Accelerated curing
Encyclopedia
Accelerated curing is any method by which high early age strength is achieved in concrete
. These techniques are especially useful in the prefabrication
industry wherein high early age strength enables the removal of the formwork
within 24 hours thereby reducing the cycle time resulting in cost saving benefits. The most commonly adopted curing techniques are steam curing at atmospheric pressure, warm water curing, boiling water curing and autoclaving
.
A typical curing cycle involves a preheating stage, known as the "delay period" ranging from 2 to 5 hours; heating at the rate of 22oC/hour or 44oC/hour until a maximum temperature of 50-82oC has been achieved; then maintaining at the maximum temperature, and finally the cooling period. The whole cycle should preferably not exceed 18 hours.
process moves more rapidly and the formation of the Calcium Silicate Hydrate
crystals is more rapid. The formation of the gel and colloid
is more rapid and the rate of diffusion of the gel is also higher. However, the reaction being more rapid leaves lesser time for the hydration products to arrange suitably, hence theprocesses is lower in comparison to normally cured concrete. This has been termed as the crossover effect.
The optimum temperature has been found to be between 65 to 70oC beyond which the losses in later age strength have been found to be considerably higher.
, which is formed by the transformation of metastable monosulfate. Delayed ettringite formation (DEF) induces expansion in the concrete thereby weakening it. DEF is promoted by the formation of the cracks which enables the easy entry of water. Therefore, a delay period is allowed to elapse before the commencement of the curing process to allow the concrete to gain a certain minimum tensile strength. The setting time of the concrete is an important criterion to determine the delay period. Generally, the delay period is equal to the initial setting time which has been found to give Satisfactory results. Lesser delay periods result in compressive strength losses.
due to the "crossover" effect. Higher temperatures would reduce the cycle time and therefore improve the economy of the manufacturing process, however, the compressive strength obtained would also be lower. Therefore, it is a trade-off between cost saving benefits and the loss in compressive strength. Depending on the type of project and economic considerations, either the cycle time is designed to suit the concrete mix or vice versa.
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
. These techniques are especially useful in the prefabrication
Prefabrication
Prefabrication is the practice of assembling components of a structure in a factory or other manufacturing site, and transporting complete assemblies or sub-assemblies to the construction site where the structure is to be located...
industry wherein high early age strength enables the removal of the formwork
Formwork
Formwork is the term given to either temporary or permanent molds into which concrete or similar materials are poured. In the context of concrete construction, the falsework supports the shuttering moulds.-Formwork and concrete form types:...
within 24 hours thereby reducing the cycle time resulting in cost saving benefits. The most commonly adopted curing techniques are steam curing at atmospheric pressure, warm water curing, boiling water curing and autoclaving
Autoclave (industrial)
Industrial autoclaves are pressure vessels used to process parts and materials which require exposure to elevated pressure and temperature. The manufacture of high-performance components from advanced composites often requires autoclave processing....
.
A typical curing cycle involves a preheating stage, known as the "delay period" ranging from 2 to 5 hours; heating at the rate of 22oC/hour or 44oC/hour until a maximum temperature of 50-82oC has been achieved; then maintaining at the maximum temperature, and finally the cooling period. The whole cycle should preferably not exceed 18 hours.
Mechanism
At heightened temperatures, the hydrationMineral hydration
Mineral hydration is an inorganic chemical reaction where water is added to the crystal structure of a mineral, usually creating a new mineral, usually called a hydrate....
process moves more rapidly and the formation of the Calcium Silicate Hydrate
Calcium silicate hydrate
Calcium Silicate Hydrate is the main product of the hydration of Portland cement and is primarily responsible for the strength in cement based materials.-Preparation:...
crystals is more rapid. The formation of the gel and colloid
Colloid
A colloid is a substance microscopically dispersed evenly throughout another substance.A colloidal system consists of two separate phases: a dispersed phase and a continuous phase . A colloidal system may be solid, liquid, or gaseous.Many familiar substances are colloids, as shown in the chart below...
is more rapid and the rate of diffusion of the gel is also higher. However, the reaction being more rapid leaves lesser time for the hydration products to arrange suitably, hence theprocesses is lower in comparison to normally cured concrete. This has been termed as the crossover effect.
The optimum temperature has been found to be between 65 to 70oC beyond which the losses in later age strength have been found to be considerably higher.
Delay period
Accelerated curing techniques invariably involve high temperatures. This may induce thermal stresses in the concrete. Further, the water in the pores starts to exert pressure at higher temperatures. The combined effect of the pore pressure and thermal stresses causes a tensile stress within the body of the concrete. If the accelerated curing process is begun immediately after the concrete has been poured, then the concrete will not be able to withstand the tensile stresses as it requires time to gain some strength. Moreover, these microcracks formed may then lead to the delayed formation of ettringiteEttringite
Ettringite is a hydrous calcium aluminium sulfate mineral with formula: Ca6Al2312·26H2O. It is a colorless to yellow mineral crystallizing in the trigonal system...
, which is formed by the transformation of metastable monosulfate. Delayed ettringite formation (DEF) induces expansion in the concrete thereby weakening it. DEF is promoted by the formation of the cracks which enables the easy entry of water. Therefore, a delay period is allowed to elapse before the commencement of the curing process to allow the concrete to gain a certain minimum tensile strength. The setting time of the concrete is an important criterion to determine the delay period. Generally, the delay period is equal to the initial setting time which has been found to give Satisfactory results. Lesser delay periods result in compressive strength losses.
Excessive temperatures
As described earlier, excessive temperatures cause a drop in the Compressive strengthCompressive strength
Compressive strength is the capacity of a material or structure to withstand axially directed pushing forces. When the limit of compressive strength is reached, materials are crushed. Concrete can be made to have high compressive strength, e.g...
due to the "crossover" effect. Higher temperatures would reduce the cycle time and therefore improve the economy of the manufacturing process, however, the compressive strength obtained would also be lower. Therefore, it is a trade-off between cost saving benefits and the loss in compressive strength. Depending on the type of project and economic considerations, either the cycle time is designed to suit the concrete mix or vice versa.