UHPFRC Strengthening of Reinforced Concrete Flexural Members Subjected to Static and Blast Loads
Creators: name: Li, Chuanjing
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2023
Subjects: UHPFRC
Retrofit
RC structures
Blast loading
LS-DYNA

description: Ultra-high performance fiber-reinforced concrete (UHPFRC) is an advanced cement-based composite with enhanced compressive strength, tensile resistance and toughness when compared to conventional concrete. Interest in the application of UHPFRC as a retrofit material has been rapidly increasing, and a few existing studies have examined the ability of UHPFRC to retrofit and strengthen existing reinforced concrete (RC) structures under static loading; however, very limited studies have examined the effectiveness of UHPFRC to improve the response of RC members under blast loading. This thesis aims at filing this research gap and investigates the behavior of UHPFRC retrofitted RC flexural members under both static and blast loads. A total of twenty-one (21) specimens, in two different series are tested. Series 1 includes nine (9) singly-reinforced beams built with high-strength concrete (HSC) and strengthened by UHPFRC to improve shear and flexural behaviour. Series 2 includes a further twelve (12) doubly-reinforced beams/columns built with normal-strength concrete (NSC), and strengthened by UHPFRC to improve response under blast, or combined blast-axial loading. Various test parameters are examined including the effects of varying retrofit types (full jacket, U-jacket or T-sided), surface roughening methods, longitudinal steel reinforcement ratio, single vs. repeated blasts, and the effects of axial loading. The results from this thesis are presented in six journal articles. Papers 1 and 2 study the effects of UHPFRC jacketing on the static and blast behaviour of the singly-reinforced HSC beams in Series 1, while Paper 3 discusses the effects of additional parameters such as: the effect of retrofit type, roughening method and steel detailing on blast behaviour. Under static loading (Paper 1), the UHPFRC jacketing was found to be effective in increasing shear resistance (by preventing shear failure), and improving flexural behaviour (by increasing strength, stiffness, ductility and overall toughness) when compared to control beams built without UHPFRC. Similarly, under blast loads (Paper 2) the use of UHPFRC jacketing prevented shear failure, and improved flexural behaviour by reducing displacements at equivalent blasts, increasing overall blast capacity, and improving damage tolerance. On the other hand, the results show that UHPFRC-retrofitted beams with low longitudinal steel ratios may be vulnerable to brittle bar fracture failures. As part of the numerical research, finite element (FE) modelling is used to predict the static and blast behaviour of the test beams using software LS-DYNA (Papers 1 and 2). The results from Paper 3, provide further insights into the effects of retrofit type (FJ, UJ and T) and roughening method on blast performance; both the UJ and FJ retrofits were found to be effective in increasing shear resistance, reducing blast-displacements and increasing blast capacity, while the benefit of the T-sided retrofit was limited by the crushing capacity of HSC concrete. The effect of roughening method was found to be negligible, except at the very late stages of blast loading. Papers 4, 5 and 6 present the experimental results from the doubly-reinforced NSC beams tested in Series 2, with a focus on the effect of UHPFRC jacketing, UHPFRC retrofitting type and Axial loading, respectively. Paper 4 shows that the UHPFRC jacketing increased the stiffness and strength of the beams under both static and blast loading, however the high bond capacity of the UHPFRC and relatively low tension steel ratio increased the vulnerability of bar rupture failure. The numerical parametric study investigates the effects of steel ratio and blast load scenario, jacket thickness and interface location on blast performance and failure model. Paper 5 confirms that the blast performance of the beams is influenced by the retrofit type, with optimal performance obtained when using full- or U-jacketing. The efficient use of localized "hinge" retrofits was also found to be effective, and reduced the vulnerability to bar rupture. The numerical parametric study investigates the effects of steel ratio and blast load scenario (single vs. repeated) on the blast performance of the beams. Paper 6, studies the effect of UHPFRC jacketing in columns tested under combined axial and blast loading. The retrofit is shown to increase blast capacity and reduce blast-induced displacements and damage, though the final failure of the columns was governed by bar rupture. As part of the numerical parametric study the effects of axial load ratio, boundary conditions, steel ratio, jacket thickness and jacket design are studied numerically and found to have significant effects on blast behaviour and failure mode.