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سال شانزدهم شماره 2 (تابستان 1402)

Corrosion of rebars is one of the most important problems in reinforced concrete structures. It may reduce the load capacity or cause failure of structural members. When corrosion occurs, the structure member can be strengthened/repaired using different methods. Due to the durability of FRP (Fiber Reinforced Polymer) materials in corrosive environments, these materials can be used to strengthen/repair the damaged reinforced concrete members against corrosion.In this study, the effect of Carbon FRP sheets on the flexural behavior of steel reinforced concrete beams in moisture and corrosion conditions is studied. For this purpose, ten reinforced concrete beam specimens with the cross-sectional dimensions of 150 x 200 mm and a length of 1400 mm were made. One specimen was considered as the control specimen without strengthening, and other specimens were subjected under accelerated corrosion conditions with different corrosion rates of 5, 10, and 15 percent. A comparison of the experimental results showed that the corrosion of longitudinal rebars affects the flexural behavior of reinforced concrete specimens. With increasing corrosion, the load capacity of the specimens decreased. Strengthening of the corroded specimens with CFRP sheets increased the load capacity of the corroded specimens up to 35% compared to the un-strengthened specimen. The results of specimens exposed to re-corrosion condition after strengthening showed that the strengthening of specimens by CFRP sheets effectively reduced further corrosion up to 26.9%.

Due to the importance of the use of pozzolans in this study, we attempted to better understand the available pozzolans in Khash region. Therefore, two different rocks from Khash region were investigated for quality assessment. These two rocks are dacite and basaltic and after mill they are called dacite and basaltic ash. In this study, two different types of concrete were designed using inorganic polymer made from the mentioned ash and their mechanical properties were investigated, which significantly showed that the basaltic ash had higher compressive strength, so that the highest compressive strength was obtained from samples made with 72 MPascal basaltic ash. However, the highest compressive strength obtained from specimens made with dacite ash is 49 MPascal. Scanning electron microscopy also showed that many particles were not reacted in samples made with dacite ash. Also, mercury injection porosity test showed that samples made with dacite ash had more porosity than basaltic samples.

In this paper, effects of curing methods (i.e. curing at ambient temperature (A), in oven (O) and in boiling water (BW)), NaOH solution concentration (i.e. 8 and 12 moles/liter) and disc thickness (i.e. 1, 2 and 3 cm) on pull-off strength (POS) of the geopolymer concrete containing Ground Granulated Blast Furnace Slag (GGBFS) and Natural Zeolite Powder (NZP) have been investigated. The results show that the value of the POS obtained from the 3 cm thickness disc is on average 2.6% higher than that obtained from the 2 cm thickness disc. This implies that the discs with thicknesses of 2 and 3 cm have given approximately equal values of the POSs. Moreover, the value of the POS obtained from the 2 cm thickness disc is on average 28.4% higher than that obtained from the 1 cm thickness one. Based on the results obtained from the 2 cm thickness disc: a) the ratios of the POSs of the samples cured in boiling water and oven to that of the specimen cured at ambient temperature have been obtained on average equal to 1.383 and 1.089, respectively. b) as 20% wt. of the GGBFS is substituted by the NZP, the value of the POS reduces by an average of 16.29%. c) as the NaOH solution concentration increases from 8 to 12 mole/liter, the value of the POS increases by an average of 14.48%. From the results, there is a strong nonlinear relationship between the compressive and pull-off strengths of the geopolymer concrete.

A more accurate understanding of the characteristics of self-compacting mortars, which have wide applications in construction projects, is essential. Today, extensive studies are conducted in this field. In the present study, the effect of two pozzolans, microsilica and zeolite, as well as the addition of superabsorbent polymer 1 (SAP) was studied in order to measure their effects on self-compacting mortar. In this research, 17 mixing plans were made with different percentages of additives. In this research, 17 mixing plans were made with different percentages of additives. Mortar setting speed test, heat of hydration, as well as compressive strength and electrical resistance tests were performed at different ages. The results showed that samples containing microsilica compared to zeolite showed a significant improvement in quality. With economic analysis, it was found that high percentages of SAP significantly increase the construction cost. Also, in the software analysis of durability, the use of 10% by weight of silica fume instead of cement resulted in an increase in useful life up to 86 years.

In this study, effect of poly vinyl alcohol (PVA) and poly propylene (PP) fiber on epoxy resin-based concrete in terms of compressive, flexural, tensile strength and deformation has been investigated. In addition, scanning electron microscope (SEM) analyses were conducted on samples. Four groups of polymer concrete with 12% epoxy resin and fiber dosage of 0.5, 1, 2 and 2,5 % were prepared for both PVA and PP fiber. The fiber length was 12 mm. Cube and prism samples with 50 mm and 50x50x300 mm dimensions were prepared and tested for compressive strength and tensile strength/ deformation at 1, 7 and 28 days. Results showed that by increase of PP dosage, compressive and tensile strength increased noticeably compared to the control sample. However, for samples containing PVA, opposite trend has been observed. Therefore, the optimum dosage of PP and PVA fiber in fiber reinforced epoxy resin-based concrete is 2.5 and 0.5% respectively. The increase in compressive and flexural strength for 0.5, 1, 2 and 2.5% PP fiber was 4.38%, 5.3%, 4.9% and 5.5% compared to the control sample, respectively. The SEM analysis showed good bonding of both PP and PVA fiber to the polymer matrix. The PP fiber had better dispersion which could be the primary factor for better physical performance.

The purpose of this paper is to evaluate the behavior of vulnerable joints of substandard RC frames, improved by Shear and flexural retrofit of beam-column joints using FRP sheets. For this purpose, the behavior of the retrofitted joints is compared with the control joints with and without special seismic details. Considering the main focus of previous studies on two-dimensional joints and not considering the real conditions including the effect of slab and transverse beam in the joints, in order to apply geometric conditions and constructional limitations, in this study the behavior of joints with slab and transverse beam effect has been extracted by experimental study. For this purpose, 2 experimental specimens of concrete exterior beam-column joint with a scale of 1/2 are made and subjected to a cyclic load. The specimens include one control specimen, without special seismic details, and the other retrofitted specimen include a joint without seismic detail, which has been improved with FRP sheets. Finally, the studied joints were modeled using OpenSees software and compared with the experimental results of the research. The results of the retrofitted joint behavior show no strength drop, stable cyclic behavior, reduced pinching effects, improved cyclic behavior due to change of the failure mode from shear failure of the joints to the formation of flexural plastic hinges in the beam. This performance can be attributed to the effectiveness of the proposed retrofit methods in promoting the behavior of vulnerable joints with real geometric conditions

In this paper, the fresh and hardened properties of mortars containing different percentages of recycled sand made from crushed concrete blocks were studied. For this purpose, natural sand was substituted with recycled sand in 0, 25, 50, 75, and 100 percentages. The same particle size distribution for both natural and recycled materials was used. Also, the water to cement ratio was considered 0.55, and the cement to sand weight ratio was considered to be 1:3. Physical properties including water absorption, water permeability, and bulk density of fresh and hardened mortar, and mechanical properties including compressive and bending strength, resistance against freezing and thawing cycles, and drying shrinkage were examined. The experimental results suggest that mortars made with recycled sand had acceptable compressive strengths all above 10 MPa. By increasing the substitution ratio, the workability and workable time of mortars were reduced. Masonry mortar made with recycled fine aggregate had acceptable resistance against freezing and thawing cycles according to ASTM C666. Results of the drying shrinkage test showed that masonry mortar made with recycled sand had more drying shrinkage than mortars made with natural sand which is a result of a more hardened cement volume ratio in the samples containing recycled sand.Overall, it can be concluded that up to 50% replacement level of aggregate can have reasonable mechanical and durability properties and may be suitable for sustainable development.

The environmental effects and the high consumption of fossil fuels in the production process of ordinary cement caused scientists to think of finding an alternative material for cement in concrete. In this regard, in the last few decades, geopolymer concrete production as a Novin was noticed by researchers in the construction industry, in the production of this type of concrete, aluminosilicate materials and active alkali solution replace cement and water, and they produce concrete with strength and strength beyond ordinary concrete. In this article, the durability properties of geopolymer-cement concrete containing 0-8% nano-silica and 1-2% polyolefin fibers have been investigated. Water capillary and weight loss were placed, also XRF and XRF spectroscopy were used to check the microstructure of the samples. The obtained results indicated the improvement of performance of concrete samples with the addition of silica nanoparticles and polyolefin fibers compared to the results of concrete containing Portland cement, microstructural investigations also confirmed the obtained results. The best performance in the results of water permeability and capillary absorption tests belonged to the age of 90 days of geopolymer concrete (containing 8% nanosilica and 2% fibers), which showed 56% and 65% improvement, respectively, compared to The control concrete was selected and the geopolymer concrete sample containing 92% slag and 8% nanosilica had the lowest weight loss of 6% after applying high heat.