CIVL1810 Chap.12 Formwork Pressure and Temporary Structures
Formwork Pressure and Temporary Structures
You can pour concrete into almost any shape you like, and something has to be that shape while the concrete decides whether it is still a liquid or already a solid. The standard resolves that by taking the lesser of two expressions for the maximum lateral pressure: a purely hydrostatic one, and one that accounts for the rate of pour, the form size, the cement type and admixtures, and the concrete temperature.
Because the equations and coefficient table are supplied in the exam appendix, the marks are entirely in using them correctly, in reading the answer as an envelope rather than a number, and in recognising that a soffit question is a different calculation altogether.
What this chapter covers
- 01
Why fresh concrete is hydrostatic near the top of a pour and capped lower down
- 02
The lesser of rule for the maximum lateral pressure of plastic concrete
- 03
The symbols: maximum lateral pressure, wet density, the form size coefficient, the rate of pour, the cement coefficient, the temperature coefficient, the concrete temperature, the form height and the poured height
- 04
The form size coefficient as 1.5 where breadth and width are under 2 metres and 1.0 otherwise
- 05
The cement coefficient table for general purpose, high early, low heat, sulfate resisting, shrinkage limited, blended cements and high replacement blends
- 06
The additive increase where a retarding or superplasticizing admixture is used, applied once
- 07
The breadth of the definition of a retarding admixture
- 08
The temperature coefficient and why 20 degrees is the reference condition
- 09
Converting a volume delivery rate into a vertical rate of pour using the plan area of the form
- 10
The bracket as the equivalent depth at which the hydrostatic line is cut off
- 11
Drawing the design envelope as a sloping segment, a corner and a vertical segment
- 12
The distinction between form height and poured height
- 13
Soffit formwork as a vertical load problem of wet concrete, formwork self weight and construction live load
- 14
Stating assumptions explicitly when a supplied value is not available
- 15
The link to stripping rules and back propping
Which expression governs, and why it matters
- +1Recall what the second expression represents. Its bracket is an equivalent depth in metres at which the concrete has stiffened enough for the pressure to stop rising, and that depth grows with the rate of pour, with the temperature coefficient and with the cement coefficient.
- +1Apply that to form B. A fast pour raises the rate of pour term directly, a cold placement temperature makes the temperature coefficient larger than one, and a retarder adds to the cement coefficient. All three push the equivalent depth deeper, so it is more likely to exceed the poured height, leaving the hydrostatic expression as the lesser of the two and therefore governing.
- +1State the site consequence. If the hydrostatic expression governs, the envelope is a straight line to the full poured depth with no constant portion, so the pressure at the base is the maximum possible for that height and the ties and form stiffness must be designed for it. Slowing the pour is the one variable a contractor controls, and it reduces the rate of pour term directly.
Key terms
- Plastic concrete
- Concrete that is still fluid, before it has stiffened. Only while it is plastic does it exert a genuinely hydrostatic pressure on the form.
- Rate of pour
- The vertical rate at which the concrete surface rises in the form, in metres per hour. It is obtained by dividing the volume delivery rate by the plan area of the form, not used directly as a volume rate.
- Form size coefficient
- A coefficient depending on the size of the formwork, taken as 1.5 where the breadth and width of the form are both less than two metres, and 1.0 for all other cases.
- Temperature coefficient
- A coefficient computed from the concrete temperature at placement, equal to one at 20 degrees Celsius, greater than one for colder concrete that stays fluid longer, and less than one for warmer concrete.
- Design pressure envelope
- The plot of pressure against depth down the form: hydrostatic from the top of the concrete to the depth given by the bracket, then constant at the maximum pressure to the base of the pour.
- Form height and poured height
- Two different heights that appear in different expressions. The form height is how tall the formwork is and appears in the second expression; the poured height is how far up the concrete is filled and appears in the hydrostatic one.
- Soffit formwork
- The deck that supports a slab while it cures. Its design is a vertical load problem combining the wet concrete, the formwork self weight and a construction live load, and it is unrelated to the lateral pressure calculation.
Formwork Pressure and Temporary Structures FAQ
Why does the standard take the lesser of two expressions?
Because fresh concrete starts as a fluid and ends as a solid, and a form has to survive both states. The first expression is the fluid case, a pure hydrostatic pressure growing with depth exactly as water would. The second describes what happens once the lower concrete has begun to stiffen, when arching and side friction develop and the pressure stops growing.
Taking the lesser amounts to saying: assume the concrete is fluid unless the rate of pour, the temperature and the mix say it will have stiffened first.
How is the admixture increase applied?
Once. Add 0.15 to the cement coefficient if the mix carries a retarder, a superplasticizer, or both, and note that having both present still earns a single addition. The definition of a retarding admixture is deliberately broad and covers plain retarders, water reducers and superplasticizers that also retard, and anything else whose use has the effect of delaying the set.
What is the fastest way to place the corner on the envelope?
Compute the bracket and read it as a depth in metres. Both expressions are the density term multiplied by something: the first by the poured height, the second by the bracket. So the bracket is the equivalent depth at which the hydrostatic line is cut off, and it gives you both the capping pressure and the position of the corner with no additional work.
Why does a soffit question ask you to state your assumptions?
Because the calculation needs a construction live load allowance that may not be in front of you. The dead loads are computable, being the wet concrete from its thickness and density plus the formwork self weight, but the live load from workers, barrows, vibrators, stacked reinforcement and the impact of placing comes from a load table.
Naming the missing quantity, stating the value you assume and saying what depends on it is what the instruction is asking for.
Exam move
Do the wall calculation until the order of operations is automatic, because the marks are procedural. Write down the rate of pour, the form size coefficient, the cement coefficient with any admixture increase, the temperature coefficient and both heights before touching either expression.
Practise changing one input at a time, since that is how the question is varied between years: the same wall with a different cement, a different temperature, a different pour rate. Learn to say which expression governed and why, because that interpretation is usually the last mark.
Then keep the soffit case separate in your mind, and rehearse writing an assumption as a named quantity with a justification and a consequence rather than as a guess.
Working through Formwork Pressure and Temporary Structures in CIVL1810? Sia is AskSia’s AI Engineering tutor — ask any CIVL1810 Formwork Pressure and Temporary Structures question and get a clear, step-by-step explanation grounded in how CIVL1810 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.
