Where metallic reinforcement is used, the life of the structure will depend on the corrosion resistance of the reinforcement. Practically all the metallic reinforcements used in construction of embankments and walls, whether they are strips, bar mats, or wire mesh, are made of galvanized steel. Epoxy coating can be used for additional corrosion protection, but it is susceptible to construction damage, which can significantly reduce its effectiveness. PVC coatings on wire mesh also provide corrosion protection, provided again that the coating is not significantly damaged during construction. When PVC or epoxy coatings are used, the maximum particle size of the backfill should be restricted to /4 in (19 mm) or less to reduce the potential for construction damage.
For the purpose of determining the sacrificial metal required (corrosion allowance), the following design life is provided, pursuant to recommendations of Task Force 27 of AASHTO-AGC-ARTBA:
Structure classification Design life, yr
Permanent structure 75
Abutments 100
Rail supporting structures 100
Marine structures 75
The required cross-sectional area of steel reinforcement is calculated using the relationships given in Fig. 8.38 for the selected type of reinforcement (strips or grids). The corrosion loss assumed is based on the following.
In 1985, an FHWA study was initiated to develop practical design and construction guidelines from a technical review of extensive laboratory and field tests on buried
FIGURE 8.38 Metallic reinforcement for MSE walls showing correction for corrosion loss.
(From the Reinforced Earth Co., with permission)
metals. The results of this research were published in December 1990 in the Federal Highway Administration report FHWA-RD-89-186, “Durability/Corrosion of Soil Reinforced Structures”:
For structures constructed with carefully selected and tested backfills to ensure full compliance with the electrochemical requirements, the maximum mass presumed to be lost per side due to corrosion at the end of the required service life may be computed by assuming a uniform loss model which considers the following loss rates:
1. Zinc corrosion rate for first 2 years: 15 ^m/yr
2. Zinc corrosion to depletion: 4 |xm/yr
3. Carbon steel rate: 12 ^m/yr
The resulting sacrificial thickness for a 75-year life based on initial galvanization of 2 oz/ft2 (86 |xm) is approximately 1.5 mm of total sacrificial thicknesses. Since this is a maximum loss rate, it is presently assumed that the reduced minimum thickness remains proportional to tensile strength and therefore no further reduction is necessary. (See Fig. 8.39.)
The select backfill materials shall meet the following requirements:
Internal friction angle. The material shall exhibit an internal friction angle of not less than 34 degrees as determined by the standard direct shear test, AASHTO T-236, utilizing a sample of the material compacted to 95 percent of AASHTO T-99, Methods C or D (with oversize correction), at optimum moisture content. Internal friction angle testing is not required for
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TABLE 8.5 Backfill Requirements Related to Durability of Steel Reinforcement
Source: From the Reinforced Earth Co., with permission. |
backfill materials that have at least 80 percent of the material greater than or equal to the /4-in (19-mm) size.
Soundness. The material shall be substantially free of shale or other soft, poor durability particles. The material shall have a magnesium sulfate soundness loss of less than /0 percent after four (4) cycles, as determined by AASHTO T-104.
Electrochemical requirements. The material shall conform to the electrochemical requirements as described in Table 8.5.
The Contractor shall furnish to the Engineer a Certificate of Compliance certifying that the select granular backfill material complies with this section of the specification. A copy of all test results performed by the Contractor, which are necessary to assure compliance with the specifications shall also be furnished to the Engineer.
Backfill not conforming to this specification shall not be used without the written consent of the Engineer.
The frequency of sampling of select granular backfill material, necessary to assure gradation control throughout construction, shall be as directed by the Engineer.
