Unbonded Beams Research Articles

Experimental investigation of full-scale post-tensioned composite AASHTO beams prestressed with carbon-fiber-reinforced polymer cables

This paper reports on an investigation of the flexural behavior of post-tensioned composite AASHTO beams under flexural monotonic and fatigue loading. Five 39.5-ft-long (12-m) post-tensioned beams with 0.76-in.-diameter (19-mm) prestressing carbon-fiber-reinforced polymer (CFRP) cables were studied. Three beams were post-tensioned with unbonded cables, and the other two beams were post-tensioned with bonded cables. The fatigue resistance and the effect of tendon profile (straight versus draped) were investigated. The results showed that the two beams with unbonded tendons survived 2.3 million fatigue cycles with an insignificant effect on the deformation and stiffness of the beams. At the ultimate load, the beams with unbonded cables showed greater deformation compared with the counterparts with bonded cables; however, the flexural capacity of the unbonded beams was lower. Different analytical models for estimating the flexural capacity of the beams were assessed against the experimental results to determine the applicability of these models to full-scale CFRP post-tensioned beams.

Experimental and theoretical study of post-tensioned unbonded beams

Paper contains data obtained during a bending test of four post-tensioned unbonded beams as well as description of geometry of test specimens and strength properties of used materials. Midspan deflection, tendon stress increase, cracking moment and ultimate bending moment were measured during the test. Crack propagation pattern was assessed as well. Ultimate bending moment, cracking moment and midspan deflection were then estimated using reinforced concrete analysis methods widespread in Russia. Comparison of the test data and calculation results shows that nowadays there are not domestic analysis methods well enough to fully estimate the unbonded post-tensioned beams.

Factors governing the fire response of prestressed reactive powder concrete beams

AbstractThis article presents the details of a fire test on eight prestressed reactive powder concrete (RPC) beams cured with 150°C hot air. The test specimens varied in load ratio, cover thickness of tendons, bonded and unbonded tendons, and partial prestressing ratio. The thermal fields, deflection behavior, effective prestress, fire endurance, failure modes, and crack patterns of the beams were observed and recorded, revealing that no obvious fire‐induced spalling occurred on the prestressed RPC beams. The fire test results indicate that the failure of prestressed RPC beams was driven by direct fire exposure and fracture of tendons and longitudinal rebars after generation of wide cracks in the beams. In extreme fire, prestressed RPC beams that were under‐reinforced at ambient temperature may fail as low‐reinforced beams. Fire resistance of the prestressed RPC beams dramatically improved with a decrease of load ratio and increase of cover thickness of tendons. Bonded prestressed RPC beams showed superior performance over equivalent unbonded prestressed RPC beams in fire. The whole‐scale transferring effect of prestress loss in unbonded beams should be considered for fire resistance performance design based on structural systems.

FLEXURAL CAPACITY FOR RC BEAMS WITH EXPOSED REINFORCEMENT

The currently available approaches on the effects of the exposed reinforcement on the flexural strength capacity of beams by other researchers are critically reviewed. These methods for estimating the flexural- compression strengths of beams with main steel exposed along all or part of the span do not give good predictions for the ordinary reinforced member. In this paper, the simple shear-compression theory introduced in earlier study is modified to the treatment of unbonded beams and a very simplistic empirical equation is proposed. The accuracy of the proposed equation is examined by comparing with results of 44 beams from literature. The comparison showed that the predictions by the proposed equation are between 0.85-1.25 to those of test results. The ratio of the experimental ultimate moments to the calculated ultimate moments by the proposed equation gives an average of 1.06 and C.O.V=0.21. The method of calculation proposed here is relatively successful in predicting the ultimate moment resistances but not in predictions of the physical behavior of the beams with exposed reinforcements.

Flexural Properties of ECC‐Concrete Composite Beam

Rebar corrosion‐induced durability issue is a major concern for bridges. The ECC cover was employed to prevent the intrusion of the corrosive agent. This paper studied the flexural behavior of ECC‐concrete composite beam. The effects of bonding at the interface and fiber mesh reinforcement on the flexural properties and cracking pattern were investigated. The strain distribution and midspan deflection were evaluated. Test results show that the bonded composite beam had a higher loading capacity. But the unbonded composite beam showed better postcrack energy absorption capacity with higher midspan deflection. The fiber mesh reinforcement could further improve the flexural properties regardless of the bonding condition. The strain at the bottom of the unbonded beam was much smaller than that of the bonded beam. The penetrated cracks were observed at the ECC layer of the bonded composited beam.

Residual load capacity of corroded reinforced concrete beam undergoing bond failure

20 rebar reinforced concrete beams, including 3 non-corroded beams, 3 unbonded beams, 9 fully corroded beams and 5 partly corroded beams, are loaded under four point static bending test. As a result of the experiment, the unbonded beams suffer arch action at any load level, and the load resisting mechanism of the 13 corroded beams which undergo bond failure changes to arch action near the later loading stage. The 13 corroded beams in the experiment fails as the maximal bond stress in the anchorage region reaches its critical value which cannot keep the balance of the arch rib. The maximal bond force in the anchorage region is taken into account as a key factor in the prediction method, as it has a great effect on the residual load capacity of the corroded beam undergoing bond failure. Based on the experimental result, the maximal bond force in the anchorage region and the residual load capacity are normalized by the values of the unbonded beam, and a model is proposed to predict the residual load capacity of the corroded beam undergoing bond failure based on arch action.

Serviceability Response of Rehabilitated Unbonded Post-tensioned Indeterminate I-Beams Consisting UHSSCC

The ultra-high strength self-compacting concrete, UHSSCC is the new generation type of concrete with a compressive strength higher than 80MPa. The application of this type of concrete on the serviceability state in CFRP strengthened unbonded post-tensioned indeterminate I-beam is monitored and the results are compared theoretically using different standards. For this aim, full scale I-beam of 9m length was cast, by UHSSCC. During the beam service load test, the stress and strain of materials, and also deflection and crack widths were monitored at different locations using different types of sensors. Based on the experimental measurements and observations, the beam serviceability response was compared theoretically by different methods. As the considerations prepared in the standards do not cover strengthening of such members with unbonded tendons, and also no trace of deflection prediction for such continuous member, one can find in the open literature. It is therefore, this investigation was planned. A comparison between theoretical and monitored results was performed for serviceability response and it was found that although, the stress of materials are well within the standards limitations for crack widths of 0.1 and 0.2 mm of bonded tendon, but the full service load is reached at a higher load, while the flexural crack, experience a width of 0.3 mm. It is also apparent that the loads corresponding to the conventional suggested deflection limits will cause to exceed serviceability state of strengthened unbonded beam, and new limitations are introduced for crack widths of 0.1, 0.2 and 0.3 mm to predict service deflection of beams.

Monitoring and theoretical losses of post-tensioned indeterminate I-beams

Self-consolidating concrete is a desirable achievement in reinforced concrete structures. This non-vibrating concrete has had a significant impact on the prestressed concrete industry because it increases productivity in the casting of durable prestressed or precast members without mechanical vibration. In the design of prestressed concrete structures, the immediate and time-dependent losses in tendon stresses are important. The main objective of this study is to investigate the losses in tendon stresses of indeterminate (continuous) post-tensioned unbonded beams consisting of two different types of concrete (high-strength self-consolidating concrete (HSSCC) and high-strength normal concrete (HSNC)). Six continuous, unbounded, post-tensioned I-beams in two groups were cast and monitored by electrical strain gauges for a duration of 60 d and the losses were measured. Results were compared with the Association of State Highway and Transportation Officials (Aashto) third and fourth edition methods, the Precast/Prestressed Concrete Institute (PCI) method and the Eurocode 2 model. It was found that the Aashto third edition, PCI and Eurocode 2 methods were conservative and the Aashto fourth edition method was non-conservative for estimating time-dependent losses in post-tensioned beams cast with high-strength concretes. The amount of experimental time-dependent losses in beams consisting of HSSCC was slightly lower than that of beams prepared using HSNC with almost the same concrete strength.

Composite Action of Concrete-Filled Rectangular GFRP Tubes

AbstractThis paper investigates the composite action and performance of rectangular concrete-filled glass fiber–reinforced polymer (GFRP) tubes in four-point bending. The study compares the performance of three different concrete-filled beam configurations with different levels of adhesive bonding of the concrete core to the GFRP tube. The three different configurations include (1) unbonded, (2) bonding of the flanges, and (3) bonding of the flanges and webs. The results show low composite action for the unbonded beam configuration up to full composite action for the completely bonded beam configuration. An analytical study is presented to provide insight into the failure mechanism for the fully bonded beam configuration and to predict ultimate capacity. For evaluation, the stiffness and strength of the fully bonded concrete-filled GFRP tube considered in this study is compared with a conventional steel reinforced concrete beam, concrete-filled steel tube, and concrete-filled GFRP tube with carbon FRP (CF...

Experimental and Theoretical Analysis on Carrying Capacity and Deflection of Bond-Retarded Prestressed Concrete Continuous Beams

Based on the experiments of four bond-retarded prestressed concrete continuous beams, an unbounded prestressed and a bonded prestressed concrete continuous beams, this paper analyzes the ultimate bearing capacity and the deflection of the continuous beams by comparing the theoretical and experimental results, shows that the bond-retarded prestressed beams has the same character with the unbonded prestressed beams when retarding mortar is not hardening; while the bond-retarded prestressed beams has the same character with the bonded prestressed beams when retarding mortar is hardening.

Flexural Strengthening of Reinforced Concrete Beams with Prestressed CFRP Plates

Four medium -scale reinforced concrete beams were constructed and tested to investigate the effectiveness of external poststrengthening with prestressed carbon fiber reinforced polymer (CFRP) plates. The various variables included the strengthening method and the amount of prestressing. The experiments consisted of one control beam, one nonprestressed CFRP-bonded beam, and two prestressed CFRP-bonded beams, all the beams were subjected to four-point bending tests. In comparison to the control beam and the nonprestressed CFRP-bonded beam, the cracking load, yield load, ultimate load and stiffness of the beams with bonded prestressed CFRP plates were increased. The failure mode of the prestressed CFRP-plated beams was not debonding, but concrete crushing. After the debonding of the CFRP plates, the behaviour of the bonded CFRP-plated beams changed to that of unbonded CFRP-plated beams due to the effect of the anchorage system. The ductility of the beams strengthened with CFRP plates having the anchorage system was considered high if the ductility index was above 3.

SHAKING TABLE TESTS ON THREE STORY PRECAST PRESTRESSED CONCRETE FRAME

This paper describes results of shaking table tests on precast prestressed concrete frames. The frame with bonded beams specimen and the frame with unbonded beam specimen were tested to observe and investigate their structural capacities including their ultimate state. Input wave were artificial one under 75Kine and actual measurement on Kobe earthquake at JR Takatori in final. The results of tests indicate that precast prestressed concrete frames had the high ductility and returnable capacities. The bond beam specimen was stronger than the unbonded one. The lateral displacement of all frames was mainly occurred at the connected potions between members. Pushover analysis can roughly simulate the test results. The model of skeleton curves was assumed as tri-linear calculated using the fiber model analysis.

Experimental Study on Anti-Impact Properties of Prestressed Concrete Beams

The dynamic displacement ductility and ultimate load-carrying capability of three partially prestressed concrete (PPC) and unbonded beams, has been experimentally investigated by applying impact load to their middle with falling hammers. The typical measurement wave curves are described. Influences of a falling hammer’s gravitational potential energy on the dynamic properties of the PPC beam are analyzed. Based on the analog simulation theory, the impact-load carrying capability of the actual unbonded PPC beam with span 12 m is gained. Results indicate that the unbonded PPC beams have favorable performance of anti-impact properties fragile destruction will not happen, and the unbonded PPC beams can be used in wide-span underground structures.

Effects of Ordinary Tension Reinforcement on the Response of Beams with Unbonded Tendons

This paper describes an analytical investigation of the flexural response of unbonded prestressed beams and the ultimate stress increase in unbonded tendons, with particular emphasis on the effects of bonded ordinary tension reinforcement. A numerical model, based on the finite element method incorporating an arc-length solution technique, is developed to predict the entire nonlinear response of concrete beams prestressed with unbonded tendons. The reliability of the proposed analysis is verified by comparing the analytical predictions with the experimental results. Two parameters relating to bonded ordinary tension reinforcement are analytically investigated. The analytical results show that an unbonded prestressed concrete beam without any ordinary tension reinforcement exhibits very poor behaviour. However, with the minimum amount of ordinary tension reinforcement provided, the flexural behaviour of the beam is greatly improved. The variable yield strength of ordinary reinforcement in combined reinforcing index is inappropriate in its current form since the calculated prediction is inconsistent with the actual trend of the variation of ultimate stress increase in tendons.

Ductility of Partially Prestressed Concrete Beam

Ductility of partially prestressed concrete beams was numerically analyzed. For a given value of reinforcement index, corresponding curvature ductility factors of an unbonded prestressed concrete beam as well as a bonded prestressed concrete beam are analyzed and compared. It is generally observed that, when reinforcement index is between 0.20 and 0.30, the ductility factor of an unbonded beam is higher than that of bonded one about 10 to 15 percent. While reinforcement index is between 0.15 and 0.20, the ductility factor of an unbonded beam is near that of bonded one. When reinforcement index we is smaller than 0.15, the ductility factor of an unbonded beam is usually lower than that of bonded one about 10 to 15 percent.

軸筋の付着の有無が鉄筋コンクリート部材のせん断性状に及ぼす影響

Monotonic loading test were conducted on four reinforced concrete short beams in order to investigate the shear transfer mechanism due to arch action. The test variables included the amount of web reinforcement and bond performance of longitudinal reinforcement (i. e. bond or no bond). The conclusions obtained from this investigation were as follows. 1) All the test beams failed due to abrupt propagation of diagonal tension crack. 2) Linear strain distribution was observed in the concrete of unbonded test beams. This result implies that the arch action does not exist in unbonded beams, which is contradicted to the theorem on arch action.

Experimental Study of Influence of Bond on Flexural Behavior of Concrete Beams Pretensioned with Aramid Fiber Reinforced Plastics

An experimental program was formulated to investigate the flexural behavior of concrete prestressed with aramid fiber reinforced plastic (AFRP) tendons. The particular focus was the influence of the bond between an AFRP tendon and concrete on the flexural response of a beam. In the main test series, pretensioned concrete beams were cast using either one of two types of AFRP tendons or steel tendons. The influence of bond was studied by testing beams with fully bonded tendons, unbonded tendons, or partially bonded tendons. It was found that, although the fully bonded beams had a high ultimate load capacity, only limited rotation occurred prior to failure. In contrast, large rotations were noted in the unbonded beams, but the strengths of these members were significantly (25%) lower than those of the fully bonded beams. The only beams that achieved both a high rotation capacity and a high ultimate load capacity were the beams with partially bonded tendons. It is suggested that the use of partially bonded tendons could provide the basis of a new design method for concrete beams prestressed with fiber reinforced plastic tendons.

ＰＣはりの塑性挙動とじん性設計に関する基礎的研究

This study was carried out for giving the basic data to establish the structural design procedure of prestressed concrete beams based on the toughness as well as to develope the structural members having high ultimate and cracking strength.Prestressed concrete beams (10×18×170cm), having various amounts of prestressing tendon and compression steel, were loaded up to complete failure. The normal strength concrete, high strength concrete and steel fiber reinforced concrete were included in the study. Also unbonded beams were tested and their test results were compared with those of the corresponding bonded beams.The principal conclusions derived were: (1) The toughness value of prestressed concrete beams could be roughly estimated by the toughness index γ, which was calculated from the sectional constant and material properties. γ became larger with an increase of the toughness of compression zone. It was noted, however, that sudden failure due to the rupture of prestressing tendon took place when γ became too large. Therefore, the upper limit of γ should be determined so as to avoid the beam failure due to tendon failure. (2) Mechanically high performance prestressed concrete beams can be made by placing a large number of re-bars in both compression and tension sides and by introducing a large amount of prestressing.

Ultimate Stress of Unbonded Tendons in Partially Prestressed Concrete Beams

U nbonded tendons are being used increasingly in partially prestressed concrete structures in China. This is because unbonded tendons usually have a lower unit cost compared to bonded tendons while also offering simplicity in construction. Tests indicate that the ultimate strength of unbonded beams can be increased by the addition of bonded nonprestressed reinforcement. Although several research papers' focus on the presence of nonprestressed reinforcernent, this practice is not currently recognized in many building codes 2 for computing the ultimate stress in unbonded tendons at beam failure. That is to say, although most designs employ at least a minimum amount of nonprestressed steel for distributing cracks in concrete and Iimiting their width, the actual influence of this added reinforcement on the ultimate stress of unbonded tendons is often neglected. For example, the minimum bonded reinforcement in AC! 318-83 is only 0.4 percent of the area of that part of the beam section between the flexural tension face and the neutral axis of the gross section, Since partially prestressed concrete is classified between fully prestressed concrete and ordinary reinforced concrete, the amount of nonprestressed steel can vary widely. Therefore, research must he conducted to determine the effect of bonded nonprestressed steel on the ultimate stress in unbonded tendons, as well as on the ultimate strength of the beam itself.

Unbonded beams on a two-parameter foundation

Unbonded beams supported by a foundation are studied. The foundation is assumed that tensile stresses cannot be transmitted across the interface between beams and their supporting subgrade. The subgrade is characterized by a two-parameter model proposed by Vlasov and Leont'ev, Pasternak and Filonenko-Borodich. Thus the shear strength of the foundation material is taken into account. With a suitable choice of parameters, analytical solutions are obtained and compared to experimental results reported by Durelli et al. The effects of the “lift up” of the beam and a discussion of the foundation parameters are also presented.