Flowable Fill: What It Is, How Strong It Gets and Uses

Flowable fill is a runny cement-based mix that is poured into a trench, hole or void instead of shoveling and compacting soil. It flows into corners and under pipes, levels itself and hardens in a few hours. Utilities, road crews and contractors use it when compaction would be slow or risky, and when a trench needs to be back in service fast. This guide explains what is in it, how strong it gets, where it is used and what to ask for when you order.

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The short answer

What it is: water, a small amount of cement or other cementitious material, and fine filler such as sand or fly ash.
Other names: controlled low strength material (CLSM), controlled density fill (CDF), lean concrete slurry and unshrinkable fill.
Strength: 1,200 psi or less at 28 days by the ACI definition; most backfill uses target 300 psi or less.
Set time: often firm enough to walk on in about 1 to 4 hours, with equipment able to cross it within about a day.
Delivered: by ready-mix truck and ordered by the cubic yard.

What flowable fill is made of

The Federal Highway Administration's guidance describes flowable fill as a slurry made from water, a cement-type binder and a fine filler. Sand is the most common filler, though fly ash, quarry fines and other fine materials are also used. Depending on the mix, a cubic yard can contain about 1,500 to 3,000 pounds of filler and about 50 to 80 gallons of water.

Cement content is low. FHWA notes that at about 3 to 5 percent cement by weight, typical 28-day strengths run about 75 to 150 psi. Fly ash is often added because its fine, round particles help the mix flow.

How strong flowable fill gets

The American Concrete Institute defines controlled low strength material as a self-compacting cementitious material with a compressive strength of 1,200 psi or less at 28 days. That ceiling is high on purpose, but in practice most uses stay much lower. FHWA notes that most applications involve strengths of 300 psi or less, and many are designed to end up about as strong as the soil around them.

Low strength is a feature, not a flaw. A utility trench backfilled with a weak, excavatable mix can be reopened later with a backhoe or even hand tools. Industry guidance commonly keeps hand-diggable mixes at around 100 psi or less. Coarse aggregate in the mix makes removal harder even at low strength, so say so when you order if future digging is likely.

Common uses

Utility trenches under roads and driveways, where compacting soil around pipes in a narrow trench is slow and settlement causes patches to sink. See why a utility cut patch keeps sinking.
Pipe bedding and encasement, because the slurry fills the space under the pipe that is hard to reach with a tamper.
Filling abandoned tanks, pipes and tunnels so they cannot collapse later.
Backfill behind walls and abutments.
Voids under slabs and pavement, and sinkholes or washouts.
Lightweight fill. Foamed mixes can weigh as little as about 20 to 80 pounds per cubic foot for building over soft soil.

Flowable fill vs concrete vs compacted soil

Concrete is designed for strength, often several thousand psi, and is used for slabs, footings and structures. Flowable fill is not a substitute for any of those.
Compacted soil or stone is cheaper by the yard but has to be placed in thin lifts and compacted, and poor compaction leads to settlement.
Flowable fill costs more per yard than soil but saves labor, needs no compaction equipment in the trench and fills shapes soil cannot.

Placement tips

Watch for floating pipe. The slurry is dense and can lift empty pipe or conduit. Crews often pour in stages or weigh and anchor the pipe.
Expect some bleed water and settling in wet mixes before they harden. FHWA notes shrinkage cracks can form at the surface as bleed water evaporates.
Protect it from freezing before it hardens.
Plan the patch. Once the fill is firm, the trench can be capped with base and asphalt or concrete.

Flowability is often checked with a slump test or a flow cone. FHWA lists a normal slump range of about 6 to 10 inches, much wetter than typical concrete; see what concrete slump means.

How much flowable fill to order

Measure the void: length × width × depth in feet gives cubic feet.
Subtract any pipe or tank you are not filling.
Divide by 27 for cubic yards.
Add a little for irregular trench walls and spillage.

Example: a trench 2 feet wide, 4 feet deep and 50 feet long is 400 cubic feet, about 14.8 cubic yards. The concrete calculator gives the same yardage for rectangular pours. Small orders can carry a short-load charge; see concrete short load fees and how many yards fit in a truck.

What to ask the ready-mix plant

Do you carry a flowable fill or CLSM mix, and what strength is it designed for?
Is it excavatable, and by hand or by machine?
Does it contain coarse aggregate?
Does it meet the local DOT or utility spec named in the permit?
What are the minimum order and short-load charges?

Questions people ask

What is flowable fill?

Flowable fill is a fluid mix of water, cement or other cementitious material and fine filler such as sand or fly ash. It is poured instead of compacted soil, levels itself, fills voids and hardens without tamping. It is also called CLSM, controlled density fill or lean slurry.

How strong is flowable fill?

The American Concrete Institute defines controlled low strength material as 1,200 psi or less at 28 days. Most backfill uses aim far lower, at 300 psi or less, and mixes meant to be dug up later stay lower still.

Is flowable fill the same as concrete?

No. It is delivered by ready-mix truck and looks like a soupy concrete, but it is designed for low strength and easy placement, not for slabs, footings or structural work.

How long does flowable fill take to harden?

Federal guidance says a typical mix with about 5 percent cement can usually support a person in about 1 to 4 hours, and construction equipment can usually move across it within 24 hours. Weather and mix change that.

Can flowable fill be dug out later?

Yes, if it is designed for it. Low-strength mixes can be dug by hand or machine. Mixes with coarse aggregate or higher strength can be very hard to remove, so ask for an excavatable mix if a utility may need access.

How much flowable fill do I need?

Measure the void in cubic feet and divide by 27 for cubic yards. A trench 2 feet wide, 4 feet deep and 50 feet long holds about 14.8 cubic yards. Add a little for irregular sides.

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