نشریه مصالح و سازه های بتنی
سال چهارم شماره 1 (پیاپی 7، بهار و تابستان 1398)

In recent decades, use of high-performance fiber-reinforced cementitious composite (HPFRCC) which has improved the performance of reinforced concrete structures and the seismic resistance and rehabilitation of their members, has been expanded. HPFRCC material under tensile loading shows strain-hardening behavior and tolerates exerted tension till it reaches to relatively high strain so that increases tensile strength and energy absorption of the structure. In this paper, the behavior of reinforced concrete beams with HPFRCC panels has been analysed. For this purpose, after verifying the samples with experimental results, reinforced concrete beams, reinforced concrete beam with a HPFRCC layer as an alternative to concrete, and samples of panels with thickness of 20, 40 and 60 mm that have been patched under the beam has been modeled by the Abaqus software and the impact of using HPFRCC material on the ductility and energy absorption of the beams has been investigated. The results showed that using of the HPFRCC panel has increased the ductility of the beam, compared to the concrete beam, by an average of 36% and compared to the concrete beam reinforced by HPFRCC as an alternative layer, by 15.55%. The amount of energy absorption of the beam which had patched panel has been increased by 39% in comparison with a beam which had a layer of HPFRCC material.

Beam-Column joints in reinforced concrete frames are one of the key elements in determining the structural behavior of a variety of loads. The role of these joints against lateral loads, especially strong seismic events, is very important and the behavior of reinforced concrete structures in past earthquakes indicates that Connections have had a significant impact on the extent and severity of the failures. On the other hand, the experience gained in past earthquakes has led to the consideration of new interconnection design schemes in the current regulations. Despite the importance of joint joints and their extensive use in steel structures, due to the lack of precise details on their use in concrete structures, they are very small. In this paper, the behavior of joints on concrete frames in two types of absorption and bonding and for various reinforced sections of reinforced beams is modeled and compared in ABAQUS software environment. The results of this study indicate that increasing the percentage of the armature at the cross section in the case of the use of clamping joints has led to an increase in connective ductility. However, by increasing the reinforcement of the reinforcement and maintaining the geometric dimensions of the beam, the ultimate degree of ductility is gradually reduced. Also, with the increase in the diameter of the reinforcement in the beam during the use of joint joints, it increases the final capacity of the connection continuously but very gradually it turns out.

Moisture and water transfer into concrete can be considered as a major threat to the durability of concrete. Damp-proofing admixtures like calcium stearate (CS) can provide a water repellent layer along the capillary pores. As a result, water and moisture transfer into concrete will be restricted by means of this layer. Accordingly, this research studied the effects of CS on the properties fresh and hardened self-consolidating concrete (SCCs). The evaluation of fresh concrete properties demonstrated that the density of fresh SCCs, passing and filling ability were reduced due to incorporation of CS. Moreover, reduction in density and compressive strength of the hardened concrete can be deemed as the major impacts of CS on mechanical properties of SCCs. In fact, the last-mentioned parameters were decreased by respectively 4% and 30% due to inclusion of 7kg/m3 CS. Furthermore, utilizing CS can not be taken into account as a constructive approach to improve the durability chracteristics under hydrostatic condition. Because no considerable improvement can to be detected in the results of electrical resistivity and rapid chloride migration tests. Eventually, CS drastically enhanced the permeability of the concrete under non-hydrostatic condition. To be more precise, addition of   7kg/m3   of CS decreased the final depth of capillary water absorption, short and long-term water absorption by respectively 60%, 65% and 15%.

In this research, a comparison has been made between the mechanical properties of cement based mixtures containing glass fiber. For this purpose, a type II portland cement and a calcium aluminate cement have been used. In order to investigate the mechanical characteristics, the tests including compressive strength, flexural strength, toughness, ultimate tensile strength, impact resistance and drying shrinkage of the mixtures were evaluated at ages from 28 days to 270 days. Glass fiber was incorporated in the mixes at replacement levels of 2 and 4 percent (percent of the total volume of the mixture). Based on the tests, the results indicate that the mixtures made with calcium aluminate had improved properties compared to the mixtures made with Portland cement. Furthermore, by adding glass fiber to the mixes, the flexural strength was increased by about 32% in 28 days compared to the plain mixture. Adding glass fibers in both types of cement also improved tensile strength, flexural toughness and impact resistance, as well as drying shrinkage.

Concrete is widely used in civil engineering because of its high compressive strength and efficiency. Low resistance of concrete against tensile stresses, especially in low confinement systems necessitates the use of steel-concrete composite systems. In concrete-filled steel tubes, the confinement of concrete is supplied by steel and the local buckling of steel is improved by concrete core. In this paper, using the Abaqus software finite element method, the numerical results of the CFST column are compared with the experimental results and have been assured of the correctness of the modeling. Then, the effect of changing the thickness of the steel and the compressive strength of the concrete on the behavior of CFST columns is investigated. To consider the effect of changing these parameters, 9 CFST column models with three different steel thicknesses and three different compressive strengths of concrete have been used. Nonlinear behavior of steel using a mixed hardening model incorporating both isotropic and kinematic hardening was used in modeling. Concrete’s stress-strain relations and compression-tensile damage parameters are fully described. The effect of increasing thickness of steel and concrete strength on structural energy absorption, initial stiffness and column capacity were investigated. According to the results of the analysis, the effect of increasing the thickness of steel on the performance of CFST columns relative to the increase in concrete strength is significant. Investigating the energy absorption capacity and the resistance of the CFST columns shows that the effect of changing the compressive strength of concrete for confinement steel with less thickness is significant. In confinement steel with high thickness, the change in concrete compressive strength does not have a significant effect on energy absorption and increases the column's strength slightly. As a result, to improve the performance of structures with CFST columns, in terms of energy absorption, stiffness and capacity, it is desirable to use suitable thickness for steel and normal strength for concrete.

High performance fiber reinforced cementitious composites (HPFRCC) are cement matrices with strain hardening behavior under tension loading. In these composites, the cement mortar with only fine aggregates is reinforced by random distributed fibers. In this material, multiple cracking in the HPFRCC occurs due to bridging mechanism of the fibers and subsequently, the strain hardening behavior is observed. In this paper, based on an experimental work, HPFRCC layers with different thicknesses and different lengths are used in lieu of normal concrete. Compressive and tensile strengths of the HPFRCC material are variable in these analytical models too. Finite element approach is used to investigate the effect of these parameters on the capacity of the reinforced concrete beam. Results show that increasing the compressive and tensile strength of the HPFRCC layers concludes to more final load and more ductility of the analytical beams. But this increasing effect is not significant. Moreover, increasing the thickness and length of the HPFRCC material concludes to more final load and more ductility in the retrofitted models. The ductility of the full HPFRCC beam is about 1.39 times more than that of the reinforced concrete beam.

Self-compacting concrete (SCC) has advantages such as flowability, filingability and having no need to vibration and better consolidation compared to conventional concrete. However, these benefits are achievable through increasing the volume of paste. Therefore, the volume of SCC paste is one of the most significant parameters in such type of concrete. In this research study, in order to investigate the fresh and hardened properties of SCC mixtures with various paste volume, three levels of cementitious materials (450 kg/m3, 495 kg/m3 and 540 kg/m3) were made keeping w/cm constant. In addition, at cementitious materials content of 450 kg/m3 and 495 kg/m3, natural zeolite was replaced 9% and 17% of cement by weight, respectively to study the influence of this material as a type of pozzolan. The experimental results indicate that increasing the SCC’s cementitious materials at a constant W/CM, does not have considerable effect on compressive strength of concrete, while increases the segregation. The results of V-funnel test show that increasing the cementitious materials content at a constant W/CM, has no prominent impact on viscosity of concrete, however partial replacement of cement by zeolite increases this parameter of SCC and has a positive effect on stability, compressive strength and resistivity.

Concrete core capability of a reinforced concrete columns to withstand compressive strain caused by the confinement pressure increases. In order to ensure proper lateral deformation capacity of reinforced concrete columns enclosing the bars should be increased according to the axial load. On the other hand, the strength and flexibility of each reinforced concrete is enhanced by improving the enclosure of plastic hinge areas. This ensures improved seismic stability during an earthquake. One of the most important characteristics of concrete structures against the forces of earthquake is their ductile behavior. Finding the values for standard plasticity due to the complexity of the structural behavior and the lack of explicit all its influencing factors, is associated with many problems. Also various regulations are expressing the different criteria for ductility. The purpose of the present study is to provide a confinement criterion based on the need for ductility. In this article, we review the various regulations, is extracted relations for design of enclosed transverse reinforcement of two type of columns, with circular and rectangular sections, In the case of medium and high ductility. In order to verify the relations, reinforcement values for designing reinforced concrete columns evaluated with the values obtained from Iranian national regulations relations. The results of the proposed relations show that the values of the design reinforcement of sections for both levels of ductility is reduced .