Indian Institute of Talent Development
Concrete is the most versatile man-made construction material of our times on account of its flow into the most complicated forms while fresh and its strength and durability characteristics when set. Concrete construction is economical considering the longevity of the structures.
Concrete Bridges are designed to be inherently durable and usually require a minimum of repair and maintenance. However, there are occasions when damage in defects requires remedial treatment to be carried out. Before carrying out any remedial measures on a concrete bridge, it is most important to identify the basic causes of deterioration. This paper is a general introduction to assist the reader in identifying likely causes of defects or deterioration, selection of correct material systems and application methods. An RCC bridge is composed of primarily three components
|Fig.1 : Bridge Components, Substructure, Superstructure and Deck||
The basic causes of deterioration originate due to factors occurring before or during construction or factors occurring post construction. It is very important that the root cause of damage to the bridge be evaluated prior to commencing repair operations. These causes are shown in Figures 2 & 3.
|Fig.2 : Causes of Deterioration Before and During Construction||Fig.3 : Causes of Deterioration occurring Post Construction|
Other faults which can require repair include: insufficient cover to steel, honeycombing or voids in the concrete, blemishes such as conspicuous or excessive blow-holes which are not structurally significant but are aesthetically displeasing.
EN 1504 Guidelines on Assessment of RCC Structures [Including Bridges]
EN 1504 is the new European Standard for the protection and repair of reinforced concrete. There are ten parts to the standard covering test methods for material properties and specification for the key repair materials, including coatings, mortars, bonding agents and injection materials. It also includes general principles for repair work and a standard for site application of products and systems. The code suggests the following steps for assessing the structure:
- Present Condition: Condition Assessment of the Bridge. Visual evaluation of the members, in case of structural damage, evaluation of the load carrying capacity of the bridge.
- The design approach: What was the design intent of the bridge? Is the bridge performing to design standards? Is it capable of performing through its design life?
- Exposure and environmental conditions the structure is exposed to: Evaluation of physical, mechanical and chemical loading the structure would be exposed to during its design life.
- Assessing the conditions and studying construction records: Evaluate the construction and maintenance records if any on the bridge structure.
- Usage Conditions and history: What was the bridge designed to be used as? Is it serving the purpose? What would be the purpose of the rehabilitation – restoring load carrying capacity or upgrading of the capacity of the bridge?
- Intended future use: What would be the use of the rehabilitated bridge?
Once the evaluations are made, the following courses of action can be taken:
- Do nothing for a certain time
- Re-analyse the structural capacity; possibly down grade the function
- Prevention or reduction of further deterioration
- Improvement, strengthening of concrete structure
- Reconstruction of concrete structure
Principles of Repair in Accordance with EN 1504
When a bridge shows signs of distress or deterioration, the following steps should be taken in principle. EN 1504 recommends the steps for the entire procedure for repairs from assessment to maintenance. A guidance note (No. 4) from the Concrete Society UK recommends:
- Assessment of damage to the Bridge [substructure, superstructure or carriageway]
- Choose Options of Repair
- Choose the appropriate repair Principles based on EN 1504: These principles address the methods for repair of both concrete and methods for mitigating corrosion of the reinforcement, based on evaluated damage.
- Choose the appropriate methods and materials of repair in accordance with the principles mentioned in Section 3 above
- Specify on-going requirements such as maintenance of records of repair, maintenance schedules and periodic evaluation of residual life of the structure
Following this a thorough QA system needs to be established to ensure the durability of repairs.
Basic Steps of Repair
When a structure shows signs of distress or deterioration, the following steps should be taken in principle. The steps are as under:
- Preliminary Investigation, detailed Investigation
- Laying out specifications for repairs
- Selection of Materials
- Surface Preparations
- Actual Repairs
- Periodical maintenance
- Maintenance of Reports etc. for future repairs
Preliminary Investigation and Detailed Investigation
|Fig.4 : Compact Kit for Testing / Diagnosis of
Deteriorated Concrete (Courtesy: MC-Bauchemie, Germany)
The first step is to conduct visual inspection of the damaged structure and to inspect the records. Further surveys to be conducted after preliminary investigation are delamination surveys, crack surveys, compressive strength tests etc. Preliminary investigation and diagnosis are carried out to ascertain the nature of damage and the extent of distress to establish feasibility of repairs. Destructive as well as non-destructive methods are at our disposal for determining the extent of distress. Figure 4 shows a compact kit for evaluation of damage
Unless the cause of distress is established, the remedial measures shall have no meaning. It is the cause that is to be rectified rather than the surface appearance of the damaged structure.
Diagnosis is actually interpretation of the results obtained from the investigations. The interpretation requires sound knowledge and experience in this field and should essentially be done by qualified engineers.
Laying out Specifications for Repair
Since the field of repairs and maintenance is a specialized one, it is very important that proper specifications are laid out for carrying out the remedial measures. The specifications should include:
- 1. Materials for repairs
- Calculations for extra reinforcement for structural repairs
- Materials for injecting the Cracks
- Guideline for surface preparations
- Steps for Repairs
- Precaution to be taken while using the materials as well as the curing procedures etc.
- Supervision and quality control at site
- Scope of work and quantities
Only proper specifications can lead to proper repairs with minimum clashes between the owner and the contractor. Methods of measurement should be unambiguous. A qualified bridge engineer, the executing contractor, the material supplier and the owner of the structure should draw up the specification of repairs together. This would ensure that the design, material, application and contractual needs of the project are completely anticipated and the repair to the bridge structure is durable.
Repair Methods and Materials
Injections For Repairs
Cracks occur in the concrete despite the fact that quality is controlled. Cracks indicated image or distress in the structure. But, not all cracks are a sign of structural failure. Cracks have to be repaired for two reasons viz. for structural or for durability purposes. The selection of material for injection requires thorough understanding of the properties of the material and functions that such a repair has to perform. In all the cases, it is imperative that the cause of crack is properly determined otherwise the selection of material willgo wrong.Injections is the first step in repair program.
The repair of cracks alone cannot guarantee the structural stability or durability of concrete and therefore, if necessary should be complimented with other treatments as per the established practices of civil engineering. Under all circumstances it is advisable to trust these types of jobs to experienced contractors having the knowledge of materials as well as experience in the use of specialized equipment. After completion of diagnosis and selection of materials for injection, the work of injection passes through following stages:
- Preparation of the crack
- Location of points for injection
- Surface sealing of cracks
- Injection of resin
- Removal of packers and plugging
- Removal of sealing material
- Final surface treatment after injection resin/grout hardens
Guideline for Material Selection
Prior to selection of a material and method to remediate a crack or void, the characteristics of the defect need to be clearly assessed. The properties to be assessed include:
- Need for Crack Filling: Enhance load transfer or stop water through crack
- Depth of Crack: Surface or deep cracks
- Crack Width
- Crack Movement: Moving or non-moving
- Condition of the Crack: Dry or damp / actively water bearing
The pattern of the cracks decides the reason for cracking which in turn reflects on the selection of base material. Width of the crack has direct bearing on the viscosity of the material required. It depends on the movements in the crack, which reflects the type of material required whether it should act as structural injection or just an elastic seal. Structural injection should be able to transfer stresses from one crack face to the other. The moisture in the crack calls for a water compatible system of injection.
Materials for Injections: Properties
All cracks are different. They vary depending on the construction material, cause, location and environment. One single system is not able to achieve durable and reliable results. Various solutions based on different materials, which are tailored to specific application needs, are now available to users. Table 1 shows selection of materials with respect to job and site conditions. Table 2 shows salient properties of injection materials.
Assessing these properties will help us in selecting the correct material and in turn determine the success of the crack repair. Internationally and in India at the moment, the following types of materials are most preferably used for crack injections:
- Epoxy resin (EP)
- Polyurethane (PUR)
- Cement slurry (CF)
- Micro fine cement suspension (CMF)
In addition to the substances above injection gels are also used in Europe for the injection of structural components. Injection gels (hydro-gels) are aqueous systems based on special acrylate or polyurethane resins. The filling of cracks and voids corresponds to the following application principles:
- Protection against the penetration of substances into the concrete structural components
- Strengthening concrete structural components
Based on Crack WidthFig.6 : Selection of Injection / Impregnation Materials
|In Brief, Table 3 below gives an idea of the type of Injection materials available and the conditions these materials can be used under. Impregnation can only be used to fill cracks in near the surface areas of horizontal or slightly tilted surfaces from above. Injection is able to fill cracks and voids going deep into the structural component. The filling of crack and voids requires minimal crack widths at the concrete surface and minimal dimensions of the voids, depending on the method of filling and the chosen filling material. Selection of materials based on crack width is shown in Figure 6.|
Injection measures comprise of the Mixing device, Injection device, Packers (filler plugs), possibly an injection hose and Insulation, if necessary. The application expertise for these materials is as important as the materials themselves.
In Conclusion, modern injection technology coupled with proper equipment can solve almost all types of rehabilitation problems thereby providing economical solution in comparison to demolition and reconstruction of structures. The specifications should be very clear and unambiguous. The specifications should at least cover points like material, viscosity, techniques to be adopted, the equipment to be employed, and type of nozzles and spacing, pressure to be applied etc. The repair of cracks is a part of repairs of damaged structures and cannot replace other remedial measures adopted for rehabilitation.
Concrete: The existing base has to be firm, stable and free from oils, impurities of all kind including form oil residues and cement laitance. The loose and damaged concrete should be removed using light hammers and chisels. Care should be taken not to remove sound concrete to provide a good base to the repair material. Only mechanical means (rotary wire brushes, grinding, chipping, grit blasting and breakers) or hydraulic methods (use of water jets 10 to 250 MPa) should be used to remove concrete. Using hydraulic is the best means, as it reduces impact of damage to the substrate concrete.
|Fig.7 : Complete Concrete Repair System
(Courtesy: MC-Bauchemie, Germany)
Reinforcement: Sand blasting or wire brushing is an effective water-free method to clean reinforcement. In this case suitable wire-brushes were used to remove the loose material, dust and laitance. Use of rust removers and water is not recommended to protect the reinforcement from further corrosion, as it may reduce the pH of the surrounding substrate and promote further corrosion. The reinforcement shall be prepared till bare metal is seen. Additional reinforcement if needed should be incorporated at this time by anchoring new steel; lapping or welding. Figure 7 shows the complete sequence of repair.
The Actual Concrete Repair System and Practical Considerations
Protection of Exposed Reinforcement
All exposed reinforcement cleaned to bare metal should be protected immediately after preparation by using suitable corrosion inhibiting active or barrier coatings. These are proprietary materials to be used to provide corrosion resistance to the cleaned reinforcement, prior to application of the polymer modified mortar / concrete system. The materials that can be used to provide corrosion protection include:
Active coatings for Reinforcement: These are coatings, which contain OPC or electrochemically active pigments, which may function as inhibitors or which may provide localized cathodic protection. Typical product that can be used is a one-component polymer modified mineral based corrosion protection coat.
Barrier Coatings: These are coatings, which isolate the reinforcement from pore water in the surrounding cementitious matrix. Typical product that can be used is a two component Epoxy Resin based Zinc Rich Primer and Coating Material for use in repairs subject to aggressive environmental and chemical attacks.
Bonding Coats: These are also proprietary materials used for bonding of fresh concrete to hardened concrete using adhesive bonded joints where it forms a part of the structure and is required to act compositely. The materials that can be used to provide bonding include:
- A one-component polymer modified mineral based corrosion protection and bonding coat, which can be used for most repair applications
- A two component Epoxy Resin based solvent free, universal bonding agent and coating for use in repairs subject to aggressive environmental and chemical attacks.
Polymer Concrete / Mortar (PCC) Repair
Small areas and patches less than 100 mm thick are usually repaired with hand / trowel or spray applied polymer modified or epoxy modified repair mortars. Some of these products are proprietary. On the other hand, these mortars can also be site batched using polymer additives. In most cases a pre-bagged manufactured ready-to-use polymer modified mortar is preferred. These manufactured products can also be available in special grades allowing thicknesses more than 40 mm to be applied in a single operation. PCC Types include:
- Site Batched Polymer Modified Mortar: This is a mixture of OPC, well graded, clean, washed, quartz sand, Styrene Acrylic or SBR based polymer additive and water as required for consistency.
- Prebaked / Manufactured Polymer Modified Mortar: The premixed / manufactured or site batched PCC for Structural Repairs to be used should have the following properties:
- Air Content: ≤ 3%
- Compressive Strength: ≥ 45 MPa
- Bond Strength to Concrete: ≥ 2 MPa
- Chloride Ion Content: < 0.05%
- Capillary Absorption: ≤ 0.5 kg/m2.h0.5
- Adhesive Bond Strength to Concrete: ≥ 2 MPa
These mortars are most suitable for repairs when the section to be replaced ranges from a depth of 5 mm to 50 mm. In case of depths more than 100 mm need to be placed, additional layers maybe needed. Follow manufacturer’s recommendations for mixing, placement, compaction and curing of the repair mortar. In case of large structural elements, a ready to use non-shrink micro concrete can be used. Cure the exposed surfaces for 7 days or more, preferably using acrylic based curing compounds (so as not to hinder bond in subsequent applications).
Fine Filling: To achieve a visually uniform surface and to provide additional preventive protection the repaired concrete surface should be fine-filled. This is done with a fine polymer modified, concrete cosmetic or fine sand and a mixing liquid composed of water and or the polymer component.
Concrete Surface Protection, Carbonation inhibitor, Coloured finishes
On completion of the work described above, the entire concrete surface must be provided with a protective coating. Such surface treatments perform several duties at the one time. Firstly, all the concrete is protected from further stress due to aggressive pollutants in the air and from progressive carbonation. Unfortunately most specifications for Repairs do not specify Anti-Carbonation Coatings for full area thereby jeopardizing the repair system. The concept of Repair is to stop or retard the corrosion process already initiated in RCC Structures. To stop further entry of CO2, SO2, moisture and chlorides in few cases the entire surface has to be coated with a system to maintain the status quo of corrosion in the structures. Selection of coating depends on exposure conditions.
These coating systems must have to a high CO2 resistance if they are to be effective in protecting against carbonation and, on the other hand, they must not have a negative effect on the buildings water vapor diffusion rate. These materials should also be water resistant, crack bridging, UV resistant and breathable. The selection of protective coatings depends on user requirements and exposure conditions. In case of structures in contact with chlorides, it may be required that the coatings are resistant to chloride ion penetration as well.
Other Techniques for Strengthening
Repair involves replacing or correcting deteriorated, damaged or faulty materials components or elements of a structure. In rehabilitation and retrofitting, we not only restore the concrete to take loads for present service conditions but we can also enhance load carrying capacities while maintaining the durability of structures. Following methods are adopted:
- Section enlargement / Jacketing
- Post tensioning
- Externally bonded reinforcement
In addition to this there are several specialized techniques used like shear strengthening, use of shear collars, confinement strengthening, span shortening techniques etc.
Section Enlargement: Jacketing
This is one of the oldest techniques used to increase the cross section of element tying additional reinforcement, which would lead enhancing to its load bearing capacity. The main thing is to bond the new material to the old element fully so that perfect load transfer and monolithic behaviour is guaranteed. This is achieved by means of through surface preparation, bonding agents as well as shear and other connections. The materials used can be: Conventional concrete/mortar, Cementitious grouts, Polymer modified cement mortars, Micro-concretes or Shotcrete.
Normally this technique is used where the damage is due to undesirable or excessive deformation. The technique of tensioning is same as through tendons but since it is done externally better inspection is possible. The flexural capacity of these structural members is increased. This requires a shear transfer mechanism and end bearing as an assembly.
Externally Bonded Reinforcement
Working principle of this system is the plate-bonding technique. When the steel plate is bonded to concrete with epoxy adhesive the structure behaves like composite structure. Two systems are commonly used:
- Steel Plate Bonding 2. GFRP/CFRP Bonding.
Steel plate Bonding: It is an efficient method of increasing flexural capacity in beams when applied to the tension side, which transfers the load to the epoxy which in turn transfers tensile loads from concrete to steel. Its case of constructability was reason for its major success. Disadvantage was cutting of steel plates to suit the geometry, its weight and problem of corrosion.
Fibre Reinforced Plastic Bonding: This is an extension of the steel plate bonding technique with tremendous advantage of light weight, ease of cutting and mould ability to suit any element and high chemicals resistance. Two systems commonly used are based on Glass Fibre reinforced plastic (GFRP), and Carbon Fibre reinforced plastic (CFRP). The rapid acceptance of this material is due to serviceability and ease of application without disturbing the structure. Wrapping of columns provides passive confinement with increase of ductility and strength. Shear strengths are also increased. Wrapping gives excellent protection against explosions. FRP plates can be bonded with epoxies to increase flexural strengths. The limitation is the use of epoxy, which can change its characteristics during thermal variation and fire.
Preventive / Periodical Maintenance
Premature failure of building components in a structure is not uncommon. To stem this, preventive maintenance is being explored today. Preventive maintenance is a part of asset management system and is required for important structures such as bridges. It seeks to forestall the deteriorating process of bridge components. Minor repairs and replacements are categorized under preventive maintenance activity. The preventive maintenance program precludes major repair work or replacement of substantial part of the structure.
Maintenance of Records
Maintenance of proper reports about the damages and subsequent repairs can offer a lot of information. This is crucial for future diagnosis of subsequent deteriorations. The records about the types of materials used at previous repairs along with other relevant technical data will clearly show the suitability for future usage as well as the durability of such materials. Documentation of this kind will go a long way to ascertain the quality of repairs, which would provide pointers for future from the past failures.
Rehabilitation of bridges is a key aspect of the bridge management system. In this case it is imperative that the owners and specifies understand the extent of the damages and the correct solutions to be used in rehabilitating the bridge. Once the process is understood, the correct systems and methodologies can be used. A combination of various systems can address any rehabilitation scenario. International codes such as EN 1504 can be referred to as they provide excellent step-by-step guidelines to address various defects.
- EN 1504: Parts 1 To 10
- IRC SP: 40
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- C. Hewlett, Methods Of Protecting Concrete, International Conference, Scotland, Sept 1990.
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- Surface Preparation And Coating Of Concrete Published By SSPC.