مجله تحقیقات بتن ایران
سال ششم شماره 2 (پاییز و زمستان 1392)

The application of fiber reinforced polymer (FRP) composites has been successfully promoted for external strengthening of reinforced concrete (RC) columns. Recent studies showed that when these jackets are formed in a wet lay-up process for confinement of columns, an average rupture strain of FRP in specimens substantially fall below those of flat coupon tensile tests. The problem is more important in square columns because of premature rupture of FRP at corners due to stress concentration. In this paper, a new method is introduced for confinement of square concrete columns as using FRP strips at corners and FRP battens at sides. In this method, FRP battens does not experience any curvature in confining of section and its behavior is very close to flat coupons. To compare the new proposed method with other FRP-confining techniques, 11 square RC columns with 133 x 133 mm cross section and 500 mm height were experimentally tested under uniaxial compression. The test parameters included influence of confining method, continuity or discontinuity of corner strips along height of column and the volume of fibers used. The experimental results clearly demonstrated that confining battens in the new method of corner strip-batten uniformly stretched under the tension stresses of confinement; therefore, uniform distribution of confining pressure on section occurred and the stress concentration at corners was eliminated. Thus, in the proposed method, better performance of FRP in confinement was observed and the compressive behavior of the strengthened column was significantly improved compared to those confined using conventional FRP wraps.

Utilization of Self consolidating concrete in construction industry has been increased due to flow ability that causing more speed and better quality of execution. Optimization mix design methods of self-consolidating concrete can reduce expenses and therefore the application of that will be increase at construction project. In this study, different methods of mix design optimization have been reviewed which can be applicable for self-consolidating concrete mix design as well as different mathematical mix designs have been compared that can be useful to optimized mix design for high strength self-consolidating concrete. In addition, with studying objective function, decision variables and its constraint parameters in concrete’s mix design such as (cement, sand, water, super-plasticizer and powder materials), have been determined. Results showed that the optimized design of high strength self-consolidating concrete’s mix design is dependent on the objective function, its constraints and other parameters of constitutive materials ratio such as water/ cement ratio, cement/fine aggregate, cement/ course aggregate, cement/super plasticizer.

In recent years, the structural concrete have been considered by many because of its advantages as low weight, improved thermal and acoustic insulations and better resistance to frost and fire. At present, Liapor or Expanded shale which produced using expansible shale by the dry process and cooking in a horizontal rotary kilns, are one of the most important artificially lightweight aggregate which have widespread application in the construction of concrete structures. In this study, order to determine the effect of diameter size on specific weight, water absorption and mechanical properties of Liapor lightweight aggregates, various tests were carried out on the three categories of gradation of Liapor. Also, by combining the lightweight aggregate and cement mortar and making concrete specimens with the 10 to 30 volume percent of lightweight aggregate and determining the uniaxial compressive strength, static and dynamic modulus of elasticity and dynamic poisson's ratio, mechanical properties the composite material made of them was evaluated. The results showed that the diameter of lightweight aggregates has significant effect on the the mechanical properties of the aggregates and concrete made of them. According to the SEM images taken from the area of contact between lightweight aggregate and mortar, it was found that the transition zone is less than 10 micrometers and therefore can not have an influence on the mechanical properties of lightweight aggregates.

In this paper, the effect of steel fibres on the mechanical properties and thermal and freezing performance of lightweight self-compacting concrete consisting of Leca and microsilica gel has been studied. For this reason, the effect of three different contents of steel fibres on rheological and mechanical properties, including compressive, tensile and flexural strengths, and on concrete performance against thermal loading and freezing has been investigated. At the first stage, ten mix designs have been made by changing the amount of water, microsilica gel, stone powder and Leca contents. Then, slump, J-ring, U-box, L-box and V-funnel tests have been done. After selecting the control mix, 7-, 28- and 90-day compressive and tensile strengths tests, 90-day flexural and thermal tests, and 28-day freezing and thawing test have been performed. Obtained results indicate that steel fibres have increased tensile and compressive strengths by 95% and 16%, respectively. They have improved thermal durability at high temperatures and have prevented the loss of compressive and tensile strengths by 13% and 92%, respectively. The existence of steel fibres has enhanced the behaviour of concrete in freezing and thawing cycles by decreasing the loss of compressive and flexural strengths by 8% and 15%, respectively.

When earthquakes occur, energy released by the earthquake gets induced into the structure as ground motion and this energy has to be dissipated for safety reasons. To release seismic energy, the structure should damage in such a way that on one hand, collapse of structure should not occur and on the other hand, after the earthquake, damage should be economically feasible to repair. To avoid the collapse of the structures and also to reduce the repair cost after the earthquake, most design codes focus on providing sufficient ductility to structure. Dissipation of large part of injected seismic energy is an important factor for a structure to be seismically resistant. In this contribution, use of fiber to improve equivalent viscous damping (EVD) of reinforced self compacting concrete (SCC) beams subjected to cyclic loading is investigated. Two types of fibers were investigated: Steel fibers and Polyphenylene Sulfide fibers. Maximum dosage of fibers was kept equal to 0.4%(Vf). The findings of this study showed that EVD of SCC is improved with the addition of fibers. Comparison of the effectiveness of two fibers investigated in this study on EVD revealed that increase in the EVD was greater with steel fibers.

Utilizing tall building with reinforced concrete core-wall is propagating nowadays. In this paper the response of a tall building of this type is studied. First, a 40-story tall building with reinforced concrete core-wall is analyzed and designed by the response spectrum procedure. Then the lower stories of the core-wall are modeled by nonlinear fiber elements and the remaining stories are modeled by elastic elements. The Nonlinear Time History Analyses is performed for a set of near fault ground motions. The results show that the overall trend of the moment diagram along the height of the structure has not agreement with the results obtained from the response spectrum analysis. In the median of the height the amount of the moment demand in the Nonlinear Time History Analyses is about the 3.5 times of the moment demands in the response spectrum analysis. In this subject the moment demand of meddle height is very big and it is very hard to remain it in the elastic region and it will need a lot of reinforcements to remain elastic. To reduce the moment demand for these areas, first one and then two plastic hinges is planned along the height of the system. The Nonlinear Time History Analyses is performed for these cases. The results show that dual and triple plastic hinge approach will reduce the moment demand about 28 and 42 percent.

Self-Compacting Concrete (SCC) is one of the innovative products in concrete technology which has been developed in recent years. SCC has gained great popularity due to its benefits such as reduction in labor and equipment costs, acceleration of construction, providing flexibility in filling highly reinforced sections and complex formworks, lowering the noise on job site and having superior surface quality.rnHowever, many researchers are still trying to optimize the mixture proportions and investigate the incorporation of new materials in SCC. The typical high content of Portland cement in SCC is one of the main challenges in optimization of its mixture proportions. This high cement consumption increases the production cost of SCC and is also undesirable from an environmental point of view, considering the large amount of CO2 emitted during Portland cement production. rnNowadays cement replacement materials are being widely used in concrete mixtures and blended cements allocate noticeable amount of cement consumption in the world. Still, use of these materials is negligible in Iran and needs more attention by construction industry. Use of cement replacement materials, depending on their properties, can bring about improvements such as reduction in production costs, rheological improvements, reduction of heat of hydration and concrete durability enhancement.rnIn this paper the results of an experimental study on effects of silica fume and Ground Granulated Blast Furnace Slug (GGBFS) on resistance of high-strength SCC against chloride ion penetration are presented. Five concrete mixtures were prepared and tests were conducted to investigate the effects on high-strength self-compacting concrete. Filling ability, passing ability and segregation resistance of fresh mixtures were evaluated using slump flow, T50 and V funnel tests; while compressive strength, electrical resistivity and water adsorption tests were carried out on hardened concrete at different ages. The results show that GGBFS replacement decreases the viscosity of concrete and consequently the needed superplasticizer dosage. Also, results imply that use of silica fume and GGBFS decreases permeability of concrete. Finally the mixture containing 10% silica fume and 10% GGBFS cement replacement are introduced as the optimum SCC mixture.

In this paper, an optimum mix design method is presented for Roller-Compacted Concrete (RCC). It is shown that the total cost of RCC designed according to ACI207 requirements can be decreased so that its strength is equal to the same strength resulting from traditional mix design. As a practical example, optimum mix design of a RCC sample with a cylinder strength of 140 Kg/cm2 that is approximately equal to the strength of experimental specimens is performed through the new method. The results are compared with those obtained from traditional mix design method and it is shown that the results are improved.