MECHANICAL AND ECOLOGICAL ASSESSMENTS OF ULTRA HIGH PERFORMANCE - FIBER REINFORCED CEMENTITIOUS COMPOSITES (UHP-FRCC) JACKETING

Ultra High-Performance Fiber-Reinforced Cementitious Composites (UHP-FRCC) has been developed to meet the recent requests for a more sustainable and longer life structures. UHP-FRCC is a highly dense, fiber-reinforced cementitious composite material that has a compressive strength of more of 150 MPa, high ductility, durability and energy absorption capacity compared with normal concrete. These high mechanical properties are achieved by the use of a high content of cementitious materials, a very low water/cement ratio and the incorporation of steel fibers. UHP-FRCC is good a solution for the repair, strengthening and seismic retrofitting of old structures. However, this high-performance material has a parallel high environmental impact: a large quantity of CO2 is released during the cement production. To overcome this issue, in this investigation industrial by-product, as fly ash, are used in order to decrease the environmental impact. The main goal of this experimental research is to simulate a retrofitting application of concrete structural columns: cylinder specimens made of normal concrete are reinforced with UHP-FRCC jackets in order to investigate their behaviour under compressive load. The ultimate load carrying capacity and the ductility will be considered as the main mechanical parameters. This research is focused on three main aspects: 1-Investigating the confinement effect, in order to find a relationship between the size of the jackets and the mechanical properties. 2-Evaluating the performances of samples with the same shape but changed material composition: cement may be replaced in different percentage by fly ash. 3-Correlating the mechanical responses of UHP-FRCC jacketing with the ecological performances. Different layouts for jackets and different UHP-FRCC mixtures have been designed. The normal concrete cylinders have a radius of 50 mm and a height of 200 mm length and the jackets have a length of 178 mm. The varying characteristic is the thickness: dimensions were considered 25 mm, 37.5 mm, 50 mm and 75 mm. Moreover, four mixtures were prepared, modifying only the percentage of cement and fly ash, being unchanged the portions of other materials: FA0, FA20, FA50, FA70, respectively with 0%, 20%, 50%, 70% of cement replaced by fly ash. The specimens were tested under uniaxial compression and the deformation is the controlled parameter. A linear relationship between the thickness of the jackets and the compressive stress has been found. So, the amount of UHP-FRCC can be then computed, and the thickness of the jacket may be adjusted to achieve the desired performances. Moreover, the mechanical properties decrease with the increase of cement substitution with fly ash. With an eco-mechanical analysis, it has been possible to identify solutions which may ensure a low environmental impact, without compromising the mechanical response. As reference sample it has been chosen that with no cement replacement and with a jacket of 25 mm. Various thicknesses of jackets made of UHP-FRCC FA50 and FA70 may ensure higher eco-mechanical performances. A design procedure has been developed to optimise the UHP-FRCC jacketing system and to jointly reduce the CO2 emissions. It has been found that, for a given maximum compressive stress, the impact of UHP-FRCC jacket has a minimum ecological impact in correspondence of the substitution rate: the range of values that guarantees both mechanical and ecological properties are between 40% and 50%.