With the increase in the construction activities in the last two decades mainly in the building sector, it has been observed that many of the structures are already showing signs of distress. In some cases repair measures become necessary even within a span of 5 to 10 years of completion of structure. This may happen due to host of factors like bad quality concreting materials poor workmanship, lack of maintenance, atmospheric effects, abuses, accidents or natural calamities. Thus retrofitting of a concrete structure becomes necessary to extend its life to ensure durability of the structure. The rehabilitation envisages restoration of structural system as close as possible to the original position. The distressed structure needs to be brought in line, level and to required strength so that it can be put into service without endangering its safety and utility.
Looking to the magnitude and complexity of the work involved in the restoration process, the requirement of various technology & materials is going to take a giant leap in years to come. The concept of retrofitting & rehabilitation has also been accepted as one of the important discipline of the construction industry.
The article presents one such case study of a clusters of buildings in Delhi where concrete had deteriorated due to poor concreting practice, quality and lack of maintenance. It also gave details of systematic repairing solutions.
Based on the study of critically affected buildings, corrosion of reinforcement has been the prime cause for deteriorations of RCC member. The corrosion occurred mainly due to leakage/overflow of water from service water tanks and leakages from roof penetrating into porous concrete has been the common cause for corrosion in all the buildings. Severe distress was also due to lack of concreting knowledge, ignorance about resultant effects & its maintenance.
Most of the residential buildings in that area are less than fifteen years of age & showed variable distress. The areas of distress in all the buildings are of same nature. Concrete at several places especially on the columns, mumty holding the water tank, staircase & roof are the main areas where the concrete clearly showed aging, severe honeycombing, poor compaction, corrosion resulting in spalling of cover concrete.
Fig.1: Distress Water Tanks, Balcony Roof & Roof
In some of the roofs, the cover was not adequate to protect from the reinforcement. Exterior Column-beam junctions of critically effected building developed vertical cracks on almost all the faces. Also column of balcony has the vertical cracks where the concrete showing aging, honeycombing, poor compaction, corrosion of reinforcement resulting in spalling of cover concrete. (Figure 1 & 2).
Fig.2: Photographs of Critically Affected Buildings
In some places especially on the mumty columns, the corrosion has been very heavy due to the ingress of water from the over flowing water tanks. The diameter of the bar has been reduced to less than 50% of its original diameter. On the staircase, the concrete was at the threshold of getting corroded. Cracks have also started developing on the slopped portion.
Majority of the distress happen to be related to severe corrosion to the reinforcement. The corrosion reaction i.e. localised breakdown of the passive film (cathode) on steel, which mainly starts from the ingress of moisture, penetration of chloride & carbon dioxide through porus concrete.
Corrosion of steel in concrete is an electrochemical process, which takes place as a result of the formation of a corrosion cell. (Figure 3 )Fig.3: Corrosion Cell
Mechanism of corrosion (Figure 4) shows the anodic reactions where metallic iron (Fe) transformed to rust (Fe2O3). The rust formation on the surface of reinforcement is accompanied by an increase in volume, may be as large as 6 times the volume of Fe. Sound concrete with pH between 11.5 & 13.5 is an ideal environment for protection of steel but as stated above alkalinity can be lost due to the formation of alkali carbonates, sulphates & chlorides & also due to anodic reaction takes place in the presence of moisture. The Anodic reaction can be explained from the following chemical reaction:
3Fe + 4H2O Õ Fe3O4 + 8H+ + 8e-
2Fe + 3H2O Õ Fe2O3 + 6H+ + 6e
Fig.4: Mechanism of Corrosion
In other words, if the carbonated front penetrates sufficiently deeply into the concrete to intersect with the concrete reinforcement interface, protection is lost and, since both oxygen and moisture are available, the steel is likely to corrode. The extent of the advance of the carbonation front depends, to a considerable extent, on the porosity and permeability of the concrete and on the conditions of the exposure. In the case of carbonation, atmospheric carbon dioxide (CO2) reacts with pore water alkali according to the generalized reaction,
Ca(OH)2 + CO2 Õ CaCO3 + H2O
It consumes alkalinity and reduces pore water pH to the 8–9 range, where steel is no longer passive.
|Fig.5 (a): Extent of Corrosion||Fig.5 (b): Addition of Steel|
Approach to Repairing Method
Since repair & rehabilitation of a structure expects to set right the damaged areas, it is therefore obvious that repairs need assured inputs of right material, quality workmanship & last but the least the proper system or specification to be adopted in a phased manner. In this case, a complete retrofitting plan of constructional defects was chalked out based on the investigation. Main treatments were required for strengthening of the structure, arrest of the corrosion, cover protection & consolidation of the concrete. Since no adequate treatment was carried out for the water tightness during the construction & its maintenance afterwards, special emphasis was given on the consolidation of the concrete. Among various techniques & newer material available for restoration of distressed concrete structures, it was proposed to use cementitious repairing material for cover & low viscous epoxy as grouting material for consolidation purely because of the structural and thermal compatibility and similar physical properties to those of the parent concrete.
Since each material has its own characteristics, it requires very careful study for selecting right material for a particular application. While selecting a system one should be careful to evaluate that whether it meets all the requirements such as placing problem, strength development & durability. For any repair & rehabilitation job, it is always better to take the manufacturer into confidence on the various aspect of the material & its suitability for that particular type of job. Based on the investigation & type of building, a detailed system for treatments were chalked out. The system included type of material, tools to be used, work schedule & rehabilitation design. The repair was carried out under strict supervision of quality control experts.
Material Used for Retrofitting
During the rectification process, large number of materials staring from corrosion inhibitor to micro-concrete were used at different stages. The selection of material does contribute to the performance of the repairing system in a long term basis.
It has been observed in many cases, the compatibility of various chemicals with cement depends on the physico-mechanical behavior of the system & plays an important role in the performance. In this case, the following products were used.
- Rust converter: it converts the rust on the reinforcement to a protective chemical barrier & dissolve the ferric oxide.
- Corrosion Inhibitor: it slows down the corrosion reaction in the concrete & help in regain its alkalinity by creating a passive film
- Low viscous epoxy: Low viscous epoxy used for densify the concrete, consolidate the structure by filling up the voids, fissures & capillaries.
- Zinc rich primer: Two component system used as an inhibitive epoxy primer used in coating of reinforcement.
- Bonding agent: Two component epoxy used as bonding agent between old & new concrete
- Micro concrete: High Performance high strength self-levelling concrete used for strengthening by jacketing various places to minimize the porosity & enhancing the density.
- Protective Coating: Polyurethane based clear coating. This coating protects the concrete from aggressive attack such as ingress of moisture.
- Acrylic Polymer: Single Component Acrylic polymer for cement based waterproofing system for providing effective & economical cementitious acrylic plaster.
Following pre-treatment guidelines were chalked out prior to the commencement of treatment. This was done to ensure the safety of the structure & also to make proper working conditions.
- a) Erection of scaffolding – This shall be done keeping in mind the locations where repairs are to be carried. Adequate precautions are necessary while erecting the scaffolding to facilitate proper working conditions.
- b) Repair to column – The repair shall be undertaken from bottom and then progressed upwards. Column length between floors shall be considered for repair only after column below are repaired.
- c) Repair to beams, slab and stair case waist slab- after completing column repairs, works of beams and slabs are to be undertaken.
- d) Before undertaking the repair work, appropriate propping must be provided preferably using steel pipes. The supporting structure should such that it should transfer the load coming on the member under consideration. Locations of props should be such that the proper force flow maintained. This aspect is important to ensure safety during the course of repairs. The slabs and beams transferring the load to the particular column shall be supported before the repair to that column is undertaken.
Methodology of Treatment
Surface Preparation of Concrete
Reinforcement was exposed by removing all loose and honeycombed concrete. All spallings were removed. The concrete was chipped off to a minimum depth of 25mm behind the reinforcement. This was required to be done to treat the corroded reinforcement. Concrete surface & reinforcement were washed with water jet to remove all soluble & insoluble salt such as chlorides present as indicated in the report.
Surface Preparation of Reinforcement
Removal of the rust from reinforcement completely by rubbing with wire brush or emery paper. Proper cleaning of the existing exposed reinforcement with rust converter. This de-rusting compound starts reacting within few minutes after application and converts the rust into a non-metallic complex compound of black colour. It forms non-porous, impermeable and moisture repelling coating of good mechanical stability. It shall be applied liberally by brush till such time it penetrates through the rust layer. Though the reaction starts within few minutes, it generally takes 6 hours to complete the same. However, next activity should start only after 24 hours.
Provision of Additional Reinforcement
In case of severe corrosion (diameter of reinforcement bar is reduced substantially say more than 20%), the affected bars were anchored with equivalent additional reinforcement by welding. This was done in consultation with structural Engineer at site. (Figure: 5b)
Application of Corrosion Inhibitor
The concrete was porus. Due to the ingress of acidic water, the pH value of the concrete slowly got reduced. In many areas the pH was observed to be below 10 when concrete tends to be acidic & accelerates the process of corrosion (refer Figure 4). This was taken care by injecting corrosion inhibitor through non reversible packers into the concrete. With this process, pH of the concrete tends to increase to safer level of 12.5 & 13. The process involved drilling of holes at a grid pattern on the concrete & non reversible packers were fixed using fast setting cementitious compound. In case of very poor concrete, the nozzles were fixed after the strengthening of the structure. (Figure 6)
Fig.6: Injection of Corrosion Inhibitor Through Non Reversible Pump
Treatment of Reinforcement
New & old reinforcement were treated with zinc rich epoxy. This anticorrosive epoxy coating acts as a barrier and also stops propagation. The coating was applied in a manner such that the dry film thickness was around 60micron.
For consolidation of weak concrete & strengthening the, pressure grouting to be carried out through the non-returnable nozzles of 6 to 12 mm dia. at a spacing of 400-500 mm c/c in zigzag pattern by drilling operation. During drilling & fixing of nozzles, precaution to be taken that damage to the concrete should be minimal. Minimum Depths of nozzle should be 40 to 50 mm. Firstly corrosion inhibitor was injected into the concrete. Once the treatment is over, subsequently low viscous epoxy was injected in to the concrete at a pressure 2 to 3 kg/cm2. Once the grouting was over, the nozzles were removed. Pressure grouting operation is to be carried out before polymer cement mortar plaster (cover concrete) and plaster.
|Fig.8: Fixing of Nozzles After Filling Up with Micro-Concrete|
Provision of Bonding Agent
Prior to the application of Cover mortar plaster/micro concrete, the existing surface was treated with a coat of epoxy based bonding agent to bond old to new concrete. Bonding agent was applied on the surface of old concrete after removing all loose concrete & some hacking was done on it. This epoxy bond coat also acts as a barrier preventing entry of chlorides and other harmful agents.
Jacketing & Strengthening of Columns Using Micro Concrete
In case of severe damages 50 mm or thickness or as per site requirement, micro-concrete to be used for jacketing the columns. Micro-concrete, cementitious non shrink free flow high strength micro concrete with ultimate compressive strength of 650kg/cm2 was used. In some cases, it was substituted with maximum of 25% of 4.75 mm downgraded aggregates. The proportion of the mix in many areas varied between 10 to 25% based on the site conditions. This was done as per the manufacturer’s instructions in the product data sheet. For bulk pouring, mechanical mixer too be used or can be mixed manually for smaller requirement. Water / Powder ratio to be adjusted between 0.14 & 0.18 depending on the site condition. Physical properties of Micro-concrete used is given in Table 1.
Repair to Columns
The first step in repairs was undertaken by repairing the column starting from the ground floor. Repair to columns was undertaken one by one. The most distressed column in the structure be taken first. The repair process shall be as under:
- Removal of all plaster over the distressed column.
- Remove cover and concrete up to 25 mm clear from the main reinforcement bar using electric cutter/pneumatic chipper or manually to completely expose the reinforcement.
- It has to be ensured that during the removal of cover, there should not be any damage to the concrete element.
- Quantification of deterioration/distress
- Treatment of columns to be taken up as per the suggestions given below
(a) In case the distress at the location is restricted to single face no jacketing will be required. Repair be carried out as given in sketch
|(b) If the distress is more than on face of a column and reinforcement is corroded less than 20 % no new reinforcement is needed only Jacketing is required as per sketch ”A” shown below. If the corrosion is more than 20 % the treatment to be done as per sketch “B”|
|A – Column Section : When Corrosion Is Less than 20%|
|(c) If Corrosion is More than 20%|
|Fig.7: Jacketing of Columns with Micro-Concrete|
c) Cover / Finished Plaster
20mm thick cement mortar plaster admixed with acrylic polymer provided on the micro-
concrete for finishing.
All the treated areas like columns to be cured using curing compound & other areas to be cured with conventional systems.
e) Protective Coating
After the curing, the treated surface are to be provided with a polyurethane based clear coating. Dry film thickness of around 100micron. This coating will protect the concrete from aggressive attack & will make the repaired surface durable for a considerable period of time.
|Fig.8: : P-61 Building before Treatment|
Fig.9 : P-61 Building During & After the Treatment
The repair rehabilitation measure suggested in the report was initially executed in one of the building in Station Workshop, Delhi. It was been periodically inspected since last two years & on the basis of the performance, The authorities had specially included the above methodology in their BOQs for other structures. It is always required to study behaviour of the restored structure and in the present case also the same assessment pattern was undertaken to check whether the affected structural elements were properly repaired or not. The performance of the rehabilitation measure was satisfactory. Some of the photographs are shown below.
The author wishes to acknowledge the contribution of project authority with their inputs while making the specification & also implementing the methodology for retrofitting of severely affected building under his zone.
- Das Supradip & et al, ‘Repair and Rehabilitation of Concrete Structure: A Case Study”, CE&CR, Mar’05
- IS 516: 1959, Method of test for strength of concrete.
- IS 456: 2000, Plain and reinforced concrete – Code of Practice.
- Das Supradip, “Repair and rehabilitation of concrete structures a case study.”: New Building Materials & Construction World,: pp 150 – 157. ( Feb’2007)
- Sivagnanam B. “ Damage Assessment and rehabilitation of concrete structures : Three case studies.”: Indian Concrete Journal ( Dec’2002 )
- Das Banabir & Rajendra Kumar (Dec’2003): Civil Engineering & Construction Review, “Rehabilitation of Leaky Basement-A case study.
- Das Supradip: Rectification & Consolidation of water retaining structure: case study
- Das Supradip, “ Repair & rehabilitation of structures- causes of distress & its solutions”, NBM&CW, pp 118 – 136 ( Feb’2014 )
- Handbook on Repairs & Rehabilitation of RCC Buildings – Central Public Works Department 2011