15 Dec 98
carrying ability of CLSM. A CLSM compressive strength of 50 to 100 psi equates to an
allowable bearing capacity of a well-compacted soil. Curing methods specified for concrete are
not considered essential for CLSM. Maintaining strengths at a low level is a major objective for
projects where later excavation may be required.
Density Unit weight of normal in-place CLSM is in the range of 115-145 lb/ft3, which is
greater than most compacted materials. A CLSM mix with only fly ash, cement and water should
have a unit weight of 90 to 100 lb/ft3. Lower unit weights can be achieved by using lightweight
aggregates, high entrained air contents, and foamed mixes. ASTM D 6023 provides a test
method for CLSM unit weight.
Settlement Settlement of compacted fills may occur even when compaction requirements have
been met. CLSM does not settle after hardening occurs.
Thermal insulation and conductivity Conventional CLSM mixtures are not considered good
insulating materials. Where insulation is desired, the mixture should be proportioned to obtain
low density and high porosity. Air entrained conventional mixtures reduce the density and
increase the insulating value. Lightweight aggregates, including bottom ash, can be utilized to
reduce density. Foamed or cellular mixtures have very low densities and exhibit good insulating
cables, high density and very low porosity (maximum surface contact area between solid particles)
are desirable. As the moisture content and dry density increase, so does the thermal conductivity.
Permeability Permeability of most excavatable CLSM is similar to compacted granular fills.
Typical values are in the range of 10-4 to 10-5 cm/sec. Permeability is increased as cementitious
materials are reduced and aggregate contents are increased (particularly above 80 percent).
Shrinkage and cracking Shrinkage and shrinkage cracks do not generally affect the
performance of CLSM. Several reports have indicated very little shrinkage occurs with CLSM.
Typical linear shrinkage is in the range of 0.02 to 0.05 percent.
Excavatability The ability to excavate CLSM at later ages is an important consideration on
many projects. In general, CLSM with a compressive strength of 50 psi or less can be excavated
manually. Mechanical equipment such as backhoes are used for compressive strengths of 100 to
200 psi. Mixtures using high quantities of coarse aggregate can be very difficult to remove by
hand even at low strengths. Mixtures using fine sand or only fly ash as the aggregate filler may be
excavated with a backhoe at strengths on the order of 300 psi. Since CLSM will typically
continue to gain strength beyond the conventional 28-day testing period, it is suggested, especially
for high cementitious content CLSM, that long term strength tests be conducted to estimate the
potential for later-age excavatability. In addition to limiting the cementitious content, entrained
air can be used to keep compressive strengths low.
e. Mix proportions. Proportioning for CLSM has largely been done by trial and error until
mixtures with suitable properties have been achieved. Most specifications available provide a
recipe of ingredients that will produce an acceptable material, although some specifications call
for performance features and leave proportioning up to the supplier. Currently, where a recipe
does not exist, trial mixtures are evaluated to determine how well they meet certain goals for