نشریه مصالح و سازه های بتنی
سال هفتم شماره 2 (پیاپی 14، پاییز و زمستان 1401)

As the industrial production of Self-Compacting Concrete (SCC) in the form of ready-mixed concrete faces challenges in different weather conditions, it is necessary to consider mixing, transportation, and casting to achieve the desired workability, mechanical, and durability properties. As a result of improper compaction and placement of SCC, a decrease in workability can result in failure to achieve the desired strength and durability. It was found that replacing mixing water with ice with 0, 10, 30 and 60% reduced the temperature of fresh concrete by approximately 1, 3, and 12°C compared to the control mix design after 110 minutes from the start of mixing. The superplasticizer amount and concrete properties were examined. After reducing the temperature of fresh concrete and maintaining its fluidity, the amount of superplasticizer decreased by 62.4%, and the result of its decrease with an increase in the amount of ice, increased the final production cost by 5.4%. In general, reducing the temperature of fresh concrete improved the rheological properties of SCC. When compared to the control mix design, the use of ice did not significantly alter the compressive strength, water absorption, and electrical resistance, except for a 17% increase in water absorption due to replacing 60% of the ice.

In general, shear reinforcement in reinforced concrete beams typically involves the use of stirrups. However, substituting stirrups with continuous rectangular spiral reinforcement can enhance construction efficiency and lower costs. Meanwhile, the adoption of fiber-reinforced concrete in concrete structures is on the rise due to its distinctive properties. To address the tensile and shear vulnerabilities of concrete, reinforcing it with fibers is a viable solution. This study explores the experimental replacement of stirrups with continuous rectangular spiral reinforcements in both traditional concrete and steel fiber-reinforced concrete (SFRC) beams. Three beams were subjected to static loading tests: the first beam, referred to as ST-NC, featured stirrups and normal concrete as a reference; the other two beams incorporated continuous rectangular spiral reinforcements with normal concrete (SP-NC) and steel fiber-reinforced concrete at a 0.75% volume fraction (SP-F0.75). The experimental findings indicate that the beam reinforced with continuous rectangular spiral reinforcements and fiber concrete exhibits improved shear resistance, energy absorption, and ductility compared to the normal concrete beam and the reference beam. The beams with continuous rectangular spiral reinforcements, using normal concrete and steel fiber-reinforced concrete, demonstrated a 23.8 and 46.5% increase in shear capacity, respectively, compared to the reference beam. Additionally, energy absorption in SP-NC and SP-F0.75 beams increased by 69 and 158%, respectively, compared to the reference beam. The ductility of the continuous rectangular spiral reinforcement beam with normal concrete and steel fiber-reinforced concrete is 0.87 and 1.17 times that of the reference beam, respectively. Notably, the results reveal a reduction in the ductility of the SP-NC beam compared to the reference beam, and the addition of 0.75% by volume of fibers to the concrete resolves this ductility weakness in the SP-NC beam. These findings underscore the advantages of employing continuous rectangular spiral reinforcements in beams constructed with steel fiber-reinforced concrete, as proposed in this study

The non-linear behavior of reinforced concrete columns under cyclic loads reduces the possibility of using classical analysis methods to check their deformation capacity. Mathematical-analytical models based on basic concepts and principles of structural mechanics or empirical-statistical models based on nonlinear regression generally lack sufficient accuracy and speed in answering such problems. The main goal of this research is to investigate the possibility of using soft computing in investigating the lateral deformation capacity of reinforced concrete columns using experimental data. For this purpose, four models are presented by using an adaptive neural fuzzy inference system and artificial neural networks. More than 100 samples of reinforced concrete columns from the PEER database were used for testing the models. The main geometric and mechanical variables affecting the lateral deformation of the column are defined as model inputs. The lateral displacements corresponding to the crushing of the concrete cover of the column and the 20% decrease in the lateral resistance of the column have been used as bending failures and output of the models. Two models are coded with the Adaptive Neuro-Fuzzy Inference System (ANFIS) and two models are modeled with the Multi-Layer Perceptron (MLP) method. Comparing the error values of ANFIS models compared to MLP models shows the appropriate ability of ANFIS in predicting the behavior of reinforced concrete columns under cyclic lateral load.

Rheological parameters of concrete, including yield stress and plastic viscosity, have a major impact on its pumpability. In the present study, the effect of temperature of mixtures with and without superplasticizer on their pumpability, has been studied through laboratory examination of yield stress and plastic viscosity of mixtures at three temperatures of 10, 20 and 30 ° C. In addition to investigating the effect of mixture temperature immediately after production, changes in these parameters over time, at the three temperatures mentioned, were also investigated.The results show that for mixtures without superplasticizer, increasing the temperature, results in increased yield stress. Additionally, the rate of yield stress increase at higher temperatures, was higher compared to that at lower temperatures. In terms of plastic viscosity, the mixture with lower temperature, had the highest amount of plastic viscosity and the rate of plastic viscosity increase over time, was greater for mixtures with higher temperatures. Calculation of the pumping pressure based on the rheological parameters, indicates that shortly after production, higher temperatures do not have a significant effect on pumpability. However, over time, the pumpability of the higher temperature mixtures will decrease, resulting in higher pumping pressure.The results indicate that for mixtures containing superplasticizers, higher concrete temperature, results in a decrease in the pumping pressure during initial times after production. However, over time, it loses this advantage and will have poorer performance compared to the lower-temperature mixtures.

Concrete pavement is been used in the aprons owing to its resistance to static loads as the main loads of aircraft. An alternative is to use interlocking concrete block pavement with advantages such as simple execution and repair and with no needs for curing. There is concern about the effect of the CBR of the subgrade on the resistance of block pavers to permanent deformation. The present study conducted to investigate the effect of subgrade (CBR) on these pavements in the apron. The Plate Load Test (PLT) performed on a 2m×2m prototype constructed as per Federal Aviation Administration (FAA) regulations. The layers of this 3D model include compacted subgrade, sub-base, crushed aggregate base and Cement-Treated Base (CTB) and 8-cm thick concrete blocks. Sensitivity analysis showed a reduction of over 12% in vertical deflection with increasing CBR from 2 to 10 and a reduction of below 4% in the presence of subgrade even with CBR=10~20%. Using subgrade with CBR of over 10 has found insignificantly increase the resistance of block pavements to permanent deformation. Concern about the risk of permanent deformation is obliterated given the key role of the Cement-Treated Base in redistributing the loads.

Basically, heavy concrete has the specific weight of about 1.5 to 2.5 times of the normal concrete. Materials with high specific weight are employed as a part of used aggregate to make heavy concrete. Heavy concrete is utilized as a protective shield against radiation. Heavy concrete is applied in the structure of nuclear infrastructures or hospitals. Ilmenite is a type of mineral that is also considered as mineral waste. Ilmenite is extracted directly from titanium mine and is found plentifully in Iran. In this study, Ilmenite is used in different volume ratios of 10%, 20%, 30%, and 40% instead of sand to make heavy concrete. Moreover, nano-silica gel, is used at the rate of 1% by weight of cement, to increase the quality and durability of concrete against the cycle of melting and freezing. The tests performed in this research include compressive strength test, tensile strength test, defining the primary and secondary water absorption rate and determination of concrete melting and freezing durability, according to ASTM-C666 standard. Results depicted that, the specific weight of heavy concrete would meet the regulations, by adding at least 20% of ilmenite instead of sand in concrete mixture. Also, adding 30% by volume of ilmenite, instead of sand, will increase the final compressive strength (90 days) by 33%. Finally, applying nano-silica gel in the heavy concrete mixture, makes the concrete stable against the melting and freezing cycles.

The role of cement fineness in the process of hydration and development of compressive strength in the early ages of cement-based materials is irrefutable and it requires that its effect be investigated by predicting models. Therefore, an extensive study including 640 cement composition (1920 cement mortar specimens) from a cement factory with different percentages of raw materials feeding to the cement kiln including SiO2, Al2O3, Fe2O3, CaO, MgO, SO3, K2O, and Na2O were used to predict the 7-day compressive strength of cement mortar by artificial neural network (ANN). To investigate the effect of cement fineness, two models have been developed in two states of with and without fineness effect. Results confirmed the significant role of cement fineness as an input parameter in the performance of predicting model. The findings of this research can be used in cement production facilities in order to reduce the laboratory costs.

The portion of phases and the quality of clinker crystal formation have a great impact on the performance of cement and concrete obtained from it; however, cement and concrete manufacturers often base their evaluation on chemical analysis or XRF and the values of phases with the help of Bogue formula. Therefore, in this extensive applied study, the relationship between the results of microscopic studies and the technical characteristics of cement and its performance in concrete has been traced. In the first stage, one hundred samples of Portland clinker type II were prepared from Tehran cement and microscopic studies and chemical analysis were performed on them. All clinker samples were grinded in industrial mill and physical and mechanical tests were performed on cement samples. Then laboratory concrete mixtures were made to evaluate the technical characteristics of cement and their performance in concrete. Based on the results of this extensive laboratory operation, innovatively, the microscopic results were summarized quantitatively (numerically). The relationship between the microscopic indicators and the properties of concrete mixtures was observed and its relationships were presented.

Compared to regular columns, short columns absorb more earthquake force due to their greater stiffness. For this reason, short columns are seriously damaged during an earthquake and sometimes cause the destruction of the entire structure. When short columns are displaced as much as long columns, shear and joint failure occurs in them due to the absorption of more energy. In this research, by using the laboratory results of a short column subjected to lateral reciprocating loads, an attempt was made to select the most optimal state with the help of steel sheets covered with different shape patterns. For this purpose, to determine the effect of retrofitting on the short column, six different installation modes of the proposed steel cover were used in abaqus software for finite element analysis. Next, the studied short column was subjected to reciprocating cyclic loading according to the standard pattern. The results of this research show that retrofitting a short column with an integrated steel sheet is able to increase the seismic resistance of the column by 49%, but the most optimal retrofitting method in terms of strength and volume of steel used is the retrofitting method with model-2 installation pattern.

In this research, the placement of energy dissipaters at the base of self-centering base-rocking walls has been investigated. For this purpose, structures with 4-, 8-, 12-, 16- and 20-floors were investigated. The nonlinear dynamic behavior of these structures was investigated subjected to 22 Far-Field (FF), 14 Near-Field without Pulse (NF-non Pulselike), and 14 Near-Field with Pulse (NF-Pulselike) seismic ground-motions. The models have been analyzed in two dimensions via OpenSees software. The considered seismic ground-motions are scaled and applied to the structure at DBE and MCE levels. The results showed that changing the location of energy absorbers can be effective in the value of the moment and shear demands of walls as well as the response of residual drift and roof acceleration. By increasing the distance of the energy dissipaters from the middle of the section wall, the moment and shear demands could be reduced about 14% and 13% at the DBE level, and about 19% and 17% at the MCE level, respectively. Furthermore, the reduction values of the moment and shear demands of the structures under NF-Pulselike and DBE or MCE levels have been observed more than other seismic ground-motions. Finally, increasing the distance of the energy absorbers from the middle of the wall, the accelerations of the roof decrease.

The curing ambient temperature and the amount of Silica Fume are the effective factors on the mechanical properties of reactive powder concrete (RPC). In this research, the effects of the curing ambient temperature and also the effects of the amount of silica fume on reactive powder concrete have been investigated. The autoclave device has not been used so that these concretes can be used in different environments as in-situ concreting. The prepared specimens were cured by immersion in water. To investigate effects of the curing ambient temperature, specimens were cured in water at three temperatures of 27, 60, and 80°C and were tested for compressive, tensile, and flexural strength at the age of 3, 7, and 28 days. The results show that the curing ambient temperature has a significant effect on the early strength of RPC, which increases strength at high temperatures. In the curing in water, it was found that 60°C is an optimum temperature; Because the specimens with the curing ambient temperature of 80°C show higher compressive, tensile, and bending strengths, but the increase in strengths compared to the curing ambient temperature of 60°C is very small. The compressive strength of the specimens cured in the water pool with temperature of 80°C has increased by 43.7%, 43.5%, and 25.9% at the ages of 3, 7, and 28 days, respectively, compared to the specimens cured at temperature of 27°C; Also, the increases in bending strength in the mentioned cases are equal to 51.5, 53.4 and 25.8% respectively. Similar results are also observed in tensile strength. In the next step, to study the effect of the changes in the amount of silica fume, five concrete mixtures with different amounts of silica fume were produced. These values were considered as a percentage of cementitious material (cement + silica fume) of 0, 10, 15, 25, and 35%. The specimens made in this step were only cured in a water pool with temperature of 27°C at the ages of 3, 7, and 28 days and were tested for compressive, tensile, and bending strength. It should be noted that at this stage, cementitious material (cement + silica fume) is fixed and is equal to 1008 kg / m3 of RPC. The results indicate that the replacement of silica fume has increased the strengths, but the optimal value is in the range of 15 to 25%. The compressive strength of the specimens with 25% replacement of silica fume at the ages of 3, 7, and 28 days has increased compared to the specimens with 10% replacement of silica fume by 66.3%, 40.9%, and 64.0%, respectively. These increases are equal to 100.8, 93.8 and 107.7 percent, respectively, compared to the specimens with zero percent replacement (without silica fume)

In the present study, due to sustainable development and eco-friendly aims, alkali activated slag and fly ash were used as the geopolymer binder instead of cement in self-compacting concretes. The alkaline solution consisted of sodium hydroxide and sodium silicate solutions. Binder content was between 500-700 kg/m³ and water to binder (W/B) ratio varied from 0.45 to 0.48. Na₂O percentages were in the range of 5 - 7% and three ratios of coarse to fine aggregate (25/75, 30/70, 35/65) were used. Design of experiments was based on orthogonal L9 array of Taguchi methodology. A total of nine main mixes and three validation mixes were prepared. Fresh concrete characteristics and mechanical properties including compressive strength, splitting tensile strength, modulus of elasticity and water absorption were evaluated. The results demonstrated that the binder content and coarse to fine aggregate ratio were the most influential parameters on the mechanical properties. The highest compressive strength, splitting tensile strength and modulus of elasticity were belonging to the mixture with low W/B ratio and higher binder content by 58 MPa, 4.5 MPa and 32 GPa, respectively. Moreover, Taguchi method has a good capability (R²≥0.967) to analyze and predict the mechanical properties of self-compacting geopolymer concrete.