Early age autogenous shrinkage and long-term drying shrinkage of fibre reinforced concrete.

The role of fibres against cracking in concrete because of volumetric deformations has become popular over the years. Various researchers have reported the crack-control benefits of fibres in both fresh and hardened concrete. The volumetric changes of concrete are broadly divided into early-age and long-term deformations. The volume change in concrete begins with the hydration reactions when water is mixed with cement. Contrary to the long-term shrinkage deformations, the early-age deformations are usually ignored in design of concrete structures because of lesser magnitude. In autogenous conditions, capillary pressure in pores result in significant deformation, thus creating potential for cracking. Especially in concretes with low water-to-cement ratio, capillary pressure can cause significant autogenous shrinkage deformations. In the absence of moisture evaporation, hydration reactions cause noteworthy thermal heating, followed by cooling in fresh concrete. Moreover, thermal deformations manifest in the form of expansion and contraction in fresh concrete, which produce internal stresses that increase the vulnerability of cracking. The aim of this thesis was to study the effects of different types of fibres on the early-age autogenous and long-term drying shrinkage deformations of concrete. A test setup called Schleibinger Bending-drain was used to record early-age (2days) autogenous deformations of fresh concrete immediately after mixing. The test results were used to study the physical processes controlling different stages of the early-age autogenous deformation in relation to the effect of fibres and concrete composition. The early-age autogenous deformations in Plus and SR cement concrete were significantly reduced by the use of 0.38% by volume of steel and plastic fibres as compared to plain concrete. However, 0.38% by volume of plastic and glass fibres used in White cement concrete had a small effect on the early-age autogenous deformations. Other than fibres, the factors affecting the early-age autogenous deformations were different cement types (hydration heat) and the shrinkage-reducing admixture. The lesser amount of bleed-water in White cement concrete resulted in large magnitude early-age autogenous deformation. The use of fibres at 0.38% by volume did not have a prominent effect on the 56 days unrestrained drying shrinkage deformation. The general opinion that fibres allow more moisture escape by bridging the pores could not be established as similar average water loss (kg/m3) was recorded for both the plain and fibre reinforced concrete mixtures. Moreover, the magnitude of early-age autogenous deformations was approximately 19%, 22% and 51% of the total combined deformations (early-age and long-term) for Plus, SR and White cement concretes, respectively