Overlays are applied to the surface of concrete and can be 6 mm or thicker. They can be bonded, partially bonded, or unbonded to the concrete surface. Various materials can be used for overlays, such as polymer concrete, Portland cement concrete, epoxies, polyesters, and polymer-modified concrete. Overlays can change the appearance, texture, and elevation of the original concrete surface, and are available in a variety of colors and finishes. They can bridge non-moving cracks, but active cracks may reappear. Overlays are commonly used as a wearing course to provide protection against water intrusion and chloride ion penetration, as well as abrasion resistance. They can also be used for decorative purposes. Overlays are particularly useful for sealing areas with a large number of cracks, where individual treatment of each crack would be costly. It’s important to note that unbonded overlays only seal the surface of cracks and do not fill them deep inside. One common type of unbonded overlay is a two or three-ply membrane of felt with tar between the plies, covered with gravel, concrete, or bricks for protection. Unbonded overlays are typically used for sealing dormant cracks. However, for active cracks, extensible overlays are necessary as they have the ability to elongate and shorten. For dormant cracks, almost all types of overlays work well. Both bonded overlays and surface treatments can be used to repair dormant cracks in structural slabs and pavements.
Overlays are applied onto concrete surfaces through methods such as placement, troweling, screeding, or spraying, and can be done in one or multiple layers. When analyzing existing structures, the additional dead load of overlays must be considered. Overlays can be designed to work together with the existing structure, and additional reinforcement like welded-wire fabric or reinforcing steel fibers may be added. It’s important for the surface to be clean, sound, and appropriately roughened for proper bonding of the overlay. Bonded overlays are not suitable for surfaces with active cracking or structural movement, in which case unbonded overlays may be used. Prior to overlay placement, the surface must be thoroughly cleaned and free of laitance, weak material, grease or oil, and a bond coat made of finer material from the overlay or an epoxy adhesive is applied immediately. Polymer-modified Portland cement mortar or concrete, as well as silica fume concrete, are commonly used for overlays. The polymers used are typically styrene butadiene or acrylic latexes, added in quantities of 15 to 20 percent by weight of the Portland cement. The minimum thickness of overlays can be around 30mm and they can be used to cover fine dormant cracks. Polymer-modified overlays are mixed, placed, and finished within 15 minutes in warm weather, and may require 24-hour moist curing. Vapor barrier overlays should not be used on slabs-on-ground in freezing climates. Thin, cementitious overlays are susceptible to plastic-shrinkage cracking due to their high surface-to-volume ratio, which causes rapid evaporation under drying conditions. Cracking in bonded overlays can be caused by tearing of the surface due to late finishing operations, plastic shrinkage from excessive drying, differential movement between the deck and overlay due to temperature differences or drying shrinkage, or existing cracks reflected through the overlay. Testing of the in-place concrete should be done before surface preparation to determine the modulus of rupture, and after the overlay is completed to determine bond strength. Common types of overlay systems include: [list the types of overlay systems here].
Unbonded or partially bonded:
Overlay systems are commonly used in various structures such as bridge decks, plaza decks, parking garages, and office spaces. These systems serve several purposes, including providing wearing courses and creating slopes for drainage over waterproofing membranes. Additionally, they can be used to enhance the architectural appearance of exposed decks or driveways.
Overlay systems can be made of various materials such as asphalt, asphalt concrete, or cement concrete. However, it’s important to note that asphalt and asphalt concrete overlays have some limitations. They are porous and do not provide waterproofing, which can lead to deck deterioration due to freezing and thawing. Therefore, it’s essential to consider the specific requirements of a project and choose the appropriate type of overlay system that meets those needs.
Bonded PCC overlay:
Bonded Portland cement concrete (PCC) overlays are a type of concrete layer that is typically placed horizontally onto an existing concrete surface. These overlays serve various purposes such as repairing spalled or disintegrated surfaces, providing protection, increasing the cover over reinforcing steel, enhancing slip resistance, leveling floors, repairing damage from abrasion, freezing, or fire, and restoring deteriorated pavements.
In some cases, Portland cement overlays may be reinforced with fibers or other additives. This can improve the strength and durability of the overlay. Low-slump concrete overlays and bonded PCC overlays can also be used to protect against deicing salts and to increase the load-carrying capacity of the underlying concrete.
Overall, bonded PCC overlays are a versatile and effective solution for repairing and enhancing concrete surfaces. They offer a range of benefits, from improving safety and functionality to increasing the lifespan of concrete structures. With the right preparation and application, bonded PCC overlays can be a valuable investment for many different types of construction projects.
Surface treatments:
Low solids and low-viscosity resin-based systems, which utilize materials like urethanes, epoxies, polyesters, and acrylics, are suitable for sealing concrete surfaces that do not experience traffic. For optimal results, it is recommended to first address cracks by injecting them with epoxy or by routing and sealing them prior to applying the surface treatment. The thickness of the treatment can vary from 1 to 50 mm, and additional aggregates can be sprinkled on top to enhance skid-resistance.