As our cities and municipalities grow older, the utility and wastewater infrastructure and corrosion preventive program for underground asset protection are aging with them. Municipalities and Utility Companies are struggling to handle rehabilitation measures and effective corrosion prevention programs to deal with prolonging the life of their buried assets. More municipalities are in the process of undertaking rehabilitation projects to restore underground structures and protect them against further corrosion. Many of these municipalities are taking a proactive approach. A few regions in the US, especially those in the southwest, gulf coast, and southeast are starting to require that the concrete manholes, vaults, large diameter pipelines, and structures first be lined with a corrosion resistant material before going into the ground. The warm, humid climate in these regions is a breading ground for significant corrosion in existing underground structures. Preventive maintenance programs are being initiated to prevent or slow the rate of corrosion and degradation and to ensure that utility work and wastewater construction cost are spent efficiently.
Precast concrete vaults, such as those used by sewage, electric, gas, steam, telecommunication, and fiber optic utility companies are not exempt from the effects of corrosion within their underground vaults. If ground water can leach in, the structure is susceptible to corrosion attacking the concrete. Concrete vaults are everywhere – under the streets & sidewalks, in our parks, and along our roadways. Road salt, de-icing materials, petroleum products, lawn fertilizers and anything blended with rain water and run-off can infiltrate these structures.
Precast concrete is still the most widespread choice when selecting a material for underground utility vaults. Precast concrete has several advantages over vaults made from steel, fiberglass, and cast-in-place concrete. First, precast concrete is non-toxic and noncombustible so it will not loose its structural integrity in the event of a fire. Second, due to their weight, they will remain in the ground and ground water will not raise the structure or move it creating disruption to the connections, lines, or service.
Vaults that are produced in quality controlled plants are now covered by ASTM Standards that regulate the design, installation and inspection of these vaults. They are manufactured to be water tight, environmentally sound, and will strengthen over time. Installation of these vaults is much easier and more cost effective over cast-in-place concrete, which is much more labor intensive and costly to install. With precast structures you are able to backfill right away and not have to wait 28 days until the structure is fully cured, adding to installation downtime. Since water by nature can contribute to a corrosive environment, rust can be a major weakness in some underground storage systems. Concrete, by its nature, is vulnerable to cracking. In most cases, precast concrete tanks rely on sealants or adhesives to seal the seams of the tank. When there is ground movement around the tank causing a shift or settling effect, or when the tank is improperly installed, these seams are susceptible to leaking. Concrete tanks, whether precast or cast-in-place, are susceptible to deterioration if the steel reinforcement rusts or is exposed through cracks in the concrete causing both groundwater contamination and groundwater infiltrating into the tank. Exposure to hydrogen sulfide (H2S) gases, present in wastewater tanks, will also lead to corrosion damage to a concrete tank and steel reinforcement, thereby limiting the precast structure’s or tank’s useful life.