Considerations for Selecting Manhole Repair Systems
As the nation's manholes become increasingly in need of repair, manhole rehabilitation has become more commonplace.
By Denis Pollak and Mike Oriol
As the nation's manholes become increasingly in need of repair, manhole rehabilitation has become more commonplace. Large cities may rehab dozens or even hundreds per year. Since it is much less expensive to rehab manholes than to excavate and replace them, it can be expected that manhole rehabilitation will continue to be a growing segment of the trenchless sector.
In order to produce the highest quality manhole rehabilitation, the designing engineer must combine his/her knowledge of the manhole conditions and available materials and technology. The engineer must use the present condition of the manhole to guide the overall rehab design. Defects must be defined and the main failure mode must be determined in order to select the appropriate rehab method. This aspect of the project is essential to the ultimate success of the project. A rehab with poor specifications or a material selection that does not properly match with the current manhole conditions is doomed to failure from the outset.
Fortunately, with recent advances in technology, engineers have a myriad of information available to them as they design manhole rehabilitation projects. Manhole inspection software can synchronize between GIS systems to provide users with the most up-to-date view of manhole and pipeline networks. A single database can contain links to manhole locations, GPS points, photos, traffic needs, flow conditions, real time data and maintenance history. All of this information gives the engineer and the contractor more information for the rehab, making them better equipped to do the job right and leaving less room for error.
Asking the Right Questions
There are many factors to consider when it comes to selecting the proper rehabilitation method for a particular manhole. The following questions must be answered before a final decision can be reached.
- What is the substrate condition? (Its tensile strength? Is it a bond inhibiting material?)
- What are the owner requirements (noise, vibration, dust)?
- What are the material requirements (adhesion, abrasion, chemical resistance)?
- What are the application conditions (dust, water, small spaces)?
Contractors who bid on the project need to be informed of any application constraints that exist within the manhole structure. Having a clear line of communication with the potential contractors will allow the bids to be as detailed and inclusive as possible. This will lessen the likelihood of costly change orders due to unexpected application constraints.
In addition, it is imperative to evaluate environmental conditions such as the gases that are present in the manhole structure, sewer flows, traffic control and bypassing criteria.
If there is an environment that can not be controlled, such as constant moisture, live flow, low oxygen levels or high levels of hydrogen sulfide (H2S), these factors will play a huge role in material selection. The needs of the manhole environment must also be taken into account, including protection from H2S, abrasion resistance, impact resistance and necessary life span of the coating or lining.
SpectraShield™ liner is a spray applied silicone modified polyurea system that is used to rehabilitate and protect wastewater structures such as manholes.
Once the conditions of the manhole have been determined, it is time to evaluate the various rehabilitation methods and decide which technique is best suited for the manhole's particular environment.
This stretchable epoxy coated polyester eliminates inflow and infiltration (I/I), provides structural reinforcement and can be installed without confined space entry on simple structures. Cured-in-place liners work best in locations where there is a potential for infiltration that can not be addressed in an effective amount of time. It is also a good option when increasing the structural integrity of the manhole is important. Cured-in-place liners provide a good monolithic surface that prevents water from entering the structure.
These coatings are a good option for manholes with varying degrees of concrete deterioration (0.5-inch to 2-inches). Cementitious coatings are appropriate for rehabilitation projects where bypass pumping or flow control is limited. These coatings do well with damp surfaces and the presence of moisture in the atmosphere. They typically require minimal surface preparation beyond high pressure and/or detergent cleaning. Additives, such as calcium aluminate can provide pH buffering properties and reduce the rate of concrete degradation in the presence of hydrochloric acid. In addition, some additives, such as fiberglass flakes, help protect against corrosion and increase structural integrity. Cementitious coatings with close to 100% calcium aluminate tend to give the best performance, as the portions of the coating that do not contain calcium aluminate are the weakest links.
Cementitious coatings can be spray applied, pumped and troweled or spin cast. Spin casting is an automated process that spins the coating material onto the wall. On the topside, there is a rig with a pump. On the bottom there is a spinning head that rotates.
Spin casting can provide uniform thickness down the manhole if the spin casting machine is guided through the manhole at a constant rate. However, spin casting can not be used where any bends or irregularly shaped portions of the structure are present. The benefit to spin casting is that it can be done without confined space entry for application. The coating will have to be troweled smooth if required by the specifications. (It is typically recommended that cementitious coatings have a smooth finish in order to reduce microbiological-induced corrosion).
Mechanical Seals, Inserts and Liners
The materials used for mechanical seals, inserts and liners range from PVC, fiberglass and HDPE. They are typically grouted in place after installation by applying mortar to ½ inch thickness and physically pushing the liner onto the surface. The mortar moves into the existing annular space and acts as the mechanism to lock the liner into place, providing the desired level of adhesion.
Mechanical seals, inserts and liners are a good option for larger diameter manholes. They provide a physical barrier against corrosion, and if applied properly, have records of longevity. They can, however, be more expensive than other rehabilitation methods.
Polymer Modified Coatings
Polymer Modified Coatings can be spray applied, trowel applied or spin cast. They are a good option for odd shaped structures and in manholes where the atmosphere can be controlled. A benefit to using 100% solids polymer modified coatings is that they do not require a super-smooth surface for good adhesion. Some polymer modified coatings can be applied up to 250 mils, reducing the concern about pinholes in the coating, due to voids in the concrete, which can occur with thinner film coatings. These coatings can get into nooks and crannies, especially when spray applied. Spray application allows for better coverage, as the gun can be set to account for the different types of angles that may exist in the manhole.
Flex-Seal Utility Sealant, available from Sealing Systems Inc., is a plural component aromatic urethane designed to prevent inflow/infiltration and to provide corrosion.
Polymer modified coatings require environmental controls and stringent surface preparation. Full-time inspection is also recommended. Most coatings of this type are moisture and temperature sensitive and can pinhole on concrete if the environment is not properly controlled.
A decision matrix can be helpful in determining the best rehabilitation approach. The matrix should include manhole conditions and needs, current information regarding available materials and technology, project management and quality control requirements, as well as inspection and grading protocol.
Choosing the appropriate material and technique for the rehab is vital to extending the life of the manhole, as well as ensuring cost efficiency. The selected method should provide the desired long-term solution to the problem, as well as be contractor-friendly with proven success in the field. Investigating case histories is an extremely important part of the selection process, as is working with a contractor who has experience with the chosen material and whose work has a proven track record.
Once the decision has been made regarding the appropriate rehabilitation method, it is time to begin preparing the manhole for application of the selected material. Surface prep is essential so that there is proper adhesion of the coating or liner to the substrate. Surface prep also removes surface contaminants and creates a surface profile suitable for adequate adhesion. In recent years, this requirement has become an even more important part of the rehabilitation process. Newer coatings tend to have less surface tolerance than coatings such as coal-tar and lead-based coating that were used in the past. The newer, more environmentally friendly coatings do not have natural-born adhesion to the substrate. Therefore, now there is more of a dependence on mechanical adhesion versus chemical adhesion, making surface prep all that more essential.
Sauereisen manufactures sealants, corrosion barriers and substrate repair materials for the protection and restoration of wastewater infrastructure.
Surface preparation methods include detergent washing/scrubbing, pressure washing, acid etching, grinding, abrasive blasting, and saw-cutting for coating terminations. The specific standards and surface prep methods depend on the substrate, the condition of the existing coating and the new coating that is to be applied. For example, if the concrete is in relatively poor condition, pressure washing and a light abrasive blast may be enough to create the proper surface profile. If it the concrete is in fairly good or new condition, more profiling may be necessary.
Another vital aspect of surface preparation is moisture control. Prior to the rehabilitation, moisture must be controlled with chemical grouts, water stop foams, gels and/or resins. Most coatings have an adverse reaction to water and do not have the ability to displace water from the concrete surface.
It is important to take care of any existing cracks even if water isn't present at the time of visual inspection. During the rainy season, there is the potential for water to infiltrate into the manhole. In fact, it may be best to conduct condition assessments during the rainy season to ascertain the worst case scenario.
When it comes to the application of the selected rehabilitation technique, it is important to have a qualified applicator who has years of experience with the selected system.
If the rehab method includes applying a coating, the applicator must know the manhole conditions and understand the substrate. For example, in a very small space, the coating cannot be spray applied; it must be applied using a trowel or spin cast. Some porous surfaces may require a primer to minimize off-gassing and pinholing. There may also be temperature control issues – the space may need to be heated or cooled to allow for adequate cure times following application.
Inspection and quality control are vital to ensuring high quality manhole rehabilitation. Throughout the whole process, it is highly recommended that a full-time inspector be present. There are many points throughout the process where problems can be fixed before the next step. Ongoing testing should be done to identify weaknesses that can be addressed during the construction process. Visual exams, water drop testing, holiday (pinhole) testing and tensile strength testing are several ways to ensure that any potential problems are fixed before the end of the project.
As far as manhole rehabilitation is concerned, there have been many advances in coatings that will allow for more choices in rehabilitation methods. Many epoxies and urethanes can now tolerate higher levels of moisture and lower temperatures. In addition, the technology behind today's coatings and liners has evolved; they typically perform quite well over time. These products, if applied properly, can withstand 40 to 50 years of use.
The construction of new manholes is also evolving. Concrete with corrosion inhibiting additives is being used for the construction new manholes. The use of plastics and HDPE materials is becoming commonplace and there seems to be a move away from concrete to these types of materials. Plastics and HDPE are inert and do not tend to deteriorate at the same rate as concrete when exposed to sanitary sewer environments. The hope is that these types of manholes will not need to be rehabilitated as often as those made of concrete.
Manhole lids are also becoming easier to manage. These new lids are built with materials just as strong as cast-iron, but much lighter in weight. Some even have hinges that pop open once unlocked. Thirty-six-inch barrels are becoming more standard as well. Many have a double ring set-up in which the inner ring can be removed to take a look inside the manhole, and the outer ring can be removed for confined space entry. User-friendly lids and larger barrels allow for safer entry into the manhole, which may lead to more frequent condition assessments and the discovery of minor problems before they turn into larger issues.
In addition, as manholes are now seen as a vital part of the country's infrastructure, more attention is paid to the construction of new manhole structures. The specifications are more detailed and testing, such as vacuum seal testing, can be performed to ensure that the manhole was installed correctly. This type of testing can pinpoint potential problem and allow for any defects to be fixed during the construction process.WW
About the Authors:
Denis Pollak is Condition Assessment Manager for V&A Engineering, an independent consulting engineering firm specializing in corrosion engineering, coatings system management, and condition assessment services. Pollak has more than 35 years experience in project management on water and wastewater projects. He was a founding member of the California Water Environment Association Southern Sections Collection System Committee and is a former President of the CWEA, San Diego Section. Mike Oriol, P.E., is a registered civil engineer in California and is the Coatings Division manager at V&A. His experience includes the condition assessment of pipelines, concrete structures, tank coatings, and design of coatings and linings systems for water and wastewater assets.