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72 casting, plastic caps were placed on the cylinders to prevent Sample Holder evaporation of mixing water. These cylinders were then trans- ferred, cured, and tested by Hamilton County engineers or a local testing laboratory. Compressive strength data showed that while mixtures S10 and FF1 had similar compressive 152 mm strengths at 90 days, mixture CDF1 had a compressive strength more than twice the compressive strength of the other mix- 305 mm tures. The data also showed that this mixture experienced a decrease in compressive strength of approximately 100 kPa between 60 and 90 days; this loss may have been due to leach- 457 mm ing away of hydration products upon moist storage. 610 mm Corrosion activity of the buried ductile iron pipes is being monitored using the potential difference between the pipes and a copper/copper-sulfate reference electrode. The potential dif- Ductile iron ference between the electrode and the pipe is an indicator of the coupons active or passive state of the buried pipe and can be measured with a high impedance voltmeter. For this purpose, the high Figure 4.9. Ductile iron coupons attached to sample impedance voltmeter is electrically connected to the pipe holder. (connected to the wire attached to the pipe) and to the copper/ copper-sulfate electrode. The copper/copper-sulfate electrode touches the ground above the buried pipe to close the electri- Field Test at East Bay Municipal cal circuit between the electrode and the pipe. Potential read- Utility District ings are performed by the Hamilton County engineers using a copper/copper-sulfate electrode provided by the research team Introduction for this testing. The long-term strength gain and excavatability of CLSM In addition to the potential difference study, metal coupons mixtures have long been a concern for engineers at the East Bay were fabricated from a ductile iron pipe and these samples Municipal Utility District (EBMUD). A unique aspect of this were also buried in the trenches in row B to evaluate their mass concern is that, because of a shortage of fine aggregate in the loss due to corrosion. It is anticipated that their mass loss will Bay Area, most CLSM in the area contains coarse aggregate as be determined based on ASTM G 1, "Preparing, Cleaning, and well. In general, coarse aggregate is rarely used in CLSM, and Evaluating Corrosion Test Specimens." Ductile iron coupons this field test was sought to determine the effect on excavata- were attached to 0.3 m long sample holders in groups of four bility. Also, this field test was selected to gain the perspective of and placed in the CLSM when it was still in fluid state. Fig- the many utilities that are using CLSM for a range of backfill ure 4.9 shows a schematic of ductile iron coupons attached to applications. The objective of the test was to investigate con- a sample holder. structability issues related with the use of CLSM as a backfill As described in Chapter 3, CLSM is generally better than conventional fills in protecting embedded metals from corro- sion when the metals are entirely encased in CLSM. It was also Sample Holders shown in Chapter 3 that if a metallic pipe backfilled with CLSM is also in contact with the surrounding soil, the potential for setting up a galvanic cell exists due to the dissimilar media (CLSM and conventional fill). To investigate this corrosion issue, four extra sample holders, each with three ductile iron coupons, were prepared. These four sample holders were cou- pled by connecting their coupons embedded in CLSM with coupons embedded in soil, as illustrated in Figure 4.10. CLSM It is anticipated that Hamilton County and Cincinnati Water Works engineers, with the cooperation of the research Soil team, will monitor the corrosion activity for the various test- ing configurations reported herein, and it is hoped that the data Figure 4.10. Coupled sample will prove of use to them and other users of CLSM dealing with to evaluate galvanic corrosion utilities. (not to scale).