Ciria Report 108 Concrete Pressure On: Formwork Link

CIRIA Report 108 (1985) serves as the UK industry-standard guide for calculating lateral concrete pressure on formwork, utilizing a semi-empirical formula that accounts for placement rate, concrete density, and temperature. The report distinguishes between wall and column pressures and covers the impact of modern admixtures, though it may be conservative for highly flowable self-compacting concrete. The report is available through the CIRIA bookstore. Concrete pressure on formwork (R108) - CIRIA

CIRIA Report 108, published in 1985 by the Construction Industry Research and Information Association (CIRIA), is a seminal technical document providing guidance for calculating the lateral pressure exerted by fresh concrete on vertical formwork. It significantly improved upon earlier methods by accounting for modern concrete variables such as chemical admixtures, blended cements, and varying rates of placement. Quick Facts Full Title: Concrete Pressure on Formwork (Report 108). Authors: C.A. Clear and T.A. Harrison.

Core Purpose: To provide a reliable, empirical model for predicting maximum lateral pressure ( Pmaxcap P sub m a x end-sub ) to ensure safe and cost-effective formwork design.

Status: Currently considered a standard reference in the UK and internationally, often cited alongside standards like BS 5975. The CIRIA 108 Calculation Model

The report proposes that concrete pressure follows a hydrostatic distribution (increasing with depth) until it reaches a maximum value ( Pmaxcap P sub m a x end-sub ciria report 108 concrete pressure on formwork

), after which it remains constant to the bottom of the formwork. Key Factors Influencing Pressure

The model calculates pressure based on several critical parameters:

CIRIA Report 108 (1985) is a seminal guide for calculating lateral concrete pressure on vertical and inclined formwork, utilizing a trapezoidal pressure distribution model. It provides empirical formulas incorporating factors like mix composition, placement rate, and temperature, acting as a, reference for modern construction and formwork design. Read the full report details at www.sciencedirect.com

An empirical model to predict fresh concrete lateral pressure CIRIA Report 108 (1985) serves as the UK

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:

  1. The Upper Limit: Full hydrostatic pressure (rarely reached unless pouring extremely fast or using retarding admixtures).
  2. 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 Core Principle: Why Concrete Isn’t Water To

Mistake #4: Pouring in High Winds

Wind load is external, but CIRIA 108 only covers internal concrete pressure. For tall, slender formwork, wind can add 0.5 to 1.0 kN/m² of suction, stacking on top of P_max.

Common Mistakes and How to Avoid Them

Despite its clarity, CIRIA 108 is often misapplied. Here are the top five errors observed on job sites:

Common Mistakes When Applying CIRIA Report 108

Despite its clarity, engineers frequently misapply the report:

1. Rate of Rise (R)

This is the speed at which the concrete level climbs vertically inside the formwork. It is NOT the same as the pump output.

  • Low rate (0.5 – 1 m/hr): Pressure stays low. Concrete has time to stiffen before reaching great depth.
  • High rate (3 – 5 m/hr): Pressure approaches hydrostatic. The concrete is "fresh" all the way to the top.

Actionable tip: Control R by limiting concrete delivery trucks or adjusting pump speed.