###### Abstract  
The ultrasonic transmission detection method is used to investigate the applicability for the second-harmonic generation (SHG) technology of longitudinal wave to quantitatively assess carbonated concrete. The principal of this method is to use the piezoelectric lead zirconate titanate (PZT) patch to detect the second-harmonic of longitudinal waves during the concrete carbonation process and extract non-linear parameters from observed signals. Non-linear parameters of concrete with two water-cement ratios (CI (\(w/c=0.47\)), CII (\(w/c=0.53\))), two moisture contents (CI 0%, CI-W 100%), and three ultrasonic incident frequencies (50 kHz, 75 kHz, 100 kHz) were measured in this study. Results of the experiment demonstrate that non-linear ultrasonic parameters of longitudinal ultrasonic waves with high frequencies (75 kHz, 100 kHz) exhibit a better resolution regarding changes in concrete microstructure. Moisture (CI 0%, CI-W 100%) has little effect on the rate (CI: 62.73%, CI-W: 60.25, carbonation depth: 15 mm) for the change in relative non-linear parameters in the same concrete. The carbonation depth of concrete (CI (\(w/c=0.47\)), CI-W (\(w/c=0.47\)), CII (\(w/c=0.53\))) can be well reflected by the change in relative non-linear parameters. Furthermore, there exists a good fit between the relative non-linear parameters of longitudinal waves and the concrete carbonation process. The relative non-linear parameters of longitudinal waves demonstrate feasibility in the quantitative assessment of concrete carbonation. c 20222022022022022022022022022022022022202202220222202  
detected and extracted the spectral components associated with carbonation from the Raman spectrum after using Raman spectroscopy to identify spectral changes in the concrete carbonation process. The spectrum was verified to detect carbonation reaction early. During concrete carbonation, the pores and microcracks in concrete change as a result of the carbonation products [3; 4], and these changes significantly impact the material non-linearity. The feasibility of the non-linear Resonant Ultrasound Spectroscopy (NRUS) technique was verified by [PERSON] et al. [5] in concrete carbonation by measuring the resonant frequency shift of the signal in a given mode with the resonant peak amplitude [5; 6]. When large-value, high-energy ultrasonic waves penetrate non-linear media, the propagation of the ultrasonic waves exhibits a strong non-linear effect [7]. Ultrasonic waves are distorted during propagation, resulting in the formation of higher harmonics [8; 9; 10]. The products of the carbonation reaction fill the pores in carbonated concrete, resulting in an effect in the non-linear response of concrete. As such, the fundamental amplitude and the harmonic amplitude of the transmitted wave are altered by this effect [9; 11]. [PERSON] et al. [11] determined the influence of concrete carbonation on the relative non-linear parameters of the Rayleigh surface wave by using the second-harmonic (SHG) technique of the Rayleigh surface wave combined with wedge-air coupling measuring device [9; 11]. Whatever NRUS are generated by hammering or the SHG technology of Rayleigh surface wave, it qualitatively evaluates the effect of carbonation on the non-linear ultrasonic response of concrete. There is no further study on the feasibility of quantitatively evaluating concrete carbonation depth with relative non-linear parameters. In this study, the SHG of longitudinal waves and the ultrasonic penetration detection method are used. Although the SHG technology used us the same as the surface wave in this study, the detection distance of the surface wave is determined by the wavelength, whereas the detection distance of the longitudinal wave is determined by the distance between the two transducers. Subsequently, the applicability of the SHG technology of longitudinal waves to quantitatively assess carbonated concrete is verified in this study.