Fresh water weighs about 62.4 pcf. Lightweight cellular concrete and cellular grout used on Portland Metro and Pacific Northwest geotech jobs are often cast in the mid-20s to mid-50s pcf. That gap is not a footnote — it is a design and field control item whenever placement sits below the water table, in a flooded excavation, or inside a pipe or tank that can hold water.
This note is for engineers and contractors writing or reviewing cellular specs in the PNW. It is industry practice awareness plus how Volume Concrete’s Cellular Backfill line fits the conversation. It is not a buoyancy calculation worksheet and not a substitute for the engineer of record.
Why light mixes float relative to water
Cellular concrete is cement slurry plus preformed foam. The foam creates a stable air-cell matrix. The primary field knob is cast density at placement (pcf at the hose). Strength, when required, is a second acceptance criterion — not a replacement for the unit-weight number.
Industry bands that show up often on abandonment and lightweight backfill work sit well below water:
- About 27–50 pcf — common for pipe/tank abandonment, annular fill, void fill, and many foundation or abutment backfills where load reduction matters
- About 62.4 pcf — approximate unit weight of fresh water
- Higher cast densities (toward ~100 pcf) — less lightweight benefit; still lighter than many compacted soil fills, but closer to water and heavy CLSM
Any fill cast well below 62.4 pcf displaces more weight of water than it contributes. In a flooded trench, wet pipe, or tank, that means uplift, float, or migration of the plastic mass unless the engineer has staged placement, vents, ballast, anchors, or geometry to control it.
Volume Concrete’s Cellular Backfill range starts at about 27 pcf and is dialed upward toward about 100 pcf when the engineer specifies a heavier fill. A featured packet example is about 38 pcf non-permeating cellular grout — firmly in the “lighter than water” band. For planning interpolations across neat-cement and sanded bands, use the on-site density explorer — an Aerix manufacturer reference for planning. Volume designs and guarantees the mix delivered to your project specs.
What to collect before placement
Buoyancy is easiest to manage when it is named early — not discovered when the hose starts. Before dispatch, collect at least:
- Target cast density (pcf) — required; “lightweight” is not a number
- Permeable or non-permeable — different products, not a field toggle
- Minimum strength and age — only if the plans require it
- Volume (yd³) or geometry we can estimate
- Application — pipe, annular / slip-liner, tank, foundation, abutment, void
- Pump access — staging, hose run, elevation, lift limits
- Water-table / buoyancy concerns — groundwater, flood level, standing water in the void, tide or river influence if applicable
- Sand or neat if the engineer cares about mix family
- Testing / submittals expected (cast-density logs, cylinders, foam or cement certs)
Phone for dispatch: (971) 219-8604.
If groundwater or a wet void is possible, say so on the call. Higher cast density is one lever; cover weight, anchors, staged lifts, dewatering, and geometry are others. Do not assume a mid-30s pcf fill “just works” under water without that check.
Pipe and tank abandonment: wet voids are the common case
Abandoned pipes, culverts, and tanks in the Portland Metro often hold or can take water. Cellular grout is popular here because it pumps far, fills irregular voids, and keeps dead load down — often in roughly the mid-20s to mid-40s pcf band.
Buoyancy interacts with abandonment practice in three practical ways:
- Float of the plastic mass — if the void is flooded and vents are not managed, light grout can ride up or leave soft spots instead of packing the invert.
- Pipe or tank float — a light fill inside a submerged or buoyant vessel does not automatically hold the vessel down; external ballast, anchors, or staged fill may be part of the engineer’s plan.
- Venting and return — pump until material returns at high-point vents so air (and water) can leave; incomplete venting and buoyancy problems often travel together.
Put water-table notes, vent locations, and any ballast or staging requirements in the documents — not only the pcf line. Volume estimating for pipes, annular spaces, and common tanks: use the site calculators. Agree on optional overage for hose waste and irregular voids before dispatch.
Foundation and abutment backfill below flood or groundwater
Foundation, abutment, and retaining-wall backfill with cellular concrete is often chosen to cut lateral load and settlement after set. Typical planning densities for that use are often about 30–50 pcf — still well below water.
When the excavation sits below the water table or in a flood-prone corridor:
- Treat uplift on the plastic fill and fluid pressure on forms or thin walls as related but separate items
- Confirm whether temporary dewatering is part of the plan, or whether placement into a wet excavation is intentional and designed
- After set (often within roughly 8–24 hours depending on conditions), fluid hydrostatic pressure from the plastic mass drops for permanent design purposes — but groundwater uplift on the structure is still the engineer’s problem, not something the foam “solves” by itself
Closed-cell (non-permeating) mixes are common when the backfill should not become a drain path; permeable (PLDCC) mixes are for drainage fills. Say which one you want. Closed-cell is not automatic waterproofing — still drain walls as designed.
Fluid pressure while plastic, and typical lifts
While plastic, cellular concrete exerts full fluid hydrostatic pressure equal to cast density × depth. Forms, bulkheads, liners, and thin walls must resist that load during placement. Lower cast density lowers the magnitude of that pressure; it does not remove it.
Industry practice places in lifts — typically about 4 feet unless the foam system and the engineer allow more. Staged lifts also give the mass time to stiffen and can reduce uncontrolled float in a wet void. Keep workers out of the trench during placement; hose from a safe position.
Do not invent a lift schedule from this article. Put the lift limit, cure wait between lifts (if any), and form pressure assumptions in the project documents.
What belongs on the submittal conversation (no fake worksheets)
A useful submittal talk for a below-water-table cellular job usually covers:
- Target cast density and acceptance method (field unit weight at the hose)
- Permeable vs non-permeable
- Strength cylinders only if required (fragile low-density specimens; industry practice often uses 3×6 in. cylinders and ASTM C495-type care — not rod like normal concrete, not oven-dry before crush)
- Foam system awareness (Volume’s Cellular Backfill uses Aerix foam; featured mixes use Type IL cement — Ash Grove Durkee OR in the packet context)
- Acknowledgment that buoyancy, vents, ballast, and lift staging are engineer-of-record items — Volume delivers the mix to the agreed density and product type; it does not replace the buoyancy calc
- Standards often cited in literature: ASTM C495, C796, C869; ACI 523.1R; Volume mixes developed in accordance with ACI 301 and ASTM C685, C1602, C150, and C1600
That is a conversation checklist, not a certificate wall and not a downloadable buoyancy spreadsheet. If the job needs a named acceptance criterion or a stamped buoyancy check, put it in the documents.
How Cellular Backfill covers Portland Metro and the PNW
Cellular Backfill is a product line of Volume Concrete LLC. Production uses volumetric trucks and mobile silos — sand optional, permeable and non-permeable as specified. Everyday ready-mix stays in Greater Portland Metro; cellular jobs travel the Pacific Northwest when this product is on the ticket. Home base is Estacada, Oregon (city only — no plant street published here).
Confirm the jobsite on the call. Service area details live on the service area page.
Public NAP (billing / postal, not a plant street):
Volume Concrete LLC
4230 SE King Rd. #211
Milwaukie, OR 97222
Dispatch (971) 219-8604 · billing@volumeconcrete.com
Hours: Mon–Fri 7–5, Sat 7–2, closed Sun
If your Portland Metro or PNW plans call for cellular fill below groundwater or in a wet pipe or tank, write the cast density, name the water-table concern, and bring vents, lifts, and ballast into the same conversation as the mix. Soft next steps: call (971) 219-8604, open the calculators and density explorer, and confirm coverage on service area. Related notes: cast density, permeable vs non-permeable, pipe/tank abandonment, foundation/abutment backfill, and cellular vs CLSM.