# Dynamic Response of Reinforced Recycled Aggregate Concrete Pavement under Impact Loading
## 3 Experimental Results and Discussion  
### Impact Force Characteristics  
According to the test program described above, the acceleration-time history of the drop-weight was obtained from the measurement data of the A3 accelerometer, as shown in Figure 6. The acceleration-time history was used to calculate the impact force-time history. The formula is \(F(t)=ma(t)\), where \(m\) is the mass of the drop-weight that remained at 200 kg consistently in all tests. The impact force-time histories of different tests were similar in shape, showing a high magnitude peak followed by few small magnitude shocks, which were caused by the rebounding of the drop-weight after impacting the RCP specimens. Compared with other subsequent peaks, the magnitude of first impact was very high, and therefore, the impact response under first impact is the most central issue for this research [22; 23]. Figure 6: Acceleration-time histories of the (**a**) 6 MR specimen, (**b**) 7 MR specimen, (**c**) 8 MR specimen, (**d**) 7 MN specimen, and (**e**) 7M1R specimen. To assess the inertial force during the impact test, a simple integration approach is proposed; that is, to use the recorded A1 and A2 acceleration data to estimate the inertial force of the RCP specimen [26]. The acceleration-time histories of the A1 and A2 accelerometer are shown in Figure 6. The accelerometer A2 was placed further away from the impact area than was the accelerometer A1, and it was thought that the phase diversity of the acceleration between A1 and A2 reflected the lag in response, owing to the force propagating from the point of impact to the edge [42]. The RCP specimen gained significant downward acceleration immediately after impact and upward inertial force was induced. The RCP specimen was divided into three tributary integration areas according to the positions of accelerometers, as shown in Figure 7. For Area 1, the acceleration was assumed to be uniformly distributed and its value was equal to the value of the A1 sensor. The acceleration was assumed to have a linear distribution for both Area 2 and Area 3. For Area 2, the values of the inner and outer boundaries of acceleration were the values of the A1 and A2 sensors, respectively. For Area 3, since the size of the RCP specimen was much larger than that of the impact area, it could be considered that the acceleration at the far edge of the specimen was sufficiently weak to be ignored. Accordingly, the values of the inner and outer boundaries of the acceleration were the value of the A1 sensor and zero, respectively. The inertial force of the RCP specimen could be calculated by summing up the inertial forces of the three tributary areas. For all the specimens, impact force and inertial force rose immediately and reached their peak values shortly after the impact began. The impact force suddenly dropped due to the deformation and cracking of the RCP specimen. As the impact force decreased, the inertia force decreased and dissipated. The peak impact force was detected after circa 7-8.5 ms following the first contact between the drop-weight and the RCP specimen, and the result is summarized in Table 6. There was a time lag of circa 1-3 ms between the peak impact force and the peak inertia force, which was due to the stress wave propagation travelling gradually from the impact area to the far edge [25; 31; 43]. This stress wave traveled at varying speeds within the speed of sound, depending on the mass, the density and the elastic modulus of the concrete type used [44]. Comparing the impact force with the inertia force, it is obvious that the peak amplitude of impact force is greater than that of the inertia force. The reason is that most of the impact force was converted into inertia force, while a portion of impact force was balanced by ground rea  
Figure 7: Integration areas of the inertial force.