Ciria Report 108 Concrete Pressure — On Formwork __top__
CIRIA Report 108, "Concrete pressure on formwork," serves as a foundational guide for calculating the lateral pressure exerted by fresh concrete on temporary formwork structures, accounting for variables like rate of rise, temperature, and mix composition. It provides specific formulas to determine maximum pressure ( Pmaxcap P sub m a x end-sub
Where:
Post Body:
1. Check your set time hourly
Don't rely on lab data. Take a trowel to the concrete in the jump form. If it isn't green (setting), you are still at maximum pressure. ciria report 108 concrete pressure on formwork
- Hydrostatic (Fluid): Diagonal straight line up.
- CIRIA 108 (Actual): Diagonal curve that flattens out after ~3m.
- Rate R = 2 m/h
- Temperature T = 20°C
- P_max = 7.2 × (2/20) + 18 = 7.2 × 0.1 + 18 = 0.72 + 18 = 18.72 kN/m²
- High temperature – accelerates set, but wait: Higher temperature reduces E (concrete sets faster) → Actually, hot concrete sets faster, so E is smaller, so pressure is lower. Correction: This is a common trap. CIRIA 108 formulas show that colder concrete (longer E) creates higher pressure. Winter pours are actually more dangerous for formwork than summer pours.
- Retarding admixtures – Dramatically increase E. If you add a retarder for a long haul, you must reduce the pour rate.
- High cement content / Supplementary cementitious materials (fly ash, slag) – Often increase setting time.
2. The CIRIA 108 Methodology
- CIRIA Report 108: Concrete Pressure on Formwork (2017)
- Construction Industry Research and Information Association (CIRIA)
- British Standards Institution (BSI) - BS 8533-1:2017