Fresh concrete may look solid because it contains stone, sand, cement, and water, but right after placement it can act like a very heavy fluid. As it is poured into a vertical wall form, it pushes sideways on the form panels. This sideways push is called lateral formwork pressure, and it can be strong enough to bend panels or knock down braces.
Understanding this pressure helps builders keep workers safe and concrete walls straight.
Key Facts
- Fluid pressure increases with depth: p = rho g h.
- For fresh concrete, a typical density is rho = 2400 kg/m^3.
- Side pressure on a vertical form is greatest near the bottom when the concrete is still fluid.
- Total force on a rectangular form area can be estimated by F = p_avg A.
- If the pressure varies from 0 at the top to p_max at the bottom, p_avg = p_max / 2.
- Faster pour rates, colder temperatures, and wetter mixes can increase formwork pressure because the concrete stays fluid longer.
Vocabulary
- Formwork
- A temporary mold made of panels, ties, and braces that holds wet concrete until it becomes strong enough to support itself.
- Lateral pressure
- Sideways pressure exerted by fresh concrete against vertical formwork.
- Hydrostatic pressure
- Pressure caused by the weight of a fluid, which increases with depth according to p = rho g h.
- Brace
- A support member that resists forces and helps keep formwork upright and aligned.
- Setting
- The process in which fresh concrete stiffens as cement reacts with water.
Common Mistakes to Avoid
- Assuming concrete stops pushing sideways as soon as it is poured is wrong because fresh concrete can behave like a fluid until it begins to set.
- Using only the weight of the concrete straight downward is wrong because formwork must also resist lateral pressure on its vertical faces.
- Ignoring pour rate is wrong because faster filling can build up more fluid concrete before lower layers stiffen.
- Placing braces too far apart is wrong because the pressure load must be transferred safely from panels into ties, braces, and the ground.
Practice Questions
- 1 A wall form is filled with fresh concrete to a height of 2.0 m. Using rho = 2400 kg/m^3 and g = 9.8 m/s^2, calculate the pressure at the bottom using p = rho g h.
- 2 A 3.0 m wide form is filled to a height of 2.0 m. If the bottom pressure is 47,040 Pa and the pressure distribution is triangular, calculate the total lateral force using F = p_avg A, where p_avg = p_max / 2 and A = width times height.
- 3 Explain why a cold day and a fast concrete placement rate can make formwork bracing more important, even if the wall height is unchanged.