Effects Of Gradation And Moisture Content On Resistivity Of Mechanically Stabilized Earth Wall Backfill Materials

The service life of mechanically stabilized earth (MSE) walls depends on the corrosion rate of the metallic reinforcement used in their construction. The resistivity of the backfill aggregates needs to be measured accurately in order to estimate realistically the corrosion rate of the reinforcement. Resistivity testing is usually performed using the traditional soil box on the portion of the aggregates that passes a No. 10 or No. 8 sieve to either select or reject the backfill. For a more reasonable characterization of the corrosivity of coarse backfills, it is desirable to use their actual gradations. To that end, several resistivity boxes with dimensions that were double and quadruple of the original box were constructed. In addition to the three standard gradations specified by TxDOT, over twenty backfill materials sampled from sources throughout Texas were fractionated to fines, fine sand, coarse sand, and gravel. Resistivity tests were performed separately on each of these four constituents for each backfill. Test results demonstrated that the gradation and moisture content of the backfill significantly affect the measured resistivity. The results were then used to evaluate a relationship that would allow the estimation of the resistivity of any desired backfill gradations from the resistivity values of their four constituents (i.e., fines, fine sand, coarse sand, and gravel). The proposed model looks promising since the resistivity of the backfill composed of the actual gradation could be estimated with small uncertainty. The results of this study can potentially help highway agencies and contractors use a number of local quarries that are currently disqualified based on the resistivity values that obtained from only testing the materials passing No. 8/10 sieve. Lastly, the results were also used to develop a model that would allow the estimation of the resistivity of a given backfill knowing its minimum resistivity, porosity, and degree of saturation. The results were promising for estimating the resistivity of backfills at degrees of saturation higher than 50%, when the rate of corrosion becomes a concern. The model can potentially be used to estimate the resistivity of in-service wall backfill from the minimum resistivity and moisture conditions