TECHNICAL ARTICLE

REPAIR AND REHABILITATION OF BUILDINGS


Authors
Sushil K Dhawan
Chief Engineer, CPWD

Body

 The construction activity in India has been increasing geometrically without matching increase in the availability of quality inputs, in terms of materials and skilled workmen. The gap between the quality planned and the quality achieved continues to widen. The factors contributing to damages/distress due to various causes including leakage/seepage have, thus, become intrinsic right from the construction stage. Public perception about quality of construction by Government Departments is not good. This is reflected from the observations of the Director of AIIMS in one of the meetings, wherein quality was specifically emphasized with the assertion that the buildings assigned to CPWD for construction are to be planned and executed without any signs of leakage/seepage. The buildings were two hostel buildings, one for Nurses and the other for doctors of All India Institute of Medical Sciences in 1986. 

2.    Causes of deterioration and durability Aspects :
 
Concrete is a composite material comprised of embedded particles of fragments of aggregates essentially with a binding medium. In cement concrete, relevant to RCC structures, the binding medium is the mixture of hydraulic cement and water.
 
2.1         There are three stages of deterioration of RCC involving
-       Gradual loss of water tightness due to seasonal temperature variation and enhancement of interconnected porosity of cement concrete,
-       Deterioration of chemical characteristics of concrete due to ingress of aggressive environmental chemicals in intermittent presence of water and
-       Corrosion of steel reinforcement, cracking, spalling and loss of mass resulting in partial loss of strength and stiffness.
            Deterioration due to concrete permeability is on account of
                                    *           Capillary Porosity     -           High w/c ratio
                                                                                    -           Inadequate curing
                                    *           Air Voids                    -           Improper compaction
                                    *           Micro cracks              -           Loading effects
                                                                                    -           Weathering
                                                                                    -           Internal curing
                                                                                    -           Lack of after care
                                                                                    -           Secondary effects
                                    *           Macro cracks             -           Concrete Placement
                                                                                    -           Hardening process
                                                                                    -           Intrinsic chemical attack
                                                                                    -           Corrosion of reinforcement
 
3.    Inspection and Assessment of Damage: The terminology used for assessment of damage in a distressed structure is Condition Survey.
 
3.1       Objective is   (a)       To Identify     -           Causes of distress, whether due
to corrosion, fire, earthquake or any other reason including their source.
(b)       To assess    - The extent of distress
-                  The residual strength of structure.
(C)      To prioritise the distressed structural elements
(d)       To plan the effective remedies.
            3.2       Stages of condition survey –
                        (a)       Preliminary Inspection
                        (b)       Planning Stage
                        (c)        Visual Inspection Stage
(d)       Field and Lab testing Stage.
4.    Commonly observed order of deterioration of buildings
-           Wet/Water stagnating areas subjected to alternate wetting/ drying
-           Beams/Columns exposed to rain and sun
-           RCC elements located on external exposed walls/ slabs etc.
-           Terrace RCC slab with ineffective drainage, insulation, water proofing system.
-           Non-structural exposed RCC elements like chajjas, facias, fins, etc.
-           Leakage/Seepage   -           Due to ineffective drainage system.
                                                -           Due to concealed services
                                                -           Improper terrace treatment.
5.    Analysis :
5.1       Based on record of observations and test results during Condition Survey of distressed structure carried out as above, an analysis is made for following–
                        (a)       Corrosion of embedded steel.
                        (b)       Concrete quality, durability and deterioration
                        (c)        Concrete strength
                        (d)       Structural integrity.
 
5.2       Based on analysis, the need for carrying out Non-destructive Evaluation (NDE) and their location is identified. The test methods for NDE methods commonly adopted are:
- Ultrasonic pulse velocity,
                                    - Pull out test
                                    - Core test
                                    - Chloride test
                                    - Carbonation test
                                    - pH measurement
                                    - Resistivity test.
5.3       Based on interpretation of NDE results with visual observations recorded during condition survey, critical areas are identified for:
                                 - Active and Passive Repairs
                                 - Modeling of repaired comfort structure.
6.    Repair strategies have to be adopted based on local availability of technology and materials
6.1         Selection of Repair Material: While making selection of repair materials, it has to be seen that their chemical and mechanical properties are comparable to those of the substrate.
 
6.1.1     Essential parameters for repair materials
            .           Low shrinkage properties
            .           Workability,
Low air and water permeability
            .           Durability
Materials for repairs are classified based on their application and also based on their composition.
6.1.2     Classification based on Application:
·         Surface preparation
·         Chemical rust remover for concrete reinforcement
·         Bonding agents
·         Structural repair materials
·         Non-structural repair materials
·         Injection grouts, joint sealants
6.1.3     Classification based on composition
·         Polymer Modified Mortars/ Concretes (PMM/PMC): These are used to make up the damaged/lost cover concrete due to their better bond with substrate, including the reinforcement.
·         Epoxy resin mortars
·         Polyester resin mortar
·         Guniting/ shotcrete
·         Protective anti corrosive and water proofing coatings
6.2         Execution
6.2.1     Carry out structural Repairs to:
                                      (a)      RCC Slabs as per Appendix 6.1 of Hand Book.
                                      (b)      RCC beams as per Appendix 6.1 of Hand Book.
                                      (c)      RCC columns as per Appendix 6.1 of Hand Book.
                                      (d)      RCC parapet wall – The wall to be reconstructed by using super plasterizer admixed concrete mix 1:1.5:3 with W/C ratio not exceeding 0-45.
6.2.2     Carry out non-structural repairs to:
                                      (a)      Remove the vegetation growth from roof and other places to avoid further deterioration of building.
                                      (b)      Routine maintenance related defects
                                      (c)      Shift concealed service pipes to external face of walls.
7.    Protection : Protective measures for preservation and extending the service life of the structure such as
                                      .          Damp and water proofing. Depressed floor treatment.
                                      .          Step by step sequence of stages for treatment of khuras at terrace.
                                      .          Surface treatment
                                      .          Creation of Barriers
                                      .          Expansion joint treatment.
8.    Testing of structural repairs
                          8.1       Core Test
                          8.2       Carbonation Test
                          8.3       Rebound Hammer Test
                          8.4       Ultra Sonic Pulse velocity test.
                          8.5       Chemical Analysis of Concrete
9.    Case Study: It is a case of deteriorated six storeyed RCC framed Hostel Building at New Delhi
9.1         Salient Features:
·        Period of construction             :     1978-79
·        Investigation done                   :     1997
·        Type of structure                       :     Six storeyed RCC framed structure with
four wings having residential suites and a central portion comprising of staircase and lift lobby.
·        No of residential suits                    :           132
9.2         Visual Observations
1.    Spalling, cracking and splitting of concrete in RCC elements like columns, beams, slabs etc.
2.    4-6 mm wide cracks in columns at lower most storey near the ground level
3.     Cracks noticed along the reinforcement in RCC structural members and at the junction of RCC and brick work.
4.    At some locations, the diameter of reinforcement reduced due to corrosion.
5.    RCC columns gunited earlier, developed heavy cracks.
6.    The water tank at terrace had earlier leaked continuously.
7.    Spalling of concrete observed at bottom of RCC tank and on walls of tank.
8.    Seepage in the building observed at several places.
9.    Leakage observed in G.I pipelines to supply the water to kitchen.
10.GI water supply pipe lines passing through the garbage chute damaged due to impact of falling garbage, causing dampness in walls.
11.Vertical Stacks of rain water pipes were observed missing from some places.
12.Washed stone grit plaster fell at many places.
13.Grooves of washed stone grit were not finished properly and allowed water to seep in.
 
9.3         Insitu Evaluation and Laboratory Testing:
 
Carbonation Test
 
Carbonation has taken place beyond the reinforcement levels.
 
Rebound Hammer Test
 
The rebound values were as under –
     
                  Rebound Value                                     Number of Readings (in%)
                  15-20                                                                                9
                  21-30                                                                                72
                  31-40                                                                                19
 
                          Ultrasonic Pulse Velocity Test
                  Pulse Velocity (km/sec)                                    Number of Readings (in%)
                  0.0-2.5                                                                  47
                  2.5-3.0                                                                  29
                  3.0-3.5                                                                  12
                  3.5-4.5                                                                  12
                          Half-Cell Potential Test
 
Half Cell potential values were as below-
                  Potential Reading          No. of Readings (in%)        Probability of corrosion
More positive than – 200mV                            77                                      <10%
- 200 mV to 350mV                                           23                          10% to 90%
More negative than – 350 mV                         nil                                      >90%                         
                                                                                               
Chemical Analysis of Concrete
a)    Soluble chlorides ions expressed by weight of cement was 0.213% and exceeded permissible limit.
b)    Sulphate content in mix was 1.293%, which was within permissible limit.
 
9.4         Causes as concluded for deterioration and damage
1.    Poor quality of RCC
2.    Inadequate and permeable cover concrete
3.    Excessive chloride content in concrete
4.    Inadequate level of maintenance
5.    Improper workmanship of external finish.
9.5         Methods of Repair as recommended
1.    Improvement and repair to services
a)    Replace the broken/damaged drainage and water pipes
b)    Replace the worn out GI pipelines.
c)    Relocating of service lines from garbage chute to protect them from damage
2.    Structural Repairs
A)   Slabs
Provide alternate structural system comprising of RSJ’s spaced at about 1.00 metre to support RCC slabs as per Appendix 6.1
B)   RCC Beams
Carry out structural Repairs to RCC beams as per Appendix 6.1
 
C)   RCC Columns
Carry out structural Repairs to RCCcolumns as per Appendix 6.1
D)   RCC Parapet Wall
The wall to be reconstructed by using super plasticizer admixed concrete of mix 1:1.5:3 with w/c ratio not exceeding 0.45.
 
3.    Non Structural Repairs
 
a)    Remove the vegetation growth from roof and other palces to avoid further deterioration of building.
b)    Routine maintenance defects to be attended to.
c)    Conceased service pipes to be shifted to external face of walls.
 
10.Conclusion : Most important after detailed investigations is correct diagnosis.   It would need to adopt correct repair strategy. Therefore, analysis of the inspection and detailed investigations are the most important component of Repair & Rehabilitation of Buildings. Step by step methods of execution of the repair strategy including quality assurance would ensure structurally sound repair and rehabilitated structure.
Reference: Handbook on Repair & Rehabilitation of RCC Buildings – by Anil K Sharma
                     and Published by CPWD.
 
            3.5       Evaluation damage to concrete/reinforcement.
            3.6       Evaluation of building configuration,
                 
3.7         Load test for flexural member
·         Improvement and repair to services :
              - Replace the broken/damaged drainage and water pipes.
              - Replace the worm out G I Pipelines.
·         Relocating of service lines
6.                      Laying down quality assurance requirements

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