Dr K M Soni
Chief Engineer,
CPWD, West Zone I, Mumbai
Many structures have been constructed in the country without any structural or proper structural design. In addition to this, there are many structures constructed with poor quality, in terms of materials, workmanship and design. Such structures show distress after a few years of construction. Rehabilitation and retrofitting becomes a necessity in such structures, though a costly affair. In case structures are not rehabilitated in time, deterioration of such structures accelerates at a faster rate. Since a large number of such structures exist in the country particularly in semi urban and urban areas, this is high time that rehabilitation is carried out on large scale, both in government and private sectors. A brief on rehabilitation and retrofitting of such structures is given in the following.
Causes Of Deterioration Of Concrete
RCC is not considered an impervious material and therefore penetration of water and/or aggressive chemicals during the service life of structures are considered primary reasons of deterioration of concrete. Penetration of water or chemicals leads to carbonation, chloride ingress, leaching, sulphate attack, alkali silica reaction etc., and causes deterioration of the concrete and corrosion in the reinforcement. But one must understand that main reason of penetration of water or chemicals is porosity of the concrete. Therefore, it is to be examined whether sufficient precautions can be taken to avoid penetration of water.
Water penetration in the concrete is stopped by taking various measures. No concrete surface is left exposed until it is rich in mix. Roof is covered with water proofing materials as it attracts maximum quantity of water. External surfaces of concrete are plastered and then a water proofing coating is applied on the plastered surface like water proofing paints. Internal surfaces attract less water hence internal concrete surfaces are plastered or rendered and then a protective coating of paints is applied over the same. Thus, penetration of water or chemicals is not feasible as the surface becomes almost impervious with all such measures. Therefore, penetration of water or chemicals is not a direct cause of deterioration but it is the failure of the concrete or plaster or protective coating which allows penetration of water or chemicals. Thus, when construction is not carried out as per the design and specifications, it helps in penetration of water or chemicals in the concrete. Therefore, deterioration of concrete is due to poor quality of design, construction or maintenance. Poor quality gets reflected in the form of seepage, leakage, corrosion, porosity, carbonation of concrete surface etc.
Design of a structure is based on the knowledge available in terms of books, codes and ultimately designer. Normally, codes are prepared once a technology and design are proved theoretically, in the lab and also through mock up structures or prototype. Sometimes, it is observed that design philosophy changes due to unknown factors, which come in the knowledge afterwards. Therefore, codes are revised. In such cases, retrofitting of structures is required.
Once the design is finalized, execution has to be done as per the design and properties of the materials considered in the design. These are called the specifications. Thus specifications of materials are to be selected according to the design requirements and quality has to be ensured during the construction. Design issued by the structural designers is for foundation, slab, beams, and columns. Therefore, in RCC frame structures, other members are constructed according to the specifications or general practices. Suppose design of a lintel, chajja or fins is not given then at site, the same may be cast according to the specifications or general practice or even as per the will of the site in-charge. In such a case, adequate bearing may be provided in a lintel or may not be; brick work may be as per the specifications or may not be. In case these are not provided as per design requirements or specifications, they become highly vulnerable to deterioration. Similarly, in load bearing structure, in case design and detailing of bands, and corner reinforcement are not given, the same may not be provided as per the design requirements. Therefore, these members start deterioration at an early stage and even may become source of deterioration for other structural members. Poor quality construction requires frequent repair and after some time rehabilitation.
Water ingress into external RCC members is not easy. In roof, over the RCC slab, water proofing is done which does not allow water ingress. In columns and beams, over the RCC surface plaster and paint do not allow water ingress. Therefore, all RCC surfaces have protection layers. Still if water ingress is noticed in RCC members, it means that all protection layers have failed. Even after failure of protection layers, steel is protected with concrete cover and if water ingress causing corrosion in the reinforcement is noticed, it means, concrete cover has also failed. Thus, corrosion in the reinforcement means:
- Quality of paint is poor
- Quality of plaster is poor
- Quality of concrete cover is poor and/or
- Quality of RCC is poor
Quality of materials used in plaster and RCC include quality of cement, reinforcement, sand, aggregate, water and admixtures. Quality of workmanship is also equally important. Then, there is quality of procedure i.e., compaction, curing, and temperature during its placement etc. Thus, when quality is being discussed, it includes all of them. A quality structure is durable and requires normal maintenance during its service life but a poor quality structure needs heavy and frequent maintenance. Repair is to be carried out more often which is very difficult and costly affair, that too, normally carried out by small contractors and thus in such structures, rehabilitation of members becomes necessary. During earthquakes, such structures may show large distress due to repetitive dynamic loadings and thus may require seismic retrofitting also.
If quality of RCC is not poor but quality of protection layers is poor or protection layers are not provided, RCC members may be attacked by environmental factors. For example, if RCC members are not painted and surfaces remain wet and dry for considerable period, water ingress may deteriorate RCC. Thus timely maintenance and its quality are important during the service life of a structure to prevent deterioration of concrete.
Structures also get deteriorate due to aging as aging reduces various strengths and resistance to environment. Hence, aging structures require repair and rehabilitation.
Therefore, causes of deterioration of RCC can be summarised as:
- Inadequate design
- Poor quality
- Inadequate maintenance
- Aging of structures
Structures which are attacked by more number of causes as mentioned above deteriorate fast.
Decision On Repair And Rehabilitation
Cost of distressed structures depends upon the quality of the design, construction, maintenance, and aging of structures. A structure having design deficiencies requires retrofitting as per latest codes. As per the guidelines for repair, restoration, condition assessment and seismic strengthening of masonry building “As a thumb rule, if the cost of repair and seismic strengthening is less than about 30% of the reconstruction cost, retrofitting is adopted”. This cost may be only 5 – 6% of cost of reproduction of a building if only seismic members are to be provided but repair and rehabilitation is also to be carried out, it may lead to a very high cost. In case of poor quality construction, rate of distress is very high and cost of repair, rehabilitation and retrofitting is also very high. Also, such structures cannot be brought to the required quality level. For example, if the joints of brickwork have not been filled up properly, any rehabilitation work such as grouting may not fill up all the hollow joints.
Sometimes, it becomes economical to go for reconstruction but due to some other considerations such as non availability of buildings for the users, short life of structures, and litigation etc., repair, rehabilitation and retrofitting are carried out. Even after repair and rehabilitation, such structures need continuous maintenance and may not last for originally designed life.
In the structures where quality has been taken care of in the design and construction but not during maintenance period or maintenance is not carried out as per the frequency, repair and rehabilitation may be required due to distress in the structures but cost of repair and rehabilitation is comparatively low in such cases. In case of aging of structures, cost is also not very high though it depends upon the elapsed and remaining life but in case such structures have been constructed with poor quality or maintained poorly, cost becomes very high.
Structural Evaluation
Structural evaluation is carried out through condition survey and non destructive testing. Condition survey is carried out at four stages through preliminary inspection, planning, detailed visual inspection and field and laboratory testing. Non destructive testing tests include tests for insitu compressive strength like rebound hammer test, ultrasonic pulse velocity test, Windsor probe test, pull out test, core tests and load tests. Tests for chemical attack include carbonation tests, chloride test and sulphate test. Corrosion potential assessment can be made from cover meter/Profo meter, half cell method, and resistivity meter. Normally a series of tests or a combination of tests or all tests are carried out based on condition assessment and importance of the structure. Though interpretation of tests is important and to be carried out by the experts, testing procedure is equally or more important as incorrect testing procedure may provide incorrect results. Problem sometimes is that field testing is not normally carried out by the experts themselves. Sometimes results are so erratic that one may require retesting. Results also vary with the equipments used, calibration conditions of the equipments and site conditions and accessibility of the members. Hence, there are numerous factors on which correctness of indirect tests depends and thus one has to be very cautious while doing such tests and interpreting results based on them.
Materials For Repair And Rehabilitation
Selection of materials for repair and rehabilitation depends upon many factors such as:
- Technical requirements
- Cost
- Availability
- Expert’s advice
- Importance of the structure
- Balance life of the structure
- Toxicity
- Aesthetic requirements
Technical requirements of materials used for repair may include their shrinkage properties, compatibility to base materials, setting and hardening properties, workability, bond strength, thermal expansion properties, mechanical strength, curing requirements, permeability, and durability. Most of the materials used for repair are cements, admixtures, polymer modified mortars/concrete, aggregates, polymers, epoxies, resins, grouts, plasticizers, steel in the form of reinforcement, sections, nuts/bolts, wire mesh, metallic sheets, glass fibre sheets, carbon fibre sheets, geo-synthetics in the same form or in modified form.
Rehabilitation And Retrofitting Methods
Repair and rehabilitation methods are to be planned carefully and to be followed in the required sequence. Structural repairs are to be carried out first and thereafter retrofitting works for seismic requirements and then repair of non structural members. Finally, repair and rehabilitation of architectural components should be taken up. Such a sequence is followed as repair of non structural members initially may cover up structural cracks or members requiring structural strengthening.
Weak protective surfaces and materials like concrete, plaster, water proofing materials, and corroded steel are to be removed first. Repair is thereafter to be carried out by ensuring design requirements, compatibility of materials and also other factors mentioned earlier. Repair should also be compatible to design requirements. For example, if a lintel is to be repaired, it should be compatible to seismic retrofitting, if being carried out. For repair of all distressed structures, no standard method and materials may be listed but depend upon structure to structure.
During rehabilitation of distressed structures or retrofitting, following methods may be carried out:
- Sand blasting to remove rust
- Binding/adding additional reinforcement
- Binding of wire mesh
- Welding
- Anchoring to the existing members through shear keys or anchors
- Shotcreting
- Plate bonding
- Jacketing
- Fibre wrapping
- Underpinning
One should take decision judiciously on the method of rehabilitation or replacement of structural members. For example, a slab will always cost more in rehabilitation compared to replacement. Slab may require sand blasting to remove rust of the reinforcement, anti rusting coat, additional reinforcement, binding coat, welding, anchoring or shear key and shotcreting. Shotcreting itself is costlier than new slab, hence rehabilitation becomes very costly. Thus, it should be examined whether slab replacement is feasible. In case of roof, other factors like water supply system, water proofing etc are also to be considered. Sometimes, repair and rehabilitation is to be carried out in a roof slab, without disturbing water supply system to other residents, hence replacement is not feasible even if it economical. Many times, heavy “I” beams are provided to support distressed slab and wire mesh is inserted above the “I”beams (Fig. 1). Such system provides a feeling of unsafe structure. Also, there becomes a large distance between I beams, and wire mesh itself may sag after a few years. Hence, it is recommended that angle sections should be provided at the ends and T sections in between in shorter direction (Fig. 2) and wire mesh provided on top. Thereafter shotcreting can be done. Small sections at closer intervals do not allow wire mesh to sag due to small spans between beams and in future, it becomes easy to repair small portion if needed. Small portions of slabs may be rehabilitated by tying additional reinforcement but tying should be proper; else welding should be preferred with the existing reinforcement. In case wire mesh is needed, it may also be anchored in the existing slab by providing washers made of MS flats in case welding is not carried out. Mere binding additional reinforcement to the existing reinforcement with binding wire does not serve the purpose for long as binding wire gets corroded after sometime and thereafter there is no monolithic action between existing reinforcement and additional reinforcement.
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Beams cannot be replaced easily hence they are rehabilitated. Binding of additional reinforcement has to be ensured as additional reinforcement is part of the design to take up the loads. Welding, though costly, ensures monolithic action of additional reinforcement with existing reinforcement. In case of jacketing of columns, two precautions are to be ensured – one enlarging the foundation as per the requirements of new columns and other monolithic action of existing reinforcement and additional reinforcement through welding or anchoring and providing additional bent up bars (Fig. 3) or shear keys. In case of non monolithic action, cracks will appear between existing section and additionally jacketed section after some time. |
Thus, main procedure to any rehabilitation work is removal of loose materials and rusting, providing protective layer, bonding coat between old and new surfaces, providing steel members/welding/anchoring/shear keys to ensure monolithic action between old and new reinforcement and concrete work. Two surfaces may be joined through welding, anchoring, providing shear key, adhesives or bonding coat according to the surfaces and materials used in rehabilitation work.
Retrofitting is also done through plate bonding, and fibre wrap techniques. Materials used in plate may vary according to the requirements such as metallic, glass fibre or carbon fibre. Fibre wrappers are wrapped around the structural members after repair and rehabilitation of distressed members and may be of different materials such as carbon fibres, glass fibres etc.
Masonry structures are rehabilitated and retrofitted with similar materials but with different techniques. Horizontal seismic belts are provided in lieu of plinth band, lintel band and roof band. In case of RCC slab, roof band is not required. Vertical belts are provided at corners and around openings. Vertical reinforcement bar is provided at inside corners. These bands are essential for seismic retrofitting or strengthening. For monolithic action segmental arches are to be connected. Main precaution to be taken is to ensure that these belts do not get detached during shaking due to seismic forces hence anchoring of belts plays an important role. Anchoring can be done with mechanical anchors or chemical anchors. Mechanical anchors may damage brickwork and thus chemical anchors (Fig. 4) are most suitable in brickwork though mechanical anchors can be provided in strong concrete.Materials for belts may be Galvanised wire mesh or fibre sheets. Fibre sheets are costly but they do not require micro concreting/thick mortar for its covering. Also micro concrete/thick mortar over wire mesh forms bands (Fig. 5) on the surface while fibre sheets have small thickness and can be fixed with high quality adhesives.
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| Figure 4: GI Wire Mesh, Anchor and Washer for Providing Seismic Belt |
Figure 5: A Band Formed Around Opening due to Micro-Concrete/Thick Plaster over GI Wire Mesh Belt |
Conclusions
There are large number of un-engineered and engineered structures constructed with poor quality of design, materials and workmanship. Some structures are not even maintained with proper quality. All such structures are highly prone to distress and require frequent repair and rehabilitation. Such structures cannot be demolished due to high cost involved in reconstruction and non availability of assets. Thus, there is a need to develop simple and user friendly guidelines for municipalities, individuals, engineers, architects and contractors. A brief on repair and rehabilitation methods of structures including causes of distress are discussed in the paper.
References
- Rai, D C, Draft code with commentary on seismic evaluation and strengthening of existing buildings, IIT Kanpur.
- Handbook on repair and rehabilitation of RCC buildings, Central Public Works Department, New Delhi.
- Guidelines for repair, restoration, condition assessment and seismic strengthening of masonry buildings, Arya, A S.
- Soni, K.M.& Khatri, N.K.(2006). “Retrofitting of a Brick Masonry Bungalow in Lutyen’s zone”. New Building Materials & Construction World, Vol. 12, Issue 4 pp140-148.