When selecting materials for a repair job, there are numerous factors that need to be taken into consideration. The chosen materials should possess qualities such as strength, durability, and low dry shrinkage, as well as suitable thermal coefficient of thermal expansion and permeability. Additionally, the materials’ chemical and electrical properties should be compatible with the structure being repaired. It is also important to ensure that the materials undergo proper curing and are cost-effective.
Considering all of these factors is crucial to guarantee a successful and long-lasting repair work that can effectively restore the designated strength of the structure. However, it is also essential to take into account the availability of relevant materials, equipment, and skilled labor. Adequate research and planning are necessary to ensure that the repair work can be completed with the right materials and resources. By carefully selecting and utilizing appropriate materials and resources, repair work can be performed efficiently and effectively, ultimately leading to a successful and durable outcome.
11 Factors Affecting the Selection of Repair Materials
1. Strength
Bond and compressive strengths are crucial factors in the success of repairs and protective works. It is important that the strength of the base material and the repair material are either similar or that of the repair material is slightly higher. This is necessary to ensure that stress and strain can flow uniformly through the materials.
However, if both the base and repair materials have high strength, it can lead to premature failure of the repair material due to uneven stress and strain distribution. Therefore, it is important to consider the strength of both materials carefully to achieve optimal results.
Another essential aspect to consider is the bond between the underlying concrete surface and the repair material. The bond strength should be satisfactory, and if there are doubts, appropriate measures can be taken to enhance it. These measures could include using adhesives, surface interlocking systems, mechanical bonding, or a combination of these methods to improve the bond strength. A satisfactory bond strength is crucial to ensure that the repair material can withstand the stresses and strains to which it will be subjected.
2. Durability
In order to effectively repair a damaged structure, the material used must possess durability that can withstand the exposure conditions to which the structure is subjected. This means that the material must be able to resist chemical attacks and any forms of energy, such as ultraviolet rays and heat, that may be present in the environment.
Therefore, it is important to carefully select a repair material that has the appropriate level of resistance to these potential hazards. By doing so, the repaired structure can maintain its integrity and function effectively over time, without being further compromised by the same types of damage that caused the original problem. Ultimately, the choice of repair material can greatly impact the longevity and performance of the repaired structure.
3. Coefficient of Thermal Expansion
When it comes to repairing concrete, it is essential to choose a repair material that has a coefficient of expansion that closely matches that of the existing concrete. The reason for this is to ensure that no unnecessary stresses are transferred to either the bonding interface or the substrate. If there is a significant difference in the coefficient of expansion between the repair material and the existing concrete, it could lead to thermal incompatibility, which, in turn, may result in failure at the interface or within the material of lower strength. This is particularly true for overlays, where the risk of failure due to thermal incompatibility is higher. Therefore, it is crucial to consider the coefficient of expansion when selecting repair materials for concrete structures.
4. Low Drying Shrinkage
The completion of concrete base shrinkage means that repair materials must have the lowest possible dry shrinkage to maintain the bond between the repair material and the underlying concrete surface. Failure to meet this requirement could result in delamination or shrinkage crack development on surfaces, which can cause air and moisture ingress leading to steel bar corrosion. The limits for cement-based repair materials for 28-day and ultimate drying shrinkage are 400 and 1000 millionths, respectively.
One way to reduce shrinkage of cementitious repair materials is to use mixtures with low w/c, as well as the maximum practical size and volume of course aggregate. Another option is to incorporate shrinkage-reducing admixtures, or use construction procedures that minimize the shrinkage potential. Curing of the materials is very critical, especially if the thicknesses are smaller.
5. Permeability
To protect reinforcement from corrosion, it is essential that the repair materials used have low permeability. This is because aggressive substances such as carbon dioxide, water, oxygen, and industrial gases and vapors can penetrate the repair material and cause damage. Thus, materials that are highly resistant to such substances are necessary.
However, when large patches, overlays, or coatings are used with impermeable materials, it can trap moisture that rises up through the base concrete. This can cause failure at the bond or make freezing and thawing significantly more critical. Therefore, it is important that the repair or protection material used allows the concrete below to breathe.
If the repair or protection material does not allow the concrete below to breathe, the moisture that is trapped between the concrete and the impermeable repair material can cause further problems. This can lead to failure at the bond, making the repair ineffective. Additionally, the trapped moisture can make freezing and thawing much more severe, causing damage to both the repair material and the concrete beneath it. Hence, it is crucial to use materials that allow breathing of concrete below for effective repair and protection.
6. Modulus of Elasticity
The given context states that the modulus of elasticity of the repair material should match that of the existing concrete. This is important because having similar moduli of elasticity ensures that the repair material behaves in the same way as the existing concrete when subjected to stress and strain.
In cases where the repair is nonstructural, it is beneficial to use a repair material with a lower modulus of elasticity. This is because a lower modulus of elasticity helps in the relaxation of tensile stresses that are caused by restrained drying shrinkage.
It is generally recommended that the maximum modulus of elasticity for cement-based repair materials should not exceed 24 GPa. This is because exceeding this limit can result in a repair material that is too rigid and does not behave similarly to the existing concrete, leading to issues with cracking and failure.
7. Chemical Properties
The use of repair material with a pH level in close proximity to 12, which indicates an alkaline environment, can provide better protection against corrosion for embedded reinforcement. However, if the pH level is not close to 12, additional measures may be required to protect the existing reinforcement. One such measure is the application of cathodic protection, which involves the use of an external electrical current to prevent corrosion. Alternatively, reinforcement coatings may be used to provide an extra layer of protection to the embedded reinforcement. These additional measures are necessary to ensure the long-term durability and safety of the structure.
8. Electrical Properties
When repairing concrete structures, it is important to use materials that have high electrical resistance. This is because such materials can help isolate the repaired areas from the surrounding concrete, thereby reducing the risk of corrosion. In contrast, if there are differences in electrical potential between the repair material and the original concrete, it can increase the likelihood of corrosion activity around the perimeter of the repair area. This phenomenon is known as the anodic ring or halo effect, and it can lead to premature failure of the repair. Therefore, it is crucial to consider electrical resistance when choosing materials for concrete repair to ensure the longevity and durability of the structure.
9. Color and Texture Properties
Architectural concrete surfaces that require repair must be restored to match the color and texture of the surrounding area as closely as possible. Any noticeable difference in color or texture can affect the appearance of the entire structure. Thus, it is essential to conduct trials on the site to ensure that the repair material blends in seamlessly with the adjacent surface.
Before beginning the actual repair work, it is crucial to test the repair material on-site to ensure that the color and texture match the existing surface. The repair material must be carefully selected to replicate the texture and color of the architectural concrete surface. Any deviation from the original surface can result in an unsightly appearance, detracting from the overall aesthetic of the structure. Therefore, conducting trials beforehand can help ensure that the repair work is successful in blending in with the surrounding area.
10. Curing Requirement
The need for repair materials that require little to no curing is becoming increasingly important in order to reduce the amount of post-repair maintenance. Repair materials that require extensive curing can be problematic, as they may not cure properly, leading to a failure to achieve the designated strength. This can ultimately result in a repair that does not meet the necessary requirements and may require further maintenance in the future. Therefore, repair materials that require minimal curing are highly desirable as they can reduce the risk of improper curing and help ensure the repair meets its required specifications.
11. Cost of Repair Material
The use of cost-effective repair materials is preferred, but not at the cost of sacrificing the performance properties of the material.
Fig. 4: Curing of Repaired Concrete