Flowable Fill


 Flowable fill is a self-compacting low strength material with a flowable consistency that is used as an economical fill or backfill material as an alternative to compacted granular fill. Flowable fill is not concrete, nor it is used to replace concrete. Terminology used by ACI Committee 229 is Controlled Low Strength Material (CLSM). Other terms used for this material are unshrinkable fill, controlled density fill, flowable mortar or lean-mix backfill.

 In terms of its flowability, the slump, as measured for concrete, is generally greater than 8 inches (200 mm). It is self-leveling material and can be placed with minimal effort and does not require vibration or tamping. It hardens into a strong material with minimal subsidence.

 While the broader definition includes materials with compressive strength less than 1200 psi (8.3 MPa), most applications use mixtures with strength less than 300 psi (2.1 MPa). The late-age strength of removable CLSM materials should be in the range of 30 to 200 psi (0.2 to 1.4 MPa) as measured by compressive strength of cylinders. It is important that the expectation of future excavation of flowable fill material be stated when specifying or ordering the material.


 Flowable fill is an economical alternative to compacted granular fill, considering the savings in labor costs, equipment and time. Since it does not need manual compaction, trench width or the size of excavation is significantly reduced. Placing flowable fill does not require people to enter an excavation, a significant safety concern. CLSM is also an excellent solution for filling inaccessible areas, such as underground tanks, where compacted fill cannot be placed.

      Uses of Flowable Fill include:

 1.   BACKFILL -- Sewer trenches, utility trenches, bridge abutments, conduit encasement, pile excavations, retaining walls, and road cuts.

2.   STRUCTURAL FILL -- Foundation subbase, subfooting, floor slab base, pavement bases, and conduit bedding.

3.   OTHER USES -- Abandoned mines, underground storage tanks, wells, abandoned tunnel shafts and sewers, basements and underground structures, voids under pavement, erosion control, and

      thermal insulation with high air content flowable fill.

Information excerpted from the NRMCA Concrete In Practice Papers.  For more information go to www.nrmca.com  

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