Reinforced Concrete Structures Protective Systems
Protective systems are designed to prolong the lifespan of a structure and reduce the need for future repairs. These systems consist of materials and methods that offer various protective qualities, including reducing the risk of steel reinforcement corrosion, slowing concrete deterioration, limiting the penetration of moisture, chloride ions, and other contaminants into the concrete, improving abrasion and impact resistance, and providing greater resistance to other harmful attacks.
When considering a protective system, several factors must be taken into account. Firstly, life-cycle costs should be compared for the different protection systems applicable to a specific situation. The protection system with the lowest initial cost may ultimately be the most expensive when considering the costs of future repairs over the projected life of the structure. Secondly, if a protection system has a previous performance record, it increases confidence in its use.
Appearance can sometimes be a crucial factor in choosing a protective system. During the installation of the protective system, thorough supervision, testing, and visual observations must be conducted. The noise and dust levels, handling, use, and disposal of hazardous chemicals, as well as the escape of vapors into the air, must also be considered when deciding on a protective system. Additionally, local environmental laws must be followed.
The bond of the new protective system applied on the existing structure or earlier repair material must be studied. The expected lifespan of a system against prevailing atmospheric conditions should also be taken into account. Lastly, there should not be any significant medical issues for the workers or potential failure during repair work.
Factors Determining Need of Protective System
Evaluating the performance of completed repairs and protection systems requires consideration of various factors. One common factor is poor-quality concrete or inadequate cover. Concrete that is deteriorated due to excessive internal cracking, internal voids, lack of consolidation, inadequate entrained air-void system, or other substandard conditions can lead to corrosion of reinforcing steel and degradation of the structure. In repair projects, the deficient part of the concrete is removed. However, selecting an appropriate 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.
Misplaced reinforcing steel can occur during the repair or installation of a protective system. In such cases, extra material or coatings are often provided on misplaced steel at ends, corners, and hooks to ensure proper concrete cover. Additionally, cathodic protection, chloride extraction, and corrosion-inhibitor additives in repair materials can be utilized to prevent or delay future corrosion.
Water penetration is another common issue in concrete structures. It can occur through hydrostatic pressure, moisture vapor pressure, capillary action, and rain. Movement of water within concrete may be caused by cracks, porous concrete, lack of entrained air, structural defects, or improperly designed or functioning joints. This moisture can lead to corrosion of reinforcement, freezing-and-thawing damage, leakage into the interior of the structure, and potential structural damage. Therefore, when designing a protection system, efforts are made to reduce water movement and effectively control rusting of steel.
Carbonation is a process that reduces the protective alkalinity of concrete by absorbing carbon dioxide and moisture. In normal concrete, the reinforcement steel is protected by the naturally high alkalinity of the concrete, with a pH above 12, which forms a protective oxide layer around the steel and prevents corrosion. However, the absorption of carbon dioxide and water within the concrete reduces the alkalinity, and when the pH falls below 10, the chances of corrosion increase. This is especially true for bars close to the exterior surface, which are susceptible to the effects of carbonation and are not adequately protected against corrosion. To mitigate this, barrier coatings such as epoxies, latex slurries, or zinc-rich coatings can be applied on the reinforcing steel to provide protection against future carbonation, especially in cases where the concrete cover is insufficient. Alternatively, cathodic protection systems or realkalization of concrete can also be used to protect steel against future corrosion.
The anodic ring, also known as the halo effect, occurs when existing reinforcement extends from the parent concrete into a repair mortar or new concrete. This creates a difference in electrical potential at the bond line between the new and the parent concrete, resulting in accelerated corrosion of the reinforcement in the parent concrete just beyond the edge of the repair. This 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, leading to spalling of concrete. The presence of chlorides can further accelerate this process. Barrier coatings, such as epoxies, latex slurries, or zinc-rich coatings, can be applied on the reinforcing steel to partially control corrosion activity. Additionally, cathodic protection, chloride extraction, and galvanic anodes can also be used to protect steel against corrosion. However, the economic feasibility of these solutions should be considered when choosing the appropriate corrosion protection strategy.
Cracks in structures are a common issue that needs to be addressed as part of any repair or protection project. Cracks can lead to problems such as corrosion and freezing-and-thawing damage, especially in cold climates where water can infiltrate and cause further damage. Before starting any repair work, the cause of the crack should be investigated to determine the appropriate repair method.
Structural cracks need to be repaired in a way that allows for proper load transfer through the crack. Epoxy injection is commonly used to seal cracks and ensure their structural integrity. For active cracks, especially those caused by thermal changes on exterior surfaces, repairs should be done in a way that accounts for future movements. This can involve providing properly designed expansion/contraction joints to accommodate thermal expansion and contraction.
Repairing active cracks on exterior surfaces can be challenging due to various factors, including temperature sensitivity of repair materials. Most materials used for crack repair are not suitable for installation at temperatures below 4°C. It is also important to conduct repairs when the crack is close to its maximum width, as many flexible repair materials perform better under compression than tension. Various methods such as caulking, chemical grouts, elastomeric coatings, and high elongation epoxies can be used to repair moving cracks, but careful consideration of the environmental conditions and timing of repairs is crucial for successful outcomes.
Chloride or chemical attack is a process where chemical or salt solutions penetrate through concrete, leading to corrosion of the embedded steel. This can be detrimental to the integrity of the concrete. Concrete structures exposed to acids, alkalis, sulfates, or other harmful chemicals are particularly susceptible to this type of attack. To minimize the intrusion of chemicals into concrete, barrier protection systems are commonly employed.
Surface erosion is a significant concern for concrete structures such as dams, spillways, bridge decks, ramps, parking decks, industrial floors, and other traffic-bearing surfaces. It can also affect buildings exposed to acid rain or severe weather conditions, although to a lesser extent. To enhance the resistance of concrete surfaces against erosion, various treatments such as concrete overlays, surface hardeners, sealers, or other protective measures are often used. These treatments help mitigate surface erosion and protect the integrity of the concrete.