Effects of Concrete Weight on Lateral Pressure: The weight of freshly placed concrete can cause a significant amount of lateral pressure on formwork sides. As the concrete mass increases, the pressure on the form increases proportionally. This pressure is generated by the hydrostatic forces caused by the weight of the concrete in the form and the gravity of the earth. To limit this pressure, the weight of concrete should be minimized and the rate of placement should be slow.
Effects of Rate of Placement on Lateral Pressure: The rate of placement can also affect the magnitude of lateral pressure of fresh concrete on formwork sides. Rapid vertical placement of concrete can lead to a full liquid pressure for the entire depth of the form. However, when the rate of placement is slow, the concrete at the bottom of the form begins to stiffen. This reduces the lateral pressure to less than full liquid pressure by the time the concrete placement is completed in the upper parts of the form.
Effects of Vibration on Lateral Pressure: Vibration of the concrete is another factor that influences the lateral pressure of fresh concrete on formwork sides. Vibration is used to improve the quality of the concrete and reduce its porosity. However, excessive vibration can cause an increase in the lateral pressure, which can cause damage to the formwork. The formwork designer must take this into consideration when setting the vibration parameters during the design process.
Effects of Temperature on Lateral Pressure: The temperature of the concrete at the time of pouring can also influence the magnitude of lateral pressure of the fresh concrete on formwork sides. High temperatures can increase the pressure by causing the concrete to expand, while low temperatures can reduce the pressure. To limit the detrimental effects of temperature, the designer must take the ambient temperature into consideration when designing the formwork.
Effects of Chemistry and Slump on Lateral Pressure: The chemistry of the concrete mixture and the slump of the concrete can also influence the magnitude of lateral pressure of fresh concrete on formwork sides. A concrete mixture that has higher water content and higher slump can cause higher lateral pressure compared to a concrete mixture with lower water content and lower slump. To limit this pressure, the designer should ensure that the concrete mixture contains the correct amount of water, and that the slump is within the recommended range.
1. Weight of Concrete
Lateral Pressure of Fresh Concrete: The lateral pressure of fresh concrete is determined by the unit weight of the concrete. This pressure is imposed vertically on the formwork and is equal to the concrete’s unit weight multiplied by the depth of the fluid or plastic concrete.
Unit Weight Coefficient for Concrete: When the concrete slump is 175mm or less, the maximum lateral pressure can be calculated with the unit weight coefficient. This coefficient is obtained from the American Concrete Institute (ACI) 347-04 and is determined by the temperature of the concrete during placement and the rate of placement.
2. Rate of Placement
Impact of Concrete Placement Rate on Lateral Pressure: The rate of concrete placement can have a significant impact on the lateral pressure of concrete poured into formworks. This is because when concrete is first poured, it is in a liquid state and its stiffness and consolidation has not yet taken place. As such, the lateral pressure is proportional to the rate of placement.
Limiting Maximum Lateral Pressure: However, with stiffening and consolidation of the concrete, the influence of the rate of concrete placement on lateral pressure decreases. This is because the concrete is able to support itself, with the maximum lateral pressure limited to the full liquid pressure.
In conclusion, the rate of concrete placement has an effect on the lateral pressure of concrete poured into formworks, but this influence decreases as the concrete stiffens and consolidates. The maximum lateral pressure will be proportional to the rate of placement up to a limit equal to the full liquid pressure.
3. Vibration
Vibration and Lateral Pressure: Vibration causes an increase in lateral pressure on concrete forms. This is because it causes fresh concrete to behave like a liquid, filling the form and exerting pressure on its walls. The increase in lateral pressure is temporary, but typically increases by 10 to 20%.
Forms Must Withstand Increased Pressure: In order to ensure that the form is not compromised by the extra pressure, it must be designed to withstand the forces generated by vibration. Additionally, the depth of vibration must be carefully monitored during concrete placement.
Comparing Internal, External and Re-Vibration: The lateral pressure generated by internal vibration is generally lower than that generated by external or re-vibration. This means that forms must be designed with the anticipated vibration type in mind in order to ensure that it can handle the load.
4. Concrete Temperature
Impact of Temperature on Concrete Placement: The temperature of concrete during the placement process can significantly affect the lateral pressure applied to the forms. The setting time for concrete is largely dependent on temperature, with lower temperatures extending the time before the concrete is able to harden and support itself. This increases the liquid head depth, leading to a greater amount of lateral pressure. For this reason, it is important to account for cold weather when designing the form.
Optimizing Forms for Cold Weather: When working in cold weather, it is important to optimize the form to account for the extended setting time of the concrete. To do this, the form should be designed to hold more concrete, allowing for an increased liquid head depth without creating excessive lateral pressure. Additionally, the concrete mix should be adjusted to include admixtures that accelerate the setting time, such as calcium chloride or calcium nitrate, which can help to reduce the effects of cold weather.
5. Concrete Slump
What is Concrete Slump? Slump is a measure of the workability of concrete, which is the ability of concrete to flow and consolidate under its own weight. It is measured by pouring a sample of concrete into a cone-shaped mold, then measuring the distance the concrete falls. The result is expressed in inches, and can range from 0 (no slump) to 8 (very high slump).
How Does Slump Affect Lateral Pressure? When ordinary concrete is used in construction, the slump does not have a significant effect on lateral pressure. However, when self-consolidating concrete is used, the full liquid head should be considered to evaluate the lateral pressure of the fresh concrete on forms. This is due to the greater flowability of self-consolidating concrete, which can cause greater lateral pressure on forms.
6. Concrete Chemistry
Effects of Cement Type on Lateral Pressure: The type of cement used can have a significant impact on lateral pressure. According to ACI 347-04, chemistry coefficients are applied to account for the influences of cement type. For instance, type I, II, and III Cement without retarders has a coefficient of 1, whereas type I, II, and III Cement with retarders has a coefficient of 1.2.
Use of Retarding Admixtures: Retarding admixtures can also affect lateral pressure. ACI 347-04 applies chemistry coefficients to account for the influence of admixtures. For instance, type I, II, and III Cement without retarders has a coefficient of 1, whereas type I, II, and III Cement with retarders has a coefficient of 1.2.
Impact of Fly Ash or Slag Cement as a Cement Replacement: Fly ash or slag cement can be used as a cement replacement, and this can have a significant effect on lateral pressure. ACI 347-04 applies chemistry coefficients to account for the influence of these replacements. For instance, type I, II, and III Cement without retarders has a coefficient of 1, whereas type: I, II, and III Cement with retarders has a coefficient of 1.2.