# Dynamic Response of Reinforced Recycled Aggregate Concrete Pavement under Impact Loading
## 3 Experimental Results and Discussion
### Impact Force Characteristics
All RCP specimens were impacted by a free fall of 200 kg drop-weight from a constant height of 1.0 m, so in the case of ignoring friction and air resistance, when the drop-weight impacted the RCP specimen, the instantaneous impact velocity \(v_{\rm im}\) was about 4.4 m/s and the maximum impact energy \(E_{\rm im}\) was 1.96 kJ. The reported impulse \(I_{\rm p}\) is the time integration of impact force. The impulse-momentum theorem states that the impulse is equal to the change of momentum [23; 24; 25]. Thus, it is expected that the rebound velocity \(v_{\rm r}\) of the drop-weight can be calculated by the formula \(I_{\rm p}\)= \(m\times v_{\rm im}\)-\(m\times v_{\rm r}\). Once the rebound velocity \(v_{\rm r}\) is known, the residue kinetic energy of the drop-weight can be calculated. In calculating the energies for the impact test, the current study neglected the energy dissipated in the following mechanisms: the free vibration of the RCP specimen and steel strongbox, and the energy losses due to heat and noise [37; 45]. Thus, the energy absorbed by the RCP specimen can be calculated via subtracting the residue kinetic energy of drop-weight from the impact energy [27]. The calculated rebound velocities \(v_{\rm r}\), absorbed energies \(E_{\rm ab}\), and its ratio over impact energies of different tests are listed in Table 6.  
It can be seen from Table 6 that the characteristic value of impact force varies in accordance with the longitudinal reinforcement spacing, the concrete type, and the thickness. Comparing 7 MR with 7 MN, the peak impact force of 7 MR is 13.8% smaller than that of 7 MN. According to the contact theory proposed by [PERSON] [46], the force between two objects in contact is proportional to the relative elastic modulus. The 7 MR specimen showed a lower impact force, which can be attributed to a lower modulus of elasticity of the RCA mix compared with the NAC mix. The peak inertial force of 7 MN is very similar to that of 7 MR, and the difference between the two is less than 5%. This is likely due to the fact that the steel reinforcement contributed more to the stiffness of the RCP specimen in this state, thus reducing the relative influence of concrete on the overall stiffness. A tendency is observed that the peak impact force increased with the increase in reinforcement ratio. As can be seen from Figure 8, the peak impact force of 7M1R is 25.3% higher than that of 7 MR, which is the maximum value among all RCP specimens. Therefore, increasing the reinforcement ratio could improve the stiffness of the RCP specimen and have a significant effect on the impact force [22; 23]. In addition, it was observed that the peak inertial force slightly increased as the reinforcement ratio increased. Compared with 7 MR, the peak impact force of 6 MR increased by 17.1%, while the peak impact force of 8 MR decreased by 14.2%. According to the research results of [PERSON] [26], the thickness could increase the impact resistance and stiffness of the RCP specimens; therefore, an increase in the peak impact load should be also observed. The reason for this is that, although 6 MR, 7 MR and 8 MR specimens had the same reinforcement layout scheme of D8@150, the reinforcement ratio decreased with the increase in thickness. When considering the peak impact load, the influence of the thicknesses was relatively lower compared with the effect of the reinforcement ratio, which has paramount relevance. In addition, it was observed that the impact force duration slightly decreased as the reinforcement ratio increased in the 8 MR, 7 MR, 6 MR and 7M1R specimens. As can be seen from Figure 8, the peak inertia force variation rules of the RCP specimen were different. The peak inertia force of the 8 MR specimen with the largest thickness was maximum, while the peak inertia force of the 6 MR specimen with the lowest thickness was minimum.