Effect of Natural Rubber and Zycotherm on Moisture

71st RILEM Annual Week & ICACMS 2017, Chennai, India, 3rd – 8th September 2017

Effect of Natural Rubber and Zycotherm on Moisture Performance of Asphalt Mixtures P Saha Chowdhury 1, Sonu Kumar 2 and D Sarkar3 1

Department of Civil Engineering, Indian Institute of Technology Kharagpur, India L&T Construction, Gujarat-390019, India 3 Department of Civil Engineering, N.I.T Agartala, India 2

ABSTRACT Moisture damage is one of the major failure modes of the asphalt pavement. It weakens the adhesion between the aggregates and asphalt binder, which in turn reduces the structural strength of asphalt mixture. The objective of this study is to investigate the effect of ZycoTherm and natural rubber modifier on the moisture performance of dense-graded asphalt mixtures. The scope of the study includes experimental investigation on four different types of dense-graded asphalt mixtures covering control, natural rubber modified (NR), warm additive modified (WMA), and warm additive with natural rubber (WMA+NR) modified asphalt mixtures. The experimental program encompassed moisture performance evaluation of 72 specimens using Retained Stability test, Modified Lottman test, and Boiling Water test. The experimental results reveal that NR and WMA+NR-mixes produce higher resistance to moisture damages compared to the other mixtures. Furthermore, the inclusion of WMA additive into the asphalt binder has not only reduced the mixing and compaction temperatures but also increased resistance to moisture damage of the asphalt mixtures. Overall, it is concluded that natural rubber and warm additives can be used to mitigate moisture damage of dense-graded asphalt mixtures. Keywords: Asphalt mixtures, moisture damage, warm mix asphalt, natural rubber modified asphalt, zycoTherm 1

INTRODUCTION

Moisture damage is one of the major root causes of structural failure in asphalt pavement. It is caused due to the moisture ingression into the mix-matrix, which eventually affects the structural integrity and reduces the stiffness of asphalt mixtures by various moisture damage mechanisms. Further, the reduction in mechanical strength of pavement layers due to moisture facilitates other modes of distresses: rutting, fatigue, ravelling and cracking (Sengoz and Agar, 2007; Caro et al., 2008; and Cui et al., 2015). Therefore, moisture performance serves a critical role towards the overall functional and structural performance of asphalt pavement (Airey and Choi, 2002). In the last few decades, numerous studies had been carried out in the ambit of moisture performance at various segments: damage mechanism, material modification to counter moisture, aggregate-asphalt interfacial evaluation, and application of moisture repelling agents. Among all, polymer modification (Billiter et al., 1997; Lu and Isacsson 1997; Gorkem and Sengoz, 2009; Shaffie et al., 2015; Kakade et al., 2016) is one of the mainstream research to improve the moisture performance of asphalt mixtures. The modification of asphalt binder has been commonly practiced due to its ease of implementation in the field.

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Although a wealth of literature can be found on the effect of various modifiers and their associated moisture performance improvement, there is a limited literature available that targets the use of natural rubber (NR) and its effect on the moisture resistance of asphalt mixtures. Concurrently, warm mix asphalt is one the most successful sustainable paving technologies to mitigate the environmental hazards (Capitao et al., 2012; Jain et al., 2013; Kheradmand et al., 2014). Keeping the environment concerns in mind, the efficacy of warmmix additive (WMA) in conjunction with the popular material modification is yet a relatively new research area (Mirzababaei, 2016). Thus, there is certainly a need to characterize the moisture performance of WMA and NR for two reasons: (a) to assess the individual and combined moisture resistance properties and (b) to recommend the best dosages of modifiers to obtain the best performance against moisture. The research framework is presented in Figure 1. Thus, the objective of this study was to investigate the effect of WMA and NR on the moisture performance of asphalt mixtures. The scope of the study includes: x Preparation of WMA and NR modified binder and evaluation of their properties. x Estimation of moisture damage of loose mixes based on boiling water test. x Evaluation of the mixes using tensile strength ratio (TSR) and retained stability (RS). Evaluation of moisture performance

Asphalt mix characterization

Asphalt binder characterization

Penetration Marshall mix design

Moisture test

Softening Point Ductility

Mass change

Boiling water test

Indirect tensile test

Retained stability test

Viscosity

Figure 1 : Research framework. 2

MATERIALS AND EXPERIMENTAL PROGRAM

Dense aggregate gradation as per MoRTH specification along with VG40 grade asphalt was selected as a base material for the study. A varying percentage of Natural Rubber (NR) latex 1-5% was mixed with the binder by weight of asphalt binder. During this process, asphalt was heated at about 140 -150 ºC until it became liquid. The NR was slowly added into the liquid binder and was sheared with a high shear mixer at a speed of approximately 2000 rpm for 30 minutes. With regard to the WMA modification, 0.15% ZycoTherm was added into a base binder, and NR modified binder by the weight of asphalt binder. The mixing of WMA was done in a temperature range of 130-135 °C at a speed of 1000 rpm with the help of laboratory shear mixer for 15-20 minutes. The blending was done using laboratory shear mixer which is having 800-4000 rpm rotational speed with adjustable height. Determination of mixing temperature, rotational speed of mixer for mixing and period for modification are based on the past studies (Road Note 36, 1968; Vichitcholchai et al., 2012; Toraldo and Mariani, 2014; Shaffie et al., 2015; CRRI Report 2015). The basic tests were performed to understand the effect of blending and changes due to modification on all the twelve types of asphalt binders. Marshall Mix design was carried out on 216 samples (Asphalt binders=12, Asphalt contents=6, Replicates=3;

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Total=12 × 6 × 3=216) to determine the optimum trial dosages of NR for the control and WA modified asphalt binder. Boiling water, retained stability, and indirect tensile strength test was also carried out on different mixes to evaluate the moisture performance. The laboratory shear mixer, Marshall samples and a failed sample of ITS during the test is included in Figure 2.

Figure 2: (a) Laboratory high shear mixer (b) Marshall sample and (b) Failed sample of ITS. 3

RESULTS AND DISCUSSION

3.1

Characterization of asphalt binders

3.1.1 Binder property evaluation A series of consistency tests were carried out to evaluate the property of the binder and the test results are summarized in Table 1. Table 1: Characterization of NR modified asphalt binders Properties Before aging Specific gravity Penetration (mm) Softening point (°C) Kinematic viscosity (cSt) at 150°C,CSt Ductility (cm) Rolling thin film test on residue (After aging) Mass change at 163ºC (%) Reduction of penetration at 25°C (%) Increase in softening point (°C)

0

1

NR (%) 2

3

4

5

1.015 40 56.5 173 >100

1.033 36 58.3 315 >100

1.043 35 59.3 320 >100

1.054 33 60.5 351 >100

1.059 32 61.0 311 >100

1.065 30 61.3 304 >100

-0.62 22.7 4.0

-0.27 16.7 3.5

-0.26 18.8 3.0

-0.43 16.5 3.0

-0.49 19.0 3.5

-0.52 20.2 4.0

The above properties of NR modified binders are compared and conformed with the IS: 15462 (2004) modified bitumen (NRMB 40) specification. As observed, the specific gravity increased with increasing NR content indicating that the asphalt binders turned harder with NR inclusions. Thus, reduced penetration value and higher softening point ensured this change in consistency. Further, aging was conducted and then, asphalt binders were tested to evaluate physical properties to understand the aging susceptibility. Though there was no significant change in reduction of penetration due to aging, NR modified binder resulted in a lower mass change than the virgin binder. These illustrated that the oxidization and volatilization reduced due to modification.

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Concurrently, the consistency properties of six WMA+NR modified were carried out and presented in Table 2. A comparison between NR modified, and WMA+NR modified binder provided the introductory behavioural effect of these two combinations on the asphalt binder. As observed, penetration value increased in presence of 0.15% WMA at 3, 4, and 5% NR dosages. Results showed that the stiffness of asphalt binder reduced with the increase of NR dosages in presence of WMA. A similar trend of viscosity increment was found for both NR and WMA+NR modified binders where the viscosity of asphalt binders increased with increasing NR content. It was also noteworthy that asphalt binders showed higher kinematic viscosity in the presence of WMA modifier. It was due to the fact that WMA additive is a chemical additive which had little effect on rheological properties of the binder. It does not follow viscositytemperature relationship for mixing and compacting temperature of bituminous mixes. According to IRC:SP:101-2014 chemical additives are surfactants (surface active agents) that reduce surface tension between the polar aggregates and non-polar bitumen, improve wetting and reduces internal friction, and allows a reduction of 28-50°C in mixing and compaction temperatures. Further, a relatively lower degree of increment in softening point was noticed for WMA+NR binder than NR binders. Thus, the effect of aging was not very influential when asphalt binders were modified with WMA. In summary, it can be inferred that NR and WMA+NR modification of asphalt binder resulted in increased stiffness at low as well as high temperatures. It is seen in Table 2 that in the case of WMA modified asphalt binders, loss in mass is more at 163°C compared to 135°C. But 135°C is the actual simulation temperature for WMA modified asphalt binders. The short-term aging reduces with the addition of WMA in any asphalt binders. The reason is the lower mixing and compacting temperatures compared to the control and NR modified mixes which lead to the lesser oxidation of asphalt binder. Table 2: Characterization of WMA and NR modified asphalt binders Properties of asphalt binder

0

Before aging Specific gravity 1.017 Penetration 40 Softening point (°C) 56 Kinematic viscosity (cSt) 226 Ductility (cm) >100 Rolling thin film test on residue (After aging) Mass change at 163ºC (%) -0.86 Mass change at 135ºC (%) -0.4 Reduction of penetration at 25°C (%) 17.5 Increase in softening point (°C) 1.0

WMA modified NR (%) 1 2 3 4

5

1.029 36 57 339 79

1.033 36 58 367 96

1.048 38 58.5 485 99

1.060 35 59 463 98

1.066 33 60 385 >100

-0.43 -0.23 16.7 1.5

-0.46 -0.14 12.5 2.0

-0.52 -0.26 10.5 2.0

-0.57 -0.29 12.7 2.0

-0.63 -0.32 15.2 2.5

After analyzing the above results, it was observed that WMA modification of asphalt binder resulted in a slight decrease in stiffness at a lower temperature while at a higher temperature it increased. Though the chemical properties were not studied, it was predicted that the reason behind this contradictory behavior was the chemical interaction of WMA additive and asphalt binder.

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3.2

Characterization of asphalt mixes

3.2.1 Marshall mix design Optimum asphalt content (OAC) was selected on the basis of 4 % air voids level using Marshall mix design. All other mixture properties were validated at the OAC. The variations in OAC for ten asphalt mixtures are presented in Figure 3.

Figure 3: Optimum asphalt content for asphalt mixes. It was observed that asphalt modified with 4% NR reduced the OAC up to 11.6%. In the case of WMA modified asphalt, 5% NR content reduced the OAC up to 5.8%. Furthermore, it was found from the different trails of NR mixes that 4% NR of NR mixes and 5% NR of WMA+NR mixes satisfied all other volumetric criteria as mentioned in MoRTH specification. Thus, these levels of NR and WMA+NR were selected for performance evaluation in this study. 3.3

Moisture susceptibility evaluation

3.3.1 Boiling water test The boiling water test results are shown in Table 3. All the mixes including the control mix passed the boiling water test i.e. preliminary screening test. In other words, all mixes are resistant to moisture damage Though all the mixes showed higher retained coating than the specified limit as per MoRTH specification, NR and WMA+NR mixes retained more coating in comparison with control and WMA mixes. Table 3: Boiling water test of mixes Mix ID Retained coating (%) Retained Coating greater than 95% CM 95% Passed WMA 96% Passed NR 98% Passed WMA+NR 98% Passed These results showed that when NR was present in the mixes, it has significantly improved the moisture damage of the mixes. This can be explained by the strong adhesion between the NR modified asphalts and the aggregate surfaces. However, this test provides inconsistent results as it is very subjective and depends on visual observation and personal interpretation. 3.3.2 Indirect tensile strength test The Indirect tensile strength (ITS) test results are summarized in Figure 4. The dry (D) group samples were tested with no special conditioning, the wet (W) group samples were tested after conditioning for 24 hours at 60ºC without freeze-thaw cycles and frozen– thawed (FT) group of samples were tested after freeze cycle for 16 hours at -18±3ºC followed by

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thaw cycle for 24 hours at 60ºC. All the specimens were tested at 25ºC after soaking them in a water bath at 25ºC for 2 hours. Figures 4 (a-d) illustrate the effect of moisture susceptibility of different selected trial mixes as shown below. As observed in Figure 4 (a), ITSD increased with NR additive and decreased due to modification with WMA. Figure 4 (c) indicated that ITSFD increased due to modification with NR and WMA additive. The addition of NR and WMA in mixes resulted in an increase in TSRWD by approximately 4 and 15%, respectively as shown in Figure 4 (b). Furthermore, Figure 4 (d) displayed that the addition of NR and WMA in mixes increased TSRFTD by approximately 6 and 16%, respectively. It means that freezing and – thawing effect was reduced when NR and WMA were used individually. But in the case of NR and WMA combination, TSR slightly reduced. Modification of binders with NR reduced the potential to moisture damage of the mixes.

Figure 4 : (a) Indirect tensile strength test (Dry-Wet), (b) Tensile strength ratio (Dry/Wet), (c) Indirect tensile strength test (Dry-Freeze thaw), and Tensile strength ratio (Dry/ Freeze-thaw) The improvement of TSR for a mix containing WMA might be resulted to maintaining aggregate-binder interface due to active chemical interaction at an intermediate temperature as well as low temperature. 3.3.3 Retained stability test Retained Stability test results are presented below in Figure 5 (b). Unconditioned samples were kept in a water bath for 30 minutes at 60 ºC and conditioned samples were kept in a water bath for 24 hours at 60ºC before testing. Figure 5 (a) showed that stability reduced after subjecting the sample to moisture conditioning.

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Figure 5 : (a) Stability, and (b) Retained Stability of conditioned and unconditioned samples On addition to that Figure 5 (b) depicted that Retained stability was increased by approximately 3% due to modification with NR and decreased by approximately 10% due to modification with WMA. Interestingly, WMA+NR mixes showed increased retained stability. This response might be attributed to the fact that NR did not absorb water. Thus, the addition of NR to asphalt might reduce water ingress to aggregate-binder interface and consequently protect the adhesion between the asphalt and aggregate. As a result, it prevented the separation of asphalt coating from the aggregate and maintained the structural integrity of the asphalt mix. Therefore, it can be inferred that addition of NR improved the moisture performance while the addition of WMA slightly decreased the retained stability as compared to the control mix. The behavior of WMA was reverse in case of its interaction with NR. Though it was not chemically verified, it was possibly due to the presence of NR in the mix that undermined the effect of WMA and thus, increased the moisture resistance. 4

CONCLUSIONS x

x

x

5

This paper concludes that the Boiling water test results had shown to be consistent in term of identifying the stripping performance of mixes. The presence of NR in mix improved the adhesion between the aggregate and asphalt binder which led to the better moisture performance of NR modified mixes. The TSR were higher for WMA modified mixes as compared to control mixes, which indicated that WMA modification offered better resistance to stripping in different climatic conditions. However, both NR and WMA modified mixtures satisfied minimum requirement of 80% TSR. In the case of retained stability test, WMA modified mix had shown lower resistance to moisture whereas WMA and NR modified mixes produced higher moisture resistance when compared with control mixes. It can be concluded that NR incorporation in control, as well as WMA modified asphalt mixes, had sufficient potential to reduce the moisture damage which in turn improved the pavement performance. Since the moisture performance was found to be satisfactory, it is recommended to investigate the rutting and fatigue performance of WMA and NR modified mixes. Future study is needed to consider other warm-mix additives with different base binders to explore more information on this subject. ACKNOWLEDGEMENTS

Authors acknowledge Rubber board, Tripura and Zydex Industry for providing natural rubber and warm-mix additive, respectively. Authors are also thankful to Dr. Gourab Saha for his inputs in preparing the manuscript.

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REFERENCES

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