Formwork Stripping and Shoring Removal

Purpose

Stripping is released by a strength number, authorized by one named person, and never by elapsed days on a calendar. Nothing about a form tells you what is behind it. A wall that looks cured at 20 F is nowhere near a wall that looks the same at 70 F, and the crew cannot see the difference. Pull load-bearing shoring early and the failure is not a defect the shop repairs later, it is an elevated slab coming down on the people who removed its support. Pull vertical forms early and the damage is cosmetic but permanent: torn arrises, spalled corners, a snap-tie pocket that lifted a chunk of face with it. Both are prevented by the same discipline, a written release criterion and a measured value against it.

Scope

Covers stripping of vertical forms and removal of shoring and reshoring on residential and light commercial cast-in-place work: foundation walls, columns, beam sides, small elevated slabs and decks, from the release decision through hand-over of the stripped area.

It does not cover the design of the formwork or the reshoring sequence itself, which is engineered work carrying the engineer of record's stamp; slab-on-grade edge forms, which carry no structural load and are governed only by surface damage; or the making and curing of the cylinders that release the work, which the test cylinder sampling and handling SOP owns.

Roles and responsibilities

Procedure

1. Find the release criterion in writing before anyone touches a wedge. Read the drawings and specification for the condition that releases each element. Acceptance: a written criterion on site, either conditions for removal stipulated in the plans or a strength value to be demonstrated by field-cured cylinders or in-situ tests, which are the two bases 29 CFR 1926.703(e)(1) allows an employer to determine sufficient strength from. Wrong looks like a superintendent releasing on day count; stop rule, no criterion, no stripping, and the question goes to the engineer of record in writing. Hazard: nobody climbs on or loosens formwork during this step, because a form still carrying load is stored energy however routine it looks.

2. Prove the strength against that criterion. Break field-cured cylinders cured alongside the element per ASTM C31, or use a maturity method correlated to that mix. Acceptance: a measured value at or above the drawing's number, recorded with the element, date and hour; where a specification states a percentage of f'c for removing shoring under an elevated element it commonly sits in the 70 to 75 percent range, but the drawings govern and a crew that cannot find a number does not pick one. Wrong looks like a standard-cured lab cylinder used to release site work, which was cured warmer and wetter than the element and reads high. Stop rule: leave everything in place and re-break later. Hazard: none in this step for the field crew, because the shop does not break its own cylinders; a cylinder failing in compression ejects fragments, and the machine guarding and operator qualification are the testing laboratory's.

3. Set the exclusion zone and the stored-energy plan first. Clear and barricade the area below and around the element, post it, and brief the crew on the strike sequence and the fall path of every piece. Acceptance: barricade up, nobody below, the sequence read back by the person doing the work, and a plan for lowering panels rather than dropping them. Wrong looks like a trade working under an elevated slab while its shores come out. Stop rule: work does not start until the zone is clear, and it stops the moment anyone enters it. Hazard: a wall form under load releases suddenly when a wedge or tie lets go, so wedges are driven from the side rather than from underneath, nobody stands in the fall arc, and head protection is worn by everyone inside the zone.

4. Strip non-load-bearing vertical forms first, on a damage test. Take wall, column and beam-side forms, which carry no weight of concrete, before anything supporting load. Acceptance: a corner form eased off a test area leaves arrises and the face intact with no tearing or pull-out, and ACI 347R's guidance for elements not supporting weight, on the order of 12 hours at moderate temperature, is treated as a floor rather than a permission. Wrong looks like a form pried at the top with a bar, which spalls the corner it levers against. Stop rule: if the test corner tears, re-set it and wait. Hazard: stripped panels carry protruding nails and ties, so they are de-nailed or stacked face to face immediately, never leaned against a wall where they fall.

5. Strike load-bearing shoring only in the sequence the plan states, reshoring as you go. Follow the engineered strike sequence element by element, and where reshoring is required install it as the original shores come out rather than after. Acceptance: the sequence followed with each shore accounted for, reshores erected as original shores are removed per 29 CFR 1926.703(b)(8), and no reshore removed until the concrete it supports has attained adequate strength as 1926.703(e)(2) requires. Wrong looks like a crew clearing an entire bay before reshoring it because it was faster. Stop rule: stop and restore support at any point the sequence cannot be followed. Hazard: a loaded shore under a slab holds stored energy, so it is released with the jack rather than knocked out, and nobody works beneath an unsupported bay.

6. Inspect the concrete and the material before either leaves the area. Walk the stripped face for honeycomb, cold joints, exposed steel and tie-hole condition, and sort the panels and hardware. Acceptance: every defect photographed with a location before patching, exposed reinforcement reported to the engineer rather than covered, and damaged panels tagged out of service. Wrong looks like honeycomb parged over the same morning, which hides the depth of it. Stop rule: nothing that exposes steel gets patched before the engineer sees the photograph. Hazard: patching mortars are alkaline and abrasive, so waterproof gloves and ANSI Z87.1 eye protection are worn, and grinding a stripped face releases respirable crystalline silica, so it does not start here: it runs under the silica dust control SOP with a Table 1 water-fed or shroud-and-HEPA control under 29 CFR 1926.1153, and where that control is not in place the grinding does not happen on this visit.

7. Release the area in writing and hand it over. Record the release, drop the barricade, and tell the next trade the area is theirs. Acceptance: a signed release naming the elements, the strength value that supported it, the date and hour, and any element left shored with the reason. Wrong looks like a barricade quietly disappearing while two bays are still reshored, which is how another trade loads a slab that is not ready. Stop rule: partial release is stated as partial, with the shored bays marked on the ground and on the record. Hazard: reshores and posts left standing are struck-by and trip hazards in a dim basement, so they are flagged high-visibility and lighting is left in place.

The record this produces

One strip release record per element or pour area. Fields: job and element identifier, the written criterion and where it came from, the field-cured cylinder identifications with age at break and measured strength or the maturity result, the person who authorized the release with date and hour, the strike sequence used, reshoring installed and its removal criterion, the exclusion zone period, inspection photographs with locations, defects found and what was done, and any element left shored with the reason and the expected date.

It goes on the job record with the cylinder reports. The engineer reads it when a deflection or crack question opens over that bay, because the first thing asked is when the shores came out and against what number. The office reads it to know which areas are genuinely available to the next trade. The PM reads it across a project, because a job where every release lands at the minimum percentage on the earliest possible day has a schedule being carried by the concrete rather than by the plan.

One pass, with a step that failed

A residential basement with a small cast-in-place garage slab over part of it: 8 in foundation walls, and a 9 in elevated slab spanning to those walls, f'c 4,000 psi, drawings requiring 70 percent of f'c before shoring under the elevated slab comes out. Seventy percent of 4,000 psi is 2,800 psi, written on the release record before any cylinder was broken.

Step 1 passed: the criterion was on sheet S2 with the strike sequence, and a copy went in the site file. Step 4 ran first, since the wall forms carry no weight of concrete. At 16 hours a test corner came off clean with arrises intact, so the wall side forms were stripped that morning, panels de-nailed and stacked face to face.

Step 2 failed on the elevated slab. A field-cured cylinder broken at day four read 2,410 psi, which is 60 percent of the 4,000 psi f'c and short of the 2,800 psi the drawings require. The crew leader took the stop rule: shores stayed, nothing moved, and the office moved the framer rather than letting him arrive to a bay he could not load. A second field-cured cylinder broken at day six read 3,120 psi, or 78 percent, clearing the criterion with margin.

Steps 3 and 5 then ran on day six. The bay below was barricaded and posted, the sequence read back, and shores came down one line at a time with reshores installed as each original shore was pulled, per the strike order on the drawing. Step 6 found one honeycomb patch about the size of a hand at the base of the north wall with no steel visible, photographed and located before it was repaired. The release in step 7 named the wall and the slab bay, carried the 3,120 psi value, and noted the two reshored lines still standing with the date they were expected to come out.

What the day-four number would have cost if it had been read as close enough is worth stating plainly. At 2,410 psi the slab is at 60 percent of its design strength, and the load that arrives first is not the design load, it is a framing crew and a bundle of lumber landing in one spot. The failure mode is not a crack found later; it is a partial collapse with people under it, and the record afterwards would have shown a release signed against a number that did not meet the criterion printed on the same sheet.

When the site will not wait

The general contractor needs the bay today and the number is short: the answer is the number and the next break date, and if the schedule cannot absorb it, the engineer decides in writing whether a reduced load with reshoring in place is acceptable. Field-cured cylinders were never made: there is no shortcut back, so the release goes to the engineer for an in-situ evaluation rather than to a crew's judgment. A form will not release although the strength is proven: that is a bond or release-agent problem, so wedge and lower it rather than lever the concrete. And where the element sat through a cold spell, the cylinders releasing it must have lived in the same protection it did.

References

• 29 CFR 1926.703, cast-in-place concrete, including (a) formwork design and drawings on site, (b)(8) reshoring erected as original shores are removed, and (e) the removal-of-formwork determination based on plan conditions or field-cured tests.
• ACI 318, formwork removal provisions, and ACI 347R for guidance on stripping times and reshoring practice, in the editions your building department and specification adopt.
• ASTM C31/C31M for field-cured specimens. See related: the test cylinder sampling and handling SOP.
• 29 CFR 1926.501 for fall protection where stripping happens at height on construction work, and 1926.701(b) for impalement protection.
• See related: the cold weather pour standard and the silica dust control SOP for grinding a stripped face.