Concrete formworks are subjected to various loads and pressure. Concrete formwork loads and pressure calculations are described in this article.
Formworks or molds are considerably important for building constrictions by holding fresh concrete mixture at place until it get required strength by which the self weight can be sustained.
Generally, there are various loads which are possible to act on formworks. Vertical loads are one of the most significant loads that act on formworks and are due to the self weight of the formwork and casted concrete plus live load of worker in addition to their equipment.
Moreover, internal pressures which caused by the behavior of liquid fresh concrete, is acted on vertical formworks. Furthermore, it is mandatory to provide lateral bracing to achieve stability against lateral forces for example wind loads.
Concrete Formwork Loads and Pressure CalculationsFollowing are the various types of loads and pressures act on concrete formwork:
- Vertical load
- Lateral pressure of concrete
- Horizontal loads
- Special loads
1. Vertical Loads on Concrete FormworkVertical loads are exerted on formworks and may consist of dead loads such as formwork dead load, steel reinforcement embedded in the forms, molded fresh concrete and live loads such as the weight of workers, equipments, and tools. It is recommended to compute weight of materials separately in the case of heavy reinforcement to specify accurate unit weight. ACI 347-04: Guide to Formwork of Concrete specify that, to allow for workers and their placing tools such as screeds, vibrators, and hoses, at least 2.4Kpa live load should be used for the design of horizontal formworks and a minimum live load of 3.6 kPa should be employed in cases where motorized carts and buggies are utilized. Furthermore, ACI 347-04 determines combined live and dead load design of not less than 4.8 kPa or 6 kPa if motorized carts are employed. Finally, formwork selfweight is calculated using unit weight and dimensions of formwork various parts. The weight of formworks is substantially less than fresh concrete dead load and construction live load. That is why an allowance is decided as superimposed load per square meter for form work components during design. An initial assumption is made between 0.239-0.718 kPa based on experience and checked after the member is sized. This estimation is depending on the fact that common weight of formworks is 0.239-0.718 kPa.
2. Lateral Pressure on Concrete FormworkInternal pressure resulted from accumulated depth of placed concrete is imposed on vertical formworks such as walls and columns. During vibration and for short period after vibration, placed fresh concrete close to the top and to a small depth of formwork behaves like a liquid and impose lateral pressure on the formwork that is equal to the vertical liquid head. Fresh concrete is granular with internal friction but vibrations eliminate bonds in the mixture and generate liquid state. There are different reasons such as placement rate, concrete temperature, and internal frictions that affect lateral pressure of below vibration controlled depth and make lateral pressure smaller than liquid pressure head. When vertical placement is carried out at slow pace, fresh concrete could have time to start stiffening. Moreover, unless concrete temperature is low, the time to start setting is not short. Other factors such as pore water movement, creation of friction and other parameters may lead to decline lateral pressure. Various types of cement, admixtures, cement substitutes, construction practices might influence level of lateral pressure. Mostly, concrete lateral distribution pressure, which based on tests, is depicted as shown in Figure-1. The distribution begins close to the top as a liquid and reaches peak value at lower level. For design reasons, it is suggested that ultimate pressure is uniform at conservative value.
Figure-1: Typical and Assumed Distribution of Concrete Lateral Pressure on Formworks
Calculation of Lateral Pressure on Concrete FormworkACI 347-04 specify that, concrete lateral pressure is computed as per Equation-1 if fresh concrete slump value is greater than 175 mm and does not placed with normal internal vibration to a depth of 1.2 m or less.
|Density of concrete, Kg/m3||Cw|
|Less than 2240||Cw=0.5[1+(w / 2320 Kg/m3)] but not less than 0.80|
|2240 to 2400||1.0|
|More than 2400||Cw=w / 2320 Kg/m3|
|Type of cement or blend||Cc|
|Type I, II, and III without retarders1||1.0|
|Type I, II, and III with a retarder1||1.2|
|Other types or blend containing less than 70 percent slag or 40 percent fly ash without retarders1||1.2|
|Other types or blend containing less than 70 percent slag or 40 percent fly ash with a retarder1||1.4|
|blend containing more than 70 percent slag or 40 percent fly ash||1.4|
Figure-2: Schematic Bracing in Slab Formworks
Figure-3: Schematic Bracing in Walls Formworks