Influence of Ultrafine Fly Ash on the Early Age Response and the Shrinkage Cracking Potential of Concrete

In this paper, the influence of ultrafine fly ash on the early age property development, shrinkage, and shrinkage cracking potential of concrete is investigated. In addition, the performance of ultrafine fly ash as cement replacement is compared with that of silica fume. The mechanisms responsible for an increase of the early age stress due to restrained shrinkage were assessed; free shrinkage and elastic modulus were measured from an early age. In addition, the materials resistance to tensile fracture and increase in strength were also determined as a function of age. Results of the experimental study indicate that the increase in elastic modulus and fracture resistance with age are comparable for the control, ultrafine fly ash, and silica fume concretes. Autogenous shrinkage is shown to play a significant role in determining the age of cracking in restrained shrinkage tests. A significant reduction in the autogenous shrinkage and an increase in the age of restrained shrinkage cracking were observed in the ultrafine fly ash concrete when compared with the control and the silica fume concrete. Increasing the volume of ultrafine fly ash and decreasing the ratio of water-to-cementitous materials resulted in further increase in the age of restrained shrinkage cracking and a significant increase in the compressive strength.