Concrete Forming & Stripping

A concrete form has one job: hold wet concrete to the exact shape and dimensions on the drawing until the concrete has cured enough to carry its own weight and whatever load comes next. A bad form is a bad pour. No finisher, no troweler, no patch crew can save a wall that bellied out two inches at mid-height or a slab edge that wandered off the control line. The apprentice on the form crew is the first and last line of defense on every pour, and the apprentice who learns the math and the checklist becomes a journeyman carpenter years faster than the apprentice who just swings a hammer.

This guide walks through the form systems you will see in the field, Plyform grades and orientation, layout and bracing, hydrostatic pressure per ACI 347, blockouts and anchors, the pre-pour checklist, strip times, and the safety rules that keep form work from being the day somebody does not go home.

Form Systems You Will See

Forms fall into five broad categories. The apprentice who can name each on sight and knows why the foreman picked it will stand out on day one.

| System             | Typical Use                         | Reusability        |
|--------------------|-------------------------------------|--------------------|
| Job-built wood     | Footings, small walls, curbs        | 3 to 8 pours       |
| Aluminum handset   | House foundations, light commercial | 100 to 500 pours   |
| Steel ganged       | High walls, cores, shafts           | 500+ pours         |
| Modular system     | PERI, Doka, Efco, Symons walls      | 500 to 1500 pours  |
| ICF                | Insulated residential walls         | Stays in place     |

• Job-built wood is 3/4 inch Plyform on 2x4 or 2x6 studs with snap ties. Cheapest, most flexible for odd shapes. A solid apprentice skill because every crew builds at least some custom forms.
• Aluminum handset is lightweight panels 12 to 36 inches wide by 8 feet tall, set by one or two laborers without a crane. Ties pass through standard holes. Fast on repetitive work like basement walls.
• Steel ganged forms are large assemblies craned into place. Used on tall pours where form pressure is high and reuse is heavy.
• Modular systems (PERI, Doka, Efco, Symons) are engineered kits with tested ties, walers, and climbing brackets. The shop drawings spec tie spacing by pour rate. Follow the drawings, not your gut.
• ICF (insulated concrete forms) are foam blocks stacked like Legos, then filled with concrete. Foam stays in place as insulation. Common in residential energy-efficient builds.

Plyform Grade and Grain Orientation

Not every 3/4 inch plywood sheet can form concrete. The APA grades that matter:

• Plyform Class I - strongest, best face veneer, most reuses (10+ pours with care).
• Plyform Class II - lower-grade veneer, fewer reuses (3 to 8 pours).
• HDO (high-density overlay) - resin-impregnated face, slickest release, most pours.
• MDO (medium-density overlay) - paint-grade overlay, smoother finish than bare Plyform but fewer pours than HDO.

The grain rule every apprentice must know: face grain runs perpendicular to the studs/walers. Plywood is roughly twice as stiff across the face grain as with it, because the face plies take the bending load. Turn the sheet the wrong way and you lose half your panel stiffness, which shows up as a wavy wall face.

Standard sheet is 4 x 8. On a vertical wall, the 8 ft dimension typically runs vertically (matches wall height) with the face grain horizontal, perpendicular to vertical studs at 12 to 16 inches on center.

Layout Basics

Before one stud gets nailed:

1. Find the control line. The surveyor or foreman snaps a chalk line 2 ft or 4 ft offset from the finished face of the wall. Everything references that line.
2. Snap the form face. Transfer the offset from the control line to the slab or footing. Snap a clean blue chalk line for the inside face of the outer form.
3. Square the corners. String line across diagonals. For a rectangular pour, both diagonals must match within 1/8 inch for anything residential, tighter for commercial.
4. Mark stud and tie locations on the sill plate. Tick marks every 12 or 16 inches OC, long tick at every waler bay.
5. Shoot elevations. Set grade nails on every form where the top of concrete lands. No more "eyeballing from the last nail."

Stud, Waler, and Tie Spacing

A common house foundation wall, 8 ft tall, 8 inch thick, at a 4 ft/hr pour rate, 70 deg F, built from job-site lumber and Plyform:

| Component          | Typical Spec                            |
|--------------------|-----------------------------------------|
| Plyform            | 3/4 inch Plyform Class I, grain perp    |
| Studs              | 2x4, 12 inch OC                         |
| Walers             | Double 2x4, 24 inch OC vertically       |
| Snap ties          | 3000 lb, 24 inch x 24 inch grid         |
| Bracing            | 2x4 kickers every 6 ft, staked          |

For taller walls, hot weather, high pour rates, or pressurized concrete (superplasticized mixes that behave like fluid all the way up), every one of those spacings tightens. The engineered form drawings or manufacturer tables are the real answer; the table above is the starting point an apprentice should recognize.

Hydrostatic Pressure - ACI 347

Fresh concrete pushes on the form like a column of heavy liquid until it starts to set. After initial set, the column above takes most of the load from the form below. The deeper and faster you pour, the higher the peak pressure.

ACI 347 gives the design pressure for vertical walls with Type I cement and mixes slumping under 7 inches:

P = C_w x C_c x (150 + 9000 R / T)

Where:

• P = lateral pressure in psf, capped at 150h (full fluid head) and a floor of 600 psf
• C_w = unit weight coefficient (1.0 for 140 to 150 pcf concrete)
• C_c = chemistry coefficient (1.0 for Type I/II with no retarder, up to 1.4 with retarders or Type III)
• R = pour rate in ft/hr
• T = concrete temperature in deg F

Worked example. 8 ft tall wall, normal concrete (C_w = 1.0), plain Type I (C_c = 1.0), 4 ft/hr pour rate, concrete at 72 deg F:

P = 1.0 x 1.0 x (150 + 9000 x 4 / 72) = 150 + 500 = 650 psf

Check cap: 150 x h = 150 x 8 = 1200 psf. 650 is below the cap. Check floor: 600. 650 is above the floor. Design pressure = 650 psf.

A faster pour or colder concrete drives the number up. Pour at 8 ft/hr in 50 deg F weather:

P = 150 + 9000 x 8 / 50 = 150 + 1440 = 1590 psf. Above the 1200 psf fluid-head cap. Use 1200 psf. That is twice the pressure the 4 ft/hr design was built for. Tie spacing must tighten or the wall blows out.

The point for an apprentice: pour rate and temperature are the two levers that move form pressure hard. When the foreman tells a pump truck operator to slow down, this is why.

Release Agents

The form face gets a thin coat of form-release oil or emulsion before the pour so the form strips clean without tearing the concrete face.

• Apply thin. A wiped film, not a puddle. Puddles stain concrete and can trap air at the face.
• Recoat between pours. Even HDO plywood needs a fresh film after strip and clean.
• Do not spray near rebar or embeds. Oil on rebar reduces bond. Mask or wipe.
• Do not use motor oil. It is illegal on most jobs (environmental) and it staining concrete permanently is the least of the problems.

Blockouts, Penetrations, and Anchors

Anything that is not concrete has to be in the form before the pour. The apprentice usually frames these in.

• Window and door bucks. Built from 2x lumber, braced inside to resist crushing, tapered release on the inside face so the buck pops out after strip.
• Pipe sleeves. Schedule 40 PVC cut to wall thickness plus nothing, capped on both ends, anchored to studs and walers so they do not float during the pour.
• Embeds. Angle iron, plate with headed studs, weld washers. Bolted or tack-welded to a fixture that holds them at the correct offset and elevation.
• Anchor bolts. Cast-in J-bolts or headed rods on a jig at the top of the wall form. The jig - a scrap 2x6 with holes drilled at the bolt pattern - holds the bolts plumb and at the right elevation while the concrete sets. Never rely on a single nail to hold a bolt.

Pre-Pour Checklist

Walk every form with a foreman before the truck arrives. Nothing gets skipped.

• Lines: inside face of form on control line offset, corners square, diagonals within tolerance.
• Plumb: wall plumb to string or laser within 1/4 inch in 8 ft.
• Brace: kickers staked, top ties or strongbacks holding the top in line.
• Clean: no chips, mud, leaves, cigarettes, or standing water inside the form.
• Wet down: forms sprayed just before the pour in hot or dry weather to stop the wood pulling water out of the mix.
• Ties: all ties installed, threaded, wedge-pinned. Count them twice.
• Rebar: bar schedule matches drawing, supports in place, cover checked on all faces.
• Blockouts and embeds: anchored, plumb, at correct offset and elevation.
• Access and walk-path for the pump truck or chute.
• Safety: barricades, fall protection at top of wall, hard hats and eye pro, wash station for skin contact with fresh concrete.

Strip Times

ACI 347 gives minimums. The engineer of record or the project spec may require longer. Stripping early is the most common cause of a collapse that kills somebody.

| Element                         | Minimum Time (typical)   |
|---------------------------------|--------------------------|
| Wall forms (non-load-bearing)   | 12 to 24 hours           |
| Column forms                    | 12 to 24 hours           |
| Beam sides (not the bottom)     | 12 to 24 hours           |
| Slab soffits with shores left   | 7 days                   |
| Slab soffits with shores removed| 14 days (or per spec)    |
| Beam soffits, shored            | 7 days                   |
| Beam soffits, unshored          | 14 to 21 days            |

Cold weather doubles or triples these. Hot weather with accelerator may shorten them, but only the engineer calls a shortened strip time, not the crew.

Stripping Sequence

1. Remove guardrails, forms, and hardware in the order they were installed, reversed.
2. Strip non-load-bearing sides first: wall faces, column sides, beam sides.
3. Leave shores under slabs and beam soffits until the spec says otherwise.
4. Pull ties by backing off the wedge pin, then removing the tie rod. Never pry against the concrete with a wrecking bar at a tie hole.
5. Peel panels away from the face. Do not lever off a green edge; it will chip.
6. Break the tie rods at the snap-off point (job-built systems) or unthread (reusable systems).
7. Plug tie holes with a non-shrink patching mortar at the end of the day.
8. Clean and oil the panel. Stack panels face to face, no tie rods between panels.

Safety - Form Work Kills

Form collapse is listed by OSHA as a fatal hazard on 29 CFR 1926.703. Every year in the US a form crew loses a worker to a blowout, a fall inside a form, or a snap-tie whip.

• Hard hat, safety glasses, gloves, steel toe. All four, all the time.
• Watch for snap-tie whip. When a wedge is released on a loaded tie, the tie can snap and whip. Stay out of the line of release.
• Never stand on a brace, a top plate, or an unsupported walkway during a pour. Form-collapse injuries are very often the laborer who was on top of the form when it failed.
• Three points of contact on any wall ladder inside a form. Inside a wall form is a fall-from-height hazard; treat it like one.
• Fall protection above 6 ft. Guardrails around slab edges, harness and lanyard on a safety line where no guardrail is possible.
• Keep hands clear of closing seams. Panels closing together will pinch a glove off your hand.
• Never modify a manufactured form system in the field. No cutting braces, no drilling new tie holes. If the system does not fit, stop and call the engineer.
• Pump hose whip. A concrete pump hose carrying 4 inch slurry at pressure can kick like a fire hose and crush a leg. Keep the hose on the ground, have a laborer steadying it, never stand between the hose and a wall.

Day 1 Checklist

• Know which form system is in use and what the shop drawings call for.
• Grain of Plyform perpendicular to studs, face smooth and oiled.
• Stud and waler spacing matches the job drawing, not last weeks job.
• Control line offset confirmed, diagonals square, plumb to string or laser.
• All ties counted, threaded, wedged. Blockouts anchored. Rebar cover checked.
• Pre-pour walk with the foreman, signed off before the truck arrives.
• Strip time on a whiteboard in the trailer. No one strips a form without the foreman calling it.
• Every tie hole patched, every panel cleaned and stacked at end of day.

Expert Tips

• "The pour rate is a design input, not a suggestion." If the pump operator wants to pour 10 ft/hr on a form engineered for 4, stop the pour.
• "A dry form in hot weather is a cracked pour." Wet it down. Unseasoned wood will pull moisture out of the mix and cause plastic shrinkage cracks.
• "Count ties twice." Missing ties is the single most common finding on a pre-pour walk.
• "Never strip on someone elses word." Strip time goes on the drawing or the project spec, verified by the foreman, not the apprentice who thinks the concrete feels hard.
• "If a form is whistling, stop the pour." Air escaping from a form under hydrostatic load is the sound right before a blowout.