مجله تحقیقات بتن ایران
سال شانزدهم شماره 3 (پاییز 1402)

The objective of this study was to investigate the compressive strength of metakaolin-based geopolymers and slag activated with alkaline activators during periods of 3, 7, and 28 days using the response surface method (RSM). X-ray diffraction (XRD) analysis. and Fourier transform infrared spectroscopy (FT-IR) was performed to investigate the effect of each of the independent variables on the compressive strength of geopolymer samples. Geopolymer samples were made in dimensions of 20×20×20 mm, then they were kept at room temperature for periods of 3, 7 and 28 days and were subjected to mechanical evaluation at the end. The results showed that using the RSM method can provide an appropriate estimation of the response so that the R2 value greater than 90% was obtained for each response. The maximum compressive strength of geopolymer samples was measured as 10 MPa for three days and 16.57 and 22 MPa for 7 and 28 days, respectively. By increasing the molar ratio of sodium silicate to sodium hydroxide to the level of 2.5 and decreasing the ratio of metakaolin to slag, the compressive strength of geopolymer samples increased. The FT-IR results showed that when geopolymers are formed, the structure changes compared to the raw materials due to the formation of monomers and compaction. Also, the results of the XRD analysis showed the presence of a peak-like state due to the formation of an amorphous product.

The most significant difference between self-consolidating concrete and ordinary concrete is workability, which allows self-consolidating concrete to fill the mold without vibrating operations. One of the main tests in determining the workability of self-compacting concrete is the L-box test, which can be used to check the permeability and filling properties of concrete. In this study, using Flow3D software, self-compacting concrete is investigated by evaluating the results of L-box test and the time when the concrete reaches a distance of 40 cm from the gate is recorded. For this purpose, the concrete used has plastic viscosity range of 10-38 (Pa.s), yield stress range of 14-75 (Pa), specific gravity of 2000 and 2500 (kg/m3) and elastic shear modulus of 100 and 1000 (Pa). In order to model the aggregates, spherical components with a diameter of 20 mm and a specific weight of 2500 kg/m3 have been used. To save time and evaluate the effective parameters in the amount of time required for the concrete to reach a distance of 40 cm from the gate (T40cm), the test design was performed by Taguchi method and 16 experimental designs proposed by this method in Minitab software. Been investigated. After analyzing the results by signal-to-noise method, it was observed that the plastic viscosity and yield stress of self-compacting concrete have the maximum and minimum effect on T40cm, respectively, which are consistent with the results of analysis of variance.

The mechanical and chemical characteristics of concrete undergo changes in extreme environmental conditions, which often reduce the strength and durability. Therefore, the use of additives that increase the reliability of concrete in acute conditions is required. It is also important to use tests that are able to measure the permeability and resistance of concrete both in situ and in the laboratory without damaging the concrete. Therefore, it was decided to investigate the effect of some additives on the durability of concrete, which are recently widely used in Iran. There is not much research on the effect of these materials on the durability and reliability of concrete in extreme environmental conditions. In this research, by using in-situ cylindrical chamber and twist-off tests, the effect of different cycles of freezing and thawing on the permeability and surface resistance of ordinary concretes and concretes was evaluated. Contains additives. Contrary to DIN 1048 and BS EN 12390-8 standards, which require breaking concrete to measure permeability, the volume and depth of water penetration can be measured in the cylindrical chamber test without damaging the concrete. The obtained results show that by using the cylindrical chamber . Also, about 78% decrease in the strength of the surface layer of concrete occurred in the first 50 cycles and the remaining 22% occurred in the next 100 cycles. The permeability of concrete containing microsilica gel and supergel was calculated to be about 50% and 30% lower than the permeability of normal concrete, respectively.

The utilization of recycled materials in the concrete industry has become one of the most essential environmental needs today, and more technical and economic studies are being performed every day in this subject. The effect of recycled materials on the shear behavior of geopolymer concretes was investigated in this work, along with a comparison to conventional recycled concretes and the effect of recycled aggregates on both concrete groups. For this purpose, ten mixing designs with recycled aggregate replacement percentages of 0, 25, 50, 75, and 100 were made in two groups of ordinary concrete and geopolymer concrete, as well as mechanical tests such as compressive strength, modulus of elasticity, and pull-off test. A reinforced beam without a shear element was also created for each mixing design to study the effect of this type of aggregate on the shear behavior of the beam. The findings revealed that increasing the percentage of recycled aggregates causes a loss in mechanical characteristics in the designs and a reduction in beam bearing capacity, indicating that geopolymer concrete has a higher replacement capacity than regular concrete.

Many of the existing reinforced concrete structures have inappropriate details in the reinforcement, which causes problems such as low lateral displacement capacity, low energy consumption, strength deterioration, and occurrence of mechanisms in undesirable places in the structure. One of these problems is the weakness in the shear resistance of the joint core due to the lack of transverse reinforcement in the joint core. In this research, the effect of using the method of enlarging the joint core in improving the behavior of joints without transverse reinforcements in the joint core has been investigated. For this purpose, after conducting experimental studies on the control and improved connections and validating their numerical model, the control and improved concrete frames of 4 and 8 stories have been modeled in OpenSees software, and their lateral behavior has been analyzed using non-linear static analyzes under Uniform lateral loading and incremental cyclic loading were compared and evaluated. The results of the studies indicate the use of the studied improvement method in connections stirrups in the core increases the bearing capacity, reduces pinching, increases energy disipation and forms a flexural hinge in the beam.

Currently, extensive studies are carried out on polymers reinforced with FRP sheets as reinforcement of structures both internally and externally. One of the conventional retrofitting methods for reinforced concrete beams is flexural strengthening of beams with CFRP materials. In this study, a full scale reinforced concrete beam was constructed and evaluated under four-point flexural test. Afterward, a same beam was retrofitted in flexural region by monolayer of CFRP. These experimental tests were carried out to be the basis of verification for numerical specimens. For numerical study, ABAQUS software was used for modeling and analysis of totally five retrofitted reinforced concrete beams. The results show that the fully retrofitted beam with CFRP laminate and U-shape wraps has the best performance in terms of capacity and delay for emergence of cracks. Its worth to say, flexural strengthening in the middle of beam has not notable result in capacity, though, could convey the failure mechanism to out of pure bending region.

 During the service period, concrete may come into contact with harsh environmental conditions, ultra-high performance concrete can be considered as a suitable option due to its unique strength and durability. In this study, for the first time, the effect of nanoparticles of magnesium oxide and copper oxide with amounts of 0.5, 1 and 2% separately and combined with different amounts of iron slag (2.5, 5 and 10%) on the compressive strength and electrical resistance of UHPC contains a constant content of PVA fibers in two operating modes Standard and accelerated curing (immersion in water at 95°C) were studied for different ages. Moreover, the Scanning Electron Microscopy test (SEM) is conducted on the specimens to understand the concrete's microstructure better. The results show that by using the heat treatment method and microstructure modification in the presence of new nano materials, it is possible to create a higher potential than UHPC in terms of compressive strength and electrical resistivity. So that after 90 days, the compressive strength for each of the specimens containing 1% of magnesium oxide and copper oxide nanomaterials respectively increased the compressive strength by approximately 37.4% and 37.1% compared to the 7-day sample. While the growth of the compressive strength in the accelerated mode for the mentioned specimens was 16.8% and 18.8%, respectively, compared to the standard curing. In addition, by obtaining accurate experimental relationships between the two parameters of compressive strength and electrical resistivity a non-destructive method for predicting compressive strength has been presented.

In this research, the effect of the use of air-entraining admixture on the resistance and permeability of concrete materials made of Portland cement type 1-525 has been investigated. The effect of three different percentages of AEA was investigated on durability of concrete. The tests performed include the slump test, determination of volumetric mass of the fresh concrete, determining the temperature of fresh concrete, determination of the percentage of air content in concrete using the pressure method, the determination of compressive strength, the determination of volumetric electrical resistance, and determination of permeability coefficient. The results show that the amounts of used AEAs in concrete has a critical dosage (0.05% by weight of cement), so that the use of AEAs amounts less than this dosage has very little effect on creating enough air bubbles in the concrete. The use of a dosage of 0.1% AEA by weight of cement increased the workability of concrete about 26%. Moreover, the use of AEA has caused a significant increase in the workability of the fresh concrete without causing problems such as segregation and bleeding. In general, AEA led to a decrease in volumetric mass, compressive strength, electrical resistance and chloride ion diffusion coefficient of the concrete material. The results also show that the replacement of 1% of concrete's volume by the air bubbles decreases the compressive strength of concrete about 5%. The increase of the air content of concrete from 1.9% to 13.4% leads to a slight drop in the permeability coefficients of concrete.