elastic particles decrease in size [13], but a proportion always remains. This would explain some of the characteristics of these binders and the mixtures manufactured with them.
Table 3 Thermogravimetric analysis of crumb rubber.
The gradation of the crumb rubber is showed on Table 2. 100.0 90.0 80.0
Passing (%)
Table 1 Characteristics of the coarse aggregate used in the mixture.
70.0
TGA parameters
Content in the rubber (%)
Plasticizer + additives Polymer Carbon black Ash
4.67 57.41 32.22 6.02
60.0 50.0 40.0
Table 4 Components of PMB 45/80-65 R.
30.0 20.0 10.0 0.0 100
10
1
0.1
Sieve size (mm) Used Gradation
Lower Limit
Fig. 1. Gradation used in this study.
Upper Limit
0.01
Components
Proportions (%)
B 50/70 B 160/220 Crumb Rubber SBS polymer Total
44.4 44.4 8.9 2.3 100.0
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SBS manufactured in the road research laboratory of the Technical University of Madrid (UPM) was designed and tested.
PMB 45/8065
PMB 45/80-65 R
Art. 212 PG-3 [12]
EN 1426 EN 1427
53.0 65.2
51.0 66.3
45–80 P65
EN 13398
91.0
90.0
P80
Regulation
1
Penetration (10 mm) Ring and Ball Temperature (°C) Elastic recovery (%)
Table 6 Characteristics del ARBOCELÒ. Properties
Cellulose fiber
Cellulose content (%) Value of pH, 5 g/100 ml Average length of the fiber (lm) Average width (lm)
80 ± 5 7.5 ± 1.0 1100 45
2.3. Fiber The binder drainage of the asphalt mixture during transportation and laying is one of the most common problems of the SMA mixtures. This study used cellulose fiber ARBOCELÒ in percentages of 0.3% and 0.5% (over the weight of the mixture produced with PMB 45/80-65) added to the bituminous mixture in loose condition by dry process following the manufacturer’s instructions. Characteristics of the cellulose fiber used in the laboratory are shown in Table 6. In order to avoid the use of fibers in SMA mixtures, the authors worked with the hypothesis that the rubber modified binder is less susceptible to drainage than the SBS binder due to the minute devulcanized rubber particles in suspension. For this reason, the fibers were added only to the mixture with SBS modified binder.
3. Methodology In accordance with the draft specifications for SMA (SMA-DS) [11], the design procedure of the bituminous mixture was based on the air void content (EN 12697-8) of compacted samples with 50 blows/side with a Marshall hammer (EN 12697-30) [4,14,15] and posterior checking of the water sensitivity (EN 12697-12), binder drainage (EN 12697-18) and resistance to permanent deformation (EN 12697-22). The air void content is calculated using the bulk density of the test specimens in saturated dry condition (UNE-EN 12697-6) and the maximum density of the mixture is obtained using a pycnometer (UNE-EN 1697-5). The specimens are compacted in order to obtain the bulk density measurement and two determinations are carried out on the maximum density using the pycnometer. The water sensitivity test is carried out on ten cylindrical samples: five test specimens are subjected to a water bath at 40 °C during 72 h and the other five samples are kept dry. The indirect tensile strength ratio (ITSR) is determined as indirect tensile strength ratios (UNE-EN 12697-23) between both groups. In the binder drainage test (Schellenberg method) the remaining residues are determined after pouring out the mixture heated during 1 h at production temperature. Two determinations are carried out in order to obtain the average. Finally the resistance to permanent deformation is carried out on a 4 cm high flat specimen using a repetitive loaded wheel tracking test. Rutting appears in the specimen and the results of the test determine the speed of deformation (Wheel Tracking Slope). The average is calculated using two specimens. In the first instance a SMA 11 mixture was designed and tested using cellulose fiber and a commercial modified binder with SBS, type PMB 45/80-65 and named reference mixture. Subsequently the same SMA 11 with no fiber and PMB 45/80-65 R binder modified with crumb rubber from end of life tyres (ELT’s) and
3.1. Design of SMA 11 mixture with PMB 45/80-65 and 0.3% cellulose fiber Three percentages of binder 5.7%, 5.95% and 6.2% (over mixture) and an initial percentage of cellulose fiber 0.3% (over mixture) added in loose condition by dry way process were tested in order to determine the optimum asphalt content (OAC). Four test specimens were manufactured for each percentage of binder using a Marshall compactor (EN 12697-30). This content of fiber was chosen taking into account the recommendations of the manufacturer: 0.3–0.5%. According to the SMA-DS [11], the binder content should be chosen in order to generate a 4.0–6.0% air void. The volumetric properties were determined for the three contents of PMB 45/8065 (Table 7). A 5.9% of binder content was deduced by interpolation as shown in Fig. 2, in correspondence with 5.0% air voids, at the center of the interval 4.0–6.0%. In Table 8 the values obtained for 5.9% binder content of PMB 45/80-65 and 0.3% cellulose fiber are shown. 3.2. Design of SMA 11 mixture with PMB 45/80-65 and 0.5% cellulose fiber The air voids in the mixture were calculated. Given that the 5.9% of the binder corresponds to an air void of 5.5% in the interval (4–6%) this value was admitted as optimal. Table 8 shows the volumetric characteristics of the mixture. 3.3. Design of the SMA 11 mixture with PMB 45/80-65 R binder (no fiber) To manufacture a mixture with PMB 45/80-65 R a unique content of 5.9% was agreed upon. Four specimens were produced to
Table 7 Volumetric properties for different contents of binder PMB 45/80-65 and 0.3% fiber. Regulation
Air Voids, Vm (%) Voids in mineral aggregate, VMA (%) Voids filled with binder, VFB (%)
EN 12697-8
Binder content
SMA-DS [11]
5.7
5.95
6.2
5.6 19.2
4.7 19.1
4.2 19.3
4–6 P17
70.9
75.3
78.2
683
6.0 5.5
Air Voids (%)
Table 5 Binder properties PMB 45/80-65 and PMB 45/80-65 R.
y = -2.7453x + 21.172 R² = 0.9725
5.0 4.5 4.0 3.5 3.0 5.6
5.7
5.8
5.9
6.0
6.1
6.2
% Binder over the weight of mixture Air Voids
Linear (Air Voids)
Fig. 2. Determination of optimum asphalt content for 5.0% air voids.
6.3
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Table 8 Volumetric properties of the mixtures for 5.9% binder content.
Air Voids, Vm (%) Voids in mineral aggregate, VMA (%) Voids filled with binder, VFB (%)
Regulation
PMB-45/80-65 Cellulose fiber 0.3%
PMB-45/80-65 Cellulose fiber 0.5%
PMB-45/80-65 R Cellulose fiber 0.0%
SMA-DS [11]
EN 12697-8
5.0 19.1 73.6
5.5 19.5 71.8
4.8 19.0 74.8
4–6 P17 683
Table 9 Characterization of the three mixture with 5.9% of binder. Property
Regulation
PMB 45/80-65 Cellulose fiber 0.3%
PMB 45/80-65 Cellulose fiber 0.5%
PMB 45/80-65 R Cellulose fiber 0.0%
SMA-DS [11]
Indirect Tensile Strength Wet, ITSw (kPa) Indirect Tensile Strength Dry, ITSd (kPa) Tensile Strength Ratio, TSR (%) Binder drainage, D (%) Wheel tracking slope, WTSAIR (mm/103cycles)
EN 12697-23
1749.5 1831.0 95.5 0.14 0.084
1629.0 1699.8 95.8 0.05 0.056
1433.0 1478.8 96.9 0.05 0.038
–
determine the air void content. The results are shown in Table 8. No fiber was added to this mixture. Given that the mixture contains a 4.8% air void, within the interval required (4–6%) [11] in close proximity to the 5.0% air void implemented in the reference mixture, a 5.9% PMB 45/80-65 R was also approved to continue with the behaviour tests.
P90 <0.3 <0.07
8.0 7.0
Rut Depth (mm)
EN 12697-12 EN 12697-18 EN 12697-22
6.0 5.0 4.0 3.0 2.0 1.0
4. Mixture behaviour
0.0 0
Once a binder content was determined according to the air void criteria, characterization tests were carried out to evaluate the other properties required in the SMA-DS [11]. The following tests were carried out Water sensitivity (EN 12697-12) Binder drainage, Schellemberg method (EN 12697-18) Wheel Tracking test, permanent deformation (EN 12697-22)
Sample Nº1
8.0 7.0
6.0
6.0
3.0 2.0 1.0
8000
10000
Sample Nº2
Average
With 0.5% fiber, the requirements of SMA-DS [11] for the WTS parameter are met and the binder drainage minimized. The first phase of the study is deemed concluded. The 5.9% PMB 45/80-65 and 0.5% cellulose fiber are thus considered optimum values for the design of the reference mixture. Fig. 5 and Table 9 show the results to resistance to permanent deformation for the mixture without fiber. The permanent deformation of the mixture with PMB 45/80-65 R is even lower than that of the mixture with PMB 45/90-65 and 0.5% fiber. The results obtained with the PMB 45/80-65 R binder satisfy the requirements of SMA-DS [11] in terms of WTS and binder drainage, without the need to incorporate the stabilizing fiber.
7.0
4.0
6000
Fig. 4. Deformation curves, mixtures with 5.9% PMB 45/80-65 and 0.5% cellulose fiber.
8.0
5.0
4000
Number of Cycles
Rut Depth (mm)
Rut Depth (mm)
The indirect tensile strength, water sensitivity, binder drainage and permanent deformation results of the three mixtures under study are shown in Table 9. In Table 9 an infringement is observed in the permanent deformation test, a wheel tracking slope (WTS) of 0.084 mm/103 cycles rather than the 0.070 required in SMA-DS [11]. It therefore becomes necessary to raise the content of stabilizing fiber to 0.5% by weight of the mixture, within the recommended range suggested by the manufacturers of the fiber in order to accomplish the WTS specification as well as to minimize the binder drainage. Figs. 3 and 4 show the results to resistance to permanent deformation for the mixtures with 0.3 and 0.5% of cellulose fiber.
2000
5.0 4.0 3.0 2.0 1.0
0.0 0
2000
4000
6000
8000
10000
Number of Cycles Sample Nº1
Sample Nº2
0.0 0
2000
4000
6000
8000
10000
Number of Cycles Average Sample Nº1
Fig. 3. Deformation curves, mixtures with 5.9% PMB 45/80-65 and 0.3% cellulose fiber.
Sample Nº2
Average
Fig. 5. Deformation curves, mixtures with 5.9% PMB 45/80-65 R (no fiber).
M. Manosalvas-Paredes et al. / Construction and Building Materials 121 (2016) 727–732
Indirect Tensile Strength (kPa)
2000,0 1800,0
1831
1750
1700
1629
1600,0
1479
1433
1400,0 1200,0 1000,0 800,0 600,0 400,0 200,0 0,0
Wet Samples
PMB 45/80-65 with 0.3% cellulose fiber
Dry Samples
PMB 45/80-65 with 0.5% cellulose fiber
PMB 45/80-65 R without cellulose fiber
Fig. 6. Indirect Tensile Strength. Mixtures with 5.9% of binder.
5. Discussion of the results In Fig. 6, the results of the Indirect Tensile Strength have been compiled and show each one of the mixtures studied. The mixture with PMB 45/80-65 and 0.3% cellulose fiber offers the highest tensile strength compared to the other two mixtures both after being dry conditioned or after immersion. Comparing the mixtures PMB 45/80-65 and 0.5% cellulose fiber and PMB 45/80-65 R (no fiber), it is observed that the mixture PMB 45/80-65 and 0.5% cellulose fiber has a superior indirect tensile strength than the PMB 45/80-65 R both dry tested and after immersion. Consistent with the authors’ experience, the bituminous mixtures modified with tyre rubber offer slightly inferior indirect tensile strength compared to those mixtures modified with SBS polymers. This reduced traction resistance could be referred to the presence of elastic rubber particles in the modified binder [13] that have not been devulcanized and which lead to small heterogeneities that may weaken traction resistance. In certain cases this characteristic can be corrected by raising the quantity of rubber modified bitumen although there is a greater risk of plastic deformation. Regarding the fulfillment of the SMA-DS [11] requirements, the three mixtures Indirect Tensile Strength Ratio (ITSR) meet the requirements satisfactorily, in that in each case they are over the 90% stipulated. Fig. 7 compares the deformation curves of the wheel tracking test for each mixture (EN 12697-22). The values of WTS for the mixture with PBM 45/80-65 and 0.3% fiber, with 0.5% fiber and with PMB 45/80-65 R without fiber are 0.084, 0.056 and 0.038 respectively. Therefore the PMB 45/80-65 and 0.3% cellulose fiber mixture surpasses the maximum 0.070 mm/103 cycles allowed in the SMA-DS [11] meanwhile the PMB 45/80-65 and 0.5% cellulose fiber and PMB 45/80-65 R without fiber mixtures meet the specifications satisfactorily. The bituminous mixture PMB 45/80-65 R, offers the best performance regarding permanent deformations with a slope deforma8.0
Rut Depth (mm)
7.0 6.0
731
tion of 0.038 mm/103 cycles, under the 0.070 mm/103 cycles required in SMA-DS [11]. In order to understand this result, it is necessary to remember that plastic deformation tests are carried out at 60 °C. As shown in Table 5 the ring and ball temperature is slightly higher in the case of rubber modified bitumen and SBS which would explain the improved deformation performance of the bituminous mix. Over and above, the persistence of small elastic particles of devulanized rubber [13] can contribute to stabilize the bituminous mix. As far as the binder drainage results (EN 12697-18), the three mixtures met the requirements of the SMADS [11], the mixture manufactured with PMB 45/80-65 R and without fiber offered excellent results (0.05%). As mentioned, this may be explained by the presence of tiny devulcanized rubber particles and at a microscopic level by the higher molecular weight of the composites from the modified rubber. These lend the binder properties that hinder binder drainage. 6. Conclusions In Europe as well as in the United States, it is commonplace to use mineral fibers or cellulose in SMA mixtures so as to avoid binder drainage during transportation and during laying of bituminous mixtures. Nevertheless, the use of fiber results in operative complications in the manufacturing plant as well as increasing costs. The aim of this research was to evaluate the feasibility of designing a SMA mixture with a tyre rubber and SBS modified binder (PMB 45/80-65 R), without having to use the cellulose fiber stabilizers that are commonly used with the SBS modified binder (PMB 45/80-65). In view of the results obtained, it may be said that the option of PMB 45/80-65 R without fiber offers an excellent performance as regards binder drainage and permanent deformation, even better than the SMA with PMB 45/80-65 and cellulose fibers. The persistence of minute devulcanized elastic rubber particles that are suspended in the rubber modified bitumen together with the composites formed during the process, lend a higher consistency at production temperatures and service temperatures. This would explain the binder drainage test results and the permanent deformation resistance results obtained in the research. It was also observed that the water sensitivity is similar in the SMA mixture with PMB 45/80-65 R and the SMA mixture with PMB 45/80-65 and fibers. Nevertheless, the indirect tensile strength is advantageous for SMA with PMB 45/80-65 and cellulose fibers. In this case, the minute rubber particles within the binder PMB 45/80-65 R would also explain this result as they may weaken the tensile strength of the mixture. The above results are important at an industrial level since they appear to indicate that the use of bitumens modified with rubber from end of life tyres and SBS (PMB 45/80-65 R) allow to dispense with the stabilizing fiber habitually used in the bituminous mixtures manufactured with SBS modified binder (PMB 45/80-65), without the risk of plastic deformation or binder drainage.
5.0
References
4.0 3.0 2.0 1.0 0.0 0
2000
4000
6000
8000
10000
Number of Cycles PMB 45/80-65 with 0.3% cellulose fiber (average) PMB 45/80-65 with 0.5% cellulose fiber (average) PMB 45/80-65 R without cellulose fiber (average)
Fig. 7. Comparison of the deformation curves of the mixtures studied.
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