Guidelines for Culvert Selection

The following general guidelines from the Federal Lands Highway (FLH) manual should assist in determining appropriate culvert material types and necessary coatings. Other methods are available. Many state departments of transportation and local governmental agencies have published durability criteria, and this information should be used where available. A materials engineer should be consulted for important applications. Of course, the final selection must provide for structural requirements as discussed in Art. 5.8.

Concrete Pipe. Where the pH is less than 3.0 and the resistivity is less than 300 U • cm, reinforced concrete pipe should not be specified. If the sulfate concentration exceeds 0.2 percent in the soil or water, type V cement should be specified. If the sulfate con­centration exceeds 1.5 percent in the soil or water, an increased cement ratio using type V cement should be specified. The concrete cover over the reinforcement or the cement factor should be increased where there is severe abrasion.

Table 5.11 gives the minimum water side pH permitted for a concrete pipe culvert to obtain either a 50- or 75-year design service life. The table is based on research con­ducted by the Ohio Department of Transportation. Pipe size, barrel slope, and water side pH are statistically significant variables. It is interesting to note that the Ohio study found resistivity to not be a statistically significant variable.

Steel Pipe. Figure 5.28 shows a chart for determining the service life of a galvanized steel culvert under nonabrasive and low abrasive conditions. The average service life of

Minimum pH to attain
design service life*

TABLE 5.11 Requirements for Concrete Pipe Protection Pipe slope Pipe rise, in Pipe rise, mm 50 yr 75 yr Less <42 <1050 3.0 3.3 than 1% 42-72 1050-1800 3.8 4.1 >72 > 1800 4.2 4.5 1-3% <42 <1050 3.2 3.5 42-72 1050-1800 4.1 4.4 >72 > 1800 4.6 5.0 3-10% <42 <1050 3.6 3.9 42-72 1050-1800 4.5 4.9 >72 > 1800 5.0 5.4 Greater <42 <1050 4.0 4.5 than 10% 42-72 1050-1800 5.0 5.5 >72 > 1800 5.5 6.0 *Based on Eq. (4) from Transportation Research Record 1008 Article “Field Performance of Concrete Pipe Culverts at Acidic Flow Sites in Ohio.” For stream flow with a pH less than the minimums listed above, epoxy-coated concrete pipe is required. Source: From Location and Design Manual, Vol. 2: Drainage Design, Ohio Department of Transportation, with permission.

Specified thickness, in (mm)	0.052 (1.32)	0.064 (1.63)	0.079 (2.01)	0.109 (2.77)	0.138 (3.51)	0.168 (4.27)
Factor	0.8	1.0	1.2	1.7	2.2	2.6

(b)

FIGURE 5.28 Method for estimating service life of plain galvanized steel culverts, (a) Service life chart for 0.064-in (1.63 mm) thickness based on invert performance, (b) Conversion factors for other thicknesses. (From Project Development and Design Manual, FHWA, with permission)

culvert with a wall thickness of 0.064 in (1.62 mm) is displayed in terms of pH and resistivity in Fig. 5.28a. For culverts with other wall thicknesses, obtain the service life from the chart and multiply by the factors in Fig. 5.28b. Use the chart for both the out­side conditions and the inside (water side) conditions and base the design on the worst case. Generally, the inside condition controls.

For steel with a type 2 aluminum coating, the FLH manual assigns a greater service life under certain conditions. For nonabrasive and low abrasive flow, where the resis­tivity is equal to or greater than 1500 П • cm and the pH is between 5 and 9, alu­minized steel is considered to provide a service life twice that of galvanized steel as determined from Fig. 5.28.

Protective Coatings on Steel Pipe. Under nonabrasive and low abrasive conditions, the service life of galvanized steel culvert can be extended by application of protective coatings. For example, when the water side environment controls the pipe thickness, application of an asphaltic coating (a postfabrication coating by the pipe manufacturer) can add 10 years of service life to the culvert, and an application of an asphaltic paved invert in addition to the coating will add a total of 25 years. If the soil side controls, application of the asphaltic coating will add 25 years of life. Concrete lining will add 25 years of service life. Ethylene acrylic acid film coatings (a polymer precoat on the galvanized coil) with a 10-mil (0.25-mm) thickness can be expected to provide an additional 30 years of service life. Currently, there are insufficient data to predict the performance of ploymer precoated pipe under severely abrasive conditions. Concrete pavings can be designed to add service life.

Aluminum Pipe. Under nonabrasive and low abrasive conditions, where the resistiv­ity is equal to or greater than 500 H • cm and the pH is between 4 and 9, aluminum culverts can be assumed to have a service life of 50 years when the metal thickness is appropriately sized for structural adequacy.

Design for Abrasion. In moderate abrasive environments, the sheet thickness for both steel and aluminum pipes should be increased by one nominal thickness, or the invert should be protected. In severe abrasive conditions, the sheet thickness should be increased by one nominal thickness and the invert should be protected. Invert protection under severe abrasive conditions may consist of metal rails or energy-dissipating devices at the inlet. Under moderate abrasive conditions, invert protection may consist of (1) paving with port­land cement concrete or (2) asphaltic coating and invert paving with bituminous concrete.

Plastic Pipe. Under most environmental and abrasive conditions, polyethylene and polyvinyl chloride plastic pipes may be specified without regard to the pH and resis­tivity of the site. Invert protection may be required under some abrasive conditions.

Example: Minimum Thickness of Galvanized Steel Culvert. The design service life for the culvert has been set at 50 years. A site investigation of a potential location shows that the soil has a pH of 7.2 and a resistivity of 5000 П • cm. The water flow shows a pH of 6.8 and a resistivity of 4000 П • cm. Determine the minimum sheet thickness for durability.

Outside condition. In Fig. 5.28a, find the intersection of the vertical line for

5000 П • cm with the inclined line for 7.2 pH, and read the average service life of

52 years from the vertical scale at the left.

Inside condition. In like manner, for a resistivity of 4000 П • cm and a pH of 6.8,

find the average service life of 42 years.

In this example, the inside conditions control the design, and the thickness must be increased. For the 0.064-in (1.63-mm) sheet thickness, the ratio of the design service life to the anticipated service life is 50/42 = 1.2. From Fig. 5.28b, the multiplying factor is 1.2 for a thickness of 0.079 in (2.01 mm). Therefore, a thickness of 0.079 in (2.01 mm) should provide the desired service life of 50 years.

An alternative is the application of an asphaltic coating, which can add 10 years of service life when the inside condition controls. For the 0.064-in (1.63-mm) sheet thickness, 42 + 10 = 52 years. Therefore, consider an 0.064-in (1.63-mm) sheet thickness with an asphaltic coating.