مجله تحقیقات بتن ایران
سال پانزدهم شماره 3 (پاییز 1401)

The present study aims to compare the flexural behavior of reinforced concrete (RC) beams strengthened by fiber reinforced polymer (FRP) with that of fiber reinforced cementitious matrix (FRCM) sheets. Accordingly, 6 RC beams were manufactured and strengthened by FRP and FRCM. Other parameters such as strengthening method and tensile reinforcement ratio were investigated. The results show that in externally bonded reinforcement (EBR) method, strengthening by FRCM sheets leads to the improvement of load carrying capacity by 12.8% and 16.8%, respectively, in specimens with and compared to the specimens strengthened by FRP sheets. Furthermore, the failure mode of specimens strengthened by EBR method was debonding in case of FRP sheets while the failure mode of FRCM strengthened specimens was fiber rupture. In externally bonded reinforcement on grooves (EBROG) method, the load carrying capacity of specimens strengthened by FRP and FRCM was almost the same, while their failure mode was debonding with partial cover separation and complete cover separation. Finally, the experimental results were compared to the existing models and different code provisions. It was observed that by increasing the tensile reinforcement ratio in FRP strengthened beams, the predicted results by analytical models become closer to the experimental results.

Ultra-high-performance concrete (UHPC) is a type of concrete with high mechanical strengths in which silica fume, quartz powder, superplasticizer and fibers, in addition to cement and water are usually used in its mixture design. In spite of vast studies on the mechanical properties of UHPC, very few comparative studies have been made on the effects of fiber types on the mechanical characteristics of this type of concrete, subjected to high thermal changes. This study aims at providing test results for the effects of temperature changes on the mechanical properties of UHPC that are reinforced with different types of fibers. Six types of fibers, either recycled or industrially manufactured, are utilized in the mixture design. After exposure to high temperature changes, compressive strength, tensile strength in bending, ultrasonic pulse velocity and sorptivity were evaluated and compared. Results of this study can be used in other research studies and can be used in decision making in industrial projects.

Since one of the major problems is these sands, its low resistance to natural moisture and saturation conditions, a laboratory study was conducted to investigate the effect of adding polyvinyl alcohol polymer to improve the mechanical and geotechnical properties of dune sands. The results revealed that the addition of this polymer up to 0.2 weight percent increases the maximum dry weight and does not significantly change optimal moisture. The experiments also showed that by increasing the polymer content, the CBR resistance of the samples increased significantly, so in samples made with 0.5 % of the polymer, the CBR value reached 185; this amount is more than 7.5 times the CBR for neat soil. Experiments showed that by adding 2% cement to the mixture, the resistance of the samples was increased, and their resistance to scouring increased. The direct shear test results indicated that the addition of polymer also significantly increased the shear strength of the samples. Tire fibers were used to prevent this condition and made the samples more ductile. The optimum amount of fiber needed was 0.6% in this case. The results of single-axial tests also showed that adding polyvinyl alcohol increases the soil's compressive strength and shear strength considerably. For example, in a combination of 0.4% polyvinyl alcohol, 1% cement, and 0.6% tire fiber with sand, the compressive strength reached approximately 15 kg/cm2, while the compressive strength for 1% sand cement was about 0.29 kg/cm2.

Concrete that has higher specific gravity than conventional concrete is known as heavy concrete. The use of high specific gravity aggregate is the most important way for the production of heavy concrete. Due to the use of aggregates with metallic properties and high atomic number, this type of concrete has the property of shielding against harmful rays. In this research, the possibility of using iron pellets with crumb iron as a substitute for concrete aggregate to produce heavy concrete has been investigated. For this purpose, 25, 80 and 100% of aggregates have been replaced with pellets and iron shells, and to prevent the effect of this replacement on increasing the porosity of concrete, part of the mixing cement has also been replaced with micro silica. Compressive strength, tensile strength, flexural strength as well as permeability tests have been performed on the constructed specimens and the specific gravity of the produced heavy concrete has also been reported. The results show a very good effect of micro silica on improving the properties of heavy concrete. The compressive strength of heavy concrete in the presence of micro silica is up to 34% higher than conventional concrete. Tensile and flexural strengths have increased up to 50% and 12%, respectively, compared to conventional concrete if micro silica is used, respectively. The permeability of heavy concrete for each percentage of pellet and iron shell replacement was lower than conventional concrete

Today, self-healing materials are used as intelligent materials in improving the properties of structures in many industries. One of the methods of self-healing in concrete is the use of calcium carbonate precipitation mechanism due to microbial activities. This research evaluated the effects of a specific strain of Sporosarcina pasteurii bacteria as a new method on the mechanical and durability properties of self-healing concrete. For this purpose, perlite aggregate was used as a carrier, and bacteria were immobilized in it.Five different cell concentrations (1.5×109, 2.6×109, 4×109, 5.2×109, 6.7×109 cfu /ml) of bacteria were used in the concrete mixtures and compared with a control mixture without bacteria. The compressive strength, tensile strength, water permeability, and chloride ion penetration at the age of 7, 28, and 90 days were tested. Also, the microstructure of some mixtures was investigated by Scanning Electron Microscope. The test results demonstrated that the use of Sporosarcina pasteurii immobilized in perlite improved the compressive and tensile strength, reduced the permeability of concrete. Maximum increase (23%) in compressive strength was measured with 2.6×109 cfu/ml of bacteria.

In this paper, the effect of bacteria on self-healing concrete with fly ash has been studied. Sporosarcina bacteria with four different concentrations were used for evaluation. In concrete with fly ash, fly ash is replaced by 20% by weight of cement of mixing design has been used. In order to evaluate and control the specimen, destructive and non-destructive tests such as ultrasonic, permeability, water absorption and compressive strength have been used. To evaluate the effect of bacteria on self-healing of concrete cracks, concrete cracks were studied using scanning electron microscopy (SEM) and light microscope. The results show that the concentration of 10.46 × 106 is the most suitable concentration of Sporosarcina for the production of calcite and filling the pores of concrete by products such as calcium carbonate. The mentioned concentration resulted in a 36% increase in compressive strength in specimen with fly ash and a 30% increase in specimen without fly ash. The results also showed that increasing the bacterial concentration did not cause significant changes in compressive strength, water absorption, impermeability and ultrasonic wave velocity.

The most important issues of passive defense in structures is the issue of progressive collapse. Progressive collapse occurs as a result of events deliberate and administrative, military, terrorist, earthquakes and floods. In progressive collapse, due to the destruction of one member of the structure, the failure is transmitted to other members of the structure and the failure extends in a chain throughout the structure and causes the destruction part of or the entire structure. Determining the key element in the structure and strengthening can prevent progressive collapse, total destruction of the structure and the resulting damage. In this paper, 5, 10, and 15 stories structures with dual system with cantilever beam. To investigate the effect of cantilever beams and the determine key element in reinforced concrete structures with dual system by the sensitivity index method and structural analysis is used by non-linear static analysis of vertical down (Push Down Analysis). The results show that in reinforced concrete structures with dual strength system with cantilever beam, the column failure index with the longest distance from the cantilever beam (23 m) compared to other columns in each three structure 5, 10 and 15 story is 87%, 77%, and 66%, respectively, which is considered as a key element in these structures. According to results, in case of increasing the height of structure with cantilever beam with dual strength system from 20 meters (5 story) to 60 meters (15 story), the failure index of the whole structure will decrease from 87% to 66%.

In this research, it has been tried to reduce the disadvantages of GO to a minimum by modifying through the production of a polymer in a new way, while having more advantages of its presence in concrete compounds. Tensile, compressive strength and Fourier transform infrared spectroscopy (FT-IR) tests on 54 specimens with the same mix design and 0.4 water-cement ratio, including the control sample, the sample containing 0.03% by weight of graphene oxide, and the sample containing 0.03% of modified graphene oxide (copolymer) was done, the results show that adding this copolymer to concrete increases the 90-day tensile strength from 3.12 to 4.33 MPa (equivalent to 38.8%) and increases the 90-day compressive strength from 34.5 to 44.6 MPa (equivalent to 29.3%). Furthermore, the workability of concrete containing copolymer is 25% higher than concrete containing graphene oxide, which indicates its positive effect in increasing workability compared to concrete containing graphene oxide.