Reinforced Concrete Structures Protective Systems
The objective of providing a protection system is to extend the life of the structure and to reduce the number of future repairs and the rate of deterioration of the concrete structures.
Protective systems consist of materials and methods that provide the following protective qualities:
- Reduction in chances of corrosion of steel reinforcement.
- Less deterioration of the concrete.
- Less penetration of moisture, chloride ions, and other contaminants into the concrete. This can be achieved by providing surface treatments, applying electro-chemical equipment, or by modifying the PCC overlay.
- More abrasion or impact resistance.
- More resistance to other deleterious attacks.
The following factors are considered while suggesting a protective system:
1. Life-cycle costs are compared for the various protection systems applicable for a particular situation. The protection system with the lowest initial cost may actually be the most expensive when the costs of future repairs are added over the projected life of the structure.
2. In case the protection system has a previous performance record, the confidence in its use increases.
3. Appearance can sometimes be an important factor in determining the selection of a system.
4. Thorough supervision, testing and visual observations must be made during the installation of the protection system.
5. The noise and dust levels, handling, use, and disposal of hazardous chemicals and escape of vapors into air must be considered while deciding the protective system. Further, local environmental laws must be observed.
6. The bond of the new protective system applied on existing structure or earlier repair material must be studied.
7. The expected life of a system against the exposure to prevailing atmospheric conditions must be considered.
8. There must not be any serious medical problems for the working people and chances of failure during repair work.
Factors Determining Need of Protective System
The factors affecting the performance of the completed repairs and the protection system must be evaluated. The following are some of the more common factors that must be considered in a repair and protection project.
a) Poor-quality concrete or inadequate cover: Deteriorated concrete having excessive internal cracking, internal voids, lack of consolidation, inadequate entrained air-void system, or otherwise substandard conditions, may cause corrosion of the reinforcing steel and degradation of the structure.
The deficient part of concrete is removed during a repair. A properly selected protection system can improve the long-term durability of poor-quality concrete, enhance the performance of good concrete, and extend the life of any repair.
b) Misplaced reinforcing steel: During repair / installation of protective system, extra material or coatings are provided on misplaced steel at ends, corners and hooks and bars having less concrete cover. Cathodic protection, chloride extraction, and corrosion-inhibitor additives in repair materials can also be useful to prevent or delay future corrosion.
c) Water penetration: Water may penetrate into concrete by hydrostatic pressure, moisture vapor pressure, capillary action, and rain. Movement of water within concrete may occur due to cracks, porous concrete, lack of entrained air, structural defects, or improperly designed or functioning joints.
This moisture causes corrosion of reinforcement, freezing-and-thawing damage, leakage into the interior of the structure, and possible structural damage. It is tried while designing the protection system that the water movement is reduced and rusting of steel is directly controlled.
d) Carbonation: Carbonation is the reduction of the protective alkalinity of concrete, caused by the absorption of carbon dioxide and moisture. In normal concrete, the reinforcing steel is protected by the naturally high alkalinity (pH above 12) of the concrete around the reinforcement.
A protective oxide layer is formed around the reinforcing steel that helps to prevent the reinforcing steel from corroding in the presence of high alkalinity. The absorption of carbon dioxide and water within the concrete cause reduction of the useful alkalinity of concrete by a process called carbonation.
The chances of corrosion are significantly increased when pH falls below 10. The bars close to the exterior surface are subject to the effects of carbonation and are not protected against corrosion.
Barrier coatings may provide protection against future carbonation where concrete cover is insufficient. Otherwise, cathodic protection system or realkalization of concrete may be used to protect steel against future corrosion.
e) Anodic ring (halo effect): This effect is produced when existing reinforcement extends from the parent concrete into a repair mortar or new concrete. This results in an increase of the differences in electrical potential at the bond line between the new and the parent concrete.
An anodic ring or halo effect is failure that occurs due to accelerated corrosion of the reinforcement in the parent concrete, just beyond the edge of the repair. Corrosion occurs at the anode, usually in the parent concrete, as electrons are attracted to the Cathodic portion of the reinforcement in the uncontaminated repair material.
The buildup of rust produces large internal pressures at the surface of reinforcement which results in spalling of concrete. The presence of chlorides accelerates this process.
Barrier coatings on the reinforcing steel include epoxies, latex slurries, or zinc rich coatings that can partially help to control corrosion activity; but there are field-application problems. Cathodic protection, chloride extraction and galvanic anodes can also be used to protect steel against corrosion. However, economics of these solutions are to be considered.
f) Cracks: Repair of cracks is usually the first step in any repair or protection job. Water present in cracks can result in corrosion and freezing-and-thawing problems in cold climates. The reason for the appearance of a crack must be investigated before the repair work.
The structural cracks must be repaired in such a way that the load transfer can take place through the crack. Epoxy injection is used to ensure sealing of the crack. Active cracks, especially those due to thermal changes on exterior exposures, must be repaired to allow for future movements.
The cracks active for thermal movement may be repaired by providing properly designed expansion / contraction joints. The use of caulking, chemical grouts, elastomeric coatings, and high elongation epoxies can repair moving cracks. The repair of active cracks on exterior exposures can be difficult.
Most of the materials used for crack repair are temperature-sensitive and cannot be installed much below 4 °C. It is also desirable to conduct repairs when the crack is near its maximum width, because most flexible materials used in repair of active cracks perform better in compression than in tension.
g) Chloride/chemical attack: Penetration of chemical or salt solutions through concrete contributes to the corrosion of the embedded steel. The chemical attack of acids, alkalis, and sulfates, may also have a detrimental effect on the concrete. Barrier protection systems are commonly used to minimize the intrusion of chemicals into concrete.
h) Surface erosion: Erosion of concrete at the surface is a major concern on dams, spillways, and other waterfront structures, as well as on bridge decks, ramps, parking decks, industrial floors, and other traffic-bearing structures.
Usually to a lesser extent, it can also be a concern on buildings exposed to acid rain and severe weather conditions. Concrete overlays, surface hardeners, sealers, or other treatments are often used to increase the resistance of surfaces against erosion.