Developed Schmidt hammer based on jerk measurement

The evaluation of hardened concrete quality became significantly important as a result of the continuous and increasing demand on concrete. Mechanical rebound hammer is a quick non-destructive evaluation method used to test the performance of hardened concrete onsite to assure the quality of newly casted concrete or asses the performance of old concrete elements. However, rebound hammer values have uncertainty problems in detecting concrete stiffness and are less reliable. In addition, the device performance is degraded with time due to the fatigue of the mechanical spring. Consequently, this paper presents a new methodology for developing the current measurement technique of Schmidt hammer by using a jerk sensor. Jerk sensors are used in the field of analytical dynamics which includes sensing of acceleration and rate of change of force. The sensor idea is based on using a gyroscope fixed at the free end of a metal cantilever. A cantilever (L-18) of dimensions 18x5x0.5 [mm] and natural frequency 202 [Hz] was designed and modeled using ANSYS finite element analysis (FEA) software. Modal and harmonic analyses were conducted to determine the sensor sensitivity for input jerk. The sensor was applied by constructing a finite element model (FEM) of the mass-spring system inside Schmidt hammer and attaching the sensor to the hammer mass. The correlation relationship between the sensor response and the stiffness modulus of concrete was examined by conducting a transient analysis simulating the Schmidt hammer test of 6 cubic concrete specimens of different grades 20, 30, 37, 45, 50 and 60 [MPa]. Each concrete grade has a different modulus of elasticity. FEA results showed that the L-18 cantilever model has a constant jerk sensitivity of 0.047 [(deg/s)/(G/s)] in the bandwidth 0.1~60 [Hz] and a maximum sensitivity of 1.0 [(deg/sec)/(G/s)] at impact (resonance). The predicted input jerk and output angular velocity are in phase. In addition, the results proved that the stiffness modulus of concrete can be evaluated more efficiently at the impact instant using the rate of change of impact force rather than the rebound distance.