What Is Flowable Fill (CLSM) and When Should You Use It?

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Featured image with the headline What Is Flowable Fill and When Should You Use It, beside a smooth pale creamy puddle of fill settled in a square recess in sand-colored ground

What do you call a cement-based fill that levels itself, needs no compaction and still is not concrete? The ready-mix industry calls it flowable fill, and the same material is also known as controlled low-strength material (CLSM). This guide covers what it is, what goes into it, how strong it gets, how it is placed and where the published sources differ.

Quick Answer

Flowable fill, also called controlled low-strength material (CLSM), is a self-leveling, self-consolidating, low-strength cementitious fill used instead of compacted fill. It is placed by ready-mix truck, pump or chute with no compaction. NRMCA defines it as under 1,200 psi, with most applications under 300 psi.

Is Flowable Fill Concrete?

No. The National Ready Mixed Concrete Association (NRMCA) states in its CIP 17 data sheet that “Flowable fill is not concrete,” and says it is not used to replace concrete.

NRMCA describes it as a self-consolidating, self-leveling, low-strength cementitious material used as an alternative to compacted granular fill. For design, its characteristics should be evaluated like soil or other fill, not like concrete.

NRMCA reports that ACI Committee 229 calls it controlled low-strength material (CLSM). Other terms on its list are unshrinkable fill, controlled density fill, soil-cement slurry, flowable mortar and lean-mix backfill.

NRMCA also notes that without manual compaction, a trench can be narrower and workers do not need to enter the excavation. To size a pour, you can estimate your trench or void volume with our flowable fill calculator.

What Goes Into a Flowable Fill Mix, and Do the Sources Agree?

A flowable fill mix is built from cement, sand and water, with fly ash optional, and the sources do not agree on one cement amount.

NRMCA slides list cement, sand, water, an air-entraining additive and other admixtures. The aggregates need not meet ASTM C33, and NRMCA says materials that do not comply with concrete standards can be used.

Cement amounts differ by source. NRMCA’s basic mixtures use 25 to 200 lb of portland cement per cubic yard. Ohio DOT Item 613 mixes use 0, 50 or 100 lb per cubic yard, depending on the type. FHWA gives 3 to 5 percent by dry ash weight for high-fly-ash mixes. The table puts the sources side by side.

Flowable fill figures by source (not stated means the figure was not in the material read)
Property NRMCA FHWA (archived) Ohio DOT Item 613 (as read)
Cement 25 to 200 lb per cubic yard in basic mixtures 3 to 5 percent by dry ash weight in high-fly-ash mixes 0, 50 or 100 lb per cubic yard by type
Strength Under 1,200 psi by definition; most applications under 300 psi 1,200 psi ACI limit; 75 to 150 psi at 28 days for high-fly-ash mixes Alternate mixes 50 to 100 psi at 28 days
Flow Spread of at least 8 in, or flow cone time of 10 to 26 s Normal slump 6 to 10 in Not stated
Hardening 1 hour at shortest, generally 3 to 5 hours to carry a person About 1 to 4 hours (one page) or 3 to 4 hours (another) to carry a person; equipment within 24 hours Must set up within 12 hours (4 hours for Type 3)
In-place density 115 to 145 lb per cubic foot Not stated Not stated

How Strong Is Flowable Fill?

NRMCA defines flowable fill as a material under 1,200 psi (8.3 MPa), and most applications use mixes under 300 psi (2.1 MPa).

For bearing, NRMCA says 50 to 100 psi (0.3 to 0.7 MPa) gives bearing capacity similar to well-compacted soil. Test cylinders are 3 by 6 inch plastic molds, tested under ASTM D4832. Ohio DOT lists its alternate mixes at 50 to 100 psi at 28 days.

Strength also matters for later digging: NRMCA slides give about 100 psi or less for hand tools, while machines are put at 200 psi by NRMCA CIP 17 and 150 psi by archived FHWA guidance, so follow your own specification.

Flowable fill strength figures on one scale Bars drawn at 0.25 pixels per psi. Bearing similar to well-compacted soil: 50 to 100 psi (NRMCA). Hand tool digging: about 100 psi or less (NRMCA slides). Machine digging: up to 150 psi (archived FHWA) and up to 200 psi (NRMCA). Most applications: under 300 psi. Definition: under 1,200 psi. Strength figures by source, psi (one scale) Bearing like well-compacted soil: 50 to 100 psi (NRMCA) Digging by hand tools: about 100 psi or less (NRMCA slides) Digging by machine: up to 150 psi (FHWA, archived) Digging by machine: up to 200 psi (NRMCA CIP 17) Most applications: under 300 psi (NRMCA) Definition: under 1,200 psi (NRMCA) Scale: 0.25 px per psi, so 1,200 psi = 300 px. Bars start at 0 psi except the bearing bar, which spans 50 to 100 psi.
The 1,200 psi definition bar is 300 px long and the 300 psi bar is 75 px long.

How Is Flowable Fill Placed, and How Fast Does It Harden?

A ready-mix truck delivers flowable fill, and the chute can place it directly into the space.

NRMCA says it can also be conveyed by pump, chute or bucket, and that it flows long distances from the point of placement. It needs no consolidation, compaction or vibration. It does not need to be cured like concrete, but NRMCA says to protect it from freezing until it has hardened.

NRMCA treats a spread of at least 8 inches (ASTM D6103) without visible segregation as good flowability, and a flow cone time of 10 to 26 seconds as generally recommended for mixes without coarse aggregate. FHWA gives a normal slump of 6 to 10 inches. For truck planning, see our concrete truck calculator.

NRMCA slides say the time to support a person of average weight can be as short as 1 hour but generally takes 3 to 5 hours under normal conditions. Cement type and quantity, temperature, humidity and fill depth all change it. Archived FHWA guidance puts 5 percent cement mixes at about 1 to 4 hours for a person on one page and 3 to 4 hours on another, with equipment usually able to operate within 24 hours.

Flowable fill hardening times on one scale Bars drawn at 10 pixels per hour. NRMCA: as short as 1 hour, and generally 3 to 5 hours to carry a person. Archived FHWA: about 1 to 4 hours to carry a person for 5 percent cement mixes, and equipment usually within 24 hours. Hardening time by source, hours (one scale) NRMCA slides: as short as 1 hour NRMCA slides: generally 3 to 5 hours FHWA, archived: about 1 to 4 hours, or 3 to 4 on another page (5 percent cement) FHWA, archived: equipment usually within 24 hours 0 h 12 h 24 h Scale: 10 px per hour. The darker part of a range bar is the stated range; the lighter part is the time before it starts.
Walk-on times are in single hours; the FHWA equipment figure is 24 hours, or 240 px.

What Is Flowable Fill Used For?

Flowable fill is mostly used as backfill, and FHWA lists trench backfill for storm drains and utility lines as the most common use.

NRMCA CIP 17 lists these uses:

  • Backfill. Sewer and utility trenches, bridge abutments, conduit encasement, pile excavations, retaining walls and road cuts.
  • Structural fill. Foundation sub-base, sub-footing, floor slab base, pavement bases and conduit bedding.
  • Other. Abandoned mines, underground storage tanks, wells, tunnel shafts and sewers, voids under pavement, and erosion control.

The sources read for this guide do not address code approval for structural use. A compacted stone base is a different job; see the gravel base calculator.

What Are the Limits and Cautions?

NRMCA warns about floating pipes and tanks, fluid pressure on forms and walls, and limited durability.

Placing fill around or under tanks, pipes or large containers can make them float or shift, so NRMCA advises placing it in lifts and anchoring them. Archived FHWA guidance says lightweight pipes such as corrugated metal must be anchored. Ohio DOT Item 613, one state specification, also requires securing conduit against floating.

Flowable fill placed in lifts around a pipe (example, not to scale) A trench cross-section with three lifts of flowable fill labelled Lift 1, Lift 2 and Lift 3, a pipe in the middle, soil on both sides, a note that fluid fill presses on forms and walls, and a warning that fill can float or shift a pipe. Example, not to scale: fill placed in lifts around a pipe Pipe Lift 3 Lift 2 Lift 1 Soil Fluid fill presses on forms and walls Warning: fill can float or shift a pipe. Place in lifts and anchor the pipe.
NRMCA advises lifts and anchoring so pipes do not float or shift.

NRMCA says flowable fill should not be expected to resist freeze-thaw cycles, abrasion or erosion, or aggressive chemicals. Where it deteriorates in place, it continues to function like granular fill. NRMCA also advises stating the intended use and any expected future excavation when you order.

This guide does not cover cost. It read one state specification (Ohio DOT Item 613), and it did not read ACI 229R directly.

How Much Does a Flowable Fill Pour Weigh?

NRMCA gives an in-place density of 115 to 145 lb per cubic foot, so the weight of a pour moves with the mix.

The steps below are arithmetic from the calculator’s default trench preset and that source range, not a rule from a source. The preset is a 50 ft by 2 ft by 3 ft trench with 10 percent waste.

  1. Trench volume. 50 x 2 x 3 = 300 cu ft.
  2. Add waste. 300 x 1.10 = 330 cu ft, and 330 / 27 = 12.22 cu yd.
  3. Low density. 330 x 115 = 37,950 lb.
  4. Calculator default. 330 x 120 = 39,600 lb, which is 19.8 short tons.
  5. High density. 330 x 145 = 47,850 lb.

The gap between the low and high ends is 47,850 – 37,950 = 9,900 lb. Density depends on the mix, and NRMCA notes that foamed or high-air mixes are lighter.

Ready to size a pour?

Enter your trench or pipe dimensions in the Flowable Fill Calculator to get volume and weight. For other pours, browse the concrete volume and ordering calculators.

FAQs About Flowable Fill

Is flowable fill the same as concrete?

No. NRMCA states that flowable fill is not concrete and is not used to replace concrete. It is a low-strength cementitious fill that should be evaluated like soil or other fill.

What does CLSM stand for?

CLSM stands for controlled low-strength material. NRMCA reports that this is the term used by ACI Committee 229.

How strong is flowable fill?

NRMCA defines it as under 1,200 psi (8.3 MPa), and most applications use mixes under 300 psi (2.1 MPa). A strength of 50 to 100 psi gives bearing similar to well-compacted soil.

How long before you can walk on flowable fill?

NRMCA slides say as short as 1 hour but generally 3 to 5 hours under normal conditions. Archived FHWA guidance gives about 1 to 4 hours on one page and 3 to 4 hours on another for 5 percent cement mixes.

Does flowable fill need compaction?

No. NRMCA says it needs no compaction or vibration, and it does not need to be cured like concrete. It should be protected from freezing until it has hardened.

Can pipes float in flowable fill?

Yes. NRMCA warns that placing fill around or under pipes or tanks can make them float or shift. It advises placing the fill in lifts and anchoring the pipe.

What other names does flowable fill go by?

NRMCA lists unshrinkable fill, controlled density fill, soil-cement slurry, flowable mortar and lean-mix backfill as other terms for it.

Sources and Further Reading

References Used in This Article

This guide summarizes published figures and is not a mix design. Flowable fill mixes and specifications vary by agency, so follow the specification for your project. Reviewed for accuracy by Prof. Dr. Khalil Mudassar, PhD. Last updated 2026-10-19.