Dionys Van Gemert

Multi-scale approaches for the assessment of time-dependent mechanical damage in masonry. (keynote)

Although salt crystallization damage is a widespread damage process in the porous materials of our built cultural heritage, no definite solution yet exists to improve the durability of materials with respect to salt crystallization. Most research focuses on improving material properties, whereas only few studies concentrate on changing the crystallization process in order to make it less harmful.Within this last trend, recently the use of salt crystallization modifiers has been considered. Crystallization modifiers are ions or molecules that promote or inhibit crystal growth and/or change the crystal habit, thereby possibly reducing salt damage.We give in this paper experimental evidence that borax is a promising crystallization modifier for sodium sulfate and that it might mitigate salt crystallization damage when mixed in lime-based mortars. Lime-based mortars are chosen as a model system, since they are especially prone to salt crystallization damage due to their limited mechanical strength and bi-modal pore size distribution.

Creep failure of two historical masonry towers: analysis from material to structure

The collapse of several historical masonry towers has been attributed to gradually increasing mechanical damage, such as creep cracking which occurs under constant stress levels. This paper reviews theoretical, experimental and numerical methods for analysing compressive creep in historical masonry. The investigations focus on creep behaviour in ferruginous sandstone, as results are applied to understand the collapse of two historical towers in Belgium that were both constructed in local sandstone. At the level of the material, cracking, creep strains and acoustic emissions are studied in small-to-medium scale sandstone samples under dry and saturated conditions. Results indicate an acceleration of the degradation process when water absorption takes place during the secondary creep phase. At the structural level, numerical macro modelling is performed to investigate the effect of stress redistributions on creep behaviour of three-leaf masonry. Finally, experimental and numerical results are related to the collapse of two historical towers.

Strengthening of an industrial cylindrical shell damaged by a collision

This case-study concerns an old industrial reinforced concrete shell structure at the port of Antwerp. The building serves as a pilot for stowing of shipped steel and wooden products. A fork-lift hit one of the supporting columns. The impact of the collision caused the concrete to crush and the internal reinforcement to shift considerably. Some of the internal reinforcement yielded or broke. Temporary supports had to be placed to avoid collapse of the total structure. The stress situation is analyzed. A FEM model was built to gain insight into the stress distribution in the shell structure. In comparison with the undamaged situation, membrane forces, shear forces and moments did strongly increase which explains the actual damage. A solution is worked out to repair and strengthen the damaged part of the structure. The FEM-analysis demonstrates that a compression arch will arise in the top of the barrel shell to span the displaced support. This requires a tension member at the bottom side of the concrete barrel. Therefore, externally bonded reinforcement is applied to increase the tensile capacity of the perimeter beam. The web reinforcement has yielded or is broken so that it is replaced by external CFRP reinforcement. After strengthening the structure, a monitoring system is installed. Strain gauges are glued on several laminates. These measurements give feed back on the structural behaviour of the strengthened structure and the applicability of the FEM used. This case-study illustrates the load distribution mechanisms in the shell structure, the assessment of the actual condition and an appropriate intervention based on external reinforcement.