8.1 Formwork Inspection, Alignment, Tolerances & Release Agents (ACI 347 & ACI 117)

Formwork Inspection, Alignment, Tolerances & Release Agents

Forms shape the concrete and must maintain the specified geometry during placement. ACI 318-19 Section 26.11.1.2 requires the formwork design to account for the placement method, rate and construction loads. The section requires forms tight enough to inhibit loss of paste or mortar and braced or tied to maintain position and shape. ACI 301-20 assigns formwork design to the contractor. Model IBC Table 1705.3 item 14 assigns periodic special inspection of the member’s formwork shape, location and dimensions. Check the approved scope and report discrepancies to the contractor under Section 1704.2.4.

1. Verify line, grade and member details

Locate the member using the project grid and benchmark. Measure line and position relative to the applicable grid, elevation relative to the benchmark, and deviation from plumb relative to the vertical reference. A form can be plumb while displaced from its grid, so check location independently. Check the cross section between the actual contact faces, including liners or offsets that affect member dimensions.

Review the specified elevations at supports and at locations such as midspan where the detail shows camber. ACI 301-20 Section 2.3.1.7 addresses setting formwork and screed strips to produce the designated elevation, camber and contour before formwork removal, with allowance for anticipated formwork movement. Check the designated measurement stage and the project tolerances. For an exterior edge with a drip groove, check its shape and position from the approved detail.

Under ACI 301-20 Sections 2.2.1.6 and 2.3.1.2, the default chamfer material is 3/4 by 3/4 inch lumber, and the default chamfer applies to corners of permanently exposed surfaces. The default excludes beveling reentrant corners or edges of formed joints. Check any drawing requirements for a different finish or size. Verify that inserts and chamfers stay in position and that joints and openings are prepared to inhibit mortar leakage.

2. Apply the correct dimensional tolerance

ACI 117-10 Section 4.5.1 applies to the completed member. Measurements between form faces before placement help anticipate that result. For a formed 10-inch wall, the numerical thickness bounds are 9-3/4 inches and 10-3/8 inches. Check unformed beams and walls cast against soil separately under Section 4.5.2.

Worked example: Plumbness at 20 and 80 feet

ACI 117-10 Section 4.1.1 applies through 83 ft 4 in. above the foundation top or lowest support level shown. For ordinary lines, surfaces, corners and arrises, use the smaller of 0.3% of the height or 1 inch. At 20 ft, $0.003\times240=0.72$ inch, so the limit is 0.72 inch. At 80 ft, $0.003\times960=2.88$ inches, so the 1-inch cap governs.

The outside corner of an exposed corner column and grooves in exposed concrete use the separate 0.2% and 1/2-inch limits in Section 4.1.1. At 20 ft, the exposed corner check gives the smaller of $0.002\times240=0.48$ inch and 1/2 inch. For taller work, use Section 4.1.2. Local departures from a plane and form irregularities have their own checks.

3. Inspect the form contact surfaces

ACI distinguishes barrier release agents that form a physical film from chemically active agents that react with calcium ions in the paste to form a soap film. Use the specified product for the form-facing material and finish. Chemically active products can help reduce staining and surface air voids in architectural work. Verify the application and any required mockup against the specification.

Where ACI 301-20 Section 2.3.1.15 applies, follow the product instructions, apply the release agent before placing reinforcement, remove excess before concrete placement, and prevent contact with reinforcement or hardened concrete receiving fresh concrete. If oil reaches the cage or a joint face, identify the extent and verify approved cleaning and reinspection before it is concealed. Record the observed contamination and correction.