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19 strengths averaged 1.8 MPa (270 psi) for deck surface prep- FAILURES aration and 2.6 MPa (390 psi) for the overlay bond strength for the project. Alberta The Port Authority modified its contract requirements Lessons Learned from Failures and bidding procedures in order to obtain the best possible wearing surface. Epoxy resins were tested for conformance Carter (8) provides observations on lessons learned from to specifications prior to soliciting bids. Four resin suppli- TPOs installed in Alberta, but states that his comments may ers were approved with no substitutions. The resin suppliers not always be in agreement with experiences in other areas. were then formally requested to select a contractor that could provide a joint warranty with them. The warranty required 1. Failures are usually the result of constructability the contractor to repair any defects within 5 years of instal- rather than materials. If the in-place material is defec- lation. The resin supplier was required to have a full-time tive, it is usually because of improper proportioning representative on the site when work was being done. The or mixing. Using different colors for multicomponent purpose for the contract requirements was to provide the systems would help reduce proportioning errors. contractor and material supplier the incentive to take joint responsibility for the installation. 2. The moisture content of the deck is important in achieving good bond strength of the TPO to the deck. The contract required the finish surface to have a mini- Some parts of the deck, including low areas, other mum surface friction number of 60 as determined by the areas that drain slowly, and gutters, dry slower owing British Pendulum Tester. The average friction number to ponding. The moisture may prevent adequate bond obtained for the project was 71. Grooves 6 mm 6 mm ( strength development. in. in.) were cut 37 mm (1.5 in.) apart, center to center, in the transverse direction several days after the overlays were 3. Deck repair patches made with portland cement con- placed. No complaints have been received from airlines for crete need to be wet cured to reduce shrinkage and excessive rubber wear on tires. subsequent debonding. The patches should be allowed to dry sufficiently to achieve good bond to the TPO. The overlays were completed for one runway in 1998 In Alberta, the maximum depth for mortar patches is and the other in 1999. The overlays have performed well, 15 mm (0.67 in.) because deeper patches may develop with only minor repairs required. Most of the delamina- cracks around the perimeter at low temperatures. tions occurred at structural expansion joints and at locations where the overlay was very thick, about 1 in. The airport 4. The more the surface is prepared and roughened, the management and the airlines have been pleased with the per- greater the bond strength will be. Shot blasting pro- formance of the overlays (39). vides a more uniformly prepared surface than does sandblasting. Pennsylvania 5. In cold climates, thicker TPOs will fail faster than Pennsylvania DOT installed three TPOs: premixed polyes- thinner ones because of the different thermal coef- ter, an epoxy multiple layer, and an epoxy urethane mul- ficients of expansion of the concrete and the PC. tiple layer, each on two separate bridge decks. The decks were rehabilitated after being evaluated for chloride ion 6. Thin TPOs cannot be expected to provide a smooth content, corrosion activity using half-cell tests, and delami- surface on a rough deck surface. Deck patching must nation using chain drag. Problems were encountered dur- be done carefully to prevent the creation of a rough ing construction with the premixed polyester resulting in a surface. Bumps in the surface of the TPO will shorten resin-rich mixture that made it susceptible to oxidation and the life, particularly when snowplows are used. ultraviolet light degradation. A 5-year evaluation indicated significant moderate spalling, cracking, and debonding in 7. Aggregate type and grading are important for wear the polyester overlay. The two epoxy multiple-layer overlays resistance, flexibility, toughness, and crack-bridging provided good long-term performance as evidenced by the ability of the wearing surface, particularly when excellent protection against chloride and moisture intrusion. high-modulus polymers are involved. As a result of the performance, the use of epoxy and epoxy urethane multiple-layer TPOs was recommended for future 8. The method of seeding aggregate into the resin is use by Pennsylvania DOT (39). important in preventing surface ripples, wicking,