Types of Advance Cast-in-Place Concrete for Structure Repair
Cast-In-Place Fiber-Reinforced Concrete
Fiber-reinforced concrete is a specialized material that is created by adding metallic or polymeric fibers, made from materials such as steel, glass, synthetic plastics, or natural plastics, into conventional concrete. The purpose of adding these fibers is to enhance the concrete’s resistance to issues like plastic shrinkage, drying shrinkage, and cracking. This type of concrete is commonly used in construction projects where increased durability and crack resistance are desired.
The process of incorporating fibers into the concrete can be done using conventional concrete mixing methods or shotcrete methods, which involve spraying the fiber-reinforced concrete onto a surface. Fiber-reinforced concrete is commonly used in repair applications where existing concrete structures need reinforcement or restoration. By adding fibers to the concrete, the resulting material becomes more robust and better able to withstand the stresses and strains that can lead to shrinkage and cracking. This makes fiber-reinforced concrete an ideal choice for a wide range of construction and repair projects.
Fibers are often not used as the primary reinforcement in concrete, but they can be employed effectively in thin overlays where the thickness is insufficient to place steel reinforcing bars. The purpose of using fiber reinforcement in these cases is not to resist applied loads, but rather to enhance durability, minimize cracking of repair materials caused by plastic shrinkage, improve shock and vibration resistance, and increase blast resistance. However, the addition of fibers to concrete can reduce its slump and make placement and compaction more challenging. Additionally, corrosion of fibers in steel fiber-reinforced concrete can result in rust stains on the surface.
Cast-In-Place Low-Slump Dense Concrete (LSDC)
Low-slump dense concrete (LSDC) is a unique type of conventional concrete that is characterized by its moderate-to-high cement factor and a low water-to-cement (w/c) ratio of less than 0.40. Additionally, LSDC has a slump, which is a measure of its workability, of 50 mm or less. One of the key properties of LSDC is its rapid strength gain, making it suitable for applications where early strength development is desired. Moreover, LSDC has a high density and reduced permeability, making it highly resistant to chloride-ion penetration, as demonstrated by ASTM C 1202 testing. As a result, LSDC is commonly used as an overlay or wearing course in concrete repairs, providing a durable and abrasion-resistant surface that protects the underlying concrete structure.
Low-slump dense concrete (LSDC) is a type of concrete that requires special considerations during its production and curing process. To achieve the desired properties, such as high strength and low permeability, the use of a high-range water-reducing admixture (HRWRA) is necessary. This admixture helps to reduce the amount of water needed in the concrete mix, allowing for a lower water-to-cement ratio and resulting in a denser and stronger final product.
In addition to the use of HRWRA, high compacting efforts are also needed during the placement of LSDC. This typically involves using specialized equipment, such as high-frequency vibrators, to ensure proper compaction and consolidation of the concrete mixture. This step is crucial to eliminate voids and achieve maximum density, which is essential for obtaining the desired properties of LSDC.
Furthermore, adequate curing is crucial for LSDC to achieve proper hydration. Continuous moist curing for at least 7 days is typically required to allow the concrete to gain strength and durability. This involves keeping the concrete surface moist by various methods, such as covering it with plastic sheets or applying water spray at regular intervals. Proper curing is essential to prevent issues such as drying-shrinkage cracks, chloride-ion intrusion, and galvanic corrosion, which are common problems associated with LSDC.
It is important to note that the use of LSDC should be carefully considered, and environmental factors should be minimal to avoid potential issues. For instance, if there are concerns about galvanic corrosion, drying-shrinkage cracks, or chloride-ion intrusion in a particular environment, the use of LSDC may not be suitable. Therefore, a thorough assessment of the project’s specific requirements and environmental conditions should be conducted before deciding to use LSDC.
Cast-In-Place Silica-Fume Concrete
Silica-fume concrete is a type of concrete that is made by adding silica fume, a by-product of the manufacture of silicon and ferrosilicon alloys, to normal Portland-cement concrete. This modified concrete is often used instead of latex modified concrete or low-slump concrete. Silica fume is a highly efficient pozzolanic material and is typically added to the concrete mix in quantities of 5 to 15% by mass of cement to achieve compressive strengths of 85 to 105 MPa.
By adding silica fume and a high range water reducing admixture to the concrete mix, the compressive strength of the resulting material is significantly increased, while its permeability is decreased. Additionally, the density, chemical resistance, and abrasion-erosion resistance of the concrete are improved, making it suitable for a wide range of applications. Silica-fume concrete can also act as a replacement for some of the cement used in the mix.
The high strength and abrasion-erosion resistance of silica-fume concrete make it an economical solution for repairing hydraulic structures and other applications where conventional concrete may not be suitable. It is commonly used for overlays on parking structures and bridge decks to reduce the intrusion of chloride ions into the concrete.
However, there are some challenges associated with using silica-fume concrete. It can be more prone to drying and plastic shrinkage cracking, and its cost is typically higher than that of conventional concrete. It can also be difficult to achieve a steel trowel finish due to the absence of bleed water, and early wet curing is required for at least 7 days to ensure the material sets properly. Furthermore, curing temperatures must be maintained above 4°C during this period. The addition of silica fume to the mixture makes it sticky and more difficult to finish.