7.2 Formwork and Reinforcement Placement

Formwork is the temporary mold that holds plastic concrete until it cures and is self-supporting. On the NASCLA exam, formwork questions blend ACI 347 (formwork design), the carpentry/concrete reference, and OSHA 29 CFR 1926 Subpart Q (Concrete and Masonry Construction). Formwork must resist dead load, live load, and lateral pressure from fresh concrete.

Lateral pressure rises with rate of placement and falls with temperature (warmer sets faster, less pressure). Plastic concrete behaves like a fluid weighing about 150 lb/ft^3 until it stiffens.

Worked numeric (full-liquid head): A 10-ft-tall wall form, if filled faster than it sets, sees pressure at the base of: P = 150 lb/ft^3 x 10 ft = 1,500 lb/ft^2. That is why high walls are poured in lifts and forms are braced. Trap: pouring too fast can blow out a form rated for a slower rate.

OSHA 1926 Subpart Q key rules (commonly tested):

• 1926.703: Formwork must be designed, fabricated, erected, and braced to carry all loads; drawings/plans kept at the jobsite.
• Shoring and reshoring removed only when the concrete has gained adequate strength (per a registered engineer or plan).
• 1926.701(b): Protruding reinforcing steel onto/into which employees could fall must be guarded to eliminate impalement hazard. This rebar-impalement rule is a frequent trap.
• 1926.701(a): No loads on a structure until adequate strength is confirmed.

Reinforcing steel (rebar) resists tension that concrete cannot. Bars are sized in eighths of an inch by bar number.

Trap: #4 bar = 4/8 in = 1/2 inch diameter. The bar number equals diameter in eighths of an inch.

Concrete cover protects rebar from corrosion and fire. ACI 318 minimums (cast-in-place, non-prestressed):

• 3 in — concrete cast against and permanently exposed to earth (footings).
• 2 in — formed surfaces exposed to earth/weather (#6 and larger).
• 1-1/2 in — formed surfaces exposed to weather (#5 and smaller).
• 3/4 in — slabs/walls not exposed to weather or earth.

Trap: footing bars need 3 in cover when poured directly against soil.

Placement details:

• Maintain spacing with chairs, bolsters, and spacers; never place bars directly on the subgrade.
• Lap splices transfer force where bars overlap; a typical tension lap is 40 to 48 bar diameters (verify with the structural drawings). For #5 bar at 48d: 48 x 0.625 in = 30 in lap.
• Welded wire reinforcement (WWR) in slabs should sit in the upper third to control shrinkage cracking.

Trap: rebar placed too low or on the ground loses its tension function and corrodes.

Formwork Loads and Safety

Formwork must resist the lateral pressure of fresh concrete, which behaves like a fluid (~150 lb/ft³) until it sets — pressure increases with pour rate and lower temperature (slower set). Forms must be plumb, braced, tight (to prevent grout leakage), and shored. Premature form/shore removal before the concrete gains strength causes collapse — a leading cause of construction fatalities. Reshoring supports multistory slabs while lower forms are stripped.

Rebar Identification and Concrete Cover

Reinforcing bars are sized in eighths of an inch: #4 = 4/8 in = 1/2 in, #5 = 5/8 in, #8 = 1 in diameter. Grade 60 (60,000 psi yield) is standard. Concrete cover protects steel from corrosion/fire — common minimums per ACI 318: 3 in cast against earth, 2 in exposed to weather (#6 and larger), 3/4 in for slabs not exposed. Lap splices transfer force where bars overlap (length depends on bar size and f'c).

Placement, Spacing, and Support

Bars are tied in a grid and held off the form by chairs/bolsters so cover is maintained — laying rebar on the ground then "pulling it up" during the pour is a defect. Minimum clear spacing between parallel bars is generally the bar diameter or 1 in, whichever is greater, so aggregate can flow through. Welded wire reinforcement (WWR) controls shrinkage cracking in slabs and must sit in the upper third, not on the subgrade.

Common Exam Traps

• Trap: A #8 bar is 8 in. It is 1 in (8/8).
• Trap: Faster pour rate lowers form pressure. It raises pressure.
• Trap: Rebar laid on grade is fine. It loses cover; use chairs.
• Trap: Stripping shores early to reuse forms — strength must develop first.

Embeds, Dowels, and Pre-Pour Checklist

Before the pour, verify anchor bolts, embed plates, sleeves, and dowels are placed and tied so they cannot float — these set the connection to the structure above and cannot be added later. Run a pre-pour checklist: form line/grade, bracing, cleanliness (no debris/water), rebar size/spacing/cover/lap, embeds located, and access for the vibrator. A special inspector (IBC Ch. 17) often signs off before concrete arrives, because a missed embed or low-cover bar discovered after the pour means costly demolition and re-work.

Hooks, Development, and Why Bars Fail

Reinforcement only works if it is developed — embedded far enough to transfer stress without slipping. Where straight embedment is short, a standard hook (90° or 180°) anchors the bar. Insufficient lap length or development length is a classic structural defect that lets a beam fail at less than design load. Detailers follow ACI tables that scale these lengths with bar size, concrete strength, and cover, which is why field crews must never shorten laps to save steel.