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1 SUMMARY Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction This report summarizes the results of a study on the use of controlled low-strength material (CLSM) in backfill, utility bedding, void fill, and bridge approach applications. CLSM, used in lieu of compacted fill in these applications, is a highly flowable material typically composed of water, cement, fine aggregates, and often times, fly ash or other by-product materials. This study, which included substantial laboratory and field components, demonstrated that CLSM is an effective and innovative material that can be used in each of the key target applications, with good short- and long-term performance. CLSM, however, must be used in conjunction with a good quality control and quality assurance plan, and users must be aware of the unique properties of the material to avoid improper usage. This study high- lighted some of these potential issues of concern and this report provides guidance on how to recognize potential problems and take precautions to avoid them. One area that was evaluated in detail was the issue of excessive long-term strength gain, which can lead to difficulties in future excavation. Through the laboratory and field compo- nents, the parameters that impact long-term strength gain were identified, including the effects of materials, mixture proportions, and climatic conditions. Long-term strength gain and potential problems with excavatability were most commonly observed when using fly ash, especially in hot-weather applications. By recognizing that these factors have a major impact on excavatability, users can take the necessary precautions to avoid problems, such as performing more long-term strength testing and/or subjecting test specimens to elevated temperatures during curing. 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 com- pacted fill) when pipes are completely embedded in CLSM. The reduced permeability of CLSM can reduce the ingress of chlorides, and the microstructure of CLSM can improve cor- rosion resistance through changes in the pH and resistivity of the pore solution. However, a potential for corrosion exists when pipes are embedded in both CLSM and surrounding soil or conventional fill, thereby setting up a galvanic cell that can increase corrosion activity. This situation is similar in nature to metals embedded in dissimilar soils, and similar precautions can be taken to ensure the desired service life. A hallmark of CLSM technology is the ability to safely and effectively utilize a range of by- product and waste materials. The by-product materials tested in this study were found to be non-toxic. However, a protocol was developed to evaluate other by-product materials that might be more of a concern with regard to leaching and environmental impact. This approach involves the testing of total heavy metals, possibly followed by the toxicity characteristic leach- ing procedure (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).

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2 Based on the findings of the laboratory component of this project, test methods, specifi- cations, and guidelines were developed and later validated in the field testing component. Six field tests were performed throughout the United States that aimed to validate the tests, specifications, and guidelines developed under this project and to fill in the gaps in under- standing that could only be addressed through field applications. The overall findings of these field tests confirmed the above products of the research and demonstrated the bene- fits of using CLSM in backfill, utility bedding, void fill and bridge approach applications. Some of the field tests will require long-term follow-ups, especially those involving corro- sion of metals in CLSM, owing to the long-term nature of corrosion. The main deliverables emanating from this project are contained in the appendixes, including recommended test methods, recommended specifications, recommended prac- tices, and an implementation plan to push the key findings and deliverables into state high- way practice.