مجله تحقیقات بتن ایران
سال سیزدهم شماره 1 (بهار 1399)

Designing and developing rheological properties of self-compacting concrete is an important issue in concrete technology. This matter is because of the combination of varied needs in the fresh mode, extremely complexed mixture proportions, and its naturally low yield stress. Selection of chemical admixtures, especially super-plasticizers plays an important role in Designing of self-consolidating concrete (SCC). In this paper, the rheological properties of SCC samples made with various chemical admixtures in various mixing time and temperatures were examined. Super-plasticizers are strongly influenced by the ambient temperature, due to the fact that the temperature dependent on the hydration rate affects the absorbance rate of the super-plasticizer as the ability to absorb the polymer content. The rheological properties of the SCC were examined in various mixing time and temperature. After changing the temperature of the SCC to the ambient temperature, the rheology test was conducted using rheometer in 60 min. In this research, the dosage of rheological parameters and its growth rate with time have been compared in various temperatures. According to the obtained results, Naphthalene sulfonate cause to increase yield stress and to decrease plastic viscosity more than poly-carboxylate in SCC, in other words, poly-carboxylate cause better viscosity in concrete. Also, concrete containing naphthalene sulfonate has less yield stress growth rate and viscosity by increasing the time. On the other hand, slump loss of the SCC containing naphthalene sulfonate is less than the one containing poly-carboxylate. That is to say, naphthalene sulfonate protects the fluidity more than poly-carboxylate in SCC.

Post-tensioned coupled shear wall system is a new type of seismic lateral force resisting systems with self-centering capability. In this system, the coupling beam is not embedded into the walls, and coupling of concrete walls is achieved by posttensioning beam to the walls using unbonded post-tensioning (PT) strands. In this research, the cyclic behavior of post-tensioned steel coupling beam with friction devices at the beam-to-wall connection region is experimentally evaluated. Three 2/3-scale specimens were tested subjected to quasi‐static lateral loading. The specimens consisted of one control specimen without energy dissipation system and two specimens equipped with friction devices with different levels of damper normal force. Friction devices are used at the beam-to-wall connections to slip and dissipate the seismic input energy. The test results revealed that the specimens equipped with friction devices have excellent lateral stiffness, strength, and ductility, and have significant energy dissipation capacity and negligible residual displacements under reversed cyclic loading. In addition, the specimens is able to tolerate large nonlinear rotations without significant damage in the beam and the wall region. Adding friction devices increased the load-carrying capacity by 47% to 62% and significantly increased the energy dissipation capacity of the specimens. With a 30% increase in the damper normal force, the load capacity increased by about 10%, the average relative energy dissipation ratio by about 15% and the mean cumulative energy dissipation capacity by about 33%.

One of the most useful methods for improving the seismic performance of concrete columns with insufficient shear strength is the use of a steel plate around the concrete column which by decreasing the crack and direct effect on shear force tolerance lead to improve the seismic performance. In this paper, the seismic behavior of specimens under cyclic loads was investigated in terms of force-displacement behavior, energy absorption and ductility. In this regard, the finite element method was used in OpenSEES program. At first, an experimental specimen was used to validate the numerical model and then, numerical model of the finite element was developed after proper matching of the responses with the experimental model and assurance of the type of loading, displacement boundary conditions, and the type of applied elements. The main parameters in these studies were the thickness of the steel jacketing, the spacing between the stirrups and the compressive strength of the concrete. The results of numerical studies showed that in the case of use of low strength concrete and the minimum thickness of steel plate, energy absorption, ductility and maximum load-bearing capacity can be 4.40, 1.66 and 2.5 times higher than reference model.

Cracking on the concrete surface reduces the durability of the materials. The penetration of water and harmful chemicals through these cracks into concrete will cause corrosion of rebar and demolition of concrete. If we can prevent the development of small cracks when it occurs, we have prevented the creation of larger cracks, because of stress concentration. A hopeful way, is to add the agents that can heal concrete when cracks are occurred, are so called Self-healing concrete. The aim of this study is to analyze the self-healing effect of a crystalline admixture (4% by the weight of cement) in three types of environmental exposure comparing with a reference concrete. Healing was studied by means of permeability tests on cracked specimens and physical closing of the crack was observed by optic microscope and quantified through crack geometrical parameters. The studied crack openings were under 400 µm and the time set for healing was 42 days. The results show a different healing behavior depending on the exposure and the presence of the crystalline admixture, demonstrating that the presence of water is necessary for the healing reactions. Moreover, the respective rate of self-healing for all specimens at three different environmental conditions were obtained approximately 0.85 for the sinking in water, 0.59 for alternate wetting and drying and -0.265 for drying conditions.

The use of mineral additions in self-compacting concrete (SCC) as a substitute for Portland cement can not only improve the engineering properties and durability of concrete, but also reduce the environmental impacts of cement production. Nonetheless, due to the high sensitivity of SCC to its constituent materials, an appropriate study on the performance of mineral additions in the production of SCC is absolutely essential. The purpose of this study was to evaluate the fresh properties of SCC containing wollastonite powder in comparison with silica fume and natural zeolite. In this respect, 18 SCC mixtures were designed in four groups with different water/binder ratio and additions contents. Immediately after mixing, the fresh properties of concrete mixtures were evaluated for slump flow, visual stability index, T50, V-funnel time, J-ring and the sieve stability index. The results revealed that although, regardless of the water/binder ratio, the incorporation of wollastonite decreases the slump flow diameter, it can reduce the demand for high-range water reducer admixture usage in comparison with natural zeolite and silica fume. Also, the increase in viscosity and passing ability improvement were observed when wollastonite is used in SCC. Sieve stability index indicated a good segregation resistance in the presence of wollastonite.

Confining columns is one of the most commonly used methods of retrofitting structures. The confinement of concrete columns by FRP sheets is considered as one of the modern methods of retrofitting structures due to their properties. Tests conducted on a rounded-corner column by applying axial compression indicate that its behavior improves as well as the cross-section of column approaches the circular section. However, the behavior of columns confined with FRP or having transverse reinforcement has been studied by many researchers, yet study on wrapping columns by FRP when the transverse reinforcement does not satisfy the required confinement for the column has not been investigated by researchers. In this study, different models of the lateral confinement coefficient are presented. Also, by considering valid test results and the influence of different parameters such as length and width of column section, thickness of FRP sheet, the compressive strength of concrete, elasticity modulus of FRP and, radius of rounded-corner concrete a new model is presented using the Neural network in which the lateral confinement coefficient of confined columns can be predicted with high accuracy. Finally, the sensitivity analysis is carried out to evaluate the effect of each input parameter on the output parameter, and the results are presented.

Sandstone shrinks on heating, and therefore, it counteracts to some extent the expansion of the quartz grains. The loss in strength of sandstone mortars on exposure to fire is, however, often high, and therefore sandstone does not form a good fire-resistant aggregate. This paper deals with an experimental study on the post-heat performance of cement mortars containing silica fume and granulated blast-furnace slag (GBFS). In doing so, the mortar specimens were provided in standard formworks to perform compressive, tensile and flexural tests containing 7, 14 and 21% of pozzolanic materials as a replacement for cement. Moreover, for the sake of heating process, the specimens were placed in an electric furnace exposed to temperatures of 25, 100, 250, 500, 700 and 9000c with standard temperature increment and once the heating process ended and the specimens were cooled, the tests were carried out on them. Based on the results obtained, the maximum effect of utilizing granulated blast-furnace slag (GBFS) and silica fume respectively takes place in low (up to 2500c) and high (5000c and greater degrees) temperatures. Quantitatively, the compressive, tensile and flexural strengths were raised by 73 and 180%, 45 and 100%, 106 and 112%, respectively in low and high temperatures. In addition, as the temperature rises, the particles of specimens containing silica fume and granulated blast-furnace slag (GBFS) shrinks less in size compared to the reference specimen.

The mass production and distribution of plastic materials in the 1950s was one of the major achievements of mankind, that currently the human needs it as an important industry for many reasons like variety, inexpensive and availability. The environmental damage caused by fossil contaminants, the consumption of some communities and the release of them and the lack of knowledge after the end of their initial consumption period is irrational and in the medium and long time, there will be imposed heavy costs. In this research, the effect of the mixing of polymeric waste (UPVC) are reviewed in the two forms of fibers and grains and in various volumetric percentages (0, 2.5, 5, 7.5 and 10% for fiber form) and (0, 5, 10 and 15% for the grain form) in order to replace the sand in the construction of concrete and the experiments carried out such as compressive strength, tensile strength, flexural strength, and fresh concrete tests, such as slump and concrete specific gravity. The ratio of water to cement was fixed (0.45) in all mixtures and the processing of 7 and 28-days of the samples was performed according to standard laboratory conditions. The results of this study indicate that the use of these wastes in concrete improves heat resistance and up to 5% for fiber form and up to 10% for grain form has a slight reduction in the mechanical properties and specific gravity of the concrete and it improves tensile and flexural strength.

Porous concrete is one of the types of coatings used in rigid pavements. High porosity and good drainage efficiency have increased the use of this type of concrete in urban floods and runoff management. On the one hand, there is a large amount of corroded rubber produced annually in the world, which, due to the very slow rate of decomposition and contamination resulting from it, cannot be buried in the environment. The use of these worn-out tires in concrete is a good option to dispose of these wastes. In this study, experiments were conducted to investigate the effect of using scrap tires on the properties of porous concrete, and 5 modes of replacement of natural concrete aggregates with crumb rubber, including 5, 10, 15, 20 and 25 The percentage has been examined. The results show that replacement of crushed rubber and aggregates in porous concrete mixing design will reduce compressive and tensile strength. The effect of the presence of particulate rubber in the design of porous concrete mix on the reduction of tensile strength is more than its effect on the reduction of compressive strength. The results show that with increasing percentage of replacement of particulate rubber and aggregates in porous concrete, both porosity and permeability increase. Also, the compressive and tensile strength of specimens containing particulate rubber, after being placed in the sulfate medium, is lower than the compressive and tensile strength of the control sample

Today, the use of powdered materials and chemical additives to improve the mechanical, durability, and rheological properties of a variety of concretes is rapidly expanding. The use of naphthalene, polycarboxylate and melamine-based lubricating and thickening agents is one of the effective strategies to control the instability of concrete. One of the foundations of the new lubricant is the xanthan gum biopolymer. The substance synthesized from the vegetable gum has a high viscosity that is widely used in various industries. After observing the lubricating type, high consistency, and appropriate price of this material, we decided to observe its effect on concrete. In this research, 16 mixing schemes containing Pozzolan Microsilica with 10 wt% cement and different grades were investigated. A 0.25 wt% lubricant and xanthan gum additive were added to the concrete cube samples. The results show that the xanthan gum additive reduces slump and separation and increases compressive strength, due to the increased viscosity. A bonding between concrete materials and decreased pores and porosity would reduce the capillary coefficient and increase electrical resistance of the concrete. Microstructural studies of concrete, including scanning electron microscopy (SEM) experiments, showed that the permeability is reduced and structural bonds of concrete adhesive are strengthened, due to the use of xanthan gum, and as a result, the samples containing xanthan gum have less porosity than samples lacking xanthan gum.