Maintenance of Prestressed Concrete Structures from Damages and Deteriorations
Suitably designed and constructed prestressed concrete structure have demonstrated great durability performance in different environmental situations. But there are situations in which prestressed tendons of prestressed concrete structures have extremely suffered damages and deterioration. as shown in Figure-1 in which prestressed strand of prestressed concrete bridge has deteriorated heavily.
Fig.1: Prestressed Strand at Bottom Flange Seriously Corroded and Damaged
The main cause of prestressed tendon erosion and damages may be chloride ingression. After chloride ions go through concrete, it will decrease the concrete passivity and eventually with corrosion process will start with the presence of water and oxygen. Another factor that can deteriorate prestressed concrete structure is the carbonation. It occurs though chemical reaction between water and CO2 in air. The effect of carbonation might be smaller compared to chloride influence, because not only it can be prevented but also its progress is slower and its ingression depth is lower. At the design and construction stage of the structure, necessary measures can be considered to protect prestressed concrete structure from both chloride and carbonation aggression. Such protection techniques include increasing concrete impermeability by increasing concrete cover thickness to protect prestressed tendons and declining structural cracking.Most frequent prestressed concrete structural elements which are noted to have experienced deterioration are:
- Deterioration of prestressed concrete bridge deck because of salts that is employed to melt snow and improved bridge vehicular traffic condition.
- Damaging decks of parking building that is most likely due to salts used in neighboring roads or streets and the is adhered tires of the car and eventually transferred to parking structure deck.
- Because of waves and splash in the tidal zones, underside wharf decks and piles of coastal marine structures deteriorate.
- Unbonded tendons, which are wrapped around with papers and bitumastic in regions close to the coast of sea where chloride is present in the fog, have observed to experience damages.
- Graving dock wall is another prestressed concrete structural element that fact deterioration because it is flooded occasionally and then dewatered for long time and eventually it will suffer deterioration.
Fig.2: Prestressed Concrete Bridge Deck Deterioration
Fig.3: Structural Deterioration in Marine Environment
Prestressed Concrete Structures Maintenance Program
Suitable and appropriate maintenance method can be used with extreme care to maintain the strength and extend the life span of the existing prestressed concrete structures that have not been damaged or deteriorated substantially. The maintenance program for prestressed concrete structures is provided in the following sections:- For those prestressed concrete structure that the deice salt is employed whether it is by deliberate or accident, it is recommended to use water to remove the salt and eventually prevent its detrimental influence.
- It is advised to apply layer of silane for the surface to increase its impermeability against water and chloride penetration.
- It is considerably significant to fill both cracks that do not progress and those that possibly increase. The former can be filled with epoxy injection whereas the later should be sealed with a material that has flexible property for example polyurethane. This material can extend as the width of the cracks is increased.
- It is good strategy to decline relative humidity but this is only doable in enclosed spaces and sea water pump structure is an example that its relative humidity can be decreased.
- If it is possible, fill the structures with water in order prevent oxygen penetration. This will decrease the chance of chloride or carbonation attack and hence increase service life of the structure. Such structures which can be flooded with water are prestressed concrete pipes, sewers, conduits, and penstocks.
- Stitches or epoxy injection application for the repair of laminated cracks is considerably useful and provide good result.
- It is recommended to introduce coating or insulation for concrete cover in the region where the structure is subjected to the influence of freezing and thawing.
- Even though cathodic protection is provided only after prestressing steel has deteriorated, but it can be utilized by bond both prestressing steel and reinforced steel.
- Finally, utilize penetrating corrosion inhibitor for cases where the structure can be dried from water and the water in the pores of concrete does not prevent the ingression of corrosion inhibitors.