Pressure On Formwork | Ciria Report 108 Concrete

In the late 1970s and early 1980s, CIRIA undertook a massive research project, observing real-world pours in walls, columns, and slipforms. The result, published in , provided empirical evidence that concrete stiffens (develops "shear strength") as it hydrates, thereby reducing peak pressure significantly below the hydrostatic maximum.

When using SCC, many engineers use a modified CIRIA approach with a coefficient between 1.8 and 2.5, or simply default to full hydrostatic pressure (D x H) for formwork safety. 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: Mistake #1: Using the Wrong Setting Time (E) Most contractors take E from a concrete test certificate done at 20°C. If your pour is at 10°C, E might be 3x longer. Rule: Always adjust E for ambient and concrete temperature. A 5°C drop can double E. Mistake #2: Ignoring the "Jump" in Rate The formula uses average rate of rise. But if a pump starts suddenly at 4 m/hr for the first 15 minutes, the bottom formwork experiences a pressure spike. Solution: Use the peak instantaneous rate, not the average over the whole pour. Mistake #3: Overlooking Vibration Depth CIRIA 108 assumes internal vibration is stopped 1.5m below the current concrete level. If you over-vibrate (running the head too deep), you liquify the stiffened concrete, resetting the pressure to hydrostatic at that depth. 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. Mistake #5: Using CIRIA 108 for Slipforming Slipforms have their own rules. CIRIA 108’s static formulas do not directly apply to continuously moving formwork (use CIRIA 59 or equivalent instead). Practical Implementation on Site How do you turn CIRIA 108 into actionable formwork design? ciria report 108 concrete pressure on formwork

By trusting the CIRIA 108 calculation, they saved over £2 million in formwork costs and completed the walls safely and on schedule. Nearly 40 years after its publication, CIRIA Report 108 "Concrete Pressure on Formwork" remains the gold standard for rational formwork design. It shifted the industry from fearful over-design to intelligent, risk-aware engineering. In the late 1970s and early 1980s, CIRIA

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. Common Mistakes and How to Avoid Them Despite

For decades, engineers and contractors have relied on a single, authoritative document to navigate this risk:

Order a penetration resistance test (ASTM C403 / BS EN 480-2) on your specific mix at the expected site temperature.

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.