Why isn’t the lateral pressure of fresh concrete on formwork simply depth times density?
The lateral pressure of fresh concrete on formwork is not simply depth times density because the lower concrete begins to stiffen and take its own weight while placement continues above. If concrete stayed fluid indefinitely, lateral pressure would increase with depth for the full height of a pour. It does not: the pressure rises to a maximum at some depth and then stops increasing.
Where that maximum pressure falls, and how high it is, governs the formwork design — and it is a function of how fast the concrete arrives relative to how fast it sets.
What happens when concrete is placed faster than the formwork design assumed?
Placing concrete faster than the formwork design assumed raises the lateral pressure on the forms, because the lower concrete has had less time to stiffen when the upper concrete arrives, so the fluid depth is greater and the pressure higher. Formwork design pressure is calculated for an assumed rate of rise, and placement rate is the variable most often exceeded.
Crews under schedule pressure, or a pump delivering faster than planned, can exceed the assumed rate of rise substantially without anyone recognizing that the form is being loaded beyond its design. Exceeding the assumed placement rate is the single most common contributor in formwork failures during concrete placement.
How does temperature affect fresh concrete pressure on formwork?
Cold temperatures raise the lateral pressure of fresh concrete on formwork, because concrete stiffens more slowly in cold weather, so at a given placing rate the fluid depth is greater and the pressure higher. A pour placed at a rate that was safe in summer can overload the same formwork in winter.
Retarding admixtures, added to extend working time, have the same effect on formwork pressure deliberately. Both the ambient temperature and the concrete temperature at placement are therefore formwork design inputs, and both are recorded on concrete delivery tickets.
How does the concrete mix affect formwork pressure?
The concrete mix changes the assumptions behind a formwork pressure calculation, because admixtures, supplementary cementitious materials and slump all affect how fast concrete stiffens. Self-consolidating concrete remains fluid and is generally treated as exerting full hydrostatic pressure for its entire depth, which is a fundamentally different design case from conventional mix.
A mix substitution between formwork design and concrete placement — a common occurrence, and one often made for reasons unrelated to the formwork — can invalidate the formwork pressure calculation without anybody connecting the two.
How does vibration affect fresh concrete pressure on formwork?
Internal vibration raises the pressure of fresh concrete on formwork by restoring fluidity to concrete that was setting, and a vibrator inserted deeper than the current lift re-liquefies material below, raising pressure at that depth.
Over-vibration and excessive vibrator insertion depth are recognized contributors to formwork failures, and they are within the concrete crew’s control rather than the formwork designer’s. The vibration practice actually used during a pour is worth establishing from witnesses and from the pour record.
Can the failure sequence of a formwork collapse be traced back to where it started?
The failure sequence of a formwork collapse is usually legible: formwork failures typically initiate at a specific tie, wale, brace or connection and then unzip as load redistributes. The initiation point is identifiable from the debris and from the deformation pattern of the recovered formwork components.
Establishing whether the initiating formwork element failed at, below or above its rated capacity is what separates an overloaded formwork system from a defective or improperly installed component.
Does as-erected formwork match the formwork drawings?
As-erected formwork rarely matches the formwork drawings. Formwork design assumes a specific arrangement — tie spacing, wale sizes, brace locations, shore positions — and what gets erected often differs: ties omitted where they interfered, spacing opened up to save time, substituted components, or bracing left off.
Measuring the actual as-erected formwork geometry from the debris and comparing it against the formwork drawings is the core of a formwork failure analysis, since formwork capacity depends on spacing far more sensitively than most people expect.
What documents establish what was planned for a concrete pour after a formwork failure?
The documents that establish what was planned for a concrete pour are the formwork design drawings and calculations, including the assumed placing rate and temperature; the concrete delivery tickets; and the pour logs and inspection records.
Concrete delivery tickets show the mix design, admixtures, concrete temperature and delivery times, from which the actual rate of rise can be reconstructed directly.
The responsibility structure for a concrete pour also needs to be established after a formwork failure: who designed the formwork, who was required to inspect it, and who had authority to stop the pour.
What evidence should be preserved after a formwork failure?
After a formwork failure, the formwork components should be preserved as found, with the failed elements identified and undisturbed, and with enough of the surrounding system kept to establish actual spacing. Ties, couplers and connectors from both failed and intact regions should be retained for testing against their rated capacity.
The concrete delivery tickets should be secured before they are filed away or discarded, along with photographs of the pour in progress, which crews frequently take and which establish rate of rise better than any recollection.
This guidance on formwork failures is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular formwork incident requires hands-on examination by a credentialed expert.