Pima County Wastewater Treatment Plant
Date: October 2005
Location: Pima County, USA
Industry: Water/Waste Water
Challenge
The Roger Road Wastewater Treatment Plant had an expensive problem: a severely deteriorated concrete splitter box needed to be repaired, but before any work on this operating structure could begin, a temporary but costly “pump around facility” had to be installed to handle the process performed by the splitter box, says John Munden, Civil Engineering Manager with the Pima County Wastewater Management Department.
The splitter box is used to split the flow of raw sewage and direct it to various primary clarifiers, he says. The structure was to have been part of a larger project to rehabilitate other concrete structures with a calcium aluminate cement underlayment and epoxy topcoat, but the contractor handling that job declined to take on the splitter box, declaring that it couldn’t be fixed, says Allen Kauffman, the president of another contracting firm that subsequently did the work using a polymer concrete repair product and an epoxy novolac lining.
The covered splitter box is 8 ft (2.4 m) wide, 18 ft (5.4 m) long, and divided into two sections of different heights to aid in sewage separation. The turbulent contents of the splitter box encourage the development of hydrogen sulfide gas and the subsequent growth of microorganisms that attack the concrete substrate, says Kauffman. In addition, says Munden, the cover exacerbates the hydrogen sulfide problem by trapping the gas, increasing its concentration in the structure.
The deterioration of the splitter box was extensive, but not uniform, says Munden. In some areas affected by sulfuric acid attack, the concrete had lost over 50% of its original thickness. Concrete had been eaten away, exposing rebar. One wall, originally 12 in. (30.5 cm) thick, was reduced to a three-inch (7.6-centimeter) thickness, says Kauffman. The rebar in some areas had been completely corroded, he adds.
Solution
Given twelve days to do the work in November 2003, Kauffman’s company began the project by power washing the splitter box at 3,500 psi (233 bar) to clean the concrete surface and remove loose material. The deteriorated areas were then abrasive blasted with copper slag. The contractor installed a new mat rebar system, securing it to concrete with drilled-in dowels or tying it to the existing rebar. The contractor then performed another power wash with an injection of a soluble salt treatment. Blowers were set up to speed drying, and six hours later, the contractor began applying the polymer concrete repair product by trowel.
Working on one wall section at a time, the contractor applied the polymer concrete repair product in approximately two-inch (five-centimeter) thick layers, says Kauffman. The concrete repair material was allowed to set up for two hours before additional layers were applied. The contractor built the walls of the concrete structure back to their original thicknesses, in some cases taking a 1/8-inch-thick (3 mm) area back to 10 in. (25 cm). The contractor performed adhesion tests on the concrete repair product, noting that the cementitious substrate failed b e fore the polymer concrete repair product.
The surface preparation, rebar installation, and rebuilding of the concrete took four days, says Kauffman. Forced heat aided the cure of the concrete repair material, allowing the contractor to proceed with the lining application 12 hours later. Over two days, two coats of the chemical-resistant epoxy novolac lining were applied by brush and roller to a total thickness between 30 and 35 mils (750 and 875 micrometers). The epoxy novolac was heat cured over the first night and then heat cured for 24 hours following the final application, says Kauffman. According to Kauffman, Pima County had an inspector on site during all working hours.
The concrete splitter box rehabilitation was completed in less than 7 days, saving more money in “pump around facility” expenses than it cost to perform the work, says Kauffman. The contractor performed a follow-up inspection in March 2005, noting no signs of deterioration of the system. Munden says that, although a formal follow-up inspection has not been conducted on the operating splitter box, the lining system appears to be holding up well. He adds that the many of the metal components of the splitter box have been replaced and that it is slated for the installation of a ventilation system for odor control and hydrogen sulfide mitigation. Opt Co. (Gilbert, AZ) performed the surface preparation and underlayment and lining application. Novocoat (Addison, TX) manufactures the polymer concrete repair material and the epoxy novolac lining. Hold Tight Solutions Inc. (Houston, TX) manufactures the soluble salt treatment.
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