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Curing of High Strength Concrete
High Performance Concrete has very low water-cement ratio and better particle distribution due to the use of mineral admixtures, which result in significantly less pore per unit volume of cementitious materials in the mixture than the CCC. Filling of the voids by hydration product in HPC is much faster than that of conventional concrete as smaller pores needs less hydration products to fill. Therefore, moisture loss due to capillary action stops earlier in case of HPC compared to CCC under the same curing conditions. The moisture loss from HPC has been found predominant upto the first 24 hours. Owing to very low water-cement ratio and use of superplasticizer, the early stage hydration rate of HPC is higher than conventional concrete leaving less long term hydration potential. Thus curing duration after the initial moisture protection has been found to have little effect on long term chloride permeability of HPC containing microsilica or fly ash. All these indicate that the requirement of curing duration for HPC is less compared to CCC. Duration of wet curing has significance on the shrinkage of HPC, which is not the case with conventional concrete. Method of curing has similar effect on HPC both for creep and shrinkage of concrete, which are again influenced by the type and duration of curing. Curing is the most intricate part of construction of the structures with HPC. For a given level of workability, HPC has lesser quantity of water compared to the conventional cement concrete, sometimes being lower than the minimum necessary for complete hydration and self-desiccation. Therefore, loss of moisture from the concrete at an early stage leads to detrimental effects on the soundness and long term properties of the concrete. Therefore, protection against moisture loss from fresh HPC is crucial for the development of strength, prevention of plastic shrinkage cracks as well as for durability. Again, wet curing of HPC cannot be done at an early stage because this will increase the water-binder material ratio adjacent to the exposed surface causing deterioration of the concrete quality.
In one of the studies, it was found that moisture loss from HPC is maximum during the first 24 hours after placement. Fresh concrete mix of HPC is more cohesive and bleeding is very less compared to that of CCC.
Evaporation of bleed water takes place rapidly which makes HPC more prone to plastic shrinkage cracks. Critical time to start forming of plastic shrinkage cracks is around the initial setting time. Therefore ,plastic shrinkage cracks can be very serious problem under curing condition of elevated temperature, low humidity and high winds, which accelerate the evaporation of water from fresh concrete.
Therefore, to overcome this problem, curing process should start immediately after the placement of fresh HPC. Wet curing, if applied immediately, after the placement of concrete to combat plastic shrinkage cracks, as in the case of CCC, would also have harmful effects on the quality of surface layer of the hardened concrete.
In case, wet curing is applied before final setting of the concrete ,the curing water will dilute the cement paste near surface thereby increasing w/c ratio. As a result, strength and impermeability properties of concrete will be seriously hampered. Therefore, HPC should be cured at an early stage without applying water directly on the exposed surface of fresh concrete. This calls for entire curing procedure for HPC to be divided into two stages.
Therefore ,Curing of HPC is generally done in two stages-Initial curing and wet curing. Water is not used directly during the initial curing. Time of commencement of both stages of curing and their duration depends on the initial and final setting time of concrete.
It is difficult to make a general specification for curing, applicable for all weather conditions as well as for all types of structural elements.Loss of moisture from fresh HPC depends on the ambient conditions,Wind velocity,temperature and Humidity and also exposed surface area to volume ratio(s/v).Structural geometry,reinforcement layout and construction methods have bearing on the initial curing procedure.
