MC Magazine Archive

Adding Years to Manholes

Using liners to prevent concrete corrosion and groundwater infiltration.

By Jack Klein
Jack Klein is a writer specializing in construction and industry.

Precast concrete has always been the ultimate product choice used to build municipal sanitary sewage systems throughout the world. Preferred because of its strength and durability, millions of tons of precast concrete have successfully served the concrete needs of many countries for decades. In some areas, however, deteriorating conditions have created a new challenge for the precast industry: how to prevent corrosion of concrete caused by sewer gas.

Even at a time when an ever-increasing population is requiring urban municipalities to expand their wastewater collection systems, existing systems are beginning to disintegrate. A common cause of deterioration of concrete manholes er, causing corrosion of the concrete.

The amount of time and severity of concrete degradation from MICC depends on how aggressive the sewer environment is. One major factor is the amount of hydrogen sulfide gas flowing through, which can be aggravated by such other sewer conditions as heat and turbulence.

Sewage becomes septic faster in warmer climates, which creates hydrogen sulfide. Fresh sewage is not a problem because it must become septic before the MICC process begins. Though a stagnant sewer allows the sewage to become septic, in a more turbulent sewer, the gas is released more quickly.

Increased turbulent conditions may be formed in a number of ways, including structures using drop manholes; a number of pipes coming into the structure creating conflicting flows; or in the areas near lift stations, where the gases tend to back up several manholes into the system, creating high concentrations of gases. Any of these conditions can cause increased turbulence, which in turn creates more acid corrosion problems.

The MICC biological/chemical phenomenon was first isolated as far back as 1945, but there still is no solution to the destructive sulfate-concrete interactions. The only current solution is to completely protect the concrete from the harmful gases using barrier methods. Many techniques for such protection have been attempted, including applying a coating to protect the concrete from the acids and fitting a form inside the structure that does not react with the gases.

The other major concern of wastewater engineers is groundwater infiltrating sewer systems. When fresh groundwater gets into the system, it ultimately becomes wastewater, which means an increased output of energy for water treatment, the construction of over-sized water treatment plants, and other wasted expenditures.

Groundwater infiltration problems can be found almost anywhere. The most serious problems, however, occur in areas in which there are high water tables. In some cases, these water tables might be only a foot or two below ground level while the sewer system requires lift station structures running up to 30 feet deep. Thus, a condition exists in which up to 30 feet of head pressure may develop, which can cause the groundwater to force its way through cracks or joints into the sewer system.

Ironically, "drying out" a sewer system can greatly increase the problems associated with MICC, so that as more water infiltration is eliminated, more acid is produced within the system. There has been a general trend over the past few years to tighten up the manhole ring and cover by using rain dishes and gasketed rings to keep water from getting into the sewer. However, since water has a neutral pH, fresh water infiltrating the system tends to dilute the sewage so that it doesn't become as septic.

In many cases, keeping fresh water out of the system has increased corrosion in components such as manholes, which, prior to sealing of the system, might have been submerged or under water. This tends to negate the corrosion problem for those portions of the manhole that were under water. Now exposed, those sections are more prone to an attack from the sulfuric acid.

In addition, the same cracks in the rings and covers that would let rain come in to the system would also let gas get out. Since the sewer was, in effect, "vented," this created a less serious problem with MICC. However, tighter seals have helped trap gases within the sewer system, accelerating corrosion problems.

"This is a problem a lot of people have come to recognize," says Stewart Dawson, general manager of US Environmental Systems, Inc., in Miami, Florida. "They're spending billions of dollars tightening up and trying to dry up their lines. They’re working to realign the pipes and to keep the manholes from leaking. They're taking every measure possible to keep inflow to a minimum, which is necessary because treating groundwater at the treatment plant is expensive. But now, as they dry the sewers out, they see another problem. That's a greater susceptibility to MICC."

It has become paramount for municipalities to find solutions that address both the problem of MICC and that of inflow, simultaneously. Such was the case in Key West, Florida, where precast concrete manholes, some only 15 years old, had become so corroded by MICC that they had to be replaced.

"In Key West, they had a serious problem with inflow," Dawson says. "There was both freshwater and saltwater infiltration, and in many cases, there was two to three times more water getting into the system than was created by the population alone."

Ed Doty, project manager for the city of Key West, elaborates. "We have a lot of sulfides down here, and we experienced a lot of saltwater intrusion into the lines until we undertook the project of replacing our old, corroded manholes," he says. "Between the gases and the water intrusion, the manholes don't hold up very well. Our newest manholes were only about 15 years old, and we were finding a lot of problems with those."

Key West had two choices: replace the manholes every 15 years or so, or find a way to stop water infiltration while protecting the concrete from the ravages of MICC. "When we weighed out all the options, we decided manhole liners were absolutely the best way to go," Doty says. "We've done some manholes with retrofit polytriplex liners in some of the old manholes on this island, and they seem to work out okay. But when we decided these manholes had to be replaced, we didn't want to replace them again 15 or 20 years down the line. That's why we went with the liners."

According to Ed Arabian, director of engineering and product development at U.S. Precast Corporation in Medley, Florida: "Numerous coating and lining products have been used with varying degrees of success against concrete deterioration by MICC in sanitary sewer systems. In Florida, though, as well as other low-lying areas of the country, we have a dual problem: we have to protect the concrete from the acids that are generated by the microbial process that takes place in sanitary sewer systems, but those protective measures must also stand up to a great deal of back pressure. In other words, they also have to stay on the wall."

Arabian adds, "We have had just a tremendous amount of interest from municipal people who not only wanted a product they could confidently use for their new sewer projects, but for one that could be used in rehab situations. What we can offer is a system that we have engineered to not only provide corrosion protection, but will not allow any intrusion of groundwater into the manhole."

The system used in Key West's rehab project consists of the Agru Sure Grip liner. The liner is a thermoplastic, a random copolymer polypropylene (PP) material. Automobile battery manufacturers have used PP in their battery cases for years, because it stands up to the corrosive effects of sulphuric (battery) acid, high-heat conditions, exposure to various other chemicals, and it can withstand an environment of high impact and vibration.

The sheets of Sure Grip liner are unique in the way they are manufactured. This manufacturing process typically involves an extrusion process in which the profile is squeezed out through a die head. Anything added to that profile must be welded to it after the extrusion process. The Sure Grip liner, however, has 39 V-shaped anchors per square foot, which are used to hold the concrete manhole when the product is cast. This forms a strong mechanical bond between the liner and the manhole. In the Sure Grip manufacturing process, these critical anchors are not welded to the liner after extrusion, but are extruded along with the sheet, so they are formed as part of the sheet during a continuous process. The mechanical bond formed when the liner is anchored to the manhole during the casting process means the protective lining is not dependent on a chemical bond between the concrete and dissimilar materials, which is the case in most such applications, including various coating products. It also went far beyond the capabilities of first-generation liners in the United States.

That first generation of liners used a T-shaped, continuous rib to help prevent the liner from pulling away from the concrete wall. However, all thermoplastics exhibit a phenomenon called "creep," which is a stretching of the material when it is under stress for long periods of time. Because this ribbing was unidirectional in the old products, it prevented any movement from rib to rib. However, when the liner became subject to forces such as heavy backpressure, the profile would become elongated over time. This would eventually make the liner slip out of the cavity, causing delamination. Though the liners have a history of about 35 years of good performance against corrosion when used in dry environments, they were never designed to be effective against backpressure delamination.

While the fact that the PP material could stand up against the most acidic conditions was well documented, U.S. Precast was determined to find out how well the anchoring system worked in conditions where a great deal of headpressure was generated.

"We conducted our own full-scale test on a manhole prior to actually marketing the product," Arabian says. "We constructed a full-size manhole with the Agru PP liner going across the floor and up the wall and cast in four half-inch pipes at 90 degrees going around the perimeter of the manhole. Those pipes went all the way through the wall thickness to the back of the liner. Then we created a manifold between the four pipes and ran a line from the manifold back to a 40-foot standpipe, filling the standpipe with water. This gave us 40 feet of head pressure working directly against the back of the liner, trying to delaminate the liner from the concrete it was cast into. Those anchors held for the entire year that we ran the test."

After the test, U.S. Precast was assured that it had designed a liner that would stand up under conditions much more demanding than the product would ever encounter in real-life installations.

"We felt very comfortable from that point forward that we had a product we could bring to the market that would be a solution to the dual problem of acid corrosion and standing up to a great deal of back pressure as well as the ensuing problem of water infiltration," Arabian says.

Simplicity and low-cost installation are also benefits of the new liner. "At the beginning of the Key West project," Arabian says, "we went to the liner supplier and said, 'Look, we want to reduce the cost of this to the end-user as much as possible,' and we talked with them about various ways it could be done."

One solution was to minimize the amount of field welding required during the installation process. So, U.S. Precast developed the "one-piece manhole." The base section is formed with a monolithic pour, which means the base slab and the walls are poured at the same time. An integral top slab is then cast to form a one-unit structure. The interior is lined across the floor, up the wall, and across the bottom of the top slab. The top slab has a 34-inch opening in it for the manhole entrance, which also is lined. During installation, the contractor merely extracts the old manhole, puts in the new one, and then ties the pipes back into the structure.

"In essence, the entire manhole structure becomes just a big fitting," Arabian says. "All the contractor has to do at that point is brick up the new ring and cover-to-finish grade, and install the chimney seal."

The chimney seal is a rubber accordion-type boot that locks into the inside of the manhole cast-iron ring. It interlocks on the bottom of the manhole opening in the top slab, spanning the brickwork and protecting the brickwork from any corrosion. It also serves to prevent inflow through the chimney.

"We have found virtually no difference between installing the pre-lined manholes and standard manholes," says Scott Simpson, project engineer for Lanzo Construction Company in Pompano Beach, Florida. "We started the first rehab project 18 months ago and installed a total of about 100 manholes. We have around 100 more manholes due for completion on this current project in August 2000. To date, we haven't experienced any problems out of the ordinary for this kind of project."

A short-term drawback that might await municipalities planning to install this type of manhole liner is the cost. Arabian estimates that the added material, labor and the minimal amount of field welding that might be required can roughly triple the price over a standard, unprotected precast concrete manhole. When viewed in a long-term context, however, the product is a tremendous money saver.

First, the cost of the manhole itself is negligible when compared with the cost of replacing corroded manholes. The liner manufacturer, Agru, says the life span of the product is 50 years. The city of Los Angeles and Los Angeles County have both tested the Agru product, with the city putting the Sure Grip liner in the Class I category, and determined the life expectancy of the liner to be 100 or more years.

The infrastructure problems municipalities are facing now are, in large part, due to their failing sanitary sewer systems caused by corrosion. They also are constructing new systems at an ever-increasing rate. To prevent this problem from happening every 15 or 20 years, lined precast manholes could prove to be a long-lasting solution.

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