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Applications  > Controlled Density Fill
'Liquid Soil' fits tight spaces without compaction.
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control density fill CDFControlled Density Fill is a self-compacting, 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 low-strength material, flowable mortar, plastic soil-cement, soil-cement slurry, and K-Krete. CDF is defined as a material that results in a compressive strength of 1200 psi (8 MPa) or less. Most current applications of CDF require unconfined compressive strengths of 300 psi (2 MPa) or less. This allows for future excavation of the material.


Since 1964, flowable fill has become a popular material for projects such as
  • structural fill
  • foundation support
  • pavement base
  • conduit or pipe bedding
Flowable fill used at a Federal office building construction site (PCA No. 13868)Flowable fill is a versatile "liquid soil." It is placed as a flowable liquid, yet hardens and rapidly develops load-bearing properties with no compaction. The properties of flowable fill make it an economical alternative to compacted granular material due to savings of labor and time during placement. A constant supply of material will make it flow horizontally a distance of 300 feet (90 m) or more. Flowable fill also has the advantage of displacing any standing water left in a trench.
The flowable characteristics of this material mean it can readily be placed into a trench and into tight or restricted-access areas where placing and compacting fill is difficult. Flowable fill is used as a bedding material for pipe, electrical, telephone, and other types of conduits because the mixture easily fills voids beneath the conduit and provides uniform support. Flowable fill is used to fill large voids such as abandoned underground storage tanks, basements, tunnels, mines, and sewers. Other uses include paving sub-base, bridge abutment, and retaining wall backfill.
Flowable fill is an ideal pavement base material because it will not settle or rut under loads. It can be placed quickly and support traffic load within hours of placement thereby minimizing repair time and allowing a rapid return of traffic. Costs may be equal or less than the cost of using standard compacted backfill.


Like most concrete products, CDF provide many green benefits. See associated sustainability solutions and technical briefs (right) for more detail. 
Local. Materials are usually extracted and manufactured locally.  May contribute to LEED Credit M 5.

Recycled content. Fly ash, slag cement, or silica fume can substitute partially for cement, and recycled aggregate may be used. Recycled content can contribute to LEED Credit M 4.

Durable and low maintenance. CDF provides a more effective fill in many circumstances.

Reduces Site Disturbance. Because CDF flows into voids it doesn't require as extensive excavation. May contribute to LEED Credit SS 5.


CDF, or flowable fill is composed of water, portland cement, fine aggregate, and fly ash or slag cement. It is a fluid material with typical slumps of 10 inches or more, and has the consistency of a milk shake. Like most concrete, flowable fill may be mixed in central-mix concrete plants, ready-mixed concrete trucks or pugmills (small on-site mixers).

Once flowable fill is transported to the jobsite, the mixture may be placed with chutes, conveyors, buckets, or pumps, depending on the application. Flowable fill is placed continuously in most applications. For pipe bedding and backfilling, flowable fill is placed in lifts to prevent the pipes from floating. Internal vibration or compaction is not needed to consolidate mixtures. Its flowability and weight are sufficient for consolidation.

A ready mix concrete producer can aid in developing a mix design for flowable fill. However, when ordering, consider the following properties:

Strength: Applications that require removal of flowable fill 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 eight hours depending on mix design and trench conditions (e.g moisture, temperature).

Density in Place: The in-place density of normal flowable fill typically ranges from 90 to 125 pcf (1400 to 2000 kg/m3)

Flowability: Flowability can be enhanced through the use of fly ash or air entraining admixtures.

Durability: Flowable fill is not designed to resist freezing and thawing, abrasive or erosive actions, or aggressive chemicals.
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