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The steel reinforcement mostly is divided into two categories i.e.
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Primary Reinforcement or Main Steel reinforcement
Secondary reinforcement or distribution reinforcement
Main steel reinforcement bars are employed in R.C.C structures to ensure resistance against the whole design loads coming over it.
The secondary reinforcement bars are mainly employed due to durability as well as for aesthetic reasons.
This reinforcement guarantee resistance for localized areas, like limited cracking. These also offer resistance against the stresses that are created due to temperature variations
The stirrups are reinforcements that are provided laterally, to keep the main bars of the structural elements like beams and columns, in position
The stirrups may be circular, square, rectangular, helical or diamond shape based on the cross-section of the structural element.
The reinforcement bars in the corners may be L – shaped.
The caging for the structural element under consideration must be properly tied, so that during concreting no bar is disturbed from its position
The rebars are mainly provided at the junctions where the formwork of the structural element is closed as well as at the point where a new structural element needs to be bond with the former one.
Bar bending schedule provides details of reinforcement cutting and bending length. Advantages of bar bending schedule when used along with reinforcement detailed drawing improves the quality of construction, cost and time saving for concrete construction works.
Repairs and rehabilitation of concrete structures, which of late has become an activity comparable to construction itself worldwide, is mostly because deterioration of concrete due to corrosion of embedded steel.
Nearly 40% of failure of concrete structures is due to corrosion of embedded steel reinforcement.
Ideally speaking, a good concrete is supposed to provide adequate protection to the embedded steel. This is due to the protective alkaline environment (pH value as high as 12.5) provided by fresh concrete resulting in formation of protective coating on the surface of the steel, which passivates it from further corrosion.
However, over the passage of time, due to carbonation or ingress of chloride ions, pH value starts declining slowly and alkaline surrounding of the reinforcement bar is lost, heralding the corrosion process, which in turn causes cracks and spalling of concrete. It would thus be realized that the crucial factor giving quality and durability of concrete appears to be its impermeability, which can be ensured by providing sufficient cement content, low w/c ratio, complete compaction and curing. The same can be further improved by using proper admixtures and providing increased concrete cover.
There can be many causes for corrosion of reinforcement, but mostly it is related to quality of concrete, environment and quality of construction practices. So, the first step in corrosion control of rebar is to provide good quality of concrete through good construction practices. The quality of concrete materials, mixing, placing and compaction techniques and good workmanship can help control the rebar corrosion.
Methods of Corrosion Control of Reinforcement in Concrete:
1. Cement-Polymer Composite Coated Rebars (CPCC)
2. Fusion Bonded Epoxy Coated Rebars (FBEC)
3. Corrosion Resistant Steel Deformed Rebars (CRSD)
The corrosion of steel reinforcement in concrete is complex, but basically it is an electrochemical reaction similar to that of a simple battery. The composition of mild steel varies along its length and potential anodic (more negatively charged) and cathodic (positively charged) sites can be set up at various points.
Thus, it becomes necessary to provide additional protection to reinforcement steel, especially because of chloride induced corrosion (worse than carbonation corrosion) which can develop even in good quality concrete.
Maintenance of passivation is conditional on an adequately high pH of the pore water in contact with the passivating layer. Thus, when the low pH front reaches the vicinity of the surface of the reinforcing steel, the protective oxide film is removed and corrosion can take place, provided oxygen and moisture necessary for the reactions of corrosion are present.
The differences in electrochemical potential can arise from differences in the environment of the concrete. Electrochemical cells form also due to a variation in salt concentration in the pore water or due to a non-uniform access to oxygen.
When there exists a difference in electrical potential along the steel in concrete, an electrochemical cell is set up: there form anodic and cathodic regions, connected by the electrolyte in the form of the pore water in the hardened cement paste.
It can be seen that oxygen is consumed and water is regenerated but it is needed for the process to continue. Thus, there is no corrosion in, dry concrete, probably below a relative humidity of 60 percent; nor is there corrosion in concrete fully immersed in water, except when water can entrain air, for example by wave action.
The transformation of metallic iron to rust is accompanied by an increase in volume, which depending on the state of oxidation, may be as large as 600 percent of the original metal. This volume increase is believed to be the principal cause of concrete expansion and cracking. It should be noted that the anodic reaction involving ionization of metallic iron will not progress far unless the electron flow to the cathode is maintained by consumption of the electrons at the cathode; for this the presence of both air and water at the surface of the cathode is absolutely necessary.
Factors Influencing Corrosion of Steel Reinforcement
Chlorides,Ambient temperature and relative humidity,Severity of exposure, Quality of construction materials,
Quality of concrete,Cover to the reinforcement,Initial curing conditions, andFormation of cracks.
Damages to Concrete Due to Corrosion of Steel Reinforcement
Stage 1: Formation of white patches
Stage 2: Brown patches along reinforcement
Stage 3: Occurrence of cracks
Stage 4: Formation of multiple cracks
Stage 5: Spalling of cover concrete
Stage 6: Snapping of bars
Stage 7: Buckling of bars and bulging of concrete
Minimizing the Risk of Steel Reinforcement Corrosion
*Quality of Concrete
*Depth of Reinforcement Steel Cover
*Materials of Concrete Construction