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Monday, 8 June 2015

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