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Evaluation of the shear characteristics of steel–asphalt interface by a direct shear test method
Author’s Accepted Manuscript λEvaluation of the shear characteristics of steelasphalt interface by a direct shear test method Bo Yao, Fangchao Li, Xiao Wang, Gang Cheng
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To appear in: International Journal of Adhesion and Adhesives Received date: 20 October 2014 Accepted date: 8 February 2016 Cite this article as: Bo Yao, Fangchao Li, Xiao Wang and Gang Cheng, λEvaluation of the shear characteristics of steel-asphalt interface by a direct shear test method, International Journal of Adhesion and Adhesives, http://dx.doi.org/10.1016/j.ijadhadh.2016.02.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Evaluation of the shear characteristics of steel-asphalt interface by a direct shear test method
Bo Yao a,*, Fangchao Li a, Xiao Wang b, Gang Cheng b
a
Department of Civil Engineering, School of Science, Nanjing University of Science and Technology, 200
Xiaolingwei, Nanjing 210094, China b
School of Transportation, Southeast University, 35 Jinxianghe Road, Nanjing 210096, China
*Corresponding author. Tel./fax: +86 25 87794368 E-mail address: [email protected] (B. Yao). ABSTRACT Shear characteristics of steel-asphalt interface under the influences of temperature, normal stress level and tack coat material were investigated. The direct shear tests were conducted on composite specimens with epoxy asphalt (EA) and polymer modified asphalt (PMA) tack coat materials at temperatures of 25 and 60°C and normal stress levels of 0, 0.2, 0.4, and 0.7 MPa for each temperature. Results show that the failure modes include adhesive failure at the primer-tack coat interface and material failure of asphalt concrete. Steel-asphalt interface shows strain softening behavior until it reaches the sliding state. The shear strength and the shear reaction modulus increase with decreasing temperature and increasing normal stress levels. The specimens with EA tack coat provides much higher interface shear strengths than those with PMA tack coat at 25 and 60°C. In addition, the failure envelopes of the shear strength and residual shear strength were obtained for combinations of tack coat materials and temperature conditions based on the Coulomb failure law. Keywords: Steel bridge deck, Asphalt tack coat, Interface, Shear characteristics, Direct shear test
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1. Introduction In highway steel bridge construction, tack coats are usually used between the steel bridge decks and the asphalt overlays. The types of tack coat used generally fall into two categories: thermosetting materials and thermoplastic materials. The thermosetting type tack coat materials include epoxy asphalt and epoxy-polyurethane resins, and the thermoplastic type tack coat materials include polymer modified asphalt, asphalt emulsion, mastic asphalt, and asphalt multilayer membranes [1-3]. The application of tack coats has several advantages including providing sufficient bonding between the asphalt overlay and the underlying steel deck, and ensuring the two components act as an integrated unit to support the traffic and environmental loading, which is crucial for the short- and long-term performance of asphalt overlays [4-6]. A review of current practices [7-10] reveals that insufficient bond between the steel deck and the asphalt overlay may cause asphalt overlay distresses such as slippage cracking, debonding, early fatigue cracking, and reduction of overlay life. Due to very different physical and mechanical properties of steel and asphalt concrete, the incompatibility between them is pronounced and the steel-asphalt interface is generally the weakest plane of the composite system [11]. Slippage cracking of asphalt overlay is caused by a lack of shear resistance at the steel-asphalt interface and a high enough shear stress generated when heavy vehicles are accelerating, decelerating, or turning, in which case the asphalt overlay begins to slide on steel bridge deck. A well prepared and adequately treated steel deck surface before application of asphalt overlay and correct choice of tack coat materials can minimize the slippage cracking problem to a great extent. However it is desirable to have some evaluation techniques by which the interface could be characterized and tested properly. Published information [12] indicates that the performance of steel-asphalt interface have been evaluated by several test methods. Medani et al. [13] used the TU-Delft four-point shear test to characterize the response of a bituminous-based membrane material on an orthotropic steel deck bridge to shear and normal forces. Bocci and Canestrari [14,15] employed the ASTRA (Anocona Shear Testing Research and Analysis) shear test device to evaluate the shear strengths of smooth and reinforced steel interfaces coated with a polymer-modified bitumen between the steel deck and the conventional Hot Mix Asphalt (HMA). Recently, Ge et al. [16] proposed a preloading and shear loading testing method for evaluating the shear behavior of steel deck pavement pasted by GFRP sheets. However, most researchers have only focused on the strength of the steel-asphalt interface. Few studies have focused on the mechanical behavior at the interface. Three 2
laboratory-measured properties were suggested to describe the mechanical behavior at the interface: the interface shear strength, the interface reaction modulus, and the friction coefficient after failure [17]. Therefore, a more comprehensive knowledge of the actual mechanical behavior of interface is necessary to better understand the steel-asphalt composite system and more accurately compute the service lifetime of overlay structure. On the other hand, the shear tests in literature were conducted at low and intermediate temperatures (from 0 to 40°C), which is not representative of the interface environmental conditions. For the interface between the steel deck and the asphalt overlay, it is known that high temperature which the overlays experience in service, e.g. 60°C, is a more critical condition for which the interface failure is more likely to occur. Therefore, an effective test method which enables measurement of the interface shear characteristics at high temperature is necessary. The objective of this study was to evaluate the shear characteristics of two types of steel-asphalt interface by the steel-concrete interface shear (SCIS) test method. One thermosetting type interface and one thermoplastic type interface were made to observe the influence of tack coat materials on the interface shear response. The SCIS tests were conducted at temperatures of 25 and 60°C and normal stress levels of 0, 0.2, 0.4, and 0.7 MPa for each temperature. The results reported include the shear strengths, shear reaction modulus, residual shear strengths, shear stress-displacement curves, failure envelopes, and failure modes of the steel-asphalt interfaces.
2. Materials Two types of interface were evaluated in this study. An epoxy asphalt (EA) tack coat and an epoxy asphalt concrete (EAC) were used to provide a thermosetting type interface. A polymer modified asphalt (PMA) tack coat and a Gussasphalt concrete (GAC) were used to provide a thermoplastic type interface. The EA is a two-part product blended before use. Part A (used at 14.6% by weight) consists of an epoxy resin formed from epichlorhydrin and bisphenol-A. Part B (used at 85.4% by weight) is a mix of asphalt and epoxy cross-linker. After being fully cured, the product is similar to a black hard rubber. The PMA is a styrene-butadiene-styrene (SBS) modified asphalt binder graded as PG 76-22. The properties of the fully-cured EA and PMA tack coats are presented in Table 1. The EAC and GAC mixtures were prepared according to the specifications for mixtures usually laid on the steel bridge decks. The EAC mixture used another type of EA as binder with a content of 6.5% by weight. The asphalt binder of GAC mixture was an asphalt-TLA blend. The blend was composed of a Pen 20/40 3
asphalt and Trinidad Lake Asphalt (TLA) with contents of 70% and 30% by total weight, respectively. The binder content of GAC mixture was 8.5% by weight. The aggregate gradations of the EAC and GAC mixtures are provided in Fig.1.
3. Experimental program 3.1. The SCIS test The SCIS test apparatus is designed to adapt to commonly-used servo-hydraulic loading frame and environmental chamber. It consists of a shear box, a horizontal loading device for applying normal load on the interface, a horizontal load cell, a servo-hydraulic loading frame, a vertical load cell and a Linear Variable Differential Transformer (LVDT), a data acquisition system, and a computer with compatible software to record the data. To evaluate the temperature effect on the shear characteristics of steel-asphalt interface, the SCIS test apparatus is placed in an environmental chamber that can maintain temperatures ranging from -15 to 60°C with an accuracy of ±0.5 ºC. A schematic of the SCIS test apparatus is shown in Fig.2. The details of the shear box is shown in Fig.3. The asphalt concrete part of the specimen is fixed, while the steel plate is confined in the X direction only. The steel plate can moves vertically under applied load, which allows shear at the interface to take place. The application of shear load on the steel plate of the specimen results in smaller moment arms that can minimize the effect of bending moment induced by the eccentricity of the shear force to a certain extent. The movable base can be shifted in the X direction to align the steel-asphalt interface to the shear plane. This is done to ensure that the shear force is applied in the interface plane, where most of the shear displacement will occur. The shear box is restrained against rotation by two slider guides that allows the movement to take place only in the horizontal direction. A linear motion guide is placed closely attaching to the steel plate of the specimen to allow its frictionless movement in the vertical direction. To simulate the asphalt overlay upon steel bridge deck experiences a combined state of compression and shear under the action of traffic loading, the SCIS test apparatus is designed to apply shear force in the vertical direction accompanying normal force in the horizontal direction. The application of the initial horizontal load, normal to the interface plane of the specimen, is achieved through a flat jack. It provides a constant normal stress condition, which is monitored by a load cell with a reading accuracy of ±0.1N. The 4
vertical shear deformation/load, paralleled with the interface plane, is applied on the top surface of the steel plate by a servo-hydraulic actuator. A load cell connecting to the loading frame are used to read and store the vertical force continually. The shear displacements of the interface are measured by the loading frame internal LVDT housed in the loading piston with a reading accuracy of ±0.005 mm. A photograph of the SCIS test apparatus is given in Fig.4. 3.2. Fabrication of specimens The test specimen is a steel-asphalt composite structure comprising a 10 mm thick steel plate and one layer of asphalt concrete with a thickness of 55 mm. The steel plates were fabricated according to the cross section requirement of 96 96 mm. After that, they were sandblasted and coated with epoxy zinc rich primer with thickness of 60 to 80 μm measured according to method stated in GB/T 13452.2-2008 [22]. A specific steel mold measuring 300 mm 300 mm and 65 mm in thickness was used for the preparation of the composite specimens, as shown in Fig.5a. It is divided into 9 cells, each of which has a dimension of 96 96 mm based on the specimen size to be prepared. Before fabricating specimens, the steel mold was preheated in a 60°C oven for 1 hour. The hot tack coat materials were spread carefully on the surface of each steel plate using a clean scraper knife to give desired amount of tack coat. The amount corresponds to the field application rate of 0.68L/m2 and 300g/m2 for EA and PMA, respectively. Then place the steel mold on a level shelf in the 60°C oven for 10±2 minutes to allow the tack coat to self level. After that, the desired weight of EAC and GAC mixtures were mixed and laid above the tack coat followed by compaction to the specimens with laboratory roller-compactor, as shown in Fig.5b. The amount of mixtures was calculated based on the relationship between asphalt concrete maximum specific gravity, air void content, and desired height of asphalt concrete in the composite specimen after compaction. For thermosetting type interface, the steel mold was placed in a 120°C oven for 4 hours to make the EAC be fully cured before being placed on a level surface to cool to room temperature. For thermoplastic type interface, the steel mold was directly placed on a level surface for an appropriate cooling period after compaction. Finally, the compacted composite specimens were extracted from the mold. Specimens ready for testing are shown in Fig.5c. 3.3. Experimental procedure Some of the findings from the literature on tack coat applications indicate that the major factors 5
affecting asphalt interface shear characteristics are temperature, shear rate, magnitude of normal stress, HMA type, tack coat type, tack coat application rate, and surface cleanliness [23-26]. For the asphalt overlay on steel bridge deck, an overlay material is usually used together with another specific type of tack coat. In this study, one thermosetting type interface and one thermoplastic type interface were evaluated by the SCIS test. Specifically, the thermosetting type interface indicates the EA as tack coat and the EAC as overlay material, while the thermoplastic type interface indicates the PMA as tack coat and the GAC as overlay material. Two temperatures (25 and 60℃) were considered in the experimental plan to investigate the effect of temperature on the shear characteristics of steel-asphalt interface. At each temperature, four normal stress levels (0, 0.2, 0.4, and 0.7 MPa) were examined. For each combination of interface type, test temperature, and normal stress level, three replicate specimens were tested and the average values of the test results were reported. Prior to testing, specimens were placed in an environmental chamber for at least 4h in order to equilibrate to the specified testing temperature. Shear tests were conducted in displacement-controlled mode at a constant shear rate of 1 mm/min and the normal stress on the interface of the specimen was held constant during the tests. Tests were stopped after a sliding state was clearly observed or the total shear displacement reached 10 mm. The failure of the steel-asphalt interface arising from shear slip is shown in Fig.6.
4. Results and discussion 4.1. Experimental results The interface shear strength, i.e., the peak shear stress, is calculated as follows: p Tmax / A
(1)
where p is the interface shear strength, Tmax is the maximum shear load applied to specimen, and A is the cross-sectional area of the test specimen. The maximum slope of the shear stress versus displacement curve can be used to define the shear reaction modulus ( K ) as follows [27]: K /
(2)
In the SCIS test, due to the presence of the normal load acing on the interface, a residual shearing condition was clearly observed in which the shear displacement continues increasing while the shear stress keeps at a constant value defined as the residual shear strength ( r ). 6
Table 2 summarizes the average interface shear strengths, shear reaction modulus, and residual shear strengths of the composite specimens and their corresponding coefficients of variation (COVs) as well as the failure modes obtained from the SCIS tests. The typical fracture interfaces of the tested specimens are shown in Fig.7. The failure modes were revealed by visual observation of the fracture interfaces. For the composite specimens with EA tack coat at all test conditions, adhesive failure was obtained with complete separation of the EAC from steel plate (Fig.7a, Fig.7b). Cracks were detected at the primer-tack coat interface during the tests. The EA tack coat infiltrated into the EAC layer after compaction, rather than kept intact as an individual layer. No failure in the EAC was observed and the steel plate has almost no adherent EAC. For PMA tack coat, the temperature dependence is associated with a change in the failure mode of the interface. Shear tests performed at 25℃ results in an adhesive failure at the primer-tack coat interface. It is shown that the primer keep intact after the shear test while the PMA tack coat disperses in the GAC and a clear separation is observed (Fig.7c). While at 60℃, the failure mode is observed as material failure in GAC that rupture nucleates and propagates in the bulk GAC before propagating at the interface. A thin layer of GAC is attached to the interface after failure (Fig.7d). The change from adhesive to cohesive failure with increased temperature was associated with a reduction in the mechanical performance and stability of GAC. Since the performance of GAC is sensitive to temperature, it has good resistance to deformation at low temperature, but at high temperature it has a high tendency toward plastic deformation even flow. It is revealed that the interface failure mode relates to not only tack coat material, but also asphalt concrete layer closely contacting to the interface. It can therefore be concluded that the failure of steel-asphalt interface mainly involves the yield behaviour of the mixture composed of polymer adhesive and the asphalt concrete at the bottom of the overlay at elevated temperatures. The experimental results indicate that the interface shear strength, shear reaction modulus, and residual shear strength are sensitive to temperature, normal stress, and tack coat materials. The EA tack coat gives the interface shear strengths between 1.92 and 2.65 MPa at 25℃ while between 0.50 and 0.98 MPa at 60℃, as shown in Table 2. Similar results were reported by Bocci and Canestrari [14], they obtained the shear strengths of 2.3 to 2.9 MPa at 20℃ and 1.4 to 1.7 MPa at 40℃ between steel plate and asphalt concrete with EA tack coat. The COVs of the shear strength for specimens with EA tack coat are less than 8%. The narrow range of COVs is an indication of the consistent repeatability of the proposed shear test method. The values of the interface shear reaction modulus presented in Table 2 range from 2.09 to 2.64 MPa/mm at 25℃ 7
while from 0.43 to 0.73 MPa/mm at 60℃, which represents the stiffness of the interface. The COVs of the shear reaction modulus are less than 12%. The interface residual shear strength for the specimens with EA tack coat are from 0.09 to 0.38 MPa at 25℃ and from 0.08 to 0.29 MPa at 60℃ with the corresponding COVs below 14%. Considering the manufacturing complexity and variations in material properties, the COVs for the specimens with EA tack coat are acceptable. Table 2 also shows that the interface shear strengths for the specimens with PMA tack coat are between 0.36 and 0.65 MPa at 25℃ while between 0.01 and 0.10 MPa at 60℃. The shear strengths are comparable with other published results [13, 28] with some variations due to differences in geometry of specimens, test methods, and tack coat materials. The interface shear reaction modulus range from 0.41 to 0.75 MPa/mm at 25℃ while from 0.03 to 0.09 MPa/mm at 60℃, which indicates that the interface with PMA tack coat has a low resistance to shear deformation at high temperature. As for the residual shear strength, no obvious difference are found between the data obtained from the tests conducted at 25℃ and 60℃. In the case of the PMA tack coat at 60℃, the COVs of the interface shear strength, shear reaction modulus, and residual shear strength are higher since the results are rather low that a small difference in the results can highly elevate the COVs. 4.2. Shear stress-displacement curves The shear behavior of an interface between the steel deck and the asphalt overlay can be characterized by the shear stress-displacement curve. The shear stress-displacement curves for all the test conditions are shown in Fig.8 to Fig.11. It shows that the shear stress-displacement curves for specimens with EA tack coat and PMA tack coat are similar: the shear stress increases initially and then decreases with increasing shear displacement. The ascending part of the curves shows an approximately linear shape approaching to a peak value. In the post peak region, the curves show a strain softening behavior until reaching a plateau. It is worth noting that the application of a normal stress during the shear test assures a friction contribution due to the pure friction condition that occurs after the peak value has been passed. In this condition, the displacement takes place without variation in shear stress. The shear stress-displacement curves indicate ductile failure at the steel-asphalt interface which may be attributed to the fact that the asphalt tack coat materials used in this study show viscoelastoplastic behavior at normal and high temperature. It is also implied that different forms of shear stress-displacement model rather than bi-linear model usually used in 8
asphalt pavement interface analysis should be developed for steel-asphalt interfaces. It is also evident from Fig.8 to Fig.11 that the shear stress-displacement curves are variable for specimens with the identical tack coat material in different testing conditions, which indicates that the interface behavior are governed by the properties of the tack coat material, temperature, and normal stress. It is observed from Fig.8 to Fig.11 that the shear stress-displacement curves cross through three distinct zones. The first zone corresponds to the bulk linear viscoelastic behavior of the tack coat material. Due to the viscoelastic effect, the shear strength and shear reaction modulus are found to decrease with increasing temperature. The second zone corresponds to the damage initiation and evolution in the interface. The interface shear strength continues descending after a peak value. The third zone corresponds to the failure process when the shear stress leveled out to an approximately constant value and the sliding state is reached. 4.3. Effect of temperature The effect of temperature on the shear strength, shear reaction modulus and residual shear strength are presented in Table 2. It is observed that temperature has a significant effect on interface shear strength and shear reaction modulus that they significantly decrease as testing temperature increases. On average, the shear strengths and shear reaction modulus determined at 60℃ are about 25% to 40% of those determined at 25℃ for the specimens with EA tack coat. And the ratio values drop to below 20% for the specimens with PMA tack coat. The reduction in shear strength and shear reaction modulus is caused by a decrease in the tack coat material stiffness as temperature increases, indicating the viscoelastic nature of the tack coat materials. For similar interface conditions, increased temperatures result in reduced shear strengths as reported by almost all of the studies in literature [29-31]. It implies that the high temperature is a critical condition for which the interface failure is more likely to occur, and the interface shear test should be conducted at the highest temperature that the asphalt overlay on orthotropic steel bridge deck will experience in service. The data in Table 2 indicate a weak relationship between temperature and the residual shear strength. The changes in residual shear strength due to temperature increases from 25 to 60°C are much smaller compared with those in the shear strength and shear reaction modulus. Considering that the residual shear strength is primarily provided by the friction between the steel plate and the asphalt concrete, the roughness of the steel plate surface and the aggregate gradation of the asphalt concrete may be the main factors that influence the shear resistance after failure of the interface. Hence, it is possible to 9
conclude that the contribution of friction to interface shear resistance is not affected by the temperature variation. 4.4. Effect of normal stress The entire set of data collected both in shear strength and residual shear strength at all the test conditions
are shown in Table 2. From this figure, it is observed that the increase of the applied normal stress results in increased shear strength for tests carried out in this study. For the specimens with PMA tack coat, the shear strengths measured from the tests performed at the normal stress of 0.7 MPa are approximately 2 to 3.5 times higher than those measured without normal stress. While for the specimens with EA tack coat, the shear strengths measured at the normal stress of 0.7 MPa is nearly 1.1 to 1.7 times higher than those measured without normal stress. This kind of effect may be attributed to that the higher normal stress increases the interfacial contact area between the steel plate and the asphalt overlay, and thus increase the friction contribution of shear resistance. It implies that although an overloaded truck causes higher shear stress at the interface which leads to higher risk of slippage failure, meanwhile, it improves the shear resistance of the interface to some extent. The percent changes in shear strengths due to the normal stress increase from 0 to 0.7 MPa at 60°C is greater than those at 25℃, indicating that normal stress plays a more important role in the component of interface shear resistance at high temperature. It is postulated that the cohesion of the interface is reduced at high temperature and the friction effect between the steel plate and the asphalt overlay becomes a greater factor. Table 2 also shows the interface residual shear strength values for various interface types and test conditions. As mentioned before, the interface residual shear strength generates by friction between the steel plate and the asphalt overlay under the action of normal stress. Thus, no interface residual shear strength value can be obtained from tests without normal stress. It is observed that the interface residual shear strength increases with increasing normal stress. For the thermosetting type interface, it still can sustain the shear stress of up to 0.38MPa after failure. Under the same loading condition, however, the shear resistances remain only from 0.01 to 0.10 MPa for the thermoplastic type interface. By plotting the shear strength versus the normal stress which shows an approximately linear relationship, it is possible to roughly estimate the failure envelopes that can be described by Coulomb’s equation [32]: c0 n tan 10
(3)
where c0 is the cohesion that quantifies the pure shear resistance of the interface; n is the normal stress; is the internal friction angle of the interface.
For a single test temperature, the peak friction angle ( p ) and the cohesion ( c0 ) can be obtained by only varying the normal stress ( n ). Moreover, the residual failure envelope can be also plotted and characterized by the residual friction angle ( r ). Peak and residual failure envelopes for the thermosetting type interface and the thermoplastic type interface at 25°C and 60°C are shown in Fig.12. The linear regression parameters, including c0 , tan p , tan r , and the corresponding coefficient of determination R p2 and Rr2 are presented in Table 3 for all test conditions. The results show that the cohesions ( c0 ) for both interface types decrease with increasing temperature, which demonstrates the viscoelastic characteristics of the tack coat materials. Also indicated in Table 3 is that the thermosetting type interface gives higher tan p and tan r values than those from the thermoplastic type interface. It is known that the steel plate surface provides very low roughness, thus, the friction could only depends on the characteristics of the asphalt overlay materials which is in contact with the steel plate. It is observed from Fig.1 that the GAC has an extremely fine aggregate gradation with high content of mineral filler. Additionally, the tack coat binder at the interface is dispersed inside the bottom of the asphalt overlay and further weakens the interlock effect between the steel plate and the asphalt overlay. Subsequently, the friction contribution tan p and tan r to shear resistance for specimens with GAC overlay is small. 4.5. Effect of tack coat material From Table 2 it is evident that the EA tack coat provides better interface shear strengths than the PMA tack coat. The shear strengths of the specimens with EA tack coat range from 1.92 to 2.65 MPa at 25℃, which are approximately 2.7 to 3.8 times higher than those of the specimens with PMA tack coat. In the case of 60℃, the PMA tack coat gives the shear strengths of less than 0.1 MPa which are much lower than those of EA tack coat (above 0.5 MPa). For the shear strength is the critical parameter to evaluate the interface bonding performance, the PMA tack coat is not accepted by the steel-asphalt composite structure in comparison with the EA tack coat. Similar results are found on the comparisons of the shear reaction modulus between the two tack coat materials, as shown in Table 2. The higher shear reaction modulus can result in smaller interface shear displacement under the action of traffic loading and better capability of transferring the traffic loads to the steel bridge deck which is beneficial for the durability of the asphalt 11
overlay. The considerable improvement of specimens with EA tack coat in the shear resistance may attribute to the elevated mechanical property of EA, for EA is a thermosetting material that is not as sensitive to temperature variation as the thermoplastic asphalt. The modification of asphalt with epoxy resin increases its strength and temperature stability, which is helpful for the shear resistance of interface at high temperature. These observations agree well with previous findings that the shear strengths of specimens with EA tack coat are much higher than those of other types of interface including reinforced interface [14, 33].
5. Conclusions and recommendations This paper presents a laboratory study on the shear characteristics of steel-asphalt interfaces with thermosetting and thermoplastic tack coat materials by the SCIS test. The test results and the discussions presented in this paper allow the following conclusions to be drawn:
The composite specimens with EA tack coat failed at the primer-tack coat interface with complete separation of the EAC from steel plate at all test conditions. For the PMA tack coat, as the temperature increases, the failure mode is likely to change from adhesive failure at the primer-tack coat interface to the material failure of GAC.
Precision of the SCIS test method is evident from the reasonably low COVs obtained. The proposed test method is effective for determining the shear characteristics of steel-asphalt interfaces, which is evident from the consistency of test results and failure modes.
The shear stress-displacement curves for all the test conditions follow a similar pattern that the shear stress increases initially and then decreases with increasing shear displacement. The ascending part of the curves shows an approximately linear shape approaching to a peak value. In the post peak region, the curves show a strain softening behavior until reaching a plateau.
Temperature has a dominant effect on the interface shear strength and shear reaction modulus that they significantly decrease as testing temperature increases. This suggests that the high temperature is a critical condition for which the interface failure is more likely to occur, and the interface shear test should be conducted at the highest temperature that the asphalt overlay on steel bridge deck will experience in service.
The interface shear strength increase with the increasing normal stress. The percent changes in shear strengths due to the normal stress increase at 60°C is greater than those at 25°C, indicating 12
that normal stress plays a more important role in the component of interface shear resistance at high temperature. Based on the Coulomb failure law, the failure envelopes of the interface shear strength and residual shear strength were obtained for combinations of interface type and temperature conditions.
As the EA tack coat provides much better interface shear strength, shear reaction modulus, and residual shear strength than the PMA tack coat, it is recommended that the EA be used as the tack coat material between the steel bridge deck and the asphalt overlay.
Acknowledgements This work was supported by National Natural Science Foundation of China (No. 51208103). The authors would like to express their gratitude to the Southeast University Road and Bridge Laboratory for providing equipment and assistance.
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orthotropic steel deck surfaces. Transport Res Rec 2012; 2293:1-7. [16] Ge ZS, Wang YY, Hang MB. New device and methodology for evaluating the shear behavior of steel bridge deck pavement pasted by GFRP sheets. J Test Eval 2014;42(1):93-108. [17] Romanoschi SA, Metcalf JB. Characterization of asphalt concrete layer interfaces. Transport Res Rec 2001; 1778:132-139. [18] ASTM D412-06a. Standard test methods for vulcanized rubber and thermoplastic elastomers-tension. Pennsylvania, US: American Society for Testing and Materials (ASTM); 2013. [19] ASTM D648-07. Standard test method for deflection temperature of plastics under flexural load in the edgewise position. Pennsylvania, US: American Society for Testing and Materials (ASTM); 2007. [20] AASHTO T315-12. Standard method of test for determining the rheological properties of asphalt binder using a Dynamic Shear Rheometer (DSR). Washington, D.C., US: American Association of State Highway and Transportation Officials (AASHTO); 2012. [21] AASHTO T313-12. Standard method of test for determining the flexural creep stiffness of asphalt binder using the Bending Beam Rheometer (BBR). Washington, D.C., US: American Association of State Highway and Transportation Officials (AASHTO); 2012. [22] GB/T 13452.2-2008. Paints and varnishes - determination of film thickness. Beijing, China: China Standards Press; 2008. [23] Liu Y, Wu JT, Chen J. Mechanical properties of a waterproofing adhesive layer used on concrete bridges under heavy traffic and temperature loading. Int J Adhes Adhes 2014;48:102-109. [24] Ozer H, Al-Qadi IL, Wang H, Leng Z. Characterisation of interface bonding between hot-mix asphalt overlay and concrete pavements: modelling and in-situ response to accelerated loading. Int J Pavement Eng 2012;13(2):181-196. [25] Mohammad LN, Hassan M, Patel N. Effects of shear bond characteristics of tack coats on pavement performance at the interface. Transport Res Rec 2011; 2209:1-8. [26] Mohammad LN, Bae A, Elseifi MA, Button J, Patel N. Effects of pavement surface type and sample preparation method on tack coat interface shear strength. Transport Res Rec 2010; 2180:93-101. [27] Kruntcheva MR, Collop AC, Thom NH. Properties of asphalt concrete layer interfaces. J Mater Civil Eng 2006;18(3):467-471. [28] Shen HG, Zhang Y, Sheng DW, Mo LT. Experimental investigation of tensile bonding strength of Gussasphalt overlay upon steel bridge deck. Bldg Mater World 2013;34(4):29-32. [29] Xu QW, Zhou QH, Medina C, Chang GK, Rozycki DK. Experimental and numerical analysis of a waterproofing adhesive layer used on concrete-bridge decks. Int J Adhes Adhes 2009;29(5):525-34. 15
[30] Bae A, Mohammad LN, Elseifi MA, Button J, Patel N. Effects of temperature on interface shear strength of emulsified tack coats and its relationship to rheological properties. Transport Res Rec 2010; 2180:102-109. [31] Wheat M. Evalutation of bond strength at asphalt interfaces. Kansas State University; 2007 [Master thesis]. [32] Santagata FA, Partl MN, Ferrotti G, Canestrari F, Flisch A. Layer characteristics affecting interlayer shear resistance in flexible pavements. J Assn Asphalt Paving Technol 2008; 77:221-256. [33] Qian ZD, Yang YM, Song RY. A study on epoxy asphalt waterproof adhesive layer for steel bridge Gussasphalt pavement. Highw 2013; 2:1-5.
16
Tables Table 1 Properties of the tack coat materials. Material EA
PMA
a
Property Tensile strength at 23℃ (MPa) Tensile elongation at 23℃ (%) Heat deflection temperature (℃) Creep stiffness, test on (RTFO+PAV) at -12℃ (MPa) m -value, test on (RTFO+PAV) at -12℃ Dynamic shear, G* / sin( ) a, test on original binder at 76℃ (kPa) Dynamic shear, G* / sin( ) , test on RTFO at 76℃ (kPa) Creep stiffness, test on (RTFO+PAV) at -12℃ (MPa) m -value, test on (RTFO+PAV) at -12℃
* G * is the complex modulus, is the phase angle, G / sin( ) is the rutting parameter.
17
Measured value 16.6 216 -20 407 0.213 2.72 5.48 152 0.325
Test method ASTM D412 [18] ASTM D412 [18] ASTM D648 [19] AASHTO T313 [21] AASHTO T313 [21] AASHTO T315 [20] AASHTO T315 [20] AASHTO T313 [21] AASHTO T313 [21]
Table 2 SCIS test results of the steel-asphalt composite specimens. p n Tack coat COVa K COVb r COVc Failure T material (MPa/mm) (%) mode (℃) (MPa) (MPa) (%) (MPa) (%) EA 25 0 1.92 7.5 2.09 8.3 A EA 25 0.2 2.14 3.9 2.22 10.1 0.09 7.0 A EA 25 0.5 2.48 3.3 2.54 8.7 0.27 12.3 A EA 25 0.7 2.65 4.0 2.64 7.6 0.38 5.2 A EA 60 0 0.5 7.1 0.43 10.2 A EA 60 0.2 0.66 5.6 0.50 10.0 0.08 13.6 A EA 60 0.5 0.78 6.4 0.58 13.6 0.18 6.7 A EA 60 0.7 0.98 6.1 0.73 8.1 0.29 9.6 A PMA 25 0 0.36 7.1 0.41 8.3 A PMA 25 0.2 0.46 5.6 0.54 8.4 0.06 13.6 A PMA 25 0.5 0.58 6.4 0.69 7.8 0.09 16.7 A PMA 25 0.7 0.65 6.1 0.75 8.9 0.11 9.6 A 0.01 B PMA 60 0 10.7 15.3 0.03 B PMA 60 0.2 0.04 14.5 20.1 0.04 19.3 0.05 B PMA 60 0.5 0.09 11.7 21.3 0.07 18.8 0.08 B PMA 60 0.7 0.1 18.5 15.9 0.10 15.5 0.09 a b c COVs of p ; COVs of K ; COVs of r ; A = Failed through interface; and B = Failed through asphalt concrete.
18
Table 3 Parameters of the shear strength and residual shear strength envelopes. Tack coat c0 tan r tan p R p2 T Rr2 material (℃) (MPa) EA 25 1.93 1.06 0.99 0.58 0.98 60 0.51 0.64 0.96 0.41 0.96 PMA 25 0.36 0.41 0.92 0.10 0.97 60 0.01 0.13 0.96 0.12 0.95
19
Figures
Percentage Passing (%)
100
EAC GAC
80 60 40 20 0 0.01
0.1
1
10
Sieve Size (mm) Fig.1. Aggregate gradations.
20
100
Loading Frame Shear Load Cell
Actuator
Linear Motion Guide
Environmental Chamber
Slider Guides Stiffener
Flat Jack Shear Box
Base Specimen Interface
Fig.2. Schematic of the SCIS test apparatus.
21
Normal Load Cell
Data acquisition and control system
Fig.3. Details of the shear box.
22
Fig.4. Picture of the SCIS test apparatus.
23
(a)
(b)
(c)
Fig.5. Fabrication of the composite specimens for the SCIS test: (a) steel plates installed in the mold; (b) compacted asphalt concrete mixture in the mold; and (c) specimens ready for shear testing.
24
Fig.6. Shear slip failure of the steel-asphalt interface in SCIS test.
25
(a)
(b)
(c)
(d)
Fig.7. Typical interface failure modes: (a) specimen with EA tack coat at 25℃; (b) specimen with EA tack coat at 60℃; (c) specimen with PMA tack coat at 25℃; and (d) specimen with PMA tack coat at 60℃.
26
Shear Stress (MPa)
3.5
4.0
25℃,0MPa,#1 25℃,0MPa,#2 25℃,0MPa,#3 25℃,0MPa,#4 25℃,0MPa,#5
(a)
3.0 2.5 2.0 1.5 1.0 0.5 0.0
0
1
2
3
3.0 2.5 2.0 1.5 1.0 0.5 0.0
4
25℃,0.2MPa,#6 25℃,0.2MPa,#7 25℃,0.2MPa,#8 25℃,0.2MPa,#9 25℃,0.2MPa,#10
(b)
3.5
Shear Stress (MPa)
4.0
0
1
Displacement (mm)
3.5
Shear Stress (MPa)
25℃,0.5MPa,#11 25℃,0.5MPa,#12 25℃,0.5MPa,#13 25℃,0.5MPa,#14 25℃,0.5MPa,#15
(c)
3.0 2.5 2.0 1.5 1.0 0.5 0.0
0
1
2
3
4.0
3
4
3.0 2.5 2.0 1.5 1.0 0.5 0.0
4
60℃,0.7MPa,#16 60℃,0.7MPa,#17 60℃,0.7MPa,#18 60℃,0.7MPa,#19 60℃,0.7MPa,#20
(d)
3.5
Shear Stress (MPa)
4.0
2 Displacement (mm)
Displacement (mm)
0
1
2
3
4
Displacement (mm)
Fig.8. Interface shear stress-displacement curves obtained from the SCIS tests for specimens with the EA tack coat at 25°C: (a) under normal stress of 0.0 MPa; (b) under normal stress of 0.2 MPa; (c) under normal stress of 0.5 MPa; (d) under normal stress of 0.7 MPa.
27
60℃,0MPa,#21 60℃,0MPa,#22 60℃,0MPa,#23 60℃,0MPa,#24 60℃,0MPa,#25
(a) Shear Stress (MPa)
1.0 0.8 0.6 0.4 0.2 0.0
0
1
2
3
1.2
60℃,0.2MPa,#26 60℃,0.2MPa,#27 60℃,0.2MPa,#28 60℃,0.2MPa,#29 60℃,0.2MPa,#30
(b) 1.0 Shear Stress (MPa)
1.2
0.8 0.6 0.4 0.2 0.0
4
0
1
Displacement (mm) 60℃,0.5MPa,#31 60℃,0.5MPa,#32 60℃,0.5MPa,#33 60℃,0.5MPa,#34 60℃,0.5MPa,#35
(c) Shear Stress (MPa)
1.0 0.8 0.6 0.4 0.2 0.0
0
1
2
3
4
Displacement (mm)
3
1.2
60℃,0.7MPa,#36 60℃,0.7MPa,#37 60℃,0.7MPa,#38 60℃,0.7MPa,#39 60℃,0.7MPa,#40
(d) 1.0 Shear Stress (MPa)
1.2
2
0.8 0.6 0.4 0.2 0.0
4
Displacement (mm)
0
1
2
3
4
Displacement (mm)
Fig.9. Interface shear stress-displacement curves obtained from the SCIS tests for specimens with the EA tack coat at 60°C: (a) under normal stress of 0.0 MPa; (b) under normal stress of 0.2 MPa; (c) under normal stress of 0.5 MPa; (d) under normal stress of 0.7 MPa.
28
25℃,0MPa,#41 25℃,0MPa,#42 25℃,0MPa,#43 25℃,0MPa,#44 25℃,0MPa,#45
Shear Stress (MPa)
(a) 0.6
0.4
0.2
0.0
0
1
2
3
0.8
25℃,0.2MPa,#46 25℃,0.2MPa,#47 25℃,0.2MPa,#48 25℃,0.2MPa,#49 25℃,0.2MPa,#50
(b) Shear Stress (MPa)
0.8
0.6
0.4
0.2
0.0
4
0
1
Displacement (mm) 25℃,0.5MPa,#51 25℃,0.5MPa,#52 25℃,0.5MPa,#53 25℃,0.5MPa,#54 25℃,0.5MPa,#55
Shear Stress (MPa)
(c) 0.6
0.4
0.2
0.0
0
1
2
3
4
Displacement (mm)
3
0.8
25℃,0.7MPa,#56 25℃,0.7MPa,#57 25℃,0.7MPa,#58 25℃,0.7MPa,#59 25℃,0.7MPa,#60
(d) Shear Stress (MPa)
0.8
2
0.6
0.4
0.2
0.0
4
Displacement (mm)
0
1
2
3
4
Displacement (mm)
Fig.10. Interface shear stress-displacement curves obtained from the SCIS tests for specimens with the PMA tack coat at 25°C: (a) under normal stress of 0.0 MPa; (b) under normal stress of 0.2 MPa; (c) under normal stress of 0.5 MPa; (d) under normal stress of 0.7 MPa.
29
60℃,0MPa,#61 60℃,0MPa,#62 60℃,0MPa,#63 60℃,0MPa,#64 60℃,0MPa,#65
Shear Stress (MPa)
(a) 0.03
0.02
0.01
0.00
0
1
2
3
0.25
60℃,0.2MPa,#66 60℃,0.2MPa,#67 60℃,0.2MPa,#68 60℃,0.2MPa,#69 60℃,0.2MPa,#70
(b) 0.20 Shear Stress (MPa)
0.04
0.15 0.10 0.05 0.00
4
0
1
Displacement (mm) 60℃,0.5MPa,#71 60℃,0.5MPa,#72 60℃,0.5MPa,#73 60℃,0.5MPa,#74 60℃,0.5MPa,#75
(c) Shear Stress (MPa)
0.20 0.15 0.10 0.05 0.00
0
1
2
3
4
Displacement (mm)
3
0.25
60℃,0.7MPa,#76 60℃,0.7MPa,#77 60℃,0.7MPa,#78 60℃,0.7MPa,#79 60℃,0.7MPa,#80
(d) 0.20 Shear Stress (MPa)
0.25
2
0.15 0.10 0.05 0.00
4
Displacement (mm)
0
1
2
3
4
Displacement (mm)
Fig.11. Interface shear stress-displacement curves obtained from the SCIS tests for specimens with the PMA tack coat at 60°C: (a) under normal stress of 0.0 MPa; (b) under normal stress of 0.2 MPa; (c) under normal stress of 0.5 MPa; (d) under normal stress of 0.7 MPa.
30
4
Peak envelope at 25℃ Residual friction envelope at 25℃ Peak envelope at 60℃ Residual friction envelope at 60℃
1.0 (a) Interface Shear Stress (MPa)
Interface Shear Stress (MPa)
5
3 2 1 0 0.0
0.2
0.4
0.6
0.8
0.8
Normal Stress (MPa)
(b)
0.6 0.4 0.2 0.0 0.0
1.0
Peak envelope at 25℃ Residual friction envelope at 25℃ Peak envelope at 60℃ Residual friction envelope at 60℃
0.2
0.4
0.6
0.8
1.0
Normal Stress (MPa)
Fig.12. Peak and residual failure envelopes at 25°C and 60°C for: (a) the thermosetting type interface; (b) the thermoplastic type interface.
31
www.elsevier.com/locate/ijadhadh
PII: DOI: Reference:
S0143-7496(16)30018-5 http://dx.doi.org/10.1016/j.ijadhadh.2016.02.005 JAAD1795
To appear in: International Journal of Adhesion and Adhesives Received date: 20 October 2014 Accepted date: 8 February 2016 Cite this article as: Bo Yao, Fangchao Li, Xiao Wang and Gang Cheng, λEvaluation of the shear characteristics of steel-asphalt interface by a direct shear test method, International Journal of Adhesion and Adhesives, http://dx.doi.org/10.1016/j.ijadhadh.2016.02.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Evaluation of the shear characteristics of steel-asphalt interface by a direct shear test method
Bo Yao a,*, Fangchao Li a, Xiao Wang b, Gang Cheng b
a
Department of Civil Engineering, School of Science, Nanjing University of Science and Technology, 200
Xiaolingwei, Nanjing 210094, China b
School of Transportation, Southeast University, 35 Jinxianghe Road, Nanjing 210096, China
*Corresponding author. Tel./fax: +86 25 87794368 E-mail address: [email protected] (B. Yao). ABSTRACT Shear characteristics of steel-asphalt interface under the influences of temperature, normal stress level and tack coat material were investigated. The direct shear tests were conducted on composite specimens with epoxy asphalt (EA) and polymer modified asphalt (PMA) tack coat materials at temperatures of 25 and 60°C and normal stress levels of 0, 0.2, 0.4, and 0.7 MPa for each temperature. Results show that the failure modes include adhesive failure at the primer-tack coat interface and material failure of asphalt concrete. Steel-asphalt interface shows strain softening behavior until it reaches the sliding state. The shear strength and the shear reaction modulus increase with decreasing temperature and increasing normal stress levels. The specimens with EA tack coat provides much higher interface shear strengths than those with PMA tack coat at 25 and 60°C. In addition, the failure envelopes of the shear strength and residual shear strength were obtained for combinations of tack coat materials and temperature conditions based on the Coulomb failure law. Keywords: Steel bridge deck, Asphalt tack coat, Interface, Shear characteristics, Direct shear test
1
1. Introduction In highway steel bridge construction, tack coats are usually used between the steel bridge decks and the asphalt overlays. The types of tack coat used generally fall into two categories: thermosetting materials and thermoplastic materials. The thermosetting type tack coat materials include epoxy asphalt and epoxy-polyurethane resins, and the thermoplastic type tack coat materials include polymer modified asphalt, asphalt emulsion, mastic asphalt, and asphalt multilayer membranes [1-3]. The application of tack coats has several advantages including providing sufficient bonding between the asphalt overlay and the underlying steel deck, and ensuring the two components act as an integrated unit to support the traffic and environmental loading, which is crucial for the short- and long-term performance of asphalt overlays [4-6]. A review of current practices [7-10] reveals that insufficient bond between the steel deck and the asphalt overlay may cause asphalt overlay distresses such as slippage cracking, debonding, early fatigue cracking, and reduction of overlay life. Due to very different physical and mechanical properties of steel and asphalt concrete, the incompatibility between them is pronounced and the steel-asphalt interface is generally the weakest plane of the composite system [11]. Slippage cracking of asphalt overlay is caused by a lack of shear resistance at the steel-asphalt interface and a high enough shear stress generated when heavy vehicles are accelerating, decelerating, or turning, in which case the asphalt overlay begins to slide on steel bridge deck. A well prepared and adequately treated steel deck surface before application of asphalt overlay and correct choice of tack coat materials can minimize the slippage cracking problem to a great extent. However it is desirable to have some evaluation techniques by which the interface could be characterized and tested properly. Published information [12] indicates that the performance of steel-asphalt interface have been evaluated by several test methods. Medani et al. [13] used the TU-Delft four-point shear test to characterize the response of a bituminous-based membrane material on an orthotropic steel deck bridge to shear and normal forces. Bocci and Canestrari [14,15] employed the ASTRA (Anocona Shear Testing Research and Analysis) shear test device to evaluate the shear strengths of smooth and reinforced steel interfaces coated with a polymer-modified bitumen between the steel deck and the conventional Hot Mix Asphalt (HMA). Recently, Ge et al. [16] proposed a preloading and shear loading testing method for evaluating the shear behavior of steel deck pavement pasted by GFRP sheets. However, most researchers have only focused on the strength of the steel-asphalt interface. Few studies have focused on the mechanical behavior at the interface. Three 2
laboratory-measured properties were suggested to describe the mechanical behavior at the interface: the interface shear strength, the interface reaction modulus, and the friction coefficient after failure [17]. Therefore, a more comprehensive knowledge of the actual mechanical behavior of interface is necessary to better understand the steel-asphalt composite system and more accurately compute the service lifetime of overlay structure. On the other hand, the shear tests in literature were conducted at low and intermediate temperatures (from 0 to 40°C), which is not representative of the interface environmental conditions. For the interface between the steel deck and the asphalt overlay, it is known that high temperature which the overlays experience in service, e.g. 60°C, is a more critical condition for which the interface failure is more likely to occur. Therefore, an effective test method which enables measurement of the interface shear characteristics at high temperature is necessary. The objective of this study was to evaluate the shear characteristics of two types of steel-asphalt interface by the steel-concrete interface shear (SCIS) test method. One thermosetting type interface and one thermoplastic type interface were made to observe the influence of tack coat materials on the interface shear response. The SCIS tests were conducted at temperatures of 25 and 60°C and normal stress levels of 0, 0.2, 0.4, and 0.7 MPa for each temperature. The results reported include the shear strengths, shear reaction modulus, residual shear strengths, shear stress-displacement curves, failure envelopes, and failure modes of the steel-asphalt interfaces.
2. Materials Two types of interface were evaluated in this study. An epoxy asphalt (EA) tack coat and an epoxy asphalt concrete (EAC) were used to provide a thermosetting type interface. A polymer modified asphalt (PMA) tack coat and a Gussasphalt concrete (GAC) were used to provide a thermoplastic type interface. The EA is a two-part product blended before use. Part A (used at 14.6% by weight) consists of an epoxy resin formed from epichlorhydrin and bisphenol-A. Part B (used at 85.4% by weight) is a mix of asphalt and epoxy cross-linker. After being fully cured, the product is similar to a black hard rubber. The PMA is a styrene-butadiene-styrene (SBS) modified asphalt binder graded as PG 76-22. The properties of the fully-cured EA and PMA tack coats are presented in Table 1. The EAC and GAC mixtures were prepared according to the specifications for mixtures usually laid on the steel bridge decks. The EAC mixture used another type of EA as binder with a content of 6.5% by weight. The asphalt binder of GAC mixture was an asphalt-TLA blend. The blend was composed of a Pen 20/40 3
asphalt and Trinidad Lake Asphalt (TLA) with contents of 70% and 30% by total weight, respectively. The binder content of GAC mixture was 8.5% by weight. The aggregate gradations of the EAC and GAC mixtures are provided in Fig.1.
3. Experimental program 3.1. The SCIS test The SCIS test apparatus is designed to adapt to commonly-used servo-hydraulic loading frame and environmental chamber. It consists of a shear box, a horizontal loading device for applying normal load on the interface, a horizontal load cell, a servo-hydraulic loading frame, a vertical load cell and a Linear Variable Differential Transformer (LVDT), a data acquisition system, and a computer with compatible software to record the data. To evaluate the temperature effect on the shear characteristics of steel-asphalt interface, the SCIS test apparatus is placed in an environmental chamber that can maintain temperatures ranging from -15 to 60°C with an accuracy of ±0.5 ºC. A schematic of the SCIS test apparatus is shown in Fig.2. The details of the shear box is shown in Fig.3. The asphalt concrete part of the specimen is fixed, while the steel plate is confined in the X direction only. The steel plate can moves vertically under applied load, which allows shear at the interface to take place. The application of shear load on the steel plate of the specimen results in smaller moment arms that can minimize the effect of bending moment induced by the eccentricity of the shear force to a certain extent. The movable base can be shifted in the X direction to align the steel-asphalt interface to the shear plane. This is done to ensure that the shear force is applied in the interface plane, where most of the shear displacement will occur. The shear box is restrained against rotation by two slider guides that allows the movement to take place only in the horizontal direction. A linear motion guide is placed closely attaching to the steel plate of the specimen to allow its frictionless movement in the vertical direction. To simulate the asphalt overlay upon steel bridge deck experiences a combined state of compression and shear under the action of traffic loading, the SCIS test apparatus is designed to apply shear force in the vertical direction accompanying normal force in the horizontal direction. The application of the initial horizontal load, normal to the interface plane of the specimen, is achieved through a flat jack. It provides a constant normal stress condition, which is monitored by a load cell with a reading accuracy of ±0.1N. The 4
vertical shear deformation/load, paralleled with the interface plane, is applied on the top surface of the steel plate by a servo-hydraulic actuator. A load cell connecting to the loading frame are used to read and store the vertical force continually. The shear displacements of the interface are measured by the loading frame internal LVDT housed in the loading piston with a reading accuracy of ±0.005 mm. A photograph of the SCIS test apparatus is given in Fig.4. 3.2. Fabrication of specimens The test specimen is a steel-asphalt composite structure comprising a 10 mm thick steel plate and one layer of asphalt concrete with a thickness of 55 mm. The steel plates were fabricated according to the cross section requirement of 96 96 mm. After that, they were sandblasted and coated with epoxy zinc rich primer with thickness of 60 to 80 μm measured according to method stated in GB/T 13452.2-2008 [22]. A specific steel mold measuring 300 mm 300 mm and 65 mm in thickness was used for the preparation of the composite specimens, as shown in Fig.5a. It is divided into 9 cells, each of which has a dimension of 96 96 mm based on the specimen size to be prepared. Before fabricating specimens, the steel mold was preheated in a 60°C oven for 1 hour. The hot tack coat materials were spread carefully on the surface of each steel plate using a clean scraper knife to give desired amount of tack coat. The amount corresponds to the field application rate of 0.68L/m2 and 300g/m2 for EA and PMA, respectively. Then place the steel mold on a level shelf in the 60°C oven for 10±2 minutes to allow the tack coat to self level. After that, the desired weight of EAC and GAC mixtures were mixed and laid above the tack coat followed by compaction to the specimens with laboratory roller-compactor, as shown in Fig.5b. The amount of mixtures was calculated based on the relationship between asphalt concrete maximum specific gravity, air void content, and desired height of asphalt concrete in the composite specimen after compaction. For thermosetting type interface, the steel mold was placed in a 120°C oven for 4 hours to make the EAC be fully cured before being placed on a level surface to cool to room temperature. For thermoplastic type interface, the steel mold was directly placed on a level surface for an appropriate cooling period after compaction. Finally, the compacted composite specimens were extracted from the mold. Specimens ready for testing are shown in Fig.5c. 3.3. Experimental procedure Some of the findings from the literature on tack coat applications indicate that the major factors 5
affecting asphalt interface shear characteristics are temperature, shear rate, magnitude of normal stress, HMA type, tack coat type, tack coat application rate, and surface cleanliness [23-26]. For the asphalt overlay on steel bridge deck, an overlay material is usually used together with another specific type of tack coat. In this study, one thermosetting type interface and one thermoplastic type interface were evaluated by the SCIS test. Specifically, the thermosetting type interface indicates the EA as tack coat and the EAC as overlay material, while the thermoplastic type interface indicates the PMA as tack coat and the GAC as overlay material. Two temperatures (25 and 60℃) were considered in the experimental plan to investigate the effect of temperature on the shear characteristics of steel-asphalt interface. At each temperature, four normal stress levels (0, 0.2, 0.4, and 0.7 MPa) were examined. For each combination of interface type, test temperature, and normal stress level, three replicate specimens were tested and the average values of the test results were reported. Prior to testing, specimens were placed in an environmental chamber for at least 4h in order to equilibrate to the specified testing temperature. Shear tests were conducted in displacement-controlled mode at a constant shear rate of 1 mm/min and the normal stress on the interface of the specimen was held constant during the tests. Tests were stopped after a sliding state was clearly observed or the total shear displacement reached 10 mm. The failure of the steel-asphalt interface arising from shear slip is shown in Fig.6.
4. Results and discussion 4.1. Experimental results The interface shear strength, i.e., the peak shear stress, is calculated as follows: p Tmax / A
(1)
where p is the interface shear strength, Tmax is the maximum shear load applied to specimen, and A is the cross-sectional area of the test specimen. The maximum slope of the shear stress versus displacement curve can be used to define the shear reaction modulus ( K ) as follows [27]: K /
(2)
In the SCIS test, due to the presence of the normal load acing on the interface, a residual shearing condition was clearly observed in which the shear displacement continues increasing while the shear stress keeps at a constant value defined as the residual shear strength ( r ). 6
Table 2 summarizes the average interface shear strengths, shear reaction modulus, and residual shear strengths of the composite specimens and their corresponding coefficients of variation (COVs) as well as the failure modes obtained from the SCIS tests. The typical fracture interfaces of the tested specimens are shown in Fig.7. The failure modes were revealed by visual observation of the fracture interfaces. For the composite specimens with EA tack coat at all test conditions, adhesive failure was obtained with complete separation of the EAC from steel plate (Fig.7a, Fig.7b). Cracks were detected at the primer-tack coat interface during the tests. The EA tack coat infiltrated into the EAC layer after compaction, rather than kept intact as an individual layer. No failure in the EAC was observed and the steel plate has almost no adherent EAC. For PMA tack coat, the temperature dependence is associated with a change in the failure mode of the interface. Shear tests performed at 25℃ results in an adhesive failure at the primer-tack coat interface. It is shown that the primer keep intact after the shear test while the PMA tack coat disperses in the GAC and a clear separation is observed (Fig.7c). While at 60℃, the failure mode is observed as material failure in GAC that rupture nucleates and propagates in the bulk GAC before propagating at the interface. A thin layer of GAC is attached to the interface after failure (Fig.7d). The change from adhesive to cohesive failure with increased temperature was associated with a reduction in the mechanical performance and stability of GAC. Since the performance of GAC is sensitive to temperature, it has good resistance to deformation at low temperature, but at high temperature it has a high tendency toward plastic deformation even flow. It is revealed that the interface failure mode relates to not only tack coat material, but also asphalt concrete layer closely contacting to the interface. It can therefore be concluded that the failure of steel-asphalt interface mainly involves the yield behaviour of the mixture composed of polymer adhesive and the asphalt concrete at the bottom of the overlay at elevated temperatures. The experimental results indicate that the interface shear strength, shear reaction modulus, and residual shear strength are sensitive to temperature, normal stress, and tack coat materials. The EA tack coat gives the interface shear strengths between 1.92 and 2.65 MPa at 25℃ while between 0.50 and 0.98 MPa at 60℃, as shown in Table 2. Similar results were reported by Bocci and Canestrari [14], they obtained the shear strengths of 2.3 to 2.9 MPa at 20℃ and 1.4 to 1.7 MPa at 40℃ between steel plate and asphalt concrete with EA tack coat. The COVs of the shear strength for specimens with EA tack coat are less than 8%. The narrow range of COVs is an indication of the consistent repeatability of the proposed shear test method. The values of the interface shear reaction modulus presented in Table 2 range from 2.09 to 2.64 MPa/mm at 25℃ 7
while from 0.43 to 0.73 MPa/mm at 60℃, which represents the stiffness of the interface. The COVs of the shear reaction modulus are less than 12%. The interface residual shear strength for the specimens with EA tack coat are from 0.09 to 0.38 MPa at 25℃ and from 0.08 to 0.29 MPa at 60℃ with the corresponding COVs below 14%. Considering the manufacturing complexity and variations in material properties, the COVs for the specimens with EA tack coat are acceptable. Table 2 also shows that the interface shear strengths for the specimens with PMA tack coat are between 0.36 and 0.65 MPa at 25℃ while between 0.01 and 0.10 MPa at 60℃. The shear strengths are comparable with other published results [13, 28] with some variations due to differences in geometry of specimens, test methods, and tack coat materials. The interface shear reaction modulus range from 0.41 to 0.75 MPa/mm at 25℃ while from 0.03 to 0.09 MPa/mm at 60℃, which indicates that the interface with PMA tack coat has a low resistance to shear deformation at high temperature. As for the residual shear strength, no obvious difference are found between the data obtained from the tests conducted at 25℃ and 60℃. In the case of the PMA tack coat at 60℃, the COVs of the interface shear strength, shear reaction modulus, and residual shear strength are higher since the results are rather low that a small difference in the results can highly elevate the COVs. 4.2. Shear stress-displacement curves The shear behavior of an interface between the steel deck and the asphalt overlay can be characterized by the shear stress-displacement curve. The shear stress-displacement curves for all the test conditions are shown in Fig.8 to Fig.11. It shows that the shear stress-displacement curves for specimens with EA tack coat and PMA tack coat are similar: the shear stress increases initially and then decreases with increasing shear displacement. The ascending part of the curves shows an approximately linear shape approaching to a peak value. In the post peak region, the curves show a strain softening behavior until reaching a plateau. It is worth noting that the application of a normal stress during the shear test assures a friction contribution due to the pure friction condition that occurs after the peak value has been passed. In this condition, the displacement takes place without variation in shear stress. The shear stress-displacement curves indicate ductile failure at the steel-asphalt interface which may be attributed to the fact that the asphalt tack coat materials used in this study show viscoelastoplastic behavior at normal and high temperature. It is also implied that different forms of shear stress-displacement model rather than bi-linear model usually used in 8
asphalt pavement interface analysis should be developed for steel-asphalt interfaces. It is also evident from Fig.8 to Fig.11 that the shear stress-displacement curves are variable for specimens with the identical tack coat material in different testing conditions, which indicates that the interface behavior are governed by the properties of the tack coat material, temperature, and normal stress. It is observed from Fig.8 to Fig.11 that the shear stress-displacement curves cross through three distinct zones. The first zone corresponds to the bulk linear viscoelastic behavior of the tack coat material. Due to the viscoelastic effect, the shear strength and shear reaction modulus are found to decrease with increasing temperature. The second zone corresponds to the damage initiation and evolution in the interface. The interface shear strength continues descending after a peak value. The third zone corresponds to the failure process when the shear stress leveled out to an approximately constant value and the sliding state is reached. 4.3. Effect of temperature The effect of temperature on the shear strength, shear reaction modulus and residual shear strength are presented in Table 2. It is observed that temperature has a significant effect on interface shear strength and shear reaction modulus that they significantly decrease as testing temperature increases. On average, the shear strengths and shear reaction modulus determined at 60℃ are about 25% to 40% of those determined at 25℃ for the specimens with EA tack coat. And the ratio values drop to below 20% for the specimens with PMA tack coat. The reduction in shear strength and shear reaction modulus is caused by a decrease in the tack coat material stiffness as temperature increases, indicating the viscoelastic nature of the tack coat materials. For similar interface conditions, increased temperatures result in reduced shear strengths as reported by almost all of the studies in literature [29-31]. It implies that the high temperature is a critical condition for which the interface failure is more likely to occur, and the interface shear test should be conducted at the highest temperature that the asphalt overlay on orthotropic steel bridge deck will experience in service. The data in Table 2 indicate a weak relationship between temperature and the residual shear strength. The changes in residual shear strength due to temperature increases from 25 to 60°C are much smaller compared with those in the shear strength and shear reaction modulus. Considering that the residual shear strength is primarily provided by the friction between the steel plate and the asphalt concrete, the roughness of the steel plate surface and the aggregate gradation of the asphalt concrete may be the main factors that influence the shear resistance after failure of the interface. Hence, it is possible to 9
conclude that the contribution of friction to interface shear resistance is not affected by the temperature variation. 4.4. Effect of normal stress The entire set of data collected both in shear strength and residual shear strength at all the test conditions
are shown in Table 2. From this figure, it is observed that the increase of the applied normal stress results in increased shear strength for tests carried out in this study. For the specimens with PMA tack coat, the shear strengths measured from the tests performed at the normal stress of 0.7 MPa are approximately 2 to 3.5 times higher than those measured without normal stress. While for the specimens with EA tack coat, the shear strengths measured at the normal stress of 0.7 MPa is nearly 1.1 to 1.7 times higher than those measured without normal stress. This kind of effect may be attributed to that the higher normal stress increases the interfacial contact area between the steel plate and the asphalt overlay, and thus increase the friction contribution of shear resistance. It implies that although an overloaded truck causes higher shear stress at the interface which leads to higher risk of slippage failure, meanwhile, it improves the shear resistance of the interface to some extent. The percent changes in shear strengths due to the normal stress increase from 0 to 0.7 MPa at 60°C is greater than those at 25℃, indicating that normal stress plays a more important role in the component of interface shear resistance at high temperature. It is postulated that the cohesion of the interface is reduced at high temperature and the friction effect between the steel plate and the asphalt overlay becomes a greater factor. Table 2 also shows the interface residual shear strength values for various interface types and test conditions. As mentioned before, the interface residual shear strength generates by friction between the steel plate and the asphalt overlay under the action of normal stress. Thus, no interface residual shear strength value can be obtained from tests without normal stress. It is observed that the interface residual shear strength increases with increasing normal stress. For the thermosetting type interface, it still can sustain the shear stress of up to 0.38MPa after failure. Under the same loading condition, however, the shear resistances remain only from 0.01 to 0.10 MPa for the thermoplastic type interface. By plotting the shear strength versus the normal stress which shows an approximately linear relationship, it is possible to roughly estimate the failure envelopes that can be described by Coulomb’s equation [32]: c0 n tan 10
(3)
where c0 is the cohesion that quantifies the pure shear resistance of the interface; n is the normal stress; is the internal friction angle of the interface.
For a single test temperature, the peak friction angle ( p ) and the cohesion ( c0 ) can be obtained by only varying the normal stress ( n ). Moreover, the residual failure envelope can be also plotted and characterized by the residual friction angle ( r ). Peak and residual failure envelopes for the thermosetting type interface and the thermoplastic type interface at 25°C and 60°C are shown in Fig.12. The linear regression parameters, including c0 , tan p , tan r , and the corresponding coefficient of determination R p2 and Rr2 are presented in Table 3 for all test conditions. The results show that the cohesions ( c0 ) for both interface types decrease with increasing temperature, which demonstrates the viscoelastic characteristics of the tack coat materials. Also indicated in Table 3 is that the thermosetting type interface gives higher tan p and tan r values than those from the thermoplastic type interface. It is known that the steel plate surface provides very low roughness, thus, the friction could only depends on the characteristics of the asphalt overlay materials which is in contact with the steel plate. It is observed from Fig.1 that the GAC has an extremely fine aggregate gradation with high content of mineral filler. Additionally, the tack coat binder at the interface is dispersed inside the bottom of the asphalt overlay and further weakens the interlock effect between the steel plate and the asphalt overlay. Subsequently, the friction contribution tan p and tan r to shear resistance for specimens with GAC overlay is small. 4.5. Effect of tack coat material From Table 2 it is evident that the EA tack coat provides better interface shear strengths than the PMA tack coat. The shear strengths of the specimens with EA tack coat range from 1.92 to 2.65 MPa at 25℃, which are approximately 2.7 to 3.8 times higher than those of the specimens with PMA tack coat. In the case of 60℃, the PMA tack coat gives the shear strengths of less than 0.1 MPa which are much lower than those of EA tack coat (above 0.5 MPa). For the shear strength is the critical parameter to evaluate the interface bonding performance, the PMA tack coat is not accepted by the steel-asphalt composite structure in comparison with the EA tack coat. Similar results are found on the comparisons of the shear reaction modulus between the two tack coat materials, as shown in Table 2. The higher shear reaction modulus can result in smaller interface shear displacement under the action of traffic loading and better capability of transferring the traffic loads to the steel bridge deck which is beneficial for the durability of the asphalt 11
overlay. The considerable improvement of specimens with EA tack coat in the shear resistance may attribute to the elevated mechanical property of EA, for EA is a thermosetting material that is not as sensitive to temperature variation as the thermoplastic asphalt. The modification of asphalt with epoxy resin increases its strength and temperature stability, which is helpful for the shear resistance of interface at high temperature. These observations agree well with previous findings that the shear strengths of specimens with EA tack coat are much higher than those of other types of interface including reinforced interface [14, 33].
5. Conclusions and recommendations This paper presents a laboratory study on the shear characteristics of steel-asphalt interfaces with thermosetting and thermoplastic tack coat materials by the SCIS test. The test results and the discussions presented in this paper allow the following conclusions to be drawn:
The composite specimens with EA tack coat failed at the primer-tack coat interface with complete separation of the EAC from steel plate at all test conditions. For the PMA tack coat, as the temperature increases, the failure mode is likely to change from adhesive failure at the primer-tack coat interface to the material failure of GAC.
Precision of the SCIS test method is evident from the reasonably low COVs obtained. The proposed test method is effective for determining the shear characteristics of steel-asphalt interfaces, which is evident from the consistency of test results and failure modes.
The shear stress-displacement curves for all the test conditions follow a similar pattern that the shear stress increases initially and then decreases with increasing shear displacement. The ascending part of the curves shows an approximately linear shape approaching to a peak value. In the post peak region, the curves show a strain softening behavior until reaching a plateau.
Temperature has a dominant effect on the interface shear strength and shear reaction modulus that they significantly decrease as testing temperature increases. This suggests that the high temperature is a critical condition for which the interface failure is more likely to occur, and the interface shear test should be conducted at the highest temperature that the asphalt overlay on steel bridge deck will experience in service.
The interface shear strength increase with the increasing normal stress. The percent changes in shear strengths due to the normal stress increase at 60°C is greater than those at 25°C, indicating 12
that normal stress plays a more important role in the component of interface shear resistance at high temperature. Based on the Coulomb failure law, the failure envelopes of the interface shear strength and residual shear strength were obtained for combinations of interface type and temperature conditions.
As the EA tack coat provides much better interface shear strength, shear reaction modulus, and residual shear strength than the PMA tack coat, it is recommended that the EA be used as the tack coat material between the steel bridge deck and the asphalt overlay.
Acknowledgements This work was supported by National Natural Science Foundation of China (No. 51208103). The authors would like to express their gratitude to the Southeast University Road and Bridge Laboratory for providing equipment and assistance.
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References [1] Batchelor J, Paykazadi M. Application of polymer overlays to concrete and steel bridge decks. Bridge Assess Manag Design 1994;45:379-383. [2] Chen X, Qian Z, Liu X, Zhang L. State of the art of asphalt surfacings on long-spanned orthotropic steel decks in China. J Test Eval 2012;40(7):1-10. [3] Wolchuk R. Structural behaviour of surfacings on steel orthotropic decks and considerations for practical design. Struct Eng Int 2002;12(2):124-129. [4] Freitas ST, Kolstein H, Bijlaard F. Importance of interface layer on behaviour and durability of orthotropic steel decks. In: Proceedings of the international orthotropic bridge conference 2008. Sacramento, California, USA; 2008. [5] Freitas ST, Kolstein H, Bijlaard F. Parametric study on the interface layer of renovation solutions for orthotropic steel bridge decks. Comput-Aided Civ Inf 2012;27(2):143-153. [6] Kim TW, Baek J, Lee HJ, Lee SY. Effect of pavement design parameters on the behaviour of orthotropic steel bridge deck pavements under traffic loading. Int J Pavement Eng 2014;15(5):471-482. [7] Wang X, Chen X, Cheng G, Huang W. Cracking of the asphalt surfacing of the longest suspension steel bridge in China. In: Proceedings of the 24th annual Southern African transport conference. Pretoria, South Africa; 2005. [8] Bjornstad JM, Bognacki CJ, Marsano J. George Washington bridge asphalt-wearing course and bond coat analysis. In: Proceedings of the airfield and pavement speciality conference. Bellevue, WA, USA; 2008. [9] Luo S, Qian Z, Wang H. Condition survey and analysis of epoxy asphalt concrete pavement on Second Nanjing Yangtze River Bridge: A ten-year review. J Southeast Univ 2011;27(4):417-422. [10] Chen X, Huang W, Wang J, Cheng G. Damage causes of mastic asphalt pavement on orthotropic steel deck plate. J Traffic Transp Eng 2004;4(4):5-9. [11] Chen X, Huang W, Qian Z. Interfacial behaviors of epoxy asphalt surfacing on steel decks. J Southeast Univ 2007;23(4):594-598. [12] Medani TO. Asphalt Surfacing Applied to Orthotropic Steel Bridge Decks. Report 7-01-127-1. Delft, Netherlands: TU Delft; 2001. [13] Medani TO, Liu X, Huurman M, Scarpas A, Molenaar A. Experimental and numerical characterization of a membrane material for orthotropic steel deck bridges: Part 1 - Experimental work and data interpretation. Finite Elem Anal Des 2008;44(9-10):552-563. [14] Bocci E, Canestrari F. Experimental evaluation of shear resistance of improved steel-asphalt interfaces. Transport Res Rec 2013; 2370:145-150. [15] Bocci E, Canestrari F. Analysis of structural compatibility at interface between asphalt concrete pavements and 14
orthotropic steel deck surfaces. Transport Res Rec 2012; 2293:1-7. [16] Ge ZS, Wang YY, Hang MB. New device and methodology for evaluating the shear behavior of steel bridge deck pavement pasted by GFRP sheets. J Test Eval 2014;42(1):93-108. [17] Romanoschi SA, Metcalf JB. Characterization of asphalt concrete layer interfaces. Transport Res Rec 2001; 1778:132-139. [18] ASTM D412-06a. Standard test methods for vulcanized rubber and thermoplastic elastomers-tension. Pennsylvania, US: American Society for Testing and Materials (ASTM); 2013. [19] ASTM D648-07. Standard test method for deflection temperature of plastics under flexural load in the edgewise position. Pennsylvania, US: American Society for Testing and Materials (ASTM); 2007. [20] AASHTO T315-12. Standard method of test for determining the rheological properties of asphalt binder using a Dynamic Shear Rheometer (DSR). Washington, D.C., US: American Association of State Highway and Transportation Officials (AASHTO); 2012. [21] AASHTO T313-12. Standard method of test for determining the flexural creep stiffness of asphalt binder using the Bending Beam Rheometer (BBR). Washington, D.C., US: American Association of State Highway and Transportation Officials (AASHTO); 2012. [22] GB/T 13452.2-2008. Paints and varnishes - determination of film thickness. Beijing, China: China Standards Press; 2008. [23] Liu Y, Wu JT, Chen J. Mechanical properties of a waterproofing adhesive layer used on concrete bridges under heavy traffic and temperature loading. Int J Adhes Adhes 2014;48:102-109. [24] Ozer H, Al-Qadi IL, Wang H, Leng Z. Characterisation of interface bonding between hot-mix asphalt overlay and concrete pavements: modelling and in-situ response to accelerated loading. Int J Pavement Eng 2012;13(2):181-196. [25] Mohammad LN, Hassan M, Patel N. Effects of shear bond characteristics of tack coats on pavement performance at the interface. Transport Res Rec 2011; 2209:1-8. [26] Mohammad LN, Bae A, Elseifi MA, Button J, Patel N. Effects of pavement surface type and sample preparation method on tack coat interface shear strength. Transport Res Rec 2010; 2180:93-101. [27] Kruntcheva MR, Collop AC, Thom NH. Properties of asphalt concrete layer interfaces. J Mater Civil Eng 2006;18(3):467-471. [28] Shen HG, Zhang Y, Sheng DW, Mo LT. Experimental investigation of tensile bonding strength of Gussasphalt overlay upon steel bridge deck. Bldg Mater World 2013;34(4):29-32. [29] Xu QW, Zhou QH, Medina C, Chang GK, Rozycki DK. Experimental and numerical analysis of a waterproofing adhesive layer used on concrete-bridge decks. Int J Adhes Adhes 2009;29(5):525-34. 15
[30] Bae A, Mohammad LN, Elseifi MA, Button J, Patel N. Effects of temperature on interface shear strength of emulsified tack coats and its relationship to rheological properties. Transport Res Rec 2010; 2180:102-109. [31] Wheat M. Evalutation of bond strength at asphalt interfaces. Kansas State University; 2007 [Master thesis]. [32] Santagata FA, Partl MN, Ferrotti G, Canestrari F, Flisch A. Layer characteristics affecting interlayer shear resistance in flexible pavements. J Assn Asphalt Paving Technol 2008; 77:221-256. [33] Qian ZD, Yang YM, Song RY. A study on epoxy asphalt waterproof adhesive layer for steel bridge Gussasphalt pavement. Highw 2013; 2:1-5.
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Tables Table 1 Properties of the tack coat materials. Material EA
PMA
a
Property Tensile strength at 23℃ (MPa) Tensile elongation at 23℃ (%) Heat deflection temperature (℃) Creep stiffness, test on (RTFO+PAV) at -12℃ (MPa) m -value, test on (RTFO+PAV) at -12℃ Dynamic shear, G* / sin( ) a, test on original binder at 76℃ (kPa) Dynamic shear, G* / sin( ) , test on RTFO at 76℃ (kPa) Creep stiffness, test on (RTFO+PAV) at -12℃ (MPa) m -value, test on (RTFO+PAV) at -12℃
* G * is the complex modulus, is the phase angle, G / sin( ) is the rutting parameter.
17
Measured value 16.6 216 -20 407 0.213 2.72 5.48 152 0.325
Test method ASTM D412 [18] ASTM D412 [18] ASTM D648 [19] AASHTO T313 [21] AASHTO T313 [21] AASHTO T315 [20] AASHTO T315 [20] AASHTO T313 [21] AASHTO T313 [21]
Table 2 SCIS test results of the steel-asphalt composite specimens. p n Tack coat COVa K COVb r COVc Failure T material (MPa/mm) (%) mode (℃) (MPa) (MPa) (%) (MPa) (%) EA 25 0 1.92 7.5 2.09 8.3 A EA 25 0.2 2.14 3.9 2.22 10.1 0.09 7.0 A EA 25 0.5 2.48 3.3 2.54 8.7 0.27 12.3 A EA 25 0.7 2.65 4.0 2.64 7.6 0.38 5.2 A EA 60 0 0.5 7.1 0.43 10.2 A EA 60 0.2 0.66 5.6 0.50 10.0 0.08 13.6 A EA 60 0.5 0.78 6.4 0.58 13.6 0.18 6.7 A EA 60 0.7 0.98 6.1 0.73 8.1 0.29 9.6 A PMA 25 0 0.36 7.1 0.41 8.3 A PMA 25 0.2 0.46 5.6 0.54 8.4 0.06 13.6 A PMA 25 0.5 0.58 6.4 0.69 7.8 0.09 16.7 A PMA 25 0.7 0.65 6.1 0.75 8.9 0.11 9.6 A 0.01 B PMA 60 0 10.7 15.3 0.03 B PMA 60 0.2 0.04 14.5 20.1 0.04 19.3 0.05 B PMA 60 0.5 0.09 11.7 21.3 0.07 18.8 0.08 B PMA 60 0.7 0.1 18.5 15.9 0.10 15.5 0.09 a b c COVs of p ; COVs of K ; COVs of r ; A = Failed through interface; and B = Failed through asphalt concrete.
18
Table 3 Parameters of the shear strength and residual shear strength envelopes. Tack coat c0 tan r tan p R p2 T Rr2 material (℃) (MPa) EA 25 1.93 1.06 0.99 0.58 0.98 60 0.51 0.64 0.96 0.41 0.96 PMA 25 0.36 0.41 0.92 0.10 0.97 60 0.01 0.13 0.96 0.12 0.95
19
Figures
Percentage Passing (%)
100
EAC GAC
80 60 40 20 0 0.01
0.1
1
10
Sieve Size (mm) Fig.1. Aggregate gradations.
20
100
Loading Frame Shear Load Cell
Actuator
Linear Motion Guide
Environmental Chamber
Slider Guides Stiffener
Flat Jack Shear Box
Base Specimen Interface
Fig.2. Schematic of the SCIS test apparatus.
21
Normal Load Cell
Data acquisition and control system
Fig.3. Details of the shear box.
22
Fig.4. Picture of the SCIS test apparatus.
23
(a)
(b)
(c)
Fig.5. Fabrication of the composite specimens for the SCIS test: (a) steel plates installed in the mold; (b) compacted asphalt concrete mixture in the mold; and (c) specimens ready for shear testing.
24
Fig.6. Shear slip failure of the steel-asphalt interface in SCIS test.
25
(a)
(b)
(c)
(d)
Fig.7. Typical interface failure modes: (a) specimen with EA tack coat at 25℃; (b) specimen with EA tack coat at 60℃; (c) specimen with PMA tack coat at 25℃; and (d) specimen with PMA tack coat at 60℃.
26
Shear Stress (MPa)
3.5
4.0
25℃,0MPa,#1 25℃,0MPa,#2 25℃,0MPa,#3 25℃,0MPa,#4 25℃,0MPa,#5
(a)
3.0 2.5 2.0 1.5 1.0 0.5 0.0
0
1
2
3
3.0 2.5 2.0 1.5 1.0 0.5 0.0
4
25℃,0.2MPa,#6 25℃,0.2MPa,#7 25℃,0.2MPa,#8 25℃,0.2MPa,#9 25℃,0.2MPa,#10
(b)
3.5
Shear Stress (MPa)
4.0
0
1
Displacement (mm)
3.5
Shear Stress (MPa)
25℃,0.5MPa,#11 25℃,0.5MPa,#12 25℃,0.5MPa,#13 25℃,0.5MPa,#14 25℃,0.5MPa,#15
(c)
3.0 2.5 2.0 1.5 1.0 0.5 0.0
0
1
2
3
4.0
3
4
3.0 2.5 2.0 1.5 1.0 0.5 0.0
4
60℃,0.7MPa,#16 60℃,0.7MPa,#17 60℃,0.7MPa,#18 60℃,0.7MPa,#19 60℃,0.7MPa,#20
(d)
3.5
Shear Stress (MPa)
4.0
2 Displacement (mm)
Displacement (mm)
0
1
2
3
4
Displacement (mm)
Fig.8. Interface shear stress-displacement curves obtained from the SCIS tests for specimens with the EA tack coat at 25°C: (a) under normal stress of 0.0 MPa; (b) under normal stress of 0.2 MPa; (c) under normal stress of 0.5 MPa; (d) under normal stress of 0.7 MPa.
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60℃,0MPa,#21 60℃,0MPa,#22 60℃,0MPa,#23 60℃,0MPa,#24 60℃,0MPa,#25
(a) Shear Stress (MPa)
1.0 0.8 0.6 0.4 0.2 0.0
0
1
2
3
1.2
60℃,0.2MPa,#26 60℃,0.2MPa,#27 60℃,0.2MPa,#28 60℃,0.2MPa,#29 60℃,0.2MPa,#30
(b) 1.0 Shear Stress (MPa)
1.2
0.8 0.6 0.4 0.2 0.0
4
0
1
Displacement (mm) 60℃,0.5MPa,#31 60℃,0.5MPa,#32 60℃,0.5MPa,#33 60℃,0.5MPa,#34 60℃,0.5MPa,#35
(c) Shear Stress (MPa)
1.0 0.8 0.6 0.4 0.2 0.0
0
1
2
3
4
Displacement (mm)
3
1.2
60℃,0.7MPa,#36 60℃,0.7MPa,#37 60℃,0.7MPa,#38 60℃,0.7MPa,#39 60℃,0.7MPa,#40
(d) 1.0 Shear Stress (MPa)
1.2
2
0.8 0.6 0.4 0.2 0.0
4
Displacement (mm)
0
1
2
3
4
Displacement (mm)
Fig.9. Interface shear stress-displacement curves obtained from the SCIS tests for specimens with the EA tack coat at 60°C: (a) under normal stress of 0.0 MPa; (b) under normal stress of 0.2 MPa; (c) under normal stress of 0.5 MPa; (d) under normal stress of 0.7 MPa.
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25℃,0MPa,#41 25℃,0MPa,#42 25℃,0MPa,#43 25℃,0MPa,#44 25℃,0MPa,#45
Shear Stress (MPa)
(a) 0.6
0.4
0.2
0.0
0
1
2
3
0.8
25℃,0.2MPa,#46 25℃,0.2MPa,#47 25℃,0.2MPa,#48 25℃,0.2MPa,#49 25℃,0.2MPa,#50
(b) Shear Stress (MPa)
0.8
0.6
0.4
0.2
0.0
4
0
1
Displacement (mm) 25℃,0.5MPa,#51 25℃,0.5MPa,#52 25℃,0.5MPa,#53 25℃,0.5MPa,#54 25℃,0.5MPa,#55
Shear Stress (MPa)
(c) 0.6
0.4
0.2
0.0
0
1
2
3
4
Displacement (mm)
3
0.8
25℃,0.7MPa,#56 25℃,0.7MPa,#57 25℃,0.7MPa,#58 25℃,0.7MPa,#59 25℃,0.7MPa,#60
(d) Shear Stress (MPa)
0.8
2
0.6
0.4
0.2
0.0
4
Displacement (mm)
0
1
2
3
4
Displacement (mm)
Fig.10. Interface shear stress-displacement curves obtained from the SCIS tests for specimens with the PMA tack coat at 25°C: (a) under normal stress of 0.0 MPa; (b) under normal stress of 0.2 MPa; (c) under normal stress of 0.5 MPa; (d) under normal stress of 0.7 MPa.
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60℃,0MPa,#61 60℃,0MPa,#62 60℃,0MPa,#63 60℃,0MPa,#64 60℃,0MPa,#65
Shear Stress (MPa)
(a) 0.03
0.02
0.01
0.00
0
1
2
3
0.25
60℃,0.2MPa,#66 60℃,0.2MPa,#67 60℃,0.2MPa,#68 60℃,0.2MPa,#69 60℃,0.2MPa,#70
(b) 0.20 Shear Stress (MPa)
0.04
0.15 0.10 0.05 0.00
4
0
1
Displacement (mm) 60℃,0.5MPa,#71 60℃,0.5MPa,#72 60℃,0.5MPa,#73 60℃,0.5MPa,#74 60℃,0.5MPa,#75
(c) Shear Stress (MPa)
0.20 0.15 0.10 0.05 0.00
0
1
2
3
4
Displacement (mm)
3
0.25
60℃,0.7MPa,#76 60℃,0.7MPa,#77 60℃,0.7MPa,#78 60℃,0.7MPa,#79 60℃,0.7MPa,#80
(d) 0.20 Shear Stress (MPa)
0.25
2
0.15 0.10 0.05 0.00
4
Displacement (mm)
0
1
2
3
4
Displacement (mm)
Fig.11. Interface shear stress-displacement curves obtained from the SCIS tests for specimens with the PMA tack coat at 60°C: (a) under normal stress of 0.0 MPa; (b) under normal stress of 0.2 MPa; (c) under normal stress of 0.5 MPa; (d) under normal stress of 0.7 MPa.
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4
Peak envelope at 25℃ Residual friction envelope at 25℃ Peak envelope at 60℃ Residual friction envelope at 60℃
1.0 (a) Interface Shear Stress (MPa)
Interface Shear Stress (MPa)
5
3 2 1 0 0.0
0.2
0.4
0.6
0.8
0.8
Normal Stress (MPa)
(b)
0.6 0.4 0.2 0.0 0.0
1.0
Peak envelope at 25℃ Residual friction envelope at 25℃ Peak envelope at 60℃ Residual friction envelope at 60℃
0.2
0.4
0.6
0.8
1.0
Normal Stress (MPa)
Fig.12. Peak and residual failure envelopes at 25°C and 60°C for: (a) the thermosetting type interface; (b) the thermoplastic type interface.
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