Bituminous Coating
The updates from version 7.02 to version 7.03 incorporate recent changes to HDM Chapter 850 to remove Bituminous Coatings to steel pipes by industry.
Polypropylene Pipe
The update from version 7.01 to 7.02 incorporates polypropylene pipe. Polypropylene pipe was added to section 64 of the Standard Specifications.
Polymerized Asphalt Coating
The updates from version 7.0 to version 7.01 incorporate recent changes to HDM Chapter 850 to address the removal of polymerized asphalt coating to steel pipes by industry.
Abrasion
The updates from version 7.0 to version 7.01 incorporate recent changes to HDM Chapter 850 to modify abrasion level definitions.
AASHTO Load Resistance Factor Design (LRFD)
The structural requirements within AltPipe Version 7.0 are based on the AASHTO Load Resistance Factor Design (LRFD) pipe update by Structures. AltPipe Version 7.0 was specifically developed to coincide with the publication of the 2010 Standard Specifications by Office Engineer and the associated 2011 Highway Design Manual (HDM) update by Headquarters. Projects using the 2010 Standard Specifications must use AltPipe Version 6.9.
For metal pipe see HDM Tables 856.3A - P. For plastic pipe the allowable cover values will be as follows:
"Corrugated HDPE - Type S" - 15 feet for all listed diameters
"Corrugated HDPE - Type C" - 5 feet for all listed diameters
"Corrugated PVC" - 35 feet for all listed diameters
Reinforced Concrete Pipe (RCP)
The updates included in AltPipe version 6.09 are rooted in SSP 65-010_E_A06-05-09.
The updates from version 6.08 to version 6.09 includes updates to how RCP is handled in AltPipe. Firstly, the default water-to-cement ratio has been changed from 0.35 to 0.40 in accordance with industry agreement. Essentially, this means that more water is allowed in the concrete mixture. This affects the water content, expressed in percent, that is associated with the # of sacks of cement that is user-specified. The method in which the service life is calculated is not affected. This updated is stated in Para 7 of SSP 65-010_E_A06-05-09.
Secondly, under corrosive or abrasive conditions, as defined by the instructions for Para 8 in SSP 65-010_E_A06-05-09 (pH > 4.5 AND/OR Chlorides > 2,000 ppm AND/OR Sulfates > 2,000 ppm), AltPipe outputs a Minimum Cementitious Material Content table, identical to the one shown in Para 8 of SSP 65-010_E_A06-05-09. This table gives six different minimum cover, cementitious material, and water-to-cement ratio combinations to the contractor to use in their RCP design. The outputs in the table are calculated for the six different combinations in the following manner: if the calculated service life is greater than or equal to the (user-input) desired service life for one combination, AltPipe outputs the amount of minimum cementitious material content, in lbs per cubic yard, in the respective box. If the calculated service life is less than the desired service life, AltPipe outputs "N/A" in the respective box, meaning the user-input conditions will not give the desired service life, and this combination is not recommended. This process is repeated for each combination.
AltPipe is a web-based tool that may be used to assist designers in the appropriate selection of pipe materials for culvert and storm drain applications. The computations performed by AltPipe are based on the procedures and California Test Methods described in Chapter 850 of the Caltrans Highway Design Manual.
AltPipe is not a substitute for the appropriate use of engineering judgment as conditions and experience would warrant. AltPipe establishes uniform procedures to assist the designer in carrying out the highway design functions of the California Department of Transportation, and is neither intended as, nor does it establish, a legal standard for these functions. Implementation of the results and output of this program is solely at the discretion of the user.
For statewide consistency it is generally assumed that pipes/culverts will respond as estimated by the program. However, if the designer has any reason to suspect that theconditions are outside the bounds the norm — this Read Me file will provide some basis for making this determination, but generally very high or low pH or minimum resistivity, a wide fluctuation in conditions or other severe climatic/environmental situation, e.g., high-salinity soils in dry climates would be suspect — Caltrans should contact their District Hydraulics Unit, Materials Unit or the Headquarters Office of Highway Drainage Design for assistance in applying site specific considerations into the estimated service life.
AltPipe updates and supersedes the CULVERT4 computer program that has been used for helping designers estimate service life for various corrosive/abrasive conditions. AltPipe includes a more comprehensive and user-friendly format combined with a wider range of estimating tools.
AltPipe incorporates current requirements from Chapter 850 of the Highway Design Manual (HDM) for the selection of alternative pipe.
All service life projections generated by AltPipe are simply estimates based on current knowledge and the general data basis underlying the formulas within the AltPipe calculator, as well as the accuracy of any input data (e.g., for some data such as pH, the tested value can vary significantly depending upon time of year of data collection). The results provided for service life are estimates rather than absolute values and should therefore be interpreted as such and "rounded" accordingly.
The corrosion process requires an electrolyte (generally moisture) and oxygen to proceed. Very dry areas (e.g., desert environments) have a limited availability of electrolyte, and totally and continuously submerged pipes have limited oxygen availability. These extreme conditions (among others) are not well represented by AltPipe, and some adjustment in the estimated service life for pipes in these conditions should be made.
Because failure can occur at any point along the length of the pipe (e.g. tidal zones), the designer must look at the conditions and how they may vary along the pipe length - and select for input into AltPipe those conditions that represent the most severe situation along the pipe's length.
Galvanized steel pipe consists of zinc coated steel pipe. AltPipe operates based on some fairly basic assumptions for corrosion and minimum resistivity that are part of California Test 643 (PDF).
Contrary to the discussion in HDM Index 852.3 (2) Durability (PDF), on achieving design service life by first considering protective coatings prior to increasing the metal thickness, Altpipe will list all viable alternatives for achieving design service life.
Where enhanced soilside corrosion protection is needed, refer to HDM Topics 855.3 and 855.4
Structural Steel Plate: It is the opinion of the Corrosion Technology Branch that the thicker galvanized coating on SSP offers no significant additional corrosion protection and does not significantly increase the design life of the culvert, especially in culverts where the invert is exposed to abrasion. AltPipe evaluates the service life of SSPP in relation to corrosion the same way as it does with all other types of steel pipe and according to California Test 643. If thickness for structural requirements is inadequate for abrasion potential, it is recommended to apply the increased thickness to the lower structural plates of the pipe only.
In non-abrasive environments, aluminum, and the aluminum coating provided by Aluminized Steel (Type 2) pipe, corrodes differently than steel and will provide adequate durability to meet the 50-year service life criterion within the acceptable pH range of 5.5-8.5 and minimum resistivity greater than 1500 ohm-cm without need for specifying a thicker gauge or additional coating, whereas under the same range galvanized steel may need a protective coating or an increase in thickness to provide a 50-year maintenance free service life (with respect to corrosion).
Reinforcing steel can be expected to respond to corrosive environments similarly to the steel in CSP.
Waters with high chloride concentrations (e.g., marine environments) can also lead to corrosion of reinforcing steel. However, properly designed and installed RCP (i.e., minimal cracking due to handling/construction loading) will typically provide adequate concrete coverage over the reinforcing steel to provide protection to the steel, except under extreme conditions. In relatively severe acidic, chloride or sulfate environments (either in the soil or water) as identified in the project Materials or Geotechnical Design Report, the means for offsetting the effects of the corrosive elements is to either increase the cover over the reinforcing steel, increase the cement content, or reduce the water/cement ratio. The identified constituent concentration levels should be entered into AltPipe to verify what combinations of increased cover, increased cement content, or water/cement ratio reduction will provide the necessary service life (typically 50 years). AltPipe is specifically programmed to provide RCP mix and cover designs that are compatible with industry practice, and are based on their agreements with Caltrans.
Where the measured concentration of chlorides exceeds 2000 ppm for RCP that is placed in brackish or marine environments and where the high tide line is below the crown of the invert, the AltPipe input for chloride concentration is modified to 25,000 ppm.
Contact the District Materials unit or the Corrosion Technology Branch in DES for design recommendations when in extremely corrosive conditions. Non-reinforced concrete pipe is not affected by chlorides or stray currents and may be used in lieu of RCP with additional concrete cover and/or protective coatings for sizes 36" in diameter and smaller. However, non-reinforced concrete pipe is not included in AltPipe. See HDM Index 852.1(4) (PDF) for pre-cast non-reinforced concrete pipe. Where conditions occur that RCP designs as produced by AltPipe will not work, the Office of State Highway Drainage Design within the Division of Design should be contacted.
Abrasion is the wearing away of pipe material by water carrying sands, gravels and rocks (bed load) and the rate at which it occurs is dependent upon size, shape, hardness and volume of bed load in conjunction with volume, velocity, duration and frequency of stream flow in the culvert. All types of pipe material are subject to abrasion and can experience structural failure around the pipe invert if not adequately protected. AltPipe uses six abrasion levels that are defined in HDM Table 855.2A (PDF)
The Abrasion Table in HDM Table 855.2A (PDF) can be used as a "preliminary estimator" of abrasion potential for material selection to achieve the required service life, however, it uses only some of the numerous factors that contribute to abrasion; bed-load size and volume, and flow velocity. Other factors that are not used in the table should also be carefully considered. For example, under similar hydraulic conditions, heavy volumes of hard, angular sand may be more abrasive than small volumes of relatively soft, large rocks. Furthermore, two sites with similar site characteristics, but different hydrologic characteristics, i.e., volume, duration and frequency of stream flow in the culvert), will probably also have different abrasion levels. AltPipe assesses abrasion primarily by velocity and it is left to the user to adjust the service life estimate to account for bedload size, volume and angularity. The flow velocities AltPipe utilizes are based on typical intermittent flows generated by the 2-5 year return frequency flood, and not a 10 or 50-year event. For assistance in determining abrasion characteristics of a site, or when the abrasion level is severe, designers should contact their Hydraulics Unit, Materials Unit or the Headquarters Office of Highway Drainage Design.
The most common mode by which steel culverts fail is by perforation of the invert due to the effects of corrosion and/or abrasion. This failure mechanism typically occurs after the galvanizing (zinc coating) or aluminizing and any other specified protective coatings- all relatively soft materials, have been worn away by the harder stream bedload leaving the remaining steel exposed to the effects of corrosion and abrasion on the waterside of the invert. Aluminized Steel (Type 2) can be considered equivalent to galvanized steel for abrasion resistance.
The abrasion potential is dependent upon, velocity, size, shape and hardness of bedload, i.e., for velocities greater than 5 ft/s, aluminum pipe may require additional gauge thickness for abrasion or concrete invert protection if thickness for structural requirements is inadequate for abrasion potential per HDM Table 855.2A (PDF). However, AltPipe will not do this automatically for the non-abrasive levels 1 through 3 for which Aluminum pipe may be considered. Aluminum pipe is not recommened for abrasion levels 4 through 6. Aluminum may display inferior abrasion characteristics than steel in non-corrosive environments, however.
Abrasion potential for any concrete lining is dependent upon the quality, strength, and hardness of the aggregate and density of the concrete as well as the velocity of the water flow coupled with abrasive sediment content and acidity. Concrete pipes will counter abrasion through increased cover over the steel reinforcement, i.e., by adding additional sacrificial material. RCP is typically not recommended in abrasive flows greater than 10 feet per second (i.e., abrasion levels 4 through 6) but may be considered for higher velocities if the bedload is insignificant (e.g. storm drain systems and most culverts smaller than 30 inches in diameter or abrasion levels 1 through 3. See HDM Index 852.1 (PDF). The designer is encouraged to evaluate existing RCP in similar environments and discuss the thickness of sacrificial material needed to counter abrasion with their District Materials Unit to refine their service life estimate.
RCB is not included in AltPipe. However, there are significantly less limitations involved in increasing the invert thickness of RCB in the field verses increasing minimum thickness over the steel reinforcement of RCP in the plant. Therefore, RCB may be used for abrasion levels 1 through 5.
HDM Table 855.2 C has been incorporated into AltPipe. This table comprises of manufacturer applied coatings (like polymeric sheet) for steel pipe.
Not included in this table or AltPipe are contractor applied pavings (e.g. PCC in the invert of a CSP).
Besides coatings and contractor applied pavings for new pipe, there are numerous other liner and lining alternatives available to rehabilitate existing pipes that are not included in AltPipe such as plastic slipliners and Cured in Place Pipe (CIPP), both of which will provide good abrasion resistance and are not subject to corrosion effects. For further information see HDM Table 855.2A (PDF)
In addition to plastic slipliners, consideration should be given to other materials such as Polymer Mortar, fiber reinforced plastic, and other resin-based products such as Cured in Place Pipe (CIPP), which will all provide good abrasion resistance and are not subject to corrosion effects. However, these other lining materials are not included in AltPipe. For further information see HDM Table 855.2A (PDF) and Table 855.2F (PDF).
See HDM Table 851.2 (PDF) for Manning's n-Value for Alternative Pipe Materials.
AltPipe is purely a materials selection tool. It is the designer's responsibility to determine the culvert flow regime (i.e., inlet or outlet control) and size the barrel accordingly - particularly in outlet control conditions where the smoothness of the conduit may affect the diameter selected. Other hydraulic considerations which may be impacted by Manning's N-Value are:
For the maximum height of fill over RCP see Standard Plans A62D and A62DA. The following HDM table references the maximum height of cover for all other materials, most of which have been incorporated into AltPipe:
| Table | Material |
|---|---|
| 856.3A-C (PDF) | Corrugated Steel Pipe |
| 856.3D (PDF) | Corrugated Steel Pipe Arches (not in AltPipe) |
| 856.3E-G (PDF) | Steel Spiral Rib Pipe |
| 856.3H-I (PDF) | Corrugated Aluminum Pipe |
| 856.3J (PDF) | Corrugated Aluminum Pipe Arches (not in AltPipe) |
| 856.3K-L (PDF) | Aluminum Spiral Rib Pipe |
| 856.3M (PDF) | Structural Steel Plate Pipe |
| 856.3N (PDF) | Structural Steel Plate Pipe Arches (not in AltPipe) |
| 856.3O (PDF) | Structural Aluminum Plate Pipe |
| 856.3P (PDF) | Structural Aluminum Plate Pipe Arches (not in AltPipe) |
| 856.4 (PDF) | Thermoplastic Pipe (HDPE and PVC) |
HDM Table 856.5 (PDF) gives the minimum thickness of cover required for design purposes over pipe and arches. For construction purposes, a minimum cover of 6 inches greater than the roadway structural section is desirable for all types of pipe.
Where cover heights above culverts are less than the values shown in Table 856.5, stress reducing slab details available from the Headquarters Design drainage detail library may be used.
Where cover heights are less than the values shown in the stress reducing slab details, contact Office of State Highway Drainage Design or the Underground Structures Branch of the Division of Structures.
Coatings for metal culverts are designed to provide either a corrosion barrier (generally covering the entire periphery of the pipe) or a sacrificial layer of abrasive resistant material (generally concentrated in the invert of the pipe).
HDM Table 855.2 C (PDF) lists all of the plant-applied approved coatings for steel culverts and constitutes a guide for estimating the added service life that can be achieved based upon abrasion resistance characteristics only. Field application of a concrete invert lining, steel plate or other abrasion resistant linings can also be specified to increase service life due to abrasive conditions. See Index 4.5
Under most conditions, the galvanized coating on steel culverts provides adequate protection for a 50-year maintenance free service life pipe. However, for corrosive and/or abrasive conditions, the use of various protective coating products, used either individually or in combination may be required. The Department of Fish and Game (DFG) has approved the use of both polymeric sheet coating; however, DFG will restrict the use of bituminous coatings as discussed in the HDM. It should be noted that polymeric sheet coating was originally developed as a corrosive barrier although it can also provide additional protection from abrasion. The following table constitutes a guide for anticipated service life added to steel Pipe (in years) by abrasive resistant protective coating:
| Flow Vel. (ft/s) |
Channel Materials | Polymeric Sheet Coating (yrs.) | Polyethylene (CSSRP) (yrs.) |
|---|---|---|---|
| Non-Abrasive | * | * | |
| ≥ 1 – ≤ 8 (1) | Abrasive | 30 – 5 | * |
| > 8 – ≤ 12 | Abrasive | 5 – 0 | 70 – 35 |
| > 12 – ≤ 15 | Abrasive | ** | 35 – 8 *** |
| > 12 – ≤ 20 | Abrasive & Heavy Bedloads | **** | **** |
Where significant soil side corrosion and abrasion are present, a composite steel spiral rib pipe, which is externally pre-coated with a polymeric sheet, and internally polyethylene-lined, may also provide additional service life. HDM Index 853.2 (2) (a) (PDF) discusses these approved protective coatings and their application to protect against corrosion, abrasion, or both. Section 66-1.03 of the Standard Specifications outlines the requirements for the approved coatings.
Determining when a coating is needed, and what type to call for will depend on the results of the materials/geotechnical investigation and an assessment of the corrosive and abrasive potential of the site by the designer. Minimum resistivity, pH, sulfate and chloride concentrations; and the type, size and hardness of bedload materials can affect both durability and selection of an appropriate coating. In many cases, multiple coating types may be effective and as such the contractor should be given the option of selecting the most cost-effective of those that meet minimum service life requirements.
Where soil side corrosion is the only concern, polymeric coated steel pipe service life should be evaluated using Figure 854.3C (to determine steel thickness necessary to achieve 10-year minimum life of base steel), with the assumption that the (exterior) polymeric coating will provide additional protection to attain the 50-year service life requirement.
For locations where waterside corrosion and/or abrasion are of concern, coating products, like polymeric sheet can provide superior corrosive and abrasive resistant qualities.
Polyvinyl Chloride (PVC) Lined RCP is not listed in AltPipe. It is primarily used for protection from corrosion, but also provides some sacrificial abrasion resistance to RCP in lieu of additional cover and/or admixtures. PVC Lined RCP uses Polyvinyl Chloride sheet liners that cover three hundred sixty degrees (360°) of the interior surface of the pipe. It is not recommended for the extreme abrasion level as presented in HDM Table 855.2A
AltPipe does not select the appropriate pipe joint type.
Section 61 of the Standard Specifications set forth general performance requirements for transverse field joints in all types of culvert and drainage pipe used for highway construction.
Topic 854 provides guidance and Table 854.1 of the HDM indicates the alternative types of joints that are available for different arch and pipe installations.