Rust / Corrosion of Reinforcement Bars – A challenge to overcome

Mr.Sourav Dutta
Manager (Civil)
Tata Steel Ltd

Rusting/Corrosion are mainly caused due to:
1. Prolonged exposure of rebar to atmosphere/rainfall without any precaution.
2. Aggressive/saline/toxic environment.

RUST is caused by the reaction of air (O2) and moisture (H2O), and it is a common and natural phenomenon for most steel products. IT SHALL NEVER BE CAUSE OF CONCERN/REJECTION OF REBAR, AS PER BIS.

Fig  Guideline of BIS1786-2008
Fig : Guideline of BIS1786-2008

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Problems of using High Grade Steel in Constructions

Presented by:
1) Prof.M.V.Kuriakose B.Tech(Ed), M.I.E. (Retd.Principal, Govt.Polytechnic, Perinthalmanna)
2) Mr.Jismon Issac B.E (Mech) A.I.E, MBA

[Prof.M.V .Kuriakose is having around 35 years of expertise in Structural Engineering and Designing. He is an active member of Lensfed.]

[Mr.Jismon Issac has 13 years of experience in the manufacturing & Quality testing of Construction Steel bars especially High grade TMT steel bars]

Now days some contractors report a tendency among few design Engineers that they specify grade Fe-500 or higher steel in residential buildings, citing its high strength. If they design buildings considering the use of Fe 500 steel in construction, it could help reduce the volume of steel used and reduce the column size.

The rosy part aside, Fe 500 grade steel could pose quite a few site specific issues during construction, especially for small builders. Considering the reported failures and problems with grade Fe-500 or higher, it is advisable to use Fe 415 in residential and commercial buildings and Fe500 could be used only when the entire design is made according to that grade. This is explained below;

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Should design life be the same as return period for design conditions?

Design life means the minimum duration a structure is expected to last. The longer is the design life; the higher is the cost of a project. Therefore, in choosing the design life for a structure, engineers should consider the design life which generates a economical project without sacrificing the required function.

In selection of return period of certain design conditions, winds, waves, etc., one should consider the consequences of exceedance. In fact, there are normally no extreme maximum values of these design conditions and its selection is based on the probability of exceedance which is related to return period.

Therefore, design life may not be equal to return period of design conditions because their selections are based on different considerations.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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What is the difference between working stress approach and limit state approach?

For working stress approach, service loads are used in the whole design and the strength of material is not utilized in the full extent. In this method of design, stresses acting on structural members are calculated based on elastic method and they are designed not to exceed certain allowable values. In fact, the whole structure during the lifespan may only experience loading stresses far below the ultimate state and that is the reason why this method is called working stress approach. Under such scenario, the most economical design can hardly be obtained by using working stress approach which is now commonly used in the design of temporary works.

For limit state approach, for each material and load, a partial safety factor is assigned individually depending on the material properties and load properties. Therefore, each element of load and material properties is accurately assessed resulting in a more refined and accurate analysis of the structure. In this connection, the material strength can be utilized to its maximum value during its lifespan and loads can be assessed with reasonable probability of occurrence. Limit state approach is commonly used for the majority of reinforced concrete design because it ensures the utilization of material strength with the lowest construction cost input.

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A Comparative Study of Retrofitting Of R.C. Building Using Steel Bracing And Infill Walls

Prof. Pravin B. Waghmare
Acharya Shrimannarayan Polytechnic Pipri (M)- Wardha-Maharashtra

The objective of this study is to identify an efficient retrofitting method for existing open ground story reinforced concrete frame buildings. Failure of several soft-stored buildings in the past earthquakes underscores the need to retrofit existing soft-story buildings. A common cause for the collapse of multi-storied buildings is the occurrence of soft story in the ground floor due to the presence of infill walls in the upper story. During the Bhuj (Gujarat) earthquake of 6thJanuary 2001 several soft storied building failed there by confirming the vulnerability of such buildings to earthquake loading. This underscores the need to retrofit existing soft story buildings to prevent their total collapse. The existing building structures, which were designed and constructed according to early codal provisions, do not satisfy requirements of current seismic code and design practices. A two dimensional R.C. frame designed with linear elastic dynamic analysis using response spectrum method. The computer software package STAAD Pro–2005 is used for dynamics analysis technique is used to assess the performance of a (G + 4) reinforced concrete buildings, of which the ground storey is a parking facility the ground storey is 3.5m high while the upper stories giving a total height of 15.5 m. the building is located in Seismic Zone IV.

The RC frame is retrofitted by three methods namely,
1) Brick masonry infill in the ground story.

2) Steel braces in the ground story.

3) R.C. Structural wall in the ground story.

The study concludes that the building designed as per provisions of IS: 456:2000 using limit state method of design, and analyzed as per existing seismic code IS: 1893-2000 of all these three methods studied the use of structural wall in the ground story panel gave the maximum strength and ductility.

Keywords: Open ground storey, brick infill, RC wall Infill and Steel Bracing.
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