Long Term Value with Polymer Protection
Three groups of field-applied polymers are predominantly used for manhole rehabilitation: polyurethanes, polyureas and epoxies. Despite the generic similarity of some polymers, they can exhibit varying degrees of success in the field due to the application methods used and expertise involved. To better ensure long-term success, specifiers must understand product and application system advantages and disadvantages in order to make an appropriate product selection.
Optimizing rehab investments requires a unique combination of proven product, proper application equipment, experience and manufacturer support. Many systems claim one or more of these fundamental ingredients for success, but few deliver all.
Evaluating and pre-qualifying both products and installers adds assurance for a successful outcome of any project.
Although hand tools are used during surface preparation and touch-up, most experienced and qualified applicators use manufacturer approved multi-component spray equipment when performing manhole rehab, achieving the highest productivity and optimum product performance.
Surfaces to receive any coating should be cleaned and abraded producing a sound surface with adequate profile and porosity to promote a strong bond between the coating and the substrate. Generally, this can be achieved with low or high-pressure water cleaning equipment capable of 3-5,000 psi. High-pressure water jetting, abrasive blasting, grinding or acid etching are also used. Detergents and sodium bicarbonate added during water blasting may assist in removing oil, grease or other hydrocarbon residues. Mild chlorine solutions are sometimes used to neutralize the surface and diminish bacteria growth.
Active infiltration must be stopped prior to applying any coating system that intends to form a long-term bond to the substrate. Coating-compatible hydraulic cements, pressure grouting and other special techniques are used to stop leaks. Typically, large voids and bugholes are filled with quick-setting materials including cement and epoxy grouts. Cracks and joints (especially between the frame and chimney) may require a flexible seal or sealant if movement is anticipated. Repair material manufacturers should be contacted to ensure their material is acceptable for topcoating.
Most manhole rehab needs stem from poor construction, loss of mortar or bricks, corrosion and infiltration. Trenchless experts have estimated that approximately 20% of the 15 million + manholes in the U.S. need rehabilitation, with the vast majority requiring non-structural solutions. With so many products available, how do you determine which product is right for your project and how it should be applied? It's not easy, but proper inspection and evaluation of the environment, structure, existing and potential service conditions will help narrow the list of appropriate products. The table lists typical manhole conditions and describes accepted uses of polymers prevalent in manhole rehab.
Because polymers have been specified for decades to protect steel and smooth above-grade substrates, many under-engineered specifications have been duplicated for underground structure rehabilitation. Some of the largest rehab coating failures occurred simply because a good product was applied incorrectly or too thin.
Industry experts agree that a minimum of 40 mils is needed to deliver an effective monolithic surface to smooth finished underground concrete. Rougher rehab surfaces will require the same 40 mils on top of the profile peaks, meaning the average thickness needs to be much thicker (ranging from 60-300 mils). To avoid the pitfall of unnecessarily over-specifying a structural solution when a manhole is sound, suggest a specification that lists minimum and average thicknesses (i.e. 125 mils average, 60 mils minimum) to provide the monolithic protection needed.
Although spray-applied polymers have been successfully used for more than 20 years to structurally rehabilitate manholes, many remain mystified when these relatively thin, high-strength polymers yield higher physical properties than traditional alternatives.
Design thickness for structural coating systems is based upon the ability of the coating product to mechanically bond to the existing structure and long-term tensile strength (generally <600psi for cementitious repair mortars and >2,000psi for structural polymer systems). A design thickness theory was published in the Fall 1995 issue of No-Dig Engineering (Vol. 2, No. 3) and can be used as a "guide in applying sound engineering principles". Based upon the Condition II design approach less than 0.055 in. (55 mils) of a polymer coating vs. 0.5 in. of cementitious repair mortar would be adequate to repair a 10-15 foot manhole with up to ½ inch of corrosion, minor cracks and voids, and <10% cross section distortion. However, most polymer systems are specified at 125+ mils for structural applications. Condition III manholes (where collapse is imminent) should be evaluated for specific recommendations regardless of rehab product type. Greater thicknesses and the introduction of reinforcing fabrics, chopped glass or other composite fiber technology may be recommended.
There are four basic ways to test a polymer coating when used in manhole rehab: vacuum and exfiltration testing, measurement of bond strength (ASTM D4541) and spark testing. A combination of these tests and physical inspection should verify a securely bonded monolithic lining.
The many advantages of polymer systems, including superior corrosion resistance and increased life expectancy, frequently deliver the best value for manhole rehabilitation. The secret is to choose the appropriate product and have it installed by a trained applicator with the right equipment. We call this an "engineered system" destined for long-term success!
Editor's Note: This article appeared in the September 1998 issue of Trenchless Technology magazine and is reprinted here with permission.
About the Author: Joanne Hughes is vice president of <%=company%>, Tulsa, OK.