A Cost-Effective Alternative to Compacted Soil Backfill

by Steve Ragan, Director, Research and Technical Services

Controlled low-strength material (CLSM) is a self-compacted, cementitious material primarily used as a structural fill or backfill alternative to compacted soil backfill. It is often referred to by different names including flowable fill, controlled density fill, soil-cement slurry, unshrinkable fill, plastic soil cement, and flowable mortar. It is self-leveling, having the approximate consistency of pancake batter, and can be placed in one lift with minimal labor and no vibration or tamping. The American Concrete Institute (ACI) defines CLSM as having a compressive strength less than 1,200 psi, however most current CLSM applications require unconfined compressive strengths of less than 300 psi. This lower strength is more than comparable with strength of compacted soil backfill, but is necessary to allow for possible future excavation.

Since CLSM is designed to be fluid, it can easily be placed as backfill in a trench, hole or other cavity. It requires no compaction and therefore the trench width or size of excavation can be reduced. Soil backfill, even if compacted properly in the required layer thicknesses, cannot achieve the uniformity and density of CLSM. If it is necessary to backfill against retaining walls, one must be aware of the lateral fluid pressures exerted on the wall by CLSM. This can be partially addressed if necessary by designing the CLSM to set more quickly than normal, and thereby dissipate the lateral pressures imposed by the CLSM more quickly.

CLSM may also be used as structural fill such as foundation support. For example, if weak soils are prevalent under a structure CLSM is effective in distributing the structure’s load over a great area. CLSM can also provide a uniform and level surface for foundation footings and slabs that might otherwise bear on uneven or nonuniform subgrades. The CLSM’s compressive strength can be varied to match the project requirements, and therefore could even potentially reduce the required thickness or strength requirements of the concrete slab.

CLSM mixtures have also been used in pavement applications for bases, subbases, and subgrades. In these cases the CLSM mixture would be discharged directly from the mixer truck onto the subgrade between existing curbs.

CLSM is an excellent bedding material for pipe, electrical, telephone, and other types of conduits. Because it is self leveling, it readily flows around conduits to fill voids beneath the conduit and provide uniform support. A conduit encased entirely in CLSM is also protected from future damage. If the area around the conduit is being excavated at some later date, there would be an obvious change in material when the CLSM is exposed by equipment operators. This change can be made to be even more apparent by having the concrete producer add coloring to the CLSM. Different colors may be used for different conduit applications.

Both field performance and lab studies have shown that CLSM resists erosion better than many other fill materials. For example, tests which exposed CLSM and various clay fill materials to water moving at moderate velocity demonstrated the CLSM showed had less material loss and suspended solids resulting from the material loss. It is frequently used in riprap for embankment protection, and has also been used to fill flexible fabric mattresses which are placed along embankments to control erosion.

CLSM mixtures typically consist of water, portland cement, fly ash, and fine or coarse aggregates, or both. Some mixtures contain only water, portland cement, and fly ash. Although the materials used in CLSM may meet ASTM or other standard specifications, it is often not necessary to use standardized materials. The selection of materials for use in CLSM is based on cost, specific CLSM application, and the required mixture characteristics including flowability, strength, excavatability, and density. The use of fly ash improves the CLSM flowability, and can also increase strength and reduce the mixture’s bleeding, shrinkage, and permeability. Air-entraining admixtures are also often used to help improve workability, reduce bleeding, help minimize segregation, reduce the unit weight, and control strength development particularly for those CLSM applications which require low strength.

Aggregates are usually the major component in CLSM, and their type, grading, and shape can affect its physical properties. However, unlike conventional concrete aggregate that are routinely required to meet standardized specifications, CLSM aggregate need not necessarily meet these same standards to be effective. For example, manufactured sands containing up to 20% nondeleterious dust of fracture have proven to be very satisfactory in CLSM.

Strength: The unconfined compressive strength is used a measure of the load-carrying ability (i.e. bearing capacity) of CLSM. A CLSM compressive strength of 50 to 100 psi is equivalent to the bearing capacity of a well-compacted soil. Maintaining CLSM strengths at low levels is critical in those projects where future excavation will be required, and therefore it is often critical that mixtures be proportioned in a manner so as to minimize later-age strength increases.

Density: The wet density of most CLSM ranges from 115 to 145 lb/ft³, which is greater than most compacted soils. A CLSM mixture with only fly ash, cement, and water might have a density range of 90 to 100 lb/ft³. CLSM dry densities can be expected to be significantly less than the wet densities due to water loss.

Settlement: While compacted soil backfill can settle even when compaction requirements have been met, CLSM does not settle after it hardens.

Shrinkage: The performance of CLSM is not adversely affected by shrinkage or shrinkage cracks. The ultimate shrinkage of CLSM is typically in the range of 0.02–0.05 %.

Excavatability: CLSM having compressive strengths of 100–300 psi can generally be excavated with mechanical equipment such as backhoes. When this is a critical property of the in-place CLSM, particular attention must be given to the type and quantity of cement and fly ash which is used, and to the judicious use of air-entraining admixtures.

Corrosivity and Compatibility with Plastics: Electrical resistivity tests can be performed on CLSM in the same manner that soils are compared for their corrosion potential on corrugated metal culvert pipe (Caltrans test method 643). One cause of galvanic corrosion in metals is the difference in potential from backfill soils of varying composition. CLSM uniformity reduces the chance for corrosion caused by dissimilar backfill materials and by their varying moisture contents.

Polyethylene materials are commonly used as underground utility protection or as the conduits themselves. CLSM is compatible with the materials, and the fine-grained nature of many CLSM’s can help minimize scratching and nicking these polyethylene surfaces.

The degree of quality control (QC) used to monitor the production and placement of CLSM is normally dependent on previous experience with the materials used, the application, and the desired level of quality. A QC program could be as simple as visual observations of the completed work when mixtures of proven performance are used. If the application is critical, regular tests for consistency and strength may be in order. ACI 229 lists a number of ASTM tests which may be included in a QC program designed to monitor CLSM consistency, strength, and density.

Many of the benefits associated with CLSM have been previously highlighted. However, a brief summary of those and others are:

  • Readily available: Ready mixed concrete producers, using locally available materials, can produce CLSM to meet most project specifications.
  • Easy to Deliver: Ready mixed concrete trucks can deliver specified quantities of CLSM to the jobsite whenever the material is needed.
  • Easy to Place: Depending on the type and location of void to be filled, CLSM can be placed by chute, conveyor, pump, or bucket. Because it is self-leveling, it needs little or no spreading or compacting. This speeds construction and reduces labor requirements.
  • Versatile: CLSM mix designs can be adjusted to meet specific fill requirements.
  • Strong and Durable: Load-carrying capacities of CLSM are higher than those of compacted soil. CLSM is also more resistant to erosion. For use as a permanent structural fill, CLSM can be designed to achieve 28-day compressive strengths of as high as 1200 psi.
  • Can Be Excavated: CLSM having compressive strengths of 100 to 300 psi is easily excavated with standard mechanical digging equipment yet is strong enough for most backfilling needs.
  • Requires Less Inspection: During placement, soil backfill must be tested after each lift for sufficient compaction. CLSM self-compacts consistently and doesn’t need this extensive field testing.
  • Allows Fast Return to Traffic: Because CLSM can be placed quickly and can support traffic loads within several hours, it minimizes downtime for pavement repairs.
  • Won’t Settle: CLSM does not form voids during placement and won’t settle or under loading. This advantage is especially significant if the backfill is to be covered by a pavement patch. Soil or granular fill, if not consolidated properly, may settle after a pavement patch is placed and forms cracks or dips in the road.
  • Reduces Excavating: CLSM allows narrower trenches because it eliminates having to widen trenches to accommodate compaction equipment.
  • Improves Worker Safety: Workers can place CLSM in a trench without entering the trench, reducing their exposure to possible cave-ins.
  • Allows All-Weather Construction: CLSM will displace any standing water left in a trench from rain, reducing the need for dewatering pumps.
  • Reduces Equipment Needs: Unlike soil, CLSM fill can be placed without loaders, rollers, or tampers.
  • Requires No Storage: Because ready mixed concrete trucks deliver CLSM to the jobsite in the quantities needed, storing fill material onsite is unnecessary. Also, there is no leftover fill to haul away.