Ductility of Externally Prestressed Continuous Concrete Beams (original) (raw)
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Flexural Strength of Prestressed Concrete Beams with Tendon wrapped by Plastic sheets
For post-tension prestressed concrete beams, both of tendons laid in metal duct and wrapped by plastic sheets are recognized as unbonded tendon by researchers, and the same equation is used to estimate the flexural strength. But the contacting conditions between tendon and its surrounding concrete are quite different for these two kinds of unbonded tendons. This study compares the flexural strength, crack distribution and crack width of beams with these two kinds of unbonded tendons and bonded tendon. The results reveal that the structural behavior of tendon wrapped by plastic sheets is between bonded tendon and tendon laid in metal duct. A modified equation is proposed to estimate flexural strength of tendon wrapped by plastic sheets. Reducing cost and shortening construction time are the main advantages for this kind of the prestressed beams with tendon wrapped by plastic sheets. After further investigation, it may be an alternative method for the post-tension prestressed concrete structure in the future.
Shear Behavior of Concrete Beams Externally Prestressed with Parafil ropes
Although extensive work has been carried out investigating the use of external prestressing system for flexural strengthening, a few studies regarding the shear behavior of externally prestressed beams can be found. Five beams, four of them were externally strengthened using Parafil rope, were loaded up to failure to investigate the effect of shear span/depth ratio, external prestressing force and concrete strength on their shear behavior. Test results showed that the shear span to depth ratio has a significant effect on both the shear strength and failure mode of the strengthened beams and the presence of external prestressing force increased the ultimate load of the tested beams by about 75%. Equations proposed by different codes for both the conventional reinforced concrete beams and for ordinary prestressed beams were used to evaluate the obtained experimental results. In general, codes equations showed a high level of conservatism in predicting the shear strength of the beams. Also, using the full strength rather than half of the concrete shear strength in the Egyptian code PC-method improves the accuracy of the calculated ultimate shear strength.
Engineering Structures, 2013
This paper presents a brief review of equations proposed for calculating the increase in stress in external prestressing tendons at the ultimate stage. The validity of these equations to calculate the ultimate stress of external tendons in continuous beams was checked. Then, a simple method to calculate the stress in external tendons for continuous beams with or without symmetrical loading from the deformation of the strengthened member is presented and discussed. The steps of this method are simple and can be used at any stage of loading. Experimental results of 37 beams obtained from the published literature were used to check the accuracy of the proposed method. Several factors such as tendon type, internal and external prestressing forces, loading type, the presence of non-prestressed steel, tendon eccentricity/depth ratio, span/depth ratio, beam type, concrete strength, deviators number and deviator location were covered. The comparison between the actual and the calculated results showed good agreement.
Engineering Structures, 2018
This study investigates the use of carbon fibre reinforced polymer (CFRP) tendons on precast segmental beams (PSBs) to tackle the corrosion problems which are likely to occur at joint locations of PSBs prestressed with steel tendons. Up to date, the use of CFRP tendons was extensively documented for monolithic beams while their application on PSBs has not been reported yet. Three precast segmental T-section beams including two beams with unbonded CFRP and one with steel tendons were built and tested under four-point loads in this study. The test results showed that CFRP tendons can be well used to replace the steel tendons on PSBs. The beams with CFRP tendons demonstrated both high strength and high ductility as compared to the beam with steel tendons. However, the stresses in the unbonded CFRP tendons at ultimate loading conditions of the tested beams were low, ranging from only about 66% to 72% of the nominal breaking tensile strength. The type of joints i.e. dry and epoxied, greatly affects the initial stiffness of the beams but has no effect on the opening of joints at ultimate loading stage. Moreover, a comprehensive examination on four existing code equations to predict the stress in the unbonded tendons showed that the four examined codes predicted well
Construction and Building Materials, 1998
. The use of fiber reinforced polymer FRP composites is significantly growing in construction and infrastructure applications where durability under harsh environmental conditions is of great concern. Aramid fiber composite rectangular tendons have potential as reinforcements for concrete structures. Recent studies on prestressed concrete beams have showed brittle flexural failure due to the elastic rupture of the FRPs; however, for the maintenance and hazard anticipation of structures, the brittle failure mode is undesirable. In order to improve the ductility, a series of flexural tests was carried out on beams with bonded Ž . andror unbonded rectangular rebars, or with additional non-tensioned regular reinforced rebars. Test results showed that ultimate deformation in the beam with unbonded rectangular rebars was 250% that of the beam with bonded rectangular rebars, while the loading capacity was 85%. The additional placement of reinforced rectangular rebars produced a 450% higher ultimate deformation. It was found that the ductility was significantly enhanced by the use of combination of bonded and unbonded rectangular rebars. ᮊ
Flexural Design of Prestressed Concrete Beams Using FRP Tendons
Fiber Reinforced Polymer (FRP) tendons are being considered for design in structures exposed to aggressive environments or where non-metallic properties are desired. FRP tendons require considerable attention to detail during the design process. This paper presents a unified approach for the flexural design of beams with FRP tendons. Equations for flexural strength are presented, failure modes are defined, calibrations with test data are presented, and strength reduction factors are recommended. A test program validates the design approach and provides some serviceability data. Conclusions from the test program and design recommendations are provided.
CRACKING BEHAVIOR AND CRACK WIDTH PREDICTIONS OF CONCRETE BEAMS PRESTRESSED WITH BONDED FRP TENDONS
A total of six specimens were tested under four-point loading to examine the cracking behavior of prestressed concrete beams with a combination of bonded CFRP tendons and steel/GFRP reinforcements. The investigated parameters included the partial prestressing ratio (PPR), amount of prestressed CFRP tendons, types of nonprestressed reinforcements, and jacking stress levels. The characteristics of cracking propagation, cracking spacing and crack widths of specimens were presented. Test results indicated that the prestressed concrete beam reinforced with nonprestressed GFRP rebars exhibited wider crack width and larger crack spacing than those reinforced with nonprestressed steel rebars. Based on the correlative principles recommended by ACI 224R-01, a formula modified from the Frosh equation was provided for calculating the maximum crack width of concrete beams with a combination of bonded FRP tendons and steel/FRP reinforcements.
Composite Structures, 2019
Precast segmental prestressed concrete beams (PSBs) have been widely used in many elevated highway bridge projects around the world. Steel tendons at joint locations, however, are vulnerable to corrosion damages, which cause deteriorations and in extreme cases lead to the collapse of the whole structures. This study experimentally investigates the use of carbon fibre-reinforced polymer (CFRP) tendons as an alternative solution for the PSBs to tackle the corrosion issue. Four large-scale segmental T-shaped concrete beams with internal bonded or unbonded tendons and dry or epoxied joints were built and tested under four-point loading. The test results indicated that CFRP tendons showed satisfactory performances therefore could replace steel tendons for the use in PSBs. All the tested beams exhibited excellent loadcarrying capacity and ductility. Tendon bonding condition greatly affected the flexural performance of the segmental beams. Joint type had only a slight effect on the load-carrying capacity and ductility of the beams, but significantly affected the beams' initial stiffness. Unbonded tendons experienced an evident reduction in the tendon strength at the ultimate stage as a consequence of the loading type, harping effect and joint opening. Both AASTHO
Modelling of load–deflection of concrete beams internally prestressed with unbonded CFRP tendons
Magazine of Concrete Research, 2015
strain in the topmost concrete fibre in a section of cracking at cracking state å S cy compressive strain of topmost concrete fibre in a section at yielding state å dc pfu decompression strain of unbonded tendon å i pfu initial prestrain of unbonded tendon r pfb balanced prestressing reinforcement ratio for beam with bonded CFRP tendons ö S (x) distribution of linearly varying curvature at loading state S Magazine of Concrete Research Volume 67 Issue 13 Modelling of load-deflection of concrete beams internally prestressed with unbonded CFRP tendons Lee, Shin and Lee Magazine of Concrete Research Volume 67 Issue 13 Modelling of load-deflection of concrete beams internally prestressed with unbonded CFRP tendons Lee, Shin and Lee Magazine of Concrete Research Volume 67 Issue 13 Modelling of load-deflection of concrete beams internally prestressed with unbonded CFRP tendons Lee, Shin and Lee
Balanced Ratio of Concrete Beams Internally Prestressed with Unbonded CFRP Tendons
International Journal of Concrete Structures and Materials
The compression or tension-controlled failure mode of concrete beams prestressed with unbonded FRP tendons is governed by the relative amount of prestressing tendon to the balanced one. Explicit assessment to determine the balanced reinforcement ratio of a beam with unbonded tendons (q U pfb) is difficult because it requires a priori knowledge of the deformed beam geometry in order to evaluate the unbonded tendon strain. In this study, a theoretical evaluation of q U pfb is presented based on a concept of three equivalent rectangular curvature blocks for simply supported concrete beams internally prestressed with unbonded carbon-fiber-reinforced polymer (CFRP) tendons. The equivalent curvature blocks were iteratively refined to closely simulate beam rotations at the supports, mid-span beam deflection, and member-dependent strain of the unbonded tendon at the ultimate state. The model was verified by comparing its predictions with the test results. Parametric studies were performed to examine the effects of various parameters on q U pfb .