Field Performance of Copper Naphthenate Treated Hardwoods (cont)

In a test established by the Tennessee Valley Authority, Schell (1952) reported that fence post size specimens were dip treated with a 0.50% (Cu as metal) solution to an average net retention of 0.03 pcf Cu (as metal) or roughly 1/2 of the AWPA recommended retention for softwood fence posts. TVA (Anon. 1964) evaluated these fence posts over a 17+ year period. The results of the last inspection reported are shown in Table 1.

In an unpublished report, McIntyre (2001) analyzed the results of a field stake test at the Austin Carey Forest near Gainesville, Florida (AWPA zone 5) initiated by the American Railway Engineering Association (AREA) in 1957 (AREA 1975). After 15 years of exposure, the test was terminated. The test specimens were nominal 2x4x18 in. stakes of red oak, coastal Douglas-fir, or southern yellow pine. Three nominal retentions were used for this study: the AREA-recommended (100%) level, half the AREA-recommended (50%) level, and twice the AREA-recommended (200%) level. The AREA 100% retention recommended level for CuNap was 1.6 kg/m3 (Cu as metal) and 160 kg/m3 of creosote or 60:40 creosote coal tar (CCT) solution. The creosote and CCT solution generally conformed to the current AWPA (2000) Standards P1/P13 or P2, respectively. Untreated stakes of appropriate species were installed periodically, and these stakes would generally last 2-3 years before failure occurred.

Generally, the actual retentions were reasonably close to the targets except for the highest retention red oak stakes. In this case, a retention of only 146% (vs. 200%) was achieved with CuNap. As shown in Figure 3, the 50% and 100% retentions of CuNap, creosote (P1/P13), and CCT gave decay ratings that were essentially identical. However, the lower pickup at the “200%” retention resulted in a slight departure of the CuNap ratings from those of creosote and CCT, but this was to be expected since the CuNap-treated stakes had considerably less preservative actives. It should be noted that even though the CuNap stakes had only three-fourths of the relative retention, the CuNap parallels the performance of creosote and matches the 60:40 CCT in red oak stakes.

The following sections of this paper will outline the results from an ongoing study being conducted at Mississippi State University to evaluate the performance of CuNap in hardwoods.

Materials and Methods

Test stakes, nominally 19x19x1118-mm, were cut from dried red oak (Quercus spp.), sweetgum (Liquidambar styraciflua), or southern pine (Pinus spp.) and segregated into uniform density distributions by species according to AWPA Standard E7 (2000). Ten replicate stakes/species were treated using a conventional full-cell process with solutions of creosote or CuNap. The creosote conformed to AWPA Standard P2. The CuNap was a nominal 8% (Cu as metal) concentrate formulated in #6 fuel oil.

The desired creosote retentions were achieved by using toluene dilution. A similar method was used with CuNap concentrate except that a 75:25 (toluene: P9 type A oil) solvent mix was used to dilute the CuNap concentrate. This resulted in a residual oil content in the CuNap-treated stakes of approximately 96-120 kg/m3. After treatment, a 100-mm section was removed from both ends and saved for reference. The remaining piece was cut into 457-mm matched halves. One matched half was placed in the MSU Dorman Lake test plot (AWPA zone 4) and the other at the Austin Carey Forest, FL (AWPA zone 5) in 1992. Untreated controls were used as reference stakes. The stakes were evaluated annually for decay and termites according to the AWPA (2000) rating scale in Standard E7.

Results and Discussion

Typical depreciation curves for the Dorman Lake site are shown in Figure 4 for sweetgum and Figure 5 for red oak.

With sweetgum (Figure 4), the performance of CuNap-treated stakes ranging from 0.96-2.08 kg/m3 (as Cu) fell between that for creosote treatments at 120 and 176 kg/m3. For red oak (Figure 5), CuNap-treated stakes at retentions of 0.96-1.92 kg/m3 performed similarly to those treated with creosote ranging from 69-165 kg/m3. Similar results were found in the Florida test plot. Dose-response curves after 7.5 years in ground contact are shown in Figure 6 for red oak and gum. Based on a rating of 70, 60-70 kg/m3 of creosote is roughly equivalent to 1.7 to 1.9 kg/m3 (Cu as metal) in red oak. There was more variability in the gum stakes with roughly 75 kg/m3 of creosote equivalent to 1.4 kg/m3 (as Cu) in the Florida plot and 140 kg/m3 creosote equivalent to 2.6 kg/m3 (as Cu) in the Mississippi plot. The dose-response curves for CuNap were remarkably similar for both plots. Interestingly, both the CuNap and creosote curves indicated that the Mississippi site was a more severe site than was the Florida site. This is unexpected since the Florida site is in zone 5 and the Mississippi site is in zone 4.

Summary and Conclusions

The data for CuNap compare favorably with data for creosote or creosote coal-tar systems. The field data presented in this paper should represent an exposure condition more severe than that found in track with well-maintained ballast. Therefore, the data reviewed and the new data presented in this paper indicate an excellent potential for the use of CuNap as a crosstie (sleeper) preservative.

Literature Cited

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  4. Barnes, H.M.; Freeman, M.H. 2000. The performance of copper naphthenate-treated wooden pole stubs after 12 years of field exposure. In: Proceedings, International Conference on Utility Line Structures, March 2000, Ft. Collins, CO, p. 207-226.
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