Precast Parking Structure Repair: Waterproof, Traffic-Bearing Membrane Installation

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.

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.

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.

wo 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.

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.