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Repair & StrengtheningCase Studies

Abstract: Concrete being the most important and widely used construction material requires good construction practices and adequate protection to survive its intended structural design life. When asked about the service life of structures, the general con-sensus among the structural designers is they require a healthy life of more than 50 years.In reality however this life remains elusive. The potentially aggressive environments and unplanned events like fire to structures has resulted in premature damages to structures. In many cases, when related to corrosion distress, first sign of repairs are noticed within a span of 5 yrs. Reason for such damages can be many but the most common for damages to old structures are related to corrosion while for new struc-tures, poor construction quality leading to reduced concrete strength and load changes are very common.

This short article addresses the repair and strengthening of two reinforced concrete structures. The first case deals with the inspection, assessment and strengthening of newly constructed Vehicular underpasses at Rajahmundry, Andhra Pradesh. The second case deals with the mega repair and rehabilitation work of Induced draft cooling towers carried out for IL& FS thermal power company limited, Cuddalore, Tamil Nadu.

Introduction

Repair and rehabilitation for concrete structures can be clas-sified broadly into two categories: a) repair in which damages due to deterioration and cracking is corrected to restore the original structural shape and size while the core is mostly intact and b) repair which is necessary to strengthen the load carrying capac-ity of members which have become structurally deficient over a period of time. This is most often called as “strengthening”.

Case I – strengthening of vehicular underpass a2-a3 at ch. 4.98

The two vehicular underpasses are located close to the Ra-jahmundry airport in the state of Andhra Pradesh and falls under PWD. The structures consisted of RCC single deck slab of span around 20m.

Distress features: Just before the project was to be handed over and traffic started, a trial run showed development of cracks and deflection at the center portion of the bridge deck.

A closer examination by the structural consultant revealed

the need for strengthening of the deck to enhance the flexural ca-pacity and stiffness by using high modulus carbon fiber plates at the bottom of the deck slab. Sealing of cracks using low viscous ep-oxy was carried out prior to taking up strengthening application.

Zaheer Abbas Product Manager – Repairs and GroutsBASF India Ltd. Construction Chemicals Division

Strengthening scheme using high modulus carbon fiber plates

The bottom 6 meters of the slab, 3m on either side of the center line was identified as the span the required strengthening depending upon the moment profile. Additional laminates of car-bon fiber were provided as per the below drawing and anchored using steel plates bolted into the concrete substrate. The carbon laminates were inserted into grooves cut into the concrete using precision cutting and cleaning methods.

Center Line30003000

Bottom View: Deck Slab bottom Strengthening Scheme

Groove Cutting

Functional performance of bridge after strengthening

The bridge was load tested and monitored for deflection and cracking for pre and post strengthening before finally allowing the vehicular and was found to regain the deflection with no cracking. All the vehicular underpasses are fully functional now!

CONCRETE: REPAIR & REHAB

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CASE II – Repair & rehabilitation of fire effected induced draft cooling towers

The structure is an RCC Induced draft cooling tower consist-ing of fan stack supported on RCC columns and interconnected RCC beams at two levels providing vertical and lateral stability to the structure. At the base is the collecting water basin.

Distress features: A fire accident occurred at the IDCT Struc-ture (Cooling Tower) at ITPCL (Thermal Power Plant), Cuddalore District Tamil Nadu.

The accident happened during handing over of the project by contractor to client IL&FS, but due to the severe intensity of

Laminate Fixing

Final Finish

Unique characteristics of applied Mortar/micro-concrete- Dual shrinkage comp.

the fire the integrity of the structure was a question for client and PMC. For this reason IIT-Madras was roped in as an external consultant and was assigned the job of designing a scheme for the repair and rehabilitation of IDCT (Induced Draft Cooling Tow-er) structure. The job of IIT-Madras was very critical. They had to devise methodology for repair work using right product for differ-ent structural element and arrive at the BOQ.

Repair & protection scheme

Considering the vast damage incurred to the structured on account of a major fire, different areas of the IDCTs required different category of mortars depending upon the thickness of damage and the retrofitting needs.

Use of dual shrinkage compensated hand applied repair mortars: Thinking beyond compressive strength parameter, dual shrinkage compensation and quick bonding characteristic was given importance for selection of hand applied repair mortar.

CONCRETE: REPAIR & REHAB

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Unique kidney shaped PAN fibers promotes chemical adhesion be-tween cement paste and fiber

Non Drying Shrinkage crack: Mortar stability tested as per Coutinho Ring Test for Drying Shrinkage as compared to cracking in normal mortar under same test

Drying Shrinkage cracking in normal mortar on testing as per Coutin-ho Ring test

ing of damaged concrete thicknesses in concrete repairs and rehabilitation works. This typically involves spraying concrete or mortar through a spraying gun (nozzle) at high velocity on the surface.

One of the biggest disadvantage of such a system when used for repairs is the sagging of mortar due to poor grab immediately on application leading to premature delaminations which are of-ten noticed later.

For this reason a pre-bagged instead of site batched, high build, non-sag, quick setting mortar was used to repair damages.

Protection to concrete

IDCTs operates in an environments of high exposure to moisture. Hence, protection of concrete is vital to minimize cor-rosion damages to embedded steel in future. Solvented acrylic coatings was used for external protection to concrete elements by spray application.

Use of high build, quick setting pre-bagged dry shotcrete

Timely completion of job required use of material that can be applied fast without sagging even at higher thicknesses. This should also reduce time required for shuttering and deshuttering

Unique characteristics of applied dry shotcrete

Shotcreting is a technology that has proven to be of immense benefit in diverse subdomains of construction including rebuild-

Conclusion

There are number of systems available for providing repairs, protection and strengthening to concrete under different types of damages. However, selection of a system for effective perfor-mance is very important for the designers and durability of the repair scheme. The system shall be looked upon as a whole rath-er than individually considering the material properties. Also the performance characteristics of the material are very important for proper selection. For example, for repair mortars are still se-lected based upon compressive strength rather than shrinkage characteristics, it is therefore imperative for the engineer to spec-ify atleast the key performance parameters for a given system.

CONCRETE: REPAIR & REHAB