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A third of all two-component poly work is void fill — and it's won or lost on three skills: picking the right expansion-vs-strength tradeoff, doing the yield math before you quote, and respecting the heat. Taught by Andy Powell with Steve Taylor, Alchatek Technical Bootcamp.
After this module, you'll be able to choose the right foam on the expansion-vs-strength curve, calculate gallons and pounds for any void from its volume, fill large voids without creating a fire risk, and work the full application landscape — from slip-lined pipes to bridge abutments to aggregate injection.
Voids hide everywhere concrete meets soil: under pavements and garage slabs, behind bridge abutments, around buried structures, in the annular space of slip-lined pipes, under spillways. Roughly a third of all two-component polyurethane projects are void fill — making this one of the most bankable skills in the geopolymer trade.
The session opens with a lawsuit, on purpose. A foam manufacturer was sued after a factory fire: foam had been used to fill a large enclosed space in a short period, and the suit alleged the company knew of prior fire incidents and failed to provide written guidelines, warnings, and installer training.
The chemistry behind the case: polyurethane cures exothermically. In an open lift the heat dissipates; in a massive confined pour, each lift traps the heat of the one before it, and internal temperature can climb to the point of degradation — or ignition. This isn't folklore; it's printed on the Alchatek TDS: "Excessive foam build-up in confined spaces can result in high internal temperatures and expansion pressures. In large void fills, stage injections and allow cooling time between lifts."
One curve governs the whole product family: as expansion goes up, density and compressive strength go down. Pick where the job sits on that curve — maximum volume per gallon, or maximum structure per cubic foot:
| Product | Density | Expansion | Free-Rise Compressive | Pick It For |
|---|---|---|---|---|
| AP Fill 420 | 2 lb/ft³ | 35x | 22 psi (≈3,170 psf) | Maximum volume — big non-structural voids, abandoned pipes |
| AP Lift 430 | 3 lb/ft³ | 25x | 50 psi (7,200 psf) | The balance point — most structural void fill |
| AP Lift 440 | 4 lb/ft³ | 18x | 80 psi (11,520 psf) | Heavier loading, DOT and below-grade |
| AP Lift 475 | 4.75 lb/ft³ | 15x | 110 psi (≈15,840 psf) | Industrial loads — strength over yield |
Values per current TDS (rev. 1/20/2026); confined strengths run 2–4x higher — see the TDS tables. The training chart's figures vary slightly from TDS — the TDS is the authority.
The estimating consequence is direct: the cheap-per-cubic-foot product is the high-expansion one — but only when the void doesn't need to carry load. Quote 420 under a working slab and the callback eats the savings.
Every void fill estimate is three steps:
The class example uses 24x for AP Lift 430; the current TDS rates it 25x unconfined — run your own numbers off the TDS for the product and remember confined foam yields less than free-rise.
Two-component wins these on speed and force: it displaces standing water, kicks in seconds, and reaches 90% strength in 15 minutes (per TDS) — traffic-ready before the crew packs up.
Product selection follows the soil stabilization module's matrix: AP Fill 700 where permeation plus expansion helps; AP Fill 720 where sealing and bulk matter most.
Score at least 4 of 5 to unlock module completion.