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14 In the analytical portion, 3-D stress analysis software was interface using three parameters: (1) the interface reaction developed to analyze the stress, strain, and displacement of modulus, which is the slope of the shear stress-displacement composite pavement. The pavement was modeled primarily curves; (2) the maximum shear strength; and (3) the fric- as a layered system of linear elastic materials with the pos- tion coefficient after failure. They concluded that the values sibility of treating the surface asphalt layer as a linear visco- of interface reaction modulus and shear strength were not elastic material. Anisotropy and temperature effects were affected by the normal stress for an interface with a tack incorporated. Besides vertical load, the development of a 3-D coat. They were, however, affected for an interface with- computer program takes into account the horizontal shear out a tack coat. The study showed that the interface bond stresses induced on the pavement surface due to vehicle brak- might also fail in fatigue and that the permanent shear dis- ing effects (acceleration and deceleration). Using the software, placement had a linear relationship with the number of load a detailed parametric study was conducted to investigate the repetitions. effect of system parameters including layer thickness and stiff- ness on the stress-strain-displacement fields induced in the pavement. For the delaminating problem in layered pave- 2.5.2Interface Bond Strength and ments, it was found, through the analysis in this study, that Tack Coat Film Test Devices higher loading leads to higher maximum interface shear stress Table 3 describes interface bond strength and tack coat film and that increasing overlay thickness is an effective way to test devices used in the laboratory and in the field to charac- reduce maximum interface shear stress. Maximum interface terize tack coat application and performance (see Figure 9). shear stress can be found at the tire edges for a vehicle apply- In general, three test modes--shear, tension, and peel--have ing both normal and shear stresses to a pavement surface. been used in both the laboratory and the field to characterize After the maximum interface shear stress is available, it can interface/bond strengths of tack coat materials. be used to compare with the bond strength obtained through simple direct shearing testing so that an appropriate interface binder can be chosen. 2.6 Worldwide Survey The interface bond condition can seriously influence stress A worldwide survey on tack coat practices was conducted and strain distribution in a pavement structure. Hakim to better understand the current state of tack coat practices et al. (27) used falling weight deflectometer (FWD) deflec- and to design a corresponding research experiment. The pri- tion data to assess the bonding condition between bitumi- mary objective of the survey was to investigate the current nous layers. They reported that the FWD-backcalculated tack coat state of practice related to types of materials used stiffness was lower than that obtained in the laboratory. for tack coats, dilution rates of tack coat materials, residual This difference was attributed to the fact that the backcalcula- application rates, determination of rate for different types of tion procedure of modulus assumes full bonding between surfaces, methods used for tack coat distribution, and pave- bituminous layers. To address this issue, the interface shear ment failures related to tack coat application. bond stiffness was considered in a modified FWD back- A questionnaire was developed to meet these objectives. calculation method. Several studies derived interface con- The survey was organized into three main sections: tack coat stitutive models for characterizing the bonding condition materials, tack coat application methods, and character- of a pavement structure in a numerical simulation. Among ization of tack coat application. In total, 27 questions were them, the BISAR program considers the Goodman model included in the questionnaire concerning all aspects of tack for the surface and base interface (33). In this model, shear coat practices. All questions included in the survey are pre- stress is proportional to the difference in the horizontal dis- sented in Appendix A. placements of the bonding layers. Uzan et al. (22) reported that the interface reaction modulus used in the Goodman model is independent of the normal stresses at the interface. Crispino et al. (34) proposed the use of the Kelvin model to predict the viscous-elastic phenomenon of interlayer reac- Tack Layer tion under dynamic loading. Romanoschi and Metcalf (35) reported that, in the direct Layer 1 shear test, the shear stress and displacement were propor- tional until the shear stress equaled the shear strength and (a) (b) the interface failed. Based on this observation, they pro- Figure 9. An interface bond strength test specimen posed a constitutive model for the asphalt concrete layer (a) and a tack coat film test specimen (b).

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15 Table 3. Available in situ and laboratory bonding tests. Apparatus Significance and Use Procedure Specimen Test Results Lab or Remark in situ 1. Leutner Shear Test The maximum shear A vertical shear load is 6.0-in-diameter (1) Maximum Lab No normal load load and corresponding applied to a double-layered specimen cored from shear load is applied displacement are specimen with a strain laboratory- (2) measured to evaluate the controlled mode at a compacted Corresponding bonding property of constant rate of 2.0 in/min at composite (12 in maximum interface. The bonding 21.1C until failure. 12 in width by 2.8 in displacement property is used to height) determine the appropriateness of the material for use as tack coat. Shear strength of the A horizontal shear load is (1) 5.9-in-diameter Shear stress at Lab (1) Normal load 2. LTRC Direct Shear Test tack coat interlayer is applied to a dual-layer dual-layered failure is optional measured to evaluate the specimen of asphalt specimen cored from (2) Developed bonding property of tack concrete with a stress the pavement or by Louisiana coat. The bonding control mode at a constant fabricated in Transportation property is used to rate of 50 lbs/min at a given laboratory Research determine the temperature until the sample (2) To be trimmed Center (LTRC) appropriateness of the is separate. With a climate before testing to material for use as tack chamber, the temperature ensure the two ends coat. can be set in the range are flat to fit the from 20 to 80C. shear mold (3) Gap width between the shearing platens is around 1 in (25.4 mm) 3. TTI Torsional Shear Plastic shear strength in A twisting moment with (1) Dual-layered (1) Shear Lab Developed by Test torsion is measured to constant rate of 2.9 E-04 cylinder specimen strength Texas evaluate the shear radian/sec and a normal load with diameter of 6- Transportation (2) Construct resistance of the is applied on the top of a in compacted in Institute (TTI) Mohr-Coulomb interface and the quality double-layered cylinder laboratory using two failure envelopes to of the tack coat. specimen at a constant rate half-molds get the cohesion until failure. (2) Space between and the tangent of the two halves is internal friction 0.08 in (2 mm) angle 4. Florida Direct Shear Bond strength of the tack A vertical shear load is (1) Dual-layered cylinder Shear strength at Lab (1) No Test coat interlayer is applied to dual-layer asphalt specimen with diameter failure normal loads measured to evaluate the concrete specimen with of 6-in can be performance of tack strain control mode at a applied (2) Samples can be coat. constant rate of 2.0 in/min at during the roadway cores or 25C until failure. test laboratory-fabricated specimens and do not need to be trimmed to (2) Developed accommodate the device by Florida DOT (3) Gap width between shear plates is 0.19 in 5. Virginia Shear Fatigue The number of shear Cyclic shear load [a 0.015- (1) Composite cylinder (1) Maximum Lab Developed by Test (36) loading cycles at failure in deflection was applied to specimen with diameter shear stress of each Virginia is used to determine the the specimen in the form of of 3.7 is composed of cycle Polytechnic optimum application rate a 0.10-s half-sine wave, concrete core, Institute & State of asphalt binder tack at followed by a relaxation geocomposite (2) Maximum University and interface between two period of 0.9 s (the total membrane, HMA, and shear stress against the Virginia layers. cycle is 1s)] is applied at the tack coat applied on the the number of Tech geocomposite membrane interface. cycles of failure Transportation interface of dual-layer Institute (2) Concrete core is sample composed of cored from laboratory- (3) Optimal tack concrete and HMA prepared concrete slab. coat application specimens until failure at rate ambient temperature. (3) The upper HMA layer is gyratory- compacted on the top of concrete core after applied geocomposite membrane and tack coat. (continued on next page)

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16 Table 3. (Continued). Apparatus Significance and Use Procedure Specimen Test Results Lab or Remark in situ 6. ASTRA Interface Maximum interface Horizontal load is applied (1) Dual-layered Shear stress at Lab If carried out at Shear Test shear stress is measured along the interface of dual- cylindrical specimen failure different normal to evaluate the shear layered sample at constant with diameter of 3.94 in load, a Mohr- resistance property of rate until failure; Coulomb (2) Laboratory- interface. The shear meanwhile, a constant failure envelope fabricated or extracted resistance property is normal load is applied on can be obtained. from pavement used to evaluate the tack top of the specimen. coat properties. 7. Layer-Parallel Direct Nominal average shear Vertical shear load is (1) Cylindrical Tensile strength Lab (1) Shear-plane Shear (LPDS) stress and maximum applied to a composite composite specimen of can be along shear stiffness are specimen with strain control 3.94-in diameter interface or measured to determine mode at constant rate. within the (2) Laboratory- the in-layer and layers fabricated sample and interlayer shear pavement core (2) Modified by properties of asphalt EMPA, Swiss concrete layers. The in- (3) The specimen needs Federal layer shear properties are to be glued Laboratory for used to evaluate the Materials quality of the mixture Testing and and the interlayer shear Research properties are used to evaluate the tack coat properties. 8. Switzerland Pull-Off Tension strength values A tensile load is applied to (1) Cylindrical Tensile strength Lab Test is carried Test are measured to evaluate asphalt concrete specimen composite specimen of out according to the interlayer shear composed of two layers at 3.94-in diameter German testing performance between constant rate. specification (2) Laboratory- different asphalt concrete ZTV-SIB 90 fabricated sample and layers. Shear pavement core performance is used to evaluate the quality of (3) The specimen needs the tack coat and in to be glued comparison of various tack coat materials. 9. Loboratorio de Shear strength of the The dual-layer specimen (1) Cylindrical (1) Shear strength Lab (1) No normal Caminos de Barcelona tack coat interlayer is with tack coat interlay is composite specimen of load can be (2) Shear modulus Shear Test (LCB) measured to evaluate the used as a beam located over 3.94-in diameter and 7.0- applied during and the specific bonding property of tack two supports and a vertical in high this test cracking energy coat. The bonding load is applied to the (2) Developed (2) Laboratory- property is used to specimen at a constant by DOT, fabricated sample and/or determine the deformation speed of 0.05 Technical pavement core appropriateness of the in/min in the middle of the University of material for use as tack two supports until failure. Catalonia, coat. Spain 10. Wedge-Splitting Test Maximum horizontal A vertical load is applied (1) Cubic or cylindrical (1) Maximum Lab Developed by force (Fmax) and specific through a wedge to a dual- composite specimen with horizontal force Technical fracture energy (GF) are layered specimen with a interface in the middle University, (2) Specific determined to groove and starter notch and a start notch in the Austria fracture energy characterize the fracture- along the interface at a interface mechanical behavior of constant rate until complete (2) Laboratory- layer bonding. The separation of the specimen. fabricated or cored or cut fracture-mechanical from pavement behavior is used to determine the appropriateness of the material for use as tack coat. 11. Dynamic Interaction Interlayer reaction A sinusoidal shear force is Cylindrical composite The norm of Lab Developed by Test complex modulus KI* is applied to dual-layered specimen of 3.94-in Interlayer reaction University of determined for the specimen at particular diameter, cored from complex modulus Naples, Italy pavement structure temperature and given load laboratory-compacted KI* and phase analysis. The pavement frequency. twin layer slab or from angle structure analysis pavement. evaluates the capacity of the pavement and can be used to predict the remaining life of the pavement.

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17 Table 3. (Continued). Apparatus Significance and Use Procedure Specimen Test Results Lab or Remark in situ 12. NCAT Shear Test The interface shear A vertical shear force is (1) Cylindrical Bond shear Lab Developed by strength of core samples applied to dual-layered composite specimen with strength National Center is measured to evaluate specimens along the 5.9 in for Asphalt the bonding property of interface with strain control Technology (2) Height of the core pavement layers. The mode at constant rate until (NCAT) above the interface being bonding property is used failure. tested is greater than to determine the 3 in. The height of appropriateness of the each layer should be material for use as tack greater than 1.97 in, less coat. than 5.9 in. 13. HasDell EBSTTM The bond strength A shear force is applied (1) Cylindrical Bond shear Lab or Marketed by Emulsion Shear Test between two layers is along the interface until composite specimen with strength in situ R/H Specialty measured to determine failure. 5.9 in diameter and Machine, the appropriateness of Terre Haute, (2) 2.95-in x 2.95-in- the material for use as Indiana square composite tack coat. specimen 14. Traction Test Tensile strength of the A tensile force is applied at Cylindrical lab or field Bond tensile Lab or Developed by tack coat interlayer is constant rate of 54 lb/s to a sample of 4-in diameter strength in situ Ministre des measured to evaluate the cylindrical sample until Transports du bonding property of tack failure Qubec, Canada coat. The bonding property is used to determine the appropriateness of the material for use as tack coat. 15. The ATackerTM Test Shear and/or tensile A pull and/or torque force is Tack-coated plates or Tensile strength Lab or Developed by strength of tack coat applied to detach the tack- attach plate to tack- and/or shear in situ Introtek, Inc. material are measured to coated plates or detach the coated pavement strength evaluate its bonding contact plate and tack- property. The bonding coated pavement. property is used to determine the appropriateness of the material for use as tack coat. 16. UTEP Pull-Off Test Tensile strength of tack A torque force is applied to Tack-coated plates or Tensile stress at the Lab or Developed by coat material is detach the tack-coated attach plate to tack- point of failure in situ University of measured to determine plates or detach the contact coated pavement Texas at El its bonding property. The plate and tack-coated Paso bonding property is used pavement to determine the appropriateness of the material for use as tack coat. 17. UTEP Simple Pull-Off Tensile strength of tack A tensile force is applied Tack-coated plates or Tensile stress at Lab or Developed by Test coat material is directly to pull off the attach plate to tack- failure in situ University of measured to determine contact plate from the tack- coated pavement Texas at El its bonding property. The coated surface. Paso bonding property is used to determine the appropriateness of the material for use as tack coat. 18. Impulsive Hammer The vertical dynamic An impulsive loading is Pavement in field FD number In situ Under Test response of pavement applied with a hammer to development at and fractal dimension the pavement surface at Nottingham (FD) are determined to particular locations and University evaluate the bond given loading frequency. condition between asphalt layers in field. The bonding condition is used to determine the appropriateness of the material for use as tack coat. (continued on next page)

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18 Table 3. (Continued). Apparatus Significance and Use Procedure Specimen Test Results Lab or Remark in situ 19. Torque Bond Test Torque force at failure is A torque force is applied to Core sample of 3.94-in Bond strength In situ Developed by measured to evaluate the core sample from pavement or 5.9-in diameter Highway in-place bond with a torque wrench to Agency, United effectiveness of wearing failure. Kingdom course system. 20. In situ Shear Stiffness The shear strength is A rotational force is applied Pavement in field Shear strength and In situ Developed by Test measured to evaluate the to the pavement through a shear modulus Carleton shear properties of test plate, meanwhile a University, asphalt concrete normal weight is provided Canada pavements in the field. by the test equipment. Shear properties of pavement relate to the performance of the pavement. States that did not respond States that responded Figure 10. State DOTs that responded to the survey. In order to facilitate participation in the survey, the phone calls were made to ensure the respondents under- questionnaire was converted into a web-based format. stood the questions and completed all the questions on the Other forms such as PDF, MSWord, and hard-copy were questionnaire. used, based on request of the respondents. Questionnaires Remarkably, responses were received from 46 state DOTs; were sent to state DOTs, FHWA, the Asphalt Institute, field from Washington, D.C.; and from Canada (7 responses). engineers, contractors, and selected highway agencies in Other countries that participated in the survey were Den- Canada, Europe, and South Africa during the period of mark, Finland, South Africa, and the Netherlands. Figure 10 August 2005 through January 2006. Follow-up emails and indicates the state DOTs that responded to the survey.