Edge Form Failure Risk on a Sloping Site

Loads on the edge form:

1. Hydrostatic pressure from the fresh concrete. For a 900 mm deep beam placed at 1.5 m per hour, the concrete is effectively fluid for the lower part of the pour; the form must resist the lateral pressure of the fresh concrete, which increases with depth and placement rate. Faster placement and lower slump/ higher temperature change the pressure - a 1.5 m/hour rate is significant.
2. Dynamic loads from the pump. The impact and surge of concrete leaving the boom, and any surge when the line clears, add to the static pressure.
3. Wind load. Gusty conditions apply lateral and uplift loads to the form and the bracing, and can also affect the boom and the placing operation.
4. Construction loads. Workers, vibration equipment and any temporary loads on the form.

Failure modes:

• Blowout / bulging of the ply face under pressure, especially where the ply is delaminated and has reduced stiffness.
• Brace or prop failure - buckling, pull-out of fixings, or overturning under wind and concrete pressure.
• Form movement causing an out-of-tolerance or bowed edge beam.
• Collapse if the bracing is inadequate or the form is not tied back properly.

Why previous use and wind matter:

• Reused ply with delaminated edges has lower strength and stiffness and can fail locally under pressure; the form's capacity is not the same as when new. Reuse must be assessed, not assumed.
• Wind adds loads the form may not have been designed for, and gusty conditions make the pour less controllable. The bracing must be checked for the combined concrete + wind case.

Conclusion: 'looked fine last time' is not an engineering check. The form needs to be assessed for the actual pressure at 1.5 m/hour, the condition of the ply, and the wind, before the pour starts.

Before the pour:

1. Formwork inspection and sign-off. Check the ply for delamination and replace or reinforce damaged panels. Check all props, braces, ties and fixings are tight and correctly located, and that the bracing resists both concrete pressure and wind.
2. Design check. Confirm the form and bracing are adequate for the actual placement rate (1.5 m/hour) and depth (900 mm), and for wind. If there is no design, get one.
3. Pour plan. Agree the placement rate and sequence with the pump operator; place in layers that keep the pressure within the form's capacity; avoid dropping concrete from height.
4. Weather. Check the forecast; if gusts exceed the safe limit for the form, the boom or the workers, postpone. Have a wind limit agreed in advance.
5. Emergency plan. Know how to stop the pump quickly, and have people positioned to watch the form.

During the pour:

• Assign watchers to the form and bracing throughout the pour, with a clear stop signal.
• Control the rate - do not let the pump operator exceed the agreed rate; slow down if the form shows movement.
• Monitor for bulging, deflection, leaks, loose props or movement of the brace feet.
• Keep the load balanced along the beam; do not heap concrete in one spot.

Stop triggers:

• Any visible movement, bulging or deflection of the form or bracing.
• A prop or brace loosening or pulling out.
• Wind gusts exceeding the agreed limit.
• A leak of grout or concrete from the form that indicates a gap or failure.

Bottom line: the pour proceeds only after a documented formwork check for the actual loads, with rate control, watchers, a wind limit and a clear stop authority. 'Looked fine last time' is replaced by evidence and active monitoring.