جستجوی مقالات مرتبط با کلیدواژه "flexural strengthening" در نشریات گروه "عمران"

Various methods have been developed for the retrofit and strengthening of RC beams. One of the successful methods is the near-surface mounting (NSM) method. In this method, the reinforcing rebar is inserted and glued into the groove in the concrete cover. This groove is created in the tensile face of the beam. The relative protection of reinforcing materials, which are less exposed to mechanical damage, shock and fire loading is an important advantage of this strengthening method. Pre-stressing can close the cracks caused by the weak flexural strength of the member and reduce its excess deflection. The proposed strengthening method in this research was near surface mounting of pre-stressed rebars to enhance the flexural behavior of RC beams. The test program consisted of testing 5 RC beams of 200×200 ×1700 mm. One was the control sample and was tested without strengthening. Four other beams were first pre-loaded to the cracking stage and, after unloading, were strengthened by near surface mounting of the steel bars. The beams were tested under four-point bending. The applied load and the corresponding displacement in the middle of the beam were measured. In the flexural strengthening of the beams by proposed NSM method, end-anchorage and epoxy adhesive, both provided the proper bond. Tightening the end nuts with the aid of a torque meter, was the selective method to pre-stress the reinforcing bar. The results showed that this method was very effective for the flexural strengthening of RC beams, so the final load capacity of beam samples increased about twice as much as the control sample. The proposed method also led to the closure of the bending cracks and the reduction of the samples’ deflection, as well as the increase in energy absorption of the samples.

It has been used to preserve structures and extend their useful life, retrofit damaged structures. Concrete slabs, as a key structural member, play an important role in the load distribution and structural behavior, and lack of resolving the damage to concrete slabs can lead to irreparable damage. In this experimental study, the one way reinforced concrete slabs were strengthened with using high performance fiber-reinforced cementitious composite laminates in the slab's tensile side. Its lateral surfaces are then strengthened with carbon fiber reinforced polymer laminates to increase shear capacity. This study is summarized in three steps. In the first step, the mixing design and mechanical properties of fiber-reinforced cement composites were investigated. In the second step, the flexural capacity of fiber-reinforced cement composite laminates was determined separately before bonding to the slab. In the final step, shear and shear reinforced concrete slabs were tested for shear behavior investigation. The results showed that the strengthening of the lateral sides of the specimens was improved the flexural capacity, fracture pattern, stiffness, and energy absorption by examining the shear behavior of the specimens. Also for one-way slabs strengthened with fiber-reinforced cement composite laminates, if the concentrated load is applied to the slab so that the shear Span-to-effective height ratio is less than 2.5, even If it is strengthened at the lateral surfaces to increase the shear capacity of the cross-section, the failure pattern will certainly be shear.

Today, strengthening of reinforced concrete structures and especially flexural strengthening by fiber reinforced polymers (FRP) is used worldwide as an efficient way. In this study, the existing of a constant load during FRP flexural strengthening versus load removal before strengthening were experimentally investigated and compared. A preload with two different predetermined amounts, one to make the beam cracked elastically and the other to make beam deflected to plastic region, was applied before the strengthening of beam samples. All Externally Bonded Reinforcement In Groove (EBRIG) installed FRP sheets were ruptured during test and no de-bonding was observed. Pre-cracking of beams in unloaded strengthened beams caused a minor decrease in ultimate load capacity of strengthened beams. Also ductility and ultimate deformation was decreased. Flexural strengthened beams under constant load showed better ultimate strength (up to 8%) and also better ductility and energy dissipation ability.It can be concluded that FRP flexural strengthening of beams under constant load is more appropriate than load removal of beams during strengthening. The appropriate amount of load during strengthening must be studied in detail but excessive loading to plastic extents is not appropriate.

This paper shows the results of an experimental study on the behavior of rectangular reinforced concrete columns that strengthened in bending with NSM FRP bars and CFRP jacketing under combined compression axial load and cyclic lateral loading. Near surface mounted (NSM) fiber reinforced polymer (FRP) is one of the best retrofitting techniques for reinforced concrete (RC) structures. This technique is based on inserting fiber reinforced polymer bars into slits in the cover of reinforced concrete members. For this purpose, seven rectangular RC columns including one control specimen constructed and tested under constant axial load and lateral cyclic displacement. Experimental parameters include different ratios of NSM FRP bar, maximum lateral load capacity, and failure modes are describes based on the test results. The crack patterns in the specimens are also presented. Also, the study was designed to evaluate the necessity of anchoring the end of NSM FRP bars in the foundation. The test results show that a significant increase in the load carrying capacity, stiffness, and energy dissipation of rectangular RC columns can be achieved by using the NSM technique.

The old-type RC buildings constructed before 1970s are mostly designed according to gravity loads and lack seismic detailing. In these buildings, the strong column-weak beam concept is not usually observed, and due to the probability of soft-storey event, they are vulnerable to seismic loads. Therefore, the flexural strengthening of the columns is proposed as one of the top priorities in seismic retrofitting of these buildings. This paper experimentally studies the flexural strengthening of old-type RC columns with near surface mounted (NSM) technique. Six half-scale columns, including two control columns and four columns strengthened via NSM method with GFRP and steel bars, were tested under constant axial load and increasing lateral load; the effect of strengthening method and materials were also investigated. The mean flexural strength and energy dissipation capacity of the strengthened columns with GFRP bars increased respectively by 62% and 46% compared to corresponding control columns; but the equivalent hysteresis damping of these columns did not have a substantial increase. In addition, the mean flexural strength, energy dissipation capacity, and equivalent hysteresis damping of the strengthened columns with steel bars increased respectively by 86%, 197%, and 104% compared to the corresponding control columns. The results indicate that NSM technique remarkably increases the flexural strength and improves the seismic parameters of columns, especially when steel bar are used as NSM reinforcement.

Externally Bonded Reinforcement (EBR) is known as a conventional method for flexural strengthening of concrete beams with Fiber Reinforced Polymer (FRP) composites. The present study has been conducted to investigate the flexural behavior of RC beams strengthened by CFRP sheets. Eight beam specimens strengthened by EBR method having the cross sectional dimensions of 250*300 mm and length of 2200 mm with two different reinforcement ratios (low and high) were evaluated under four point bending test. The compressive strength of four specimens was 25 MPa and other four specimens were made using high strength concrete with the compressive strength of 55 MPa. Additionally, the tensile reinforcement arrangement varied in different specimens with two bar sizes. In order to strengthen the specimens, two CFRP layers with dimensions of 160*1700 were used. Based on the results of the present study, in the case of low reinforcement ratio while maintaining the total reinforcement ratio constant, by decreasing the bar size and increasing the number of bars, the load carrying capacity of the specimen increases. In contrast, in the case of high reinforcement ratio, this results in the decrease of load carrying capacity. Finally, utilizing the smaller bar size in the tensile region of the specimens with low reinforcement ratio results in a more uniform distribution of cracks in the beam.

In order to investigate the effect of Composite Fiber Reinforced Polymers (GFRP) on bending strengthening of light weight T-shaped concrete beams in this paper, nine specimens were designed, built and tested. These specimens were divided into A, B & C Groups, according to their conditions and bars arrangement and composite layers. In Group A, beams were designed to have a medium bending defects, while Groups B and C designed for maximum and minimum bending defects.In each group one beam was as reference and the others were strengthened with different numbers of FRP layers. According to observations and results, flexural strength increase of in Group A was from 81% to 116% while that was from 33% to 208% in Group B and 67% to 100% in Group C. And also the effect of using stronger epoxy and anti-slipt material to change shear failure (debonding) to flexural debonding and increasing strength was investigated in this paper.