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The use of additives for the improvement of the mechanical behavior of high modulus asphalt mixes
Construction and Building Materials 70 (2014) 65–70
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
The use of additives for the improvement of the mechanical behavior of high modulus asphalt mixes F. Moreno-Navarro a,⇑, M. Sol-Sánchez a, M.C. Rubio-Gámez a, M. Segarra-Martínez b a b
Construction Engineering Laboratory of the University of Granada (LabIC.UGR), Granada, Spain Dragados, Madrid, Spain
h i g h l i g h t s HMAM with optimal fatigue resistance behavior avoiding cracking propagation were obtained. HMAM with fibers and crumb rubber increase the resistance to plastic deformation. The use of crumb rubber increases the stiffness, bearing and stress distribution capacity. HMAM with crumb rubber or fiber is more economical than HMAM with polymer modified binders.
a r t i c l e
i n f o
Article history: Received 17 June 2014 Received in revised form 28 July 2014 Accepted 31 July 2014
Keywords: High modulus Asphalt mixes Additives Fibers Crumb rubber Stiffness Triaxial Rutting Fatigue
a b s t r a c t The high stiffness provided by high modulus asphalt mixes significantly decreases the loads transmitted by the traffic to the road foundation whilst at the same time increases their resistance against plastic deformations. Thus, the use of these types of mixtures can be an effective solution in road construction, due to the fact that they can reduce the thickness of the pavement, saving economic and material resources. Nevertheless, in some cases this high stiffness constitutes a drawback because it could reduce the fatigue resistance offered by these materials, leading to a premature appearance of cracks in the pavement, which decreases its service life. Due to this fact, the use of high modulus asphalt mixes is limited, especially in cold climates. In order to solve this problem, this research has been focused on the improvement of the mechanical performance of high modulus asphalt mixtures through the use of additives (crumb rubber and acrylic fibers), which could increase their fatigue resistance by maintaining the stiffness required. Different tests have been carried out under different temperatures for the mixtures assessment. The results obtained have shown that the incorporation of these additives could lead to a better mechanical behavior of high modulus asphalt mixes, and thus it could improve their performance. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Since their appearance in the 80s, high modulus asphalt mixes (HMAM) have been regarded as one of the most effective solutions for the construction of road and airport pavements [1,2]. This type of asphalt mixtures are composed of a strong continuous mineral skeleton, which is combined with a hard binder, in order to achieve a high capacity to absorb the loads transmitted by the traffic [3]. These characteristics provide them with a high stiffness modulus, which reduces the appearance of structural distresses by minimizing the tensile strain at the bottom of the asphalt layer, and the compressive strain on top of the subgrade [4]. ⇑ Corresponding author. Tel.: +34 958249443; fax: +34 958246138. E-mail addresses: [email protected] (F. Moreno-Navarro), [email protected] (M.C. Rubio-Gámez). http://dx.doi.org/10.1016/j.conbuildmat.2014.07.115 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
Because of this fact, the use of HMAM allows the construction of roads with major structural strengthening, which can lead to a reduction of 20–23% in the thickness of the pavement [5,6], resulting in a significant saving of asphalt binder and aggregates. Furthermore, the high stiffness provided by HMAM also increases the rutting resistance of the pavement (even higher than polymer modified bitumens [7]), which contributes to enlarge its service life. During the last decades, HMAM have been used successfully in the construction of roads and airports in many European countries, EE.UU, China, South Africa, etc. [8–10], diminishing the cumulative damage produced by the traffic in the asphalt layers. Some studies have demonstrated that their use is also effective in pavement rehabilitation, even when different rates of RAP (reclaimed asphalt pavement) are used in their manufacture [11]. Nevertheless, the high stiffness provided by HMAM can also be a problem under
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certain circumstances, such as high load amplitudes and frequencies, as well as cold climates. As HMAM are very stiff, fatigue and thermal cracking resistance can be considerably reduced, which constitutes an obstacle for their application, especially in cold regions. In this sense, the appearance of cracks in HMAM can lead to a premature failure of the base course of the pavement, and thus the end of its service life. The stiffness provided by high modulus binders makes them very resistant to plastic deformations, although other properties such as elastic recovery are considerably reduced [7]. The use of polymers as modifiers of these binders can considerably improve their performance, increasing about 5–10 times better resistance to fatigue cracking than the conventional high modulus bitumens [12]. Thus, the use of modified bitumens in the manufacture of HMAM can be a good solution in order to solve this problem. However, these types of binders are more expensive than conventional high modulus bitumens, and as HMAM are used in the thickest layer of the pavement (base course), their use can increase substantially the cost of the pavement (which limits the use of this solution). Because of this fact, it could be very interesting to find other alternatives which can improve the fatigue and cracking resistance of HMAM, providing a cost-effective and sustainable solutions that could guarantee its successful use in cold climates, and roads with high volume of heavy traffic. Based on these considerations, the aim of this work is focused on improving the mechanical behavior of HMAM by using additives that can be incorporated directly to the mixture during its manufacture. For this purpose, two cost-effective and sustainable additives (acrylic fibers, AF, and crumb rubber, CR) were used to modify the mechanical properties of a HMAM manufactured with a conventional high modulus bitumen B20/30. These two mixtures (HMAM-AF and HMAM-CR) were compared with a reference one manufactured without additives and the same bitumen (HMAMR), and with a HMAM manufactured with a polymer modified high modulus bitumen BM1 (HMAM-BM1). In order to provide a complete characterization of the mechanical performance of these mixtures, various laboratory tests have been conducted during this study: marshall tests, water sensitivity tests (including freeze/thaw cycles), wheel tracking tests, creep triaxial tests (at different temperatures), stiffness tests (at different temperatures), and fatigue four point bending tests. The results obtained showed that the use of acrylic fibers or crumb rubber can improve the mechanical behavior of conventional HMAM, and the performance of these type of mixtures can be compared with those obtained by a HMAM manufactured with a high performance polymer modified bitumen. Finally, it should be highlighted that the present study has been conducted in the framework of FATE research project (Firmes Asfálticos para Temperaturas Extremas), which is being developed in Spain and aims to design long life asphalt pavements for extreme continental climates. 2. Methodology 2.1. Materials Four different types of HMAM were studied during this research. All of them were manufactured using the same continuous mineral skeleton (Fig. 1), composed by limestone aggregates in the coarse and fine fraction, and calcium carbonate in the filler. In this sense, in order to provide a representative study of the influence of the additives in the mechanical performance of the mixtures, the same amount of bitumen was used in the manufacture of the four mixtures (5.1% over the total weight of the mixture, selected from different previous Marshall tests). Thus, the only differences between the four types of HMAM studied were the type of binder used in their manufacture (B 20/25 or BM1) and the type of additive incorporated to them (acrylic fiber or crumb rubber, Fig. 2). B 20/25 is a conventional high modulus bitumen commonly used in the manufacture of this type of mixture, while BM1 is a high performance polymer modified bitumen which also offers a high stiffness combined with a good flexibility. Acrylic fibers are additives which are incorporated into the mixture in order to reinforce it
100
% of material passing
66
80 60 40 20 0 0.01
0.1
1
10
100
Sieve (mm) Fig. 1. Grain size curve. by providing a high cohesion through a tridimensional net that increase the elasticity, the fatigue and cracking resistance, and the structural stiffness at high temperatures. They are added during the mix of the aggregates in order to guarantee a correct dispersion. On the other hand, an increase in the binder content of the asphalt mixture is not needed because acrylic fibers do not absorb bitumen. Finally, crumb rubber particles are added to the mixture using the dry process [13] with the aim of improving the fatigue and elastic properties of the mixture. Table 1 shows the main characteristics of these materials. Based on these considerations, Table 2 resumes the composition and the main characteristics of the four types of HMAM that have been evaluated during this work. The mixture HMAM-R has been established as a reference to provide a comparative analysis with the mixtures HMAM-AF and HMAM-CR, in order to evaluate the hypothetical improvement provided by the use of the additives. The percentages of acrylic fiber (0.3% over the total weight of the mixture) and crumb rubber (1.5% over the total weight of the mixture) have been chosen based on according providers recommendations, previous experiences [13,14], and results from previous preparatory tests carried out in the laboratory. The mixture HMAM-BM1 has been studied to compare if the mechanical performance offered by the mixtures modified with additives (HMAM-AF and HMAM-CR) is similar to that offered by a HMAM manufactured with a high performance modified bitumen. Based on the dosage used for each additive, considering that the same aggregates are used in the manufacture of the mixtures, an average market prices of the binders used (480 €/t for the conventional bitumen B 20/25, and 700 €/t for the polymer modified bitumen BM1) and that no modifications should be done in the asphalt plant to incorporate these additives, it can be said that the use of acrylic fibers and crumb rubber suppose a much more cost effective solution than the use of polymer modified binders. Table 3 shows that in comparison with the mixture manufactured with the polymer modified bitumen, the raw materials costs can be reduced in a 10.5% when acrylic fibers are used and in a 27.3% when crumb rubber is used.
2.2. Testing plan To analyze the influence of the additives in the mechanical behavior of the HMAM studied, different test methods and different test conditions have been considered. In this sense, the variety of the tests developed has covered the different aspects that can affect the long-performance of the HMAM during their service life. Thus, water sensitivity tests under extreme temperature conditions (including freeze/thaw cycles), resistance to plastic deformations tests (wheel tracking test and confined triaxial tests under different temperatures), bearing and stress absorption capacities (stiffness tests under different temperatures), and resistance to fatigue tests have been developed during this research. Water sensitivity tests have been carried out based on EN 12697-12 [15], although during their performance, extreme test conditions such as low compaction energy (to simulate the most unfavorable conditions during the construction in cold climates) and freeze/thaw cycles (to verify the influence of the high stiffness in the resistance against water in cold climates) have been applied. Thus, these tests involve the manufacture of six test specimens with a diameter of 101.6 mm and a thickness of 60 mm that have been compacted with 35 blows on each side by a Marshall hammer. The specimens were subsequently divided into two sets of three specimens: a dry set and a wet set. The set of dry specimens was stored at room temperature in the laboratory (20 ± 5 °C), whereas vacuum was applied to the wet set for 30 ± 5 min until a pressure of 6.7 ± 0.3 kPa was obtained, then the specimens were immersed in water at a temperature of 40 °C for a period of 72 h. After that, they are stored at 18 °C for 16 h, and then immersed in water during 24 h at a temperature of 60 °C until thaw. Fig. 3 shows the conditioning to freeze of the wet set specimens. The next step was to perform an indirect traction resistance test on each of the cylinders (in both the dry set and the wet set). This was done at a temperature of 25 °C, and after a previous period of adjustment of 120 min to this temperature. The evaluation of the resistance to plastic deformation was carried out thorough the wheel tracking test and the confined triaxial test. The wheel tracking test (EN 12697-22, [16]) involves the application of a load (700 N) on the test specimen
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Fig. 2. Additives used during this research: acrylic fibers (left) and crumb rubber (right).
Table 1 Additives properties. Property
Acrylic fibers
Crumb rubber
Color Density (g/cm3) Particle morphology Maximum size (mm) Heat resistance Cost (€/t)
White 1.17 Elongated <10 Good 3000
Black 1.15 Irregular <0.6 Good 200
(408 mm 256 mm 60 mm) by means of the repeated passes of a loaded wheel (10,000 cycles), at a temperature of 60 °C. In each of the wheel passes, the resulting deformation on the specimen is measured. The objective of the test was to determine the wheel-tracking slope (WTS, mm/103 load cycles) measured in the last 5000 load cycles. The confined triaxial compression test (EN 12697-25, method B, cyclic compression test, [17]) involves the combined application of a confining load of 120 kPa and another cyclic sinusoidal out-of-phase axial loading of 300 kPa at a frequency of 3 Hz during 12,000 load cycles. The creep and permanent deformation parameters for each specimen (cylinders with a diameter of 101.6 mm and a sawedoff height of 60 mm) are calculated, and the results obtained in the test are showed as the mean of the values obtained for a pair of test specimens. In order to provide the influence of the temperature in the mechanical response of the different mixtures evaluated, these tests have been carried out at different temperatures (40, 50, 60 and 70 °C).
Fig. 3. Freezing conditions of asphalt specimens. The bearing capacity and stress distribution has been evaluated using the stiffness modulus test (EN 12697-26, Annex C, [18]). The stiffness modulus was determined by applying a series of 15 indirect-tensile haversine-shaped load pulses lasting three seconds each. The first 10 pulses helped the specimens adjust to the load intensity and duration. The following five pulses determined the stiffness modulus of mix, which was calculated as the mean value of the 5 pulses. After this value was determined, the cylinder (with a diameter of 101.6 mm and a thickness of 60 mm, and compacted with 75 blows on each side by an impact compactor) was turned so that the modulus of the perpendicular diameter could also be calculated. The final stiffness modulus value was the mean of the two diameters. One more time, the influence of the temperature in the mechanical performance of the materials has been evaluated and this test has been carried out at 0, 10, 20, 30 and 40 °C.
Table 2 Asphalt mixtures properties. Property
HMAM-R
HMAM-AF
HMAM-CR
HMAM-BM1
Type of bitumen Bitumen content (% over the total weight of the mixture) Type of additive Additive content (% over the total weight of the mixture) Process of addition Maximum density (g/cm3) Bulk density (g/cm3) Air voids (%) Marshall stability (kN) Marshall flow (mm)
B 20/25 5.1 – – – 2.615 2.534 3.3 14.671 2.5
B 20/25 5.1 Acrylic fiber 0.3 Dry 2.625 2.515 3.5 15.146 4.0
B 20/25 5.1 Crumb rubber 1.5 Dry 2.552 2.482 3.0 14.915 3.1
BM1 5.1 SBS polymer 1.0 Wet 2.621 2.518 3.9 17.732 2.5
Table 3 Asphalt mixtures manufacturing costs. Asphalt mixture
Material
HMAM-R
B 20/25
HMAM-AF
B 20/25 Acrylic fiber
HMAM-CR HMAM-BM1
Cost (€/t)
Dosage (%)
Total cost (€/t)
Total (€/t)
480
5.10
24.48
24.48
480 3000
5.10 0.30
24.48 9.00
33.48
B 20/25 Crumb rubber
480 200
5.10 1.50
24.48 3.00
27.48
BM1
700
5.10
35.7
35.7
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Finally, the fatigue cracking resistance of the asphalt materials has been measured through the four point bending fatigue test (EN 12697-24, Annex D, [19]). Specimens of 408 50 50 mm with sawn faces were manufactured and used during the performance of this test, and a sinusoidal waveform load was applied. The tests were carried out at 10 °C, in strain control mode and at a frequency of 10 Hz. The mixtures were tested in four different strain amplitude levels, 250 lm/m; 200 lm/m; 150 lm/m and 100 lm/m, and testing three specimens in each level. Table 4 resumes the testing plan developed for each mixture.
96.7
Indirect Tensile Strength (kPa)
2500
3. Analysis of the results 3.1. Water sensitivity tests
98.3
100
91
90.7
90 2000
80 70
1500
60 50
1000
40 30
500
20 10
0
0 HMAM-R
Fig. 4 shows the water sensitivity test results obtained from the different asphalt mixtures tested. As can be observed, the incorporation of the additives tested in this research (acrylic fibers or crumb rubber) induces a decrease in the indirect tensile resistance of the high modulus asphalt mixtures, especially when crumb rubber is used (which could be due to a lack of interaction between the hard bitumen and the crumb rubber particles [20]). These results agree with those obtained by other researchers where the tensile resistance of asphalt mixtures drops when fibers [21–23] or crumb rubber [24,25] are added. The HMAM-BM1 offers the highest resistance, which proves the efficiency of the SBS polymers in the improvement of the mechanical response of the mixture. In terms of restrained resistance, the incorporation of additives in the high modulus mixtures does not offer a significant influence in the response of these materials. In this sense, all the mixtures tested offer a good behavior, even after a freeze/thaw cycle. These results also agree with those obtained by other researchers that had used similar additives [26,27].
HMAM-AF Wet Set
HMAM-CR Dry Set
HMAM-BM1 ITSR
Indirect Tensile Strength Rao, ITSR (%)
68
Fig. 4. Water sensitivity tests results.
Table 5 Wheel-tracking tests results. Type of mixture HMAM-R
HMAM-AF
HMAM-CR
HMAM-BM1
Density (Mg/m ) Specimen 1 2.515 Specimen 2 2.521 Mean 2.518
2.497 2.507 2.502
2.479 2.482 2.4805
2.508 2.498 2.503
WTS (mm/1000 cycles) Specimen 1 0.13 Specimen 2 0.175 Mean 0.153
0.111 0.07 0.091
0.091 0.068 0.080
0.082 0.151 0.117
Final deformation (mm) Specimen 1 3.243 Specimen 2 3.778 Mean 3.511
3.511 3.007 3.259
2.436 2.588 2.512
3.004 3.676 3.340
3
3.2. Wheel-tracking tests Table 5 resumes the results obtained during the wheel-tracking tests carried out. The incorporation of the additives causes an improvement in the resistance against plastic deformations. The use of a SBS modified asphalt binder (HMAM-BM1) showed a reduction in the appearance of rutting in high modulus asphalt mixtures. In this sense, it can be said that the use of crumb rubber as a modifier of the conventional bitumen (HMAM-CR) is much more effective than SBS polymers. This aspect agrees with other researches findings [28,29], and it could be due to the capacity of the elastic particles of rubber to dissipate stresses. Finally, the use of acrylic fibers as a modifier also induces an improvement in the mechanical response of the asphalt mixture against plastic deformations. The presence of this additive develops a tridimensional net inside the asphalt mixture that can distribute
the stresses generated in the material. Similar results have been obtained in this respect when polyester, polyacrylonitrile, lignin or asbestos fibers are used [14,26].
3.3. Triaxial tests Fig. 5 shows the susceptibility of the different mixtures studied to high temperatures through the use of the confined cyclic triaxial test. The use of additives considerably reduces their sensitivity to temperature (the slope in the curves is considerably reduced), thus it can offer a more stable response in different climatic scenarios. High modulus bitumens have a good performance against plastic deformations, nonetheless the results obtained in the different tri-
Table 4 Testing plan developed for each asphalt mixture studied during this research. Type of test
Standard
Temperature (°C)
Water sensitivity
EN 12697-12 [15]
25
Number of specimens 6
Observations Include a freeze/thaw cycle
Wheel tracking
EN 12697-22 [16]
60
2
–
Cyclic confined triaxial
EN 12697-25 Method B [17]
EN 12697-25, Annex C [18]
3 3 3 3 2 2 2 2 2
Confining load: 120 kPa Cyclic load: 300 kPa Test frequency: 3 Hz Number of cycles: 12,000
Stiffness modulus
40 50 60 70 0 10 20 30 40
Four point bending fatigue
EN 12697-24, Annex D [19]
10
18
Strain controlled test Test frequency: 10 Hz Strain levels: 250, 200, 150, 100 lm/m
69
2.5
10000000
2
10000000
Failure Cycle
Permanent Deformaon (%)
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1.5 1
y = 8E+16x-5.22 R² = 0.964
1000000 y = 2E+17x-5.48 R² = 0.978
100000 10000
y = 1E+18x-5.77 R² = 0.976
0.5
y = 7E+19x-6.71 R² = 0.940
1000
0 20
30
40
50
60
70
80
50
90
Temperature (°C) HMAM-R
HMAM-AF
HMAM-CR
Strain (με) HMAM-BM1 HMAM-R
Fig. 5. Triaxial tests results.
HMAM-CR
HMAM-BM1
Fig. 7. Fatigue life law obtained in the four point bending fatigue tests.
axial tests have shown that under extreme high temperatures the presence of the modifiers has an important positive effect. At these temperatures, the consistency of the bitumen is low and it is susceptible to flow under any load. The additives used provide a tridimensional net that avoids this phenomenon and provides more resistance to the asphalt material. Based on the results obtained under these tests conditions (cyclic confined loading), the SBS polymer modified bitumen shows the best response. 3.4. Stiffness modulus tests Stiffness modulus of asphalt mixtures is considerably reduced when temperature is increased (Fig. 6). In this sense, the bearing and stress dissipation capacities of these materials at high service temperatures are limited and thus, the appearance of plastic deformations is more probable. On the contrary, when service temperatures are considerably low asphalt mixtures offer a high stiffness that could lead to a premature fatigue cracking failure. Because of this fact, the presence of additives in asphalt mixtures should reduce the stiffness at low temperatures (in order to avoid the appearance of cracking) and increase it at high temperatures (in order to avoid the appearance of rutting). Results obtained in the tests carried out at different temperatures show that the presence of crumb rubber (HMAM-CR) and SBS polymer (HMAM-BM1) increase the stiffness of the mixture at any temperature. The presence of these additives reduces the strain induced to the material under any load due to their resilient response. This aspect agrees with the results obtained during the evaluation of the resistance to plastic deformation (which are improved when these additives are used), and with those obtained in other studies [14,28]. Special attention should be paid to the case of acrylic fibers (HMAM-AF). In this case, the use of this type of additive reduces the stiffness of the asphalt mixture making it more flexible (as occurs when cellulose
Sffness Modulus (MPa)
HMAM-AF
50000
Table 6 Resume of the results obtained in the fatigue four point bending tests.
e 106 e 106 max. e 106 min. a b R2
HMAM-R
HMAM-AF
HMAM-CR
HMAM-BM1
115 120 109 2 1017 5.487 97.85%
122 129 116 8 1016 5.222 96.64%
115 121 110 7 1019 6.713 94.01%
120 126 114 1 1018 5.773 97.60%
or polyester fibers are used [21,30]). As observed in previous tests, this aspect has not had a negative effect in the resistance to plastic deformations, so if both results are combined, it could be said that an improvement in the general performance could be achieved. 3.5. Fatigue four-point bending tests Fig. 7 and Table 6 resume the results obtained in the fatigue four-point bending tests. As expected, the use of a polymer modified bitumen (HMAM-BM1) improves the fatigue resistance of asphalt mixtures, especially at low strain levels. At these strain levels, the presence of crumb rubber (HMAM-CR) also exerts a positive effect on the resistance to fatigue of the mixture, although at high strain levels (which can appear when high defections are appeared in the pavement) this resistance is considerably reduced due to the high stiffness of these materials. According to the results obtained in the stiffness tests, the use of acrylic fibers (HMAM-AF) improves the fatigue resistance of high modulus asphalt mixtures at any strain level. In accordance to these results, similar improvements in fatigue life have been also achieved when other types of fibers are used in asphalt concretes [30]. Based on the results obtained, HMAM manufactured with this additive could offer a mechanical response as good as those manufactured with a high performance polymer modified bitumen (BM1). 4. Conclusions
5000
500 -10
0
10
20
30
40
50
Temperature (°C) HMAM-R
HMAM-AF
HMAM-CR
HMAM-BM1
Fig. 6. Stiffness modulus tests results.
60
High modulus asphalt mixtures suppose an interesting choice for the construction of road pavements. Nevertheless, under certain climate conditions, its mechanical performance is limited due to their low flexibility and low stress relaxation capacity. This problem can be solved using polymer modified binders, but its application is reduced due to high costs associated to this solution. Because of this fact, one of the objectives of the FATE research project has been the assessment of the feasibility of the use of acrylic fibers and crumb rubber additives as an alternative for the improvement of the mechanical response of high modulus asphalt
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mixtures. This paper describes the laboratory works carried out in this sense at the University of Granada (Spain) in the framework of this project. Based on the results obtained, the following conclusions can be drawn: – The use of crumb rubber or acrylic fiber as modifiers in the manufacture of high modulus asphalt mixtures is a more economical solution than the use of high performance polymer modified binders. – The use of these additives slightly reduces the mechanical resistance of the high modulus asphalt mixtures in terms of indirect tensile stress. Nevertheless, they do not exert any negative effect in the water susceptibility of these materials, even after a freeze/thaw cycle. As the four mixtures studied offer a low sensitivity to water and ice effects, the use of the additives do not exert a significant different in this term. – The presence of acrylic fiber and crumb rubber increase the resistance to plastic deformations of high modulus asphalt mixtures and also reduces their susceptibility to the temperature. The results obtained in the wheel-tracking and confined cyclic triaxial tests show that the mixtures manufactured with these additives offer a similar resistance to rutting and climate changes than that offered by a high modulus asphalt mixture manufactured with a polymer modified bitumen, and better than the conventional high modulus mixture. – The addition of crumb rubber to high modulus asphalt mixtures increase their stiffness, which is a positive effect in terms of bearing and stress distribution capacities. Nevertheless, when these materials are subjected to high strain this aspect could exert a negative effect in their fatigue resistance, inducing a premature failure of this type of mixtures when deflections in the pavement layers are high. The addition of acrylic fibers to the high modulus asphalt mixtures reduces their stiffness, but as commented, this aspect does not cause any negative effect in their plastic deformation resistance. On the contrary, this additive provides an improvement in the flexibility of the asphalt mixture, which results in a considerable increase of its fatigue life (even bigger than that offered by an asphalt mixture manufactured with a high performance polymer modified bitumen).
Acknowledgements The present article has been conducted within the framework of the FATE research project (Firmes Asfálticos para Temperaturas Extremas, IPT-2012-0977-370000), funded by the Ministry of Innovation and Science of Spain, inside the National Plan of Science Research, Development and Technological Innovation 2012–2015, and co-funded by the FEDER funds. References [1] Nunn ME, Smith T. Road trials of high modulus base for heavily trafficked roads. England: Thomas Telford; 1999. [2] Corté JF. Development and uses of hard grade asphalt and of high modulus asphalt mixes in France. Transportation Research Circular No. 503, TRB, National Research Council, Washington D.C.; 2003. p. 12–30. [3] Maupin GW, Diefenderfer BK. Design of a high-binder-high-modulus asphalt mixture. Research Report VTRC 07-R15, Virginia Transportation Research Council; 2006. [4] Lee HJ, Lee JH, Park HM. Performance evaluation of high modulus asphalt mixtures for long life pavements. Constr Build Mater 2007;21:1079–87.
[5] Catalogue des structures types de chaussées neuves de la direction des routes. LCPC/SETRA; 1998. [6] Ruiz A. Mezclas de Alto Módulo. Madrid: Jornadas para Jefes de Obras de Probisa; 2002. [7] Geng H, Clopotel CS, Bahia HU. Effects of high modulus asphalt binders on performance of typical asphalt pavements structures. Constr Build Mater 2013;44:207–13. [8] Diefenderfer BK, Maupin GW. Field trials of high-modulus-high-bindercontent base layer hot-mix asphalt mixtures. Research Report VTRC 17-R2, Virginia Transportation Research Council; 2010. [9] Nkgapele M, Denneman E, Anochie-Boateng JK. Construction of a high modulus asphalt trial section Ethekwini: South Africa’s first practical experience with design, manufacturing and paving. In: Proceedings of the 31st Southern African Transport Conference (SATC); 2012. [10] Liu YQ. Application technology study on high modulus hot mix asphalt concrete. Liaoning Province: Commun. Res. Inst; 2007. [11] Miró R, Valdés G, Martínez A, Segura P, Rodríguez C. Evaluation of high modulus mixtures behavior with high reclaimed asphalt pavement (RAP) percentages for sustainable road construction. Constr Build Mater 2011;25:3854–62. [12] Bankowski W, Tusar M, Wiman LG. Laboratory and field implementation of high modulus asphalt concrete. Requirements for HMAC mix design and pavement design. Sustainable Pavements for European New Members States, SPENS. European Commission DG Research, 6th Framework Programme, Sustainable Surface Transport; 2009. [13] Moreno F, Rubio MC, Martinez-Echevarria MJ. Analysis of digestion time and the crumb rubber percentage in dry-process crumb rubber modified hot bituminous mixes. Constr Build Mater 2011;25(5):2323–34. [14] Abtahi SM, Sheikhzadeh M, Hejazi SM. Fiber-reinforced asphalt-concrete – a review. Constr Build Mater 2010;24:871–7. [15] EN 12697-12: Bituminous mixtures. Test methods for hot mix asphalt – Part 12: Determination of the water sensitivity of bituminous specimens. AENOR, Asociación Española de Normalización y Certificación, Madrid; 2009. [16] EN 12697-22: Bituminous mixtures. Test methods for hot mix asphalt – Part 22: Wheel tracking. AENOR, Asociación Española de Normalización y Certificación, Madrid; 2008. [17] EN 12697-25, Method B: Bituminous mixtures. Test methods for hot mix asphalt – Part 25: Cyclic compression test. AENOR, Asociación Española de Normalización y Certificación, Madrid; 2006. [18] EN 12697-26, Annex C: Bituminous mixtures. Test methods for hot mix asphalt – Part 26: Stiffness. AENOR, Asociación Española de Normalización y Certificación, Madrid; 2012. [19] EN 12697-24, Annex D: Bituminous mixtures. Test methods for hot mix asphalt – Part 24: Resistance to fatigue. AENOR, Asociación Española de Normalización y Certificación, Madrid; 2012. [20] State of California Department of Transportation. Asphalt rubber usage guide. Materials Engineering and Testing Services, September 30; 2006. [21] Oda S, Fernandes JL, Ildefonso JS. Analysis of use of natural fibers and asphalt rubber binder in discontinuous asphalt mixtures. Constr Build Mater 2012;26:13–20. [22] Ferroti G, Pasquini E, Canestrani F. Experimental characterization of highperformance fiber-reinforced cold mix asphalt mixtures. Constr Build Mater 2014;57:117–25. [23] Anurag K, Xiao F, Amirkhanian S. Laboratory investigation of indirect tensile strength using roofing polyester waste fibers in hot mix asphalt. Constr Build Mater 2009;23:2035–40. [24] Akisetty C, Xiao F, Gandhi T, Amirkhanian S. Estimating correlations between rheological and engineering properties of rubberized asphalt concrete mixtures containing warm mix asphalt additive. Constr Build Mater 2011;25:950–6. [25] Xiao F, Amirkhanian S, Shen J, Putman B. Influences of crumb rubber size and type on reclaimed asphalt pavement (RAP) mixtures. Constr Build Mater 2009;23:1028–34. [26] Xu Q, Chen H, Prozzi JA. Performance of fiber reinforced asphalt concrete under environmental temperature and water effects. Constr Build Mater 2010;24:2003–10. [27] Moreno F, Rubio MC, Martinez-Echevarria MJ. The mechanical performance of dry-process crumb rubber modified hot bituminous mixes: the influence of digestion time and crumb rubber percentage. Constr Build Mater 2012;26(1):446–74. [28] Moreno F, Sol M, Pérez M, Martín J, Rubio MC. The effect of crumb rubber modifier on the resistance of asphalt mixes to plastic deformation. Mater Des 2013;47:274–80. [29] Kök BV, Çolak H. Laboratory comparison of the crumb-rubber and SBS modified bitumen and hot mix asphalt. Constr Build Mater 2011;25:3024–212. [30] Ye Q, Wu S, Li N. Investigation of the dynamic and fatigue properties of fibermodified asphalt mixtures. Int J Fatigue 2009;31:1598–602.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
The use of additives for the improvement of the mechanical behavior of high modulus asphalt mixes F. Moreno-Navarro a,⇑, M. Sol-Sánchez a, M.C. Rubio-Gámez a, M. Segarra-Martínez b a b
Construction Engineering Laboratory of the University of Granada (LabIC.UGR), Granada, Spain Dragados, Madrid, Spain
h i g h l i g h t s HMAM with optimal fatigue resistance behavior avoiding cracking propagation were obtained. HMAM with fibers and crumb rubber increase the resistance to plastic deformation. The use of crumb rubber increases the stiffness, bearing and stress distribution capacity. HMAM with crumb rubber or fiber is more economical than HMAM with polymer modified binders.
a r t i c l e
i n f o
Article history: Received 17 June 2014 Received in revised form 28 July 2014 Accepted 31 July 2014
Keywords: High modulus Asphalt mixes Additives Fibers Crumb rubber Stiffness Triaxial Rutting Fatigue
a b s t r a c t The high stiffness provided by high modulus asphalt mixes significantly decreases the loads transmitted by the traffic to the road foundation whilst at the same time increases their resistance against plastic deformations. Thus, the use of these types of mixtures can be an effective solution in road construction, due to the fact that they can reduce the thickness of the pavement, saving economic and material resources. Nevertheless, in some cases this high stiffness constitutes a drawback because it could reduce the fatigue resistance offered by these materials, leading to a premature appearance of cracks in the pavement, which decreases its service life. Due to this fact, the use of high modulus asphalt mixes is limited, especially in cold climates. In order to solve this problem, this research has been focused on the improvement of the mechanical performance of high modulus asphalt mixtures through the use of additives (crumb rubber and acrylic fibers), which could increase their fatigue resistance by maintaining the stiffness required. Different tests have been carried out under different temperatures for the mixtures assessment. The results obtained have shown that the incorporation of these additives could lead to a better mechanical behavior of high modulus asphalt mixes, and thus it could improve their performance. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Since their appearance in the 80s, high modulus asphalt mixes (HMAM) have been regarded as one of the most effective solutions for the construction of road and airport pavements [1,2]. This type of asphalt mixtures are composed of a strong continuous mineral skeleton, which is combined with a hard binder, in order to achieve a high capacity to absorb the loads transmitted by the traffic [3]. These characteristics provide them with a high stiffness modulus, which reduces the appearance of structural distresses by minimizing the tensile strain at the bottom of the asphalt layer, and the compressive strain on top of the subgrade [4]. ⇑ Corresponding author. Tel.: +34 958249443; fax: +34 958246138. E-mail addresses: [email protected] (F. Moreno-Navarro), [email protected] (M.C. Rubio-Gámez). http://dx.doi.org/10.1016/j.conbuildmat.2014.07.115 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
Because of this fact, the use of HMAM allows the construction of roads with major structural strengthening, which can lead to a reduction of 20–23% in the thickness of the pavement [5,6], resulting in a significant saving of asphalt binder and aggregates. Furthermore, the high stiffness provided by HMAM also increases the rutting resistance of the pavement (even higher than polymer modified bitumens [7]), which contributes to enlarge its service life. During the last decades, HMAM have been used successfully in the construction of roads and airports in many European countries, EE.UU, China, South Africa, etc. [8–10], diminishing the cumulative damage produced by the traffic in the asphalt layers. Some studies have demonstrated that their use is also effective in pavement rehabilitation, even when different rates of RAP (reclaimed asphalt pavement) are used in their manufacture [11]. Nevertheless, the high stiffness provided by HMAM can also be a problem under
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certain circumstances, such as high load amplitudes and frequencies, as well as cold climates. As HMAM are very stiff, fatigue and thermal cracking resistance can be considerably reduced, which constitutes an obstacle for their application, especially in cold regions. In this sense, the appearance of cracks in HMAM can lead to a premature failure of the base course of the pavement, and thus the end of its service life. The stiffness provided by high modulus binders makes them very resistant to plastic deformations, although other properties such as elastic recovery are considerably reduced [7]. The use of polymers as modifiers of these binders can considerably improve their performance, increasing about 5–10 times better resistance to fatigue cracking than the conventional high modulus bitumens [12]. Thus, the use of modified bitumens in the manufacture of HMAM can be a good solution in order to solve this problem. However, these types of binders are more expensive than conventional high modulus bitumens, and as HMAM are used in the thickest layer of the pavement (base course), their use can increase substantially the cost of the pavement (which limits the use of this solution). Because of this fact, it could be very interesting to find other alternatives which can improve the fatigue and cracking resistance of HMAM, providing a cost-effective and sustainable solutions that could guarantee its successful use in cold climates, and roads with high volume of heavy traffic. Based on these considerations, the aim of this work is focused on improving the mechanical behavior of HMAM by using additives that can be incorporated directly to the mixture during its manufacture. For this purpose, two cost-effective and sustainable additives (acrylic fibers, AF, and crumb rubber, CR) were used to modify the mechanical properties of a HMAM manufactured with a conventional high modulus bitumen B20/30. These two mixtures (HMAM-AF and HMAM-CR) were compared with a reference one manufactured without additives and the same bitumen (HMAMR), and with a HMAM manufactured with a polymer modified high modulus bitumen BM1 (HMAM-BM1). In order to provide a complete characterization of the mechanical performance of these mixtures, various laboratory tests have been conducted during this study: marshall tests, water sensitivity tests (including freeze/thaw cycles), wheel tracking tests, creep triaxial tests (at different temperatures), stiffness tests (at different temperatures), and fatigue four point bending tests. The results obtained showed that the use of acrylic fibers or crumb rubber can improve the mechanical behavior of conventional HMAM, and the performance of these type of mixtures can be compared with those obtained by a HMAM manufactured with a high performance polymer modified bitumen. Finally, it should be highlighted that the present study has been conducted in the framework of FATE research project (Firmes Asfálticos para Temperaturas Extremas), which is being developed in Spain and aims to design long life asphalt pavements for extreme continental climates. 2. Methodology 2.1. Materials Four different types of HMAM were studied during this research. All of them were manufactured using the same continuous mineral skeleton (Fig. 1), composed by limestone aggregates in the coarse and fine fraction, and calcium carbonate in the filler. In this sense, in order to provide a representative study of the influence of the additives in the mechanical performance of the mixtures, the same amount of bitumen was used in the manufacture of the four mixtures (5.1% over the total weight of the mixture, selected from different previous Marshall tests). Thus, the only differences between the four types of HMAM studied were the type of binder used in their manufacture (B 20/25 or BM1) and the type of additive incorporated to them (acrylic fiber or crumb rubber, Fig. 2). B 20/25 is a conventional high modulus bitumen commonly used in the manufacture of this type of mixture, while BM1 is a high performance polymer modified bitumen which also offers a high stiffness combined with a good flexibility. Acrylic fibers are additives which are incorporated into the mixture in order to reinforce it
100
% of material passing
66
80 60 40 20 0 0.01
0.1
1
10
100
Sieve (mm) Fig. 1. Grain size curve. by providing a high cohesion through a tridimensional net that increase the elasticity, the fatigue and cracking resistance, and the structural stiffness at high temperatures. They are added during the mix of the aggregates in order to guarantee a correct dispersion. On the other hand, an increase in the binder content of the asphalt mixture is not needed because acrylic fibers do not absorb bitumen. Finally, crumb rubber particles are added to the mixture using the dry process [13] with the aim of improving the fatigue and elastic properties of the mixture. Table 1 shows the main characteristics of these materials. Based on these considerations, Table 2 resumes the composition and the main characteristics of the four types of HMAM that have been evaluated during this work. The mixture HMAM-R has been established as a reference to provide a comparative analysis with the mixtures HMAM-AF and HMAM-CR, in order to evaluate the hypothetical improvement provided by the use of the additives. The percentages of acrylic fiber (0.3% over the total weight of the mixture) and crumb rubber (1.5% over the total weight of the mixture) have been chosen based on according providers recommendations, previous experiences [13,14], and results from previous preparatory tests carried out in the laboratory. The mixture HMAM-BM1 has been studied to compare if the mechanical performance offered by the mixtures modified with additives (HMAM-AF and HMAM-CR) is similar to that offered by a HMAM manufactured with a high performance modified bitumen. Based on the dosage used for each additive, considering that the same aggregates are used in the manufacture of the mixtures, an average market prices of the binders used (480 €/t for the conventional bitumen B 20/25, and 700 €/t for the polymer modified bitumen BM1) and that no modifications should be done in the asphalt plant to incorporate these additives, it can be said that the use of acrylic fibers and crumb rubber suppose a much more cost effective solution than the use of polymer modified binders. Table 3 shows that in comparison with the mixture manufactured with the polymer modified bitumen, the raw materials costs can be reduced in a 10.5% when acrylic fibers are used and in a 27.3% when crumb rubber is used.
2.2. Testing plan To analyze the influence of the additives in the mechanical behavior of the HMAM studied, different test methods and different test conditions have been considered. In this sense, the variety of the tests developed has covered the different aspects that can affect the long-performance of the HMAM during their service life. Thus, water sensitivity tests under extreme temperature conditions (including freeze/thaw cycles), resistance to plastic deformations tests (wheel tracking test and confined triaxial tests under different temperatures), bearing and stress absorption capacities (stiffness tests under different temperatures), and resistance to fatigue tests have been developed during this research. Water sensitivity tests have been carried out based on EN 12697-12 [15], although during their performance, extreme test conditions such as low compaction energy (to simulate the most unfavorable conditions during the construction in cold climates) and freeze/thaw cycles (to verify the influence of the high stiffness in the resistance against water in cold climates) have been applied. Thus, these tests involve the manufacture of six test specimens with a diameter of 101.6 mm and a thickness of 60 mm that have been compacted with 35 blows on each side by a Marshall hammer. The specimens were subsequently divided into two sets of three specimens: a dry set and a wet set. The set of dry specimens was stored at room temperature in the laboratory (20 ± 5 °C), whereas vacuum was applied to the wet set for 30 ± 5 min until a pressure of 6.7 ± 0.3 kPa was obtained, then the specimens were immersed in water at a temperature of 40 °C for a period of 72 h. After that, they are stored at 18 °C for 16 h, and then immersed in water during 24 h at a temperature of 60 °C until thaw. Fig. 3 shows the conditioning to freeze of the wet set specimens. The next step was to perform an indirect traction resistance test on each of the cylinders (in both the dry set and the wet set). This was done at a temperature of 25 °C, and after a previous period of adjustment of 120 min to this temperature. The evaluation of the resistance to plastic deformation was carried out thorough the wheel tracking test and the confined triaxial test. The wheel tracking test (EN 12697-22, [16]) involves the application of a load (700 N) on the test specimen
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Fig. 2. Additives used during this research: acrylic fibers (left) and crumb rubber (right).
Table 1 Additives properties. Property
Acrylic fibers
Crumb rubber
Color Density (g/cm3) Particle morphology Maximum size (mm) Heat resistance Cost (€/t)
White 1.17 Elongated <10 Good 3000
Black 1.15 Irregular <0.6 Good 200
(408 mm 256 mm 60 mm) by means of the repeated passes of a loaded wheel (10,000 cycles), at a temperature of 60 °C. In each of the wheel passes, the resulting deformation on the specimen is measured. The objective of the test was to determine the wheel-tracking slope (WTS, mm/103 load cycles) measured in the last 5000 load cycles. The confined triaxial compression test (EN 12697-25, method B, cyclic compression test, [17]) involves the combined application of a confining load of 120 kPa and another cyclic sinusoidal out-of-phase axial loading of 300 kPa at a frequency of 3 Hz during 12,000 load cycles. The creep and permanent deformation parameters for each specimen (cylinders with a diameter of 101.6 mm and a sawedoff height of 60 mm) are calculated, and the results obtained in the test are showed as the mean of the values obtained for a pair of test specimens. In order to provide the influence of the temperature in the mechanical response of the different mixtures evaluated, these tests have been carried out at different temperatures (40, 50, 60 and 70 °C).
Fig. 3. Freezing conditions of asphalt specimens. The bearing capacity and stress distribution has been evaluated using the stiffness modulus test (EN 12697-26, Annex C, [18]). The stiffness modulus was determined by applying a series of 15 indirect-tensile haversine-shaped load pulses lasting three seconds each. The first 10 pulses helped the specimens adjust to the load intensity and duration. The following five pulses determined the stiffness modulus of mix, which was calculated as the mean value of the 5 pulses. After this value was determined, the cylinder (with a diameter of 101.6 mm and a thickness of 60 mm, and compacted with 75 blows on each side by an impact compactor) was turned so that the modulus of the perpendicular diameter could also be calculated. The final stiffness modulus value was the mean of the two diameters. One more time, the influence of the temperature in the mechanical performance of the materials has been evaluated and this test has been carried out at 0, 10, 20, 30 and 40 °C.
Table 2 Asphalt mixtures properties. Property
HMAM-R
HMAM-AF
HMAM-CR
HMAM-BM1
Type of bitumen Bitumen content (% over the total weight of the mixture) Type of additive Additive content (% over the total weight of the mixture) Process of addition Maximum density (g/cm3) Bulk density (g/cm3) Air voids (%) Marshall stability (kN) Marshall flow (mm)
B 20/25 5.1 – – – 2.615 2.534 3.3 14.671 2.5
B 20/25 5.1 Acrylic fiber 0.3 Dry 2.625 2.515 3.5 15.146 4.0
B 20/25 5.1 Crumb rubber 1.5 Dry 2.552 2.482 3.0 14.915 3.1
BM1 5.1 SBS polymer 1.0 Wet 2.621 2.518 3.9 17.732 2.5
Table 3 Asphalt mixtures manufacturing costs. Asphalt mixture
Material
HMAM-R
B 20/25
HMAM-AF
B 20/25 Acrylic fiber
HMAM-CR HMAM-BM1
Cost (€/t)
Dosage (%)
Total cost (€/t)
Total (€/t)
480
5.10
24.48
24.48
480 3000
5.10 0.30
24.48 9.00
33.48
B 20/25 Crumb rubber
480 200
5.10 1.50
24.48 3.00
27.48
BM1
700
5.10
35.7
35.7
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Finally, the fatigue cracking resistance of the asphalt materials has been measured through the four point bending fatigue test (EN 12697-24, Annex D, [19]). Specimens of 408 50 50 mm with sawn faces were manufactured and used during the performance of this test, and a sinusoidal waveform load was applied. The tests were carried out at 10 °C, in strain control mode and at a frequency of 10 Hz. The mixtures were tested in four different strain amplitude levels, 250 lm/m; 200 lm/m; 150 lm/m and 100 lm/m, and testing three specimens in each level. Table 4 resumes the testing plan developed for each mixture.
96.7
Indirect Tensile Strength (kPa)
2500
3. Analysis of the results 3.1. Water sensitivity tests
98.3
100
91
90.7
90 2000
80 70
1500
60 50
1000
40 30
500
20 10
0
0 HMAM-R
Fig. 4 shows the water sensitivity test results obtained from the different asphalt mixtures tested. As can be observed, the incorporation of the additives tested in this research (acrylic fibers or crumb rubber) induces a decrease in the indirect tensile resistance of the high modulus asphalt mixtures, especially when crumb rubber is used (which could be due to a lack of interaction between the hard bitumen and the crumb rubber particles [20]). These results agree with those obtained by other researchers where the tensile resistance of asphalt mixtures drops when fibers [21–23] or crumb rubber [24,25] are added. The HMAM-BM1 offers the highest resistance, which proves the efficiency of the SBS polymers in the improvement of the mechanical response of the mixture. In terms of restrained resistance, the incorporation of additives in the high modulus mixtures does not offer a significant influence in the response of these materials. In this sense, all the mixtures tested offer a good behavior, even after a freeze/thaw cycle. These results also agree with those obtained by other researchers that had used similar additives [26,27].
HMAM-AF Wet Set
HMAM-CR Dry Set
HMAM-BM1 ITSR
Indirect Tensile Strength Rao, ITSR (%)
68
Fig. 4. Water sensitivity tests results.
Table 5 Wheel-tracking tests results. Type of mixture HMAM-R
HMAM-AF
HMAM-CR
HMAM-BM1
Density (Mg/m ) Specimen 1 2.515 Specimen 2 2.521 Mean 2.518
2.497 2.507 2.502
2.479 2.482 2.4805
2.508 2.498 2.503
WTS (mm/1000 cycles) Specimen 1 0.13 Specimen 2 0.175 Mean 0.153
0.111 0.07 0.091
0.091 0.068 0.080
0.082 0.151 0.117
Final deformation (mm) Specimen 1 3.243 Specimen 2 3.778 Mean 3.511
3.511 3.007 3.259
2.436 2.588 2.512
3.004 3.676 3.340
3
3.2. Wheel-tracking tests Table 5 resumes the results obtained during the wheel-tracking tests carried out. The incorporation of the additives causes an improvement in the resistance against plastic deformations. The use of a SBS modified asphalt binder (HMAM-BM1) showed a reduction in the appearance of rutting in high modulus asphalt mixtures. In this sense, it can be said that the use of crumb rubber as a modifier of the conventional bitumen (HMAM-CR) is much more effective than SBS polymers. This aspect agrees with other researches findings [28,29], and it could be due to the capacity of the elastic particles of rubber to dissipate stresses. Finally, the use of acrylic fibers as a modifier also induces an improvement in the mechanical response of the asphalt mixture against plastic deformations. The presence of this additive develops a tridimensional net inside the asphalt mixture that can distribute
the stresses generated in the material. Similar results have been obtained in this respect when polyester, polyacrylonitrile, lignin or asbestos fibers are used [14,26].
3.3. Triaxial tests Fig. 5 shows the susceptibility of the different mixtures studied to high temperatures through the use of the confined cyclic triaxial test. The use of additives considerably reduces their sensitivity to temperature (the slope in the curves is considerably reduced), thus it can offer a more stable response in different climatic scenarios. High modulus bitumens have a good performance against plastic deformations, nonetheless the results obtained in the different tri-
Table 4 Testing plan developed for each asphalt mixture studied during this research. Type of test
Standard
Temperature (°C)
Water sensitivity
EN 12697-12 [15]
25
Number of specimens 6
Observations Include a freeze/thaw cycle
Wheel tracking
EN 12697-22 [16]
60
2
–
Cyclic confined triaxial
EN 12697-25 Method B [17]
EN 12697-25, Annex C [18]
3 3 3 3 2 2 2 2 2
Confining load: 120 kPa Cyclic load: 300 kPa Test frequency: 3 Hz Number of cycles: 12,000
Stiffness modulus
40 50 60 70 0 10 20 30 40
Four point bending fatigue
EN 12697-24, Annex D [19]
10
18
Strain controlled test Test frequency: 10 Hz Strain levels: 250, 200, 150, 100 lm/m
69
2.5
10000000
2
10000000
Failure Cycle
Permanent Deformaon (%)
F. Moreno-Navarro et al. / Construction and Building Materials 70 (2014) 65–70
1.5 1
y = 8E+16x-5.22 R² = 0.964
1000000 y = 2E+17x-5.48 R² = 0.978
100000 10000
y = 1E+18x-5.77 R² = 0.976
0.5
y = 7E+19x-6.71 R² = 0.940
1000
0 20
30
40
50
60
70
80
50
90
Temperature (°C) HMAM-R
HMAM-AF
HMAM-CR
Strain (με) HMAM-BM1 HMAM-R
Fig. 5. Triaxial tests results.
HMAM-CR
HMAM-BM1
Fig. 7. Fatigue life law obtained in the four point bending fatigue tests.
axial tests have shown that under extreme high temperatures the presence of the modifiers has an important positive effect. At these temperatures, the consistency of the bitumen is low and it is susceptible to flow under any load. The additives used provide a tridimensional net that avoids this phenomenon and provides more resistance to the asphalt material. Based on the results obtained under these tests conditions (cyclic confined loading), the SBS polymer modified bitumen shows the best response. 3.4. Stiffness modulus tests Stiffness modulus of asphalt mixtures is considerably reduced when temperature is increased (Fig. 6). In this sense, the bearing and stress dissipation capacities of these materials at high service temperatures are limited and thus, the appearance of plastic deformations is more probable. On the contrary, when service temperatures are considerably low asphalt mixtures offer a high stiffness that could lead to a premature fatigue cracking failure. Because of this fact, the presence of additives in asphalt mixtures should reduce the stiffness at low temperatures (in order to avoid the appearance of cracking) and increase it at high temperatures (in order to avoid the appearance of rutting). Results obtained in the tests carried out at different temperatures show that the presence of crumb rubber (HMAM-CR) and SBS polymer (HMAM-BM1) increase the stiffness of the mixture at any temperature. The presence of these additives reduces the strain induced to the material under any load due to their resilient response. This aspect agrees with the results obtained during the evaluation of the resistance to plastic deformation (which are improved when these additives are used), and with those obtained in other studies [14,28]. Special attention should be paid to the case of acrylic fibers (HMAM-AF). In this case, the use of this type of additive reduces the stiffness of the asphalt mixture making it more flexible (as occurs when cellulose
Sffness Modulus (MPa)
HMAM-AF
50000
Table 6 Resume of the results obtained in the fatigue four point bending tests.
e 106 e 106 max. e 106 min. a b R2
HMAM-R
HMAM-AF
HMAM-CR
HMAM-BM1
115 120 109 2 1017 5.487 97.85%
122 129 116 8 1016 5.222 96.64%
115 121 110 7 1019 6.713 94.01%
120 126 114 1 1018 5.773 97.60%
or polyester fibers are used [21,30]). As observed in previous tests, this aspect has not had a negative effect in the resistance to plastic deformations, so if both results are combined, it could be said that an improvement in the general performance could be achieved. 3.5. Fatigue four-point bending tests Fig. 7 and Table 6 resume the results obtained in the fatigue four-point bending tests. As expected, the use of a polymer modified bitumen (HMAM-BM1) improves the fatigue resistance of asphalt mixtures, especially at low strain levels. At these strain levels, the presence of crumb rubber (HMAM-CR) also exerts a positive effect on the resistance to fatigue of the mixture, although at high strain levels (which can appear when high defections are appeared in the pavement) this resistance is considerably reduced due to the high stiffness of these materials. According to the results obtained in the stiffness tests, the use of acrylic fibers (HMAM-AF) improves the fatigue resistance of high modulus asphalt mixtures at any strain level. In accordance to these results, similar improvements in fatigue life have been also achieved when other types of fibers are used in asphalt concretes [30]. Based on the results obtained, HMAM manufactured with this additive could offer a mechanical response as good as those manufactured with a high performance polymer modified bitumen (BM1). 4. Conclusions
5000
500 -10
0
10
20
30
40
50
Temperature (°C) HMAM-R
HMAM-AF
HMAM-CR
HMAM-BM1
Fig. 6. Stiffness modulus tests results.
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High modulus asphalt mixtures suppose an interesting choice for the construction of road pavements. Nevertheless, under certain climate conditions, its mechanical performance is limited due to their low flexibility and low stress relaxation capacity. This problem can be solved using polymer modified binders, but its application is reduced due to high costs associated to this solution. Because of this fact, one of the objectives of the FATE research project has been the assessment of the feasibility of the use of acrylic fibers and crumb rubber additives as an alternative for the improvement of the mechanical response of high modulus asphalt
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mixtures. This paper describes the laboratory works carried out in this sense at the University of Granada (Spain) in the framework of this project. Based on the results obtained, the following conclusions can be drawn: – The use of crumb rubber or acrylic fiber as modifiers in the manufacture of high modulus asphalt mixtures is a more economical solution than the use of high performance polymer modified binders. – The use of these additives slightly reduces the mechanical resistance of the high modulus asphalt mixtures in terms of indirect tensile stress. Nevertheless, they do not exert any negative effect in the water susceptibility of these materials, even after a freeze/thaw cycle. As the four mixtures studied offer a low sensitivity to water and ice effects, the use of the additives do not exert a significant different in this term. – The presence of acrylic fiber and crumb rubber increase the resistance to plastic deformations of high modulus asphalt mixtures and also reduces their susceptibility to the temperature. The results obtained in the wheel-tracking and confined cyclic triaxial tests show that the mixtures manufactured with these additives offer a similar resistance to rutting and climate changes than that offered by a high modulus asphalt mixture manufactured with a polymer modified bitumen, and better than the conventional high modulus mixture. – The addition of crumb rubber to high modulus asphalt mixtures increase their stiffness, which is a positive effect in terms of bearing and stress distribution capacities. Nevertheless, when these materials are subjected to high strain this aspect could exert a negative effect in their fatigue resistance, inducing a premature failure of this type of mixtures when deflections in the pavement layers are high. The addition of acrylic fibers to the high modulus asphalt mixtures reduces their stiffness, but as commented, this aspect does not cause any negative effect in their plastic deformation resistance. On the contrary, this additive provides an improvement in the flexibility of the asphalt mixture, which results in a considerable increase of its fatigue life (even bigger than that offered by an asphalt mixture manufactured with a high performance polymer modified bitumen).
Acknowledgements The present article has been conducted within the framework of the FATE research project (Firmes Asfálticos para Temperaturas Extremas, IPT-2012-0977-370000), funded by the Ministry of Innovation and Science of Spain, inside the National Plan of Science Research, Development and Technological Innovation 2012–2015, and co-funded by the FEDER funds. References [1] Nunn ME, Smith T. Road trials of high modulus base for heavily trafficked roads. England: Thomas Telford; 1999. [2] Corté JF. Development and uses of hard grade asphalt and of high modulus asphalt mixes in France. Transportation Research Circular No. 503, TRB, National Research Council, Washington D.C.; 2003. p. 12–30. [3] Maupin GW, Diefenderfer BK. Design of a high-binder-high-modulus asphalt mixture. Research Report VTRC 07-R15, Virginia Transportation Research Council; 2006. [4] Lee HJ, Lee JH, Park HM. Performance evaluation of high modulus asphalt mixtures for long life pavements. Constr Build Mater 2007;21:1079–87.
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