Effects of cover thickness & cracking
In reinforced concrete structures, sufficient cover of concrete has to be provided to avoid exposure of reinforcement to aggressive environmental conditions and consequent rusting and deterioration of the cross sectional area in the structural elements. The most common construction defect, particularly in buildings, is lack of adequate thickness of cover.
It provides the nominal cover requirements to meet:
ü The durability requirements
ü Specified period of fire resistance
Requirements of concrete cover
The protection of the steel in concrete against corrosion depends upon an adequate thickness of good quality of concrete
Construction & supervision deficiencies are the major cause of defects, leading to cracking. It has been found that 36% of the defects were due to these causes. A well-known expect on structural failures said that, he never found a failure caused by poor concrete, but he had never investigated one that did not contain poor or interior concrete. This comment related to collapses of structures, but when the definition of failure is extended as we are doing here, the quality of concrete does become much more important. For example, the protection afforded to steel is greatly of concrete depended on the compaction & curing of the cover concrete.
Survey revealed that construction defects could be grouped into 4 classes.
A. Deficiencies in the control of concrete materials, batching & mixing
ü Use of salt water as mixing water ü Excess fines in the aggregates
B. Inadequate preparation before concreting.
ü Salt water contamination of reinforcement
ü Lack of cover to reinforcement
C. Inadequacies of placing & subsequent treatment
ü Plastic cracking & settlement cracking ü Lack of curing
D. Faults of construction planning &procedure
ü Overloading of members by construction loads ü Loading of partially constructed members
ü Differential shrinkage between section of construction ü Omission of designed movement joints
ü Unexpected behavior and restraint during prestressing