Section 2: Estimated Service Life

Corrugated Metal Pipe and Structural Plate

Determine the service life of corrugated metal structure by calculating the service life of the exterior and interior of the pipe using the site characteristics for the soil and water discussed in the previous section. The overall service life will be the lesser of the interior service life or exterior service life. The service life of a corrugated metal conduit is expressed by the sum of the base metallic coating, post applied coating, and paving or lining service life, as in Equation 14-1 and Equation 14-2:
EquationObject334401
Equation 14-1.
EquationObject335402
Equation 14-2.
where:
  • SL
    INT
    = service life of the interior of the pipe
  • SL
    EXT
    = service life of the exterior of the pipe
  • SL
    BMCI
    = service life of the base metallic coating interior
  • SL
    BMCE
    = service life of the base metallic coating exterior
  • SL
    PACI
    = service life of the post applied coating interior
  • SL
    PACE
    = service life of the post applied coating exterior
  • SL
    LI
    = service life of the paving or lining interior

Corrugated Steel Pipe and Steel Structural Plate

The base metallic coating data provided in this section are limited to the following values for galvanized metals:
  • 6 < pH < 8
  • resistivity ≥ 2,000 ohm-cm
  • soft waters considered hostile when resistivity ≥ 7,500 ohm-cm
For aluminized type 2, the following values apply:
  • 5.0 < pH < 9.0; Resistivity > 1,500 ohm-cm
  • soft waters not considered to be a problem
Estimate the service life for the interior base metallic coating using Equation 14-3
EquationObject336403
Equation 14-3.
The basic interior service life for 18-gage corrugated galvanized metal pipe is provided in the table following Equation 14-4 for pH values of 7.3 and lower and using the equation for pH values in excess of 7.3.
EquationObject337404
Equation 14-4.
where:
  • L
    i
    = interior years
  • R
     = resistivity (ohm-mm)

Exterior Coating

Estimate the service life for the basic exterior base metallic coating using Equation14-5.
EquationObject338405
Equation 14-5.
The basic exterior service life (L
e
) for 18-gage corrugated galvanized metal pipe is provided in the table following Equation 14-6 for pH values of 7.3 and lower and using the equation for pH values in excess of 7.3.
EquationObject339406
Equation 14-6.
Exterior Durability for 18-Gage CMP (years)
pH
Resistivity (ohm-cm)
1,000
1,500
2,000
2,500
3,000
4,000
5,000
7,500
10,000
7.3
54.8
59.6
63.1
65.8
67.9
71.4
74.1
78.9
82.4
7.0
34.6
39.4
42.9
45.6
47.7
51.2
53.9
58.7
62.2
6.5
23.9
28.8
32.2
34.9
37.1
40.5
43.2
48.0
51.5
6.0
18.0
22.9
26.3
29.0
31.2
34.6
37.3
42.1
45.6
5.8
16.2
21.0
24.5
27.2
29.3
32.8
35.5
40.3
43.8
5.5
13.8
18.6
22.1
24.8
26.9
30.4
33.1
37.9
41.4
5.0
10.4
15.3
18.7
21.4
23.6
27.0
29.7
34.5
38.0
Heavier gage metal has more sacrificial metal and, therefore, a longer anticipated life under given conditions. The table below provides coating thickness/gage multipliers for use in Equation 14-1 and Equation 14-2 for the respective gage and metallic coating. The resulting values are not exact but allow a systematic comparison of relative durability of the various metals and gages used in design.
Thickness Multipliers for Steel Conduit
Gauge
Item 460 - CMP
Item 461 - Structural Plate
Thickness
Factor
Thickness
Factor
in. (mm)
Galv
Alt 2
in. (mm)
Galv
18
0.052 (1.32)
1
3.6
**
**
16
0.064 (1.63)
1.3
3.9
**
**
14
0.079 (2.01)
1.6
4.2
**
**
12
0.109 (2.77)
2.2
4.8
0.109 (2.77)
2.24
10
0.138 (3.50)
2.8
5.4
0.138 (3.50)
2.84
8
0.168 (4.27)
3.4
6
0.168 (4.27)
3.54
7
**
**
**
0.188 (4.78)
3.81
5
**
**
**
0.218 (5.54)
4.42
3
**
**
**
0.249 (6.32)
5.05
1
**
**
**
0.280 (7.11)
5.68

Corrugated Aluminum Pipe and Aluminum Structural Plate

The service life of aluminum pipe and aluminum structural plate is a function of the pitting rate of the aluminum, which is less than 0.013 millimeter per year in the following environmental limits:
  • 4.0 ≤ pH ≤ 9.0
  • resistivity ≥ 500 ohm-cm
  • resistivity ≥ 25 ohm-cm (provided a free draining backfill material)
  • no upper resistivity limits; soft waters not a problem
Estimate interior service life (SL
BMCI
) and exterior service life (SL
BMCE
) using Equation14-7.
EquationObject340407
Equation 14-7.
The following table shows gage thickness and available structural plate thickness.
Aluminum Pipe Gage Thickness
Item 460 – CMP
Item 461 – Structural Plate
Gage
Thickness
Gage
Thickness
(in)
(mm)
(in)
(mm)
18
0.048
1.22
**
**
**
16
0.06
1.52
**
**
**
14
0.075
1.91
**
**
**
12
0.15
2.67
**
0.1
2.54
10
0.135
3.43
**
0.125
3.18
8
0.164
4.17
**
0.15
3.81
**
**
**
**
0.175
4.45
**
**
**
**
0.2
5.08
**
**
**
**
0.225
5.72
**
**
**
**
0.25
6.35

Post-applied Coatings and Pre-coated Coatings

The following table provides anticipated additional service life for post-applied and pre-coated coatings (SL
PACI
 and SL
PACE
) for use in Equation14-1 and Equation14-2.
Post-applied and Pre-coated Coatings Guide to Anticipated Service Life Add-On (additional years)
Coating
Interior (SL
PACE
)
Exterior (SL
PACE
)
Abrasion Level
Level 1
Level 2
Level 3
Level 4
Bituminous
8-10
5-8
0-2
0
30
Polymer 10/10
28-30
10-15
0-5
0
30

Paving and Lining

The following table provides additional service life for applied paving and lining (SL
l
) for use in Equation14-1.
Post-applied Paving and Lining Guide to Anticipated Service Life Add-On
Paved Or Lined
Interior
Exterior
Abrasion Level (SL
LL
)
Level 1
Level 2
Level 3
Level 4
Bituminous Paved Invert
25
25
25
0
N/A
Concrete Paved Invert
40
40
40
25
N/A
100% Bituminous Lined
25
25
25
0
N/A
100% Concrete Lined
50
50
50
35
N/A

Reinforced Concrete

There is little technical data on methods to estimate service life for reinforced concrete. In department experience when cast-in-place and precast reinforced conduit is used in appropriate environments, service life exceeds the original design life of the project (typically in excess of 50 years).
Durability of reinforced concrete can be affected by acids, chlorides, and sulfate concentrations in the soil and water. If the pH value is 6.5 or less, the use of porous concrete pipe with shell thickness of 1 in. (25 mm) or less is not advisable. If the pH value is 5.5 or less, use of reinforced concrete without a protective coating of epoxy or other acceptable coating is not advisable.
Salt content of the soil and water can have a detrimental effect on reinforced concrete because the salt (with its chloride constituent) can permeate the concrete in time, threatening the embedded reinforcing steel. Sulfate content in the soil or water can have a detrimental effect on reinforced concrete facilities. The following table presents a guide for adjusting cement type and factor for sulfate content in soils and runoff.
Guide for Sulfate Resisting Concrete
Water-soluble sulfate in soil sample (%)
Sulfate in water sample (ppm)
Type of cement
Cement factor
0 - 0.20
0 - 2,000
II
Minimum required by specifications
0.20 - 0.50
2,000 - 5,000
V
II
Minimum required by specifications
7 sacks
0.50 - 1.50
5,000 - 15,000
V
II
Minimum required by specifications
7 sacks
over 1.50
over 15,000
V
7 sacks

Plastic Pipe

To date, the department has minimal long-term experience with plastic pipe applications. More information will be provided as the department becomes aware of appropriate information. However, this lack of information should not preclude the possible use of plastics that conform to AASHTO and ASTM specifications if there is solid indication that the particular installation will meet service life expectations.