Ciria Report 108 Concrete Pressure On Formwork
Before CIRIA 108, engineers primarily relied on hydraulic pressure formulas, assuming that fresh concrete behaved like a liquid (Pressure = Density x Depth). While this approach (often called the "hydrostatic" model) is safe, it is wildly uneconomical. It assumes that until concrete hardens, every inch of height exerts full fluid pressure.
The introduction of Self-Compacting Concrete (SCC) in the late 1980s and 1990s presented a challenge to many heritage formwork documents, including CIRIA 108. SCC is highly fluid, designed to flow through congested reinforcement without mechanical vibration. Conventional wisdom suggested such fluid concrete would behave exactly like water, exerting maximum hydrostatic pressure.
Back to Basics: Why CIRIA Report 108 Still Rules Concrete Pressure Design
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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.
). Instead of assuming a uniform hydrostatic distribution, the report utilizes a bilinear or tri-linear pressure envelope to model reality accurately. Step 1: Determine the Materials and Blends Factor ( C1cap C sub 1 The coefficient C1cap C sub 1
K = [36/(20+16)]² = (36/36)² = 1.00
This influence is perhaps most visible in how engineers deal with Self-Compacting Concrete (SCC). Because SCC flows so easily, there was a concern that it might behave like a heavy liquid and create full hydrostatic pressure on formwork. However, research has consistently shown that . Studies from the early 2010s found that for standard pouring methods, maximum SCC pressures were generally less than the values predicted by CIRIA 108, offering a valuable safety margin.
): The temperature of the concrete at the time of placing. Lower temperatures slow down the setting time, keeping the concrete in a fluid state longer and increasing pressure. Concrete Density (
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 Before CIRIA 108, engineers primarily relied on hydraulic
To fully appreciate the significance of Report 108, one must rewind to its predecessor: (published in 1965 by the then Cement and Concrete Association). By the early 1980s, the construction industry had undergone a seismic shift. Traditional Ordinary Portland Cement (OPC) concretes were increasingly being replaced by mixes utilizing chemical admixtures (superplasticizers, retarders) and blended cements (such as Pulverized Fuel Ash (PFA) and Ground Granulated Blastfurnace Slag (GGBS)).
While newer materials like Self-Compacting Concrete have pushed the boundaries of the report's original data set (requiring supplemental guidance like CIRIA C660), the fundamental mechanics established by Clear and Harrison remain valid. For the engineer on site tomorrow, CIRIA 108 is the first line of defense against one of construction's oldest problems: keeping wet concrete where it belongs.
So, the design pressure is: $P_d = 25.0 \cdot 2.154 = 53.85 \text kN/m^2$. The introduction of Self-Compacting Concrete (SCC) in the
Always apply appropriate safety factors for material strengths and construction tolerances as specified in local building codes. 6. Limitations and Future Trends
