مجله تحقیقات بتن ایران
سال سیزدهم شماره 3 (پاییز 1399)

One of the most effective methods for strengthening and repairing damaged corrosion elements is the use of fiber reinforced polymers. In this research, an experimental study was conducted to investigate the effect of GFRP on the retrofit of corroded RC beams. For this purpose, 15 concrete beams were constructed with two types of hooked metal fibers and polymeric macros with a volume percentage of 0% and 0.5%. The samples were tested at three levels of corrosion of 0%, 7% and 9%. An accelerated corrosion test was used from a 3% salt pool. Finally, corroded beams retrofitted with a GFRP composite polymer fiber layer were subjected to a flexural loading test. Based on experimental results, corrosion reduces the ductility of the specimens and the use of GFRP increases the ductility and bearing capacity of the specimens and reduces the number of cracks created and increases the depth of them and more focus on failure and damage is within the range of sheet separation.

Concrete structures undergo behavior change and instability at elevated temperatures. In most cases, these behavioral variations and instability are related to the changes in C-S-H and (C-A-S-H nanostructures, which is an important part of the cement paste hydration process and plays a crucial role in determining the strength and mechanical properties of concrete.. Application of aluminum dross as a type of pozzolanic material to cementitious composites can improve concrete behavior at elevated temperatures. Consideringly, this article aims to investigate the effects of elevated temperatures on concrete containing different percentages of aluminum slag from the microstructural perspective. For this purpose, cubic samples containing different percentages of aluminum slag powder were prepared. The specimens were cured in a humidity bath for 28 days and then heated for 60 min at temperatures of 25 to 750 °C. To evaluate the mechanical properties, the percentage of weight and compressive strength changes of concrete samples were investigated. Scanning electron microscopy (SEM) images were also used to observe the microstructural properties of the samples at different temperatures. According to the results, in samples containing slag less than 10% due to its activation as well as the hydration process and the formation of C-A-S-H and C-A-H nanostructures, the portlandite phase was formed which has improved the compressive strength. On the other hand, the elevated temperature and the occurrence of the dehydroxylation process lead to water decomposition within the bonds of C-S-H nanostructure which subsequently results in the change in compressive strength behavior.

Using fiber-reinforced plastic (FRP) to retrofit reinforced concrete columns is an impressive way. In this research, the effects of using 1.2 kg/m3 micro monofilament polypropylene fibers of 12-millimeter length on compressive strength of Specimen confined by FRP are studied. Upon mechanical and chemical tests on materials, the best mix plan is chosen followed by making one cubic meter of concrete prepared in batching machine. Half of the concrete was made with polypropylene fibers while the other half did not contain any fibers. As many as 24 cylindrical samples of 150*300 mm were taken from each. Composite coir fibers were used to increase bearing strength and for confinement. The samples were tested under axial static loading and were compared using their load-displacement curves. The results show that FRP-confinement improve mechanical properties of polypropylene fiber concrete including concrete stiffness, compressive strength, ultimate strain and ductility index and also delayed the final break.

Concrete shear resistance is one of the most important parameters in concrete structures design. Suitable understanding of concrete behavior against shear forces has always been a problem for researchers. Many different methods have been developed for measuring and predicting shear strength. One of the most common method, is to find a relationship between compressive and shear strength. In this study, an attempt was made to provide a practical relationship between compressive and shear strength using an experimental design with compressive strength in the range of 20-30 MPa which resulted in 50 compressive and shear concrete specimens. Gene expression Programming (GEP) method has been used to estimate shear resistance and five relationships have been provided. In order to select the best relationship, in addition to considering error measurement parameters such as correlation coefficient,, Willmotts index, and etc ., the criterion of simplicity and easy use of the relation is also considered.

Studies have shown that one of the leading causes of earthquake inflicting irreparable damage has been the high weight of structures. Using FRP composite sheets -due to the proper mechanical and physical properties to reinforce and improve concrete members in load-bearing, removal of concrete in the deflection and increasing the ductility of concrete - is an appropriate option. The purpose of this study is to strengthen voided concrete slabs with AFRP, CFRP and GFRP. In this study, 10 specimens of reinforced concrete with dimensions of 150×70×20 cm were investigated. The results demonstrated that the amount of load capacity in the Network strengthened specimen with CFRP compare to the control sample 175% and, in Linear, strengthened specimen with AFRP in comparison with the control sample 153% increased. This value in comparison with Solid specimens increased by about 96% for samples having been strengthened with CFRP and 91% for those strengthened with AFRP. The maximum displacements have been presented in the specimen strengthened with CFRP (236%) compared to that of the control sample and in the specimen strengthened with GFRP (99%) compared to that of the control sample. The stiffness amounts in Linear specimen strengthened with CFRP 225% compared to that of the control sample and in Network specimen strengthened with AFRP 136% compared to the that of control sample also increased.

The technology of making self-compacting concrete has been introduced in the world in recent years due to its high quality in high-strength areas. Because this type of concrete has a high cement consumption, one of the cement alternatives that has been considered by researchers is zeolite pozzolan. However, so far the persistence of this type of concret in chloride areas such as the coasts of the Persian Gulf has received less attention.The study program in this paper includes fresh and hardened concrete tests for different percentages of zeolite replacement. The method of this research is to create mixtures for determining the extent of cement materials in order to meet the conditions for acceptance of fresh and hardened concrete behavior according to the existing regulations. U-shaped box, V-shaped funnel, slump flow, J-ring and GTM screen stability have been tested on fresh zeolite-based self compact concrete specimens(SCC). Also, the water absorption, chlorine ion penetration and water pressure penetration depth for hardened concrete were performed. The results of this study indicate that the addition of zeolite pozzolan significantly increases the viscosity of fresh concrete and improves its rheological properties. According to the results of experiments, the replacement of 5 to 15 percent cement with zeolite improves the performance conditions of fresh concrete itself. The best results for concrete durability at 10 and 15 percent are obtained. On the other hand, an excessive increase in zeolite levels results in lower workability and slump, leading to higher levels of excess superplasticizer and water.

One of the most suitable materials for using in construction is self-compacting concrete. This type of concrete, not only due to the lack of the need for vibration but also because of lesser weight resulting from using lightweight aggregate instead of gravel, could be a proper option to be utilized in earthquake-prone areas. Especially in cases where the existing structure, including the building, needs to be retrofitted and repaired, the presence of lightweight concrete with no need to compaction can be auspicious. Thus, in this study, 32 mix designs of self-compacting concrete were used as research. In these mix designs, the type of lightweight aggregate, both type and amount of pozzolan were changed to evaluate the effect of each one on the bond strength properties of concrete. The concretes were placed in acidic environments, and then the bond strength was assessed using push-out and splitting-prism methods. From the results, it is shown that pozzolan containing 5% of metakaolin and 10% of zeolite has the best and worst rheological properties, respectively. The rate of reduction and changes in the bond strength in sulfuric acid are lower than those of hydrochloric acid. Specimens containing scoria show higher bond strength values. Furthermore, by exposure to acidic environments, after 7 days the specimens containing scoria and after passing 28 and 60 days, the specimens containing lica experienced the least reduction of bond strength

In this research, rocks were selected from different mines to evaluate the engineering properties of In this research, rocks were selected from different mines to evaluate the engineering properties of aggregates in concrete and then used for making concrete. At first, the physical and mechanical properties of the selected aggregates are determined in the laboratory, and then the aggregates are subdivided into sand-sized aggregates and some aggregate properties such as shape, porosity, impact value, compressive value and aggregation are determined. Then, using a fixed mixing scheme, the aggregates were prepared from concrete and the mechanical properties of the prepared concrete such as compressive strength, tensile strength and elastic modulus were determined after 7, 28 and 90 days. the effect of each of the aggregate properties on the concrete strength is investigated. This percentage is a useful guide for identifying the desired aggregates as well as the tests required to explore the aggregate resources. The results show that the resistance properties, abrasion, specific gravity and porosity, shape and texture, durability and stability of aggregates have the most effect on concrete strength, respectively.

The application of fiber and nano to improve the performance of concrete is common in the construction industry. In this study, the glass and steel fibers were selected, and their performances were investigated in combination with nano-montmorillonite. The purpose of this paper is to use glass fibers instead of steel fibers with the same strength. For this purpose, the weight ratio of nano-montmorillonite to cement was varied from 0 to 2.7%, and the weight ratio of fibers to cement was varied from 0 to 4.5%. Compressive strength tests were performed at the ages of 7, 28, and 90 days, flexural strength and water absorption tests were performed at the ages of 28 days. Nano-montmorillonite has increased the compressive strength of steel fiber concrete by 8 percent and the flexural strength by 15 percent. The influence of nano-clay on the mechanical properties of glass fiber concrete is greater than that of steel fiber concrete. Nano-montmorillonite has increased the compressive strength of glass fiber concrete by 40 percent and the flexural strength by 19 percent. Comparison of the effect of Nano-montmorillonite on the behavior of glass fiber reinforced concrete and steel fiber reinforced concrete shows that nanoclay not only improves the strength of the concrete but also eliminates the strength reduction caused by increasing volume fraction of glass fiber. The compressive and flexural strength tests showed that the optimal amount of steel and glass fibers is 3 percent. The optimum amount of nano-montmorillonite is dependent on the type of fiber and experiment.

Around the world, sugarcane production complexes annually produce about 1.2 million tons of surplus bagasse, which is burned due to the lack of conversion industries. In today's advanced world and due to advances in various scientific fields, the concrete industry has also evolved. Concrete containing pozzolan has different applications due to its special properties, which are differentiated according to its specific weight and compressive strength. Due to the importance of this issue, in this article, the effect of bagasse stem sugar pulp on the mixing of structural concretes in sulfate and chloride environments is investigated. For this purpose, eleven laboratory samples are used with a percentage of pozzolans of 0 to 55% is studied as a variable parameter. Finally, laboratory experiments showed that the addition of bagasse pozzolan in concrete increases the mechanical strength of concrete in chloride and sulfate environments and also the optimal percentage of bagasse pozzolan that causes the highest compressive strength in Concretes containing this pozzolan are 25% by weight of cement