Application of Strain Hardening Cementitious Composites for retrofitting unreinforced masonry

Abstract

Starting from the late eighties induced seismic activity is present in the Northern part of the Netherlands. Cause of these earthquakes is the extraction of natural gas. In this region, most of the building stock comprises out of unreinforced masonry (URM) buildings. Multiple experimental campaigns are performed since 2014 to investigate the structural integrity of these type of structures. Both static and dynamic, full-scale and smallscale, experiments are executed to identify both in-plane and out-of-plane failure mechanisms. Strengthening, or retrofitting, of these already existing structures, is of interest and different researchers propose multiple techniques. From the literature, it is found that the application of strain hardening cementitious composites (SHCC) results in the required improved structural performance of the retrofitted masonry. The SHCC material is characterised by high ductility in the range of 3-7%, tight crack widths of around 60 μm and a relatively low fibre content equal to 2% (maximum by volume). In this experimental campaign small-scale, both inplane and out-of-plane (i.e. shear strength and flexural strength, respectively), experiments are performed on masonry samples retrofitted using a single-sided applied SHCC overlay. Different thicknesses of this overlay, masonry surface preparations, and overlay curing conditions are considered. With the help of these experiments the material properties of retrofitted masonry are investigated. From the initial shear strength experiments, it is concluded that the retrofitting approach did not work as intended. Results of all the different specimen examined shows similar capacities to the non-retrofitted masonry triplets. With the help of digital image correlation, it is shown that for all of the retrofitted specimen, the overlay material is not activated during the tests. Additional experiments and numerical simulations are performed to investigate the several parameters possibly influencing these experiments. The bond strength of the interface between the SHCC and the masonry substrate is determined to be too low. This is one of the main factors resulting in the absence of cracks in the retrofitting overlay. Additionally, the high cracking strength of the SHCC mixture considered, and the applied pre-compression, have influenced the cracking behaviour of the retrofitting overlay. Besides the shear strength of the retrofitted masonry material also the flexural strength is investigated with multiple out-of-plane four-point bending tests. Retrofitted masonry beams are used for these experiments. The plain masonry beams are not able to carry their self-weight, therefore all of the additional capacity is accommodated to the applied overlay. The experimental results are governed by shear failure. For eight out of ten specimens debonding of the masonry units is prevalent. Two of the retrofitted masonry beams showed flexural failure. A thicker overlay will result in higher flexural strength. Again, the bond strength between the overlay and the masonry substrate seems to be governing. No numerical analyses are performed to analyse these experiments in more detail. From this research, it is concluded that a single-sided SHCC overlay can be a very attractive method for retrofitting unreinforced masonry. However, both the interfacial bond strength and the material properties of the SHCC material must be taken into account. The bond strength must be sufficiently high, and the cracking strength of the overlay material sufficiently low, for the overlay to be activated. With the help of numerical simplified micro-models, the requirements of both parameters can be estimated.