Strengthening of Concrete Structures and When its Required?

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Strengthening of concrete structures must be considered when the existing structure deteriorates or any alteration to the structure has to be made due to which the structure may fail to serve its purpose. Concerns must be taken to existing materials, often in deteriorated condition, loads during strengthening and to existing geometry. In some cases it can also be difficult to reach the areas that need to be strengthened. When concrete structural strengthening is to be undertaken all failure modes must be evaluated. Strengthening a structure for flexure may lead to shear failure instead of giving the desired increased load bearing capacity. It should also be noted that not only the failure mode of the strengthened member is important. If a critical member in a structure is strengthened, another member can become the critical one. Because of changed stiffness in an undetermined structural system the whole structure must be investigated. The strengthening should also designed with consideration to minimize the maintenance and repair needs. When a strengthening is designed the consequences from loss of strengthening effectiveness by fire, vandalism, collision etc. must in addition be considered.

The existing documentation of the structure is often very poor and sometimes even wrong. It might be necessary to redesign the structure with the probable former codes that were active when the structure was built. This can give enough knowledge about the structural mode of action, otherwise field investigations must be undertaken to provide an understanding of the structure. The design of a strengthening however must fulfill requirements in the codes of today. It is not only the financial and structural aspects that should form the basis for decisions of strengthening method, but environmental and aesthetic aspects must also be considered.

Ductility of Concrete Structures for Structural Strengthening

Most fibre composites are linear-elastic material without any defined yield point. Structures on the other hand should be designed to fail in a ductile way or at least with adequate warning signals preceding a potential collapse. Ductility can be defined as capability of a structure to deform while still carrying the load even when the maximum load bearing capacity is exceeded. It is important to distinguish between material ductility and structural ductility. Steel bars with short anchorage can be an example of brittle failures even though steel is considered to be a ductile material. Material properties and structural ductility are not directly dependent, and linear-elastic materials may increase the ductility of a structure. A concrete beam reinforced in bending with steel bars is often considered to have a very ductile behavior. However, consider the same beam subjected to a fatigue load that causes high strains in the steel both in compression and tension. The loading will make the structure to fail in a brittle way, but even worse the normal behavior of the structure. Consider the same beam, with fatigue load, and strengthened with a linear elastic material, due to strengthening the stresses in the steel bars will decrease and it will not fail in fatigue, instead the ductile behavior is regranted. Work has been carried out on many different types of structures to restore or increase the flexural capacity, which gives that the structure will be loaded close to its maximum shear capacity. One of the chief concerns is that shear failures are often very brittle with no, or only small warnings preceding the failure because of the higher elastic energy built up compared to what it had before strengthening. On the other hand, a structure with a brittle failure in shear may be strengthened so that the failure mode will change to a more ductile and friendly mode.

Need of Structural Strengthening for Concrete Structures

Concrete structures need to be strengthened for any of the following reasons:

• Load increases due to higher live loads, increased wheel loads, installations of heavy machinery, or vibrations.
• Damage to structural parts due to aging of construction materials or fire damage, corrosion of steel reinforcement, and/or impact of vehicles.
• Improvements in suitability for use due to limitation of deflections, reduction of stress in steel reinforcement and/or reduction of crack widths.
• Modification of structural system due to elimination of walls/columns and/or openings cut through slabs.
• Errors in planning or construction due to insufficient design dimensions and/or insufficient reinforcing steel.

Strategies for Structural Strengthening

When a structure is to be strengthened there are several aspects to consider. The figure shows a schematic example of a structure that had inadequate load bearing capacity due to a design fault already present before it was taken into service. It was then strengthened slightly above the desired performance level. After some time the structure was damaged due to an accident, collision, fire or overload that damaged the system to a level where performance requirements were not fulfilled. The damages were then repaired to a new satisfactory performance level. Later, the demands on the structure were changed, higher load bearing capacity was required, and the structure needed to be strengthened to a higher performance level to meet these demands. By a third strengthening it was possible to meet the new demands and keep the structure in service.

Fig 1. Performance History of a Structure

Without considering the deterioration the need for strengthening may not be that complicated. Insufficient performance due to a design fault, accident or increased demands can quite clearly be identified. When deterioration is significantly prevalent it becomes more complicated. For a new structure that is inadequate due to a design or constructional fault, the size of the problem is more or less well known and the desired life of the structure can also be quite clearly expressed. The selection of suitable strengthening methods can nevertheless be complicated. For older structures in need of strengthening, the situation becomes even more complex. One important issue is the remaining life of the structure. It is not always valid to strengthen a structural part to give it 50 years remaining life if the foundations, for instance, will only function for another 10years. For example a road network may be changed within 5 years due to a larger infrastructure project. If a bridge on the existing road needs to be repaired to provide satisfactory reliability in the meantime, it would be very cost ineffective to replace the old bridge with a new. In this case the bridge, if possible, should be repaired and the repaired bridge does not need to have a lifespan longer than 5 years.

Fig 2. Deterioration and Strategies for Strengthening

With deterioration in mind the strategy for strengthening becomes more complicated. This is schematically illustrated in the figure 2.The performance level of the structure is slowly decreasing, but it still fulfills its performance requirements. New demands are placed on the structure, but the time being it still fulfills the performance requirements. The decrease of performance will in the nearby future result in the structure being inadequate, marked by X.The rate of degradation can be different in different cases. However, the structure must be upgraded, in this case by strengthening. Read More: Protective Systems for Reinforced Concrete Structures Maintenance of Prestressed Concrete Structures from Damages and Deteriorations Repair of Post Concreting Defects in Structures and Methods of Repair