Pretensioned Beams Research Articles

Effects of exposure to elevated temperatures on the transfer length of pretensioned concrete elements

Pretensioned concrete elements are widely used in bridges and precast buildings, which may be exposed to fire. The residual performance of a pretensioned element that has survived a fire event is dependent on the effects of elevated temperatures on steel, concrete, and the bond between these materials. This paper investigates changes in the transfer length of pretensioned beams due to exposure to elevated temperatures. Ten large-scale pretensioned concrete beams with different cross-sectional dimensions and strand layouts were fabricated. The transfer length of these elements was measured using demountable mechanical strain gages. The specimens were subjected to elevated temperature cycles with maximum temperatures of 500 and 700 degrees for two hours. After natural cool down, changes in the transfer length of specimens were determined. The results showed that heat exposure leads to an increase in the transfer length by up to 35% and 42% for maximum temperatures of 500 and 700 degrees Celsius, respectively. Strand arrangement, compressive strength of concrete, cross-sectional dimensions, and clear cover were found to affect the sensitivity of elements to heat exposure. A simplified equation was proposed for estimating the transfer length of pretensioned concrete beams that survive elevated temperatures, which may be used for structural assessment applications.

Experimental and Numerical Study of Strengthening Prestressed Reinforced Concrete Beams Using Different Techniques

This study aimed to evaluate the static response of prestressed reinforced concrete beams strengthened in their flexure and shear properties using different strengthening techniques, steel plates, and externally bonded woven carbon fiber fabric (WCFF). The experimental work involved testing twenty large-scale prestressed reinforced concrete beams with a length of 3000 mm, and cross-sections measuring 400 mm in height and 200 mm in breadth were cast in the factory and tested in the laboratory. Four beams without prestressing served as the reference beams; two unbonded pre-tensioned beams served as the control beams, and the remaining fourteen beams were strengthened with steel plates and externally bonded woven carbon fiber fabric (WCFF). Eight of the beams were strengthened with 4 mm thick steel plates and tested under a monotonically increasing load with manual readings recorded. The remaining six beams were strengthened with 0.5 mm thick WCFF and tested under a monotonically increasing load with manual readings recorded. The variables considered included the strengthening techniques (FRP composite sheets, steel plates), the types of strengthening (slices, U-shaped), and the flexural and shear capacities of the strengthened beams. All the implemented strengthening techniques yielded enhancements in both the flexural and shear strength outcomes of the beams compared to their respective controls. The most significant increase in load capacity, whether in terms of ultimate load or first crack load, for the prestressed concrete beams’ flexure properties occurred when strengthening with U-shaped steel plates. Additionally, the greatest reduction in deflection at the point of reaching the maximum load for the prestressed concrete beams, in terms of their flexure properties, was observed when strengthening with U-shaped steel plates. Similarly, the maximum load increase for the prestressed concrete beams, in terms of their shear properties, was achieved through strengthening with U-shaped woven carbon fiber fabric wrapping. Furthermore, a finite element model was created to simulate various experimental specimens. The finite element model’s results exhibited harmony with the experimental results, affirming the efficacy of the presented finite element model.

Fatigue performance of CFRP reinforced pretensioned prestressed beams

This study presents an experimental investigation on the fatigue performance of CFRP reinforced pretensioned prestressed beams. A total of 10 specimens were designed and tested under monotonic or fatigue conditions. The investigated variables included the prestressing level, fatigue load range, and the absence or presence of novel additional aluminum alloy ribs (ARs) anchorage. Specimens using a high prestressing level or subjected to a low fatigue load range would have a reduced deflection and crack width development compared to those in their counterparts. In addition, the fatigue loading was found to cause cumulative bond damage of pretensioned FRP bar to concrete and finally led to an uncontrollable slippage failure of the beam. Such bond issue was demonstrated to have been avoided by using the ARs anchorage at beam ends. The applicability of three theoretical models proposed for steel reinforced beams to predicting the deflection of CFRP reinforced pretensioned prestressed beams under fatigue loading were evaluated, and CEB-FIP model was found to be the most suitable one to describe the deflection development of the tested specimens.

Flexural behaviour of prestressed reinforced concrete beams strengthened with carbon plate

This paper presents the effect of carbon plate length on the deflection and ultimate moment of resistance of a prestressed flexural member. Numerical and experimental results are also included. Three typical failure modes were observed in the experimental programme: bending failure, end shear failure and peeling off of the carbon plate. The geometry of the beam was chosen so that three typical failure modes occurred when different carbon plate lengths were applied. The experimental results show that for the selected geometry of the prestressed concrete beam, the ultimate moment capacity of carbon-fibre-reinforced polymer (CFRP) strengthened beams is up to 3.66 times that of beams without CFRP reinforcement. However, the stiffness can be increased by as much as 52% using CFRP. Non-linear analyses of pre-tensioned beams, with and without CFRP strengthening, were compared with respect to the ultimate limit state. The experimental test results and numerical calculations showed very good agreement in terms of the yield load and ultimate load.

Comparative Study of High-Performance Concrete Characteristics and Loading Test of Pretensioned Experimental Beams

High-performance concrete (HPC) is subjected to wide attention in current research. Many research tasks are focused on laboratory testing of concrete mechanical properties with specific raw materials, where a mixture is prepared in a relatively small amount in ideal conditions. The wider utilization of HPC is connected, among other things, with its utilization in the construction industry. The paper presents two variants of HPC which were developed by modification of ordinary concrete used by a precast company for pretensioned bridge beams. The presented variants were produced in industrial conditions using common raw materials. Testing and comparison of basic mechanical properties are complemented with specialized tests of the resistance to chloride penetration. Tentative expenses for normal strength concrete (NSC) and HPC are compared. The research program was accomplished with a loading test of model experimental pretensioned beams with a length of 7 m made of ordinarily used concrete and one variant of HPC. The aim of the loading test was to determine the load–deformation diagrams and verify the design code load capacity calculation method. Overall, the article summarizes the possible benefits of using HPC compared to conventional concrete.

Tests on prototype pretensioned natural stone beams

Stone has over millennia been used for structures which naturally develop compression under gravity loads. This study focuses on segmental stone beams with artificially induced compression. Prototype specimens were fabricated using epoxy adhesive for longitudinal shear transfer between prestressed steel strands and pre-cut Valange limestone blocks with mortar inter-block joints. Each specimen was prestressed by a single strand, to enable focus on the fundamental mechanics. Each strand was tensioned against the stone, then released after the epoxy became structurally active, so this is a form of pretensioning which eliminates external anchors and elastic shortening losses, and which enables agile construction because it can be implemented either at the factory or on site. Specimen details were informed by pullout tests. One specimen type entailed concentrically pretensioned square-section stone blocks, the other eccentrically pretensioned rectangular-section blocks. One concentric-strand specimen was strain-gauged near both ends to enable identification of the transfer zone, while an eccentric-strand specimen was instrumented at midspan with strain and displacement gauges to quantify pretensioning-induced camber and strains. Vibration tests were conducted on both specimens, then an eccentric-strand specimen was tested to ultimate in three-point flexure while a concentric-strand specimen was subjected to fire. The results reveal good damping, a high and consistent elastic modulus for the stone based alternately on local and global data, beneficial delay of external load-induced cracks at the joints due to the pretensioning and protection of the epoxy by the stone over an extended period in fire. The ultimate limit state under load entailed gradual spalling of stone in the peak compression zone followed by a dominant crack which was asymmetric about midspan. These results strongly hint at the viability of pretensioned stone beams for low-carbon floor applications.

Experimental Study on Flexural Behavior of CFRP Reinforced Pre-Stressed Concrete Beams with Initial Cracks

In order to explore the flexural behavior of CFRP reinforced pre-stressed concrete (PC) beams with initial cracks, 6 pre-tensioned beams were manufactured. Then the beams were pre-loaded to crack with 40% and 60% ultimate load respectively, and the beams were strengthened by CFRP under the conditions of load holding and fully unloaded. After that, the four-points bending tests were performed, and beam section strains, flexural capacities and cracks were analyzed. The results demonstrate that the ultimate load of CFRP reinforced beams increased by 10%~18%, and the ultimate loads of CFRP reinforced load holding beams were 3% and 6% lower than that of CFRP reinforced non-destructive beam, and the inhibiting effect of CFRP on cracks was weakened, the hysteresis strains should be considered for them. In this paper, the fiber hysteresis strains were calculated by the method of full section decompression moment, and the flexural capacities of CFRP reinforced PC beams were calculated, which are in good agreement with the test results.

Influence of Shear Reinforcement on Thin Web Pretensioned Concrete Beams

The objective of this paper is to present the results of the experimental study which was carried out on influence of shear reinforcement of pretensioned concrete thin web beams made with M50 grade concrete. Four pretensioned beams B000, B400, B200 and B100 of the same length, cross section and prestress force were fabricated with no stirrups, 400 mm c/c, 200 mm c/c and 100 mm c/c spacing of stirrups, respectively. The effect of varying percentage of shear reinforcement on cracking load, crack propagation, deflection, ultimate capacity, failure mode and beam ductility was examined. Deflection of the each test beam was measured up to ultimate load at an incremental loading of 2.5 tonne (25 kN). The test results were analyzed for cracking loads and ultimate loads and compared with theoretically estimated values. It has been observed that the loads are found to be increased while increasing percentage of web reinforcement. All the shear reinforced beams exhibited delay in appearance of first crack when compared with beam B000. The reason for the delay in the magnitude of first crack load is recognized as the contribution of shear reinforcement arresting the diagonal shear cracks in the shear zone. The energy abortion capacity of the beams increased while decreasing in the stirrup spacing in the shear span. The beam B100 which was reinforced by double the amount of the web reinforcement than the beam B200 showed a satisfactory margin of safety above 1.50 and ductility response of 3.81.

Action of Moving Loads on the Bernoulli-Euler and Timoshenko Beams

The dynamic behavior of Bernoulli-Euler and Timoshenko beams subjected to a moving concentrated load is considered. Deflection expressions are found in the form of convergent series for a pretensioned beam and for a beam on an elastic foundation. We determine the parameter ranges for which the results of these two models are coincident. It is shown that, under certain conditions, a resonance is possible in such a system.

Assessment of the Effect of the Pre-cast Beams IG Axial Spacing on the Effort Bridge Span

Abstract In designing of reinforced concrete bridges are often used pre-cast of pre-tensioned or post-tensioned prestressed members. In professional practice, when designing bridge span pre-cast of pre-tensioned prestressed beams IG type, different axial beams spacing’s are used and they are joined by a reinforced concrete slab. The paper presents a comparative analysis of an influence of the axial spacing of IG type beams in the cross-section of the bridge, on the bridge span effort, using standards PN-85/S-10042 and PN-EN 1992-2:2010. The optimal axial spacing of pre-cast IG type beams is presented, while maintaining the standard conditions of ULS (Ultimate Limit State). Such a solution has a positive impact on the costs of the entire project, significantly lowering the global investment cost.

Losses of prestressed forces of pre-tensioned precast composite beams

Losses of prestressed forces of pre-tensioned precast composite beams

THE BENEFIT OF THE NEW SUPER U-BEAM USED FOR SECTION 4 OF A WEST COAST EXPRESSWAY

The West Coast Expressway is approximately 315.8 km in length and is separated into 15 sections of varying lengths. The concessionaire is West Coast Expressway Sdn. Bhd. Jacobs was involved in Section 4, totaling 5.59 km in length and including 7 directional ramps. These structures feature a new ‘Super U-Beam’ section whereas standard pre-tensioned precast beams such as the ‘inverted T’, ‘I’, ‘M’ and ‘U’ beams are typical options for span lengths ranging from 25 m to 40 m. The ‘Super’ U-beams are larger than standard U-beams and feature wide flanges. This arrangement results in a significant reduction in the number of beams necessary as well as a reduction in all the major material quantities. This paper will further discuss the technical details and advantages of the new ‘Super U-Beam’.

Prestress Loss Distributions along Simply Supported Pretensioned Concrete Beams

The prestressing forces in prestressed tendons undergo a process of reduction over a period of time. A common assumption is that prestress loss is constantly distributed throughout the span of a simply supported pretensioned concrete beam. The purpose of this work is to investigate the accuracy of this assumption. The types of prestressed concrete beams investigated in this work include the following three typical types of tendon profile: (I) straight strands, (II) single-point depressed, and (III) two-point depressed. The major findings derived from this work are: (1) The total prestress loss is not constantly distributed throughout the span of a simply supported pretensioned concrete beam with any of the three types of tendon profiles, (2) The variation of prestress loss along the span of a pretensioned beam caused by elastic shortening of concrete or creep of concrete is much more significant than that caused by shrinkage of concrete or relaxation of tendons, and (3) The type of tendon profile in a simply supported pretensioned concrete beam has significant effects on the pattern of prestress loss distribution along the beam.

15 m 실규모 프리스트레스트 콘크리트 보에서 2,400 MPa 강연선의 전달길이 평가

In this study, 14.8 m long full scale pretensioned prestressed beam using 2,400 MPa PS strand was fabricated and the transfer length ofthe strand was investigated by measuring strains on nearby reinforcement steel bars. The test results of the beam were compared withsmall size specimen tests with a strand from previous researches (Oh and Kim, 2000; Yim et al., 2015). The full scale test showedsmaller transfer length and smaller difference in the transfer lengths between at cut (sudden release by flame cut) and dead ends thansmall specimens with a strand. Evaluation on the equation of transfer length proposed by Oh et al. (2014), based on test and analysisresults of transfer length of 1,860 MPa strand considering concrete compressive strength and concrete cover depth, was used tocalculate the transfer length of 2,400 MPa PS strand in full scale beam as well as small specimens with a strand. Comparison of theevaluation and test results presented that the equation overestimates the transfer length of 5~9% in small specimen tests, however, morethan 30% in full scale beam specimen for 2,400 MPa PS strand.Key words : 2,400 MPa PS strand, Pretension, PSC, Transfer length

Optimization of Pretensioned Steel Fiber Reinforced Concrete Beam

Pretensioned concrete beams are used as a main load bearing member for composite bridges with a span to 30 m. The advantage of longitudinal prefabrication technology of beams for small span bridges is quick installation, savings of straight supporting scaffolding of centers and formwork. The amount of labour with formwork, reinforcement and concrete including work with scaffolding of centers on site is reduced at a minimum. During searching applications of steel fiber reinforced concrete (SFRC) suitable for this kind of structure a pretensioned concrete beam suitable for a bridge bay with a span from 12 to 15 m has been chosen for an investigation. Three types of beam were manufactured for experimental tests. The beams were supposed to be a part of a bridge bay with a composite slab. These pretensioned beams were made of SFRC. In case of the experimental tests, a cast-in place concrete cover from plain concrete was casted on the top of the beams. The cast-in place concrete cover simulated a top composite slab. The bearing capacity of the beams with the cast-in place concrete cover was tested until their destruction. The tested beams showed higher bearing capacity than it was determined by a theoretical calculation. The beams also demonstrated high safety against collapse during structure overloading. The process of the experimental testing was also simulated on a numerical nonlinear model and then the results were compared. The result comparison of the both types of tests did not show any significant irregularities.

A theoretical and experimental investigation on large amplitude free vibration behavior of a pretensioned beam with clamped–clamped ends using modified homotopy perturbation method

In this study, large amplitude free vibration behavior of a pretensioned Euler–Bernoulli beam is investigated both theoretically and experimentally. The Hamilton's principle is used to derive the beam governing equation of motion. By implementing the Galerkin's method and assuming the clamped–clamped boundary condition, the partial differential equation is converted to an ordinary nonlinear differential equation. Because of the large coefficient of the nonlinear term, the new homotopy perturbation method proposed by He, is modified to solve the governing nonlinear equation. Comparing the first- and the second-order approximate solutions of the modified homotopy perturbation method (MHPM) and those available in the literature demonstrates that the second-order MHPM leads to a more accurate solution which is valid for a wide range of the vibration amplitudes. The results have been validated by the experimental tests and the MHPM method. Also, the results show that an increase in the vibration amplitude and/or the pretension load increases the fundamental resonance frequency ratio. Besides, it would decrease with increasing the beam slenderness ratio.

Flexural tests on pre-tensioned lightweight concrete beams

Eight pre-tensioned beams made using lightweight concrete with a design compressive strength of 35 MPa were tested under two symmetrical third-point concentrated loads. The main variables investigated were the eccentricity and effective prestress of the strands and the partial prestressing ratio. The flexural capacity of the beam specimens was compared with predictions obtained from simple equations using the equivalent stress block combined with different formulas proposed for the stress of bonded tendons at the ultimate strength of beams and from non-linear strain compatibility analysis. Test results showed that the normalised flexural capacity measured from pre-tensioned lightweight concrete beams with a reinforcing index below 0·15 was comparable to that of pre-tensioned normal-weight concrete beams with a similar reinforcing index, while that of a pre-tensioned lightweight concrete beam with a reinforcing index of 0·207 was lower than that of pre-tensioned normal weight concrete beams with a reinforcing index of 0·2. As a result, the increasing rate of normalised flexural capacity against the reinforcing index in pre-tensioned lightweight concrete beams was slightly lower than that observed in pre-tensioned normal weight concrete beams. The flexural capacity of pre-tensioned concrete beams predicted using an equivalent stress block was marginally influenced by the values determined from different formulas for the stress of bonded tendons at the ultimate strength of beams, although the ratio of the measured and predicted flexural capacities was commonly lower in pre-tensioned lightweight concrete beams than in pre-tensioned normal weight concrete beams.

Experimental Study on Creep Behavior of Prestressed Concrete Beams with Pretensioned Bent-Up Tendons for 600 Days

The prestressed concrete beam with pretensioned bent-up tendons combines many advantages of the straight-line pretensioned prestressed beams and the curvilinear posttensioned beams. To improve its application in bridges, long-term deformation should be exactly predicted and controlled. Three prestressed concrete beams with pretensioned bent-up tendons XPB1,XPB2, XPB3 were fabricated, and XPB1 and XPB3 were put in standard curing room, but XPB2 were put in outdoor environment. All the experimental beams were simple supported under long-term loads. Their deformation such as creep strain and long-term deflection in mid-span section were observed about 600 days. By analyzing the influence factor and time-history law of creep coefficient and long-term coefficient, creep coefficient equation of the test beams was fitted and compared with the code model such as ACI209R-92, CEB-FIP MC90 and 86 model proposed by China Academy of Building Research, and the ±10% margin error of this model was pointed out. All these results may offer the initial value of long-term deformation of the prestressed concrete beams with pretensioned bent-up tendons.

Local Stress Analysis and Corner Radius Optimization of PPSCB

To research the local stress state of the polyline pretensioned beams, the 3-D nonlinear finite element model is established by FEA software ANASYS. The results show that the existence of the inflection point on the prestressed reinforcement is the main reason of local stress distribution, and the corner radius R is the primary influential factor of the local stress distribution phenomenon. Then, the variation law of the stress and stress distribution by the corner radius R is illustrated clearly. With the increasing of R, the maximum local stress has decreased, and the stress distribution curve has increasingly become smoothly and gently. Considered the variation law of local stress and various of construction requirement in engineering practice, R=600mm is selected as the optimal radius.

New Simulation Method of Reinforcement Simulation in FEA

During the research of local stress distribution of polyline pretensioned beams by 3-D nonlinear finite element method, a problem of curve prestressed reinforcement simulation is discovered. If the line of prestressed reinforcement is curve, the calculated result of local stress distribution is greatly affected by element size, because of the tectonic stress concentration by the coupling between the linear element and body element. Then, the validity and applicability of the model couldn't be judged. To eliminate the harmful effect, the curve prestressed reinforcement is determined to simulate by body element. The result shows that it will solve the problem thoroughly, and the calculated result is truthful and reliable. This research presents a new thought and method about the local stress analysis of the prestressed concrete structure.