7.3 Formwork, Shores, Reshores & Removal

7.3 Formwork, Shores, Reshores & Removal

Reinforced and prestressed concrete structures rely on temporary formwork systems to hold fresh concrete in its intended geometric shape until it achieves self-supporting strength, and internal steel reinforcement to resist tensile, shear, and flexural stresses. In commercial construction across Georgia, general contractors must manage the structural design of vertical and horizontal formwork under ACI 347, enforce reinforcing bar placement tolerances and clear cover standards under ACI 318 and ACI 117, execute code-compliant lap and mechanical splices, and oversee high-strength post-tensioning (PT) stressing operations with strict elongation tolerances.

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Formwork Design & Lateral Concrete Pressures (ACI 347)

Formwork must be engineered to safely support vertical dead loads (concrete self-weight of 150 pcf for reinforced concrete plus formwork dead load), construction live loads (workers, buggies, equipment: typically 50 to 75 psf), and lateral fluid pressures exerted by fresh, unhardened concrete.

                 VERTICAL FORMWORK STRUCTURAL ASSEMBLY
                 
       Fresh Concrete
      ┌──────────────┐ ◄─── Plywood / Steel Sheathing (Direct Contact)
      │              │
      │  ██████████  │ ◄─── Vertical Studs (Supports Sheathing)
      │  ██████████  │
      │  ██████████  │ ◄─── Horizontal Walers (Double 2x4 or 2x6)
      │              │
      │   ════════   │ ◄─── Form Tie / Snap Tie (Resists Internal Hydrostatic Burst)
      │              │
      │  ██████████  │ ◄─── Vertical Strongbacks (Aligns & Stiffens Walers)
      │  ██████████  │
      └──────────────┘ ◄─── Diagonal Braces & Kickers (Resists Wind/Lateral Loads)

ACI 347 Lateral Concrete Pressure Formulas

Fresh concrete acts as a quasi-fluid until initial set occurs. Maximum lateral pressure ($P_{max}$) exerted on vertical wall and column forms depends on the rate of placement ($R$ in feet per hour), the fresh concrete temperature ($T$ in °F), the unit weight coefficient ($C_w$), and the admixture/chemistry coefficient ($C_c$):

1. Column Forms (All Pour Rates) & Wall Forms with $R < 7 ft/hr$:

$P_{max} = C_w C_c [ 150 + (9000 R) / (T) ]$

(Subject to a minimum of $600 C_w psf$ and a maximum of $w h$, where $w = 150 pcf$ and $h = pour height in feet$.)

2. Wall Forms with $7 ft/hr <= R <= 15 ft/hr$:

$P_{max} = C_w C_c [ 150 + (43400) / (T) + (2800 R) / (T) ]$

(Subject to a minimum of $600 C_w psf$ and a maximum of $w h$.)

3. Wall Forms with $R > 15 ft/hr$ or Rapid Self-Consolidating Concrete (SCC):

$P_{max} = w * h = 150 * h (Full Hydrostatic Fluid Pressure)$

Key Formwork Components & Stripping Milestones

• Sheathing: Plywood (B-B Plyform, HDO/MDO overlaid plywood) or steel panels in direct contact with concrete.
• Studs & Walers: Studs support the sheathing directly; double horizontal walers transfer loads from studs to form ties.
• Form Ties: Internal tension rods (snap ties, coil ties, taper ties) engineered to resist lateral concrete pressure. Rated by safe working load (SWL, typically 2,250 lbs to 9,000+ lbs with a 2:1 safety factor).
• Shores & Reshores: Shores are vertical compressive posts (timber or adjustable steel props) supporting suspended slabs and beams. Reshoring is the process of removing original shores and replacing them with snug, unloaded props to distribute new construction loads across multiple cured lower slabs without inducing permanent structural deflection.
• Removal and reshoring criteria: There is no universal calendar-day stripping table that overrides the approved formwork and reshoring drawings. Remove forms and shores only when the concrete has enough verified strength for its self-weight and every construction load that will be imposed. The engineer's criteria, field-cured tests or other approved strength evidence, temperature history, member span, post-tensioning sequence, and loads from upper floors all matter. Reshores are placed and sequenced so that load is transferred without overstressing a young slab; they are not assumed to be loaded identically to original shores.

Pre-pour and removal control

Before placement, compare the formwork drawings with actual tie spacing, bracing, pour rate, lift sequence, concrete mixture, pump location, construction joints, embeds, openings, and access. Self-consolidating concrete or a delayed-setting mixture can maintain fluid pressure longer than a conventional mix, so the form designer must know the mixture and planned placement rate. During placement, monitor alignment, leakage, movement, tie distress, and the actual rate; stop work when behavior departs from the design assumptions.

Treat removal as a temporary-works operation. Record the approving person, required strength, test evidence, slab temperature, stripping zone, and reshoring sequence. A lower floor may already carry its own dead load plus shores from several levels above. Premature removal can redistribute that stack into a slab never designed for the construction stage. The exam trap is to choose a familiar number of days. The defensible answer is the approved design and verified strength for the loads at that time.