| Controlled Low-Strength Material
1964, the U.S. Bureau of Reclamation documented the first known
use of controlled low-strength material (CLSM). Plastic soil-cement,
as the Bureau called it, was used as pipe bedding on over 320 miles
(515 km) of the Canadian River Aqueduct Project in northwestern
Texas. Since 1964, CLSM has become a popular material for projects
such as structural fill, foundation support, pavement base, and
CLSM is a self-compacted, cementitious material used primarily
as a backfill in lieu of compacted backfill. Several terms are currently
used to describe this material, including flowable fill, controlled
density fill, flowable mortar, plastic soil-cement, soil-cement
slurry, K-Krete, and other names. CLSM is defined as a material
that results in a compressive strength of 1200 psi (8 MPa) or less.
Most current CLSM applications require unconfined compressive strengths
of 200 psi (1.4 MPa) or less. This lower strength requirement is
necessary to allow for future excavation of CLSM.
The term CLSM can be used to describe a family of mixtures for
a variety of applications. For example, the upper limit of 1200
psi (8 MPa) allows use of this material for applications where future
excavation is unlikely, such as structural fill under buildings.
Low density CLSM describes a material with distinctive properties
and mixing procedures. Future CLSM mixtures may be developed as
anticorrosion fills, thermal fills, and durable pavement bases.
CLSM is composed of water, portland cement, aggregate, and fly
ash. It is a fluid material with typical slumps of 10 inches (254
mm) or more. It has the consistency of a milk shake.
most concrete, CLSM may be mixed in central-mix concrete plants,
ready-mixed concrete trucks or pugmills. Once CLSM is transported
to the jobsite, the mixture may be placed using chutes, conveyors,
buckets, or pumps depending upon the application and its accessibility.
A truck often can be discharged in less than 5 minutes. A constant
supply of CLSM will keep the material flowing and will make it flow
horizontal distances of 300 feet (91 m) or more. Although CLSM may
be placed continuously in most applications, care must be taken
when backfilling around pipes. For pipe bedding and backfilling,
CLSM is placed in lifts to prevent the pipes from floating. Internal
vibration or compaction is not needed to consolidate CLSM mixtures.
Its fluidity is sufficient to consolidate under its own weight.
The fluidity/flowability and self-compacting properties of CLSM
mixtures make CLSM an economical alternative to compacted granular
material due to savings of labor and time during placing. CLSM is
also an all-weather construction material—it will displace
any standing water left in a trench—making it a ideal material
for many projects.
The primary application of CLSM is as structural fill or backfill
in place of compacted soil. The flowable characteristics of CLSM
mean that it can readily be placed into a trench and into tight
or restricted-access areas where placing and compacting fill is
difficult. CLSM also makes an excellent bedding material for pipe,
electrical, telephone, and other types of conduits because the mixture
easily fills voids beneath the conduit and provides uniform support.
CLSM will not settle or rut under loads, making the material an
ideal pavement base. Additionally, CLSM can be placed quickly and
support traffic load within hours of placement-minimizing repair
time and allowing a rapid return to traffic. CLSM may be equal to
or less than the cost of using standard compacted backfill.
Since 1979, the Iowa Department of Transportation has used CLSM
to structurally modify more than 40 substandard bridges by converting
them into culverts. CLSM is also used to fill large voids such as
old tunnels and sewers. In a Milwaukee project, 830 cubic yards
(635 cubic meters) of CLSM were used to fill an abandoned tunnel.
A ready-mixed concrete producer can aid in developing a mix design
for CLSM. However, when ordering CLSM, consider the following:
- Strength: Applications that require removal of CLSM at a later
date usually limit the maximum compressive strength to less than
200 psi (1.4 MPa).
- Setting and Early Strength: Hardening time can be as short as
one hour, but can take up to 8 hours depending on mix design and
- Density in Place: Density of normal CLSM in place typically
ranges from 90 to 125 pounds per cubic foot (1440 to 2000 kg/cubic
- Flowability: Flowability can be enhanced through the use of
fly ash or air entrainment.
- Durability: CLSM materials are not designed to resist freezing
and thawing, abrasive or erosive actions, or aggressive chemicals.
More information can be found at the
National Ready Mix Association Web site.