1. What is gypsum concrete (“gypcrete”), and how is it different from regular concrete?
Gypsum concrete is a poured floor underlayment made from gypsum cement, sand, and water. “Gyp-Crete®” is a Maxxon trade name that became the trade’s generic word, the way Kleenex did — other major brands include FIRM-FILL® (Hacker), Levelrock (USG), and Treadstone® (Formulated Materials). Compared to standard concrete it is lighter (roughly 110–120 lb/ft³ versus about 150), self-leveling, faster to place, and poured thinner — 3/4″ to 3″. It is not a wear surface: it is the fire-rated, sound-rated, dead-flat layer that goes under finish flooring in wood-frame construction. Its light weight is the reason it can go on wood framing at all.
2. Why does my architect’s floor assembly require it?
Two code reasons. Fire: gypsum concrete is an integrated component of over 100 UL-rated floor/ceiling assemblies with 1–4 hour ratings, and it has zero flame spread per ASTM E84. Sound: the IBC requires STC 50 and IIC 50 (45 field-tested) between dwelling units, and in wood-frame multifamily the practical path to those numbers is a gypsum topping, usually over an acoustic mat. If your project is wood-frame apartments, condos, hotels, or senior living, the assembly your architect specified almost certainly depends on it.
3. What is the difference between STC and IIC?
STC (Sound Transmission Class) rates airborne sound — voices, TV, music — and is defeated by mass. IIC (Impact Insulation Class) rates impact sound — footsteps, dropped objects — and is defeated by decoupling, meaning a resilient mat that breaks the rigid path between the finish floor and the framing. Gypsum concrete over an acoustic mat addresses both in one assembly: the topping supplies the mass, the mat supplies the decoupling. The two ratings are independent — an assembly can pass one and fail the other, which is why both are specified.
4. What PSI should be specified, and what do the ASTM numbers mean?
Gypsum underlayments run from 2,000 to over 4,000 PSI compressive strength, tested per ASTM C472. Which end you need depends on the finish floor and traffic: glue-down and floating floors in residential units are typically fine at the lower-to-middle range, while high-traffic and commercial applications push higher. The number that matters is the one in your spec — UL and acoustic assemblies are engineered around a minimum PSI at a given thickness. The quiet industry problem is that hand-batched pours frequently miss the specified PSI through over-watering or over-sanding. That is the specific problem our computer batching exists to eliminate.
5. How long before we can walk on it, and when can flooring go down?
Light foot traffic is typically fine within hours of the pour — usually the same or next day — so other trades are back to work quickly. Finish flooring is a different question: the slab must dry to the flooring manufacturer’s moisture requirement, which commonly takes 5 to 7 days under decent drying conditions and can stretch to weeks if the pour was over-watered or the building is cold and sealed up. Two practical notes: keep air moving and heat on during dry-down, and know that a weighed-water pour dries on a predictable curve, which lets you actually schedule the flooring installer instead of guessing.
6. Why do some gypcrete floors turn out soft or dusty?
Almost always because of the batching, not the product. Extra water and extra sand both dilute the gypsum crystallization that gives the material its strength — the result is low PSI, a surface that powders under traffic, and adhesion failures when flooring goes down. It is the number-one failure mode in this trade, and it is invisible on pour day; a wet, rich batch flows beautifully. This is why we weigh every ingredient of every batch by computer against the specified mix design. The mix cannot drift, so the floor cannot inherit the drift.
7. What is a batch report, and why should I care?
For every batch we pour, our Smart Batch® GC-1 records the actual weights of gypsum, sand, and water, plus mix duration and timestamps, and archives it all to a cloud server. QC reports are emailed automatically and compiled for your closeout file. You care because floor disputes surface months after the pour, when memory is the only witness — unless there is a record. With a per-batch log, “was it poured to spec?” is a lookup, not an argument. No hand-batching crew can produce this document, because with hand-batching the data never existed.
8. Can you pour over radiant heat tubing?
Yes — radiant encapsulation is a core use of gypsum concrete and one of our standard scopes. The tubing is stapled to the subfloor, pressurized for the pour, and encapsulated in typically 3/4″ to 1.5″ of material over the tube. Gypsum’s density and full-contact encapsulation transfer heat evenly across the floor. Consistent batch density matters more here than anywhere: density variation between batches shows up later as uneven floor temperature. Weighed batching keeps every batch at the same density by construction.
9. How much area can you pour in a day, and what does that do to my schedule?
Up to 40,000 square feet per day — the rig produces a weighed half-ton batch every 75 seconds, sustaining 13–20 cubic yards per hour, so the pump is never waiting on the mix. In practice that means a typical garden-style building is a pour day or less, and a large podium building is a handful of pour days rather than weeks. Fewer pour days means fewer trade conflicts, less weather exposure, and a shorter gap between dry-in and flooring.
10. What do you need from the site before pour day?
The standard punch list: building dried in (roof and windows), subfloor swept and sound with penetrations plugged, framing inspections passed on the areas being poured, heat available in cold weather, water supply, and access for a 25-foot trailer within pumping distance. We walk the deck with your super before the pour and flag anything that would compromise the result. Our plant sets up in under 10 minutes, so once the site is ready, we are pouring almost immediately.
11. What does gypsum concrete cost per square foot?
Honest answer: it depends on thickness, mix design (PSI), acoustic mat scope, building height and pumping distance, project size, and location — which is why per-square-foot numbers quoted without plans are worth what you paid for them. What we will commit to: send your plans through the estimate request page and you get a real, itemized number, typically within days, with the pour-day count and mobilization plan attached. On total cost, remember the failure math — a re-pour or a flooring adhesion failure costs multiples of any per-foot difference between bids.
12. How far will you travel, and how does mobilization pricing work?
We pour throughout Washington, Oregon, Idaho, and Montana from our Puyallup base. The batch plant is fully self-contained — its own power, its own weighing system — so distance affects travel logistics, not pour quality. For projects outside the Puget Sound core we schedule coordinated pour windows and, where possible, bundle nearby projects into one mobilization to share the cost. Mobilization is a line item you see in the bid, not a surprise.
13. Do you install the acoustic mat too, or just the topping?
Both, as one scope. The mat and the topping are a single tested assembly — the rating assumes a specific mat under a specific topping thickness — so splitting them between subs is how ratings quietly die. We install the sound mat and pour the gypsum over it with one crew, one schedule, and one responsibility line.
14. Can gypsum concrete be used outdoors, on balconies and breezeways?
Gypsum concrete itself is an interior material — it is not for weather exposure. Exterior elevated decks get a different system we also self-perform: lightweight concrete sloped for positive drainage, a waterproofing membrane, and a pedestrian traffic coating as the wear surface. Same crew, same mobilization, one scope for interior floors and exterior decks — which eliminates the classic two-sub finger-pointing when a deck leaks.
15. When should we bring you into the project?
Earlier than you think — ideally during design or early bidding. At that stage we can catch assembly mismatches (a drawing calling for a rating the specified build-up cannot hit), advise on thickness and mat selection against your framing and door heights, and reserve a pour window before your framing schedule collides with ours. It costs nothing and has saved our GCs real change orders.
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Request a Free Estimate · (253) 271-7135 · office@soundfloorstoday.com