Early age setting, shrinkage and tensile characteristics of ultra high performance fiber reinforced concrete

Recently, ultra high performance fiber reinforced concrete (UHPFRC) with compressive strength over 180 MPa and tensile strength of 10 MPa has been developed. Since UHPFRC not only has superior strength, ductility and durability but also reduces the self weight of structures by decreasing the cross sectional areas needed, as such the research for applying UHPFRC to structural members is being widely performed. However, UHPFRC shows different material behavior to normal concrete and conventional high performance concrete. For example, it displays high early age autogenous shrinkage as well as rapid surface drying and surface cracking because of its low water–binder ratio and addition of high fineness admixtures. In this study, therefore, to precisely evaluate the basic material properties of UHPFRC at an early age, penetration resistance, shrinkage, tensile and ultrasonic pulse velocity (UPV) tests were performed. Test results indicate that paraffin oil well prevents the rapid water evaporation on the surface when the penetration resistance test of UHPFRC mortar was performed. The restrained shrinkage stress starts to develop when penetration resistance was 1.5 MPa. Thus, the time-zero of autogenous shrinkage measurements of UHPFRC was defined as when the penetration resistance becomes 1.5 MPa, this is 0.6 and 2.1 h before the initial and final sets, respectively. The tensile strength and elastic modulus were measured from near the initial set using tensile test apparatus and prediction models for tensile strength and elastic modulus of UHPFRC were proposed. Finally, the UPVs at the proposed time-zero, initial and final sets were defined and the very early age strength developments were also predicted using UPV.