Concrete is known to be a very versatile and reliable material, but some construction errors and construction negligence can lead to the development of defects in a concrete structure. These defects in concrete structures can be due to poor construction practices, poor quality control or due to poor structural design and detailing.
Common types of defects in concrete structures are honeycombing, form failure or misalignment of formwork, dimensional errors, rock pockets and finishing errors.
1. Honeycomb and Rock Pockets
Honeycomb and rock pockets appear on the concrete surface where voids are left due to the failure of cement mortar to fill spaces around and among coarse aggregates.
Causes of honeycomb and rock pockets involve poor quality control during mixing; transporting; or laying of concrete, under or over-compaction of concrete, insufficient spacing between bars, and low cement content or improper mix design.
Honeycomb and rock pockets may reduce durability because they expose the reinforcement to the environment which may reduce the strength of the concrete sections.
If these defects are minor, they can be repaired by using cement mortar grout just after the removal of the formwork. If the repair work is delayed for more than 24 hours, epoxy bonded concrete replacement should be used.
2. Defects due to Poor Formwork Installation
Formwork installation errors include misalignment, movement, loss of support, failure of forms that can lead to cracking and structural failure.
Settlement cracks develop due to concrete settlement caused by the loss of support during construction. Inadequate formwork support and premature removal of formwork are the major causes of loss of support during construction.
Defects due to formwork placement mistakes can be repaired with surface grinding to maintain the verticality of the structure if the error is minor. In case of major error, the concrete member shall be repaired by removing the concrete in the defective area and then reconstructing that portion of the structural member using suitable methods.
3. Defects due to Concrete Dimensional Errors
Dimensional errors in concrete structures occur either due to the poor centering of a structural member or due to deviation from the specifications. In that case, the structural member can be used if it is acceptable for the intended purpose of the structure or can be reconstructed if it doesn’t suffice.
4. Defects due to Finishing Errors
Finishing errors in concrete structures can involve over-finishing of the concrete surface or addition of more water or cement to the surface during finishing of the concrete. This results in the porous surface which makes the concrete permeable resulting in less durable concrete.
Poor finishing of concrete results in the spalling of concrete from surface early in their service life. Repair of spalling involves removal of defective concrete and replacement with epoxy bonded concrete.
5. Shrinkage Cracks
The formation of shrinkage cracks in concrete structures is due to the evaporation of water from the concrete mixture. The severity of this issue is based on the amount of water in concrete (as water quantity increases, the number of shrinkage cracks increases), weather conditions, and curing regime.
This problem can be tackled by considering suitable curing regime and adding a suitable amount of water to the concrete mixture.
6. Defects due to Poor Reinforcement Placement
Errors during reinforcement installation could lead to serious concrete deterioration. For instance, inadequate chair bars and insufficient tying of reinforcement would cause rebar movement which may lead to inadequate concrete cover and reduction in effect depth of the concrete section. As a result, the durability of the concrete structure is compromised and the structure would be vulnerable to chemical attacks.
Bugholes or surface voids are small regular or irregular cavities formed due to the entrapment of air bubbles in the surface during placement and consolidation. They commonly occur in vertical cast-in-place concrete like walls and columns.
Both the number and size of bugholes vary and depend on form-facing material and condition, release-agent type and application thickness, concrete mix characteristics, and placement and consolidation practices.
Bugholes are considered as defects if their width and depth exceed 3.81cm and 1.27cm respectively.
Read More: Concrete Repair / Protection Guide