The severe deterioration of the CA mortar layer may lead to the movement of the upper concrete slab and compromise the safety of the speedy train. In this study, a specimen simulating the structure of high-speed rail track slabs was built with delaminated cracks embedded inside the CA mortar layer. In the specimen, the defects are located at either top concrete slab-CA mortar interface or CA mortar-bottom concrete slab interface. The lateral size of the defect and mortar-thickness are also experimental variables. The impact-echo tests were performed above the defected and good-bond locations. Normalized amplitude spectra were obtained by dividing amplitude of the impact-echo spectrum to the amplitude of the spectrum corresponding to the modified displacement of the Rayleigh wave. The dominant peak frequency and amplitude in the normalized amplitude spectrum were closely related to the existence of the delaminated defect underneath the test point. The results showed the defects could be assessed more easy for a test performed on the region containing the thinner CA mortar layer. Simple indices were developed to identify the soundness of the CA mortar layer. The indices are obtained based on the difference of the experimental peak amplitude and frequency to the ones calculated from the previously developed formulas for a plate without substrates. As a result, the present technique requires no experimental baseline for the crack assessment.