As one of the most important building materials used in construction, proper care and use of concrete is critical to the success of a project. From finding the best concrete forms for sale to placement and curing, there is a science to properly using concrete that sets some structures apart from others. One critical element is knowing the right protocol for curing your concrete after using formwork in order to achieve the right results. If you are wondering when the best time to remove formwork from elevated slabs is, this guide can help.
About Formwork Removal
Formwork refers to a type of casting mold or box that serves as a container into which fresh concrete is poured and then compacted. Once the concrete hardens, the formwork and concrete formwork ties are removed, leaving a solid mass in the shape of the inner face of that formwork. There are a wide variety of concrete forms for sale in today’s market, so it is important to choose one that matches the shape and size of your desired structure.
The process of removing formwork is also called strike-off, and should only occur after concrete components achieve their final and sufficient strength. Removing the formwork should also be done carefully to assure stability and avoid damage.
Determining When to Strike Off Formwork
There are a number of factors to consider when deciding if it is time to remove formwork, primarily based on how quickly the concrete used gains its strength.
Factors to consider include the following:
Grade of Concrete: The richer the grade of concrete, the rate of development of strength increases, leading to a quicker time to sufficient strength.
Grade of Cement: Higher grades of cement offer quick setting and shorter times to gain full strength.
Type of Cement: The cement type used in concrete mixtures can impact strength development. For example, low heat cement requires more time to set.
Temperature: High temperatures of concrete and the surrounding environment during placement can allow for concrete to gain strength in shorter time periods. Similarly, winter will usually be a time that the process is slower.
Size of the Concrete Member: Larger-sized concrete section members are able to gain strength in a shorter time than smaller sections.
Before a structure is considered complete and subjected to all loads assumed during structural design, the members are subjected to self-weight and construction loads during the process. It is essential to calculate the behavior of the structure under both self and construction loads. When this is not possible to calculate, the characteristic strength of cube of equal maturity to the structure required at the time of formwork removal can be calculated.
Estimating Strength of Concrete
Before removing formwork, you will want to be sure the strength of concrete is what you expect. Tests on concrete cubes or cylinders should be carried out. These cubes and cylinders should be prepared from the same mix as that of the structural members and cured under the same circumstances of temperature and moisture of the original structural member.
Once it is confirmed that the concrete has gained sufficient strength to withstand the design load, then it is safe to remove formworks. If possible, it is always better to let the formwork sit for extra time to help with curing. Delaying is recommended if there is a risk of freeze-thaw damage or formation of cracks due to thermal contraction after striking.
Proper Removal of Framework
Formwork removal can be extremely hazardous and should be carried out properly to avoid not only structural damage, but also danger to people in the vicinity. Quality engineers should inspect the site before starting the removal process to ensure safety.
Removal should be carried out sequentially and in the proper pattern, with the approval of the site in charge and engineers. The beam and joist units should be the last pieces removed. Once the removal process is complete, any exposed surfaces should be covered with polythene bags or wet bags so that the concrete is fully cured.
If formwork is removed early or improperly, you may notice the sides bulge due to concrete that hasn’t hardened. Evaporation of water will start immediately and curing will have to begin again immediately. If sufficient strength hasn’t been gained, the structure may deflect or in some cases, collapse. When deflection occurs, it will remain permanently and be difficult to repair.