Concrete structures are expected to last a certain number of years and, in order to meet the expected lifespan, the concrete must be able to withstand structural loading, fatigue, weathering, abrasion and chemical attack.
Eddie Correia, executive vice president of Chryso Southern Africa, says that the duration and type of curing applied during the setting process will play a determinant role in enabling the concrete to achieve the strength and durability that is required to meet the design parameters of the structure.
Correia explains that curing is the process in which the concrete is protected from loss of moisture and kept within an acceptable temperature range. “This process is key to producing a final product with increased strength and decreased permeability, and is important in mitigating cracks which allow open access for harmful materials to bypass the low permeability concrete near the surface and this can adversely affect durability,” he says.
“When effective curing of concrete is applied, the amount of cement required to achieve the given strength or durability is reduced or in some instances can be replaced with supplementary cementitious materials,” Correia continues.
With cement being the most energy-intensive portion of a concrete mix, this reduction leads to a cost reduction as well as a lower carbon footprint. In addition, sound curing methods using quality products can enhance sustainability by eliminating the need for resource-intensive conditioning treatments, particularly those that are not compatible with the environment.
Curing methods are divided into two phases; one prior to the initial set and these are applied continuously during the bleeding of the concrete. The other is after the initial set and final surface finishing and is generally applied before the final set.
Temperature can be an important factor as the rate of hydration and therefore strength development is faster at higher temperatures. Correia says the temperature of placed and compacted concrete should not be allowed to fall below 5°C because this will result in the slowing down or even halting of the hydration process with the result that the concrete will take longer to gain strength, delaying form or mould removal and subsequent construction. Concrete will freeze at temperatures below 2°C.
In situations where the concrete temperature is expected to drop below 2°C, an air-entraining agent from the CHRYSO® Air range can be added to protect it from freezing or thawing conditions. Air bubbles act as a pressure relief valve allowing moisture within the concrete to freeze and expand into the bubbles, thereby preventing the cracking and spalling of concrete.
Correia says that in addition to protecting the new concrete from extremely low temperatures, it is also important to reduce the temperature differential between the core and outer surface of the concrete to an acceptable level while the concrete is gaining strength to avoid thermal cracking.
Protection from moisture loss due to evaporation is easily achieved using Chryso® ProFilm 19. Correia says this is an evaporation reducer that offers contractors a reliable controlled way of preventing the rapid evaporation of water. Suitable for use on fresh concrete prior to the initial set, it produces an effective continuous barrier film over the concrete surface. It can be applied after compaction and initial striking of concrete.
A range of other Chryso® Curing Agents is available to use after the initial set and final surface finishing. These include Chryso® Cure Acrylic, Chryso® Cure WB (wax based), Chryso® Cure WP (pigmented) and Chryso® Cure HPS – all are suitable for use on pavements and slabs. The first three are used on columns beams and walls, while Chryso® Cure Acrylic is most appropriate for the top of columns, beams and walls.
More information from Kirsten Kelly, Tel:+27(0)11 3959700/www.chryso.com
