Ciria Report 108 Concrete Pressure On Formwork Page

CIRIA Report 108 (1985) provides the industry-standard semi-empirical model for calculating lateral concrete pressure on formwork, incorporating variables like placement rate and mix design to avoid over-engineering. The report, which superseded earlier guidelines, defines maximum design pressure ( Pmaxcap P sub m a x end-sub

) based on concrete density, rate of rise, and temperature coefficients, distinguishing between wall and column applications. Detailed information on the 1985 report is available at CIRIA. Concrete Pressure Analysis per CIRIA 108 | PDF - Scribd

Published in 1985, CIRIA Report 108 (R108) serves as a primary industry standard for calculating the lateral pressure of fresh concrete on formwork. The report provides an empirical formula to determine maximum pressure based on variables like concrete density, rate of rise, and temperature, which remains relevant for ensuring safe, cost-effective formwork design. For full details, visit CIRIA. Concrete pressure on formwork (R108) - CIRIA

3. Key Variables Explained

To effectively use CIRIA Report 108, an engineer must master the inputs: ciria report 108 concrete pressure on formwork

Critical Factors That Influence R and E

The magic of CIRIA 108 lies in accurately capturing Rate (R) and Setting Time (E). Misjudging these is the primary cause of formwork failure.

Common Mistakes When Applying CIRIA Report 108

Despite its clarity, engineers frequently misapply the report:

The "CIRIA 108" Curve

Most engineers memorize the rule of thumb: Pressure is 110 kN/m² or ( 2.4 \times R ), whichever is lower. But the report details five distinct concrete types (ST1 to ST5) and their rheology. 3 Lessons from the Report for Today’s Site 1

For modern mix designs (high workability, superplasticizers): Be careful. CIRIA 108 assumes "vibration" limited to 1.5m below the top. If you over-vibrate or use self-compacting concrete (SCC), the pressure reverts to full hydrostatic.

❌ Mistake 4: Misapplying to slipforming or jump forms

CIRIA 108 is intended for stationary forms. For slipforming, dynamic pressures are higher—consult separate guidance.

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. dynamic pressures are higher—consult separate guidance.

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.

2. Key Content and Methodology

The report is structured to take the user from the fundamental properties of concrete to the final design pressure.

• Theoretical Basis: The report establishes that fresh concrete behaves as a fluid for a limited time. The pressure profile is not hydrostatic throughout; instead, it peaks at a certain depth and remains constant below that point (the "envelope" theory).
• The "CIRIA Method": The core of the report is the pressure calculation formula. It identifies the Critical Rise Rate and the Temperature of the concrete as the primary variables.
• Mix Proportions: A significant contribution of R108 is its categorization of concrete types. It distinguishes between:
  • Ordinary Portland Cement (OPC).
  • Concrete containing Pulverised Fuel Ash (PFA).
  • Concrete containing Ground Granulated Blastfurnace Slag (GGBS).
  • Retarded mixes. This differentiation is crucial because these admixtures alter the stiffening time of the concrete, directly affecting the pressure exerted on the forms.