REPAIR STRATEGIES
B. Durable Repair Application:
Preparation of the repaired surface-
The most important surface characteristics of the receiving substrate are its roughness soundness, cleanliness and moisture condition prior to application of the repair material. The first step in the repair to be carried out is the removal of the damaged concrete. It is very important to select a method most appropriate for the specific in-situ condition. Any method that weakness the sound concrete and create micro cracking should be avoided. Otherwise, the durability and bond will be decreased by these defects. Commonly used methods in-situ include: sand blasting, chipping with jack hammers, and hydro demolition among which, the last is highly recommended. A sound surface if adequate roughness can be created by this method.
Higher plastic shrinkage of the repair material near the interface should be avoided. This requires that the substrate be pre wetted for at least 7 hours prior to the application of the repair material in order to decrease the absorption and expansion of substrate caused by the uptake of moisture from the repair material. Otherwise, the higher uptake of moisture by the substrate after the repair material is cast may lead to higher plastic shrinkage of the repair material near the interface and higher expansion of the substrate, and there by resulting in the possible debonding of the repair material at an early age.
The application method and surface preparation are equally important considerations with regards to the performance of the repaired structures. The repair method adopted not only affects the resultant quality of the repair material, but also the quality of the interfacial transition zone, shotcrete seems to be an ideal method because it has good compatibility with the substrate concrete. Further ore, good compaction with a relatively lower water/cement ratio of the repair material can be achieved using the shotcreting process. This ensures good/high mechanical properties of the component parts and durability of the repair structure.
Bonding agents
Use of polymer bonding agents is not recommended as their modulus of elasticity is substantially different from that of the substrate. However, use of a cementitious bonding agent with a low water/cement ratio may be considered. This type of bonding agents not only has good compatibility with the substrate and repair material, but can also alleviate the effects of differential shrinkage and thermal movement between the repair material and concrete substrate thus, enhancing the bond strength and durability.
Curing of Repair Material
Excessive loss of water may result in higher shrinkage (Plastic and drying) and cause debonding failure of the repair material at an earlier age. Therefore, specification of proper curing after completion of the repair is very important. Curing time should be at least the same as that adopted for usual concrete practice or in accordance to manufacturers recommendations if a commercially available material is used due to the restraint afforded by the substrate.
C. Evaluation of the Repairs
Behaviour of the interfacial transition phase
The formation of the interfacial transition phase is affected by many factors. Defects such as micro-cracks and pores may be formed within this phase caused by the differential movements between the substrate and repair material and a lack of aggregate interlock action between the two materials. Its mechanical behaviour and durability affect directly the performance of the repaired members in service. Thus, the mechanical properties and durability of this phase should be evaluated after the completion of the repair work.
Behavior of the repaired structure
Differential movements between the repair material and they may result in the cracking of the repair material and thereby decrease the flexural stiffness and durability of the repaired members.
The degradation in flexural stiffness of the required beams under static and cyclic loading was related to the appearances and development of cracks. However, the presence of steel fibres within the repair materials may improve resistance against cracking and fatigue resistance. Therefore, the flexural stiffness can be enhanced and the deflection of the repaired members reduced. At the same time, the fatigue resistance of the interface between the repair material and substrate may also be evaluated by cyclic loading test of representative samples. The test results can also form a database for the formulation of guidelines for use in practice.
Assessment Procedure for Evaluating Damages in Structure and Repair techniques
For assessment of damage of a structure the following general considerations have to be take account.
- Physical inspection of the damaged structure.
- Presentation and documenting the damage.
- Collection of samples and carrying out tests both in situ and in the lab.
- Studying the documents including structural aspects.
- Estimation of loads acting on the structure.
- An estimate of environmental effects including soil-structure interaction.
- Diagnosis.
- Taking preventive steps not to cause further damage.
- Retrospective analysis to get the diagnosis confirmed.
- Assessment of structural adequacy.
- Estimation of future use.
- Remedial measures are necessary to strengthen and repair the structure.
- Post repair evaluation through tests.
- Load test to study the behavior.
- Choice of course of action for the restoration of the structure.
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