High-strength high-ductility concrete (HSHDC) is a new category of high-performance fiber-reinforced cement composites. This type of concrete uses ultra-high molecular weight polyethylene (UHMWPE) fibers, enabling a combination of high compressive strength and high ductility. However, the effects of various material ratios, such as slag, silica sand, and fine silica sand that significantly influence HSHDC’s behavior have yet to be fully investigated. This study aims to examine the influence of these material ratios in optimizing both ductility and compressive strength in HSHDC. Notably, this research marks the first production of HSHDC in Iran. HSHDC offers considerable advantages in strengthening reinforced concrete (RC) structures due to its enhanced ductile behavior compared to fiber-reinforced polymer (FRP). It has a high potential capacity to reduce rupture risk while increasing both ductility and load capacity in RC beams. One of the limitations of FRP-strengthened beams is the occurrence of unfavorable failure modes, such as debonding, which can prevent full utilization of the high tensile capacity of FRP. As a result, the method of installing the strengthening composites to RC beams plays an important role in determining their failure mode and flexural performance. This study is the first to eva‎luate various installation and casting methods for strengthening RC beams with HSHDC, aiming to achieve optimal interaction and performance with the substrate concrete. In this research, HSHDC mixture were investigated with four water-to-cementitious materials ratios (0.16, 0.18, 0.20, 0.22), four slag-to-cement ratios (0.5, 0.75, 1, 1.25), four silica sand-to-cement ratios (0.4, 0.6, 0.8, 1), and three fine silica sand-to-silica sand ratios (0.25, 0.5, 0.75). These mixtures were eva‎luated for compressive strength and toughness, all demonstrating ductile behavior characterized by strain hardening and microcracking. The best performing mixture, with a water-to-cementitious materials ratio of 0.20, slag-to-cement ratio of 1.0, silica sand-to-cement ratio of 0.6, and fine silica sand-to-silica sand ratio of 0.75, achieved compressive strength and modulus of rupture of 113.2 MPa and 18.1 MPa, respectively, with an ultimate deflection of approximately 1/12 of the span length. However, this mixture was not selected for RC beam strengthening due to its high viscosity and associated challenges in production, molding, and surface finishing. For the strengthening of RC beams, mixtures without fine silica sand were chosen, resulting in compressive strength and modulus of rupture of 91.5 MPa and 15.7 MPa, respectively, with an ultimate deflection of approximately 1/19 of the span length. Six RC beams were then subjected to four-point bending tests, including a control beam, beams strengthened with prefabricated HSHDC panels using the EBR, EBROG, and mechanical anchoring methods, as well as beams strengthened with in-situ HSHDC using the EBROG and nailing methods. All beams strengthened with HSHDC exhibited flexural failure, accompanied by panel rupture. Compared to the control beam, the beam strengthened with HSHDC panels using mechanical anchors method achieved the greatest increase in load-bearing capacity (39%), while the beam strengthened with in-situ HSHDC using the EBROG method demonstrated the highest increase in energy ductility index (48%) and displacement ductility index (64%).

داود مستوفي نژاد , عليرضا سلجوقيان

محمود معصومي

محمدرضا افتخار , آلاء ترابيان اصفهاني