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50 The main objective of this project was to evaluate the effects of pavement surface type, tack coat material type, and resid- ual application rate on the ISS and pavement performance by using the LISST method. Results of this project will be used to support the potential implementation of the LISST method developed in NCHRP Project 09-40 in field projects by state DOTs. Effects of interface bonding on short-term pavement performance were also evaluated. The experimen- tal program encompassed 10 field projects in six states that included 33 in-service test sections to quantify the effects of the selected variables. Field projects included new, existing, and milled HMA pavement surfaces, and grooved PCC pave- ment surfaces. Each field project included at least one SS and one RS nontracking tack coat materials and thereby created one or more pairs of tack coats for the comparison purpose. 5.1 Conclusions During the course of the experimental program, tack coat distributor truck calibration and pavement surface texture measurement were performed in each field project before overlay construction. Moreover, FWD tests and distress sur- veys were conducted both before and following the overlay construction. HMA overlay construction used different types of tack coat materials at various residual application rates between the overlay and underlying pavement layers. In total, six types of emulsified tack coat materials were evaluated. Quality and rate of tack coat application were evaluated dur- ing construction. Samples were cored from the test sections to measure the ISS, and tack coat materials were collected for laboratory characterization. These measurements were used to validate the proposed test method and criterion and to relate the measurements with the observed field perfor- mance. Furthermore, an ISS predictive model was developed; the model may be used by practitioners during the planning of overlay construction activities, that is, selection of tack coat type and application rate given the project conditions. On the basis of the findings of this project, the following conclusions were drawn regarding interface bonding: ⢠For the effect of emulsified tack coat material type, non- tracking RS tack coats with stiff base asphalt cement exhib- ited the highest ISS, and SS resulted in the lowest. However, nontracking RS tack coats with soft asphalt cement showed similar interface bonding when compared with SS tack coats because of similar rheological properties of the residual asphalt binders. ⢠For the effect of pavement surface type, it was observed that the ISS was largely dependent on the type of pavement surface receiving tack coat (i.e., HMA versus PCC) and pavement surface texture (i.e., milled versus nonmilled). In general, milled HMA surface yielded the highest ISS, fol- lowed by new HMA, existing HMA, and PCC surface types. ⢠For the effect of residual application rate, within the evalu- ated residual application rate range, the mean ISS improved with the increase in the residual application rate for all tack coat types and for all pavement surface types. ⢠For the effect of service time, interface-bonding strength increased with service time in all field projects and for all pavement surface types. This phenomenon was primarily attributed to tack coat curing, which was more pronounced with SS tack coat materials on new HMA pavement sur- faces. Further, the curing effect of tack coats increased with the increase in the residual application rate. ⢠Regarding FWD test results, the mean center deflection decreased with service time in all field projects and for all pavement surface types. This phenomenon was primarily attributed to the densification effect of HMA overlays due to in-service trafficking and improved interface bonding with service time. ⢠Regarding short-term pavement performance, laboratory measured interface shear strength values correlated well with short-term cracking performance of field pavements. Test sections in all field projects performed satisfactorily C H A P T E R 5 Summary and Conclusions
51 with regard to rutting and surface cracking, except a few test sections that did not meet the minimum recommended interface shear strength threshold value of 40 psi. ⢠Regarding the developed nonlinear ISS predictive model, good agreement between the measured and predicted inter- face shear strength values was demonstrated. 5.2 Implementation of Research Products ⢠This study validates the use of the LISST to determine ISS between pavement layers. With this test setup, effects of pavement surface types, tack coat material types, tack coat application rates and methods, and service time can be evaluated accurately. Hence, this test method may be used for the quality control and quality assurance testing of the tack coat construction processes and to evaluate interface- bonding conditions of in-service pavements. ⢠A minimum ISS threshold value of 40 psi was identified as an appropriate criterion for satisfactory pavement per- formance. Thus, 40 psi may be used as a specification tool to avoid premature debonding between pavement layers. Further, it is recommended that interface shear testing should be completed within seven days from the period of securing cores. ⢠The developed predictive model can be used by practitio- ners during the planning phase to select tack coat type and application rate given the conditions in the field. Default values are provided to assist in the use of the predictive model in the field. In addition, caution should be exercised in using the ISS prediction model for values of param- eters outside the ones used in model development.
