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 super plasticizer, 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 micro-silica 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.
1. Initial curing method:
The intricate part of HPC curing process is the initial curing.The objective of the initial curing is to prevent moisture loss from the fresh concrete till the time wet curing is started.Construction procedure,characteristics of the exposed surface,exposed surface/volume ratio,and the environmental condition have significant bearing on the initial curing methods for HPC.The concrete
surfaces,which are generally encountered for curing can be categorized principally into two types,namely Type-1 surface-where the exposed surface of fresh concrete which will be exposed to the service condition and /or environment after curing.The exposed surface area to volume ratio of this type is high.Examples are-Slabs,shell roofs,beam,etc.
Further ,this type of surface can be sub-categorised as:
Type –1 A surface :Where the concrete surface on which the finishing work is to be carried out at the time of construction ,viz. Shell segment of the dome;
Type –1 B surface :The concrete surface for which no finishing work is specified,Viz.Concrete pavements.
Type – 2 surface –Where the concrete beams are of large depths,Viz. the ring beam of IC Dome,etc.This type of surface has lower exposed area to volume ratio.In this case,exposed surface of the segmented pours except the last one of these segmented pours have reinforcement extended for the next pours and to form the construction joints.Latent needs to be removed from the exposed surface in order to achieve quality construction joints.
Moisture loss from Type-1 surfaces are generally more that that from Type-2 surfaces considering the ratio of surface area to volume.Therefore separate methods for initial curing of each of these surfaces are important.
1.1. Initial curing for Type-1 surfaces:
Curing compound has not been found to be very effective for initial curing. Immediately after the placement of fresh concrete, water sheen (bleed water) appears on the top of the concrete surface. If curing compound is spread before this water sheen dries, local ponding of the curing compound mixed with the water sheen occurs on the concrete surface.Again,allowing the water sheen to be completely evaporated may be harmful for the long-term properties of concrete expecially in dry and hot climate.It was seen in many cases that ,cracks with random orientation develops on the membrane formed by the curing compound when it dries up.These cracks are not plastic shrinkage cracks.These cracks are responsible for making the curing compound ineffective in preventing the moisture loss from the exposed surface of fresh HPC.
Finishing work on this type of surface is to be done during the period of initial curing.It cannot be started immediately after the placement of fresh concrete.In the case of Kaiga and Rajasthan nuclear power plant constructions,this time has been reported as about 2 hours after the placement.The surface finishing work has to be completed prior to the commencement of initial setting process of HPC.The surface needs to be covered prior to starting surface finishing work.Duration of surface finish work has to be as minimum as possible. Many literatures have reported that-covering of fresh HPC by plastic sheet is an efficient method for initial curing.
1.2. Initial curing for Type-2 surface :
Two additional considerations for initial curing of Type –2 surface are :
1)the exposed surface has reinforcement extending out of it, and
2) the surface is to be treated to remove the latent for concreting the next pour over it in order to achieve good quality construction joint.
It is not feasible to spread any coversheet over this type of surface as in the case of Type-1 surfaces and the latent on the exposed surface needs to be removed for achieving good construction joints. Green cutting seems to be the best method for removing the latent from the concrete surface. However, it is to be done after the final setting time so that the additional water available from the water-air jet of green cutting on the surface does not cause harm on the quality of concrete. Based on various studies done, it is recommended that the curing procedure for various types of surfaces.
The rate of achieving strength at early stage is faster for HPC compared to CCC.The above two aspects give rise to two contradictory situations and the best way is to overcome this is by retarding the setting of the exposed surface of HPC. Surface retarders can retard the setting process of HPC adjacent to the exposed surface to a depth of 4-5 mm even 12 hours after the placement of concrete. Quality of retarder has bearing on the depth of retardation and the uniformity of green cut surface.
Application of surface retarder could effectively prevent the formation of plastic shrinkage cracks upto the time of final setting. Moisture loss is less when concrete is covered which is obvious. In one of the case studies ,at Kaiga Nuclear power plant in 1998 ( Karnataka),where M 60 grade HPC was used , potential of appearing plastic shrinkage cracks was found to be higher in finished surface compared to the one without finishing.
2. Final curing method (Wet curing) :
For final curing, wet curing as adopted for conventional concretes, such as ponding water on the exposed surface or covering the exposed surface by wet burlap and keeping it wet by continuous sprinkling of water has been found to be effective.
Curing Duration for HPC:
The initial curing of HPC should be started immediately after the placement of fresh concrete and continued till the final setting of the concrete. A better proposition may be to extend it about an hour after the final setting time. The initial curing is followed by wet curing. Total curing duration of HPC is the sum of the initial curing duration and wet curing duration out of which the second part is the longer one.
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. Overall, considering the above, a curing duration of 7 to 10 days seems to be necessary for HPC though curing duration of about 1-2 days could be sufficient from strength gaining .