Precast parking structures are subject to sub-structure concrete deterioration due to the high number of control joints. Typically constructed with double tees and inverted tee beams supported on cast-in-place or precast columns, the structural pieces are cast off-site. These pieces are then erected by a crane-supported construction crew. At the intersection of each precast piece is a control joint. Original construction detailing usually includes installation of an elastic caulking system to seal the joints from water infiltration. However, there may be problems with the caulking system that lead to failure: the caulking may be poorly installed; thermal movement at the joint may overcome the caulking elastic properties; or the caulking degrades due to sun exposure, traffic wear, and poor maintenance practices. Water infiltration of these joints initiates corrosion in the reinforcing steel at critical structural areas. This leads to expensive concrete repairs.
Older precast parking structures may have a concrete topping field cast on top of the double tee system. Some precast suppliers currently provide pre-topped tee systems. In either case, the concrete deck is subject to deterioration from many factors such as environmental conditions, salt application rates, traffic counts, and original construction quality. Urethane traffic membrane systems, when properly selected and installed, can protect parking structures from water infiltration and concrete deterioration.
Restruction Corporation recently installed a urethane traffic coating on a 75,000 sq. ft., single-structural level, precast parking garage located in Golden Colorado. The garage, opened in 1993, is a truncated triangular shape, creating a pie-piece appearance with the interior “tip” cut off. Four, 60 ft. double tee spans and one, 45 ft. double tee span make the garage’s east to west radius dimension 285 lineal feet. There are 4 interior “t-beam” lines supporting the double tees. The garage is 435 ft. wide at the outside parapet, a precast “L-beam” tee support. The interior garage width, at the pie tip cut-off, is 100 ft. Two traffic entrance/exits are located along the north edge. An expansion joint, running east to west, separates the garage into two independent structures.
A thin, bonded epoxy overlay and joint re-caulking maintenance project was completed at an earlier date. However, the epoxy system was poorly installed, leading to large-scale de-bonding, excessive wear, and generally poor performance. Caulking detailing created additional problems as the low elongation epoxy system was applied over the joints. Due mainly to the pie shape and single-structural level configuration, bearing pad “walk-out” was occurring and indicated high movement at some control joint locations. Water infiltration at control joints had caused structural concrete deterioration at tee bearing locations and along inverted tee beam ledges. Structure owners consulted with both a traffic membrane supplier and Restruction Corporation, an experienced installer. The owner eventually selected a system based on factors such as elastic properties, traffic wear durability, waterproofing characteristics, and expected weather conditions at time of installation.
The urethane traffic coating system was selected instead of a thin bonded epoxy overlay system, due to better elastic properties and anticipated movement at the control joints. The system basecoat has ASTM D 412 measured elongations of 595%. During planning it was discovered that this choice involved lower overall material and installation costs when compared to available MMA protection systems. Several high-quality manufacturers of urethane traffic coating materials keep price and performance competitive.
At both entrances, approximately 3100 sq. ft. each, traffic counts were very high. BASF Masterseal MS200 basecoat, applied at 25-mil thickness, followed by two coats of BASF Masterseal MS 275 were installed to provide maximum waterproofing protection. Each coat of the MS 275 was applied at 40-mil thickness. Flint rock aggregate, size #3, was broadcast into the wet MS 275 film for maximum wear protection. Two coats of MS 295 topcoat, a harder, lower-elongation urethane, were applied with #16-30 sand broadcast for slip protection. The MS 295 provides first-line, tire wear protection for the underlying waterproofing layers. Target thickness for the MS 295 was 20-mils. A five-year warranty was provided to cover issues related to installation or material failures. Regular inspections were scheduled in order to catch excessive topcoat wear early, thereby providing maximum waterproofing durability and service life.
Finally, MS200 (25-mil thickness), followed by two coats of MS 295 (20-mil thickness), each with sand broadcast, was installed on the remaining 68,800 sq. ft. of elevated parking deck. The overall thinner application was more cost efficient and will provide concrete protection for many years if properly maintained. The thinner application will minimize cracking risk at the control joints. Reinforcing fabric and stripe coating were installed at every control joint as specified by the manufacturer. Reinforcing fabric was used to mitigate control joint movement from degrading the traffic coating system.
The MS200 single-component basecoat material was selected primarily due to slower curing conditions during extended cold or wet weather. Each subsequent coat must bond to the previous coat for a durable traffic coating. Having a long “open time” base coat allowed work to continue even under difficult winter weather conditions. The MS295 topcoat is a two-component material that uses a chemical reaction to accelerate curing. Subsequent layers will bond to each other readily if clean and dry, thus making application less weather-dependent. The MS295 also provides maximum protection from ultra-violet light degradation and is an excellent selection for Colorado, where sunshine is prevalent and strong.
Material Installation Considerations
Bonding the traffic coating materials to the concrete deck and each subsequent layer was required for successful installation. A previously installed, poorly bonded epoxy protection system had to be removed and the remaining surface prepared prior to coating installation.
Removal tests were completed to measure production and to determine how much concrete could be removed without causing excessive damage to the substrate. Three-foot diameter stand-up grinding equipment with diamond grinding bits was selected.
Grinding rates were targeted for 2000 sq. ft. per machine per day. This rate was selected to remove nearly all the poorly bonded epoxy coating and over 50% of all existing coating. After the grinding, cut wire shot blasting was used for final surface preparation.
Two quality control tests were implemented after grinding and shot blasting. Initially, six separate ICRI 210.3R Tensile Bond Pull-off tests were performed on the prepared surface. The test failure results ranged from full concrete failure to partial remaining existing epoxy coating. Bond results were 141,148,243,382,432, and 450-psi. A result of 200-psi was considered a very good result regardless of failure mode. After reviewing the lower results, additional inspection and coating removal was completed. The testing, added inspection and removal of poorly bonded existing epoxy materials were recommended by Restruction Corporation. The owner recognized the value of this additional inspection work.
Basecoat-adhesion quality control tests were also completed after initial grinding and shot blasting. These tests are qualitative and have no measurable value. A small area of basecoat is applied to the prepared deck surface. Next a 1″ wide strip of joint reinforcing fabric is embedded into the basecoat. The tab of fabric is pulled after curing. As the fabric is pulled, basecoat adhesion is observed. This test was also considered acceptable and provided value to the owner.
Interlayer bonding was also tested using the ICRI 210.3R method. Three random locations were selected for interlayer bond testing. Cores were cut through the completed coating and into the concrete deck surface. Testing dollies were adhered to the completed coating using a standard, fast-set epoxy. The dollies were pulled to failure using a calibrated jack. Gauge pressure at failure was recorded and converted to coating tensile failure values. All three tests failed within the concrete surface at values of 392,430, and 546-psi. These results meant that the weakest layer in the coating system was the concrete deck surface. The layers of the coating, bonded together, were stronger than the concrete deck. These results were considered excellent.
The project was started in October and completed in mid-February. Selecting the single-component basecoat system, along with modifying the coating to include two topcoats of the harder, aliphatic topcoat, ensured a successful outcome for all parties. Detailed attention to weather forecasts allowed for material application at lower, but dry, environmental temperatures. Using flint rock and the manufacturer-recommended layering system at the high-traffic entrances will provide good coating durability. Most importantly, testing was performed at critical decision points and provided assurance of desirable results. Restruction Corporation will continue to monitor the coating performance over the 5-year warranty period.