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7 Examinations of spalled material usually showed a tiator or curing agent, curing temperature, and binder con- layer of concrete beneath the epoxy PC. tent. AASHTO Guide Specifications (6 ) show the minimum · Decks that were rated higher before overlay placement times for curing that have been successfully used for TPOs. performed better based on an inspection rating system for both the deck and overlay. W hite and Montani (9) point out the importance of ten- · Longer span bridges were more likely to have problems sile elongation and note that polymers that have good tensile with overlays owing to the greater deflections that can elongation at room temperature may have poor elongation at cause deck or overlay cracking. More flexible epoxies are low temperatures. They recommend 20% elongation of the recommended for longer spans. Bridges with girders that polymer at 40°F and 30% when tested at 73°F in accordance were wide flange sections or post-tensioned I-beams had with ASTM D638 (10 ). overlays with higher ratings than bridges with plate girders. Gaul (11) reports on the use of epoxy asphalts over the previous 25 years. He describes the properties of epoxy REPAIRS asphalt and provides guidance on proper installation. Cracks Aggregates TPO contracts in Alberta in the period 1985 to 1987 required It is important that aggregates used in TPOs be dry (less than flexural cracks to be repaired (8). Flexural cracks were dis- 0.2% moisture), angular-grained silica sand or basalt, and tinguished from shrinkage cracks by (1) straightness in con- free from dirt, clay, asphalt, and other organic compounds. trast to shrinkage cracks, which tend to be more curved and AASHTO Guide Specifications (6 ) give recommended randomly oriented; (2) location in negative moment region; gradings for multiple-layer, slurry, and premixed overlays. and (3) presence of stains on the bottom of the bridge, which indicated that the cracks were full depth. The cracks were Fontana et al. (12) indicate that an increase in aggregate routed and filled with a flexible epoxy that was said to have moisture to 1% by weight can significantly decrease the "probably performed as designed until exposed to very low strength of the PC from which it is made. They state that temperatures." In 1988, "band-aid" repairs were developed. the addition of 1% silane coupling agent by weight of the These repairs involved debonding the wearing surface along monomer (methyl methacrylate) can significantly offset up the crack, strengthening the wearing surface with tensile to 1% moisture in the coarse aggregate in the reductions in reinforcement centered over the crack. One repaired bridge strength and freezing and thawing that normally would be originally had about 600 lineal meters (2,000 ft) of flexural expected. cracks and 6,000 lineal meters of shrinkage cracks; after 4 years, about 100 lineal meters (325 ft) of cracks have reflected Polymer Concrete Application Rates through the TPO. Another repaired bridge had 3,000 lineal meters (10,000 ft) of crack repairs, and about 70% of the Typical PC application rates for the three different types of cracks have reflected through the TPO. Carter concludes overlays are given in AASHTO Guide Specifications (6 ). that, "the extreme cold, as well as the flexibility and live load deflection of this deck, make crack repairs appear futile" (8). INSTALLATION Sprinkel (4) states that "large cracks should be filled ahead of time with a gravity fill polymer that is compatible Surface Preparation with the overlay." Surface preparation procedures need to be approved only Concrete when test patches, 0.3 m by 0.9 m (1 ft by 3 ft), constructed with approved materials are shown to have a minimum aver- It is important that concrete with chloride ion content greater age tensile rupture strength of three tests per patch, based on than 0.77 kg/m3 (1.3 lb/yd3) at the level of reinforcing be the procedure in ACI 503R (13), of 1.7 MPa (250 psi) (4). If removed and replaced before placing the overlay (4). the failure in the concrete occurs at depths greater than 6.4 mm (0.25 in.) and at stresses less than 1.0 MPa (150 psi), it is important that the concrete be removed and replaced with MATERIALS USED IN OVERLAYS higher quality concrete, followed by surface preparation and placement and testing of new test patches. When stresses Resins at failure occur between 1 MPa (150 psi) and 1.7 MPa (250 psi), the overlay can be placed, but the engineer determines Typical resins are epoxy, polyester, and methacrylate (4). whether part or the entire base concrete needs to be replaced. The curing time is a function of the type and amount of ini- A visual inspection needs to be made before the overlays are
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8 placed to ensure that the surface is properly prepared, dry, passes are adequate to provide the required surface prepara- and free of dust or other contaminants. tion to achieve a minimum tensile bond strength of 1.7 MPa (250 psi) or a failure area at a depth of 64 mm (0.25 in.) or Substrate Tensile Strength more into the base concrete greater than 50% of the test area. The test result might be based on the average of three tests on The deck must have a minimum rupture strength of 1.0 an overlay test patch of at least 0.3 m by 0.9 m (1 ft by 3 ft). MPa (150 psi) based on the test method in ACI 503R (13) or One test result could be obtained for each span or 418 m2 (500 ASTM C1583 (14) to receive an overlay; otherwise, the con- yd2) of deck surface, whichever is the smaller area, as shown crete must be removed and replaced so that a sound substrate in Figure 2. The cleaning procedure could be approved if the is available to bond to the TPO (15). test requirements are met for each test panel (6, 17). Because the temperature of the overlay can affect the test result in that Repair of Substrate adhesion decreases at higher temperatures, the temperature of the overlay at the time of the test should be recorded (17). The substrate concrete needs to be patched, and large cracks [greater than 1 mm (0.04 in.)] in width need to be repaired (15). It is important that concrete with chloride ion content greater than 0.77 kg/m3 (1.3 lb/yd3) at the level of the rein- forcing steel be removed and replaced before placing the overlay (16, 17 ). Surface Cleaning "The surface should be cleaned by shot blasting (Figure 1) and other approved cleaning practices to remove asphaltic material, oils, dirt, rubber, curing compounds, paint, car- bonation, laitance, weak surface mortar, and other detrimen- tal materials that may interfere with the bonding or curing of the overlay" (15). Along edges of the deck and other areas that cannot be cleaned by shot blasting, grit blasting might be used. FIGURE 2 Apparatus for performing pull-off test to evaluate surface preparation or overlay bond strength. Test for Adequacy of Surface Preparation Methods of Application The test method described in ACI 503R (13) and ASTM C1583 (14) could be used to determine whether the size of It is important that TPOs be placed the same day that the shot, flow of shot, traveling speed of machine, and number of surface is shot blasted to ensure cleanness. Areas of the deck FIGURE 1 Shot-blast preparation of the surface.
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9 that are not shot blasted the same day could be shot blasted and kg/m 2 (5.21 lb/yd 2) of silica flour. This slurry mix is again just prior to installing the overlay (4). applied with a gauge rake to ensure proper depth of place- ment (Figure 4). Gap-graded aggregate (as used in multiple- Multiple-Layer Overlays layer overlays) is broadcast onto the surface. A binder seal coat at 0.68 kg/m2 (1.25 lb/yd2) is applied. The binder con- Multiple-layer overlays are constructed by application of tent is approximately 24% by weight of the overlay (primer the binder (resin or monomer system) to the deck surface and seal coat). The thickness is about 7.9 mm (0.31 in) (15). by spray, squeegee, or broom and broadcasting gap-graded aggregate to excess over the fresh binder-covered surface (Figure 3). After the binder has cured, the loose aggregate is removed from the deck and a second layer is applied. The first layer consists of approximately 1.1 kg/m 2 (2 lb/yd 2) of binder and 5.4 kg/m 2 (10 lb/yd 2) of aggregate. The second layer consists of approximately 2.2 kg/m2 (4 lb/yd2) of binder and 7.6 kg/m2 (14 lb/yd 2) of aggregate. The resin content is approximately 25% by weight of the overlay. The thickness is about 6.4 mm (0.25 in.) (15). FIGURE 4 Using gauge rakes to control thickness of slurry overlay. (Courtesy : Virginia Center for Transportation Innovation and Research.) Premixed Overlays Premixed overlays are installed by mixing about 12% binder with the aggregates. A primer is usually applied to the sur- face at an approximate rate of 0.41 kg/m2 (0.75 lb/yd 2) to improve the bond strength (Figure 5). The polymer concrete is placed and a vibratory screed is used to strike off and con- solidate the PC (Figure 6). In some applications, continuous batching and paving equipment has been successfully used to place premixed PC (Figure 7). A suitable skid resistance can be achieved by placing grooves in the fresh PC (Figure 8) or by broadcasting aggregates onto the fresh PC surface (Figure 9). The thickness is about 13 mm (0.50 in.) (15). FIGURE 3 Workers spraying resin followed by application of aggregates in multiple-layer TPO. Slurry Overlays Slurry overlays are constructed by applying a primer of monomer or resin system at an approximate rate of 0.41 kg/ FIGURE 5 Priming deck with initiated high-molecular-weight m2 (0.75 lb/yd2) followed by a slurry mixture of about 2.6 kg/ methacrylate prior to placement of PPC. (Courtesy : American m2 (5 lb/yd 2) of binder, 3.8 kg/m 2 (7 lb/yd 2) of silica sand, Civil Constructors.)