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There are several factors that may lead to the failure of formwork during and after concrete placement. Improper bracing can cause formwork failure since the weight of fresh concrete is no longer supported by formwork system. Unanticipated incident can make member to fail and subsequent overloading or misalignment of other members results in the failure of the entire formwork structure.
Moreover, omissions in the assembly detail is another factor that cause concrete formwork failure. This is because localized weakness and overstress would occur which destabilize the formwork system. Furthermore, workmanship errors due to haste, indifference, or lack of knowledge is another cause of failure.
Finally, formwork failures can lead to loss of life, delay in construction, and increase construction cost. That is why familiarizing with causes of failure and considering proper prevention strategies are substantially crucial. Good practice in designing, constructing, and handling formwork can provide safety as well as efficiency and economy.
1. Improper Stripping and Shore Removal
Early striping of forms and careless practices in reshoring can lead to failure of formworks and total failure of specifically in multistory building in which progressive collapse is highly likely.
Sometimes, premature striping causes sagging of partially cured concrete. This leads to crack development and create maintenance issues. Insufficient spacing and size of reshores may lead to a formwork collapse during construction as well as damage of the concrete structure.
2. Inadequate Bracing
Insufficient cross bracing and horizontal bracing of shores are common causes of formwork failure. This is because poor bracing generates lateral force and hence create lateral deformation of supporting members. When a failure occurs at one point, inadequate bracing may permit the collapse to extend to a large portion of the structure and multiply the damage.
3. Inadequate Control of Concrete Placement
Failure to regulate properly the rate and order of placing concrete on horizontal formwork introduce unbalanced loadings and consequent failures of formwork.
So, concrete temperature, concrete chemistry, rate of vertical placement of concrete, and vibration of concrete are factors affecting the development of lateral pressures that act on the forms.
If concrete temperature drops during construction operations, the rate of concrete placement needs to be decreased to avoid a buildup of lateral pressure overloading the formworks.
4. Inadequate Concrete Strength Development
It is another factor that has been reported to be the cause of concrete formwork system. The failure of hyperbolic cooling tower, which is located in Ohio/ USA, during its construction is compelling example of failures due to insufficient concrete strength development.
A jump form system was supported by anchors cast in the previous day's concrete placement. The forms were linked together around the inside and outside of the circular shell structure. After raising the forms and during the casting of an additional ring level, the concrete placed the previous day failed at the anchors, causing the scaffolding and forms to collapse.
5. Vibration and Impact
Vibration and impact due to passing traffic, movement of workers and equipment on formwork, and vibration because of concrete consolidation can displace supporting shores or jacks of formwork system and cause total failure of the forms.
6. Unstable Soil under Mudsills and Out of Plumb Shores
Unstable soil and out of plumb shores are another factor that jeopardize the stability of the formwork system. Unstable soil would lead to differential settlement and the ability of out of plumb shores to support loads declined greatly. Therefore, verticality of shores and the sufficient ability of soil to support loads shall be ensured during the construction of formwork system.
Supporting elements of formwork should not be placed on frozen ground. This is because moisture and heat from the concreting operations or changing air temperatures may thaw the soil and allow settlement that overloads or shifts the formwork. Finally, site drainage must be adequate to prevent a washout of soil supporting the mudsills.
7. Lack of Attention to Formwork Details
Assemblage errors such as insufficient nailing, failure to tighten the locking devices on metal shores, inadequate provisions to prevent rotation of beam forms where slab forms frame into them on one side, inadequate anchorage against uplift for sloping form faces, and lack of bracing or tying at corners can cause failure of formworks even if its design is completed in the best possible way. So, skilled and trained labor shall assemble the formwork system under great supervision.
8. When Formwork Is Not at Fault
There are cases in which the failure of the formwork system during building construction is not related to formwork faults. Factors such as incorrect assumption regarding types of soil in design stage, inadequate shear reinforcement placement, and placement of ducts at high stress region in slabs are reported to be the causes of failures.