What is the Mechanism of Drying Shrinkage of Concrete?
Water molecule has thickness is considered as 1% of particles size of gel which indicates a linear dimension change of cement paste upon complete drying. The actual value observed was up to 400 X 10-6 but theoretically these may be 10000 X 10-6.
But specific surface of cement paste cured by high pressure steam is low; the microcrystalline structure suffer lesser shrinkage of (5~10) times than normally cured cement paste. Sometimes 17 times less shrinkage are observed. The coarse natural stone shows less drying shrinkage relative to fine grained stone, which is the example of influence of gel particles on drying.
We can relate shrinkage or fraction of it with removal of water from intercrystalline structure. It is found to change of space between lattice of hydrate of calcium silicate from (1.4~0.9) nanometer on drying. Hydrated tetra calcium aluminate & calcium sulfo-aluminate also show identical behavior but at different extent.
Thus we are not sure that whether intra-or-intercrystalline moisture movement produces shrinkage. But as both Portland cement and high-alumina cement, sometimes pure ground granulated calcium mono aluminate also used to make paste and all exhibit similar behavior especially in shrinkage, the basic cause behind shrinkage should be considered due to physical structure rather than due to mineralogical and chemical character.
A relation between shrinkage and mass of lost water are shown in Figure.1. In case of neat cement paste, shrinkage is proportional to water loss as there has no capillary water within the paste, only water adsorbed is removed.
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| Figure 1: Relation between shrinkage of specimen and water loss (by mass),made of cement paste (added pulverized silica, dried after curing for 7 days at 21oC)
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However, when pulverized silica is added to mix and to which high water/cement ratio is required (to have greater workability), it has capillary pores (even after complete hydration). Capillary water is important as this water don’t result shrinkage after removal.
After removal of capillary water, adsorbed water is removed. When adsorbed water is removed, shrinkage is taken place as in the case of neat cement paste which is represented by final slope of every curves in Figure 1.When aggregates added to concrete having some water in their pores with large cavities, even more variation in shape of curves are observed in shrinkage Vs water loss plot.
Loss of water also depends on sample size. In concrete specimens, Mensi et al. developed a general pattern of water loss with variation of distance from surface at which drying starts. He made an assumption that diffusion rate of vapor have proportionality relation with √t1,( t1=time elapsed).
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| Figure 2: Water loss from various sizes of specimen (prisms, ambient relative humidity=55%) |
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| Typical periods of drying of concrete |
In following table time required to dry concrete from different distance from surface to loss evaporable water (80% removal) are listed. In interpreting data for calculating water loss due to shrinkage, there have further complication with formation of surface cracking on specimen. In laboratory condition small specimens have less surface cracking.
In full size concrete members, actual surface cracking obviously affects effective shrinkage. There have also internal redistribution of stresses; cracking also possibly increase rate of water loss.