Test of loaded and unloaded natural stone masonry walls exposed to fire

Abstract

This paper describes two full-scale tests carried out on two natural stone masonry walls subjected to fire loading, the first one was thermally loaded only (exposed to fire during 120 min without failure, then a natural cooling phase was observed) while the second one was thermally and mechanically loaded (exposed to fire and loaded up to failure with a compressive force at its bottom end). The experimental results showed that the thermal bowing was greatly affected by the in-plane mechanical loading. A water vaporization plateau at about 100 °C has also been observed, showing the influence of the hydric phenomena on the temperature distribution across the wall thickness. Analysis of cracking and failure of the walls showed that crack initiated in the vertical joints and propagated vertically through the blocks, which is quite typical for masonry in compression. Furthermore, the observed fire behaviour of natural stone masonry walls has been compared to the fire behaviour of newer, manufactured materials such as hollow bricks, concrete blocks or reinforced concrete walls.

Appendix

A preliminary estimation of the strength (by unit length) of a natural stone masonry wall at ambient temperature, N_R, may be carried out according to EN 1996-1-1 [35] as follows:

$$ N_{R} = \phi_{i} \times t \times f $$

(1)

where:

• $\phi_{i}$ is a reduction factor that accounts for geometric imperfection, buckling risk and transversal horizontal loading when applicable. Here, it is lower at mid-height of the wall ($\phi_{i} = 0.74$) than at its extremities ($\phi_{i} = 0.9$).

• t = 0.2 m is the thickness of the wall;

• f is the local strength of the block/mortar assembly, empirically determined by:

  $$ f = K \times f_{b}^{0,7} \times f_{m}^{0,3} $$

  (2)

where:

• K = 0.45 for natural stone;

• f_b = 7.4 MPa is the compressive strength of stone;

• f_m = 2.5 MPa is the compressive strength of mortar used in this calculation (as a mortar class M2.5 is required for soft stones).

Here, the limit load of the wall, without taking into account partial safety factors, is N_R = 341 kN/ml. A 250 kN/ml load corresponds to 73.3% of this empirical limit load at ambient temperature.