On Portland Metro and Pacific Northwest jobs, “flowable fill” and “cellular concrete” get used as if they were the same product. They are not. Both can place as a fluid and set in place. The difference that shows up on a spec, a pump, and a settlement calculation is unit weight — and whether that weight was designed down with preformed foam.
This is for specifiers and contractors picking a fill. It is not a mix design or a substitute for the project engineer.
What cellular concrete actually is
Cellular concrete — also called cellular grout or lightweight cellular concrete (LDCC) — is a cementitious mix with a foamed cell structure. The usual production path is a cement slurry plus preformed foam. The slurry is the carrier. The foam is how you get a specified cast density instead of a soil-like unit weight.
Volume Concrete’s Cellular Backfill line is made that way: Type IL cement slurry, preformed foam from Aerix Industries AERLITE / AERLITE-iX, sand optional, permeable or non-permeable as specified. Production is volumetric trucks and mobile silos — on-site, not plant-batched only. Home base is Estacada, Oregon (city-level; no plant street). Cellular jobs travel the Pacific Northwest.
The spec knob is cast density at placement, in pounds per cubic foot. Strength, permeability, and pumpability follow the mix, but density is what you call out first. Cellular Backfill starts at about 27 pcf and is dialed up — typically toward 100 pcf — when the engineer specifies a heavier fill. A common mid-range job example on our mix packet is about 38 pcf, non-permeating. Packet maturity for that example is about 104 / 144 / 180 psi at 7 / 14 / 28 days — the design Volume stands behind when that mix is ordered.
Cellular grout is not polyurethane foam fill. PU foam is a polymer. Cellular Backfill is cementitious grout that sets like concrete at the density you specified.
What typical CLSM / flowable fill is
Controlled low-strength material (CLSM), often called flowable fill, is also a self-leveling cementitious fill. In everyday use it is a sand–cement slurry (sometimes with fly ash or other fines) designed to flow into a trench or annulus and set to a low excavatable strength.
Unless the specification adds foam, that mix is still relatively heavy. Unit weights commonly sit in the soil-to-lean-concrete range — useful for mass, confinement, or a trench backfill that does not need compaction lifts. It is the right product for a lot of utility work. It is the wrong product when the reason you are filling the space is to take load off a pipe, tank, abutment, or weak subgrade.
Saying “we’ll use flowable fill” does not automatically get you a lightweight cellular mix. If the engineer wanted LDCC, the spec has to say so — density, foam, and all.
When each tends to make sense
Lean toward cellular concrete / cellular grout when:
- You need load reduction. A 27–40 pcf fill is a different dead load than a 100-plus pcf slurry — on poor soils, over buried structures, or anywhere settlement is already tight.
- The work is pipe or tank abandonment, void fill, or slip-liner / annular grouting. A light, pumpable grout can occupy the space without stacking extra mass on the host.
- The fill is foundation or abutment backfill and the design is trying to cut load, not add it.
Lean toward conventional CLSM / flowable fill when:
- The design wants a heavier cementitious fill and does not need foam.
- The trench is standard utility backfill and compaction effort is the problem, not unit weight.
- The specification already names CLSM and does not call out a cellular density.
Both can appear on the same project. The mistake is treating the words as interchangeable on the bid form.
Spec knobs worth collecting before the truck shows up
A useful quote or mix callout for cellular work usually includes:
- Cast density (pcf) at placement — the primary knob. “Lightweight” is not a number.
- Permeable vs non-permeable. Permeable mixes are meant to let water move through the set mass. Non-permeating mixes are common when the fill should not become a drain path — a frequent callout on slip-liner and pipe decommissioning. The engineer picks this.
- Strength, if the design needs it. Do not invent a psi table. If the packet or engineer gives a 7/14/28-day target, write it down. Density still comes first.
- Volume in cubic yards, or a geometry we can estimate. On-site volume calculators cover full-pipe abandonment, annular fill, and common tank shapes — geometric estimates only.
- Pump access and hose run. The mix is made to pump. The job still needs a place to stand the truck.
- Water table and buoyancy. See below.
The density explorer interpolates published Aerix ASTM C495 values — a manufacturer reference for planning. Volume designs and guarantees the mix delivered to your project specs.
Buoyancy and lift thickness
Fresh water weighs about 62.4 pcf. Any fill cast well below that is lighter than the water it can displace. In a flooded pipe, tank, or excavation, that is not a trivia fact: the grout can float or lift the host if you do not stage the placement.
Treat buoyancy as a design item whenever specified density is ≪ 62.4 pcf. Anchors, staged filling, vents, and a written sequence belong to the engineer and the contractor.
Placement is usually done in lifts. A typical starting point is about 4 feet unless the foam system and the engineer allow more. Thicker lifts are possible with the right foam. They are not a default. Ask before you assume a single-shot fill on a deep tank.
Standards to be aware of — not a certificate wall
Literature and test methods specifiers often see include ASTM C495, ASTM C796, ASTM C869, and ACI 523.1R. Volume’s Cellular Backfill mixes are developed in accordance with ACI 301 and ASTM C685, C1602, C150, and C1600.
That is standards awareness, not a certificate wall. If the job needs cylinders, a submittal, or a named standard, put it in the documents. We will not invent extra marks.
How we deliver it in the Portland Metro and PNW
Cellular Backfill is a product line of Volume Concrete LLC. Everyday ready-mix stays in Greater Portland Metro. Cellular work is mobile, so we take jobs anywhere in the Pacific Northwest — Oregon, Washington, and Idaho — when the mix is this product. Confirm the jobsite on the call.
Public NAP (billing / postal, not a plant street):
Volume Concrete LLC
4230 SE King Rd. #211
Milwaukie, OR 97222
(971) 219-8604
If you are writing a spec or pricing abandonment and backfill, call dispatch with density, permeable vs not, volume, and access. Same facts live on the service area page, the calculators, and the density tools.