Porosity evolution of granitic rocks used for the bridge construction

Granitic rocks are generally ranked to very low porosity (open porosity of fresh, non-weathered granite should be below 1 vol. %) and durable rocks. Favourable physical properties dictate their choice for monumental works and important infrastructural projects such as bridges.

Current study focuses on detailed analysis of porosity evolution in granitic rocks used for the construction of two road bridges (making part of cultural heritage of the Czech Republic) over the Vltava River in Prague (Bohemian capital) in the second half of 19^th c. After 120-150 service, many of the stone ashlars of load-bearing parts exhibit quite extensive decay phenomena; however, mostly close to the exposed surfaces. In order to understand causes of decay, several tens of samples obtained from decayed surfaces and from original source quarries were studied by various methods, namely mercury intrusion porosimetry (MIP) and optical and scanning electron microscopy.

Based on the results, porosity of granites in the stone masonry significantly increased compared to that of source rocks: two principal types of granites show open porosity 0.53-0.82 vol. %, and 1.03-1.15 vol. % respectively. In the case of granitic samples from studied bridges, their open porosity ranged from 2.13 to 6.42 vol. %. Most of the pores rank to coarse pores and macropores (according to IUPAC terminology), content of mesopores is negligible.

Significant increase of porosity reflects dynamics of decay process in a specific microclimate in Vltava River valley and polluted atmosphere of the city. However, pre-quarrying history of granites is another important factor: the examined rock types belong to the oldest members of the Central Bohemian Plutonic Complex of Variscan age, and numerous discrete hydrothermal alteration phenomena present in the studied rocks can promote their susceptibility to decay as well.