جستجوی مقالات مرتبط با کلیدواژه « الیاف cfrp » در نشریات گروه « عمران »

Deep beams are members considered to have span to depth ratio less than 4. These beams are widely used in different sorts of structures including dams, reservoirs, silos, caissons and high-rise buildings. The cracking mode of deep beams is mainly dependent on their boundary conditions. Due to the rule of shear failure, deep beams are designed against shear. So, strengthening and repairing these beams has always been important to improve shear resistance. One of the applied methods is the use of CFRP fibers for repairing and strengthening deep beams. These fibers can be fabricated in the form of strips, sheets and rebars. Due to the constructional limitations, CFRP strips are frequently installed on the external surface of beams. For this reason, the failure of strengthened beams coincides with debonding of CFRP strips. The former experiments indicate that strengthening deep beams with CFRP strips is useful for improving their behavior. These experiments also indicated that parameters including strengthening angle, shear span to depth ratio and the method of CFRP installation can affect the strength increment due to CFRP. One of the other remarkable parameters that affects the behavior of reinforced concrete beams is size effect. This parameter matters when the geometry of struts and nodal zones remains slender in deep beams. To assess the intensity of size effect in deep beams, the force strength of beam must be normalized based on the compressive strength of concrete and the beams sections area. This study was conducted to investigate the effect of CFRP strengthening on the size effect on deep beams. It was aimed to use explicit dynamic analysis method in Abaqus software so as to model and analyze 53 CFRP-strengthened deep beams with evaluation of previous experiments. In this method, due to the absence of excessive iterations within each analysis step, the number of analysis steps is increased. The so-called method is also appropriate for simulating quasi static models. To reach the purpose of study, the specimens of three different experiments were modeled and analyzed to evaluate the assumptions of numerical modeling. After the evaluation conditions were satisfied, 53 deep beams were modeled in Abaqus software. The specimens were subjected to two incremental point loads and were divided into four-member groups with depths of 400 mm, 600 mm, 800 mm and 1000 mm. The beams shear span to effective depth ratios are 0.5, 1 and 1.5; The compressive strength of concrete also varied from 24.8 MPa to 35 MPa. Since changing the width of deep beams does not affect the intensity of size effect, the beams width was considered constant and equal to 80 mm.The results of the study indicate that strengthening deep beams with CFRP strip or sheet is suitable for reducing the size effect; In addition, increasing compressive strength of concrete and keeping the loading plate constant can amplify size effect of deep beam. Increasing shear span to effective depth ratio of beam caused the size effect to be decreased. Strengthening deep beams with both angles of 45 and 90 degrees was appropriate for decreasing deep beams size effect.

Concrete is the most widely used building material in construction industry worldwide and its constituents are easily accessible everywhere. However, cement industry, as the producer of the primary binder of concrete, is one of the effective sources of environment degradation. Cement production needs extraction of mineral resources and burning fuel and causes extensive greenhouse emission due to disintegration of raw materials. Cement production alone is responsible for 7% of global CO2 emission with estimated annual growth of 4%. Toward environmental sustainability, one way is partially or totally replacing cement by waste or byproducts of other industries such as fly ash, ground granulated blast-furnace slag (GGBS), waste water, metakaolin, and silica fume. Geopolymer is a cementitious material with comparable characteristics to those of ordinary cement produced by alumina- and silica-rich waste materials. Therefore, it does not require energy-intensive and pollutive calcination process. Geopolymerization is formed by reaction of silica-alumina under an alkaline solution which creates three dimensional Si-O-Al-O polymeric chains to attain compressive strength, compared to the ordinary cement which develops calcium silicate hydrates (C-S-H) as the main adhesive. Extensive research has conducted on geopolymer concrete. However, more investigations are needed to better understand characteristics of geopolymer concrete containing additives. Fibers are proved to have a positive effect on mechanical strength of concrete. As well, fillers such as microsilica can improve mechanical and durability of concrete. Moreover, most studies in this area are focused on fly ash-based geopolymers and the investigations on GGBS-based geopolymer are rare in the literature. In this study, mechanical and durability of GGBS-based geopolymer concrete containing CFRP fibers and microsilica is investigated. Different concrete samples with 0-3% CFRP fibers and 0-10% microsilica are prepared and experimentally tested. Sodium Hydroxide (NH) and Sodium Silicate (NS) solutions are used as alkali activators. 8 M NH as well as NS with 14.7 Na2O and 29.4 SiO2are used with the NS/NH ratio of 2.5. Since no standard exists for mix design of geopolymer concrete, proposed mix design by Venkatesan and Pazhani (2015) is used. Alkaline to binder ratio of 0.4 is selected with 430 kg/m3binder.The specimens were tested after 28 days of curing. Next, mechanical and durability tests including compressive strength, tensile strength, ultrasonic pulse velocity, water absorption, RCPT, and acid resistance are conducted on the samples. Also, microstructure of the geopolymer concrete is investigated. Results of experimental tests show that, compressive and tensile strength of geopolymer samples decrease by adding microsilica. However, 5% microsilica is the best value to enhance mechanical properties of geopolymer concrete. On the other hand, microsilica can enhance durability properties of geopolymer concrete so that adding 5% microsilica causes moderate improvement of water absorption and chloride penetration. The greatest impact of microsilica is on acid resistance by which adding 5% microsilica resulted in 67% improvement of compressive strength loss. However, unlike the microsilica, CFRP fibers have detrimental effect on mechanical properties and durability of the geopolymer concrete since adding fibers can yield disruption of concrete integrity .On the other point of view, microstructure study shows that all the specimens exhibit micro cracks that can inversely affect the performance of concrete. Also, SEM images show that there is not a strong bond between CFRP fibers and binder paste which yields lower performance of concrete specimens containing fiber.

Due to existence of the facility ducts, such as elevators, heating, and cooling systems in buildings, creating of the openings in the reinforced concrete slabs is inevitable. Usually, small openings have negligible effects on the slab performance whereas the large openings can affect significantly on the slab behavior and can cause the stress concentration adjacent to the openings. In some cases, ignoring this issue can cause serious problems in the integrity, stiffness and even stability of structures. One of the rehabilitation and strengthening methods of such slabs is the use of Fiber Reinforce Polymers sheets (FRPs). In this research, the influence of the lateral and corner openings as well as the impact of using a kind of FRPs known as CFRP (Carbon Fiber Reinforce Polymer sheets) for strengthening of two-way slabs were investigated. The obtained results indicate that this method can be successfully employed in remedy operations for mentioned slabs. CFRP sheets can recover the lost flexural rigidity of the slab considerably and even in some cases can improve it in comparison to that of homogeneous slab (slab with no opening). The results also show that by implementing this method the flexural rigidity of the slab can be increased between 12 to 30 percent in the case of slabs with lateral opening and between 1 to 10 percent in the case of slabs with the corner opening relative to the original weakened slab (slab with opening without strengthening).