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Why the services are needed from a Licensed Structural Audit during repair? Society can manage on their own.
First of all, Society bye law no. 77 specifies Structural Audit as a mandatory (necessary and binding to Society) requirement. It stipulates that if the age of a building is 15 to 30 years, Structural Audit must be carried out once in 5 years and for buildings older than 30 years, it should be carried out once in 3 years. Today, well informed decisions can be taken by Society Managing Committee towards the Building Structural repair and painting works to be undertaken.
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Who carries these Civil Structural Audits and PMC services?
Structural Audit is commissioned by appointing a Panelled Consulting Structural Engineer registered with the BrihanMumbai Municipal Corporation (BMC). The Consultant carries out as a visual survey of the building covering its Terrace top areas to , external faces, stilts, staircase internals, inside flats, shops establishments and ancillary structures such as Lift room, pump room, compound wall, etc. Repair history of the building and specific observations/ experiences of the members are also noted. Critical observations, probable causes of distress, remarks on structural health and recommendations for further action are given in the Structural Audit Report.
STRUCTURAL AUDIT CONSULTANT– PERIODICALLY:
Periodic “Structural Audit”, of the building and compliance thereon, also positively restricts the BMC (or others /owners), from declaring the building as “dilapidated” and ultimately from receiving notices of eviction from the building. Such notices would also mean revocation of buildings “Occupancy Certificate” and increase in Water Charges.
STRUCTURAL AUDIT – CHECKING:
“Structural Audit”, of the building means EVERYTHING connected to the conduct of a Building, which includes strength of the buildings Columns, Beams, Pillars, Iron Bars & Plaster, Sewage discharge systems, Water pipeline systems, Electrical cabin and wiring system, Lifts, Podiums ….
a) Structural Audit parameters will vary for Residential buildings, Commercial buildings and Industrial buildings.
b) Some “quack” type Structural Auditors, perform a superficial check and issue a manipulated Structural Audit Report, to the Society, with the connivance of the apathetic Society office bearers.
STRUCTURAL AUDIT – RESTORE TO NORMALCY:
A Structurally unfit building shall be prone to sudden collapse without any warning, at the first instance of an earthquake, which in turn means risking the Life and Property of the buildings residents and other people in the vicinity.
a) At present times, there are several technological techniques available, along with highly experienced technical professionals, where old buildings can be easily refurbished /restored, to complete normalcy, thus peacefully extending the life of old buildings by further 20-40 years.
b) This helps in gross monetary savings, while still retaining the emotions and sentiments attached to the building and its co-residents.
ILLEGAL ALTERATIONS – BUILDING COLLAPSES:
Around July-2007, a 25 year old building named as “Laxmi Chhaya” (at Borivali West), just simply collapsed in the early morning hours, just because some greedy shop-owner/s had reduced the girth of the buildings Column & Pillars. This resulted in loss of Life and Property of the buildings residents and other people in the vicinity. Today due to courtesy of Human Apathy, everything is back to normal and the losses of human lives are apathetically and conveniently forgotten by all concerned. No lessons learned.
WHAT SHOULD BE CONDUCTED DURING STRUCTURAL AUDIT?
A “prudent” Structural Engineer would reflect the following in his Structural Audit Report, highlighting the relevant violations under the Municipal Laws, while keeping upfront the Approved Plans of the Building:
a) Any changes and contraventions of the Approved Plans /Availability of Sanctioned Plans. Whether Building has been built in conformity to the Sanctioned Building Plans,
b) Availability of approved & sanctioned building Plans, IOD, CC, OC,
c) Changes made to the buildings Columns, Beams, Pillars,
d) Changes made in place of WC, Bathroom, Kitchen, installation of loft water-tanks,
e) Extension OR Covering of Balcony,
f) Removal of Internal walls between rooms,
g) Internally Amalgamating (joining) of two flats, by removing partition walls /doors,
h) Installing over-protruding Grills, Sheds, Chajjas,
i) Conversions of basements OR Stilt /podium parking for any other usage (e.g. Offices, Shops …. )
j) Existence of unauthorized Lofts & Mezzanine Floors in the building,
k) Any other encroachment of Common areas, Refuge areas and Society premises,
l) Installation of illegal Mobile Towers and Hoardings, and adverse effect of same on the building,
m) Whether the building has appropriate Drainage /Sewage lines connected to the Mains,
n) Existence of Open-Well, Bore-Well and other clandestinely built sub-ground level storage water tanks,
o) Common Electrical Wiring system,
p) Changes in Internal /External Drainage /Sewage lines,
q) Ground and Overhead Water Tanks, Water Meters and supply pipelines,
r) Water logging around periphery of the Building and reverse incline level of ground,
s) Detailed report on the repairs and restorations, that is required in the Building, in terms of the Approved Plans of the building.
Note: Structural Engineer /Auditor shall clearly outline each Alterations with graphic images and photographs and the repairs /rectifications /restorations that needs to be done in the Building.
STRUCTURAL AUDIT TECHNIQUES :
The Structural Engineer, mandatory must be a NEUTRAL authority, more so since any manipulations or fabricated or untrue Structural Audit Report, would mean, criminal prosecution against the Structural Engineer. An experienced & competent Structural Engineer would conduct several tests to determine the extent of corrosion, distress and loss of strength in concrete & steel. Some of the tests means:
a) Concrete Core Cutting & Compression testing for columns, beams and slabs for Strength
Assessment of concrete.
b) Half Cell Potential test for determining the probability of corrosion in the embedded steel.
c) Carbonation test for carbonation depth measurement for Steel.
d) Ultrasonic Pulse Velocity Test (UPV) for Strength Assessment of concrete.
e) Integrity tests for pile foundations and various testing techniques.
The Building Structural Auditor has to inspect the Fire Fighting Systems (as applicable) with the relevant approved vendor and prepare recommendatory reports, highlighting the repairs and restorations required, for due compliance of the building Owner /Society.
Failure to install the mandatory Fire Fighting Systems, and to keep maintaining them in working condition is mandatory, failing which the Fire Department is within its jurisdiction to disconnect Electricity and Water connections, besides revoking the FIRE NOC for Occupancy Certificate granted to the building.
Presently Earthquake resistance buildings are built & designed as per Indian Standard Codes, means “Earthquake Code IS 1893-2002”.
a) Under the parameters of “Indian Standard Codes”, the island city of Mumbai is located in Earthquake (Seismic) Zone III.
b) Some very old buildings, which are not built for Earthquake resistance, can still be retrofitted appropriately to make it literally earthquake resistant, subject to the building being structurally fit. This preemptive /precautionary measure will help in avoiding sudden collapse of buildings, during Earthquake/s, and giving enough time to clear the building, thus helping in saving Life & Property.
FAILURE TO CONDUCT REPAIRS:
IF the building Owners /Society, consistently fails to carry out all the repairs and restorations highlighted in the Structural Audit Report, THEN the BMC is empowered to conduct the repairs and restorations, on its own and recover the cost of such “repairs and restorations” from the building Owner /Society. Failure to carry out relevant repairs by the Society, would also lead to revoking of “Occupancy Certificate”, of the building, thus leading to double the water charges u/s 92 of the BMC Act. The BMC is empowered to file prosecution u/s 488 of the MMC Act and levy Penalty, for failure to comply with the repairs, mandated in the Structural Audit Report. Criminal proceedings under Indian Penal Code can also be filed on the Mg. Committee members, for their deliberate failures and endangering the Life and Property of the building residents.
TABLING OF STRUCTURAL AUDIT REPORT:
After the Structural Audit of the Society building is duly concluded, the Society Mg. Committee has to distribute a Copy of the same to each member at the cost of the Society and then duly call a Special General Body meeting, to deliberate on the Structural Audit Report and the costs for repairs and restoration highlighted by the Structural Engineer. The said Repairs and Restoration has to be mandatory complied and the General Body of the Society has NO OPTION to avoid the same. The General Body CANNOT pass any resolution to avoid the said Repairs and Restoration, under the Structural Audit Report. The repairs & restorations to the building (as applicable) has to be started WITH or WITHOUT the approval of the General Body. Hence the “Tabling of the Structural Audit Report”, before the General Body thou necessary, has no relevance under the Municipal Corporation Act. It has to be mandatory complied with (repairs) even if the General Body does not sanction repairs, due to any reasons, whatsoever.
Note: IF the building owner /society do not complete the repairs /restorations, as per the directions in the Structural Audit Report within 6 month of submission of Audit Report, THEN they are also liable to be punished under section 471 & 352(B) of the MMC Act.
NON-AVAILABILITY OF BUILDING PLANS & OC :
In the event, IF the building’s Sanctioned /Approved Plans and/or Occupancy Certificate (OC) are not available, THEN the building owner (Society) has to conduct the necessary documentations and arrange /make the buildings plans etc….
The buildings Occupancy Certificate can be procured, using various compliance parameters, BUT ONLY after repairing & restoring all illegal alterations /amalgamations.
PREEMPTIVE MEASURES :
Even in newer buildings, the building owners (Society) should themselves consider to conduct “Structural Audit” of their buildings, every Five years, as precautionary measures against encroachments of common spaces /balconies /niche areas /flower-bed areas and unauthorized alterations in WC, Bathrooms & Kitchens or Amalgamating two Flats.
Such preemptive Structural Audit would help in timely repairing or reversing back any such unauthorized alterations /changes, and avoid hairline fractures in the Columns, Beams, Pillars, Walls and Floors.
The compliance of above, will mean preserving & increasing the overall life of the buildings. This also serves to save Life, Property & Money of the building residents.
If we are serious about the Structural Audit, how do we carry it out? What do we tell to our Structural Engineers? What are our expectations?
STEP 1 :
It is imperative that we must have Architectural and Structural plans of the bldgs. it will be helpful if we have detailed structural calculations including assumptions for the structural design. The assumptions can also include the allowable live loads; Whether the bldg. is designed for residential, commercial, light industry or heavy industry and whether any future provision for adding new floors is considered? What type of Earthquake loads are considered? Which I.S. Code requirements have been met?
STEP 2 :
If the Architectural plans and Structural plans are not available, the same can be prepared by any Engineer by measuring the size of the bldg. & locating the
position of the columns, beams and size of all such structural elements.
STEP 3 :
Inspection of the Bldg. - A detailed inspection of the bldg. can reveal the following :
1. Any settlements in the foundations.
2. Visual cracks in columns, beams and slabs
3. Concrete disintegration and exposed steel reinforcements – photographs can
4. Slight tapping with hammer can reveal deterioration in concrete.
5. Extent of corrosion in reinforcement.
6. Status of Balconies – sagging, deflection, cracks?
7. Status of Architectural features viz. chhajjas, fins, canopies etc.
8. Cracks in walls indicating swelling in R.C.C. members or distress or
deflection or corrosion.
9. Leakages from terrace & Toilet blocks.
10. Leakages & dampness in walls resulting into cracks and corrosion.
11. Changes carried out affecting structure.
Toilet blocks - Added or changes made?
Change of user – from Residential to Commercial to Industrial?
Change of Partition Walls?
12. Status of lift and lift machine room – Type of Maintenance Contract, renewal
13. Status of electrical wiring from meter room to all the flats. Substation status.
Any explosion in the meter room, substation?
14. Status of overhead & underground water tanks - capacity. Leakages, cracks
& frequency of cleaning, status of pumps.
15. Plinth protection in the compound including status of drainage, water pipes
& pumps. How much the Ground was flooded during recent monsoons?
16. External paint – When last painted and type of paint.
17. Status of repairs & last repaired.
What was repaired?
Who was the Agency?
How much was spent for repairs?
18. Bldg. plans available? When approved?
Occupation Certificate available?
Structural Plans available? Structural Stability Certificate available?
Structural Calculations available?
19. Last Structural Audit prepared?
STEP 4 :
Tests Recommended :
It is important that various tests are carried out in the old bldgs. This will give an idea about the extent of corrosion, distress and loss of strength in concrete & steel.
Tests may include:
1. Concrete Core Cutting & Compression testing for columns, beams and slabs for Strength Assessment of concrete.
2. Half Cell Potential test for determining the probability of corrosion in the embedded steel.
3. Carbonation test for carbonation depth measurement for Steel.
4. Ultrasonic Pulse Velocity Test (UPV) for Strength Assessment of concrete.
This can be useful for simple foundations.
5. Integrity tests for pile foundations.
STEP 5 :
Highlight the critical areas and how to go for repairs. For e.g.
1. No. of columns requiring immediate attention including treating rusted steel,
adding new steel, jacketing of columns etc. – Repairing foundations, repairing
2. Attending of beams and slabs wherever required.
3. Attending water proofing of terrace, toilet blocks.
4. Attending cracks in external walls and providing good quality of paint.
The critical areas highlighted need to be attended immediately.
STEP 6 :
Earthquake Criteria :
Mumbai is located in Earthquake Zone III as per Indian Standard Codes. The Earthquake Code IS 1893-2002 provides rigorous analysis and designs of Bldg. structures so that it can withstand the Earthquake forces. It may be possible to
retrofit the old buildings, so that they do not collapse during Earthquake; but may develop some cracks and allow enough time for people to escape. Thus saving precious lives.
STEP 7 :
Compliance of Audit requirements.
Audit is a good thing, but in itself Audit is not sufficient. It is important that the findings and/or recommendations of audits are implemented satisfactorily, within a stipulated time limit and are certified by Structural Engineers; Otherwise the Audit findings will remain on paper. Is it a costly process? Of course, this is going to be costly; but human lives are important and they need to be saved at any cost.
If your bldg. is more than 15 years old, it is important that rigorous audit is carried out every five years. After 30 years, every 3 years structural audit should take place. This will be a continuous process as it is difficult to guarantee future life of old bldgs. However, regular Audits and implementing audit findings will avoid sudden collapse of bldgs. and save thousands of life. This process will also increase the future life of bldgs.
Non Destructive Tests: Structural Audit – Recommended Tests
The following NDT tests are required to be carried out on structural elements. However,
it is important that the testing scheme is prepared based on preliminary survey of the
Core tests to determine the estimated equivalent in situ compressive strength & to establish correlation between Rebound hammer test & in situ strength of concrete.
Rebound Hammer test to estimate the in situ compressive strength of cover concrete.
USPV test to assess the integrity of concrete.
Carbonation test to assess the depth of carbonated concrete.
Half cell potentiometer test to determine the probability of active corrosion.
Cover test to assess the cover provided to RCC structural members.
Steps to the Core Tests :
The reinforcement is detected at planned location with the help of Rebar Locator called Profometer to avoid cutting of reinforcement.
The Core cutting equipment is fixed at the planned location & core is extracted.
The Cores are transported to the laboratory & visual observations of cores are recorded for interpretation purpose. Reinforcement bars, if encountered, are cut off.
The Cores are removed from water cut to the required L/D ratio of 2, wherever possible, exactly perpendicular to the longitudinal axis.
Both the ends are prepared by grinding up to the tolerance limit as specified by Clause 4:8 of BS 1881:Part 120: 1983 for flatness & parallelism.
A thin layer of plaster of Paris is applied to ends to ensure proper contact.
Now the cores are ready for compression testing.
Rebound Hammer Test:
Steps to carry out Rebound Hammer Test :
The test is performed as per guidelines given by IS : 1331 (Part 2): 1992 & BS 1881: Part 202: 1986 to estimate the in situ strength of concrete based on the correlation established between in-situ strength at the particular location & rebound numbers.
1. The plaster is removed at test locations.
2. For testing, smooth, clean, dry surface without any defect like Honeycombing cracks and hollow sound is selected.
3. The area of approx. 300 mm x 300 mm is rubbed with carborundum stone to remove loosely adhering scales, or remains of plaster mortar, if any.
4. In this area 12 points at approximate 30 mm apart are selected in grids.
5. By holding the rebound hammer at right angles to surface of the concrete member, 12 readings are taken at selected points.
6. Of these readings, abnormally high & abnormally low results are eliminated & average of the balance readings is worked out.
7. Taking into consideration the factors influencing hardness of the concrete surface like moisture condition of the surface, carbonation, test location within the member, direction of test etc. corrected rebound number is worked out.
8. The compressive strength of concrete against each rebound number is obtained from graph prepared on correlation established between rebound numbers at core test locations & equivalent cube strength values.
9. The statistical analysis is carried out for this set of values of compressive strengths obtained by above method.
Steps to carry out the USPV test :
1. The plaster is removed at test locations wherever required.
2. For testing, smooth, clean, dry surface without any defect like honey combing, cracks, and hollow sound is selected.
3. The area of approx. 300 mm x 300 mm is rubbed with carbonation stone to remove loosely adhering scales, or remains of plaster mortar, if any.
4. Two points are marked on opposite faces of the concrete members. (At exactly opposite locations for direct transmission of ultrasonic pulses).
5. Grease is applied as a coupling medium to ensure proper contact of the transducers with concrete surface so that ultrasonic pulse is transmitted through the medium without much disturbance.
6. Now both the transducers are held at correct test locations by applying constant pressure & ultrasonic pulses are transmitted through the concrete.
7. The machine displays the time taken to travel the known path in microseconds.
8. The velocity is calculated from the reading obtained against each known path.
9. Following velocity criterion for concrete quality grading is given by IS 13311 (PartI) : 1992.
Criterias for grading of Concrete Quality
Ultrasonic Pulse Velocity by Cross probing (Km/Sec.) Concrete Quality Grading
1. Above 4.5 Excellent
2. 3.5 to 4.5 Good
3. 3.0 to 3.5 Medium
4. Below 3.0 Doubtful
CARBONATION TEST PROCEDURE :
The powder of concrete is obtained by drilling inside into concrete at selected location. Then the collected powder is made moist & then phenolphthalein indicator is dropped on it to check any colour change. If the colour changes to pink, indicates that concrete is not affected by carbonation & if no colour change is observed, indicates concrete is affected by carbonation.
Half-cell Potentiometer Test Procedure:
The half-cell potentiometer consists of a rigid tube, which contains a copper rod immersed in a copper sulphate solution. This is connected to a voltmeter and another live wire connection comes through voltmeter to connect it to rebar. To start the experiment firstly the live wire is connected to a rebar of the test specimen and the rigid tube is put on the surface of concrete and the reading of voltmeter is taken. Reading gives the potential difference between the electrodes. From the value of the potential difference, corrosion status inside the concrete can be predicted. The possibility of active corrosion is found out according to guideline below :
Half cell potential (mV) reading Percentage chance of active corrosion
< -350 90%
-200 to –350 50%
> -200 10%
COVER METER TEST PROCEDURE:
The instrument used is PROFOMETER - 4, Rebar Locator Model S, manufactured by M/s. PROCEQ SA, Switzerland, which is able to perform following functions :
To locate the bar accurately.
To assess the clear cover to the bar.
To calculate bar diameter of the selected bar.
The instrument works on magnetic principle & has limitations of spacing between bars to identify the bars individually.The limitation of rebar locator instrument to identify bars, its diameter is that depth of rebar’s from concrete surface should be less than to 70 mm depth & spacing of bars should be more than 150 mm.
Sample Visual Observations
Chajjas are severely affected by corrosion.
Severe corrosion cracks are developed in columns.
Top level slab is severely affected by corrosion, cover of concrete has spalled down and steel is exposed.
Front side Chajja throughout the length of structure is severely affected by corrosion.
Top level beams are affected by corrosion.
almost 100% columns in the top floor have corrosion related distress.
Inferences & Remedial measures
1. Polymer Modified Mortar Treatment
2. Jacketing to columns – Micro-concrete.
3. Recasting of Slabs/ Chajjas
4. Water proofing Treatment