# Evaluation of the Functional Performance of Paving Materials Based on the Driving Wheel
## 4 Results and Discussion
### Tire-Pavement Noise  
Two microphones were used to collect the tire-pavement noise of the specimens under the set number of loading repetitions, and the variation of the tire-pavement noise for different mixtures was obtained through calculation and analysis using MATLAB software (7.0, MathWorks, Natick, MA, USA). The A-weighting filter was adopted to mimic the noise response of the human hearing system. The test results are shown in Figure 13.  
Figure 12: Spalling mass-loss rate under different wheel-load repetitions.  
Figure 13: Tire-pavement noise of mixtures under different wheel-load repetitions.  
The tire-pavement noise increased with increasing wheel-load repetitions for all materials. The growth rates of the noise sound pressure for OGFC-13 and SMA-13 were lower than those for AC-13. At 70,000 wheel-load repetitions, the tire-pavement noise for the OGFC-13 mixture and AC-13 mixtures increased by 1.7 dB(A) and 3.5 dB(A), respectively, compared to the initial. The tire-pavement noise of the micro-surfacing materials increased slowly; the noise sound pressure of all three micro-surfacing materials had increased by only 0.6-0.7 dB(A) by the end of the test.  
According to the absolute value of the noise, OGFC-13 produced the lowest tire-pavement noise, followed by SMA-13. The AC-13 mixture produced the lower noise in the early stages of the test, but by the end of the test, the noise level was close to that of the micro-surfacing material. Furthermore, for the cold-mix mixtures, the micro-surfacing materials exhibited a worse tire-pavement noise performance. The coarser the gradation of the micro-surfacing material, the higher the tire-pavement noise. Therefore, tire-pavement noise reduction should be one of the main research directions for the optimization of micro-surfacing materials. According to the Environmental Quality Standard for Noise (GB3096-2008), there are five categories of acoustic environment functional areas [42]. Among them, the daytime noise limits of category ii (residential and commercial areas), category iii (industrial area), and category iv (arterial roadside area) are 60 dB(A), 65 dB(A), and 70 dB(A), respectively. Therefore, micro-surfacing was recommended to be applied on the arterial road only. The OGFC-13 mixture was the best material for pavement in residential and commercial areas. AC-13 and SMA-13 mixtures could be used in the industrial area.  
The noise frequency spectrum of various pavement materials was extracted using MATLAB software. Considering that the MS-3 material is the most widely used micro-surfacing in China, the noise frequency characteristics of AC-13, SMA-13, OGFC-13, and MS-3 were analyzed (Figure 14). The frequency of tire-pavement noise for AC-13, SMA-13, OGFC-13, and MS-3 exhibited distinct characteristics. The noise frequency of the four materials was a wide continuous spectrum, the main frequency range was 200-5000 Hz, and the spectrum peak value was between 600 Hz and 1000 Hz. The spectrum peak value of AC-13 was the largest, followed by SMA-13, MS-3, and OGFC-13. Considering that human hearing is sensitive to noise in the frequency range of 500-2000 Hz, the noise frequency curves of the four materials in this frequency range can be subdivided into the frequency range of 700-1600 Hz, which is the region where the air pump effect occurs. The OGFC-13 produced the lowest tire-pavement noise in this frequency range, followed by SMA-13.  
Because the OGFC-13 material had a higher connected porosity, it dissipated more noise energy to greatly reduce the pump noise between the tires and the pavement. SMA-13 had fewer voids than OGFC-13, but the texture depth of SMA-13 was better than that of AC-13 and MS-3. Therefore, the tire-pavement noise of SMA-13 was also reduced in the low-frequency band compared to AC-13. The sound pressure of the MS-3 micro-surfacing material was substantially higher than that of the other hot-mix materials over the entire frequency range.