Curing of Concrete in Construction works
Apart from bricks, which continue to control the construction business, concrete is the most often utilized man-made material today for constructing various structures. Custom-made concrete is now feasible because to the development of numerous grades and types of blended cements, as well as the use of admixtures.
Despite developments in concrete technology, many of its users are unaware of the need of ‘curing’ and its application to specific requirements in order to achieve the desired concrete characteristics.
Good curing allows for longer hydration of cementitious components and the formation of a well-developed micro-structure in concrete, allowing it to operate to its maximum capacity. It is generally understood that while improper curing has no visible effect on compressive strength, it has a major effect on concrete durability.
The primary component of concrete is Portland cement. To minimize greenhouse gas emissions, more additional cementitious materials, such as pulverized fuel ash and crushed, granular blast furnace slag, are being used to partially substitute cement.
Lately, High Performance Concrete (HPC) has gained popularity due to the inclusion of Silica fume, which makes the concrete more cohesive with little or no bleeding during placement and compaction.
Partially replaceable materials can improve durability, minimize the risk of thermal cracking in mass concrete, and have lower embedded energy than ordinary Portland cement (OPC). However, the use of such mixed cements requires a longer curing time to achieve the desired strength than OPC.
Similarly, the use of HPC requires an early start of initial curing.
Permeability, which determines the degree to which concrete is damaged by outside forces and causes internal reinforcements to corrosion, is one of the most crucial factors affecting concrete’s durability.
Ponding/immersion, spraying/fogging liquid applied evaporation reducers, and saturated wet covering are some techniques that maintain water level in the concrete mix throughout early time of hardening. Evaporation reduces are considered as first curing techniques among these liquid treatments and spraying/fogging techniques.
Concrete can be covered or protected with heavy paper or plastic sheets to prevent mixing water from evaporating from its surface, or membrane-forming curing agents can be used.
The other techniques for increasing strength development involve heating and wetting the concrete, often using steam or heating coils. Depending on the size and shape of the section, the concrete’s age, and the materials’ availability, a mix of the proposed techniques is used, preferable for quick construction under factory-controlled circumstances. The amount of hardening required to prevent a given operation from harming a concrete surface determines the time of each approach.
The early loss of moisture due to evaporation will be quick, and plastic shrinkage cracks shall form, for rigid pavements and airfield pavements that have a high surface area to volume of concrete.
The strength, abrasion resistance, and durability of pavement will be affected by a continuous loss of moisture and a reduction in degree of hydration. The whole exposed surface must be covered and kept moist until the specified curing time is complete and/or the necessary qualities of concrete have formed in order to prevent plastic shrinkage cracks.
After curing, the surface of cast-in-place bridge substructures, superstructures, and retaining walls must be protected from excessive loss of moisture by covering wet cloth or hessian with plastic sheets until the surface dries below the sheet.
It is also advised that the water used to cure concrete be drinkable, which is costly owing to the usual lack of water supply in an urban setting. In Germany, recycled water from the recycling of unset/discarded concrete is used as mixing water in Ready Mix Concrete operations.
However, raw water from sewage facilities is not yet widely accepted until chloride and sulphate levels are lowered by secondary or tertiary treatment and human safety is ensured due to the presence of microbiological and chemical contaminants.
Strength and setting time, with limitations on chlorides and sulphates content, are the performance requirements for mixing concrete in other industrialized nations.
However, there is a possibility of using recycled sewage water after secondary and tertiary treatment for concrete curing. While water curing after final setting of concrete is well recognized, the use of liquid membrane-forming components for final curing is left to the manufacturer’s specifications.
Sheets or liquid membrane forming materials used over concrete to prevent evaporation loss are examples of curing materials. A mixture of plastic film attached to absorbent cloth assists in the retention and distribution of moisture between the film and the concrete surface.
Wax or other organic materials thinned with a solvent can be used to make liquid membrane forming compounds. Other comparable compounds are also utilized, such as those based on water-soluble solids or a water emulsion.
It is advised that the liquid membrane forming compound be applied at right angles to each other. They must be applied shortly after the surface with shine has been finished. When utilizing curing compounds to decrease moisture loss from surfaces, moisten the exposed surfaces immediately after de-shuttering until the curing compound is applied.
When the evaporation rate equals or exceeds the rate of bleeding during cold weather, the concrete evaporation rate increases and the rate of evaporation decreases.
Precautions are taken in hot and dry areas to ensure that nearby partially hardened concrete and form-work do not absorb water from freshly laid concrete. The ice cubes and chilled water used to cool the concrete mix should not include any harmful chemicals, and the mix should not be excessively cold to cause rapid moisture evaporation.
When temperatures are high with wind and/or low humidity, an evaporation reducing coating may be placed once or twice during the finishing process to lessen the possibility of plastic shrinkage cracking.
Placement techniques and schedules should be adjusted to evening or nighttime with sunshades. During the first few days, continuous water curing is advised to minimize volume fluctuations caused by alternate wetting and drying.
For initial prestressing in the some precast members, accelerators and steam curing are used to expedite concrete strength development. However, when different types of blended cements are used to meet performance standards, a minimum curing duration must be specified.
Temperature differences must be high in mass concrete structures such as piers, abutments, and heavy footings, and this is worsened when cements with high cementitious constituents are used.
The hardened vertical surfaces of such members shall be sprayed with water, or when forms are still in place, the form ties may be loosened and water allowed to run down within the form.
The minimum curing duration for un-reinforced portions constructed entirely of ordinary Portland cement is two weeks. When using ground granulated blast furnace slag or the other pozzolana, the curing duration is extended to three weeks. Continuous curing is advised for RCC piers and abutments for 7 days or until 70% of the required compressive strength is achieved.
There are a variety of in-situ tests available to assess the air-void content, temperature (with sensors), and maturity of concrete throughout the hardening process, in addition to slump, compaction factor, flow table tests, and so on, which will be beneficial in making corrections to the curing procedure.
The necessary levels of strength and/or durability are determined by chemical composition, cementitious fineness, w/c ratio, proportion of mix, aggregate characteristics, chemical and mineral admixtures, concrete temperature, and curing procedure efficiency.
As a result, it is suggested that in mass concrete constructions, a Thermal Control Plan be developed that covers, among other things, the maximum temperature limit, installation techniques, expected temperature during hardening, insulation and curing methods, and correction methods. This type of design will meet the curing requirements of roadway construction.
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