CORROSION PROTECTION OF STEEL REINFORCEMENT IN RB AND RCC CONSTRUCTION

Scope

1.1. This standard (revised) specifies the recommended practice for surface preparation, surface pretreatment and anti-corrosive treatment based on inhibited and sealed cement slurry as an in-situ process for corrosion protection of mild steel reinforcement/HYSD bars in conventional reinforced concrete structures and conventional .reinforced brickwork constructions.  This practice is also applicable for non-prestressing steels (mild steel reinforcement/HYSD bars) used in prestressed concrete structure.

1.2. This standard applies only for in-situ corrosion protection of miId steel reinforcement/ HYSD bars after all bending and shaping operations are completed. However, this does not preclude the possibility of using this practice as a factory process.

 

2. Anticorrosive treatment process sequences - The anticorrosive treatment should necessarily include the following sequential steps

(a) Surface preparation (de-rusting) - Since presence of oil, grease, dirt, heavy scale and rust will adversely affect the performance of any anticorrosive treatment, it is essential to adopt suitable surface preparation technique. Surface preparation can be either by acid pickling or by sand blasting.

(b) Surface pretreatment - Surface preparation should be immediately followed by a surface treatment step to ensure temporary protection during the time lag between the de-rusting and finish coating. This pretreatment should not adversely affect either the adhesion of the finish coal or corrosion performance. (c) Inhibited cement slurry coating - A minimum of 2 coats should be applied to ensure full coverage.  (d) Sealing treatment - Sealing treatment should make the coating harder and less permeable. A typical flow diagram is shown in Fig.1,

 

3. Procedure for surface preparation

3.1. Sand blasting - Sand blasting of the steel surface to SAE 21 /2 if standards cm be done

3.2. De-rusting by picking - The pickling weld should be preferably based on hydrochloric acid and should include an efficient inhibitor to prevent base metal attack.

A typical de-rusting solution should have the following characteristics

(a)   Inhibitor efficiency should not be less than 97 per cent (b) Weight loss of a polished standard mild steel reinforcement / HYSD bars specimen when dipped in the solution for 10 minutes should not be more than 2 g per sq. metre. (c) Specific gravity when tested using a standard specific gravity bottle should be around

(b)    

4. Procedure for surface pre-treatment - De-rusted should be immediately convened by using phosphating treatment. It should be ensured by a suitable rinsing process that no residual acid is remaining on the surface at the time of phosphating. A typical phosphating composition of brushable consistency should have the following characteristics:

(a) Coating weight when tested as per test procedure A given hereunder should be around 4.5 gms./Sq.m.  (b)   Density of the product should be in the range of 1.22 to 1.4 kg/litre. (c)   Presence of fungicide shall be tested by dissolving the jelly in deionized water; the resulting solution shall be yellow in colour. (d)  Presence of phosphating chemical in jelly shall be ensured with ammonia molybdate test.  (e) pH of the composition when tested in a standard specific gravity bottle should be

around 2.5 ±0.1  (f)  Nail scratch tea should clearly leave a mark on the specimen. This indicates ihe

existence of the coating.

 

5. Procedure for inhibited cement slurry coating

5.1. A typical inhibitor admixture used should have the following characteristics:

(i) It should in liquid form ready for mixing with ihe ordinary Portland cement. (ii) Specific gravity when tested using a standard specific gravity bottle should be  1.04 ±0.02  (iii)  pH when tested using pH meter should be 12.75  ±0.25  (iv)  Tolerable limit for chloride in inhibitor- admixture when tested using anodic polarization technique (as per lest procedure B) should be 300 ± 25 ppm

5.2. Ordinary Portland cement - This should conform to IS 269 and should be sieved to pass through 75 microns IS sieve.

5.3. Cement and inhibitor admixture should be mixed in specified proportion to have suitable consistency. Coating should be applied preferably by brushings. However, under specific circumstances spraying dipping is also allowable.

5.4. Sufficient time tag should be allowed in between successive coatings to ensure final setting of the undercoat. A minimum of 6 to 12 hours may be necessary.

5.5. A minimum of two coats with sufficient time lag in between should be applied. 

 

6. Sealing treatment - The sealing treatment should be performed over the coated surface immediately after final selling of the top coat. Sealing treatment can be applied by brushing, spraying or dipping.

6.1. A typical sealing solution should have the following characteristics

(a)   Specific gravity when tested using a standard specific gravity bottle should be 1.09 ±0.02

(b)  pH when measured using a pH meter should be -12.25  ±0.25

(c)  Tolerable limit for chloride in sealing solution using anodic polarization technique should be 450 ± 25 ppm (as per test procedure B).

 

7. SPECIFICATIONS FOR FINISHED END PRODUCT

7.1. Finished coating when visually examined should be fairly uniform in thickness and should be devoid of any defects such as cracks, pinholes, peeling, bulging etc. No surface area should be left uncoated. No rust spot should be visible to the naked eye.

7.2. Thickness of the coating - The minimum thickness of the coating shall be 200 microns. Preferable range is 200 to 400 Microns.

7.3. Bond strength of the coated rebar - The bond strength of the coated rebar and concrete shall not be less than that specified in IS:456-2000 as per clause No. 0,2,5, II and 44.1.2 and tested as per IS;2770(Part-1).

7.4. Hardness of the coating when measured using a pencil hardness tester shall be around 5H to7H.

7.5. Tolerable limit for chloride in 0.04 normal NaOH medium using anodic polarisation technique (as per test procedure B) shall be around 4500 to 5000 ppm.

7.6. No film failure as evidenced by evolution of hydrogen gas at the cathode or appearance of corrosion products at the anode shall lake place during one hour of testing (as per test procedure C).

 

8. General remarks - It is advisable that severely rusted and heavily pitied reinforcements are not accepted for treatment.

 

Test procedure A

Determination of phosphate coating weight.

 7.5 cm x 2.5 cm or 7.5 cm x 5 cm mild steel polished and degreased specimens are to be used for this test. First the blank loss of unphosphated specimen is to be found out. For this, the initial weight (W₁) is accurately weighed. The specimen is kept immersed in the Clark's solution or patented inhibited derusting solution for I minute, The specimen is removed, rinsed in distilled water and dried using hot air blower. The specimen is immediately weighed (W₃). The difference between W, and w, is termed as blank loss. Another specimen (polished and degreased) is brushed with phosphating jelly and kept for 45 minutes.  Then the specimen is washed free of jelly, rinsed in clean water and dried using hot air blower. The phosphated specimen (W₁) is accurately weighed. After weighing, the specimen is kept immersed in Clark's solution or patented inhibited de-rusting solution for one minute. Then the specimen is removed, rinsed in distilled water and dried using hot air blower. The specimen is immediately weighed (W₄). Coating weight = W₃ - W₄ - blank loss.

Fig 1000/1 Flow diagram for anti-corrosive treatment of reinforcement bars

Test procedure B

Anodic polarisation technique.

Mild steel reinforcement / HYSD Bar lest specimens of size 10 mm in dia and 100 mm in length with stems of size 5 mm in dia and 50 mm in length is polished, degreased and sealed at bottom edge and at the stem with suitable sealers like wax, lacquer.  Then test specimen ii kept immersed in test solution and potential is monitored using high impedance multimeter against suitable reference electrode such as saturated calomel electrode/ copper-copper sulphate electrode. After getting stabilised potential using appropriate current regulator (0-100 mA), the lest specimen is anodically polarised at a constant current density of 290 mA cm using a platinum/ stainless steel/TSIA/ polished mild steel reinforcement/HYSD Bar as cathode. Potential with timee is followed for 5 minutes after current is applied. The maximum chloride concentration upto, which the potential remains constant for 5 minutes, is taken as a measure of tolerable limit.                                                                

Test procedure C

Resistance to applied voltage test                                     

Two mild steel reinforcement / HYSD bars of size not less than 10 mm in dia and 800 mm in length shall be given anti-corrosive treatment as per specified procedure.  The end of the rebar's shall be soldered with insulated copper electrical connecting wire (14 gauges) to serve as electrical contact point.  Coated rebus at the two ends shall be sealed with an insulating material to a length of 25 mm at each end. Test area shall be the area between the edge of the bottom sealed end and immersion line which shall not be less than 25 mm in this case.

The coated rebar's shall be suspended vertically in a non-conductive plastic container of size not less than 150 mm X 150 mm square and S50 mm high.  The rebar shall be so suspended as to have a clearance of 25 mm at bottom, 45 mm at the sides and 40 mm in between the rods.  Container shall then be filled to a height of 800 mm with aqueous solution of 7 percent NaCI. A potential of 2 V in between the coated rods shall be impressed for a period of 60 minutes using a high resistance volt meter for direct current having an internal resistance of not less than 10 mega Ohms and having a range upto 5 V (minimum).  Storage batteries may be used for impressing the voltage. During these 60 minutes of testing, there shall not be any coating failure as evidenced by evolution of hydrogen gas at the cathode or by appearance of corrosion products of iron at the anode

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

Testing of steel

•         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