Flowable fill concrete is a self-compacting cementitious slurry consisting of a mixture of fine aggregate or filler, water, and cementitious material which is used as a fill or backfill in lieu of compacted-soil backfill. This mixture is capable of filling all voids in irregular excavations and hard to reach places (such as under undercuts of existing slabs), is self-leveling, and hardens in a matter of a few hours without the need for compaction in layers.
Flowable fill is sometimes referred to as controlled density fill (CDF), controlled low strength material (CLSM), lean concrete slurry, and unshrinkable fill.
Flowable fill materials will be used as only as a structural fill replacement on VA projects. Unless otherwise noted, flowable fill installed as a substitution for structural earth fill, shall not be designed to be removed by the use of hand tools.
The materials and mix design for the flowable fill should be designed to produce a comparable compressive strength to the surrounding soil after hardening, making excavation at a later time possible to produce the compressive strength indicated for the placed location, as determined by the Engineer.
Main Ingredients of flowable fill concrete are cement, sand, water with fly ash GGBF slag, stable air generators.
Properties of Flowable Fill Concrete
It has high lump (nearly 9 inch) but the strength is low, commonly 100 PSI (0.7 Mpa) in 28 days. It has low cementitious mix, does not hydrate and it settles and dewaters.
The less water does not equate to faster hardening and too little water will not allow proper settlement. So, the more the concrete is wetter, the more it is better.
Therefore, specification offers flexibility of its mix design, minimum mix design testing and little or no field testing required.
Compacted-Soil Backfill Concerns
Lack of compaction causes most call-backs to repair settlement problems. Improper backfill is the number one complaint of public works directors. Study indicates that 65% of utility cut restorations were unsatisfactory.
Other Concerns is compaction around pipes for support.
Number of compaction lifts:
- Increases project construction time.
- Continuous testing.
- Requires continuous inspection.
- Safety of workers.
- Moisture content.
- Storage of materials on site.
- Inconsistent soil test results.
Benefits of Flowable Fill
- Reduced in-place costs- less labor & equipment
- Minimized settlement
- Easily removable
- Year-round availability
- No compaction required
- Self leveling
- Increased worker safety
- Can Be color-coded for utility identification
Flowable Fill Applications
- Structural Fill
- Other Uses
Flowable Fill as Backfill
It is used where speed of construction is advantageous and where better performance is required. Ideal for use in tight or restricted areas where placing soil fill is difficult.
- Trenches — utilities, primarily under roadways
- Bridge abutments
- Pipe bedding
- Building excavations
- Wet conditions
Flowable Fill as Structural Fill
- Road base
- Mud jacking
- Sub footing
- Floor slab base
Other Uses for Flowable Fill
- Abandoned sewers, tanks, pipe, etc.
- Environmental remediation
- Slope stabilization
Typical Areas Which are Difficult to Compact
- Road Cuts
- Under & Around Pipes
- Underground Tanks
- Foundation Wall Backfilling