Petru Mihai

The accumulation of end-of-life tyres represents a growing environmental burden, motivating the incorporation of tyre rubber as a partial aggregate replacement in concrete. However, the flexural performance of reinforced rubberized concrete (RuC) beams at high rubber contents and elevated longitudinal reinforcement ratios remains insufficiently investigated. This study experimentally examined the flexural behaviour of RC beams cast with conventional concrete and RuC incorporating 40% and 60% replacement of small coarse aggregate by volume, at longitudinal reinforcement ratios of 1.5% and 2.0%, approaching the over-reinforced range. Eighteen beams were tested under four-point bending until failure by concrete crushing, with deflection, crack development, and material properties monitored throughout. Rubber incorporation reduced the modulus of elasticity and compressive strength, corresponding to a Eurocode 2 class downgrade from C45/55 to C35/40, while increasing peak axial strain. At 1.5% reinforcement, increasing rubber content progressively enhanced deflection capacity and ductility, with RuC60 beams reaching a ductility coefficient of 2.52 versus 2.43 for conventional beams. At 2.0% reinforcement, this benefit was suppressed, with ductility converging to 1.40–1.47 across all mixes as compression zone crushing increasingly governed failure. These findings indicate that the structural benefits of high-rubber-content RuC are most pronounced in moderately reinforced beams and diminish as sections approach the over-reinforced regime, informing the applicability envelope of RuC for load-bearing flexural elements.

The flexural behavior of reinforced rubberized concrete beams was assessed, and it was demonstrated that they exhibited a constant performance decline with an increase in rubber content. Numerical simulations are critically important in the study and engineering of concrete elements due to several key reasons, as follows: to allow engineers to anticipate the behavior of concrete components under diverse loads; to help elucidate intricate mechanisms such as crack initiation, propagation, and fracture processes; and to explore new materials, geometries, and reinforcement layouts without the need for extensive prototyping. This paper presents both experimental and numerical investigations on the flexural behavior of conventional and rubberized concrete reinforced beams. The parameters of the research included the percentage replacement of natural aggregates by rubber particles and the change in the longitudinal reinforcement ratio. The results showed an increase in the load-carrying capacity and a decrease in the midspan deflection with an increase in reinforcement ratio. Substituting natural aggregates with rubber particles resulted in a slight decrease in the load-carrying capacity but an increase in the midspan deflections. Numerical simulations using ATENA v5 software predicted the load-carrying capacity, failure mode, and cracking patterns of the reinforced concrete beams.

In the past decades, nanomaterials have become one of the focal points in civil engineering research. When added to cement-based construction materials (e.g., concrete), it results in significant improvements in their strength and other important properties. However, the final mix characteristics depend on many variables that must be taken into account. As such, there is no general consensus regarding the influence upon the original material of certain nano-sized additives, the optimum dosage or the synergistic effect of two or more nano-materials. This is also the case for titanium dioxide (TiO₂) and nanoclay (NC). The paper focuses on reporting the existing research data on the use of the above-mentioned materials when added to mortar and concrete. The collected data is summarized and presented in terms of strength and durability properties of cement mortar and concrete containing either TiO₂ or NC. Both nano-materials have been proven, by various studies, to increase the strength of the composite, at both room and elevated temperature, when added by themselves in 0.5%~12% for TiO₂ and 0.25%~6% for NC. It can be inferred that a combination of the two with the cementitious matrix can be beneficial and may lead to obtaining a new material with improved strength, elastic and durability properties that can be applied in the construction industry, with implications at the economic, social and environmental levels.

The vertical structural components of RC frame buildings are the fundamental elements that ensure lateral stability of entire system under exceptional seismic loading situations. In this study, various scenarios of transverse reinforcement of RC columns for this structural system were numerically studied. A variation in the position and quantity of reinforcement in the same section of the column was considered. Thus, a better interaction of the concrete with transverse reinforcement was recorded for the volumetric confinement situation. The structure exhibited significant ultimate displacements compared to the classic reinforcement system. The new reinforcement system has been numerically validated and can be applied in further experimental studies.

In the first part of the current study, the effectiveness of the transversal cross-section reduction method for RC beams in marginal areas (by means of mechanical drilling) was validated. The said method “encourages” the formation of plastic hinges at the beam ends and, at the same time, allows for taking into account the bending stiffness of RC slabs, which is exerted upon the RC beams. In these conditions, the second part of the current research study (i.e., the current manuscript) highlights the real mode of reducing the lateral stiffness of the slabs upon the RC beams. These elements form a common body, together with the beam–column frame node. The same method as in the first part of the study—“weakening” the plates in the corner area through vertical drilling, without affecting the integrity of the reinforcing elements—was used. The analytical MR RC frame model, studied by means of the comparative method, highlights the efficiency of the transversal cross-section reduction method for RC slabs. Basically, the directing of the plastic deformations from the weakened slab areas towards the marginal areas of the reinforced concrete beams takes place. The beams rotate as far as the weakened slab areas allow its plastic deformation, thus being possible to observe the partial conservation effect of the beam–column frame joint. Furthermore, for the analytical model with the maximum number of vertical holes in the corner areas of the concrete plate, minimal plastic deformations are recorded for the marginal areas of the concrete columns. A partial conservation of the formation mechanism of the “beam-slab-frame node” common rigid block is also noted. Consequently, the dissipation of the seismic energy is made in a partially controlled and directed manner, in the “desired” areas, according to the “Strong Columns—Weak Beams” (SCWB) ductile mechanism of the lateral behavior to seismic actions for reinforced concrete frame structures. The mechanism is specified in current design norms for RC frame systems. The effectiveness of the method for reducing the transversal section of the RC plates in the corner areas by means of transversal drilling is highlighted and validated from the perspective of the local and global ductile seismic response of reinforced concrete frame structures. A significant reduction in the bending stiffness of the slabs upon the beams and a real development of the plastic hinges in the marginal areas of the beams (together with partial implications and plastic deformations) were observed.