15 Dec 98
CONTROLLED LOW STRENGTH MATERIAL
WITH COAL-COMBUSTION ASH AND OTHER RECYCLED MATERIALS
1. Purpose. This Engineer Technical Letter (ETL) provides background and an example
specification for controlled low strength material (CLSM). CLSM has many applications but has
been primarily used as a flowable, self-compacting fill in place of a compacted fill. The example
specification contains provisions for using fly ash, bottom ash, and other recycled materials within
the CLSM mix.
2. Basic Documents and References. To provide a state-of-the-practice specification for use by
Corps of Engineers and Army practitioners, this ETL is based on a recently-updated specification
for CLSM by the State of New York Department of Transportation (NYSDOT), which is
performance-based and incorporates lessons learned from earlier field experiences. NYSDOT
has indicated that industry concurs with its requirements, which, while limited to a specific region
of the US, demonstrates the acceptance of the specification among both suppliers and owner
representatives. This ETL is also based on ACI 229R-94, "Controlled Low Strength Materials
(CLSM)." In addition, ASTM standard test methods have been developed for CLSM. These and
other references are listed in the Appendix.
3. Background. Extensive background and guidance on controlled low strength material is given
in ACI report 229R-94. That report has been used liberally in the preparation of this section.
a. Material description. As described by ACI 229R-94, CLSM is a self-compacting,
cementitious material used primarily as a backfill in lieu of compacted fill. Often called flowable-
fill, it should flow easily in construction and gain sufficient strength to be walked on and buried
within a day. CLSM is especially popular as a excavatable material for backfilling trenches and
other hard-to-compact locations. To allow for future excavation most current CLSM applications
require unconfined compressive strengths of 50 to 300 psi. However, CLSM is defined in a
broader sense by ACI 116R, "Cement and Concrete Terminology," as materials that result in a
compressive strength of 1200 psi or less. The upper limit of 1200 psi allows use of this material
for applications where future excavation is unlikely such as structural fill under buildings.
Most CLSM mixes are currently designed to incorporate marketable components such as
water, cement, fly ash, and natural sand. However, recent research and experience shows that
currently non-marketable or non-standard components such as non-ASTM-conforming fly ash and
coal-combustion bottom ash may be used as long as the materials have been tested and found to
satisfy the intended application. The example specification in this ETL contains provisions for
using fly ash and recycled aggregate, including bottom ash.
While CLSM is a cementitious material that has much in common with concrete, it is not