What is meant by Dry Shrinkage? How to prevent it? How is it different from creep?
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After unloading, creep recovery takes place, but it is partial, because of aging. In practice, creep during drying is inseparable from shrinkage. The difference called the drying creep or Pickett effect (or stress-induced shrinkage) represents a hygro-mechanical coupling between strain and pore humidity changes.
Drying shrinkage occurs within the cement paste fraction of the concrete. Therefore the occurrence of drying shrinkage cracking can be reduced by using a concrete with a lower water content or by increasing the aggregate volume where possible to minimise the cement paste volume.
Dry shrinkage is defined as the contracting of a hardened concrete mixture due to the loss of capillary water. This shrinkage causes an increase in tensile stress, which may lead to cracking, internal warping and external deflection, before the concrete is subjected to any kind of loading.
Dry shrinkage or drying shrinkage occurs due to loss of moisture in the concrete. There are different types of water in a hydrated cement paste. Dry shrinkage occurs due to the loss of capillary water.
This water can be found in voids of sizes ranging from 5 nm to 50 nm in the hydrated cement paste. Due to the small size of the voids, the water is held inside by surface tension or capillary tension. Due to the bond created by the surface tension, evaporation, or any other form of removal of this water causes significant volume changes to the structure. This volume changes causes dry shrinkage to the structure.
Dry shrinkage is the contraction or shrinkage of hardened aging concrete due to the loss of capillary water mainly through evaporation. It depends on the amount of water used at the time of mixing and can cause serious loss of strength and major deflections and cracking.
The standard consistency or standard water content that the cement will completely use up for Ordinary Portland cement is 28%. But now, it has got two problems. Firstly, it so turns out that the cement in the concrete doesn’t hydrate completely at 28% moisture even though after pretty good mixing. To improve it, you’d need to hand mix 100g samples for 10 minutes each as we do for the standard consistency test. This isn’t feasible. Also, even then, there won’t be complete hydration. To ensure complete hydration, we must use around 45% to 55% water content for cement. Secondly, the concrete has very poor workability at 28% w/c and needs at least 45% water to be well workable. Now, the cement will use up around 28% water, and the rest will remain there as capillary water. This water, when evaporates with age, causes drying shrinkage.
The effects are cracking, deflection, and loss of strength before the service life of the concrete members.