Understanding Concrete Formwork: When building concrete formwork, it is essential to have a basic understanding of how concrete exerts pressure on the formwork. Understanding the lateral forces that concrete exerts on the formwork will help in designing and constructing the formwork effectively.
Designing Concrete Formwork: Designing concrete formwork requires consideration of the lateral forces exerted by the concrete on the formwork. For best results, the formwork should be designed to withstand these lateral forces.
Constructing Concrete Formwork: Constructing concrete formwork is a complex process and requires careful consideration of the lateral forces exerted by the concrete. To ensure that the formwork is built correctly, the formwork should be designed to withstand these lateral forces.
Concrete Formwork Design Considerations
Lateral concrete pressure on formwork is affected by:
- Concrete Pour Rate and Lateral Concrete Pressure on Formwork: The concrete pour rate affects the lateral concrete pressure on formwork, as it determines how quickly the concrete is placed in the form. The pour rate should be adjusted based on the height of the pour and the weight of the concrete, as well as the temperature, type of cement, vibration, concrete slump and chemical additives.
- Weight of Concrete and Lateral Concrete Pressure on Formwork: The weight of the concrete also affects the lateral concrete pressure on formwork. The amount of pressure exerted on the formwork increases with the weight of the concrete, and should be taken into consideration when determining the pour rate.
- Temperature and Lateral Concrete Pressure on Formwork: Temperature is another factor that affects the lateral concrete pressure on formwork. Higher temperatures cause concrete to expand, which can increase the pressure on the formwork. Conversely, lower temperatures can cause the concrete to contract, resulting in reduced pressure on the formwork.
- Type of Cement and Lateral Concrete Pressure on Formwork: The type of cement used in the concrete pour can also affect the lateral concrete pressure on formwork. Different types of cement have different properties that can affect the concrete’s strength and ability to withstand pressure.
- Vibration and Lateral Concrete Pressure on Formwork: Vibration can also affect the lateral concrete pressure on formwork. Vibrations can cause the concrete to settle, resulting in an increase in pressure on the formwork.
- Concrete Slump and Lateral Concrete Pressure on Formwork: The water-cement ratio of the concrete, known as the concrete slump, also affects the lateral concrete pressure on formwork. A low slump indicates a dryer mix, which can lead to increased pressure on the formwork.
- Chemical Additives and Lateral Concrete Pressure on Formwork: Chemical additives can also affect the lateral concrete pressure on formwork. These additives can alter the properties of the concrete, including its strength and ability to withstand pressure.
Height of Concrete Pour:
Understanding the Pressure of Liquid Concrete The height of concrete pour is a major factor in determining the pressure it exerts against the forms. As the height of the concrete increases, so does the pressure. This is because the weight of the concrete above each point of the form creates a direct pressure. The thickness of the wall does not affect this pressure.
Concrete Pour Rate: How Increasing the Rate Affects Form Pressure
When the pour rate is increased, the pressure of the liquid concrete against the forms increases accordingly. However, if the concrete begins to harden before the pour is complete, the full liquid head will not develop and the pressure against the forms will be less than if the pour were finished before any of the concrete hardened. Once the concrete has hardened, it cannot exert more pressure on the forms even if liquid concrete continues to be added.
Weight of Concrete: Examining the Impact of Unit Weight
The unit weight of the concrete is also a factor in the pressure it exerts against the forms. Lightweight concrete will exert less pressure than normal weight concrete. This is due to the difference in mass between the two types of concrete. The diagrams below illustrate how the pressure of form pressure varies depending on the unit weight of concrete.
1) Formwork Design: Formwork design plays a critical role in the hardening time of concrete. The shape and size of the formwork will determine the pressure exerted against the forms, which will affect the rate at which concrete hardens. Designing the formwork to meet the pressure from the concrete is essential to ensure the concrete hardens properly.
2) Concrete Mix Design: The mix design of the concrete is also an important factor for the hardening time. Factors such as cement type, aggregate size, water content and admixtures all affect the rate of hardening. Adjusting the mix design to meet the concrete’s desired properties can help to ensure the concrete hardens in the desired amount of time.
3) Rate of Placing: The rate of placing the concrete plays a significant role in the hardening time. If the concrete is placed too quickly, the pressure exerted against the formwork can be too great and cause the concrete to harden too quickly. The rate of placing should be adjusted according to the formwork design and the concrete mix design.
4) Temperature: Temperature is an important factor in the hardening of concrete. Most formwork designs are based on an assumed average air and concrete temperature of 21°C. When temperatures drop, the hardening of concrete is delayed and the rate of placing must be adjusted accordingly. Ideally, the concrete should be poured at temperatures between 16°C and 38°C to ensure proper curing.
5) Type of Cement: The type of cement used in the concrete mix can affect the rate at which the concrete hardens. A high early strength cement will harden faster than a normal cement and will require a faster pour rate. When using a cement that alters the normal set and hardening time, the pour rate should be adjusted accordingly.
6) Vibration: Vibration plays a key role in the hardening of concrete. Internal vibration helps to consolidate the concrete and make it act more like a liquid. Re-vibration and external vibration can result in higher form loads, and these types of vibration require specially designed forms.
7) Concrete Slump: The slump of the concrete can also influence the rate at which it hardens. If the slump of the concrete is very low, it will act less like a liquid and will transmit less pressure against the formwork. When using concrete with a slump greater than 100 mm, special formwork should be designed to resist the full liquid head.
8) Chemical Additives: Chemical additives, such as retarders and plasticizers, can also affect the hardening time of concrete. When using these additives, be sure to refer to the vendor’s application data to determine the correct pour rate and formwork design.