Formwork is a critical component of concrete construction, as it provides temporary support and shape to fresh concrete until it sets and gains strength. One of the key considerations in formwork design is the lateral pressure exerted by fresh concrete. Several factors influence this lateral pressure, including the weight of the concrete, the rate of placement, vibration, temperature, chemistry, and slump. Since freshly placed concrete behaves temporarily like a fluid, it exerts a lateral liquid pressure on vertical formwork surfaces, making these factors critical for safe and efficient construction. This article delves into these factors and their implications for formwork design.
Weight of Concrete
The weight of concrete is a fundamental factor in determining lateral pressure. The pressure is directly proportional to the unit weight of concrete and the depth of the fresh concrete. Mathematically, the lateral pressure is equal to the unit weight of the concrete multiplied by the depth of the fluid or plastic concrete.
For slumps of 175 mm or less, the maximum lateral pressure is influenced by the concrete’s temperature during placement, the rate of placement, and a unit weight coefficient. These coefficients are specified in design standards such as ACI 347-04, which provide essential guidance for formwork engineers.
Rate of Placement
The rate at which concrete is placed affects the lateral pressure initially, but its influence decreases as the concrete begins to stiffen and consolidate. A rapid vertical placement can result in the full liquid pressure being exerted over the entire depth of the form. Conversely, when the rate of placement is slower, the concrete at the bottom of the form begins to stiffen as more concrete is added above. This stiffening reduces the lateral pressure below the full liquid pressure by the time the concrete at the top is placed.
Pumping allows for rapid placement of concrete, which can increase lateral pressure significantly, especially if the forms are filled to their full height before any stiffening of the concrete occurs. It is, therefore, essential for designers to consider the placement rate and adapt their designs to accommodate the anticipated pressures.
Vibration
Vibration, particularly internal vibration, is commonly used during concrete placement to consolidate the material and remove trapped air. While this improves the quality of the concrete, it temporarily increases lateral pressure by 10–20%. During vibration, fresh concrete behaves like a liquid to the full depth of vibration, increasing the load on the formwork.
Re-vibration and external vibration methods generate even greater lateral loads compared to internal vibration. Consequently, formwork must be designed to withstand these additional loads. Additionally, controlling the depth and intensity of vibration during placement is critical to maintaining both concrete quality and structural safety.
Concrete Temperature
The temperature of the concrete at the time of placement has a significant impact on lateral pressure. Temperature influences the setting time of the concrete; lower temperatures delay stiffening, allowing more concrete to be poured before the lower layers begin to support themselves. This increases the liquid head depth and, in turn, the lateral pressure on the formwork.
Cold weather conditions, in particular, must be carefully accounted for in formwork design. Engineers need to anticipate the slower setting times and ensure that the formwork can withstand the higher pressures resulting from these conditions.
Concrete Slump
Slump, a measure of concrete workability, affects lateral pressure primarily when self-consolidating concrete (SCC) is used. For ordinary concrete, slump does not significantly influence lateral pressure. However, SCC, due to its high flowability, behaves like a fluid throughout placement, requiring the evaluation of the full liquid head when determining lateral pressure.
This unique property of SCC makes it essential for designers to consider its behavior and adjust their formwork designs accordingly, as SCC exerts greater pressure compared to traditional concrete mixes.
Concrete Chemistry
The chemical composition of the concrete mix can also significantly influence lateral pressure. Factors such as the type of cement, the use of retarding admixtures, and the inclusion of fly ash or slag cement as partial replacements for traditional cement all play a role.
For instance, retarding admixtures slow the setting process, extending the time during which concrete behaves like a liquid. Standards such as ACI 347-04 provide chemistry coefficients to account for these effects. For example, cement types I, II, and III without retarders have a coefficient of 1.0, whereas the same cements with retarders have a coefficient of 1.2. These coefficients help designers accurately estimate lateral pressure based on the specific chemistry of the concrete mix.
Conclusion
The lateral pressure exerted by fresh concrete on formwork is a complex phenomenon influenced by multiple interrelated factors. The weight of the concrete, rate of placement, vibration, temperature, slump, and chemistry all play crucial roles in determining the magnitude of this pressure. Understanding these factors and accounting for them in the design process is essential for ensuring safe and efficient construction.
By following established guidelines such as ACI 347-04, designers can accurately calculate lateral pressure and develop formwork systems that withstand the loads imposed during concrete placement. This ensures not only the structural integrity of the formwork but also the quality and safety of the overall construction process.