8.3 Form Ties, Walers, Bracing & Pre-Pour Cleanliness

Form Ties, Walers, Bracing & Pre-Pour Cleanliness

Wall and column forms need a load path for fresh concrete pressure and construction loads. Read the approved formwork plan before checking the installed system. Compare member marks, spacing, connections and support with the design assumptions. ACI 318-19 Section 26.11.1.2 covers placement loads, tightness and the bracing or ties that maintain position and shape.

1. Trace the pressure load path

Snap ties leave an embedded portion after the removable ends are broken off. She-bolt systems use removable outer bolts with an inner tie, and taper ties are withdrawn from the formed wall. Use the approved system’s instructions for removal and treatment of the resulting opening. Where specified, ACI 301-20 Section 2.2.1.2 calls for ties with integral water barrier plates or another acceptable positive barrier. Check the tie detail and placement for that wall.

Under the unmodified ACI 301-20 Section 2.2.1.2 default, ferrous ties in SF-2 and SF-3 finishes have a breakback of at least 3/4 inch. Use coated or corrosion-resistant ties when specified or if the specified breakback is less than 3/4 inch. Sections 2.3.2.7 and 5.3.7.2 address tie-hole treatment. Check the finish and patching requirements. For the specified portland-cement repair mortar, clean and dampen the holes. Use accepted alternatives in accordance with the applicable instructions.

2. Choose the lateral pressure case

In ACI 347R-14 Section 4.2.2.1a, the inch-pound full liquid head at a point is $wh$, where $w$ is the concrete unit weight in lb/ft³ and $h$ is the depth in feet of fluid or plastic concrete above the point. For $w=150$ lb/ft³, a 10-ft head produces 1,500 psf and a 20-ft head produces 3,000 psf. Use the unit weight assumed in the design for the actual mixture.

For conventional concrete, use Table 4.2.2.1a(a) to choose the equation. Measure slump for this purpose after admixtures are added. For the table only, a column has no plan dimension exceeding 6.5 ft, and a wall has at least one exceeding 6.5 ft.

For equations (b) and (c), use concrete temperature $T$ in °F, placement rate $R$ in ft/h, and chemistry and unit-weight coefficients $C_c$ and $C_w$ from their tables.

For these equations, apply the minimum $600C_w$ psf, subject to the upper limit $wh$. This upper limit applies to these equations. A separate base-pumped case can exceed the full liquid head. Section 4.2.2.2 recommends full head for self-consolidating concrete unless its effect on form pressure is understood through measurements or previous studies and experience. Section 4.2.2.4 recommends full head plus at least a 25% surge allowance for concrete pumped in at the base. The 4-ft internal vibration criterion used for pressure selection is distinct from the ACI 309R-05 guidance on penetrating into a previous plastic layer.

Worked example: Compare placement rates

A 12-ft conventional wall form uses 150 lb/ft³ concrete at 60°F. Slump is 6 inches, internal vibration depth is 3 ft, and assume $C_c=C_w=1.0$. At a rate of 6 ft/h, use (b). The calculation is $150+9000(6)/60=1050$ psf, between 600 and $150(12)=1800$ psf. At 10 ft/h, use (c), which gives $150+43400/60+2800(10)/60=1340$ psf. At 16 ft/h, use full head, 1,800 psf. If the design specifies a 6-ft/h limit, observe and report an actual rate above that limit even when the concrete remains visually stable. Refer changes to the responsible parties under the approved procedure.

3. Check cleanout access and closure

ACI 301-20 Section 2.2.2.6 requires temporary openings where needed for cleaning and inspection. Locate them from the form plan or accepted procedure so concealed areas can be checked. Before placement, verify removal of debris and standing water, a sound and prepared joint where applicable, and secure closure of the access openings. Compare closure, tie and bracing details with the approved plan and report deviations. A clean form and correct dimensions are separate observations.