Causes of Concrete Formwork Failures: The process of construction of concrete structure requires a setup of a formwork to support the concrete loads when the pouring process takes place. the formwork stays in place until the concrete is hardened and gains some strength to make it capable of carrying its own loads. Usually, formworks are removed after 7 to 15 days of casting depending on the type of concrete and the precautions taken during construction. Engineers design formworks for the expected loads during the construction such as construction materials, fresh concrete, equipment, labour, and also wind loads especially in open areas.
Thus, forms must carry on the acting loads without failure or too much sagging. However, sometimes we hear about accidents where the formwork failed during the construction of a building (during and after concrete placement). The collapse of formworks is considered a serious accident that can lead to severe injury of workers and it can reach to death. Add to this the delay in construction and increase in construction cost. Look at the photo below to see an example of how bad things might go for. So, it is important for engineers to understand the causes of failure so that they can take precautions and prevent this type of accidents.
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Causes of Concrete Formwork Failures
There are many causes of failures of formwork. Following is a partial list:
Improper, or lack of, design of formwork
Engineers design formworks to carry the expected loads during the construction process. If design was improper, formwork is going to collapse. Designers should consider all the anticipated actions on formworks. Also, the formwork designer should not omit the safety factor and calculations. Before construction, the design of formwork should be approved by a licensed engineer.
Inadequate shoring is a major reason for formwork collapse, where impact loads from concrete castings and other effects trigger the failure of vertical shores. Installing of shoring has to ensure a continuous load path from the formwork to the foundations or other structural elements that can support the formwork and fresh concrete.
Inadequate strength of form material
In some cases, the elements of formwork might not have proper cross-sections to provide sufficient strength against the loads imposed on the formwork.
Premature stripping of formwork
Premature stripping of forms, premature removal of shores, and careless practices in reshoring can cause disastrous results.
In case of multistory buildings, collapse of shoring can cause progressive collapse in which the collapse of a floor will impact the floor directly below it causing its collapse in the process continues until the lowest floor. This has a domino effect and by the impact load from the weight of upper floors debris, lower floors also collapsed subsequently in less than an hour.
In other cases, early striping of formworks leads to severe deflection of partially cured concrete. This causes significant cracking zones and may cause the slabs unsuitable for service. In such cases, floors may get demolished and reconstructed, if sagging was severe.
Inadequate lateral bracing
The more frequent reason of formwork collapse are factors that induce lateral forces or displacements of the shoring. Improper cross bracing and horizontal bracing of shores is a major factor responsible for formwork accidents. In addition, shoring with overloading at the top is vulnerable to eccentric or lateral loading. Diagonal bracing enhances the stability of such a structure.
When a failure happens in one-part, inadequate bracing may permit the collapse to extend to a large portion of the structure and multiply the damage. One major objective of bracing is to prevent such a minor accident or failure from becoming a disaster.
Unstable soils under mudsills
Unstable soils under the mudsills is another reason for formwork collapse. The mudsills are considered a base for a shore or post in formwork. The mudsills could be a timber plank, a frame, a small footing, or pedestals.
Formwork should transfer its loads safely through vertical members to the solid ground. The ground must be capable of carrying the load without settlement. Shores and mudsills must not stay on the frozen ground because moisture and heat from the concreting operations may thaw the soil, causing settlement of shores that will lead to overloads and/or shifts the formwork.
Improper vibration or consolidation of concrete
Forms sometimes fails when their supporting shores or jacks are displaced by the vibration caused by passing traffic, the movement of the workers and the equipment on the formwork, and the effect of vibrating concrete to consolidate it. Diagonal bracing can help prevent failure due to vibration.
Some failures have taken place when the concrete pipeline (used for pouring the concrete) has been supported on the shoring towers without the designers being informed of such an arrangement. The pipeline induces vibration to the shoring towers for which they may not be designed. The vibration sometimes cause struts or supports to loosen leading to the failure of the support system.
Improper rate of concrete placement
Casting concrete at a faster rate than design rate causes higher impacts on the formwork. If formwork can not support this load, failure happens.
There are other causes for formwork failures including improper connections, inadequate bearing details, errors in placement of reshoring, failure to follow codes and standards, modifications of vendor-supplied equipment, and negligence of workers or supervisors.
Types of Formwork Failure
Hadipriono and Wang (1986) conducted a study of 85 collapses of formworks. They divided the causes of failure into three types:
- Enabling causes, which are defined as events that lead to deficiencies in the design and/or construction of the formwork.
- Triggering causes, which are accidental factors that result in the collapse of formwork.
- Procedural causes, which are hidden causes apart from the enabling or the triggering factors.
Hadipriono and Wang (1986) summarized the causes of formwork failures as shown in the following table, along with the number of occurrences. Looking at the number of occurrence of accidents, almost half of the building formwork collapses were because of deficiencies in vertical shores.
Formwork for Concrete, M. Hurd
Formwork for Concrete Structures, K. Jha