Initial study to determine the tensile material properties of fresh concrete

The early age cracking of concrete often occurs in flat concrete elements such as bridge decks and slabs exposed to conditions with high evaporation. These cracks, which include plastic shrinkage and plastic settlement cracks, have a premature detrimental effect on the durability of concrete structures. Cracking occurs when mechanisms such as differential settlement and capillary pressure cause the development of tensile stresses in the concrete which exceeds the tensile strength of the concrete. A fundamental understanding of the tensile properties of fresh concrete and the variables that affect them are imperative in order to understand and reduce the risk of early age cracking. In addition, the tensile properties such as ultimate tensile strength, tensile strain capacity and Young's modulus all as a function of time are also required as input parameters for the numerical analysis of early age cracking of fresh concrete which can ultimately be used to predict and prevent cracking. This paper gives a summary of the most significant work done in this field over the last few decades. This is followed by the description of initial results achieved with a newly built test setup which aims to measure the tensile material properties of fresh concrete specimens at ages from 1 hour after mixing and onwards. Although the new test setup showed promise it still encountered several issues which are discussed in the paper. These issues were mainly due to the difficulty of handling and testing plastic concrete at such an early age. Finally, an improved test setup is proposed based on the knowledge gained from the initial test setup as well as the literature study. This improved test setup is currently under construction and forms part of a larger ongoing research project at Stellenbosch University aimed at controlling the early age plastic cracking of concrete.