Flowable fill, a fluid, low strength material, is a cost effective alternative for project owners, specifiers, and contractors to meet a project's backfill or structural fill needs.

A flowable slurry-like material with the consistency of pancake batter, flowable fill is self-leveling, can be placed in one lift with minimal labor and no vibration or tamping, and reaches 95% or more compaction within a few hours of placement, according to Rick Meininger, vice president of research for the National Ready Mixed Concrete Association.

Due to reduced labor, reduced excavation and reduced equipment needs, flowable fill offers lower in-place costs for all your backfill projects.

The editors of Concrete Construction magazine, in their May 1991 issue, included the following 15 reason for owners, specifiers, and contractors to use flowable fill.

1. Readily Available

Ready mixed concrete producers, using locally available materials, can produce flowable fill to meet most project specifications.

2. Easy to Deliver

Ready mix trucks can delivery specified quantities of flowable fill to the jobsite whenever the material is needed

3. Easy to Place

Depending on the type and location of void to be filled, flowable fill can be placed by chute, conveyor, pump, or bucket. Because flowable fill is self-leveling, it needs little or no spreading or compacting. This speeds construction and reduces labor requirements.

4. Versatile

Flowable fill mix designs can be adjusted to meet specific fill requirements. Add more water to improve violability. Add more cement or fly ash to increase strength. Admixtures can be added to adjust setting times and other performance characteristics. Adding foaming agents to flowable fill produces a lightweight, insulating fill.

5. Strong and Durable

Load-carrying capacities of flowable fill typically are higher than those of compacted soil or granular fill. Flowable fill also is less permeable, thus more resistant to erosion. For use as a permanent structural fill, flowable fill can be designed to achieve 28-day compressive strengths of as high as 1200 psi.

6. Can be Excavated

Flowable fill having compressive strengths of 50 to 100 psi is easily excavated with conventional digging equipment yet is strong enough for most backfilling needs.

7. Requires Less Inspection

During placement, soil backfill must be tested after each lift for sufficient compaction. Flowable fill self-compacts consistently and doesn't need this extensive field testing.

8. Allows Fast Return To Traffic

Because flowable fill can be placed quickly and can support traffic loads within several hours, it minimizes downtime for pavement repairs.

9. Won't Settle

Flowable fill does not form voids during placement and won't settle or rut under loading. This advantage is especially significant if the backfill is to be covered by a pavement patch. Soil or granular fill, if not consolidated properly, may settle after a pavement patch is placed and forms cracks or dips in the road.

10. Reduces Excavating

Flowable fill allows narrower trenches because it eliminates having to widen trenches to accommodate compaction equipment.

11. Improves Worker Safety

Workers can place flowable fill in a trench without entering the trench, reducing their exposure to possible cave-ins.

12. Allows all-Weather Construction

Flowable fill will displace any standing water left in a trench from rain or melting snow, reducing the need for dewatering pumps. To place flowable fill in cold weather, heat the material using the same methods for heating ready mixed concrete.

13.  Reduces Equipment Needs

Unlike soil or granular backfill, flowable fill can be placed without loaders, rollers, or tampers.

14. Requires No Storage

Because ready mix trucks deliver flowable fill to the jobsite in the quantities needed, storing fill material onsite is unnecessary. Also, there is no leftover fill to haul away.

15. Makes Use of a Waste By-Product

Fly ash is a by-product produced by power plants that burn coal to generate electricity. Flowable fill containing fly ash benefits the environment by making use of this industrial waste material.

Recommeded Flowable Fill Specification...

1. Description 

This section specifies the requirements for flowable fill used for trenches, support for pipe structures, culverts, utility cuts and other works where cavities exist and where firm support is needed for pavements and structural elements. Flowable fill may also be used to fill abandoned water and sewer lines, abandoned fuel tanks, and where any granular compacted fill is needed as is approved by the Engineer. 

2. Materials 

Materials shall meet the requirements specified in the following sections of the Standard Specifications: 

Portland Cement, Type I, II, or III ASTM C150 

Fly Ash* ASTM C618 

Fine Aggregate/Mineral Filler ** 

Air Entraining Admixtures As Per Manufacturer's Specifications 

Light Weight Cellular Admixture As Per Manufacturer's Specifications 

Water Potable 

* LOI limits do not apply. 

** Any ASTM or non-ASTM sands or mineral fillers with 100% passing the 1/2" sieve may be considered which produce an acceptable flow and desired performance characteristic. 

3. Mix Proportions 

The contractor and/or the material supplier shall be responsible for producing a flowable mixture using these guidelines and adjusting the mixture design as called for by the circumstances or as directed by the Engineer. 

For excavatable mixes, the flowable fill material should be proportioned to produce a 28-day compressive strength of 75-100 psi (not a specification requirement). 

Typical mix design proportions for normal weight flowable fill are as follows: 

1. Portland Cement 30-70 lbs/yd3 

Fly Ash 250-400 lbs/yd3 

Fine Aggregate/Mineral Filler 2000-3000 lbs/yd3 

Air Content 0-70% 

Water 600 lbs/yd3 

2. Portland Cement 30-70 lbs/yd3 

Mineral Filler 2000-3000 lbs/yd3 

Water 800 lbs/yd3 

Note: Weights or volumes of each material should be adjusted to approximate one cubic yard of flowable fill. For 
Light Weight Cellular flowable fill proportions, consult with manufacturer's recommendations. 

The percentage of cement may be increased above these limits only when early strength is required and future removal of the product is not required. The mix proportions shall be adjusted for removability, pumpability, flowability and setting characteristics. Set times are a product of consolidation of the flowable fill due mainly to dehydration. A decrease in soil permeability, an increase in air content or excessive fine material will cause an increase in the setting time of flowable fill. 

4. Testing 

Testing of flowable fill in the field is not usually necessary. In the event that testing is required, follow the recommendations of ACI 229. 

Modified Slump Test 

Apparatus: 

Scoop, measuring tape, flat edge, 3" x 6" cylinder mold open at both ends, and a flat non-absorbent surface. 

Procedure: 

1. Set mold upright on flat surface. 

2. Scoop representative sample of flowable fill. 

        1.Fill mold with sample without tamping and then strike off the top with flat edge to form a flat surface.

        2.Clear any residue from around the bottom of the mold. 
        3.Lift the mold straight up for a duration of 3 seconds, allowing sample to spread on the flat surface. 

6. Measure the spread's diameter to the nearest 1/2". A spread of 7-9 inches is considered flowable. 

Air Content 

Use ASTM C231 procedures where applicable. Unit weight method may also be desirable with high air content mixtures. 

Compressive Strengths 

No testing shall be done for compressive strengths in the field. 

5. Production and Placing 

Flowable fill shall be produced and delivered using standard concrete construction equipment and practices. Placing flowable fill shall be by chute, pumping, or other method approved by the Engineer. 

6. Construction Requirements 

The flowable fill shall be discharged directly from the mixer truck into the space to be filled. No compaction or vibration of the material is required. The mix may be placed part depth or full depth as conditions dictate. Care is to be taken to prevent pipe from floating. Formed walls or other bulkheads shall be constructed to withstand hydrostatic pressure exerted by the plastic flowable fill. All necessary means to confine the material within a designated space shall be provided. Curing of the flowable fill is not required. 

7. Limitation of Operation 

No flowable fill shall be placed on frozen ground. Mixing and placing of the material is acceptable in freezing temperatures. At the time of placement the flowable fill shall have a temperature of at least 40 degrees F. When flowable fill is placed in freezing temperatures, the material should be covered with blankets overnight. When paving over flowable fill in cold weather, any frozen material on the surface can be scraped off and removed prior to paving. 

8. Acceptance 

The flowable fill shall be proportioned and placed as specified herein. In general, the strength desired is the maximum hardness that can be excavated at a later date using conventional excavating equipment. Failure to do so will result in non-compliance. 

The flowable fill shall be left undisturbed until the material obtains sufficient strength. Sufficient strength for paving is achieved when the flowable fill can support the weight of foot traffic without apparent deformation. Sufficient strength for supporting vehicular traffic is 2.5 tons per square foot as measured by a pocket penetrometer. 

This specification is for information purposes only. Actual job specifications should be accepted by a Professional Engineer