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| Title | BEHAVIOR OF HIGH STRENGTH FIBER REINFORCED CONCRETE BEAMS |
| Type | MSc |
| Supervisors | IBRAHIM G. SHAABAN; FOUAD B. A. BESHARA |
| Year | 2007 |
| Abstract | In recent years, high strength concrete (HSC) has found many applications not only in columns of high-rise buildings, but also in a broad range of long-span flexural members. However, the maximum potentiality of HSC cannot be fully realized in structures due to the brittleness of the material and the serviceability problems associated with the resulting reduced cross sectional dimensions. In the present work, the effect of steel fibers inclusion on the flexural behavior of HSC beams is investigated under monotonic static loading. Comprehensive experimental program study was performed to study the different flexural response characteristics of high strength fiber reinforced concrete (HSFRC) beams with different fiber contents and aspect ratios, fibers cocktail, and fiber zoning. Finally, semi-empirical formulae were developed for the ultimate strength analysis and design of HSFRC beams in bending. In the experimental program, eleven simply-supported beams were tested under two symmetrically concentrated loads. All beams were reinforced by high-strength tensile steel bars as tension reinforcement, mild steel bars as secondary steel, and vertical mild steel stirrups as web reinforcement. The mean compressive strength of concrete matrix was 80 MPa. Using under-reinforced section approach, the longitudinal tension steel ratio was evaluated as 0.87%. The research objectives were to study the effects of fibers inclusion on the load-carrying and moment-resisting capacities, flexural stiffness, load-deflection curves, load-steel strain relationships, displacement and strain ductilities, failure mechanisms, and crack propagation and width. Moreover, the measured load-displacement response of the tested beams was used to define several indices to measure the flexural toughness of HSFRC at different loading levels. The effects of different fiber volume fractions (vf = 0.5%, 1.0% & 2.0%), fiber aspect ratios (lf/Φ = 50, 60 & 80), depths of fiber inclusion zones (partial, full), and polypropylene-steel fibers cocktail were studied on the flexural response. The testing results indicated that the flexural strength and toughness, displacement ductility, steel strain ductility, and secant stiffness, increase considerably with the increase of fiber content or fiber aspect ratio. It delays the crack propagation process and reduces the crack width. The increase of fiber volume from 0.0% to 2.0% leads to an increase in ultimate moment by 21.0%, in displacement ductility by 17.0%, in toughness by 77.0%, and in steel-strain ductility by 49.0%. The increase of fiber aspect ratio from 50 to 80 leads to an increase in ultimate moment by 20.0%, in displacement ductility by 15.0%, in toughness by 68.0%, and in steel-strain ductility by 41.0%. The use of 2.0% fiber content reduces the crack width by 29.0%, and the use of fiber aspect ratio 80 with fiber volume 1.0% decreases the crack width by 30.0%. The flexural response of the partially reinforced fibrous beams, where steel fibers were included only in the tension zone, was close to that of the fully reinforced fibrous beams. The addition of polypropylene fibers to HSFRC beams has a minor effect on the improvement of the measured structural response. Based on strain compatibility and equilibrium conditions for HSFRC sections in joint with idealized compression and tension stress blocks, simple formulae were developed for the ultimate flexural strength analysis and design of HSFRC beams. The stress blocks were given by suitable empirical functions for the compressive and post-cracking strengths of HSFRC. The accuracy of the proposed procedure was checked by comparing the calculated flexural strengths with the measured results, and with the experimental data reported in the literature. A very good correlation was found for different concrete strengths, steel reinforcement ratios, steel fiber parameters, and different sizes of beam specimens. |
| Keywords | |
| University | Benha |
| Country | Egypt |
| Full Paper | download paper |
| Title | BEHAVIOR OF REINFORCED CONCRETE CONTINUOUS DEEP BEAMS |
| Type | PhD |
| Supervisors | IBRAHIM G. SHAABAN; FOUAD B. A. BESHARA |
| Year | 2012 |
| Abstract | Deep beams are structural elements which frequently occur in practice as transfer girders, pile caps, tanks, folded plates and foundation walls, often receiving many concentrated loads and transferring them to a small number of reaction points. In the present work, comprehensive experimental studies were performed to study the response of reinforced concrete continuous deep beams (RCCDB's) till failure with different shear span-to-depth (a/d) ratios, concrete compressive strength (fcu), tension steel ratios, shear reinforcement ratios, and bond condition. The obtained experimental results are compared with design code predictions in flexure and shear. Also, the testing results are compared with the predictions of nonlinear finite element (FE) analysis using the well established program; ANSYS 10. Finally, a modified strut-and-tie model (STM) was developed as analysis and design tool for RCCDB's. In the experimental program, eighteen RCCDB's were tested. The test specimens were divided into two main groups as flexural beams and shear beams. Each group contains nine equal-span beams. Different values of (a/d) ratios were used as 1.25, 1.0, and 0.8. Tested beams were made by (fcu) ranged from 25 MPa to 35 MPa. Ratio between top and bottom longitudinal tension reinforcement (As-ve/As+ve) ranged from 1.0 to 1.35. For two flexural specimens, two steel layers were used as tension top reinforcement. Anchorage length for all specimens was 800 mm except for one specimen was 300 mm. Vertical and horizontal shear reinforcement ratios are ranged from 0.0% to 0.6%. The research objectives were to study effect of the test variables on the structural response characteristics of RCCDB's which include load-carrying capacity, stiffness, load-deflection curves, strain distribution for flexural and shear reinforcement, cracking patterns, and failure modes. The testing results indicated that the strength and stiffness reduction was prominent in case of lower fcu and higher a/d and that the variation of strains along the main longitudinal top and bottom bars was found to be dependent on the a/d. For tested beams having small a/d, horizontal shear reinforcement was always more effective than vertical shear reinforcement. For the vertical web reinforcement, a major redistribution of strains occurred for beams with a/d > 1 only. For the horizontal web reinforcement, major strain redistribution occurred for beams with a/d < 1. Increasing (a/d) ratio from 1.0 to 1.25 resulted in a decrease in the first flexural cracking, first diagonal cracking, and ultimate loads by about 20.0%, 18.0%, and 12.0%, respectively. The comparison between test results and current design codes indicated that American (ACI) and Egyptian (ECP) codes underestimate the shear capacity of RCCDB's. The obtained experimental field moments are higher than that obtained from the theoretical predictions for both ECP and ACI codes. The predictions of load-deflection response as well as the cracking patterns using the nonlinear FE analysis show a good agreement with the testing results. The FE predicted successfully the ultimate loads, displacement ductility, stiffness changes and failure mechanisms for deep beams with different variables. The proposed STM accounts for the effects of concrete compression softening, longitudinal flexural top and bottom steel, web reinforcement and bearing elements. Comparison of the results of the proposed STM with 60 test results indicates that the model generally performs well in predicting the ultimate load carrying capacities for RCCDB's. The overall average value of the ratio between the experimental strength to the predicted strength is of value 1.09 and a standard deviation of 0.12. The STM of the current design codes underestimate the strength of continuous RC deep beams. The predictions are consistent and accurate for RCCDB's with different geometrical properties, concrete compressive strengths and total reinforcement ratios. The proposed STM predicts well the reaction at the internal support compared to the experimental results. The overall average value of the ratio between the experimental reaction to the predicted reaction is of value 1.04 and a standard deviation of 0.14. |
| Keywords | Deep Beams; shear; flexure; strut-and-tie; continuous beams |
| University | Benha |
| Country | Egypt |
| Full Paper | download paper |
