Background: Controlled Low-Strength Material (CLSM), also known as flowable fill, is a cementitious fill placed without compaction, typically by mixer truck. It is prepared by mixing a variety of materials such as hydraulic cement, fine aggregate, fly ash or other similar by-products, and water. There are many uses of this type of fill including utility and manhole backfill and bedding, wall backfill, and void filling (such as in abandoned tunnels and sewers).

Conventional backfill in trenches and around small structures usually involves placement of aggregate material in thin layers with labor-intensive compaction. Poorly constructed backfill or lack of control of compaction often creates excessive settlement of the road surface and may produce unacceptable stresses on buried utilities and structures. Use of CLSM removes the necessity for mechanical compaction with the associated worker safety hazards. It can also provide more efficient placement and may permit reduced trench dimensions.

Usually CLSM incorporates locally available, recyclable by-products, such as fly ash or bottom ash. Depending on the specific application, CLSM requirements may include the following: compatibility with different types of utilities; compatibility with different pipe types; absence of undesirable environmental effects; flowability with minimum segregation; strength consistent with easy removal; and rapid but limited strength gain. Because the use of CLSM is evolving, the key issues for this research are to define the CLSM design and construction criteria for proper backfill envelope or fill material placement in specific applications; understand how the characteristics of its constituents may relate to the properties of CLSM; and determine how the required properties may be monitored in the field.

Objective: The objectives of the research are to (1) define the properties of CLSM (controlled low-strength material) necessary for its use as utility and manhole bedding and backfill, wall backfill, void filling, and bridge approaches; (2) for these applications, define test methods and develop criteria for the necessary properties of CLSM, including its corrosion potential and possible environmental impact; (3) define the relationships between the properties of CLSM and its constituents; (4) define field methods to monitor in-place properties of CLSM for construction acceptance; and (5) prepare design criteria and construction guidelines for CLSM to take advantage of its properties for backfill, utility bedding, void fill, and bridge approaches.

Tasks: Specific research tasks to reach these objectives include the following.

Phase I: (1) Complete the work plan approved in Project 24-12, analyze its results, draw conclusions, and recommend a set of practical test methods required to design or approve CLSM for backfill, utility bedding, void fill, or bridge approaches and a set of field procedures to monitor the constructibility of CLSM in these applications. (2) For the properties and test methods identified in Task 1, establish specification criteria and limits for the use of CLSM as backfill, utility bedding, void fill, or bridge approaches. (3) Using the results of the previous tasks, prepare provisional design criteria and construction guidelines to take advantage of the properties of CLSM that affect CLSM's constructibility and performance as backfill, utility bedding, void fill, and bridge approaches. Consider the normal variety of utility types, pipe types, trench dimensions, surrounding soil types, wall types, and other key design parameters found in street and highway projects. (4) Submit an interim report, within 4 months of the contract award date, summarizing the results and conclusions of Tasks 1 through 3. In the report, propose a detailed, Phase III work plan for a field experiment to be conducted in cooperation with several state highway agencies to evaluate and verify the provisional design criteria and construction guidelines, test methods, and specification criteria. The research agency will be expected to meet with the NCHRP approximately 1 month later to obtain approval of the report and the work plan before beginning Phase II.

Phase II: (5) Carry out the work plan approved in Phase I, analyze its results, draw conclusions, and make revisions to the provisional design criteria and construction guidelines as required. (6) Prepare a final report that summarizes findings, draws conclusions, and documents the research products. Present the provisional design criteria, construction guidelines, specification criteria, and recommended test methods in AASHTO standard format. Provide an implementation plan for state highway agencies to use in incorporating the research products into practice.

Status: The project is complete.

Product Availability: The project final report has been published as NCHRP Report 597: Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction and NCHRP Web-Only Document 116: Corrosion Study and Implementation Plan for NCHRP Report 597.