Housekeeping is essential to extend the life of any structure
BV Bhedasgaonkar, Consulting Engineer believes that the lack of housekeeping, non-engineered alteration to structures (particularly steel structures), cutting members and making punctures in slabs for ducts and pipes without adequate structural modifications are major causes of distress.
Kindly explain to our readers in simple terms what is meant by structural restoration or retrofitting of a structure. How does it differ from architectural restoration?
All structural components such as columns, beams, slabs, walls, domes, vaults, retaining walls, load bearing walls, shells of chimneys, silos, tanks, etc that carry the loads in a structure, undergo distress at earlier or later in their life span. Relieving this distress is done by repairing and retrofitting the structural components.
Architectural restoration, on the other hand, concerns mainly with restoring the functional components of the structures such as non-load bearing walls, windows, doors, plasters, tiling, plumbing, electrical etc.
Repairs and retrofitting of RCC structures are representing a substantial portion of total expenditure in RCC Construction. In developed countries this means about 50 to 60 per cent of total outgo. In developing countries like India where most of the construction in industries, mass housing, infrastructure has been done over the last 30 to 40 years, the percentage is less. But this is only likely to go up.
Repairing and retrofitting, as it has been understood and executed till now, has been mainly to restore the structure so that further damage is prevented. With some of the relatively new techniques it is now possible to strengthen the structure so that they perform at their design parameters and in some cases are able to deliver even higher design performance.
Why is structural restoration required at all?
The structures require restoration because they undergo distress. This distress could be on account of several causes, few are listed below:
The structure is under designed meaning some loads are not anticipated or accounted for in the design. This is the flaw at the design stage itself.
Additional loads are envisaged after the structure is in use such as capacity increase, change in earthquake zones requiring higher demand.
Poor quality of concrete in as constructed structure (one of the most common causes).
Corrosion damage, weathering etc.
Some of these causes may lead to functional shortfall (such as leakage) or even structural shortfall leading to structural instability.
As a result of the distress the structure is required to undergo restoration.
What are the different techniques used to assess the health of a structure? Explain in brief.
One of the main techniques for assessment is a very detailed visual observation of the structure. The observation done by an experienced eye can reveal the defects, probable causes of defects and give an idea about possible ways to arrest the damage due to defects and bring the structure back to original condition.
This is usually followed by non-destructive testing of concrete and reinforcement inside concrete. The techniques include rebound hammer, ultrasonic pulse velocity tests, core tests and carbonation tests, etc. on concrete to assess integrity of concrete and strength of concrete. The reliable strength arrived from the tests can be compared with the design grade. For assessing the reinforcement condition Half-cell potential test, resistivity tests are done to assess level of corrosion. Ground Penetration radar scanning is done to map the reinforcement spacings and Diameters which can then be compared with the original drawings. Tomography is used to scan the concrete structure to identify voids, cavities, and honeycombs inside concrete.
In steel structures the NDT includes assessing remaining thickness of corroded steel elements with ultrasonic technique to arrive at remaining cross sections, testing of welds using magnetic particle induction, die penetration tests for welds to check integrity and quality of weld etc.
Third and most important part is assessing the capacity of members and checking whether these are capable of resisting the design loads or there is some shortfall. Reanalysis of structure is required in many cases based on observed and assessed quality (through NDT). Once the shortfall is known a suitable strengthening scheme either for defective elements or for the entire structure (such as seismic capacity shortfall).
How can one arrive at the cost to be incurred on structural restoration?
It is possible to arrive at the cost to be incurred on structural restoration. This can be done after the damages are mapped and quantified after the technique explained above. As built drawings are required for accurate quantification. In absence of the drawings such drawings are required to be made as measured structural drawings and some assumed reinforcement patterns and quantities as per standard design practices. The quantum of damages observed in visual observations can be misleading as the actual damaged areas could be higher during actual repairs once all defective portions are removed, chipped off. Therefore, the accuracy of estimates has limitations. However, with experience an estimate to about 20 per cent accuracy is possible.
What is the general impression about different structures in a cement plant that is about 10-12 years old?
Usually in a span of 10 to 12 years, no defects or distress is likely to occur in any structure in a cement plant. If there are any inherent defects at construction stage itself or if there is any gross neglect on housekeeping, then only such defects occur. These include deflection and or cracking in slabs where material gets deposited and not cleaned from time to time. Examples are silo top roofs, conveyor galleries, roofs of conveyor galleries and storage sheds such as stockpiles. The defects could include damage to roofing sheets, deflections of structural steel elements and corrosion of elements embedded inside deposited materials, leakage from silo roofs and formation of lumps of stored material. Plants located in coastal areas may need a review of the condition of the structures after a period of 5 to 6 years.
What are the different structures in a cement plant that are more prone to damage? Why?
The structures that are more likely to get damaged are all the material handling structures such as conveyors, transfer towers, crushers, and material storage structures such as storage gantry structures and clinker stockpiles (usually the underside of the roof of this structure is inaccessible except in shutdown). The defects arise due to vibrations, material depositions, abrasion, exposure to corrosive weather and rains, etc. Storage structures such as silos are also prone to corrosion of reinforcement due to exposure to rainwater. Also, in the Clinker storage structures, the defects are due to abrasion, deposition of dust on structural elements, high temperature etc.
Are housekeeping and structural damage related to each other? How?
Housekeeping is essential to extend the life of any structure. Lack of housekeeping leads to deposition of material which may get wet during rains and keep corroding the concrete or steel structure underneath. Since the condition of the structure is not seen the damages go undetected till one fine day partial or total collapses occur. Leakage arresting is also an essential part of housekeeping in any structure particularly flat portions receiving rains such as silo tops, floors of various platforms and floors etc. Leakages if not checked in time lead to corrosion of reinforcement in concrete as well as corrosion of steel elements (through leakages in roofing sheets). A lot of major retrofitting can be avoided if the housekeeping is done periodically and defects, if any, are attended from time to time.
How typically a restoration work of a silo or a preheater tower is carried out once the damage is ascertained?
In silos the typical defects are due to poor quality of concrete (usually slip forming) and patch repairs. Inadequate reinforcement spacing is also observed in many silos. In prestressed silos, lack of prestress is found to lead to structural damage. In these structures attending all the defective concrete portions and repairing these with polymer modified mortars or micro-concrete is required. Comprehensive anticorrosive treatment to reinforcement inside is essential prior to that.
If the capacity shortfalls are found due to inadequate / corroded reinforcement, then strengthening (usually in hoop direction is required. External post tensioning using steel wires and cables, or Carbon and Glass fiber wrapping are resorted to. Overall protective coating from outside is also essential in silos which are restored.
In preheaters defects mentioned above are encountered. Lack of verticality of structural elements such as columns is required to be assessed. The technique most commonly sided to correct the effect is jacketing. One of the main problems in preheaters is seepage through cracks in slabs. Preheater is usually an open structure and rainwater accumulates on floors. There is also high temperature and there are punctures in slabs for ducts and pipes. Invariably there is severe corrosion of slab reinforcement. This can best be arrested by providing floor impregnation from top, corrosion protection and repairs from soffit side. Sometimes recasting slabs with lost steel formwork is found to be faster and economical.
Have the number of queries you normally receive gone up during pandemic? How are you using technology to reduce the number of visits to a plant location?
The queries have increased in number because the plants have the possibility of getting shut down due to lack of demand during the pandemic. With travel restrictions it is becoming inconvenient to inspect the structure by making a visit. We have been able to arrive at the possible defects using detailed videos and photos shared by clients. In 80- 90% of the cases, it is possible to provide a solution. The clients are also accepting this concept of virtual consultancy.
What are the different techniques / materials you prefer in the restoration job?
The techniques and material used for restoration change from problem to problem. But usually for comprehensive anti-corrosive treatment we use thorough cleaning such as sand blasting, migrating corrosion inhibitors, protective coating on old and new reinforcement, epoxy bonding agents and section make up using polymer modified mortar (for thin repairs up to 50 mm ) and micro-concrete( for thicker repairs). Protective coating is also recommended by use depending upon the situation and these include acrylic coating, Epoxy coating and Polyurethan coatings.
For upgrading load carrying capacity we have resorted to Carbon and Glass fiber wrap, Jacketing, shotcreting (for large continuous areas like silos). For silos post tension is also used. In some cases, steel plate jacketing is also convenient.
In your opinion, what are the reasons for poor attention to the structures of a cement plant?
The structures are an equally important part of the plant as the equipment. Equipment gets attended periodically and usually in shutdown most equipment needing overhaul are attended to. Lack of housekeeping, non-engineered alteration to structures (particularly steel structures), cutting members and making punctures in slabs for ducts and pipes without adequate structural modifications are major causes of distress.
For repairs to structures sometimes the shutdown time available is inadequate. Therefore, the structures need to be repaired in running condition. Not adequate fund allocation is done for repairs to structures in the annual budget. Usually for an integrated plant annually about Rs 4 to 5 crore are required to be allocated for structural upkeep, maintenance and protection. A dedicated housekeeping team is required to attend to the plant on a continuous basis.
Do the plants carry out structural audits of the entire plant? Is there any regulation that will compel plants to carry out structural audits at a certain frequency? How is it monitored?
Structural audits have become quite common now. Plants do carry out the audit every 4 to 5 years. However, the implementation of the recommendations on audit are kept on the back burner by not allocating adequate funds. This leads to structural defects getting more severe and resulting in accidents.
The structural stability certificates required to be submitted to the factory inspector periodically are mandatory but usually are not based on comprehensive structural audit. Clients who are conscious of the quality and safety of the structures and personnel carry out comprehensive structural audits including NDT. These are usually done once in 4 to 5 years.
Any typical challenging structural restoration you have carried out? Please give details without mentioning the names.
There are several case studies. These include storage silos in many cement plants including post tensioning using steel cables and Fiber wrapping. Cases with vertical misalignment in slip forming have also been designed. Strengthening of raw mill structure with inherent defects using jacketing was done in a few plants. Strengthening of preheater structure including foundations was carried out in case of a preheater where capacity (and height of preheater was to be increased) enhancement was envisaged. Other few strengthening cases with challenge were to strengthen floor plates in buildings on account of poor concrete which is achieved using additional structural concrete on top and fiber wrapping.
What kinds of damages are seen in the housing colonies of a cement plant? Do you see preventive maintenance happening there?
In housing colonies of cement plants, the defects are usually functional such as cracks in walls, leakage, settlement of floors in black cotton soils etc. Not much structural damage is required to be attended.