Structure Repair with Cast-In-Place Conventional Concrete
The most common and cost-effective method of repairing defective concrete is by replacing it with new concrete placed conventionally. However, repairs made with conventional concrete may deteriorate again due to various reasons such as acid attack, aggressive water attack, or abrasion-erosion. To extend the service life of the repaired concrete, modifications can be made by adding materials such as silica fume, acrylics, styrene-butadiene latex, or epoxy to Portland cement concrete. When choosing a repair material, it’s important to ensure compatibility with the existing concrete substrate, and using the same materials and proportions as the original construction is often the best option.
Conventional concrete is typically composed of Portland cement, aggregates, and water, with admixtures used to entrain air, accelerate or retard hydration, improve workability, reduce mixing water requirements, increase strength, or alter other properties of the concrete. By modifying the composition of the concrete, its performance and durability can be enhanced, resulting in a longer service life for the repaired structure.
Pozzolanic materials, such as fly ash or silica fume, can be combined with Portland cement in order to reduce costs and provide specific properties, such as reduced early heat of hydration, improved later-age strength development, reduced permeability, or increased resistance to alkali-aggregate reaction and sulfate attack. When repairing concrete to minimize shrinkage cracking, it is important to use repair concrete with a low water-to-cement ratio (w/cm) and a high coarse aggregate content. Conventional concrete is commonly used for repairs involving relatively thick sections (greater than or equal to 50mm) and large volumes of repair material. It is suitable for partial and full depth repairs and is commonly used for repairs of slabs, walls, columns, piers, hydraulic structures, bridge decks, and parking structure decks. It is also suitable for repairs in marine environments due to the typically high humidity in such environments reducing the chances of shrinkage. Conventional concrete is readily available, economical, and relatively easy to produce, place, finish, and cure. It can have similar properties as the original concrete and can be easily placed underwater using well-recognized techniques. However, it is not recommended for repairs where the causes of the original distress have not been removed, as new shrinkage can cause separation of the overlay if the existing concrete has already gone through most of its expected shrinkage. Conventional concrete also has limitations under hot and cold climates. Low-slump concrete overlays, also known as PCC overlays, are modified mixtures designed to produce denser and more durable concrete. They may increase the load-carrying capacity of the underlying concrete and provide a protective barrier against deicing salts. The thickness of an overlay depends on its function, ranging from 40mm to any reasonable thickness. Portland-cement-based overlays are suitable for resurfacing spalled or cracked concrete surfaces, increasing cover over reinforcing steel, adding slip resistance, leveling floors, repairing concrete surfaces damaged by abrasion, freezing, or fire, and rehabilitating deteriorated pavements. They have good bond characteristics to a properly prepared substrate and increased durability due to lower w/c. These systems are less expensive than modified concrete systems. However, Portland-cement-based overlays are not recommended in situations where the original damage was caused by chemical attack, as the chemical attack may continue in the future. Portland cement overlays are commonly used as a wearing course on parking garages and plaza decks to improve drainage, and may also be used in conjunction with traffic or pedestrian elastomeric membrane systems.