Abstract
Concrete structures need rehabilitation or strengthening for various reasons with its age. A systematic approach adopted by TCE to assess the health of the structures in the existing fertilizer plant has helped to determine the probabilistic structural performance of the various elements of the buildings for the intended purpose along-with proper maintenance. Such approach enhances the life of the structure or part thereof with economical retrofitting of the structure
Introduction
One of the India’s leading phosphate-based fertilizer plant manufactures complex fertilizer grades. The plant is located on a coastal belt and has raw material unloading system, storing facility, manufacturing plants, conveying systems, bagging & packing plants, product storage / godowns, utilities, infrastructure facilities etc. The plant was in operations for more than three decades. The environment around the plant is highly corrosive due to the nature of raw materials, finished products/by-products and the saline atmosphere in the coastal area.
TCE was appointed by the customer for assessing the structural health of the various buildings / structures of their above-mentioned plant.
TCE’s scope of services was visual inspection, technical survey, ND Test as applicable, to record observations and submit detailed report based on observations. The scope also included to conduct Non-Destructive Testing (NDT), if need arises and include the results, regarding the overall health of the structures and suggest the remedial measures to minimize the effect of existing distress and prevent further distress / damages.
Defining a purpose of Structural Health Study
As a first step towards this study, TCE deployed a team of experienced persons to have face to face discussion with the concerned persons in the plant to understand overall plant operations, and to know the reasons for structural health study.
It is necessary to define the purpose of structural health assessment and strengthening of the existing structure. Unless this is known, it would be very difficult to adopt a focused approach towards rehabilitation work.
Site Visit and Visual Inspection Survey
TCE deployed a team of qualified and experienced engineers to site for visual inspection and survey of the entire plant. They worked in groups and covered the visual inspection of entire plant in about a month’s time
All the accessible areas are visually inspected. The elevated parts of structures such as roof trusses, are inspected using binocular and evidences are recorded by optical/digital cameras.
In some of the stores / godowns, staging is provided by customer for inspection at higher elevations. Sample checks of purlins & trusses are carried out.
TCE collected available drawings from customer, discussed with plant in-charge, about structural modification & repairs carried out by them and problems faced in past.
The data collected during the visual inspection at site was collated along with the photographs taken to substantiate the findings. The type of distresses was listed and are as given below.
During the investigations, the need was felt for carrying out further detailed investigations such as Non-destructive tests to assess residual strength of concrete, extent of deterioration and to collect other useful data.
Finally, detailed inspection report giving findings along-with photographs, any other evidences of repairs or structural modifications carried out in the past, conclusions and recommendation on strengthening work along with estimated cost was submitted to the customer.
Detailed Investigation
The detailed investigation, based on the recommendations given in the inspection report, was carried out to substantiate the findings during the visual inspection. The detailed investigations involved following tests to evaluate the residual concrete strength, which one of the important parameters in the assessment of the structural health.
| Parameters | Recommended Tests |
| Concrete Strength | Cores; Pull-Out; Pull-off |
| Concrete Quality | Ultrasonic Pulse Velocity; Radiography; Rebound Hammer; Penetration Resistance; Core |
| Corrosion of reinforcement bars | Half Cell Potential; Cover Depth; Carbonation Depth; Chloride Concentration; Resistivity |
Diagnosis of Structural Health
Though there may be multiple ways to classify the parameters of structural distresses, based on the finding of visual inspection and detail investigation test results, the structural distresses were rated as “Very Good”, “Good”, “Fair”, “Bad” and “Critical” based on their degree of distress. These ratings were derived from the co-relation between the different NDT test results such as Average Rebound Number, Ultrasonic Pulse Velocity etc. and the quality of concrete.
Every structural element was diagnosed and tagged as per the significance of parameters as indicated below.
| Parameters | Significance or Meaning of Parameters |
| “Very Good” or “Excellent” | No distresses are observed. Pulse velocity > 4.5 km/sec |
| “Good” | Indicates distresses which are less critical currently but may need attention in near future. Pulse Velocity > 3.5 km/sec < 4.5 km/sec |
| “Fair” or “Medium” | Indicates distresses which may lead to further deterioration in structural strength if not rectified timely. Pulse velocity > 3.0 km/sec < 3.5 km/sec |
| “Bad” or “Doubtful” | Indicates distresses which are of serious nature and resulted in deterioration of structural strength and thus need to be rectified immediately. Pulse velocity < 3.0 km/Sec |
| “Critical” | Indicates distresses which are of very critical and may result in failure of structural member or part of the structure which may cause loss of property and even loss of life if not rehabilitated urgently. Requires dismantling and re-building of the members. Not worth carrying out Ultrasonic Pulse Velocity test buy may be necessary to carry out other tests. |
Once the parameters of structural distresses were assigned to various parts of the structure, the “Fit to Purpose” strengthening philosophy was formulated.
Replacement / Repair philosophy
Though there are no set rules to decide repair or replace philosophy, the factors such as purpose of the strengthening with its cost economics, investment and feasibility of strengthening work will drive further work. Following principles were adopted to decide the “Repair” or “Replace” methods.
Depending upon the severity of the damages in the concrete members, it was necessary to understand the behavior of the concrete members under various loads acting on it and then recommend the right methods for strengthening. Accordingly following three methods of strengthening or rehabilitation of concrete members were recommended.
Recommended Maintenance Schedule
To avoid further deterioration post rehabilitation, TCE recommended a regular maintenance of the plant and structures for longer life.
| Maintenance Item | Description of Maintenance | Frequency |
| Structural Study / Audit / Assessment and issuance of Structural Stability Certificate through a qualified Chartered Engineer | Every 3-5 Years based on local statutory and legal norms | |
| Debris Removal | Clear gutters and down-take pipes for blockages | Half Yearly and preferably before monsoon |
| Dust Removal | Clean floors, canopies, chajjas, roofs, beams, steel members with brushing or brooming or blowing air or water jetting. | Min quarterly if dust accumulation is large else once in a year |
| Rain water Gutters & Down take pipes | Clean rain water gutters and down take pipes for leakages and repair to stop leakages. | Once in six months and preferably before monsoon |
| Crack filling or Joint Sealing | Check and fill cracks or joints around window sills, door frames, and in siding gaps. | Yearly |
| Plaster | Check and repair spalling plaster on the exterior face of building with water-proofing compound. | Once in 3 years |
| Painting of Structural Members | Check for rusted surface, peel offs, cracking. Repaint if necessary as per paint manufacturer’s guidelines. | Every 5 years or situation warrants whichever is earlier |
| Water Seepage | Check for water stains on the walls or underneath of slabs or in the exterior overhangs or soffits and repair the cause of seepage. | Yearly and preferably before monsoon |
| Plinth Protection | Check for damage, slope, etc., Repair or replace if necessary. Remove vegetation growth around the building. | Yearly |
| Acid resistant Flooring | Check for cracks, tears, de-bonding with sub-base, etc., Repair or replace if necessary | Yearly |
| RCC Pedestals | Check for damage, cracks, bonding of base plate, insert bolts, spalling of concrete, corrosion of reinforcement, rust stains, etc., Repair or replace if necessary | Yearly |
| Machine & Equipment Foundations | Check grout below base plates for damage, shear/vibration cracks, insert bolts, de-bonding of base plate / acid resistant lining over foundation etc., | Yearly |
| Base plates, Insert Plates, Gusset plates | Check section thickness, level of corrosion, etc., Repair/replace/apply anti-corrosive paint, if necessary | Yearly |
| Weld, bolt or riveted connections | Check section thickness, Repair or replace if necessary | Yearly |
| Structural Members- Column, Beam, Bracing, Truss | Check section thickness, level of corrosion, alignment etc., Repair or replace if necessary. | Yearly |
| Structural Items-purlin, runners, J-hooks | Check section thickness, level of corrosion, alignment etc., Repair or replace if necessary | Yearly |
| Jacketing to Structural Columns | Check for damage, cracks, spalling of concrete, de-bonding with steel members etc. Repair or replace if necessary | Every 3 years |
| RCC Members Column, Beam, slab, Chajja, Trestles, RCC walls to Godowns & Silo | Check for cracks in members, spalling of concrete, corrosion of reinforcement, rust stains etc., Repair if necessary. | Yearly |
| Grating, hand railing, Chequered plates | Check for corrosion damage, connections with structural members etc., Repair or replace if necessary | Yearly |
Conclusion
Structural strengthening or retrofitting projects are very demanding and challenging as it requires good domain knowledge and experience for understanding & corelating the test results for assessment of the structural health with respect to safety and stability.
It is imperative that the Consultant / Engineer identifies and understands the reasons of distresses before formulating rehabilitation program and provides clear details to the end users. The root causes of structural distresses shall be eliminated to the extent possible.
Consultant shall evaluate cost economics of carrying out strengthening works or demolishing the building and re-building it, with respect to overall safety and integrity of the structure. For achieving the satisfactory performance of the structure, the rehabilitation measures implemented shall be well supported by the effective maintenance schedule.
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