Question

Why is it important to monitor the structural health of an RCC structure, the codal provisions, stages and steps to carrying out structural audit. Lists different types of steps to be carried out in a lab, pre and posts in the structural audit, your recommendation and references

Answer 1

Structural Audit is an overall health and performance checkup of a building like a doctor examines a patient. It ensures that the building and its premises are safe and have no risk. It analyses and suggests appropriate repairs and retrofitting measures required for the buildings to perform better in its service life. Structural audit is done by an experienced and licensed structural consultant.

Purpose of Structural Audit

 To save human life and buildings
 To understand the condition of building
 To find critical areas to repair immediately
 To comply with statutory requirements
 To enhance life cycle of building by suggesting preventive and corrective measures like repairs and retrofitting.

Bye-Laws

As per clause No.77 of revised Bye-Laws of Cooperative Housing Societies: “The Society shall cause the ‘Structural Audit’ of the building as follows:
 For building aging between 15 to 30 years once in 5 years
 For building aging above 30 years Once in 3 years.

Stages in Carrying Out Structural Audit

1. Study of architectural and structural drawings, design criteria, design calculations, structural stability certificate of existing structures
2. If architectural and structural drawings are not available, as built drawings can be prepared by an engineer
3. Visual Inspection
a) Need of visual inspection:
 to recognize the types of structural d efects
 to identify any signs of material deter ioration
 to identify any signs of structural distress and deformation
 to identify any alteration and addition in the structure, misuse which may result
in overloading
b) Scope of visual inspection
The inspection report should reveal the following listings along with photographs and
sketches.
•) General information of the building
 Name and address of the building
 Number of stories in each block of building
 Description of main usage of building viz. Residential, commercial, institutional
 Maintenance history of the building
•) Structural System of the building
 Sub structure: Settlement of columns or foundations, Settlement of walls
and floors, Deflection and cracks in Retaining wall, Soil bearing capacity through trial pits or from adjacent soil data
 Super structure: Materials used and framing system of structure, identification of the critical structural members like floating columns, transfer beams, slender members, rusting of exposed steel and its extent.
 Mention the status of all building elements like beams, slabs, columns, balconies, canopy, false ceiling, chajja, parapet and railings with respect to parameters deflection, cracks, leakages and spalling of concrete.

Likewise, verify the status of water tank, staircase, lift and lift machine
room.
•) Addition or Alterations in the building
 Identification of change of occupancy
 Alteration or addition of partition walls
 Alteration or addition in loadings- stacking
 Alteration or addition of toilets, water tank
 Alteration or addition of balcony
•) Dampness and leakages
 Detect the dampness in walls
 Identify the leakages in Terrace, toilets, plumbing lines, drainage lines and
overhead tanks.
4. Non Destructive and Destructive Testing

In addition to visual inspection, the real strength and quality of a concrete structure
need to be checked with non-destructive tests. A number of non-destructive tests (NDT) for concrete members are available to determine present strength and quality of concrete. Some of these tests are very useful in assessment of damage to RCC structures subjected to corrosion, chemical attack, fire and due to other reasons. These tests have been put under four categories depending on the purpose of test as under:
a) Concrete Strength
 Rebound Hammer Test: To measure surface hardness of concrete
 Ultrasonic Pulse Velocity Test: To assess homogeneity of concrete, to assess strength of concrete qualitatively, to determine structural integrity
 Core Sampling and Testing: To measure strength, permeability, density of concrete.
b) Chemical Attack
 Carbonation Test: To assess depth of carbonation and pH of concrete
 Chloride Test: To assess total water/acid soluble chloride contents
 Sulphate Test: To assess total water/water soluble sulphate contents of concrete
c) Corrosion Potential Assessment
 Cover Meter: To measure cover of reinforcement, diameter of reinforcement and spacing of reinforcement
 Half Cell Method: To assess probability of corrosion in the embedded steel
 Permeability Test: To assess permeability of concrete due to water and air

d) Homogeneity and integrity Assessment
 Ultrasonic pulse velocity for determination of cracks and discontinuities
5) Core Testing
This is direct method of assessing strength of concrete. In this method cylindrical core
samples are taken from existing structures. The cores are visually inspected and tested
in laboratory to check its compressive strength.
6) Pushover Analysis
Generally Push over analysis is used to understand the existing capacity of structure
for seismic and gravity loading which will show different occupancy levels like
immediate occupancy, life safety and collapse prevention. The seismic evaluation of
existing buildings compares their capacity against earthquake demand at specific site
and concerns the potential earthquake-caused risk to building systems and elements
that are closely related to human life safety.
7) Identification of critical areas in building
Based on the above inspection, analysis and test results, the report should conclude the
critical areas that need immediate repairs and retrofitting. For example: number of
columns requiring immediate repair and strengthening, repair of critical slab and
beams, water proofing of terrace, toilet blocks, cracks in walls or structural elements
etc.
Post Structural Audit

1) Repairs
Based on the audit findings and recommendations different measures of repairs and
strengthening are carried out. According to ACI 546R-04, to repair is to replace or correct deteriorated, damaged, or faulty materials, components, or elements of a structural system. From this point of view, repair may be divided into structural repair and serviceability repair. The former refers to the restoration of lost sectional or monolithic properties of damaged members, while the later refers to the restoration of structural surfaces to a satisfactory operational standard. Obviously, poor design, poor construction, poor maintenance, incorrect usage, new environmental influences or an intended increase of the loading or extension of the structure’s lifespan can make repair and/or strengthening necessary. Excluding technical considerations, the ultimate choice of method of repair and
strengthening of a concrete structure may also be influenced by factors like overall quality of repairs and the size of individual repairs, access for repair, relative cost, ease of application, available labor skills and equipment and client requirements including future maintenance and economic considerations

2) Strengthening and Retrofitting
Strengthening is the process of restoring the capacity of damaged components of structural concrete to its original design capacity, or increasing the strength of structural concrete. Strengthening of a concrete structure may be required due to several reasons:
 Change of usage which may cause over-stress in the structural member.
 Serious materials and structural deteriorations which cause structural members to be no longer able to carry the imposed loads with an adequate factor of safety
 To increase the capacity for seismic resistance if the building is not designed for it or the structure does not fulfill current design requirement corresponding to seismic zones, R factor or so.
Strengthening of structural members can be achieved by replacing poor quality or defective material with better quality material, by attaching additional load-bearing material, such as high quality concrete, additional steel, thin steel plates, various types of fiber reinforced polymer sheets, and so on, and by the redistribution of the load such as by adding a steel supporting system. The purpose of strengthening is to increase the load-carrying capacity or stability of a structure with respect to its previous condition.