The strength of concrete in existing Italian tunnels

To assess the performances of existing Reinforced Concrete (RC) tunnel linings, the current structural analyses need the definition of material properties, especially of concrete strength. Such parameter is usually measured by means of destructive tests on concrete cores drilled from the lining, or by means of indirect non-destructive analyses, such as the acoustic emission techniques or the rebound hammer test. Nevertheless, in several cases, a rough estimation of the compressive strength of concrete is sufficient, without performing any test on the structures. This is the case of more than 500 highway tunnels built in northern Italy, which date back to the Sixties of last century and are currently in service. Accordingly, the strength-for-age curves, introduced to calculate the average strength (and the percentiles) of a concrete cast in a specific year, can be used for concrete linings. They are based on the results of about 500,000 tests performed on concrete cubes since 1897 and stored in a database available at the Politecnico di Torino (Italy). As a result, through the strength-for-year curves, a rapid assessment can be performed and the priorities of retrofitting, necessary to mitigate the risks associated with the service of RC tunnels, can be better identified.