Ciria Report 108 Concrete Pressure On Formwork Hot! ❲99% Essential❳
CIRIA Report 108 (1985) provides a standard, non-hydrostatic method for calculating lateral concrete pressure on vertical formwork by accounting for placement rate, temperature, and concrete mix design. The model determines the maximum pressure based on either full hydrostatic pressure or a formula covering the "arch effect," which is widely recognized in standards like BS 5975. For more information, visit the official CIRIA R108 overview Concrete pressure on formwork (R108) - CIRIA
3 Lessons from the Report for Today’s Site
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.
2. The "CIRIA Bypass" If you have a slow-setting retarder (common in summer), CIRIA 108 allows for zero pressure relief. Your formwork ties must be designed for full fluid head.
3. Height matters The report clearly states that for walls > 4m, the maximum pressure reaches a plateau. Pouring faster above 4m doesn't increase pressure—it just risks a blowout at the bottom. ciria report 108 concrete pressure on formwork
The Core Principle: Why Concrete Isn’t Water
To understand CIRIA 108, you must abandon the "liquid assumption."
Water exerts pressure equally in all directions. Concrete, however, is a granular material with thixotropy (it thickens when left undisturbed) and cohesiveness. Once the concrete begins to set, it forms an arching action against the formwork.
CIRIA 108 defines the Maximum Lateral Pressure (P_max) as a value between two extremes: CIRIA Report 108 (1985) provides a standard, non-hydrostatic
- The Upper Limit: Full hydrostatic pressure (rarely reached unless pouring extremely fast or using retarding admixtures).
- The Lower Limit: Zero pressure (fully hardened).
The determining factor is internal friction, which the report links directly to the rate of rise (R) and the initial setting time (E) of the concrete.
The Problem CIRIA 108 Solved
In the 1970s and 80s, engineers realized the "fluid head" method was wasting millions on over-engineered formwork. Concrete stiffens as it hydrates. CIRIA 108 introduced the concept of "pressure relief" based on setting time.
Formwork Height
Report 108 distinguishes between the height of the formwork and the pressure envelope. In tall columns, the pressure might reach a maximum peak and then drop off near the bottom because the concrete there has set. In short walls, the pressure might be hydrostatic because the pour finishes before the concrete has time to stiffen. 3 Lessons from the Report for Today’s Site 1
Executive Summary
CIRIA Report 108 remains the definitive guide for calculating lateral pressures of fresh concrete on vertical and inclined formwork in the UK construction industry. While it predates the current European standards (EN 12812), it is widely regarded as the most practical and reliable document for this specific aspect of temporary works design. It bridges the gap between complex rheological theory and the pragmatic needs of a site engineer.
Mistake #5: Using CIRIA 108 for Slipforming
Slipforms have their own rules. CIRIA 108’s static formulas do not directly apply to continuously moving formwork (use CIRIA 59 or equivalent instead).
