Wind energy turbines and offshore hydrocarbons platforms rely on injected concrete grout to support and transfer loads between steel substructures. Deterioration of this grout under large operational stresses can lead to the loss of bonding, formation of gaps, crushing and the loss of grout from the annulus between the steel substructures. In this paper, the integrity of the grout between two steel panels is experimentally tested using a low-frequency ultrasound backscatter method [1]. The experimental results and modelled outcomes [1] are compared for grout condition classes including: good condition, gaps between the grout and either steel panel and the complete absence of grout. Pearson’s correlation coefficients of over 83% were observed when comparing the notch magnitudes and frequencies and on the modelled and experimental reflectance spectra from front and rear gap, and missing grout conditions. Kolmogorov-Smirnov similarity tests on the modelled and experimental notch magnitudes indicate a 20% significance on the rear gap and front gap spectra and a 10% significance on the missing grout spectra. The significances of these tests support the potential application of backscattered low-frequency ultrasound for grout condition inspection. However, development of automated condition recognition algorithms based either on spectral characteristics or on time-localised spectral features of the backscatter, is required to make routine inspection commercially viable.