Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
89 Conclusions This report summarized the key findings of a multi-year re- search project on CLSM for use in backfill, utility bedding, void fill, and bridge approach applications. The research in- volved both a major laboratory component and field compo- nent. Through these efforts, several key deliverables have been produced, including a recommended suite of tests methods (Appendix B), specifications (Appendix C), recommended practices (Appendix D), and an implementation to drive these deliverables into practice (Appendix E). Significant progress was made in this project to better under- stand the behavior of CLSM and to evaluate the properties that most impact performance. The following list presents some of the main overall findings from this project: 1. Suitable test methods exist or were developed under this project to measure most of the key CLSM properties affect- ing performance in the four target applications. Appen- dix B describes the tests recommended to evaluate relevant fresh, hardened, and durability properties of CLSM. 2. Predictive models were developed to predict the water demand and compressive strengths for a range of CLSM mixtures. This information can be helpful in designing mixtures for applications where strength may be a key lim- iting factor, such as in the use of excavatable CLSM. 3. The effects of temperature on strength gain of CLSM mix- tures can be very pronounced, especially when using Class C fly ash. One should be aware of this increased strength gain, especially when CLSM is being used in a hot climate and when future excavatability may be required. Keeping this strong temperature dependence in mind and accounting for it in design and construction can help to effectively produce excavatable CLSM. Trial batching and testing at elevated temperatures help to gain insight into long-term strength gain in field applications, especially when fly ash or other supplementary cementitious materials are used. 4. There is no single parameter that adequately predicts exca- vatability. Compressive strength can serve as a useful sur- rogate value in some cases, but one should try to capture the long-term strength gain when applying strength as a pre- dictive tool. Basing long-term strength gain on short-term laboratory testing can be problematic for some CLSM mix- tures (especially those containing fly ash). Calculating a re- movability modulus shows promise in predicting excavata- bility. Lastly, the dynamic cone penetrometer was found to be a valuable method of assessing CLSM in the field and estimating ease of excavatability. 5. Significant research was performed on the corrosion of metallic pipe materials in CLSM. In general, CLSM was found to be beneficial in reducing corrosion (compared to typical compacted fill) when pipes are completely embed- ded in CLSM. The reduced permeability of CLSM can re- duce the ingress of chlorides and the microstructure of CLSM can improve corrosion resistance through changes in the pH and resistivity of the pore solution. A potential for corrosion exists when pipes are embedded in both CLSM and surrounding soil or conventional fill, because a galvanic cell is set up that can increase corrosion activ- ity. This case is similar in nature to metals embedded in dis- similar soils, and similar precautions can be taken to ensure the desired service life. 6. The by-product materials tested in this study were found to be non-toxic. However, a testing program was proposed to evaluate other by-product materials that might be more of a concern with regard to leaching and environmental im- pact. This method involves the testing of total heavy metals, possibly followed by TCLP (if the total heavy metals are above certain threshold values), and possibly followed by leachate testing from CLSM containing the subject material (if the TCLP values exceed certain thresholds). 7. Due to the high fluidity of CLSM mixtures, floating of pipes or unintentional shifting of utilities may occur during place- ment; users should take precautions to avoid this result. C H A P T E R 5 Conclusions and Suggested Research
90 Precautions are addressed in the specifications and guide- lines developed under this project for backfill applications. Suggested Research As is the case for any research project, all of the important issues can not be studied, or at least not in the level of detail de- sired. This research made important gains in many of the tech- nical areas relevant to CLSM, and the findings from this study have shed light on issues that require even more study. The following list presents some of the areas for suggested future research: 1. More long-term monitoring is essential in truly assessing the corrosion of metals in CLSM. Field tests initiated under this project will continue to be monitored, and the relevant find- ings will be communicated to the appropriate AASHTO committees. 2. Other data and information from field performance of met- als in CLSM should be gathered and synthesized to better quantify the service life in various environments. 3. Other durability issues, such as frost heave (or other frost- related issues), should be studied in more detail to deter- mine if long-term field performance can be assured in cold climates, with severe freeze-thaw cycles. 4. Information should be gathered on problems encoun- tered in field applications related to excessive long-term strength gain that have hindered excavation. Information on materials, mixture proportions, and engineering prop- erties (e.g., strength, unit weight, etc.) should be gathered in order to further elucidate the factors that most con- tribute to excavatability problems. 5. More detailed information on productivity and speed of CLSM, compared to conventional compacted fill, should be collected to better quantify the benefits of using CLSM in the four key applications studied in this project.
