Ciria Report 108 Concrete Pressure On Formwork May 2026

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: Concrete Pressure on Formwork (1985) is a seminal industry standard used to calculate the lateral forces exerted by fresh concrete on vertical formwork. It replaced the older CIRIA Report 1 (1965) to better account for modern developments like chemical admixtures and blended cements. Core Calculation Methodology

The report uses a simplified empirical equation to determine the maximum characteristic pressure ( cap P sub m a x end-sub

), assuming a hydrostatic pressure envelope that reaches a constant maximum value toward the bottom of the pour. ScienceDirect.com The design pressure is calculated as the smaller of: Full Hydrostatic Pressure: is density and is height). The CIRIA Formula: (simplified version). Key Variables in the Formula: (Rate of Rise): The vertical speed at which concrete is placed (m/h). cap C sub 1 (Size/Shape Coefficient):

Dependent on the formwork's dimensions and shape; it distinguishes between walls and columns. cap C sub 2 (Material Coefficient): ciria report 108 concrete pressure on formwork

Accounts for the type of cement and use of admixtures (e.g., retarders). (Temperature Coefficient):

Adjusts for the temperature of the concrete at placement; lower temperatures typically result in higher pressures due to slower setting. Independent Design House Key Takeaways and Practical Use Concrete pressure on formwork - BS5975:2019 - Prontubeam

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

CIRIA Report 108 (1985) serves as a foundational technical guide for calculating lateral concrete pressure on formwork, offering methods based on concrete mix, placement rate, and temperature. While widely adopted for temporary works design and standards like BS 5975, the method has limitations with highly flowable or self-compacting concrete. For the full report, visit CIRIA Bookstore AI responses may include mistakes. Learn more Form pressure generated by fresh concrete

CIRIA Report 108 (1985) establishes a widely used semi-empirical method for determining maximum lateral concrete pressure on vertical formwork, focusing on factors like placement rate, temperature, and mix design. While it remains a foundational guide, limitations exist regarding high-performance concrete, pumping surge pressures, and modern self-compacting concrete (SCC), often requiring a full hydrostatic approach for the latter. For a detailed summary of the report, visit Studocu. Form pressure generated by fresh concrete CIRIA Report 108 (1985) is a seminal guide

Practical Implementation on Site

How do you turn CIRIA 108 into actionable formwork design?

Step 1: Get a realistic "E". Order a penetration resistance test (ASTM C403 / BS EN 480-2) on your specific mix at the expected site temperature.

Step 2: Calculate your maximum allowable Rate (R). Rearrange the formula: R_max = P_allowed / (1.2 × D × E) If your formwork is rated for 80 kN/m², you solve for R to determine the maximum trucks per hour.

Step 3: Install a rate-of-rise indicator. Use a simple plumb line mark on the formwork with a time log. Or use modern IoT sensors that trigger alarms if the pour rate exceeds your R_max.

Step 4: Plan the pour sequence. Consider "horizontal layering" (pouring in lifts of 1-2 meters with a 30-minute delay between lifts). This allows lower layers to set, drastically reducing pressure on the bottom tie-rods. acting as a

Step 5: Monitor during the pour. Have a ready-mix engineer track the concrete temperature. If the truck arrives cooler than expected, recalculate P_max immediately.

2. Additives and Retarders

Retarding admixtures extend the time before initial set. This effectively increases R/T ratio, raising pressure. The report suggests applying a temperature correction or using an "effective temperature" lower than the actual concrete temperature.

1. Slump Influence

Higher slump (e.g., self-consolidating concrete) increases fluidity and delays stiffening. CIRIA 108 recommends adjusting the coefficient C1 upward for slump >120 mm. For SCC (slump flow >600 mm), many designers conservatively revert to hydrostatic pressure.

The Concept of "Stiffening"

The genius of Report 108 lies in its classification of concrete based on "stiffening time." The report recognizes that concrete does not set instantly. It defines categories (Tables within the report) that correlate temperature and mix properties to how quickly the concrete transitions from a fluid to a solid state.

Crucially, the report places a "cap" on pressure. Even if the rate of rise is high, the pressure will not increase indefinitely. It levels off once the concrete at the bottom has stiffened enough to support the load of the concrete above it.