Author: Lily Oom

All you need to learn about Striking Formwork

Formwork or Shuttering is indispensable in both private and commercial construction projects. This unique feature serves the purpose of a mould for concrete features in the project. And the mixed concrete is poured into the formwork to take shape. It also holds it together until the concrete is cast and dry. At the same time, before you remove the formwork, you must be sure that the concrete structure is strong enough to bear the weight of the part where the formwork was holding.  Visit https://www.bkhgroup.com.au/formwork/ to read about Striking Formwork.

As crucial as the formwork is to construction, the project manager and other stakeholders need to understand some basic principles of using them. However, this article will only focus on what special formwork is, how to remove the formwork and the minimum period it may take. So, let us follow carefully.

What is Striking Formwork?

Whenever you hear of Striking formwork, it refers to removing the formwork after the concrete has been cast and set. When the concrete has achieved its recommended strength and other properties to support its weight and the pressure exerted on it, the formwork or shuttering has served its purpose. In other words, shuttering removal is also essential for the curing process to work. 

When you evaluate the formwork after removal, which is still valuable for reuse, you can safely keep it for future projects. At the same time, depending on the side forms and the removed case of slabs and beams, you may allow the formworks to stay in the place for more extended periods. 

Factors that affect the Period of Striking of Formwork

Striking of the formwork has a designated time that must be removed, and this fantastic time depends on many factors. At the same time, the unique period and related factors may also vary from one building project to another. In addition, some critical controlling factors that determine when the formwork or shuttering is to be removed are listed below.

  1. Concrete type in the structure

When concrete contains a blast furnace slag (GGBS) or pulverized-fuel ash (PFA), such concrete can take a relatively longer striking period. On the other hand, the standard Portland cement concrete and similar ones may not take as long as the earlier mentioned cement types. GGBS and PFA cement types take a longer time to gain enough strength against the OPC. But the implication is also that when it finally gains enough power, it can become reliable. Meanwhile, employing rapid hardening types of cement can significantly reduce the level to which the concrete can attain strength while speeding up the hardening process.

  1. The weather of casting concrete

When the weather is cold, the rate of setting the concrete and attaining stiffness can be delayed when trying to reduce the striking time. For instance, ground slabs can take more time to gain strength and stiffness than the typical weather concreting process for concrete structures. It is also different from delaying troweling operations.  When you have higher stability in the concrete, it also shows a shortened period of striking off the formwork.

  1. Exposure conditions of the site

When the temperature of the structures increases, it takes a reduced required time to remove the formwork from the concrete. At the same time, it is also capable of the proper strength to withstand the exerted weight. It can also attain this power level more quickly as long as you can effectively control the cracking of the structure from affecting the inner concrete. 

  1. The best method for curing concrete

When the concrete inside the formwork is well cured, then the process can effectively reduce the striking time before the formwork is removed. And as soon as the formwork structure is removed from the formwork, the concrete curing should begin. The curing of the concrete should be secured when there are the right insulations in place that can protect the structure’s surface against extreme temperature changes. 

  1. Treatment and admixtures to concrete 

As a stakeholder in using concrete formwork, you can also reduce the striking time when adding some admixtures to the concrete structures. For example, some of the recommended admixtures are accelerators that efficiently reduce the setting time of the cast concrete. 

Essential factors to consider before removing formwork 

Before even planning to strike out formwork from a structure, certain aspects must be held sacrosanct. Some of these factors are mentioned below.

  1. When using slabs and beams, you can remove fit formwork conveniently in-situ with the strength of the concrete to be 10 N/mm2. On the other hand, it should be able to take up to two times of the stress or weight that will be exerted on it
  2. In general, the best practice of removing shuttering is to be done after assurance that it is strong enough under all conditions. The strength must be self-supporting and capable of bearing loads that are imposed on it
  3. To access the internal strength of the cast concrete, you should also perform all the pull-out tests for the cubes through standard procedures. They can also be cured under similar conditions as the in-situ concrete, as achieved by matching the cube samples when immersed in water. The water temperature is also compared to the structure.
  4. The proprietary quick-strip is also a valuable component in removing the shuttering that has completed its job. This process also can be done by propping or disturbing the structure in any way. 
  5. When the weather is cold, concrete formworks for walls, beam sides and columns must be given more than enough time. The reason is that if this striking of the formwork is done too early, you may still find what we know as ‘green concrete’ underneath it, and it can be easily damaged.
  6. All striking processes for formwork must be carefully carried out to avoid all kinds of damage due to inaccurate projections. In the case of early removal that is found to be wrong, the repair of the damage is to be carried out immediately on the formwork. It may include obliterating the formwork and reinstalling it. 

Classification of Formwork according to their Structures

Depending on how you apply formworks to the structures, there are different kinds of formwork we have. Meanwhile, there are other ways of classifying formworks, including the materials they are made from and other properties. But this particular article will focus on the actual application of this formwork in a significant construction project. Let us consider these formworks one after the other. Click here to read more about Formwork Structures.

Foundation formwork

The design of foundation formwork can be in diverse ways which can be different from each other. For instance, there is a stark difference between erecting formwork for individual foundations and strip foundations. The former is often designed as socket foundations. Meanwhile, the entire design choice is determined by the height of the foundation formwork and the desired size you intend to make.

Similarly, the kind of formwork that you employ for individual foundations can also be similar to the column formwork. At the same time, this formwork that is applicable to strip foundations can also be identical to the entire formwork. Ideally, sheeting panels that comprise formwork bearers appear as walers and are also relevant to the foundation formwork. They can also be secured through the rim type of walers.

Concerning bracing, there is a need of supplying squared and round timbers, including the diagonally arranged boards. Recall that formwork ties are done using tie wires or metal screws to drive them adequately. Nevertheless, do not forget that there is also a need to adhere to the specified dimensions in the approved drawing. Meanwhile, the components of the foundation formwork include the following

  • Formwork sheeting
  • Stull
  • Waler
  • Post
  • Thrust-board
  • Concrete bottom
  • Tiw wire

Wall formwork

A wall formwork comprises vertically arranged upright timbers which serve as the formwork bearers. These bearers also connect the sheeting boards to the concrete side that are nailed together. Also, the side formwork boards on either side can be diagonally braced in the upright timbers—similarly, every third vertical timber on the cleats with horizontally arranged walers. Meanwhile, the opposite walers are also tied together at specified distances. 

In the building process, you can replace the sheeting boards with prefabricated sheeting panels. At the same time, one can provide the cleaning holes at the foot of the formwork. Finally, a wall formwork in the vertical section comprises the following

  • A stull
  • A screw tie
  • Waler
  • Bracing component
  • Post
  • Cleaning hole
  • A thrust boards

Ceiling formwork

The ceiling formwork is the kind of formwork that one finds in the basic construction structures or buildings. Using formwork sheeting also comprises sheeting boards or prefabricated sheeting panels. Meanwhile, this formwork sheeting also lies on the squared timber formwork bearers, arranged on the primary bearers. These bearers can carry off all the forces of the round timber columns.

At the same time, in the case of smaller rooms, the primary bearers are synced with both columns to form a trestle. It also uses diagonal board bracings to take up the horizontal acting forces on the structural formwork. It also has round timber columns on double wedges, serving as stripping aid and correction devices. A vertical section of a ceiling formwork also includes the following.

  • Formwork sheeting
  • Main bearer
  • Formwork bearer
  • Cleat
  • Wall
  • Bracing
  • Support wedges
  • Column 

Beam formwork

Any formwork that will serve as the beam formwork must have been prefabricated with the formwork sheeting parts, including the sheeting bottom and side sheeting panels. Moreover, the individual components of the beam formwork must be specified in the construction project. And the prefabrication process of the formwork sheeting requires some level of preparation, especially if you have to manufacture them on site.

For instance, the formwork sheeting bottom with the side panels has sheeting boards nailed together with the cover straps. The measurements of the sheeting bottom on the two sides of the reinforced concrete columns depend on the beam’s size. The sheeting thickness with its cover straps also has a thrust board width of about 100 mm. You can also place the sheeting bottom on pedestal support, a formed trestle that comprises a connected waler with two columns through the cleats.

Column formwork

Column formworks are similar to beam formworks, which are the prefabricated components of the sheeting boards with dimensions connected to the cover straps. Also, it contains the sheeting panels in the foot rim, anchored by steel bolts in the soil. This foot rim also comprises double-nailed boards. It is also essential to measure the foot rim with the exact location where the column will be placed. This feature also has the same functions as the thrust-board for laying the proper foundation or beam formwork. 

In addition, it may also have a lateral cleaning hole inside the foot of the formwork. This feature serves to remove impurities within the formwork before placing the concrete inside it. At the same time, one should provide both sides of the column with the formwork to enhance the erection of a reinforcement. This stage is essential if there is a need to erect a steel reinforcement inside the column formwork.

The Preparation and Treatment of a Formwork after Use

There is a need to check and clean the formwork to perfect it even before placing the concrete inside it. After the cleaning process is completed, there is also a need to close up and tighten the cleaning holes in the wall and column formworks. This closure will also help to optimize the maximum pressure of the concrete mix. As a result, the sheeting pieces of the cleaning holes can be put against the thrust board and the formwork bearers.

At times, there may be a need for an additional formwork bearer that is to be mounted. And after casting the concrete, there is a need to continue to water on the formwork. At the same time, it extracts less water from the concrete mix from the formwork to make the stripping or removal much more straightforward. But it is pretty vital to ensure that there are no changes to the formwork, especially when placing the concrete inside it.