Age-Dependent Strength Assessment of Low Calcium Fly Ash Concrete Based on Ultrasonic Pulse Velocity and Rebound Hammer Number Measurement

In the present research, the ultrasonic pulse velocity (UPV) and rebound hammer number (RHN) are employed to assess the age-dependent compressive strength of low calcium fly ash concrete. Simultaneously, the results are compared with the compressive strength obtained through compression tests. The concrete cylinders having dimensions of 100 × 200 mm were cast to carry out the present investigation. The experiments were performed on eight different concrete mixes with fly ash content varying from 0 to 60%. The age-dependent compressive strength, UPV, and RHN were carried out at different ages, varying from 7 to 180 days. The compressive strength of fly ash concrete mixes obtained using RHN is significantly lower than the experimental values, particularly for higher cement replacement with fly ash in concrete. However, the values of compressive strength obtained through UPV are relatively similar for different percentages of fly ash and at all ages. New models have been proposed to evaluate the age-dependent compressive strength of concrete containing varying percentages of fly ash by using UPV and RHN. The proposed models are adequate for the strength assessment of existing concrete structures containing any percentage of fly ash at any age. The integrity and quality of the structural concrete up to significant depth can be determined with the help of UPV, whereas RHN determines the surface hardness of the concrete. These models will benefit designers and practicing engineers in determining fly ash concrete’s strength without extracting the compressive strength of concrete through a compression test.