Surface strength: definition and testing by a sand impact method

Many degradation processes affecting surfaces of buildings that are part of the Cultural Heritage manifest themselves by detachment of the component particles. The most important cases are the crystallisation of salts, the formation of ice, the intrusion of living forms, and the impact of particles transported, or at any rate mobilised, by the wind. This work presents a physical analysis of the process of grain detachment at microscopic level. It is suggested here that the impact of solid particles on the surface may be used as the basis on which to construct a method to measure the mechanical strength of surfaces, also in relation to the assessment in quantitative terms of the performance of consolidants currently used in the conservation sector. The experiments showed that the approach was effective in discriminating the different parameters used in the test (diameter of impacting particles, position of surface, operational pressure of nozzle).