Formwork are a crucial part of reinforced concrete (RC) design. It supports the concrete until concrete gains sufficient strength. Depending upon the shape, formwork can be as simple as a cubic box to as complected as spline surface. While design of formwork is itself a major field of concrete engineering, here in this article, the impact of bad formwork on service life of the RC structure will be explored mainly focusing in the mechanical strength.
The most common problem in field during construction is the misalignment of formwork. Besides visually disturbing, if it exceeds the specified tolerance can change the behavior of the structure. Obviously, misalignment affects the thickness of concrete cover. When the cross section is bigger than the designed, it may add to overstressing of the structure due to additional dead weight, while if the cross section is thinner than the designed, it may not be provide sufficient strength.
For example in the case of beam, when the cover thickness changes due to formwork misalignment, in the compression side, the concrete section cannot generate the design strength. Such kind of over-stressed section are susceptible to creep and fatigue and reinforcement in such region are more likely to buckle in case of dynamic loading. If there is insufficient cover in the tension side, it may not create a strength problem but the steel bar will be near to the external environment and thus high chance of corrosion. This will ultimately, decrease the service life.
Leakage of cement paste from the fomwork joints is another common construction problem. This generally results in honeycomb in surface when formwork is removed. Additionally, such removal of cement paste will not allow the hardened concrete structure to have the desired strength as there will be lack of bonding between the aggregates.
Friction between the formwork and hardened concrete not only results in difficulty in removal of formwork but sometimes such a high impact force is required that can result in cracking of concrete surface and in worst case spalling off of the concrete occur. Construction workers generally try to hide such defects by plastering the surface, however, such damage to surface of concrete from the beginning of service life can result in expedited degradation.