Reinforcement steel- Types and corrosion control



Types of steel
         Hot rolled bars
1.     Hot rolled plain round mild steel bars (MS)
2.     Hot rolled ribbed mild steel bars (Not recommended)
3.     Hot rolled high strength deformed bars (HYSD)
4.     Thermo mechanically treated (TMT)
         Hot rolled cold twisted deformed bars like tor steel
         Hard drawn steel wire fabric


         Check the material test certificate (MTC)
         Yield strength
         Ultimate tensile strength
         Elongation
         Bend test and reverse bend test


Requirements as per IS 1786
Property
Fe 415
Fe 500
Yield strength
415 N/mm2
500 N/mm2
Ultimate tensile strength
485 N/mm2
545 N/mm2
% Elongation
14.50
12
Bend test up to 22 mm
3D
4D
Bend test over 22 mm
4D
5D
Re bend test up to 10 mm
5D
5D
Re bend over 10 mm
7D
8D


Mass per unit length
Dia of bar
Weight
8
0.367-0.423
10
0.574-0.660
12
0.844-0932
16
1.499-1.660
20
2.393-2.544
25
3.734-3.965
28
4.685-4.975
32
6.121-6.499


Storing of reinforcement
         Different dia stored in different area.
         Separate supports so that they will not bend.
         Stored above ground level to avoid corrosion
         If stored for longer period they must be covered with some covering.
         Any rust material must be removed from steel before cutting and bending.
         Any material which is affecting the bond strength of steel must be removed from steel.


Structural steel sections
Hot rolled steel sections
         Angle sections
         Channel sections(ISJC, ISLC, ISMC)
         I sections(ISJB, ISLB, ISMB, ISHB)
         T sections
         Plane metal sheets, corrugated sheets, sheet piles, rail sections, flats of varying width, Cold formed light gauge steel sections.



Reinforcement corrosion

Reactions during corrosion


Effect of corrosion on RCC
         Cracking
         Delamination
         Spalling
         Loss of structural strength


Main causes of corrosion
Carbonation
         Caused because of CO2 gas. It will react with Ca(OH)2 to form calcium carbonate. This will reduce the alkalinity of concrete.
         Rate of carbonation depend upon following factors
1.     Relative humidity
2.     Grade of concrete,
3.     Permeability of concrete,
4.     Whether concrete is protected or not,
5.     Depth of cover,
6.     Time
         Measurement of extent of carbonation can be done by treating concrete with phenopthalein in diluted alcohol. If carbonation is not there colour will be pink and if it is carbonated concrete will remain uncolored
Chloride attack
         Caused because of reduction in alkalinity of concrete due to the attacks of chloride ions. They mainly enter into concrete from
1.     Deliberate addition of admixtures
2.     Use of salty water for mixing the concrete
3.     Chloride contaminated aggregates
4.     Attack of sea water
5.     Attack of chemicals


Relationship between w/c, depth of cover and time in years for carbonation depth to reach the reinforcement
W/C ratio
Depth of cover in mm
15
20
25
30
0.45
100
100
100
100
0.50
56
99
100
100
0.55
27
49
76
100
0.60
16
29
45
65
0.65
13
23
36
52
0.70
11
13
30
43


Limit of chloride content of concrete as per IS 456- 2000
Sr. No.
Type or Use of concrete
Maximum total acid soluble chloride content Expressed as Kg/m3 of concrete
1
Concrete containing metal and steam cured at elevated temperature and prestressed concrete
0.4
2
Reinforced concrete or plain concrete containing embedded metal.
0.6
3
Concrete not containing embedded metal or any material requiring protection from chloride
3.0


Corrosion control

Design and detailing
         Low permeability concret
         Nominal cover to the reinforcement
         Use of mineral admixtures
         Crack width control

Alternative reinforcement
         Fusion bonded epoxy coating
         Galvanized reinforcement
         Stainless steel reinforcement

Fusion bonded epoxy coating
         It is basically epoxy coating applied on to the reinforcement.
         Applied in plants and not  on site.
         Coating thickness 130 to 300 micron.
         Looks greenish in colour
         First bars are sand blasted and then bars are heated up to controlled temperature and then coating is applied.


Limitations

         After coating if u do cutting and bending some time it may cause damage to the reinforcement coating.
         The coating is not possible on sites.
         Any defect in reinforcement coating  may cause localized corrosion of the bars.
         Epoxy coating will not provide resistant to UV rays and hence bars should not be kept exposed in sun for longer time.
         The bars can not be welded.
         The coating may get damaged during the vibration of concrete.
         The epoxy coating is costly as costly as reinforcement.


Galvanized reinforcement
         Hot dip galvanized coating bars. Used only for aggressive exposure conditions, precast construction.
         It produced by dipping the steel bars in molten zinc.
         When zinc corrodes it will form Hydrated zinc oxide which will act as a insulator which will further prevent corrosion.
         Better bond strength
         Steel accessories which are used in RCC like fittings and inserts which are partially exposed should be galvanized.
         This coating will ensure design strength of steel, avoid spalling of concrete and corrosion of reinforcement.
         IS 12594-1988


Stainless steel reinforcement
         Stainless steel is highly corrosion resistant steel which contain almost 12 % of chromium. This chromium creates and invisible coating which will avoid corrosion of the reinforcement bars.
         It is available in four types
         Ferritic- Low carbon steel with  less than 17% of chromium
         Ferritic austenitic- Low carbon steel with  less than 22 to 28 % of chromium and 4 to 8 % of nickel.
         Austenitic-Low carbon steel with  less than 17% of chromium and 8 % of nickel
         Martensitic.carbon content high as 1.2 % and chromium content of about 12 to 18 %
         Very good corrosion resistance property because of presence of chromium.
         Good strength and ductability and weldability.
         The main disadvantage is that they are quite costly as compared to normal reinforcements.


Coatings on concrete
         Polymer concrete overlays
         Latex modified concrete overlays
         Silica fume concrete
         Waterproof membrane or coating


Coatings

Polymer concrete coatings

         This is a coating of a material where Portland cement is replaced by polymers.
         It avoid the ingress of water and chloride ions and avoid the corrosion.
          It is expensive and difficult to prepare and hence mainly used as a repair material.
         Coating thickness is almost of about 12 mm.
          
Latex modified polymer coatings

         This is basically mixture of normal portland cement concrete of very low water cement ratio in which polymer latex emulsion are added in that.
         Water present in the emulsion hydrates the cement and polymer increase the bonding capacity of cement paste.
         This coating is generally applied in thickness ranging from 40 to 55 mm.


Silica fume concrete

         It is basically normal portland cement concrete in which silica fume is added.
         The silica fume is 10% by weight of cement.
         Plasticizers are required to mix in this concrete.
         This concrete is having low water content and hence very less permeability. Because of which it avoid the ingress of water and chloride ions in the concrete.
         The main disadvantage of this concrete is that there are lots of plastic shrinkage cracks in this concrete. Hence special precautions are required to avoid such plastic shrinkage cracks in this concrete.
          
Water proof membrane

         These are basically water proof coatings applied on to the concrete.
         These chemicals are made from combination of four additives like binder, inert filler, liquid solvents and admixtures.
         The performance of such coating mainly depend upon the type and selection of material, exposure conditions, working methods etc.


Corrosion inhibitors

         They are of three types
                                                             i.      Anodic inhibitors- Chromates, nitrites, alkali phosphates, silicates and carbonates.
                                                           ii.      Cathodic inhibitors- Zinc, magnesium, manganese and nickel
                                                        iii.      Organic inhibitors- Amines, esters, Sulphonated blocks.
         Among all most widely used is calcium nitrite.
         It is added in the concrete as admixture.
         Dosage 10 to 30 litres per m3 of concrete


Chemical or Electro chemical method
         Cathodic protection
         Realkalisation


Cathodic protection

         Two types of anode are used. One is sacrificial anode or impressed current anode.
         Sacrificial anode systems consist of
                                                        iv.      Strip or mesh type sacrificial anode made up of zinc, aluminium or magnesium alloy.
                                                           v.      Ion conductive backfill (hydrogel and polymer system)
                                                        vi.      Shielded electrical leads to be taken from anodes embedded steel reinforcement  sacrificial anode.
         Mostly used in case of under ground structures and underwater structures.
         Chloride ions move away from steel reinforcement as they are drawn towards the externally placed anode.


Realkalisation

         Realkalisation is electrochemical process in which used to restore the alkalinity of concrete which will prevent corrosion of reinforcement.
         Realkalisation of concrete maintains the ph of concrete almost up to 12 initially.
         Realkalisation is performed by applying an electric current between the reinforcement and anode mesh placed temporarily in an electrolytic solution on the concrete surface.
         This electrolyte solution which is mainly sodium carbonate is transported into the carbonated concrete and maintain the alkalinity of concrete



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