Dr. Arkadiusz Kwiecien, Professor, Institute of Structural Mechanics, Department of Civil Engineering, Cracow University of Technology, Poland

Title: Reduction of stress concentration by polymer flexible joints in seismic protection of masonry infill walls in RC frames

Abstract: Currently, most common building materials as concrete and masonry are characterized by relatively high stiffness (Young’s module greater than 1 GPa) and brittle behavior. Taking into consideration large scale structural elements carrying loads, high stiffness is advantageous property because allows constraining deflections. On the other hand, high stiffness is accompanied by brittleness, manifested by sudden failure of structural material (without warning). If safety of users is required, this kind of behavior is rather avoided in structural elements, especially in seismic areas, where ductile behavior is needed.

Focusing on microscale behavior of concrete and masonry, matrices of these materials include structural discontinuities (e.g. at an interface between gravel and mortar in concrete), being triggers of strain and stress concentrations when the material is loaded. Peaks of stress concentrations overcome the material strength with load increase, forming micro-cracks and next some of them are linking into a main crack. The rest of micro-cracks make the brittle material weaker in surrounding of the newly formed crack. In brittle materials, cracks develop and propagates quickly and may cause failure of structural elements. This is the reason why cracks have to be protected against propagation and repaired.

One of the repair methods is filling of the cracks using bonding materials, which join cracks’ sides, stopping their development. There are used various stiff bonding agents as epoxy resins or cementitious based fluids (of Young’s module greater than 1 GPa and of linear characteristic), applied mostly by injection in external repair. Stiff inject bonds the crack but micro-cracks in its surrounding are developing under stress concentrations with reloading of the structure, creating a new main crack fast beside, often at the load level lower than the primary one. In such case, the repair using stiff bonding agents do not increase or even recover the global strength of the cracked structure, because is unable to reduce peaks of stress concentrations.

A new innovative solution in external repair is using of flexible bonding agents filling cracks, which stiffness is much lower (of Young’s module less than 1 GPa) than the bonded structural material. Moreover, these flexible materials are able to withstand high deformations and are of nonlinear characteristic, resulting in reduction of stress concentrations and their redistribution. Such properties of the flexible repair bond allows for increasing of load carrying capacity of the cracked structural element made of brittle materials. In the paper, reduction of stress concentrations by polymer flexible joints in seismic protection of masonry infill walls is presented, basing on laboratory tests.

Keywords: polymer flexible joints, composite strengthening, reduction of stress concentration, seismic protection, masonry infill walls