Ciria Report 108 Concrete Pressure On Formwork Guide

The report categorizes concrete mixes into groups based on their components and setting characteristics. Engineers look up the constituent materials (OPC, GGBS, Fly Ash) and the presence of admixtures to select the appropriate mix coefficient, which scales the pressure calculations according to the specific chemical behavior of the pour. Step 2: Calculate the Operational Variables Gather the site-specific parameters: : Rate of concrete rise ( : Concrete temperature at placement ( ∘Craised to the composed with power C : Concrete density ( kg/m3kg/m cubed : Total vertical height of the pour ( : Minimum internal dimension of the formwork section ( Step 3: Compute the Empirical Formwork Pressures

Identify the percentage of GGBS, PFA, or chemical retarders to select the correct

CIRIA 108 provides specific formulas to calculate the maximum pressure ( Pmaxcap P sub m a x end-sub

Fresh concrete behaves as a fluid, generating lateral pressure on formwork analogous to hydrostatic pressure. However, this pressure is rarely truly hydrostatic because concrete begins stiffening and developing internal shear resistance almost immediately after placement. The key difference is that formwork can be designed for pressures significantly lower than full hydrostatic head, provided the rate of concrete placement and other influencing factors are properly managed.

A 2025 critical review highlighted that current design standards, including CIRIA 108, ACI 347, and GB50666, have significant limitations in addressing the multifactorial coupling mechanisms and time-dependent evolution of lateral pressure in concrete formworks. This indicates ongoing research needs. ciria report 108 concrete pressure on formwork

Segmented formulas based on pour rate, chemistry, and wall vs. column classification.

Comparison graph of concrete pressure on formwork showing CIRIA 108 plateau vs hydrostatic pressure.

Moderate (Uses chemistry factors for cement type and accelerators) High (Uses consistency classes and setting time tEt sub cap E Accounts for cross-sectional dimensions ( Ignores section size (assumes infinite wall width) Focuses primarily on setting time and pour height Hydrostatic Cutoff Explicitly applied based on pour height Explicitly applied based on pour height Applied based on consistency and setting thresholds Modern Relevance and the Transition to CIRIA Guide C751

When a code offers a new method, engineers ask: "Is it accurate?" Subsequent academic research has consistently validated the R 108 method. A comparative study testing predicted pressures against experimental data found that CIRIA 108 predictions were generally accurate, though specific variations occurred depending on geometry. The report categorizes concrete mixes into groups based

For instance, one study comparing the predicted pressures for various wall types (Wall 1, 4, 5, and 6) against actual experimental measurements found underestimations ranging from . While a 17% underestimation might sound concerning, it should be noted that this falls within typical safety factor tolerances for temporary works, provided that proper site controls (especially regarding Rate of Pour and Temperature) are enforced. The report remains a "best practice" benchmark, though it relies on the contractor maintaining the specified placement rate.

CIRIA Report 108 is built on the understanding that fresh concrete is a —it behaves as a liquid when agitated but gains shear strength as it rests. The key variables affecting lateral pressure are:

Wall height H = 2.0 m Density D = 24.5 kN/m³ Placement rate R = 2.0 m/hr Concrete temperature T = 20°C C₁ = 1.00 (wall) C₂ = 0.45 (OPC with retarder)

P.J. Clear Publisher: Construction Industry Research and Information Association (CIRIA) Year: 1990 (Reprinted with minor amendments 1995) However, this pressure is rarely truly hydrostatic because

Consequently, the actual lateral pressure profile deviates from the hydrostatic line, peaking at a certain depth below the top surface before decreasing or stabilizing toward the bottom. CIRIA Report 108 provides the exact mathematical framework needed to determine this peak pressure. Key Variables Influencing Formwork Pressure

The $C_2$ value is vital because it tailors the calculation to the specific concrete mix. This is where CIRIA 108's strength lies. By selecting the appropriate $C_2$ value, you account for how the concrete's chemistry affects its setting time and, consequently, the pressure it exerts.

Fresh concrete stiffens faster when it is hot and slower when it is cold. CIRIA 108 incorporates this via the formula:

The formwork supplier designed ties at 1.2 m horizontal × 1.5 m vertical spacing, versus 0.6 × 0.8 m for hydrostatic. Material savings: 60% less tie hardware, lighter walers, and faster assembly. The pour completed without any deflection or leakage. This project alone saved over £15,000 in formwork materials.

Input the planned rate of rise and placing temperature to find the active structural coefficients. Compute Pmaxcap P sub m a x end-sub : Run the CIRIA 108 empirical formulas.

So, the design pressure is: $P_d = 25.0 \cdot 2.154 = 53.85 \text kN/m^2$.