Flexure of continuous HSC beams with external CFRP tendons: effects of fibre elastic modulus and steel ratio

The results of a theoretical study on the flexural behaviour of continuous high-strength concrete (HSC) beams prestressed with external fibre reinforced polymer (FRP) tendons are presented. A previously developed numerical model is extended to the analysis of continuous HSC beams with external FRP tendons. A numerical test is conducted on two-span externally prestressed beams made of HSC with compressive strength of 90 MPa. The external tendons are assumed to be carbon FRP (CFRP) composites covering a wide range of modulus of elasticity. Various levels of nonprestressed steel ratio are used. Comprehensive aspects of behaviour of such type of beams are examined. The results show that CFRP with high elastic modulus of 500 GPa mobilizes quite different structural responses compared to those with normal elastic modulus, and that the amount of nonprestressed steel affects remarkably the behaviour of such beams. The study also indicates that some moment redistribution knowledge valid for conventional continuous concrete beams may not be applicable to continuous HSC beams with external FRP tendons

Factors affecting moment redistribution at ultimate in continuous beams prestressed with external CFRP tendons

A numerical investigation of redistribution of moments in continuous concrete beams prestressed with external carbon fiber reinforced polymer (CFRP) tendons at failure loads is described. A finite element analysis (FEA) model is introduced, and an extensive parametric study is carried out on two-span continuous beams. The factors examined in this study include the content of non-prestressed steel, tendon eccentricities, tendon area, effective prestress, span-to-height ratio, concrete strength, CFRP modulus of elasticity and load type. The results obtained from FEA are compared with those obtained from various codes. The study shows that the importance of some factors is not reflected in the codes. When used to calculate the degree of moment redistribution in these beams, the parameter εt (net strain in extreme tension steel) seems to be more reasonable than the parameter c/d (ratio of neutral axis depth to section effective depth). A simplified equation for calculating the degree of moment redistribution at ultimate is proposed

Finite Element Modeling and Analytical Study on the Behavior of Beams Prestressed with Unbonded Tendon

Documentos apresentados no âmbito do reconhecimento de graus e diplomas estrangeirosThere are two types of unbonded prestressed concrete beams according to tendon layout: unbonded internally prestressed concrete beam and externally prestressed concrete beams. Because of the advantages of simple construction, structural safety and economy, the unbonded prestressing technique has been widely used in the new construction of large civil engineering structures. Also, Many engineering structures at service especially bridges are suffering from fatigue or structural harm due to structure aging, corrosion of reinforcement bar and overweight. It is very effective to strengthen these harmed or fatigue structures using the externally prestressing technique. Analysis of beams prestressed with unbonded tendon is much more complicated than that of bonded concrete beams. As to bonded concrete beams, complete bond between steel and concrete is always assumed, so analysis can be performed based on plane section hypothesis. However, the strain compatibility between tendon and its adjacent concrete is no longer valid when unbonded internally or externally unbonded concrete beams are analyzed. The precise analysis of unbonded beams performed should be based on structural compatibility condition using iterative method. In this dissertation, analysis model for beams prestressed with internal unbonded or external tendon based on large deformation finite element theory is developed, and material and geometrical nonlinearities are involved. By replacing the effect of internal unbonded or external tendon with equivalent nodal loads of beam element, analysis can be performed with ordinary bonded reinforced concrete beams, whose internal forces are caused by externally applied loads and equivalent nodal loads from unbonded tendon together. Fibre integration method is initially proposed to cope with arbitrary concrete section of beam element and to derive the section tangent stiffness matrix. Computational efficiency is remarkably improved. Conventional strip method is also utilized in this dissertation. Based on these, standard finite element formulae are established by utilizing nonlinear plane beam element. Two numerical examples are analyzed to verify the validity of the proposed model. It is difficult to analyze unbonded prestressed beams by commercial finite element programs with conventional methods. To solve this problem, a commercial finite element program, ABAQUS, was used to develop an analysis model for concrete beams prestressed with internal unbonded or external tendon. The finite element model included two types of main element: beam element for concrete and truss element for internal unbonded or external tendon. The end nodes of main elements were connected with internal constraints named MPC in ABAQUS. Spring elements with very large stiffness were set up at the place of the deviators of externally prestressed beams, or along the span with relatively little space of unbonded internally prestressed beams. The modified Riks method is utilized to trace the entire structural response of beams prestressed with unbonded tendon from zero to ultimate loads. The reliability of the analysis model is verified by analytical results of typical test beams in comparison with experimental ones. Eight series 76 beams prestressed with internal unbonded tendon and two series 21 beams prestressed with external tendon and 3 comparative beams prestressed with internal unbonded tendon are designed to conduct parametric study. The designed beams are analyzed using the finite element model proposed in this dissertation. Influence of various parameters including ratio of nonprestressed reinforcement, ratio of unbonded prestressing tendon, span-depth ratio, effective prestress, cylindrically compressive strength of concrete, ultimate tensive strength of prestressing tendon, yield strength of nonprestressed reinforcement and loading pattern on the behavior of unbonded prestressed concrete beams and the ultimate stress in tendon, fps, are investigated in detail. Parameters having important influence on second-order effects of externally prestressed concrete beams as deviators, tendon configuration and load type are evaluated. The study makes an in-depth understanding and recognition on the working behavior and structural mechanism of beams prestressed with internal unbonded or external tendon. Therefore, a sound basis is offered for optimal design of unbonded prestressed beams and for future modification in new code equation for fps

Numerical assessment of the nonlinear behavior of continuous prestressed steel-concrete composite beams

This paper evaluates the flexural performance and quantifies the secondary moments in two-span prestressed steel-concrete composite beams. A nonlinear model capable of simulating the full-range nonlinear behavior of continuous prestressed composite beams is validated against experimental results. A parametric numerical investigation is then conducted to examine the effectiveness of strengthening a continuous steel-concrete composite beam with external tendons of different cross section areas. In addition, secondary moments in continuous prestressed composite beams having different tendon layouts under symmetrical and unsymmetrical loads are investigated. The results indicate that external prestressing not only increases significantly the ultimate load-carrying capacity but also improves the moment redistribution ability of continuous steel-concrete composite beams. Moreover, the analysis shows that significant secondary moments are present in continuous prestressed composite beams throughout the loading history. It is therefore necessary to consider secondary moments in the strength design of this structural typology.</p

Time-dependent assessment and deflection prediction of prestressed concrete beams with unbonded CFRP tendons

This paper presents the assessment of the time-dependent behavior and the prediction of the long-term deflection of concrete beams prestressed with internal unbonded carbon fiber reinforced polymer (CFRP) tendons. A numerical model for the time-dependent analysis of concrete beams prestressed with unbonded tendons is calibrated against experimental results. Parametric numerical simulations are then conducted on simply supported unbonded prestressed concrete beams subjected to long-term sustained loads to investigate the effect of using CFRP tendons instead of low-relaxation steel ones, the magnitude of the initial prestress, the loading conditions, and the quantity of the compressive reinforcing steel. The results show that the long-term prestress loss of beams with CFRP tendons is considerably higher than that of beams with steel tendons. Moreover, it is shown that increasing the quantity of compressive reinforcing steel leads to a substantial decrease in long-term downward deflection. A modification of the ACI 318-14 equation is proposed to predict the time-dependent deflection of prestressed concrete beams with unbonded FRP or steel tendons

Numerical evaluation of prestressed steel-concrete composite girders with external FRP or steel tendons

The use of external FRP tendons instead of steel ones for prestressing steel-concrete composite girders is evaluated. A nonlinear model for prestressed steel-concrete composite (PSCC) girders is calibrated against experimental results. Numerical simulations are then performed on singe-span and two-span PSCC girders. The investigated variables are the tendon type and the prestress level. In particular, CFRP, AFRP and conventional prestressing steel tendons are compared for prestress levels ranging from 0 to 60%. The results demonstrate that the behavior of PSCC girders with CFRP and steel tendons is similar, while AFRP tendons result in lower ultimate load and higher deformation capacity. In addition, the influence of prestress level on the moment at the center support of continuous PSCC girders is marginal due to the presence of secondary moments. The study also shows that the influence of secondary moments on moment redistribution is less pronounced in PSCC girders with AFRP tendons than in PSCC girders with CFRP or steel tendons.</p

Effect of Tendon-Related Variables on the Behavior of Externally CFRP Prestressed Concrete Beams

This work assesses the flexural performance of prestressed concrete beams with external carbon fiber-reinforced polymer (CFRP) tendons, focusing on tendon-related variables. A finite element analysis (FEA) method is verified. A numerical parametric analysis of prestressed concrete beams with external CFRP tendons is carried out. Four tendon-related variables are considered, namely, the area, initial prestress, depth and elastic modulus of tendons. The analysis shows that flexural ductility decreases as the tendon area, initial prestress or elastic modulus increases but is insensitive to the tendon depth. The ultimate tendon stress increment (Δσp) is influenced by all of the four variables investigated. JGJ 92-2016 (Chinese technical specification for concrete structures prestressed with unbonded tendons) significantly underestimates Δσp and, hence, is over-conservative for the strength design of these beams. An equation is proposed for calculating Δσp, taking into account all four variables investigated. An analytical model is then developed to estimate the flexural strength (Mu) of prestressed concrete beams with external CFRP tendons. The proposed analytical model shows good agreement with FEA, i.e., the mean discrepancy for Δσp is 0.9% with a standard deviation of 11.1%; and the mean discrepancy for Mu is -1.6% with a standard deviation of 2.1%.FCT 2022.04729.CEECIN

Numerical Study of Using FRP and Steel Rebars in Simply Supported Prestressed Concrete Beams with External FRP Tendons

This study aimed at examining the feasibility of using fiber-reinforced polymer (FRP) rebars instead of steel ones in prestressed concrete beams (PCBs) with external FRP tendons. By applying an experimentally validated program, numerical tests were performed on simply supported PCBs, with investigated variables including rebars&rsquo; type and area. Three types of rebars were considered, i.e., carbon, glass FRPs (CFRP, GFRP), and reinforcing steel. The ratio of tensile rebars ranged from 0.22% to 2.16%. The results indicated that the beams with CFRP rebars exhibited better crack mode and higher ultimate load than the beams with GFRP or steel rebars. GFRP rebars led to considerably higher ultimate deflection and tendon stress increment than steel rebars. In addition, several models for calculating the ultimate stress in unbonded tendons were assessed. An analytical model was also proposed to predict the tendon stress at ultimate and flexural strength in externally PCBs with steel and FRP rebars. The model predictions agreed well with the numerical results

A novel strong and durable near-surface mounted (NSM) FRP method with cost-effective fillers

Conventional near-surface mounted (NSM) FRP methods rely on expensive and less durable epoxy-filler bond to transfer tensile force from FRP reinforcements into concrete elements, generally failing to fully develop the tensile strength of FRP reinforcements. In this paper, cost-effective and potentially more durable fillers (e.g. cement-based paste and ceramic tile adhesive) combined with anchorages have been proposed to improve conventional NSM FRP methods. Experimental results demonstrate that the proposed NSM methods are able to make full use of FRP reinforcements, suggesting excellent compatibility between fillers and the proposed method. Further mechanical improvements can be achieved by evenly distributing the FRP material of one groove into multiple grooves. Moreover, the study tends to use high-viscosity cement filler for facilitating FRP installation especially on the bottom or side of concrete elements. The application of flexural reinforcement on the side of concrete elements have also been explored, aiming to deliver a much easier FRP installation. It is believed that the paper presents feasible improvements for NSM methods. Test results suggest that those improvements can achieve a stronger and potentially more durable NSM method with cost-effective fillers