Kaspar Willam

A new retrofit technique for unreinforced masonry infills in non-ductile reinforced concrete frame structures is being investigated. The proposed technique uses a thin layer of sprayable, ductile fiber-reinforced mortar material referred to as Engineered Cementitious Composites, or ECC. The aim of the retrofit is to hold the masonry wall together to delay infill and frame strength degradation and enhance system ductility. Proof-of-concept experiments on retrofitted masonry prisms and beams demonstrated large increases in strength and ductility. Quasi-static, in-plane, cyclic testing of small-scale frames with masonry infills including 3 retrofit designs demonstrated a 10x increase in ductility in one design relative to the unretrofitted system. Results were used to design a retrofit for a 2/3-scale, in-plane test, where a 2.6 x increase in ductility was achieved. Several finite element modeling approaches are under investigation for use in system predictions and further retrofit analyses.

This paper summarizes some of the findings of a research project that investigates the seismic performance of masonry-infilled, non-ductile, RC frames, including the development of reliable analytical methods for performance assessment and effective retrofit techniques. Quasi-static tests were conducted on small and large-scale, single-story, single-bay, RC frames infilled with brick masonry walls with and without openings. Some of the infill walls were strengthened with an engineered cementitious composite material. Furthermore, two 2/3-scale, three-story, two-bay, masonry-infilled, RC frames were tested on a shake table. One was tested with no retrofit measures, and the other had infill walls strengthened with the engineered cementitious composite and fiber reinforced polymeric material in the first and second stories, respectively. The tests have demonstrated the effectiveness of the retrofit measures. Computation models that combine smeared and discrete cracks have been developed and validated by the experimental data. Some of the experimental and numerical results are presented in this paper.

Stress transfer from concrete into steel is one of the fundamental mechanisms behind the significant ductility of reinforced concrete. Considering the low value of cracking strain of plain concrete, it is the stress transfer that takes place during debonding when the steel strain approaches the yield limit at a level which is an order of magnitude larger than the cracking limit of concrete. This technical note examines the difference of “tension softening” of plain concrete versus “tension stiffening” of reinforced concrete which describes the stress transfer mechanism from concrete into the embedded reinforcing bars. Using three-dimensional finite-element simulations the fracture energy arguments of Mode I versus Mode II are reexamined when the axial response behavior of plain concrete in tension softening is compared with that of reinforced concrete exhibiting tension stiffening in long and short tension members.

Under extreme temperature exposure thermal softening of stiffness and strength properties as well as thermal expansion and drying shrinkage of cement-based materials have come to the forefront in the safety assessment of tunnel and jet fuel fires in high rise buildings. It is indeed unfortunate that several accidents in the recent past have reinforced the need for a more comprehensive understanding of concrete and steel materials exposed to fire environments. Thereby the transient nature of rapid heating and cooling and the thermal mismatch between concrete and steel may introduce extensive spalling in reinforced concrete because of pore pressure build up in regions of severe temperature gradients. In this paper we explore the temperature sensitivity of concrete and steel and their effects on the thermal collapse of a reinforced concrete benchmark structure (RCS) representative of a fire scenario. Specifically we review the mismatch of thermal expansion and the thermal softening of the mechanical response behavior of concrete under heating and cooling. In the sequel, we illustrate some of these issues with the thermal collapse of a RC box structure subjected to an increasing temperature gradient. Whereas structural collapse is well understood using limit analysis concepts of plastic yielding in skeletal structures, failure under combined hygro-themio-mechanical load histories deserves more attention in view of the absence of upper and lower bound theorems for softening and transient conditions. Computational failure studies using finite element simulations introduce additional issues which give raise to fundamental questions what does constitute collapse, and how does numerical failure relate to physical collapse in the presence of viscous regularization and global stabilization strategies. In the last part of the paper we explore the classical collapse problem of a portal frame for which plastic limit analysis techniques provide well-established reference solutions to validate numerical failure simulations using the incremental damage-plasticity model for concrete in ABAQUS.

In order to investigate the performance of conventional concrete at different loading rates, experimental results of strength enhancement were obtained under tension and compression tested under different loading speeds. G-mix specimens prepared at the Tyndall Air Force Base were used to determine the range of loading rates of the servo-hydraulic equipment at hand and to obtain experimental results with a series of indirect tension and uniaxial compression tests. For interpretation of the experimental observations, a comprehensive triaxial concrete model was extended from rate independent elasto-plasticity to rate dependent visco-plasticity using the Duvaut-Lions overstress formulation. The visco-plastic concrete model was adopted to explore the dynamic strength enhancement in tension, compression and shear in terms of a single viscosity or rather relaxation time. In concrete the failure properties, i.e., the triaxial strength and the failure mode, depend not only on the load path, but also on the loading rate especially at high speed impact. In this paper an effort was undertaken to assess the diffuse and localized failure modes of the triaxial concrete model and its visco-plastic extension.