STRUCTURAL DISTRESS IN EXISTING PLANT AND STUDY FOR REMEDIAL ACTION

By: Suparna Ghosh, Suman Sen, Manos De

Introduction

Plant structures are designed and constructed to resist loads envisaged to be carried based on its function and local environmental conditions. Additional requirement for the desired durability of the various structural elements are determined based on the operating conditions like exposure to high temperatures, steam, chemical fumes, fatigue effects from moving equipment and other operational situations. The former requirement is adequately catered for by designing the structure for the applicable loading conditions and adopting the various design specifications given in governing codes of practice. The latter requirement is ensured through good detailing, adopting best engineering practice and ensuring compliance to quality control procedures set out for the project. When the adoption of good practices gets neglected during execution due to various reasons, the structural elements start to deteriorate in appearance and functionality much before the design life manifesting various types of distress like cracks, spalling, rusting and flaking. Such distressed elements need to be remedied to increase working life and maintain the plant operation. Such studies are carried out by specialists with deep knowledge of structural behaviour, understanding of the basic principles of good detailing and engineering practice, causes of distress in members, repair techniques and chemicals along with their suitability of application and limitations for use. TCE has a wealth of knowledge and experience in carrying out such studies and suggesting suitable remedial measures for strengthening of the structural members and extending the working life.

Scope of Health Study of Structures in Coke Plant

In the assignment described below, TCE was involved in health study of existing structures in a coke making plant. The various structures covered in the study included Coal Tower, Wet Quenching Tower, Boiler inlet Damper Building, Coke Wharf, Conveyor Tunnels, Quenching Pond, Blending Bunker building, Quenching and Pusher Track foundations, Battery Front and End benches, Wagon Tippler, Crusher House, Pump Houses, Electrical Buildings and other plant auxiliary buildings. In this plant, the various structures in process area are subjected to exposure conditions like high temperature from hot coke, steam from the quenching operation of the hot coke, exposure to acidic and phenolic compounds present in the hot water discharged during quenching process, moving equipment like track mounted carriages, cranes and monorails.

Field Investigation work for Condition Assessment

The site investigation scheme comprised visual inspection by design engineers to identify distressed structures. This was followed by detailed measurements through survey agency and non-destructive testing methods like rebound hammer, ultrasonic pulse velocity, crack width measurements, core test and half-cell potential test. The principle causes for such distress were found to be poor construction quality, improper cover, inadequate anchorage to reinforcement at junction, and exposure to harsh operating environment. Appropriate repair methodologies were selected based on nature and severity of damage, analysis of root cause of damage, criticality of the member with respect to performance of the complete structure and durability requirements for the service life.

Analysis of Investigation results and Engineering Solutions

Some of the major types of deterioration observed and the proposed remedial measures are highlighted in below table:

Sl. No. Type of deterioration Remedial measure
Shear crack in monorail supporting RCC beams in Settling Pond After preparation of concrete sub-surface, carbon fibre wraps pre-soaked in epoxy has been applied after application of epoxy primer.
Spalling of concrete with exposed corroded reinforcement on walls of Quenching Towers After cleaning of the concrete surface, old corroded reinforcement has been replaced with zinc coated reinforcement bars, epoxy bonding agent applied on concrete and finished with polymer modified repair mortar.
Encasing of structural supports for cantilever walkways Fixing of dowels in concrete, install zinc coated reinforcement, apply epoxy based concrete bonding agent and free flowing concrete has been pumped after erection of water tight shuttering.
Repair of major cracks and honeycombs in concrete Holes for inserting nozzles made of 10 mm dia PVC pipes are made along the crack line. After fixing the nozzles with an adhesive, injection grouting through nozzles.
Water seepage on basement floor of Wagon Tippler Polyurethane resin injection injected through nozzles at specified locations followed by injection foam grouting inside the voids. For permanent sealing, elastic polyurethane injection resin has been grouted through these nozzles.

 

These solutions were provided in the form of detail drawings showing photographs of the distressed structure and the scheme for remedial action with detail specification of proposed material to be used and sequence of activities for the rectification work for proper guidance to execution. Some photographs depicting condition of distressed structures and its appearance after rectification work are shown below.

Photographs

Quenching Tower Wall

Wall repair with nozzle grout

Wall after repair

Quenching Tower beam

Micro-concreting of beam

Thermobond coating

Boiler Damper column

Carbon Fibre Wrapping

Plaster over wrapping

 

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