Sumeet Agarwal,
Head-Recovery,
Sustainable Environment and Ecological Development Society, New Delhi
Buildings, with poor quality of construction or in various states of disrepair or dilapidation, are a common sight in cities. Many of these buildings, that serve as living, working and learning spaces, in addition to being eyesores, are highly vulnerable to disasters as evidenced by incidents of building collapse accompanying disasters in India and other countries. The extensive damage to buildings in the earthquake in Gujarat in 2001(3) and in Haiti in 2010(2) illustrate consequences of neglect of building construction, maintenance and repair issues. Considering the high disaster vulnerability of India and high population densities in cities, issues of substandard building stock need to be prioritized to avoid a catastrophe. Based on damage need for repair can be structural, non-structural or both.
Non- structural Damage
Repair of non-structural componentsis generally taken up after structural repairs have been carried out. Such repairs generally help to restore architectural features of the building and make it possible to develop a more congenial living/working environment and tore-use the spaces but they do not enhance the structural strength of the building and may, in fact, conceal the defects of the building making it difficult for engineers to address and remove the defects at a later date. Non-structural repair activities generally include the following:
- Repair to electrical fittings and wiring
- Repair of doors, windows and glass panes
- Patching up of cracks and fallen plaster
- Repair of plumbing and gas pipes
- Re-building of non-structural elements like boundary walls and chimneys.
- Replacement or rearranging of roof tiles.
- Re-laying of flooring.
- Redecoration- white washing, painting etc.
Structural Damage
Extensive structural damage can be addressed through repair or reconstruction such damage can be avoided through timely repair or retrofitting. The decision of strengthening or retrofitting v/s reconstruction needs to be made after careful consideration of the advantages and disadvantages. Replacement or reconstruction of damaged or unsafe buildings by reconstruction is generally avoided for the following reasons:
- Higher cost than that of strengthening or retrofitting
- Preservation of historical architecture
- Maintaining functional, social and cultural environment.
As a thumb rule if the cost of repair and strengthening is less than about 50 percent of the reconstruction cost, retrofitting is adopted. It may be possible to retrofit a structure in shorter time as compared to reconstruction and would have a less disruptive effect on the occupants of the building. However, incorporating provisions of relevant codes during construction of a building works out much cheaper than retrofitting a structurally inadequate buildings.
Strengthening of Buildings
Strengthening generally does not limit itself to increasing the strength of deteriorated or damaged structural members, but also considers behavior of the entire structure. It may also be taken up specifically to improve the resistance of the buildings to disasters, like earthquakes. The seismic resistance of existing buildings, which do not meet seismic strength requirements of earthquake codes, in force, due to original structural inadequacies and material degradation due to time or alterations carried out during use over the years can be upgraded to the level of present day codes. Strengthening procedures aim at the following:
- Increasing the lateral strength in one or both directions by reinforcement, increasing wall areas or increasing number of walls and columns.
- Improving unity of the structure by providing better connections between its resisting elements so that forces generated by vibration of a building can be transmitted to members, which can resist them. Connection between roof and wall and intersecting walls are examples of elements whose connections need to be strengthened.
- Elimination of features that are a source of weakness to the structure or lead to concentration of stresses in some members. Abrupt changes in stiffness from one floor to the other, large openings in walls without peripheral reinforcement, asymmetrical plan distribution of load resisting members, abrupt changes in stiffness from one floor to the other are all examples of defect of this kind.
- Avoiding likelihood of brittle modes of failure by proper reinforcement and connection of resisting members.
- Since signs of deterioration may not be visible due to cosmetic maintenance work, assessments and tests of the structural members would be necessary to reveal the nature and extent of deterioration. A detailed damage assessment would help to determine:
- If the structural condition of the building is amendable for repair, whether continued occupation of the building is permitted, if whole or part of the structure requires demolition.
- Need for detailed damage assessment of structural components ( mapping of crack pattern, distress location, crushed concrete, reinforcement bending/ yielding, etc.). Non-destructive techniques could be employed to determine the residual strength of the members
- Temporary supporting arrangement of the distressed members so that they do not undergo further distress due to gravity loads(1).
- Based on the assessment of the building and available cost and time the appropriate strengthening measures can be decided.
Assessment
Various ways of conducting assessments of structures are as under:
- Non Destructive tests (NDT)
- Visual inspection of building
- Study of relevant drawings and documents
- Discussion with personnel involved in design and construction of the building
Brief details of each of the methods are given below:
Non-destructive Tests
Non Destructive tests can accurately determine the retrofitting and repair techniques required for various structural members in a structure. Different types of NDT methods are:
- Rebound Hammer Test
- Ultrasonic Pulse Velocity test
- Core extraction
- Pull out test
- Carbonation test
Visual Inspection of Buildings
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Detailed visual inspection of buildings can provide adequate information about structural distress of the building and the following is to be seen during the visual inspection of a building:
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Study of Relevant Drawings and Documents
Study of drawings and documents of a building can provide a wealth of information on a building. It would help to determine the additions/ alterations to the building and design omissions or inadequacies if any. The drawings also help to determine the structural system in areas of a building made inaccessible by furniture, machines or fixtures and may also provide information about codes used in the structural design, helping to determine if the building conforms to current codes or not.
Discussion with Personnel and Study of Documents
- Study of soil test reports.
- Study of the structural and architectural drawings of the structure.
- Study of recommended grade of concrete and building material specifications.
- Results of tests such as core tests, brick tests etc.
- Issues faced during construction.
Case Study: Assessment for Retrofitting of an Office Building in New Delhi
The author was part of a team that conducted an assessment of a large office complex in New Delhi. The details of the office building cannot be mentioned for contractual reasons but the assessment process, findings and recommendations will be described. The description seeks to demonstrate the application of assessment processes described above, the kind of structural deterioration that a building can undergo and the possible strengthening interventions that can be proposed in such cases.
The assessment team decided on the following methodology for the work:
- Conduct a visual inspection of the building to detect visual signs of distress.
- Carry out a condition survey of the building by conducting Non Destructive Tests on the structure.
- Recreate as-built structural drawings of the building.
- Carry out soil testing to ascertain strength and condition of foundation strata.
- Carry out structural analysis of the building to ascertain structural integrity of the building, existing grade of concrete and existing steel reinforcement.
- Recommend remedial and strengthening measures to ensure satisfactory performance of the building
The Complex
The office complex, consisted of five multi storied buildings constructed between 1962 and 1984.The building was highly vulnerable to earthquakes since the area, where it was located, has been marked as a high seismic hazard zone by Seismic Hazard Micro zonation map, published by Earthquake Risk Evaluation Centre, Indian Meteorological Department. One of the reasons for designating the area as a high hazard zone was due to liquefaction potential of the soil.
Visual Inspection of the Buildings
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A visual inspection of the buildings was first carried out. Available drawings were also studied and relevant personnel were interviewed for structural issues faced. The inspection and study revealed the following information on the buildings:
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Issues
The following main issues were revealed during a visual inspection of the complex:
- A bridge connecting building 1 and building 3 had been added and subsequently a fire escape staircase was also added. No structural information is available about these elements although they have a bearing on the structural performance of the building.
- Incidents of plaster and concrete pieces falling from shading louvres and overhangs had been reported and surface cracks were noticed on louvers and overhangs of the buildings.
- The maintenance personnel informed that the roof slab of building 3 was 75mm thick.
- The personnel also informed that a number of walls between columns had been removed and replaced with large glass windows for aesthetic purposes.
- Intermediate walls between rooms and corridor have been removed to convert them into halls.
- Roof spaces of buildings were found to have been used for installation of solar panels mounted on concrete stubs bolted to reinforcement of the roof slab. The panels have not only added to the dead load on the building but also added a factor of wind load exerted on the large panels bolted with a single hook.
- An additional steel frame has been constructed on the roof space of building 4 to extend the roof area and almost hangs from the roof and the rear wall of the building. The frame was meant to create additional space for the AHU as the roof space is occupied with solar panels.
- Large deep cracks were noticed on the bottom of beams and slabs of the buildings. A discussion with the maintenance staff revealed that large lumps of concrete had also fallen from the beam, which at the time of inspection was concealed under a false ceiling. An additional pillar also had to be provided to support the cracking beam.
- An additional floor was added to building 5 with independent external columns and beams.
Inferences from Visual Inspection
- Modifications to the building and additional live loads are significant in view of the fact that change in Seismic zone in Delhi in 2002 shall alter the factor of safety applied in the building design originally. Since the upgrading of the zone calls for a higher design factor under building code provisions for safety of structure it needs to be analysed against changed loads.
- Structural integrity of the building connections of smaller built additions like the fire escape staircase that have been inserted in the existing structure are a cause for concern.
- Changes made to the structure of the building like removal of walls between columns and replacement with glass windows would have an impact in the event of seismic activity since the columns have been deprived of the bracing provided by the intermediate walls.
- Distress that has developed in various structural elements of the complex over the years needs to be addressed.
The visual inspection of the building was followed by the following tests.
Soil Test
Field penetration tests and liquefaction studies were carried out and the findings are:
- Though liquefaction is not anticipated in the foundation soil during an earthquake, the factor of safety against liquefaction is found to reduce as the ground is inundated with water and the soil is completely saturated. Hence proper drainage of the premises needs to be ensured so that the ground does not get flooded.
- The value of standard penetration resistance blow count is less than 15 in the top 6 metre of ground and below 6 metre its value increases. Hence, it would be advisable to improve ground strength at shallow depth by grouting.
Non-destructive Tests
The following non-destructive tests that were carried out.
- Rebound Hammer Test as per IS: 13311 (Part-2)-1992 for determining the compressive strength of concrete.
- Ultrasonic Pulse Velocity Test as per IS: 13311 (Part-1)-1992 for ascertaining the quality of concrete.
- Drilling out Concrete Cores, preparing the test specimens and testing the cores as per IS: 516-1959 for determination of compressive strength of concrete.
- Carbonation Test for determining the depth of carbonation of concrete.
- Corrosion Potential Assessment by conducting half-cell potential test as per ASTM: C876 – 1991 for assessing the risk of corrosion of reinforcement.
- Rebar Locator Test for establishing the existing depth of concrete cover over reinforcement.
- Chemical Tests on concrete in the laboratory for determining the following parameters:
- Chloride content as per IS : 14959 (Part 2) – 2001
- Sulphate Content as per relevant B.S.
- pH value.
The test results from one of the buildings has been described below for illustrative purposes. The NDT results from other buildings were found to be the similar and have not been described to avoid repetition.
Building 1
Rebound Hammer Test
The test results reveal that the concrete in various structural elements of the building at different floor levels falls in M 10 & M 15 grade categories. However, at first floor level the grade of concrete in all structural elements falls in M 15 grade. Since 10 Nos. test results out of total 19 Nos. fall in M10 to M15 grade, keeping safety considerations in view, for structural analysis for seismic assessment of Main Building, the overall grade of concrete for RCC works can be considered as M12 grade.
Ultrasonic Pulse Velocity Test
Reviewing the test data, it is seen that the ultrasonic pulse velocity investigations carried out on foundation footing of column & various structural elements at different floor levels of the Main Building indicate that overall quality of concrete can be considered to fall in ‘Doubtful’ to ‘Medium’ category reflecting unsatisfactory workmanship with presence of likely internal deficiencies.
Concrete Strength
As per IS:456-2000, the concrete in the member represented by a core test shall be considered acceptable if the average equivalent cube strength of the cores is equal to at least 85 % of the cube strength of the grade of concrete specified for the corresponding age and no individual core has a strength less than 75 %. The concrete is seen to fall in M 10 to M15 grade.
Carbonation Test
The carbonation test was conducted on foundation footing of column & various RCC elements at all floor levels, & the depth of carbonation has been found to range from 20 mm to 80 mm. The tests have revealed that the carbonation has penetrated up to or beyond the reinforcement bars at most of the locations.
Half-cell Potential
The results of the half-cell potential survey indicate that the likelihood of corrosion of reinforcement at present in various structural members at different floor levels is varying from less than 10 % to more than 90%.
Cover Meter Test
There is no deficiency in depth of cover recorded as compared to the requirement of minimum concrete cover as per IS: 456-2000.
pH value, sulphate and chloride analysis
As per Clause 8.2.5.2 of IS:456-2000, for RCC containing embedded metal, max. total acid soluble chloride content expressed as Kg/m3 of concrete should not be more than 0.60. As such, all the test results in respect of chloride content are within permissible limit.
As mentioned earlier above tests were conducted on other buildings, including building 2 but have not been included to avoid repetition. However, considering the state of structural elements, test results and height of the buildings strengthening measures were proposed for buildings 1 & 2, which are given above.
Strengthening Measures Proposed
The following measures are presented as sample.
Building 3
- The columns would be required to be strengthened heavily with new reinforcement and concrete jacketing with micro-concrete. Similarly, the beams would require strengthening using FRP laminates/ concrete jacketing. These would further increase the size and weight of the structural members and therefore, the foundations would be further overburdened requiring their sizes to be increased and strengthening.
- For earthquake resistance, the foundations would require to be thickened and tied with beams along with strengthening and enlargement.
- The strength of concrete is found to be very poor for all the structural members, which would require pressure grouting using low viscosity epoxy grouts.
- The building aesthetics may not be maintained because of resizing of columns and beams.
Building 4
- The columns would require strengthening with new reinforcement and concrete jacketing with micro-concrete. Similarly, the beams would require strengthening using FRP laminates/ concrete jacketing. These would increase the size and weight of the structural members and therefore, the foundations would be required to be enlarged.
- For earthquake resistance, the foundations would require to be tied with beams along with strengthening and enlargement.
- The strength of concrete is found to be very poor for all the structural members and the same would need to be improved adopting pressure grouting using low viscosity epoxy grouts.
The building aesthetics may not be maintained because of resizing of columns and beams.
Building 5
The building arrangement appears to be non-engineered and the seismic resistivity of the structure is doubtful.The extent of strengthening requirement for all the Buildings would be extremely high and would require almost strengthening from foundation to columns, beams & slabs, viz., strengthening of each structural member.
It is obvious from the above description of the building that it is highly vulnerable to earthquakes due to the following factors
- Ageing
- Unplanned modifications
- Non- conformity to present day codes
The building would sustain heavy damage during a severe earthquake as strength of concrete is less than acceptable limit with medium to doubtful quality. Corrosion of reinforcement will also continue to increase as depth of carbonation is quite high. In addition to the deterioration the building has been constructed under old codes and needs to be upgraded to conform to present day codes and revised seismic zoning. In order to improve the resilience of the building. Structural members need to be treated in multiple ways which include removal of loose concrete, protection of existing reinforcement steel from corrosion, jacketing with concrete/steel/ carbon fibre reinforced polymer or a combination of these. The appropriate treatment would need to be decided on the basis of cost and structural design. The example also illustrates the extent of deterioration that can occur in a building if left unaddressed. The treatment to the structural members would not only be expensive but would also affect the functioning of occupants of the building. Thus timely assessment, repair and treatment of buildings is essential.
References
- National Building Code of India
- https://www.newscientist.com/article/dn18406-why-the-haiti-quake-killed-so-many/
- https://architexturez.net/doc/az-cf-21244
- http://pwd.delhigovt.nic.in/pims/right_to_info/handbook.pdf
- https://www.ripublication.com/ijcer_spl/ijcerv5n4spl_05.pdf
- https://www.nicee.org/iaee/E_Chapter9.pdf
- IS 13935, 2009: Seismic evaluation, repair and strengthening of masonry buildings- Guidelines