Reconsideration of Unbonded Concrete Overlay Design Methodology and Bonded Concrete Overlay Minimum Required Bond Strength Based on Field Performance



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In-depth evaluations and reconsideration on determination of Unbonded Concrete Overlay (UBCO) thickness design through the literature reviews, comparison between field performance and finite element analysis results. As such the existing Portland Cement Concrete (PCC) overlay design methods were reviewed in terms of the basic design concepts and limitations as well. American Association State of Highway Transportation Officials (AASHTO) design method, Corps of Engineers design method (1958), Portland Cement Association (PCA) design method (1985), United Kingdom (UK) method and many of implementation reports were described and discussed. The original source showing the use of the exponent n=2 in the equation for unbonded concrete cannot be traced. The earliest literature found concerning the applicability of equation for unbonded overlays was a publication of American Concrete Institute (ACI). However, the thickness obtained from these assumptions are not corresponded with field performance based on various field testing, so a modified equation for UBCO thickness design was developed by using the ratio of tensile stress at the bottom of existing slab to stress at the bottom of overlay slab (R1, R2 and R3) obtained from finite element analysis. The analysis results table was developed, which are 270 cases and on the basis of the information, five examples showed the fairly reasonable thickness results compared with the existing design methodology. Also based on deflection analysis, AASHTO 1993 design method was re-considered, concluding that field performance of UBCO is shown as BCO performance in comparison with statewide deflection data. An intensive evaluation of Bonded Concrete Overlay (BCO) performance by bond strength testing was conducted in US 287 Bowie, US 281 Wichita Falls and LP 610 South Houston and based on field testing data, traffic analysis on each testing section and literature reviews on collection of construction practices information for recommendation of bond interface between the existing and overlay slab for the minimum required bond strength were conducted through various publication on not only recommendations for bond interface, but construction practice information also. To support a logical deduction, traffic data on each project location was analyzed in terms of 18-kips truck traffic volume. As such, the results of reasonable minimum required bond strength, but more closed to empirical results, could be traced. This result can be applied to very limitative condition, in other word, only applicable to field testing locations because all traffic data in Texas was not analyzed and bond strength very depends on other aspects as well such as composite of concrete or how to treat the bond interface. Therefore, 200 psi which were recommended in USA seems to be reasonable bond strength, but it should be considered with traffic analysis and how to proceed construction practice on site.



PCC pavement, bonded concrete overlay, unbonded concrete overlay, overlay thickness design, bond strength