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54 Durability Evaluation Phase 2C of the Recommended Practice consists of dura- bility evaluations across all available material pipe types in the userâs pipe system inventory. 10.1 Overview of Approach Highway drainage pipe systems deteriorate over time due to in-service loading and environmental exposure. Processes such as abrasion and corrosion can lead to impairment of structural and hydraulic performance and reduce the service life of drainage pipe systems. A key requirement of a rational process to allow bidding of alternative drainage pipe systems is an ability to predict the service life of a drainage pipe system, that is, the EMSL. Different methods for estimating EMSL are available in the technical literature and there is no widespread consensus on the most accurate method for any given pipe material type. Different methods will provide different levels of accuracy depending on how similar the conditions are between the pipe systems being evaluated and the pipe systems and conditions included in the development of the method. Additional details regarding application of the recommended EMSL evaluation methods are provided in Appendix C. Highway drainage structures are designed with the goal of providing a minimum DSL. Different drainage pipe system materials respond to environmental conditions in different ways, and thus have different definitions for when the end of the service life is reached. For a design to be technically acceptable, the EMSL must be greater than or equal to the DSL. Durability performance of existing drainage structures in the same watershed or under similar environmental conditions may also be used as a guide to anticipated durability perfor- mance. An inspection program and data management system would facilitate the use of in-service durability performance results in the durability evaluation of new systems. Such com- parative evaluations are to be considered a complimentary approach, and should be used in conjunction with the quan- titative methods described in this chapter. Durability evaluation in the Recommended Practice is performed in the sequence shown in Figure 26. 10.1.1 Step 1aâPerform Abrasion Evaluation This step requires use of Table 2 in the Recommended Practice to determine what limitations, if any, on pipe material selection are a result of the abrasion level determination made in Phase 1. Abrasion potential is a function of several factors, including pipe material, frequency and velocity of flow in the pipe, and composition of the bedload. The most comprehensive abrasion evaluation methodology is the method developed by Caltrans (White and Hurd 2011). Caltrans defines six levels of abrasion for preliminary estima- tion of abrasion potential based on flow velocities and bedload characteristics. Only some of the more relevant factors are considered in Table 2 and additional factors may need to be considered when assessing abrasion potential. 10.1.2 Step 1bâPerform Baseline EMSL Evaluation Apply the appropriate service life prediction model for the specific pipe material type. While this topic is the subject of on-going research and refinement, the Recommended Practice relies on a range of prediction models that are currently in use, with the recognition that these will be improved over time as more agencies adopt alternative drainage pipe bidding systems and additional applied research is undertaken. Due to the complexity of different pipe materialsâ performance and associated deterioration mechanisms, not all current predic- tion models have the same degree of reliability and so caution must be exercised in their application. C H A P T E R 1 0
55 Figure 26. Durability evaluation procedure. Level Pipe Material Guidance 1 No restrictions on material types due to abrasion. 2 Generally, no abrasive resistant protective coatings needed for steel pipe. Polymeric, polymerized asphalt, bituminous coating, or an additional gauge thickness of metal pipe may be specified if existing pipes in the same vicinity have demonstrated susceptibility to abrasion and thickness for structural requirements is inadequate for abrasion potential. 3 Steel pipe may need an abrasive resistant protective coating or additional gauge thickness if existing pipes in the same vicinity have demonstrated susceptibility to abrasion and thickness for structural requirements is inadequate for abrasion potential. Aluminum pipe may require additional gauge thickness for abrasion if thickness for structural requirements is inadequate for abrasion potential. Aluminized steel (Type 2) not recommended without invert protection or increased gauge thickness (equivalent to galv. steel) where pH < 6.5 and resistivity < 20,000. 4 Steel pipe will typically need an abrasive resistant protective coating or may need additional gauge thickness if thickness for structural requirements is inadequate for abrasion potential. Aluminum pipe not recommended. Aluminized steel (Type 2) not recommended without invert protection or increased gauge thickness (wear rate equivalent to galv. steel) where pH < 6.5 and resistivity < 20,000 if thickness for structural requirements is inadequate for abrasion potential. Increase concrete cover over reinforcing steel for reinforced concrete box (RCB) (invert only). Reinforced concrete pipe (RCP) generally not recommended. Corrugated and high density polyethylene (HDPE) (Type S) limited to > 48" min. diameter. Corrugated HDPE Type C not recommended. Corrugated PVC limited to > 18" min. diameter. 5 Aluminum pipe not recommended. Aluminized steel (Type 2) not recommended without invert protection or increased gauge thickness (wear rate equivalent to galv. steel) where pH < 6.5 and resistivity < 20,000 if thickness for structural requirements is inadequate for abrasion potential. Closed profile and Standard Dimensional Ratio (SDR) 35 polyvinyl chloride (PVC) liners are allowed but not recommended for upper range of stone sizes in bedload if freezing conditions are often encountered, otherwise allowed for stone sizes up to 3 in. Most abrasive resistant coatings are not recommended for steel pipe. A concrete invert lining or additional gauge thickness is recommended if thickness for structural requirements is inadequate for abrasion potential. See lining alternatives below. Increase concrete cover over reinforcing steel for RCB (invert only). RCP generally not recommended. 6 Aluminum pipe not recommended. Aluminized steel (Type 2) not recommended without invert protection or increased gauge thickness (wear rate equivalent to galv. steel) where pH < 5.5 and resistivity < 20,000. None of the abrasive resistant protective coatings are recommended for protecting steel pipe. A concrete invert lining and additional gauge thickness is recommended. See lining alternatives below. Corrugated HDPE not recommended. Corrugated and closed profile PVC pipe not recommended. RCP not recommended. Increase concrete cover over reinforcing steel recommended for RCB (invert only) for velocities up to 15 ft/s. RCB not recommended for bedload stone sizes > 3 in. and velocities greater than 15 ft/s unless concrete lining with larger, harder aggregate is placed (see lining alternatives below). SDR 35 PVC liners (> 27 in.) allowed but not recommended for upper range of stone sizes in bedload if freezing conditions are often encountered, otherwise allowed for stone sizes up to 3 in. Source: Caltrans Highway Design Manual, Table 855.2A. Table 2. Recommended abrasion guidance (from the Recommended Practice).
56 10.2 Principles and Definitions DSL requirements are typically based on the type of high- way facility and the attendant difficulty of providing repair or replacement. Failure to understand and address the potential effects of aggressive soil/effluent conditions or highly abrasive bedload can shorten the actual service life of a culvert. The following are some of the influences that must be included in any estimation of service life: â¢ pH (hydrogen-ion concentration) of the surrounding soil, groundwater, and streamflow â¢ Resistivity, chloride, and sulfate concentrations in the soil â¢ Resistivity, chloride, and sulfate concentrations in the groundwater and streamflow â¢ Size, shape, hardness, and volume of bedload â¢ Volume, velocity, and frequency of streamflow in the culvert â¢ Material characteristics of the pipe barrel and any linings for coatings â¢ Anticipated changes in the watershed upstream of the culvert (such as development, mining, and logging activities) â¢ Possible effects of severe climates Tests performed on concrete pipe have generally shown excellent wear characteristics. Although high velocity flow will induce abrasion regardless of the size of bedload particles, tests performed on concrete pipe have shown that cobble and larger sizes will induce higher wear rates than sands and gravels. Larger rocks apparently impact with enough force to break away minute particles of the wall. The use of high quality aggregate (i.e., aggregate that is harder than the anticipated bedload hardness) in the concrete mix can greatly enhance the resistance to wear of the concrete. Likewise, manufacturing methods that lead to a denser concrete mix, such as roller com- pacted or spun concrete, or higher compressive strength con- crete can exhibit increased resistance to wear. Where velocities are known to be high and a bedload is present, additional concrete cover over the reinforcing steel is recommended. The presence of a very high or very low pH environment will accelerate the abrasive effects of any bedload conditions. Steel culverts are the most susceptible to the dual action of abrasion and corrosion, particularly where thinner walled pipes are used. Abrasion accelerates the normal rates of corro- sion by removing protective surface coatings (e.g., an applied protective coating or a previously corroded surface) and expos- ing fresh metal to renewed corrosion. Once the thin protective coating on a steel pipe is worn away, whether it is zinc or another substance, exposure to low resistivity and/or low pH environments can dramatically shorten the life of a steel culvert. Although aluminum culverts are occasionally specified to combat corrosion, plain aluminum is typically not recom- mended for abrasive environments because tests indicate that aluminum can abrade as much as three times the rate of steel. Abrasive effects are typically countered in metal pipes by using protective coatings, invert paving, added metal thickness, or a combination of these measures. Plastic culvert materials (both PVC and HDPE) exhibit good abrasion resistance. Since plastic is not subject to corrosion, it will not experience the dual action of corrosion and abrasion. Plastic pipes, like metal pipes, have relatively thin walls and thus the rate of wear must be carefully evaluated with the material thickness. The documented abrasive resisting capabilities of plastic pipe are primarily based on tests using small aggregate sizes (gravels and sands) flowing at velocities in the range of 2 to 7 ft/s. The effects of large bedload particles (cobbles and larger) and/or high velocity flows are not well defined as a result of limited data. Additionally, because of their more recent emer- gence as a culvert product, plastic pipes have generally not had rehabilitative strategies developed specifically for them. Some of the more popular current strategies (e.g., invert paving) are not effective with plastic pipes because of the smooth surface of the plastic and an inability to achieve a satisfactory bond. Although generally unpredictable from a design standpoint, there are other physical factors besides corrosion and abra- sion that can shorten the life of drainage culverts. The loss of structural integrity can sometimes be traced to a defect in the manufacture of the pipe, improper construction techniques, or the effects of a large storm event. More commonly, though, the loss of structural integrity occurs over many years and is related to such factors as soil piping, seepage, soil movement, scour, and backfill soil loss. These processes can gradually reduce the culvert strength and support and make it suscep- tible to catastrophic events such as floods. Plastic pipe materials are also subject to certain limiting conditions that are a less significant consideration in selecting other culvert types. Among these conditions are use under deep fills, extended exposure to sunlight (specifically ultraviolet radiation) for some types of plastic, and a higher potential for damage from improper handling and installation. Plastic pipe is also flammable. When used where the poten- tial for roadside brush fires is high, end treatments using con- crete headwalls or specially attached end sections will limit the possibility of fire damage. However, due to their light weight and corresponding ease of handling, plastic pipes lend themselves to installation in remote and/or hard to reach locations where other materials would not be suitable, and they usually have a more rapid installation than heavier products. Both PVC and HDPE come in ribbed and/or corrugated shapes and smooth, solid-wall profiles; however, culvert and storm drain applications will generally call for the higher strength attained by the ribbed or corrugated designs. Even with the higher strength plastic profiles, material creep is pos- sible, and care must be taken to provide a uniform, compacted backfill around the culvert. In flexible pipe installations, the
57 full strength of the culvert installation relies heavily on the support obtained from the backfill material. Minimum cover applications must also be evaluated carefully when selecting this material. 10.3 Concrete Pipe Systems 10.3.1 DSL The service life of reinforced concrete pipe is typically the period from installation until reinforcing steel is exposed, or a crack signifying severe distress develops. 10.3.2 Evaluation Methods The following table lists methods that can be used to deter- mine EMSL values for reinforced concrete pipes. The EMSL values obtained using these different methods can vary widely so the Recommended Practice selects the lowest EMSL value from the methods used. The limitations and range of param- eters for which each method is applicable are summarized in the following table: Methods for Determining EMSLs for Reinforced Concrete Pipe Durability Method Reference Notes Ohio DOT Model Potter 1988 Based on large data set over wide range of pH and size values. Includes an abrasive component Hurd Model Potter 1988 Method developed for large diameter pipes in acidic environments Hadipriono Model Potter 1988 Method includes wide pH range Florida DOT Model Florida DOT, Optional Pipe Materials Handbook, 2012 Considers corrosion to be the only mechanism of degradation Concrete culverts are constructed in a large variety of round, elliptical, arch, and rectangular box sizes and have the abil- ity to withstand a wide range of loading and environmental conditions. There are no definitive design methods for esti- mating concrete culvert service life. As a result, the designer is required to make judgments about the severity of the envi- ronmental conditions and the offsetting nature of a variety of design accommodations. One method of accommodating a harsh environment is the addition of extra sacrificial concrete cover over the reinforcing steel. Typically, where severe abrasion is anticipated, at least 2 in. of additional concrete cover is recommended. Sulfate-resisting concrete or high density concrete should be used where acids, chlorides, or sulfate concentrations in the surrounding soil or water are detrimental. Generally, if soil and/or water have a pH of 5.5 or less, concrete pipe should be required to have extra cover over the reinforcing steel or a protective coating, and cast-in-place pipe should not be used. Additional concrete cover is also used to protect reinforcing steel in reinforced concrete pipe in situations where they are exposed to aggressive soils or water. The concrete cover can be increased to inhibit moisture from penetrating to the steel. 10.4 Steel Pipe Systems The design of metal culverts starts with the selection of the proper thickness to handle the loading conditions. Where corrosion and/or abrasion are expected, design charts and empirical data are used to determine if additional metal thick- ness (heavier gage) or some type of protective coating will extend the service life to an acceptable range. With perhaps the greatest variety of shapes and sizes available, including round, elliptical, and arch, there will typ- ically be some metal culvert to fit a given culvert installation. Since metal culverts have been in service for many years, its history of use has enabled researchers to develop relatively well-defined parameters to govern its use and estimated life. Currently, there are more well-defined methods to estimate the service life of steel culverts than any other type of material. Unfortunately, these existing methods deal much more with the potential effects of corrosion than abrasion. The basic assumptions used to determine service life for standard metal pipes may also be extended to metal structural plate pipes (AASHTO M 167/M 167M for steel and M 219/ M 219M for aluminum). One advantage of metal plate is the ability to specify thicker plates for installation in the invert of the structure while keeping the rest of the plates thinner (meeting structural loading requirements only) for economy. This provides greater protection where corrosion and abrasion will typically be most severe. Protective coatings have been used for many years primarily to protect steel culverts against the effects of corrosive action. Only recently have products become available that exhibit ade- quate bonding and wearability characteristics that make them attractive for abrasion resistance. Selection of an appropriate coating will require consideration of the pH and resistivity ranges to be encountered (both on the soil and water side of the culvert) and the potential for abrasion. Soil side protection of culverts will often provide up to 25 years of additional service life where conditions are not unduly severe. However, where the primary concern is on the water side, due to the dual action of abrasion and corrosion, additional service life may be as little as 1 to 2 years. Often, a combination of protective coatings is used to increase the expected years of life. Any applied coating is only as good as its bond with the base culvert material. Zinc galvanizing consists of the application of a thin layer of zinc to the steel by a hot-dip process. This most common
58 protective coating is not particularly abrasion resistant and has been shown to provide corrosion protection primarily when the site pH is within a range of 5.5 to 8.5. Similar to galvanizing, aluminizing is the hot-dip appli- cation of a thin layer of aluminum to both sides of the steel sheet. Unlike galvanizing, the aluminizing process (Type 2 onlyâconforming to AASHTO M 274M) creates an alloy layer between the exterior aluminum and the steel. The result is a protective coating with abrasive resistant characteristics that are similar to zinc galvanizing. From a corrosion resistance standpoint, aluminized steel pipe (Type 2) is typically recommended for use when pH values are between 5.0 and 9.0, and resistivity is above 1,500 ohm-cm. Recent data from industry evaluations of in-field performance of culverts with over 40 years of service verify that, within the prescribed environmental limits, aluminized steel pipe (Type 2) can provide effective corrosion resistance. 10.4.1 DSL The DSL of corrugated metal pipes will normally be the period in years from installation until deterioration reaches the point of either perforation of any point on the culvert or some specified percent of metal loss. Different methods used to estimate service life use different definitions of service life. 10.4.2 Evaluation Methods 10.4.2.1 Galvanized Steel Pipes A number of methods are available for estimating the EMSL of galvanized steel pipe. The California method is the most widely accepted and is recommended for use if no state- or location-specific research is available that indicates another method is more suitable. The other methods are modifications of the original California method. The following table lists the methods that can be used to determine EMSL values for plain galvanized steel pipes: 10.4.2.2 Aluminized Type 2 Steel Pipe The following table lists the methods that can be used to determine EMSL values for aluminized Type 2 steel pipes: Methods for Determining EMSLs for Plain Galvanized Steel Pipe Durability Method Reference Notes California Method California Test 643, Method for Estimating the Service Life of Steel Culverts, 1999 Includes combined effects of corrosion and abrasion. Based on soil/water pH and resistivity. Service life of pipe considered to be until time of first perforation. American Iron and Steel Institute (AISI) Method Handbook of Steel Drainage and Highway Construction Products, AISI, 1994 Modification of California Method. Service life of pipe considered to be until 25% thickness loss in the invert. Federal Lands Highway Method Federal Lands Highway, Project Development and Design Manual, 2008 Modification of California Method. Increase the EMSL by 25% after first perforation. Colorado DOT Method CDOT-2009-11, Development of New Corrosion/ Abrasion Guidelines for Selection of Culvert Pipe Materials, 2009 Calibration of California Method to state-specific conditions with a limited data set. Florida DOT Method Florida DOT Optional Pipe Materials Handbook, 2012 Modification of California Method to include a minimum steel thickness of 16 gage. Methods for Determining EMSLs for Aluminized Type 2 Metal Pipe Durability Method Reference Notes Florida DOT Method Florida DOT Optional Pipe Materials Handbook 2012 Based on anticipated soil/water pH and resistivity 10.5 Aluminum Pipe Systems The following table lists the methods that can be used to determine EMSL values for aluminum pipes: Methods for Determining EMSLs for Aluminum Pipe Durability Method Reference Notes Florida DOT Method Florida DOT Optional Pipe Materials Handbook 2012 Based on estimated corrosion rates due to pH and resistivity When installed within acceptable pH and soil resistivity ranges (typically 4.0 to 9.0 and > 500 ohm-cm, respectively) aluminum pipe (AASHTO M 196/M 196M) can provide a significant advantage over plain, galvanized steel pipe from a corrosion standpoint. It is therefore possible to use alumi- num pipe in lieu of a thicker walled or coated (and thus more expensive) steel pipe. Because aluminum is softer than steel, it is more susceptible to the effects of abrasion. This is particularly true for higher velocity flows that produce a scraping action, as opposed to lower velocity flows that allow the bedload to roll over the
59 culvert surface. Where high velocity flows (15 ft/s or greater) carrying a bedload are prevalent, use of aluminum should be carefully evaluated. As with all metal pipes, invert loss is caused by a combination of abrasion and corrosion and, thus, the severity of both conditions must be considered. 10.6 Thermoplastic Pipe Systems The most commonly used thermoplastics are PVC and HDPE. These materials are largely resistant to the chemical and corrosive elements typically found in soils. Empirical data regarding the durability of thermoplastic pipes is limited when compared with the data available for pipe material types that have longer histories of service. Slow crack growth and oxidative/chemical failure have been identified as the primary long term failure mechanisms for corrugated HDPE pipes, but no methods based on service histories have yet been developed for serviceable life predictions for these materials. The long term performance of thermoplastic pipes is highly dependent on the quality of the installation. Estimated ser- vice lives assume that pipes are installed in compliance with specifications and that such compliance is confirmed by post- installation inspection. Agencies typically assign an estimated service life of between 50 and 100 years for thermoplastic pipes manufactured in accordance with the relevant AASHTO standards and installed in accordance with relevant specifications. 10.7 HDPE Pipe Systems 10.7.1 DSL The service life of thermoplastic pipe may be considered at an end when excessive cracking, perforation or deflection has occurred. Generally constructed with helical, annular corrugated, or ribbed profiles according to either AASHTO M294 or ASTM F894, HDPE pipes are available in round configurations only. Due to its ability to withstand corrosive attack, HDPE pipe has found wide use in mining applications and other severe environment locations. Where HDPE has its major weakness is where high temperatures are of concern. Material creep can occur where water temperatures exceed 140Â°F (extremely rare for culvert or storm drain applications). HDPE compounds used in pipe manufacture are combined with ultra-violet inhibitors that help retard ultraviolet degradation and are not subject to the long term exposure problems that some other plastics can experience. Joints for HDPE pipe are available in both band type (split couplers) and tongue and groove designs. It is often recom- mended that a factory applied neoprene gasket or construction applied filter fabric wrap be used at the joints to enhance the soil tightness of the split coupler design. 10.8 PVC Pipe Systems 10.8.1 DSL The service life of thermoplastic pipe may be considered at an end when excessive cracking, perforation, or deflection has occurred. Generally similar in application to the more commonly used HDPE pipe, PVC pipe (AASHTO M304) is equally resistant to corrosive environments and only slightly more susceptible to abrasion, particularly where pH is very low (<4). How- ever, considering the typically greater pipe wall thickness of PVC compared with HDPE, service life can be equal if not longer. Typically available in a ribbed, corrugated, or profile wall (either open or closed cell) design, PVCâs higher stiffness will generally allow its use under deeper embankments than HDPE pipe of similar configuration. PVC pipe for culvert and storm drain applications is available only in round configurations. PVC pipe products do not usually incorporate a high level of ultraviolet light inhibitors; thus, they can be susceptible to long term breakdown when continuously exposed to sunlight. Typically, this translates to brittle material (impact resistance is reduced, but tensile strength is only minimally affected) at exposed culvert ends, and is one reason why PVC is more popular in storm drains than in culvert applications. Exposure issues can be overcome to a large degree if concrete endwalls are used or where corrugated metal pipes are used at the exposed culvert ends. PVC pipe will also become brittle from exposure to cold (less than 37Â°F) temperatures. This requires that extra care be taken when handling the pipes if installations will take place during the winter season. 10.9 Other Rigid Pipe Materials The following other (non-concrete) rigid pipe materials are used to a lesser extent in highway drainage practice for new installations and/or are present in historic in-service inventories: â¢ Ductile iron pipe â¢ Fiberglass pipe â¢ Vitrified clay pipe The EMSL values for these materials can be established by past performance history or by application of the above listed methods for pipes with equivalent component materials. In the absence of reliable prediction models, it would be prudent to assign conservative EMSL values, in consultation with the pipe suppliers, until further research and documented case
60 studies are available or until evaluation methods become avail- able and widely accepted. 10.10 Other Flexible Pipe Materials The following other (i.e., non-metal, HDPE, or PVC) flexible pipe materials are used to a lesser extent in highway drainage practice for new installations and/or are present in historic in-service inventories: â¢ Metal reinforced HDPE pipe â¢ Polypropylene pipe â¢ Fiberglass pipe (can be rigid or flexible dependent on installation) The EMSL values for these materials can be established by past performance history or by application of the above listed methods for pipes with equivalent component materials. In the absence of reliable prediction models, it would be prudent to assign conservative EMSL values, in consultation with the pipe suppliers, until further research and documented case studies are available or until evaluation methods become avail- able and widely accepted. 10.10.1 Step 2âAdd-On Additional Service Life Due to Protective Measures Coatings and or invert protection are often applied to culvert pipes (predominantly to metal pipes) to increase their service life. Many different coatings exist, the main types of which are listed below: â¢ Asphaltic/Bituminous â¢ Fiber-bonded bituminous â¢ Asphaltic mastic â¢ Polymerized asphalt â¢ Polymeric sheet â¢ Concrete Guidance on the additional service life due to the applica- tion of coatings on corrugated steel pipes can be found in the most recent version of the National Corrugated Steel Pipe Association (NCSPA) Pipe Selection Guide (NCSPA 2010). Predetermined service life add-on values depend on the abra- sion characteristics and type of coating. Add-on service life year values typically range from 10 to 80 years. A table sum- marizing recommended service life add-ons for supplemental pavings and coatings recommended by the metal pipe industry (NCSPA 2010) is shown in Figure 27. 10.10.1.1 Asphaltic Coatings Several different types of asphaltic-based coatings are cur- rently being used. Most do not provide extensive protection against abrasion but can be applied to both metal and concrete culverts. Coating thickness is typically measured over the inner crests of the corrugations on metal pipe. Because of the limited abrasion resistance, these coatings provide their greatest ben- efit where soil side corrosion is the most likely item of concern or where bedload is not present. Besides limited abrasion resistance, most asphalt coatings experience problems where the culvert is exposed to sunlight. Ultraviolet rays and temperature extremes often result in the development of cracks that expose the bare metal and eventually break the bond of the coating. However, asphalt does not appear to be affected by the various ranges of pH typically encountered in culvert installations. Asphalt coatings can be flammable. Where the risk of fire is high, concrete end walls or other âinsulatingâ end treatments should be considered. All asphalt coatings require that special Figure 27. NCSPA recommended values for service life add-ons provided by supplemental pavings and coatings (NCSPA 2010).
61 care be taken during the application process. The pipe must be cleaned thoroughly and brought to an elevated temperature to ensure proper bonding to the culvert. Special care should also be taken during shipping and installation to ensure that the coating is not damaged or removed. 10.10.1.2 Bituminous The most common asphalt coating is the hot-dip application (ASTM A 849) of bituminous material (AASHTO M 190M). This type of coating often covers the entire inside and out- side of the culvert and provides corrosion protection. Typical minimum application thickness is 0.05 in. This typical appli- cation provides very little protection against abrasion, and where flow velocities exceed 6.5 ft/s will provide almost no additional service life. To improve the abrasive characteris- tics of bituminous coatings, the addition of extra thickness of bituminous material over the entire inside (bituminous lining) or only the invert area (bituminous invert paving) may be specified. This type of treatment will typically involve asphalt paving to provide a minimum thickness of 1â8 in. above the crest of the corrugations for at least 25 percent of the circum- ference of round pipe and 40 percent of the circumference for pipe arch. Due to both the air quality concerns over the hot-dipping process and the water quality concerns related to bitumen impact on fish habitat, some regulatory agencies have placed restrictions on the use of bituminous coatings, and their use in practice is decreasing. 10.10.1.3 Fiber Bonded Bituminous To create better bonding characteristics so that the bitu- minous coating will better withstand severe environments, a fiber mat is embedded into molten zinc galvanizing while it is being applied to steel sheets. Asbestos has been used as the fiber material but is generally being replaced with newer materials, such as aramid (ASTM A 885). Bituminous material is then applied in the standard fashion, developing a strong bond with the protruding fibers. Although still not highly resistive to abrasion, this process does enhance the corrosion resistance of metal pipes in severe conditions. Marine environments are typical of the conditions that can make fiber bonded pipe cost-effective. 10.10.1.4 Asphalt Mastic Asphalt mastic (AASHTO M 243M) is typically not used in conjunction with lining or invert paving. Asphalt mastic can be substituted for bituminous coatings and is applied (ASTM A 849) to the same minimum thickness with a spray application. Like bituminous coatings, there are environmental concerns regarding its use and abrasion resistance is minimal. 10.10.1.5 Polymerized Asphalt Polymerized asphalt (ASTM A 742/A 742M) is primarily an abrasion resistive coating that will provide some corrosion resistance benefits for metal pipes. Applied in a hot-dip pro- cess (ASTM A 849) to a minimum thickness of approximately 0.05 in., polymerized asphalt is applied to only a 90 degree portion of the pipe that is centered about the invert. Independent testing has indicated a service life extension of several times that of bituminous coatings. Since only a portion of the pipe is coated, extensive soil side corrosion concerns, continuous immersion, or use near saltwater environments may pose problems. However, the polymerized asphalt is compatible with other asphalt coatings in combination and has received acceptance from some environmental regulatory agencies. 10.10.1.6 Polymeric Sheet Coating Protection to metal culvert pipes can be provided by poly- mer coatings, which have good corrosive resisting properties. A laminate film is applied over the protective metallic coating (typically galvanizing) and is generally 10 to 12 mils thick (0.01 to 0.012 in.). The coating is often applied on both sides of the pipe (water and soil sides) but can also be applied to only one side, and is applied to the steel prior to corrugating. Polymer coatings also typically provide more abrasion resis- tance than bituminous coatings provide. Independent studies of the durability of these coatings are not available and guidance on the use of polymeric coatings is given by industry and trade groups representing manufac- turers and suppliers of polymer coatings. The NCSPA 2012 report on the performance of polymer coated steel pipes does however present performance inspection data across a wide range of environments from studies conducted in parallel with a number of state agencies. pH, resistivity, and abrasion level (FHWA) are typically used to determine the most appropriate coating type for a speci- fied service life. Polymer coatings are not recommended for use in applications where the FHWA abrasion level is greater than 3. One drawback of polymer coatings is that they are suscep- tible to damage from impacts and gouging, with the damage to the coating typically occurring during construction and installation. Where damage has occurred, the pipe wall will not be supplementarily protected leading to localized increased rates of corrosion. Corrosion will typically not spread away from the area of initial localized damage. A solution to this problem is to apply a touch-up after construction, however,
62 the quality and consistency of these repairs remains a concern for many agencies. 10.10.1.7 Concrete Coatings Primarily used with metal culverts (ASTM A 849) to act as sacrificial material for abrasion resistance, concrete can be placed in the invert area of the pipe to a thickness of between 3 and 6 in. The thickness and width of coverage are variable, based on typical flow depth and anticipated abrasive potential. Although the concrete may be placed directly against clean pipe material, steel reinforcing bars or wire fabric is often welded to the metal pipe before concrete placement. 10.10.2 Step 3âSelect Final EMSL This step requires selection of EMSL Values for use in design evaluations. Where more than one method of estimated EMSL is used, to allow for automation in the process, the Recommended Practice selects the lowest EMSL values for use in design. 10.10.3 Step 4âCompare EMSL to DSL The EMSL design value obtained from the previous step is then compared with the DSL. If the EMSL is greater than the DSL, then the pipe option is determined to be acceptable from a durability standpoint. If the EMSL is less than the DSL, the pipe option does not meet the durability evaluation criteria and is eliminated. Failure of individual pipe systems to meet durability require- ments will not disqualify entire pipe classifications, as other similar pipe system options that provide higher EMSL values based on increased wall thickness, additional/different coat- ing, improved concrete mix design, or other factors will be independently assessed against the DSL. No currently available method provides the designer with an exact estimate of service life. One of the best ways to esti- mate service life is to investigate existing drainage facilities near the project site. Unless upstream watershed charac- teristics have been altered since existing culverts have been installed, to include new aggressive conditions, investigations that show a particular pipe product has successfully met or exceeded its DSL (or has shown such minor deterioration over a lesser period of years to indicate the capability of attaining or surpassing the DSL) in a like environment will give the designer more useful information than other service life analyses. Service life can also be affected by debris damage or ero- sion caused by major storm events, improper manufacture or handling of the culvert, and incorrect laying or backfilling of the culvert. These issues may often be the cause of culvert distress or failure, but are difficult to predict and are not cur- rently accounted for in estimating service life.