Credit: Tnemec Co. Inc.
The weak laitance layer—typical of acrylic modified, portland-based, and calcium aluminate-based cementitious resurfacers—should be removed prior to topcoating with high performance coatings.
Owners and engineers faced with rehabilitating corroded concrete surfaces in their water or wastewater treatment facility generally focus on selecting a coating that offers long-term protection. Although manufacturers continue to develop coatings that can withstand highly corrosive environments, proper preparation and restoration of the concrete surface often is overlooked. Knowing a few important aspects of concrete restoration and integrating them into the construction process will help ensure long-term coating performance and substrate protection.
The first step in refurbishing degraded concrete is decontaminating the surface to remove areas imbued with acids, salts, treatment chemicals, wastewater scum, or other substances that may interfere with the coating. The surface then should be blasted to remove any weakened concrete. The most effective way to meet both requirements is to blast the surface using high-pressure water cleaning (>5000 psi).
Alternately, mechanical abrasive blasting using media such as metallic slags or silica sand can be used. However, this method of preparation should be preceded by properly decontaminating the surface by first neutralizing and then removing the material using high-pressure water cleaning.
The second step is determining what type of resurfacing material best suits the project's requirements in terms of substrate rehabilitation, curing/hydration requirements, and budget constraints. By providing a contiguous surface for the protective coating topcoat, resurfacing materials are critical in restoring concrete substrate. This, in turn, enables a protective coating to be applied in a pinhole-free, monolithic manner.EVALUATING MATERIALS
The most common and least expensive resurfacing product is hydraulic portland cement. Its curing process requires water within the mortar mixture to remain in contact with the cement particles for proper hydration. The surface must be kept wet by either constant “misting” or applying a curing compound designed to form a membrane on the surface. Either method must be employed for a minimum of seven days to prevent moisture within the concrete from escaping before it properly hydrolyzes with the cement. A minimum thickness of ½ inch generally is required for the portland cement matrix to properly form a cohesive structure. Once fully cured and before any coatings are applied, the topmost layer of the resurfacer must be removed. This is accomplished by mechanical surface preparation, such as abrasive blast cleaning.
The top outer layer of the mortar is removed for two reasons. First, any membrane curing compound left on the surface can interfere with coating adhesion. Second, the weak layer of fine cement particles found at the surface of concrete, called “laitance,” must be removed to expose sound substrate. Laitance forms from the troweling of a cementitious resurfacer and working the fine particles to the surface. Additionally, these fine materials are carried to the surface with “bleed water,” the excess water that exudes to the surface immediately following placement. This layer does not have the same physical strength and must be removed to prevent a “weak link” in the coating system when topcoated with a high-performance coating.
Hydraulic calcium-aluminate-based cementitious resurfacers cure faster and resist mildly acidic environments better than traditional portland-based products and cure faster. Depending on the mix design, most calcium-aluminate-based products require a minimum of 24 to 48 hours of wet or membrane cure to sustain the rate and degree of hydration necessary to achieve optimum physical strength. This faster cure time basically results from the calcium-aluminate cement's smaller particle size, enabling quicker hydrolization with water. Laitance still forms at the surface of this type of material, so it must be removed with any curing compounds prior to coating application.
Another way to facilitate better hydrolization and decrease cure times is to incorporate acrylic polymers into the cement formulation. These types of products work by enveloping water molecules with acrylic resin that slows evaporation, keeping them in proximity to the cement particles. These products modified with acrylic polymer require only 48 to 72 hours of wet or membrane cure.