Durable concrete structures

Durable concrete structures

7:05 AM, 19th May 2018
Durable concrete structures
Professor A S Khanna is from the Department of Metallurgical Engineering & Materials Science, Indian Institute of Technology (IIT) Bombay.

By Professor A S Khanna

Corrosion of reinforced concrete structure with steel rebars is a matter of great concern. In RCC structure, reinforced steel bars undergo distress due to embedded steel bars. Though, initially, when steel bars are reinforced, steel does not corrode as the concrete aggregates which consists of oxides of calcium, has alkaline condition (pH between 10-12.5). In this pH range, the steel forms a passive layer and hence does not corrode.

However, as the time passes moisture (from rain or high humid conditions) and pollutants such as chlorides (from coastal environment), carbon dioxide, sulphur dioxide (urban environment), penetrate through the concrete cover, reach concrete/steel interface, reduce pH below 10, it leads to the initiation of corrosion process.

Corrosion results in the formation of voluminous corrosion products (hydro-oxides, carbonates or sulphates) leading to stress generation and thereby causing distress, initially in the form of a pin holes, followed by small cracks and finally to the spallation of plaster, exposing the rebars.

What is then the strategy of making distress free RCC structure which can give long durable life?

The schematic of a reinforced concrete structure can be shown as in Figure 1, below:

                                                                 Fig. 1 Schematic of the RCC structure

Thus, for enhancing the durability of a concrete structure we need to design a concrete structure with following properties:

1. Create a suitable barrier protection at concrete cover so that the pollutants from environment cannot enter the concrete cover.

2. Delay the diffusion or transportation of moisture and pollutants through concrete cover.

3. Create a very strong natural obstruction on steel surface so that the pollutants are not able to react with steel and form voluminous corrosion products.

Step 1

As far as the first step is concerned it is simplest and requires a corrosion resistant coating on the concrete surface which can resist the aggressive environment (moisture, chlorides, carbon dioxide or sulphur dioxide) to stay on the concrete cover or even allow it to penetrate through the concrete cover. These are usually modified epoxy coatings, for example with MIO, with a top coat of acrylic polyurethane coatings of the choice of any desired colour.

Step 2 requires two things:

(a) Best concrete cover designing such as a concrete of density M60 or more. Higher density helps in two ways:

(i) it helps in delaying the diffusion of pollutants and

(ii) a dense concrete will take a much higher stresses from the high volumes of corrosion products formed at concrete-steel interface and hence delay in initiation of initial pin holes, followed by micro-crack formation, leading to main crack and spallation.

(b) Further modification of concrete cover can be done by

(i) taking a mixed size of aggregates;

(ii) adding concrete straighteners such as silica fumes and

(iii) the most important, adding inhibitors to the concrete admixture.

The latter helps in delaying the diffusion of moisture and pollutants by acting as obstacles and thus delaying the process as discussed in (a).

However, the main factor responsible for increasing the durability of concrete structure is the activity at point “B”.

There are several technologies which Reinforced concrete structures are made and the most common ones are :

1. Use of TMT or CRS bars.

2. Use of Coatings, especially Fusion Bond Epoxy coating which is well known today for many important structures such as Bandra Sea-link, the most latest one and many flyovers in coastal towns in India.

3. Stainless steel Rebars

TMT bars are being wrongly projected as corrosion protection technology, which actually is not. The thermo-mechanical treatment only gives a 5-10 micron of martensitic hard layer which has no direct relation to corrosion.

Use of CRS - Corrosion-resistant steels is half backed technology which is again being wrongly imposed as corrosion resistant steel by just adding 1.5-2.4 wt.% of some elements such as Cr, Ni, Cu and Mo. Experience has shown that such modification enhances life just by one to one and a half times, and therefore not suitable to provide long durable structure lasting more than even 30 years or more. This partial modifications initially shows lower corrosion rate but in long run, it is harmful as shown in a study (Fig. 2).

Fig. 2 shows how an inadequate concentration of alloying elements helps initially to lower corrosion but enhances corrosion at a later date (lower curve) and where the concrete is already having some chlorides, there is no effect at all (upper curve)

Fusion Bond Epoxy provides, perhaps one of the best coated rebar with all excellent properties required for a concrete structure but it cannot provide long durable life of even 50 years or more, as coatings have usually design life of 30 years. More over handling of FBE coated bars is a big challenge. Thus in order FBE coated bar be used for RCC structures, it must be assured a complete checking of damage to FBE coating before pouring concrete.

Use of Zinc based galvanised rebars can be other choice. This is a technology very well practised in USA, Australia and has given distress free structure for more than 50m years. The main advantages of Galvanized rebars is that it gives a coating which in addition to barrier properties also provides cathodic protection to steel bars. Handling is not a problem and bond strength is excellent.

The biggest challenge today in the country to use Galvanized rebars is its availability and vendors who can coat long rebars in large quantities. Today galvanized rebars are almost matching the cost of fusion bond epoxy coated bars.

Stainless steel is perhaps, the most acceptable corrosion resistance material which remains un-attacked even in a highly polluted environment. This mainly protects the RCC structure at point “B” as shown in Figure 1 above, due to the formation of a strong chromium based passive layer.

The main requirement of stainless steel to be used for rebars in RCC structures is, therefore, a suitable composition which can form a strong passive layer. This can happen by adding about 11.5 percent of chromium in steel as usually seen by several ferritic stainless steels ( 400 series Stainless steels). In the same way, there are austenitic and Duplex stainless steels which are formed by a combination of Cr & Ni and Cr, Ni & Mo.

The passive layer in latter cases is even more stronger and hence these stainless steels are very costly. Thus a cost-effective solution for stainless steel rebar is a ferritic stainless steel with a minimum 11.5 percent of Cr and which passes Fe500D specifications that is it must have a yield strength of above 500.

A detailed study, carried out in our lab on a ferritic steel with 11.5 percent of Cr, shows that the passive layer on the stainless steel remains stable even after exposing to 3 percent NaCl solution Fig. 3 [1].

Fig. 3 Corrosion of Ferritic Steel ( 11.5 percent Cr) in various concentrations of Chlorides. It is confirming that that even with 3 percent of Chlorides which is 3000 ppm the 11.5 percent Cr SS is still showing passivity, confirming its long life [1].

Such a high levels of chlorides, reaching at concrete- steel interface may take very long time, close to hundred years if a design strategy as shown above in Fig. 1 is strictly followed with a confirmation of following best concreting practices. Thus as per the latest Indian Standard IS 16651:2017 for stainless steels, following stainless steels are listed as given in table 1:

Grade

C

Mn

Si

Cr

Ni

P

S

Other Elements

405

0.08

1.0

1.0

11.5-14.5

-

0.04

0.03

0.1-.3 Al

410L

0.03

1.0

1.0

11.5-13.5

0.6

0.030

0.040

 

429

0.12

1.0

1.0

14.0-16.0

-

0.04

0.03

 

430

0.12

1.0

1.0

16.0-18.0

-

0.04

0.03

 

446

0.20

1.5

1.0

23.0-27.0

-

0.04

0.03

0.25 N

Table 1 List of the recommended stainless for rebar as per Indian Standard IS 16651:2017

Thus AISI type 410L is a good choice based upon the chemical composition and cost effective ness. A detailed analysis on cost-effectiveness is given in the enclosed paper.

Conclusions

There is large choice for making durable concrete structures with proper choice and selection of reinforcement steel. In a polluted environment in India especially in C3 and C4 environment, use of TMT and CRS is very risky. With the kind of concreting practices and post maintenance strategies, TMT and CRS bars cannot give a distress free life of more than 10-15 years and after which there is problem of repair and maintenance. Hence alternative protected materials such as Fusin Bond Epoxy coated and Hot Dip Galvanized bars would be of better choice. FBEC coated technology is a great technology provided one can take care of handling problems and quality inspect bars before pouring concrete.

Using modifications as suggested in Fig. 1 above, still one cannot use FBEC for a durability of more than 50 years. HDG rebars is a still better choice with almost same cost with a durability of 60-100 years. However, for a distress free, and maintenance free structure of more than 100 years, the cost effective solution is Ferritic Stainless steel reabrs. Also for a structural life of more than 150-300 years in C4 and C5 environment, higher grade stainless steels such as Duplex stainless steels are the best solution.

Author: Professor A S Khanna is from the Department of Metallurgical Engineering & Materials Science, Indian Institute of Technology (IIT) Bombay.

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Reference

  1. A.S.Khanna, Stainless Steel- an ultimate choice for reinforced bar in concrete structure, Proc. American Concrete Soc., Bombay, 2003.

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