EFFECT OF USING SMOOTH GLASS FIBER REINFORCED BARS ON THE RESPONSE OF CONCRETE BEAMS
AMR MOHAMED IBRAHIM MOSTAFA;
Abstract
Six beams reinforced with GFRP bars were initially casted and tested in three point loading; the GFRP bars had a fixed nominal diameter of 9.64 mm, tensile strength of 550 MPa and elastic modulus of 48 GPa. The compressive strength of the concrete mix was 30 MPa, the strain gauges and the LVDTs were fixed in place and the tensile and compressive strains and the deflection along the beam span were measured. All the beams failed due to sudden brittle failure at the load position. The load-strain relations were presented with the strain profile along the height of the beams and the load- maximum deflection curves of the beams.
Three beams were retested as there was slippage of the bars at the end of the beam, so the beam ends were casted by non-shrinkage grout and these three beams were re-tested with reduced span and with shear span to depth ratio less than 1.5 to decrease the stresses on the bars and it was found that the failure mode did not change. A comparison of the test results and the experimental failure load with the flexural capacity, shear capacity and cracking load of the beams was done and it was found that the experimental load was less than the flexural capacity of the beams and less than the shear capacity computed from equations (5.6) to (5.10).
Then the experimental load was compared to the cracking load of the beams and it was found that three of the beams failed at approximately the cracking load while the other beams failed at a load level higher than the theoretical cracking load of the beams. Then an analysis of bond between the used GFRP bars and the concrete was conducted and it was found that the bond stress calculated from the pull-out test results was less than the bond strength calculated using different equation, thus it was realized that the bond between the bars and the concrete had a strong influence in the failure mode that occurred in the tested beams.
6.2 Conclusions
1. ACI 440.1R-06 design method for calculating shear strength of concrete beams reinforced with GFRP bars provided very conservative results as the computed load was less than the experimental load although the failure was not a shear failure.
2. The CSA S806-02, JSCE (1997) and CNR-DT 203 (NRC 2006) methods for calculating the concrete contribution to the shear strength of beams reinforced with GFRP bars are more accurate in case of shear span/depth ratio (a/d) ≥ 3, while they provide conservative results for concrete shear strength for beams with (a/d) < 3 as the computed loads were less than the experimental although the failure was not a shear failure.
3. There is no consistency between the different codes for calculating the shear strength of concrete beams reinforced with GFRP bars.
4. The failure load in the beams with depth ≤ 500mm was (44-65)% of their flexural capacity.
5. The cracking load of concrete beams reinforced with GFRP bars is different than that of concrete reinforced with conventional steel bars which indicates that the cracking stress of concrete reinforced with steel bars ( ) is different in concrete reinforced with GFRP bars.
6. It is not recommended to use smooth GFRP bars in reinforced concrete unless the bar is coated with rough sand and epoxy before casting of concrete as a rough epoxy coating significantly increases the bond between the bars and the concrete or using machined GFRP bars instead or using steel plates at the end of the bars to increase their anchorage.
7. Using sand and epoxy coating on the surface of the GFRP bars tripled the bond stress between such bars and the concrete (of compressive strength 30 MPa).
8. The smooth surface texture cause internal bond failure of the GFRP bars when subjected to tension during loading of the concrete beams which results in the slippage of the bars from the concrete causing a brittle failure.
6.3 Recommendations and Suggestions for Future Research
1. It is recommended to use higher reinforcement ratios when using this type of reinforcement (with no yielding plateau) in concrete beams.
2. Study of the behavior of concrete beams reinforced with epoxy coated GFRP bars.
3. Study the effect of beam size on the concrete contribution to shear strength.
4. More investigation of the flexural behavior and moment redistribution of concrete beams reinforced with FRP bars.
Three beams were retested as there was slippage of the bars at the end of the beam, so the beam ends were casted by non-shrinkage grout and these three beams were re-tested with reduced span and with shear span to depth ratio less than 1.5 to decrease the stresses on the bars and it was found that the failure mode did not change. A comparison of the test results and the experimental failure load with the flexural capacity, shear capacity and cracking load of the beams was done and it was found that the experimental load was less than the flexural capacity of the beams and less than the shear capacity computed from equations (5.6) to (5.10).
Then the experimental load was compared to the cracking load of the beams and it was found that three of the beams failed at approximately the cracking load while the other beams failed at a load level higher than the theoretical cracking load of the beams. Then an analysis of bond between the used GFRP bars and the concrete was conducted and it was found that the bond stress calculated from the pull-out test results was less than the bond strength calculated using different equation, thus it was realized that the bond between the bars and the concrete had a strong influence in the failure mode that occurred in the tested beams.
6.2 Conclusions
1. ACI 440.1R-06 design method for calculating shear strength of concrete beams reinforced with GFRP bars provided very conservative results as the computed load was less than the experimental load although the failure was not a shear failure.
2. The CSA S806-02, JSCE (1997) and CNR-DT 203 (NRC 2006) methods for calculating the concrete contribution to the shear strength of beams reinforced with GFRP bars are more accurate in case of shear span/depth ratio (a/d) ≥ 3, while they provide conservative results for concrete shear strength for beams with (a/d) < 3 as the computed loads were less than the experimental although the failure was not a shear failure.
3. There is no consistency between the different codes for calculating the shear strength of concrete beams reinforced with GFRP bars.
4. The failure load in the beams with depth ≤ 500mm was (44-65)% of their flexural capacity.
5. The cracking load of concrete beams reinforced with GFRP bars is different than that of concrete reinforced with conventional steel bars which indicates that the cracking stress of concrete reinforced with steel bars ( ) is different in concrete reinforced with GFRP bars.
6. It is not recommended to use smooth GFRP bars in reinforced concrete unless the bar is coated with rough sand and epoxy before casting of concrete as a rough epoxy coating significantly increases the bond between the bars and the concrete or using machined GFRP bars instead or using steel plates at the end of the bars to increase their anchorage.
7. Using sand and epoxy coating on the surface of the GFRP bars tripled the bond stress between such bars and the concrete (of compressive strength 30 MPa).
8. The smooth surface texture cause internal bond failure of the GFRP bars when subjected to tension during loading of the concrete beams which results in the slippage of the bars from the concrete causing a brittle failure.
6.3 Recommendations and Suggestions for Future Research
1. It is recommended to use higher reinforcement ratios when using this type of reinforcement (with no yielding plateau) in concrete beams.
2. Study of the behavior of concrete beams reinforced with epoxy coated GFRP bars.
3. Study the effect of beam size on the concrete contribution to shear strength.
4. More investigation of the flexural behavior and moment redistribution of concrete beams reinforced with FRP bars.
Other data
| Title | EFFECT OF USING SMOOTH GLASS FIBER REINFORCED BARS ON THE RESPONSE OF CONCRETE BEAMS | Other Titles | تأثير استخدام أسياخ الألياف الزجاجية الملساء علي سلوك الكمرات الخرسانية | Authors | AMR MOHAMED IBRAHIM MOSTAFA | Issue Date | 2015 |
Recommend this item
Similar Items from Core Recommender Database
Items in Ain Shams Scholar are protected by copyright, with all rights reserved, unless otherwise indicated.