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Services | Flowable Fill
What is flowable fill?
Flowable fill is an engineered, low strength fill material containing water, Class F fly ash, sand and portland cement. It is self-leveling, self-compacting and non-settling and has characteristics that compete favorably with traditional materials such as compacted soil, soil-cement and concrete.
Flowable fill is delivered in a slurry form that resembles very workable concrete and provides for an in-place product that is equivalent to a high-quality compacted soil without the use of compaction equipment and related labor. The flowable nature of the product is derived from the spherical particle shape of fly ash when mixed with enough water to lubricate the particle surfaces.
The compressive strength can be designed to range from 1200 psi to 50 psi. With compressive strength less than 150 psi, flowable fill can be readily excavated. Flowable fill mixes in the 50 to 70 psi range have at least two to three times the bearing capacity of well-compacted earthen backfill material. Strength development in flowable fill mixtures is directly related to the amount of cementitious material and water content. Most high fly ash content mixes only require from three to five percent portland cement to develop 28-day compressive strength in the 50 to 150 psi range. Water is added to achieve a desired flowability; at a given cement content, increased water content results in a slight decrease in compressive strength.
Bleeding and subsidence are possible in flowable fill mixes due to evaporation of the bleed water and absorption into the surrounding soil. The shrinkage may occur both laterally and vertically up to 1/8-inch/foot, but no additional shrinkage or long-term settlement of flowable fill occurs after initial set. Prior to hardening, flowable fill mixes are self-leveling.
Applications
Flowable fill can be designed to meet a wide variety of needs. Common applications include the following:
- Sewer and utility backfill
- Bridge abutment backfill
- Culvert and trench backfill
- Bedding for slabs and pipes
- Fill for embankments, bases and subbases
- Fill for abandoned storage tanks, shafts and tunnels
Advantages
- Generally delivered in conventional ready-mix trucks
- Self-leveling – does not require compaction
- Less labor intensive than conventional backfill methods
- Allows for faster placement
- Eliminates excavation for underground backfill
- More durable than compacted soil
- Less permeable and more erosion resistant than compacted soil
- Excavatable using conventional equipment
- Rapid bearing strength allows faster return of traffic loads
- No voids are formed during placement and around embedded structures
- Reduced settlement and rutting under loading
- Allows narrower trenches
- Improved worker safety – placement without entering trench
Cost competitive
Flowable fill is generally economically favorable to traditional materials because it can reduce project time with fast and easy placement of materials along with less equipment and labor.
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