Located in Niagara Falls, N.Y. and built in 1980, Covanta's Niagara Resource Recovery Facility processes 2,250 tons of waste per day, selling the steam to adjacent chemical facilities and the electricity to the Niagara Mohawk Power Company. Originally designed as a cast-in-place reinforced concrete structure, the existing elevated floor consisted of a 6-inch overlay placed on precast double-tees with spans that varied from 36-40 feet, depending on location. No special anchor or QC procedures were used to ensure that the overlay was bonded to the double-tees. As a result, the slab was deteriorating and was experiencing significant cracking and most of the floor had typical through-cracks on 2-foot-by-2-foot grids with liquids seeping through the cracks. The owner decided to replace the overlay with a reinforced concrete overlay and strengthen the elevated tipping floor to carry more live load in the form of larger equipment. The area to be repaired was approximately 4,000 square feet; the double-tees are approximately 5 feet wide with 28-inch deep and 4-inch wide stems and approximately a 4-inch slab.
| What is a Tipping Floor? Most solid waste facilities (transfer stations, material recovery facilities, recycling plants and waste-to-energy plants) have a "tipping floor." The tipping floor is an area of the facilities' floor where trucks drive in, "tip" solid waste onto the floor and drive out. The solid waste is then sorted and moved by machinery to transfer ports. In a waste-to-energy facility, the sorted waste is moved to a transfer port that feeds into the energy conversion section of the plant. Most tipping floors are constructed with concrete and are either slab on grade or elevated slab. Tipping floors take a lot of abuse from the solid waste being dropped onto the floor and then moved around the floor by heavy machinery. Additionally, chemicals and other substances in the solid waste seep into the concrete and cause corrosion, delamination and spalling. Because of the abuse that tipping floors take, many solid waste facilities use an iron aggregate topping that is more abrasion & chemical resistant than regular or high-strength concrete. For more on tipping floor repair, please go to http://www.tippingfloors.com/ |
Structural Preservation Systems (SPS) was contacted to develop a solution that would increase the live load capacity of the tipping floor by:
1. Repairing the deteriorated topping slab by replacing it with an 8-inch bonded RC topping slab
2. Installing Fiber Reinforced Polymer (FRP) sheets on the double-tee stems
An evaluation of existing design loads concluded that a retrofit design based on a modified HS20-44 would be adequate. The design also considered the loader used to process trash in the facility which is a rubber tire vehicle with a front axle weight of 34,280 pounds. Per AASHTO, a live load distribution factor was used in analysis that assumes that 80 percent of the load is supported by the loaded double tees, with 20 percent distributed to adjacent double tees. A live load impact factor of 1.2 was incorporated in the analysis.
In order to have adequate capacity, the new reinforced concrete overlay must be acting monolithically with the double-tees and require additional reinforcement to supplement the concrete to concrete bond, and provide the necessary horizontal shear transfer. This was achieved using steel dowels installed on the top side of the double-tee slab, along the full span of the double-tees. The lack of these dowels in the original overlay is one of the main reasons for the debonding and excessive cracking of the original floor system.
Based on preliminary calculations, it was found that the existing double-tees with new bonded reinforced concrete overlay would still be deficient in bending and shear for the new design loads. Different options were then evaluated for the upgrade of the existing floor, including the use externally bonded FRP, external post-tensioning, and concrete enlargement of the stems.
The strengthening solution included a 7 ½-inch bonded concrete overlay, reinforced with top and bottom mats of #5 bars placed at 8 inches on center in each direction. The bending and shear capacities were supplemented with externally bonded strips of carbon FRP applied to the soffit and sides of the double-tee stems. To begin the repair/strengthening project, SPS first removed the existing 6-inch topping slab. While the topping slab was removed, FRP was installed on the underside of the double-tees.
During bonded slab installation, 1½-inch deep joints were installed on a 20-foot-by-20-foot grid. To prevent joints from spalling due to high point loads, the joints were filled with a low modulus epoxy with Shore A hardness of 80 and Shore D hardness of 50.
All FRP design was specified to be achieved in accordance with the American Concrete Institute (ACI) Guidelines ACI 440.2R-02. All applicable environmental and bond-related reduction factors and other design requirements were established to comply with ACI 440.2R-02. FRP design properties were specified to be in accordance with ACI 440.2R-02, Section 3.3 that stipulates the design properties based on testing of 20 specimens.
The project was completed within two weeks during February, 2008 and allowed the facility to remain operationalDuring bonded slab installation, 1½-inch deep joints were installed on a 20-foot-by-20-foot grid. To prevent joints from spalling due to high point loads, the joints were filled with a low modulus epoxy with Shore A hardness of 80 and Shore D hardness of 50.