The concrete surfaces of hydraulic structures can deteriorate severely when continuously or frequently overflowed by water containing solid particles; this process is called hydro-abrasion. Kinetic energy introduced to the concrete surface by moving solid particles leads most probably to near-surface damage of the concrete in the form of micro-cracks. In predicting the abrasion resistance and the remaining lifetime of concrete structures subjected to hydro-abrasion, a sound knowledge of the mechanisms leading to the wear of concrete on different levels of observation is needed. Besides the hydro-abrasion tests morphological analysis of the concrete microstructure was performed in order to attain better understanding of the wear mechanisms. In this first part of the paper the experimental results are presented and discussed. Based on the attained insights and data, an existing abrasion prognosis model for abrasion was extended, which now allows for a quantitative prediction of the surface wear. This model will be described in the second part of the article.