Category: Repair / Rehabilitation
Location: Tucson, AZ
Size: 5,000 sq. ft.
Masonry Used: Around 5,000 sq. ft. of Carbon Fiber was used to reinforce the existing masonry in lieu of replacement.
Submitted by: HJ3 Composite Technologies
Manholes are traditionally built out of masonry or concrete and allow access to local sewer systems. Sewer systems that are considered 'sanitary' or 'combined' (transporting storm water and human waste) almost always produce Hydrogen Sulfide gas, which is a by-product of human waste decomposition. This gas rises up the manhole shafts, interacts with the Thiobacillus bacteria found on the concrete, and produces sulfuric acid. In this highly corrosive environment brick can degrade rapidly. This degradation threatens the structural integrity of the entire manhole. Additionally, spalling, or flaking of the substrate, occurs and may fall into the sewage system. Too much infiltration most often leads to disruption in sewage system due to backups in the pipes, and eventually may lead to a costly shut down for clean-up.
In this particular case there were four brick manholes in need of structural repair. The manholes surrounded a golf course and were considered combined sewers, as they were part of the city's storm water and sanitary sewer system. These manholes showed significant corrosion and cracking, due to corrosive Hydrogen Sulfide gas exposure. In addition to the internal corrosive conditions the manholes experienced structural failure caused by external forces from the surrounding soil. Both repair and replacement options were considered.
Instead of replacement, the manhole repair system was recommended by the hired contractor and approved by the county. The surface of the manholes was abraded utilizing a high-pressure, abrasive blast to remove loose brick and clean the substrate down to intact, rough aggregate. The surface was then cleaned to remove any residual dust and contaminants with a high-pressure water blast. A quick-set grout was used to resurface the brick substrate. All cracks were injected with CarbonSeal low-viscosity crack injection epoxy resin.
After surface preparation and patching, the CarbonSeal system was bonded to the entire surface area of the manholes. To protect the strengthened manhole from gasses and chemicals, the CarbonSeal top coat was applied. The carbon fiber liner was spark-tested and top coat was reapplied to cover all pinholes until the liner was determined to be pinhole-free.
Replacement of the manholes would have brought significant disruption to traffic on a busy street, as a 10'x10' hole would have had to been cut for each manhole. Traffic control costs would have quadrupled, and amenities for the workers would have been required. Repair with high strength carbon fiber to the interior walls of the manholes created sufficient tensile reinforcement of the masonry walls. The repair system saved the county over 80% of the cost of replacement, and provided a long-lasting repair that will not require any future maintenance.
Owner: Pima County Regional Wastewater Reclamation Department
- Structural Engineer: Olley Scholer, HJ3 Composite Technologies
- General Contractor: Adam Sedgeman, KE&G Construction, Inc.