What is Flowable Fill?

A low strength material mixed to a wet, flowable, slurry used as an economical fill or backfill material, placed by pouring it into the cavity to be filled. Slumps, measured in the ordinary way, are generally 8 in. or higher. It is self-leveling. With a consistency similar to pancake batter, it can be placed with minimal effort and no vibration or tamping. It hardens and develops strength.

ACI Committee 229 calls it “Controlled Low Strength Material” (CLSM) – it is not considered concrete. Other names used for this material are – “flowable mortar” and “lean-mix backfill”. If it is anticipated or specified that the flowable lean-mix backfill may be excavated at some point in the future, the strength must be much lower than the 1200 psi which ACI uses as the upper limit CLSM. The late-age strength of removable CLSM materials should be in the range of 30 to 150 psi as measured by compressive strength in cylinders.

Why is CLSM Used?

Flowable CLSM mixtures are an economical alternative due to the saving of labor and time over placing and compacting soil or granular materials.

Uses of Flowable Fill Include:
BACKFILL: Sewer Trenches, Utility Trenches, Bridge Abutments, conduit Trenches, Pile Excavations and Retaining Walls.
STRUCTURAL FILL: Foundation Sub-base, Subfooting, Floor Slab Base and Pipe Bedding.
OTHER USES: Filling Underground Storage Tanks, Wells, Utility company vaults, Voids Under Pavements, Sewers and Manholes, and to stabilize Muddy Conditions.

How is Flowable Fill Ordered?

Ask for it by intended use and indicate whether it is required to be easily removed later. Ready mixed concrete producers have developed mix design proportions for flowable CLSM products that take into consideration the products ultimate use.

Strength (for later removability): At least 20 psi is needed at 3 days and 30 psi at 28 days (ASTM C 403 Penetration Resistance Numbers of 500 to 1500) should be obtained to assure required bearing capacity in-place as a fill. However, later age strength must be limited to assure convenient removal with power equipment.

Strength (when it can or must be higher): When higher strength fills are required or can be tolerated because removal is not required, use higher cement contents.

Setting and Early Strength may be important where equipment, traffic, or construction loads must be carried. Judge settling by scraping off loose accumulations of water and fines on top and see how much force is necessary to cause an indentation in the material. ASTM C403 penetration can be run to estimate bearing strength.

Density in place is usually in the 115 to 145 lb/cu ft range, higher than that obtained from most compacted soils or aggregates. If lightweight fills are needed or if greater thermal insulation is needed, high air-entrainment, foam materials, and/or lightweight aggregates can be used.

Flowability of CLSM is important, so the mixture will flow into place and consolidate due to its fluidity without vibration or puddling action.

Durability. CLSM fill materials are not designed to resist freezing and thawing, abrasive or erosive actions or aggressive chemicals. If this is required, use high quality concrete. Fill materials are usually buried in the ground or otherwise confined. If CLSM deteriorates in place it will continue to act as a granular fill.

How is Flowable Fill Delivered and Placed?

CLSM is delivered in ready mixed concrete trucks and placed easily by chute in a flowable condition directly into the cavity to be filled or into a pump for final placement. Keep the drum agitating. For efficient pumping, some granular material is needed in the mixture.

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