مجله تحقیقات بتن ایران
سال دوازدهم شماره 4 (زمستان 1398)

Concrete as the most consuming material Construction has long been a response to the growing needs of the human community, whose parameters are constantly changing. Scientists' research over the past two decades has led to the emergence of a new type of concrete with high properties and high compressive strength, which is known as concrete with ultra high performance. The main goal of the present study is to achieve a mixing design with a compressive strength suitable for ultra high performance concrete, without the use of fibers. First, the basic mixing plan has been considered, and with the change in the type and size of materials and also by applying different curing, the compressive strength of the samples has reached 212 MPa. To create ductility and absorption of energy in concrete made with mixing design The present research has used polyvinyl alcohol fibers, polypropylene fibers and steel fibers as a single and hybrid for the reinforcement of concrete. Maximum usable fibers, ductility, and compressive strength that make the fibers mentioned Single and hybrid for this concrete are determined and compared with each other. The results of this experimental study showed that samples containing 2% fiber had the best mechanical performance, as well as samples of 1.5% polypropylene fibers and 0.5% polyvinyl alcohol fibers The most ductility for concrete has been created.

Alkali activated slag (AAS) binders can be used as a replacement to Portland cement. AAS concrete is made by slag activation with alkalis without the use of Portland cement. Due to the effect of alkalis on the properties of AAS concrete, in this study the effect of alkali concentration and sodium silicate modulus of alkaline solution on the setting time, slump, compressive strength and drying shrinkage of AAS concrete were investigated experimentally. For this purpose, 20 mix designs with different ratios of alkali concentration and sodium silicate modulus were cast and the setting time of paste and slump were tested in fresh state and in addition, concrete compressive strength of 7, 28 and 90 days and drying shrinkage of concrete samples up to 400 days, were measured. In most mixes, with increasing the alkali concentration and the sodium silicate modulus of the activator solution, the slump of fresh concrete was increased and the rate of the initial compressive strength development of the mixtures was enhanced but the setting time was lowered. However, in high alkali concentrations and silicate moduli, a decrease in compressive strength and an increase in setting time were observed with increasing the parameters of activator solution. However, with increasing the sodium silicate modulus and the concentration of the alkaline solution, the initial shrinkage rate was increased, but it was observed that the effect of the sodium silicate modulus is much higher than the alkali concentration on the final amount of the drying shrinkage of the samples.

Nowadays, manufacturing of high strength concrete is not an arduous work due to progress of concrete technology. In executive projects, the construction of the concrete with a compressive strength of 60 to 70 MPa is efficient but expensive. Increasing the strength of concrete reduces its ductility and causes brittle behavior in concrete. Confinement of concrete with glass and carbon fiber can decrease the brittle behavior. In this study, the behavior of the concrete with a compressive strength of 60 MPa made with non-standard aggregates in most construction workshops enclosed with carbon fiber coatings was tested. The behavior of this type of concrete was studied using the analytical methods presented by the researchers in this field such as Atard and Ciyan. There is an appropriate adaptation between theoretical and experimental results. The obtained results indicated the high effect of carbon fiber coating on the rate of ductility and compressive strength of concrete. A high-strength, economic, and fully executive concrete with a compressive strength of over 100 MPa and perfect ductility was constructed by this method

Structures dynamic characteristics including natural periods, mode shapes and damping ratios are the most important factors of the structure's response to earthquakes. Numerical models used to determine the dynamic characteristics of structures are usually based on simple assumptions. The effects of non-structural components such as infill walls and also the soil structure interaction effects are not considered, as well as the damping rate in the structure depends on the materials consumed and the construction methods that can be measured only by field testing on the structure. In this research, forced vibration test was utilized to determine the dynamic characteristics of a six-roof building located at Rasht's Motahari Avenue. In this test, an exciter was placed on the 5th floor near the center of mass of the floor and the structure was vibrated at specific frequencies. The structure response was recorded by accelerometers and the data obtained from the experiments were analyzed by the SeismoSignal software. The dynamic characteristics of the structure, such as period and main frequencies, were obtained in both directions, as well as the damping and torsion of 5th floor. The results obtained based on forced vibration testing were compared with the empirical values of the Iranian Code 2800 and indicated that the main period obtained by forced vibration test on the actual structure is less than the calculated period according to the Code 2800. In addition, the amount of damping coefficient obtained the test was also less than the amount suggestion in the Code 2800.

High-Performance Fiber Reinforced Cement Composites (HPFRCC), are mortars exhibit strain hardening behavior in the tensile test also have features such as flexibility, Durability, and high energy absorption capacity. In this paper, the effect of the fiber on the characteristics of the HPFRCC mechanical properties, the Fracture pattern and the energy absorption capacity have been investigated. Since there is no specific standard test available for HPFRCC, first, 18 mixing designs with different material proportions were made, the mixing that was the most economical and has the most appropriate mechanical properties has been selected. Finally, the selected mixing was evaluated with 60 specimens in three states of non-fibers, one and two percent fibers at different ages. The results indicate that with increasing age of specimens in all three conditions, the tensile and compressive strength increased, also the pattern fracture of the specimens has been improved by adding the fibers has moved from the deep cracks and the fragmentation of the specimens to the surface cracks .Also energy absorption capacity increased by 1 and 2 percent fiber, 24 and 52 percent, respectively. Then the relations for determining the compressive strength at different ages and the calculation of tensile strength with compressive strength for plain concrete presented in the standard were also evaluated for HPFRCCs.

Concrete behavior at high temperatures has significant implications for structural safety under specific loads and for measuring the load-bearing capacity of a structure for its continued utilization; however, its behavior varies with changes in temperature. Thus, this paper aims to investigate the effect of different temperatures on the strength parameters of concrete made with Portland cement from a nanostrucrual viewpoint based on the nanostrucrual changes in C-S-H. Accordingly, 300 samples were cured for 1, 3, 7, 14, and 28 days in a moisture room. After that, all samples were exposed to temperatures of 25, 50, 100, 200, 300, 500, 700, and 900 degrees Celsius for two hours. The changes in length and weight, compressive strength, and cracking behavior in the concrete samples were studied. Moreover, scanning electron microscopy (SEM) was used to analyze the microstructural behavior of samples at different temperatures. Based on the results, the behavior of C-S-H nanostructure causes the changes in length, weight, and compressive strength of the samples to be dependent on the C-S-H nanostructure behavior. With the onset of C-S-H decomposition due to heat, the compressive strength and weight decrease and the cracks spread. The results indicated that the compressive strengths of the 14- and 28-day samples reduced from 262 kg/cm2 and 270 kg/cm2 to 36 kg/cm2 and 44 kg/cm2, respectively. SEM analysis indicated that this reduction was due to the complete decomposition of the C-S-H nanostructure and Portlandite in the cement structure.

The present study examines the behavior of concrete coarse-grained and fine-grained (raw concrete and concrete mortar) containing tungsten oxide Nanoparticles as well as the performance of concrete beams repaired with concrete mortar containing tungsten oxide Nanoparticles. For this purpose, two types of designs, including primary concrete and concrete mortar containing 0.5, 1.5 and 2.5 weight percent of cement were used. The compressive and tensile strength tests 0f 3, 7, 28 and 63 days specimens and 1 , 3, 7 and 28 days specimens were carried out on the primary and mortar concrete, respectively. Four-point bending strength tests on the primary concrete by 28 and 63-day duration and concrete mortar was performed on 28 day. For the repair of concrete beams, the contact surface of concrete and concrete mortar were prepared in a smooth and coarse manner and subjected to a four-point bending test in order to evaluate compatibility between the repair materials and the primary concrete. The results showed that with the increasing presence of nanoparticles in raw concrete and mortar, increase compressive strength, flexural tensile, by compared to the control samples and about the repairing beams with rough surface samples have a flexural strength more than the sampler with a smooth surface that part of this increase in flexural strength can be used to increase the compressive and tensile strength of mortar and other restorative effect of tungsten oxide nanoparticles.

The problem of failure brings about considerable financial and living costs to various societies every year. The issue came under a scientific scrutiny since the beginning of the twentieth century. For the macro scale analysis of concrete structures, concrete is assumed as a homogeneous material, whereas in reality concrete contains various components such as cement paste, sand and air, which are important for concrete behavior determination. Therefore, in the current research, to calculate concrete failure parameters, cement paste and 4 different aggregate sizes were modelled to examine the effect of aggregates in the cement paste. According to the random distribution of the aggregates in the cement paste, 100 different types of the aggregate distribution in the cement paste were modeled to evaluate a wide range of responses. By examining the results obtained from 100 numerical samples of the bending beam under the three-point test, an unexpected result was obtained. It is normally expected that only the failure coefficient of the first mode should be created under this experiment. However, the failure coefficients of the second and third modes were also created due to the distribution of aggregates. The amount of these modes are probably dependent on aggregates and their location. The existence of all three failure modes contributes to the inconsistency of energy release rate distribution in the thickness, and the final result is the early failure of the sample.

The improvement properties of concrete such as strength and durability are functions of number, type, size and the way of void connections in hardened cement paste. The permeability parameter has an important role in structures such as tanks and water concrete reservoirs. In the other side, seep the destructive materials into the concrete decrease the concrete compression strength. So for prediction the concrete durability are two basic properties. Nowadays utilization of various additives for increasing the durability and quality of concrete has been growth. Nanostructure materials, based on the special behavior characteristics, are widely used in industrial division. In this study the effect of Titanium dioxide Nano materials on the concrete compression strength and permeability has been investigated. The mix design utilized in tests has been prepared with the water-cement ratio equal to 0.5 and cement content value equal to 40 kg/m3. In addition by adding the nanoparticles with various percent into mix design, new concrete mixtures have been produced. The experimental results indicate that adding the Titanium dioxide particles up o optimum percent into concrete, causes the significant increase in compression strength and noticeable decrease in concrete permeability with respect to evidence samples. Adding the particles more than optimum percent makes reduction in the concrete compression strength in comparison with the sample with optimum percent. Some applicable relationships have been presented in this study to predict the compression strength and permeability of concrete versus various Nano particles percent.

In this study, the effect of different amounts of AEA and amount of cement on the capillary water absorption of concrete was studied. For this purpose, 15 concrete mix designs with w/c of 0.45, Dmax=19mm and 4 levels of cement: 275, 325, 350, 375 and 400 kg/m3, and three levels of AEA( zero, 0.01% and 0.03% of cement weight) was considered. Various experiments included: total air, density and slump on fresh concrete and capillary water absorption on specimens of hardened concrete were studied. The results showed that in non-AEA mixtures, the total air content decreased and the density increased with increasing cement content. By increasing the amount of AEA, in mixtures with 275 and 325 kg/m3 of cement, the amount of total air of mixture decreased and the density increased, but for mixtures with higher than that, the total amount of air was increased and the density decreased. The results on specimens hardened concrete showed that increasing the cement content increased the amount of capillary water absorption. Also, for concrete samples with cement of 275 and 325, by increasing the amount of AEA, the amount of capillary water absorption increased, while for concrete samples with cement of 350, 375 and 400 kg/m3, decreased. Based on results, it was found that in concrete lining in conveyance canals, amount of cement should not be less than 330 kg/ m3. There is not difference between 0.01% and 0.03% of AEA, and therefore, an amount of 0.01% cement weight is recommended.