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109 INTRODUCTION This appendix to NCHRP Project 20-05 Synthesis Topic 45-01, Service Life of Culverts, provides a summary of the most commonly accepted independent (i.e., not published by a pipe trade organization) quantitative service life calculation meth- ods for concrete and metal pipes. No known methods are in use to calculate the estimated material service life (EMSL) of thermoplastic pipes. The EMSL of thermoplastic pipes is based on the material performance specifications and details of the resins used in the pipe-manufacturing process. The materials are thus generally assigned a fixed EMSL regardless of the environmental parameters at the site. Thermoplastic culvert pipes for highway drainage applications are usually assigned EMSL values between 50 and 100 years. REINFORCED CONCRETE PIPE METHODS Concrete culverts are constructed in a large variety of round, elliptical, arch, and rectangular box sizes and have the ability to withstand a wide range of loading and environmental conditions. No definitive design methods estimate concrete culvert service life. As a result, the designer is required to make judgments about the severity of the environmental 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 or water have a pH of 5.5 or less, concrete pipes should be required to have extra cover over the reinforcing steel or a protective coating. Table B1 lists methods that can be used to determine EMSL values for reinforced concrete pipes. The EMSL values obtained using these different methods can vary widely and because no specific national guidance is available, each agency must select which EMSL value(s) to use for design from the range of available methods. The limitations and range of param- eters for which each method is applicable are described in detail for each method. TABLE B1 METHODS FOR DETERMINING EMSLS FOR REINFORCED CONCRETE PIPE Durability Method Reference Notes Ohio DOT Model Potter (1990) Based on large data set over a wide range of pH and size values. Includes an abrasive component. Hurd Model Potter (1990) Method developed for large-diameter pipes in acidic environments. Hadipriono Model Potter (1990) Method includes a wide pH range. Florida DOT Model Drainage ManualâOptional Pipe Material Handbook (FDOT 2012) Considers corrosion to be the only mechanism of degradation. Hurd Model The Hurd model was developed for use at sites with pH values of 7 or lower, and is given by the following equation: APPENDIX B Summary of Service Life Calculation Methods
110 where: EMSL = estimated material service life (years), pH = pH of the water, Slope = pipe invert slope (%), Sediment = sediment depth in pipe invert (inches), and Rise = vertical pipe diameter (inches). The Hurd model was developed for conditions where the pH is less than 7.0. For conditions where the pH is greater than 7.0, the primary degradation mechanism that forms the basis of the Hurd model was assumed to not occur. As such, for pH values greater than 7.0, the EMSL is reported to be conservatively estimated as a value less than the EMSL with a pH value of 7.0 (Potter 1988). Hadipriono Model The Hadipriono model is applicable to sites with pH values between 2.5 and 9, and is given by the following equation: where: EMSL = estimated material service life (years), pH = pH of the water, Slope = pipe invert slope (%), and Rise = vertical pipe diameter (inches). Ohio DOT (ODOT) Model The ODOT model comprises two separate equations, depending on the pH level. For pH values between 2.5 and 7: For pH values greater than or equal to 7: where: EMSL = estimated material service life (years),
111 pH = pH of the water, Slope = pipe invert slope (%), Sediment = sediment depth in pipe invert (inches), Rise = vertical pipe diameter (inches), Flow = velocity rating number (1 â rapid, 2 â moderate, 3 â slow, 4 â negligible, 5 â none), and K = abrasive constant (0.9 â without abrasive flow, 1.19 â with abrasive flow). Florida DOT (FDOT) Model The FDOT model includes a number of parameters, such as the concrete cover depth and specifications of the concrete mix design. The equation is given as where: EMSL = estimated material service life (years), C = sacks of cement per cubic yard, D = depth of concrete cover over reinforcing steel (inches), K = chloride concentration (ppm), W = total water percentage in the concrete mix (%), and S = sulfate content (ppm). This equation was developed for a 60-in.-diameter pipe. The adjustment factors shown in Table B2 must be applied depend- ing on the actual pipe size. TABLE B2 FDOT CONVERSION FACTORS FOR DIFFERENT-SIZED CULVERTS Pipe Diameter (in.) Factor Pipe Diameter (in.) Factor 12 0.36 48 0.76 18 0.36 60 1.00 24 0.41 72 1.25 30 0.48 84 1.51 36 0.54 96 1.77 42 0.65 108 2.04 Figure 6-4 (Figure B1) and Table 6-5 (Table B3) of the FDOT Optional Pipe Material Handbook (February FDOT 2012) illustrate the use of this equation and provide a chart showing the relationship between service life, chloride concentration, and pH.
112 FIGURE B1 Estimated service life versus pH and resistivity for 60-in.-diameter concrete culverts, S = 1,500 ppm (FDOT 2012). TABLE B3 ESTIMATED SERVICE LIFE VERSUS PH AND CHLORIDES FOR 60-IN.-DIAMETER REINFORCED CONCRETE CULVERTS AT 1,500 PPM SULFATE CONCENTRATION (FDOT 2012)
113 METAL PIPE METHODS The design service life 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. Galvanized Steel Pipe 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. Table B4 lists the methods that can be used to determine EMSL values for plain galvanized steel pipes. TABLE B4 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 (Caltrans 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 Con- struction 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 (FHWA 2008) Modification of California method. Increases 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 con- ditions with a limited data set. Florida DOT Method Florida DOT Optional Pipe Material Handbook (2012) Modification of California method to include a mini- mum steel thickness of 16 gage. NCSPA Recommendations Pipe Selection Guide (NCSPA 2010) Includes combined effects of corrosion and abrasion. Based on soil/water pH and resistivity. Service life of pipe considered to be until time of invert perforation. Utah DOT Method UDOT-IMP-76-1, Pipe Selection for Corrosion Resistance (Leatham and Peterson 1977) Result of Utah DOT study of 58 installations. The method considers corrosion alone through the follow- ing four parameters: minimum soil resistivity, pH, total soluble salts, and sulfate content. 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). 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. California Method A chart useful for application of the California method is presented in Figure B2. The following equations can also be used: For pH values greater than 7.3: For pH values less than 7.3: where R is the minimum resistivity (ohm-cm).
114 The resulting EMSL value must be multiplied by a factor depending on the gage thickness (Table B5). TABLE B5 GALVANIZED STEEL PIPE GAGE THICKNESS FACTORSâCALIFORNIA METHOD Gage 18 16 14 12 10 8 Factor 1.0 1.3 1.6 2.2 2.8 3.4 FIGURE B2 Chart for estimating years to perforation of steel culverts using California method (Caltrans 1999). AISI Method The AISI is very similar to the California method, with a different definition of the conditions that occur at the end of the useful service life. A chart useful for application of the AISI method is presented in Figure B3. The following equations can also be used: For pH values greater than 7.3: For pH values less than 7.3: where R is the minimum resistivity (ohm-cm).
115 The resulting EMSL value must be multiplied by a factor depending on the gage thickness (Table B6). TABLE B6 GALVANIZED STEEL PIPE GAGE THICKNESS FACTORSâAISI METHOD Gage 18 16 14 12 10 8 Factor 1.0 1.3 1.6 2.2 2.8 3.4 FIGURE B3 Chart for estimating average invert life using AISI method (AISI 1994). Federal Lands Highway (FLH) Method The Federal Lands Highway method is also a modification of the California method. A chart useful for application of the FLH method is presented in Figure B4. The following equations can also be used: For pH values greater than 7.3: For pH values less than 7.3: where R is the minimum resistivity (ohm-cm).
116 The resulting EMSL value must be multiplied by a factor depending on the gage thickness (Table B7). TABLE B7 GALVANIZED STEEL PIPE GAGE THICKNESS FACTORSâFLH METHOD Gage 18 16 14 12 10 8 Factor 0.8 1.0 1.2 1.7 2.2 2.6 FIGURE B4 Chart for estimating service life of plain galvanized steel using Federal Lands and Highway method (FHWA 2008). FDOT Method The FDOT method is also a modification of the California method. A chart and table useful for application of the FDOT method are presented in Figure B5 and Table B9, respectively. The following equations can also be used: For pH values between 7.3 and 9.0: For pH values between 5.0 and 7.3: where R is the minimum resistivity (ohm-cm).
117 The resulting EMSL value must be multiplied by a factor depending on the gage thickness (Table B8). TABLE B8 GALVANIZED STEEL PIPE GAGE THICKNESS FACTORSâFDOT METHOD Gage 18 16 14 12 10 8 Factor -- 1.0 1.3 1.8 2.3 2.8 FIGURE B5 Estimated service life versus pH and resistivity for 16-gage galvanized steel using FDOT method (FDOT 2012).
118 TABLE B9 DESIGN SERVICE LIFE VERSUS PH AND RESISTIVITY FOR 16-GAGE GALVANIZED STEEL CULVERT PIPE USING FDOT METHOD (FDOT 2012)
119 Utah DOT Method The Utah DOT method was published in 1977 and is based on a study of 58 pipe culvert installations that were evaluated for durability characteristics and assigned a pipe rating to aid in numerical analysis and correlation with environmental soil and water conditions. Minimum soil resistivity, pH, total soluble slats, and sulfate content are interdependent parameters affecting pipe corrosion. The Utah DOT method monograph is presented in Figure B6. FIGURE B6 Utah DOT material selection criteria for metal pipe (Leatham and Peterson 1977).
120 Additional Service Life Due to Coatings Additional service life due to protection by coatings is generally included by adding on a predetermined number of years to the calculated service life using one of the aforementioned methods. Predetermined service life add-on values depend on the abrasion characteristics and type of coating. Add-on service life year values can range from 10 to 80 years. The summary table (Table B10) from the Ministry of Transportation of Ontario (2007) provides an example for that agency of the allowable additional service life values used for various coatings. TABLE B10 EXAMPLE OF DESIGN GUIDELINES FROM THE MINISTRY OF TRANSPORTATION OF ONTARIO (2007) Aluminized Steel (Type II) Pipe FDOT Method The FDOT method for estimating material service life of aluminized (type II) steel can be applied using Figure B7 or Table B12. The following equations can also be used: For pH between 5.0 and 7.0: For pH between 7.0 and 8.5:
121 For pH between 8.5 and 9.0: where R is the minimum resistivity (ohm-cm). The resulting EMSL value must be multiplied by a factor depending on the gage thickness (Table B11). TABLE B11 ALUMINIZED STEEL (TYPE II) GAGE THICKNESS FACTORSâFDOT METHOD Gage 18 16 14 12 10 8 Factor -- 1.0 1.3 1.8 2.3 2.8 FIGURE B7 Estimated service life versus pH and resistivity for 16-gage aluminized steel type II using FDOT method (FDOT 2012).
122 TABLE B12 DESIGN SERVICE LIFE VERSUS PH AND RESISTIVITY FOR 16-GAGE ALUMINIZED STEEL CULVERT PIPE USING FDOT METHOD (FDOT 2012) Aluminum Pipe Estimates of service life for aluminum pipe can be made based on an FDOT method, applied through the use of Figure B8 or Table B13. The EMSL valued depends on the minimum resistivity, pH, and gage thickness. The end of useful service life is defined as the time to first perforation. No explicit equation was found for these relationships. 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 cor- rosion standpoint. It is therefore possible to use aluminum 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 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.
123 FIGURE B8 Estimated service life versus pH and resistivity for aluminum using FDOT method (FDOT 2012). TABLE B13 DESIGN SERVICE LIFE VERSUS PH AND RESISTIVITY FOR 16-GAGE ALUMINUM CULVERT PIPE USING FDOT METHOD (FDOT 2012)
