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20 high porosity, and resin-rich areas that are not ther- weight methacrylate had low bond strengths. Anecdotal mally compatible with the concrete. It is important experiences of others confirmed that some epoxies tend to that the aggregate be allowed to spread out and fall lose bond with time when placed over a high-molecular- downward into the resin, with the dust and fines car- weight methacrylate. ried off in the air. In hot weather, it is important that aggregate not be introduced too quickly; rather, it needs to be slowly and evenly built up on the surface STRESSES IN OVERLAYS until no wet spots are visible. Analytical Method for Calculating Thermal Stresses 9. Static mixers and paddle mixers are not foolproof. The viscosity of the resin components are affected Choi et al. (40 ) present a method for calculating interfa- by temperature and hot or cold weather may affect cial stresses and axial stresses in TPO overlays because the mixing process. Routine calibration checks must of changes in temperature. The method assumes (1) that be made. there are linearly elastic stresses in overlay and substrate, (2) that the effect of the very thin adhesive layer is negli- Case History of a Failure gible, and (3) that the composite beam (overlay and sub- strate concrete) is subjected to a uniform temperature Carter (8) reports that a bridge overlaid in 1985 had many change and the difference in the thermal coefficients for problems even though only experienced contractors were the overlay and concrete remain constant during the tem- allowed to bid for the job. The contractor provided a 5-year perature change. The governing differential equations and warranty, but problems occurred quickly. The surface prepa- solution are presented. The three types of stresses that can ration by the contractor was rejected twice; the final sur- be determined from this analysis are the interfacial shear face preparation was a compromise. The contractor did not and normal stress and the axial stress in the overlay shown notify the resin manufacturer until the job was well under- in Figure 10. The shear and normal stresses are maximized way, the contractor's workmanship was poor, and there was near the end or boundary, which can be the edge of the little input from the manufacturer. The warm weather that overlay, a crack or a joint, which explains why delamina- lowered the viscosity of the resin combined with improper tion always starts near one of these boundaries as shown seeding methods resulted in a glassy, resin-rich surface that in Figure 11. The axial stress in the overlay starts at zero varied in thickness by as much as 6 mm (0.25 in.) across stress at the boundary and within a short distance from the a 75-mm (3-in.) core. Water had infiltrated some of the boundary reaches its maximum stress. resin drums. After 3 years, 180 m2 (1936 ft2) of surface had debonded, beginning in the first winter. Of the 180 m 2 that The stresses are shown to be a function of the ratio of the were repaired, 40 m 2 (430 ft2) have since failed along with coefficient of thermal expansion of overlay to concrete, the 60 m2 of additional TPO. temperature change, the ratio of overlay thickness to sub- strate thickness, and the ratio of modulus of elasticity of the Texas overlay to the substrate. As each of these increases, the shear, normal, and axial stresses increase. Thus, overlays that are Two short overlay test sections in Texas failed within a thinner and less stiff will produce smaller stresses with the short time of installation because the high-molecular-weight same temperature change. Graphs are provided to simplify methacrylate primer was allowed to pond near the edge of the determination of stresses, and examples are given to a sloping deck and form a thick film. The thick film had a illustrate the method in Figures 12, 13, and 14. The graphs very high coefficient of thermal expansion, and it delami- are developed for differential strain because of temperature nated over a large area, requiring replacement with an epoxy change, T = 500 10 -6 in./in., and a given substrate modu- overlay (22). lus, Es = 4 106 psi. In the graphs, to is the overlay thickness, ts is the substrate thickness, and Eo is the overlay modulus. Panama, Canal Zone T is the difference in the coefficients of thermal expan- sion for the two materials, overlay and substrate, times the The Bridge of the Americas was overlaid with epoxy slurry change in temperature. The analytical values are compared TPO. High-molecular-weight methacrylate was used to with experimentally determined stresses. Examples are seal the cracks on the bridge before installing the TPO. shown in Appendix B. After 1 or 2 years, a considerable portion of the overlay had delaminated based on visual observations. Laboratory Letsch (41) shows measured stresses resulting from dif- tests sought to duplicate the application. The initial tensile ferential strains in the substrate and the overlay and indicates bond tests gave good strengths, but when they were per- that curing, shrinkage, and thermal changes can result in formed after several months on the same specimens, the stresses in the overlay and substrate. Strains were measured specimens that had been primed with the high-molecular- with a cracking frame.