Prestress Level Research Articles

Flexural strengthening of reinforced concrete beams using external tendons

Strengthening of reinforced concrete (RC) beams is the important issue in the civil engineering practice. One of the techniques that can be applied in strengthening of such structures is prestressing using unbonded tendons. Despite the fact that many research studies have been done in the subject of external prestressing there are still some gaps in knowledge in this issue. These gaps concern especially the effect of preloading, precracking and strengthening sequence on the ultimate load and load–displacement behaviour of strengthened RC beams.The paper presents experimental and numerical research on reinforced concrete beams strengthened with external unbonded tendons. The results of experimental investigation performed on four point bending beams that were preloaded and precracked before strengthening are briefly described and discussed. On the basis of experimental observations a three-dimensional model of the beams was developed. The model considers the nonlinear behaviour of concrete, steel bars and prestressing strands. For the calibrated numerical model a parametric study was performed. The influence of the following parameters on the ultimate load, load–deflection and stress increase in tendons of beams were taken into account: concrete class, steel ratio, loading configuration, preloading level, strand shape, cross-section shape, deviator configuration.The experiments show that external prestressing is a very efficient technique for the flexural strengthening of reinforced beams. Preloading, precracking and the strengthening sequence have a minor influence on the ultimate stress increases in strands and the load-bearing capacity of beams after strengthening. It was also demonstrated that strengthening under existing external loading has a positive influence on the serviceability (deflection) limit state. The proposed numerical model confirms the good predictive capabilities for all experimentally tested beams and all considered load and structural stages. The parametric study demonstrated that the initial prestress level has a moderate influence on strengthening efficiency. It was also found that for elements that are characterised by cross-sections without wide compressive zones (i.e. rectangular cross-sections) the compressive strength is one of the key parameters that governs the ultimate load and ductility of the strengthened beam. The considered schemes of loading (two concentrated forces and uniformly distributed load) and the shapes of the tendons (in the form of broken or straight line) have a very moderate influence on the load-bearing capacity and load–displacement behaviour. The second order effects are more pronounced for beams with lower tendon eccentricities. These effects were activated after yielding of the main reinforcement. Finally, it was shown that before yielding of the main reinforcement, increases of stresses in tendons are a strong function of the stiffness of a beam and do not depend on the eccentricity of tendons or the type of supporting of tendon along the span of the beams.

Flexural behaviour of notched steel beams strengthened with a prestressed CFRP plate subjected to fatigue damage and wetting/drying cycles

Applying a prestressed Carbon Fibre Reinforced Polymer (CFRP) to strengthen deficient steel structures is a favourable approach. However, fatigue loading and hygrothermal exposure can weaken adhesive bonding. A theoretical analysis of the interfacial stress distribution at the notch location was first carried out to explain the mechanism of delay of the interfacial debonding of notched steel beams after prestressed CFRP strengthening. Then, the prestress loss and flexural behaviour of specimens (with a prestress level of 25%) subjected to fatigue damage (FD) and/or wetting/drying cycles (WDCs) with 3.5 wt% NaCl solution were further experimentally investigated. Theoretical results showed that prestressed CFRP strengthening can significantly reduce the interfacial stress at the notch site and delay interfacial debonding. The CFRP prestress loss mainly occurs in the hardening stage of the adhesive layer, and the total prestress loss is less than 2.7% for specimens subjected to WDC exposure, indicating the effectiveness of the end anchorage system. The adhesive microcracks at the notch induced by FD and adhesive bonding degradation caused by WDCs significantly affect the interfacial debonding. In addition, NaCl solution penetration accelerated by microcracks causes further degradation of interfacial bonding. Nevertheless, the end anchorage system showed promising stiffness and load-carrying capacity behaviour for the specimens subjected to FD, WDCs, and the combined effects. The mixed failure mode of CFRP fracture and CFRP extraction from the end anchorage demonstrates that the strength of the CFRP plate was thoroughly utilized for the specimens with prestressed CFRP.

Impact responses of precast hollow reinforced concrete beams with prestress tendons using high-fidelity physics-based simulations

This study investigates the dynamic response, damage mechanisms, and residual capacity of precast hollow reinforced concrete beams with prestress tendons (HRCBPT) under impact loads by using High-Fidelity Physics-Based (HFPB) finite element (FE) models. The accuracy of the numerical model developed in this study has been fully calibrated against two different drop-weight impact tests available in open literature, including a scaled hollow reinforced concrete beam (HRCB) and a full-scale reinforced concrete (RC) beam. The results in this study show that under low to moderate impact loads, the flexural cracks and global deformation of the HRCBPT are similar to the hollow beam with normal reinforcement. Under high impact loads, although these beams exhibit similar diagonal shear cracks near the impact location, the HRCBPT experiences more local concrete spalling damage on the bottom flange, higher residual capacity index, and smaller reaction force at supports owing to the contribution of the initial prestress force. The results also suggest that the maximum compressive stress at the bottom flange of the section should be smaller than 17.6% of its compressive strength in order to maximise the beam’s impact-resistant capacity, while the prestress level in the tendon should be lower than 65% of the tendon yield strength to prevent premature failure under moderate impact conditions. Furthermore, different combinations of the impact velocity and impact mass would generate different impact force profiles on the beam resulting in dissimilar impact responses and residual capacity. In the design analysis, together with the critical impact location at the mid-span, the impact location close to the supports which may cause critical shear response of the beam, also needs be considered. In this study, the impact in the vicinity of the support yields the most severe damage to the beam resulting in the lowest residual capacity.

Peculiar aspects of cracking in prestressed reinforced concrete T-beams

In order to study the cracking of prestressed reinforced concrete T-shaped beam structures, the authors planned and carried out a full-scale experiment with five variable factors. The following factors were chosen as variable factors: the relative span of the shear, the ratio of the table overhang width to the thickness of the beam rib, the ratio of the table overhang thickness to the working height of the beam section, the coefficient of transverse reinforcement, the level of prestressing in the working reinforcement. The article describes the cracking process and the destruction of test beams. It was found that the loading level of an opening of inclined cracks is 53% larger than the loading level of a normal crack opening. Mathematical models of bending moments and transverse forces of cracking were built using the “COMPEX” software. Also, the mathematical models of the crack opening width and the projection length of a dangerous inclined crack were obtained. These models are based on the experimental data. Analysing the obtained models, the complex influence of variable factors on the main parameters of crack formation and crack resistance was established. In particular, it was found that the prestress level in the working reinforcement has the greatest effect on the bending moment of cracking. In this case, the value of the shear force of cracking significantly depends on both the prestressing level in the reinforcement and the relative span of the shear. On the basis of the experimental data, the empirical expression is obtained for determining the projection of a dangerous inclined crack for prestressed reinforced concrete T-shaped beams. The resulting equation can be used to calculate a shear reinforcement.

RC beams strengthened by prestressed CFRP plate subjected to sustained loading and continuous wetting condition: Flexural behaviour

Time-dependent prestress loss of RC beams strengthened by a prestressed CFRP plate subjected to sustained loading and continuous wetting condition was investigated in a previous study, which showed obvious deflection and crack development after long-term service. Thus, the degradation of flexural behaviour is investigated in this paper for the concern of safety. Eleven RC beams were prepared with different prestressing levels (0, 20% fu and 40% fu), environmental conditions (dried and wet), and sustained loading levels (0, 94 kN and 120 kN). After a year of exposure, four-point bending tests were conducted. The results showed that the cracking load of the strengthened beams was decreased by up to 23.9% after suffering the sustained loading, while the reductions for both the yield load and the ultimate load were insignificant. In addition, moisture exposure can improve the cracking load and stiffness by 6.8% and 26.1% respectively, but the effect on the yield load and ultimate load was not obvious. Moreover, the ultimate utilization ratio of CFRP plate in the 20% prestressed specimens can be decreased by approximately 8.5% due to moisture exposure, whereas there was no change in the higher prestressed specimens. Consequently, the test results reveal that strengthening RC structures by using prestressed CFRP plate could be a reliable method in a moist environment. Finally, a model to predict the bearing capacity was proposed based on the test results and a satisfactory agreement was reached.

Flexural performance of pretensioned centrifugal spun concrete piles with combined steel strands and reinforcing bars

Pretensioned spun high-strength concrete (PHC) piles have been widely used in building foundations in many parts of the world. However, PHC piles are also known to have poor lateral deformation and ductility capacity and hence are not suitable for seismic regions. To improve the horizontal load bearing and deformation capacities, pretensioned centrifugal spun concrete piles with steel strands alone (hereafter referred to as PSC piles) or a combination of steel strands and mild steel deformed rebars (referred to as PSRC piles) have been developed. PSRC piles are expected to benefit on the one hand from steel strands with high deformation capacity, and on the other hand from deformed steel bars with good bond with concrete and hence better control of cracks. This paper presents a comprehensive study on the comparative flexural performance of PSC and PSRC piles. The bending tests of PSC piles and PSRC piles were firstly conducted on six full-scale pile specimens with three different pile diameters, respectively. The flexural performances are evaluated in terms of crack resistance, flexural and deformation capacities, as well as crack distribution. The results show that the cracking bending moment of PSRC piles is similar to that of PSC piles, but the ultimate bending moment is about 55% higher due to the addition of deformed rebars. The general cracking behavior of PSRC piles exhibit remarkable improvement comparing to PSC piles, with an extended crack region, denser distribution of cracks, and much reduced maximum crack width. Following the experimental study, a theoretical method is proposed for the calculation of the flexural strength and flexural stiffness of prestressed concrete piles considering the presence of both steel strands and deformed rebars. Furthermore, a finite element (FE) model has been developed to simulate the flexural performance of the piles. The accuracy of the FE model is validated through comparing with the experimental results, and subsequently parametric analyses are conducted to explore the influences of three parameters, including the prestressing level of the prestressing strands, prestressing strand ratio and deformed steel bar ratio, on the flexural performance of PSRC piles.

Determination of Prestress in Circular Inhomogeneous Solid and Annular Plates in the Framework of the Timoshenko Hypotheses

Determination of prestress fields in structures is of the utmost importance, since they have a significant impact on operational characteristics, and their level and distribution must be strictly controlled. In this paper, we present modeling of bending vibrations of solid and annular round inhomogeneous prestressed plates within the framework of the Timoshenko hypotheses. New inverse problems of prestress identification in plates are studied on the basis of the acoustic response subjected to some probing load. To solve direct problems on calculating oscillations and amplitude-frequency characteristics, a computational Galerkin-method-based scheme has been developed. In order to treat the inverse problems, we use a special projection approach based on the constructed weak problems statements, which makes it possible to determine the desired characteristics in the given classes of functions. The developed techniques for solving direct problems are implemented in the form of software packages realized via Maple. For both solid and annular plates, we estimate the sensitivity of the amplitude-frequency characteristics the values of which are used as the additional data in the inverse problems to a change in the prestress level; we conclude that the most favorable frequency range should be selected in the resonance vicinity. We have conducted a series of computational tests on reconstructing the plate’s prestresses of various levels and distribution patterns (decreasing, increasing, sign-changing laws). The results of computational tests revealed that the technique developed allows for the determination of the prestresses with a low error for two cases: when the cause of prestress formation and its type are known and when arbitrary prestress changing laws are considered.

Strengthening of timber beams with pretensioned CFRP strips

The article presents the results of research on the strengthening of timber beams with the use of pretensioned CFRP strips. In the beginning, the review of literature regarding both passive and active strengthening methods of timber beams with steel bars, steel mats and FRP strips is presented. The review is followed by the description of a test program concerning the strengthening of solid timber beams with pre-tensioned composite strips. In four series of beams various levels of prestress were applied. The article discusses all stages of beams production, including the prestressing process. The behavior of beams over 28 days after prestressing is reported. The results of four-point bending of strengthened beams are described. The ultimate forces, mechanisms of failure as well as the “load–deflection” relationships in individual beam series were analyzed in comparison with the reference beams. The possibility of prestressing of solid timber beams and the reduction of their deflection due to prestress was proved.

A Low-Cost Prestress Monitoring Method for Post-Tensioned RC Beam Using Piezoelectric-Based Smart Strand

This study proposes a cost-effective prestress monitoring method for post-tensioned reinforced concrete (RC) beams using a smart strand. Firstly, the concept of a piezoelectric-based smart strand and its implementation for prestress force monitoring are developed. The smart strand is prepared by embedding inexpensive and high-sensitivity electromechanical impedance (EMI) sensors in a steel strand. Next, the feasibility of the proposed method is experimentally verified for prestress force monitoring of a simple supported post-tensioned RC beam. A smart strand prototype is fabricated and embedded into a 6.4 m RC beam which is then prestressed with different levels. For each prestress level, the EMI responses of the smart tendon are measured and the EMI features are extracted for prestress force monitoring. The results showed that the EMI signals of the smart strand showed strong resonant peaks that varied sensitively to the prestress level of the beam. The prestress change in the prestressed RC beam was successfully estimated by using linear regression models of the EMI features.

Design of ductile H-anchorage for strengthening reinforced concrete beams with prestressed FRP

Most FRP strengthening methods increased load capacity at the cost of decreased ductility. A review of 551 specimens from 64 literatures shows that the ductility of FRP strengthened RC beams experienced an average decrease of ductility by 37.2%. To achieve balanced improvement of ductility and load capacity, a previously proposed H-anchorage is adopted to prestress FRP laminates and to strengthen reinforced concrete (RC) beams in flexure. A total of ten specimens were cast and tested under four-point bending, the test results are presented and discussed. The effects of concrete strength, bond scheme, and prestress level of FRP were investigated. The H-anchorage was found to improve the utilization of FRP in tension, achieve great increase of load capacity, and maintain satisfactory ductility. Partial bonding of prestressed FRP effectively improved the flexural performance within the elastic range, and the load capacity generally increased with increasing prestress level of FRP laminates. The increase of ductility was more prominent in the specimens using high-strength concrete than those using low-strength concrete. During the loading process, the deformed concrete section remained planar. Based on this observation, an analytical model is proposed to estimate the load capacity, reaching good agreement with the experimental results. Finally, a design method is proposed for optimizing the configuration of H-anchorage to ensure optimum performance in terms of load capacity and ductility.

Displacement Control of Nonlinear Pin-Jointed Assemblies Based on Force Method and Optimization

The geometry of the structure and internal bar forces under varying service load situations or prestressing actually impact on the structure’s serviceability. Or else, prestressed cable structures are defined by the joint locations and delicate to external stimuli due to their high flexibility and light weight, and the shape distortion is supposed unacceptable. Consequently, based on the force method, the displacement control through updating coordinates of the structure and optimization on the form of quadratic programming is suggested to attain the nonlinear displacement control (NDC) of the geometrically nonlinear systems (including prestressed cable structures). The technique preserves the prestress level in the structural bar elements to any level required. The proposed method has been compared with other prior NDC techniques corresponding to the requirements. It shows that the computational results of the current approach are in good response with the previous NDC techniques. The method can exactly achieve the target values without any significant errors for the corresponding outcomes. Simultaneously, the technique keeps the prestress level of the structural members to the targeted level. The proposed approach technique also proved to control both small and large displacement without any remarkable error.

Experimental research on the bending performance of locked coil wire rope and Galfan strand

As a key component in prestressed structures, the bending stiffness of a cable significantly affects the dynamic response characteristics, such as vibration control and cable tension measurements using the vibration method. Based on the beam theory, this study reports experimental results on the bending performance of locked coil wire ropes and Galfan strands. Quasi-static bending tests under different end treatments and prestress levels were conducted, using four common specifications of locked coil wire ropes and three specifications of Galfan strands, to infer the effective flexural rigidity and to study the flexural performance of the cables. The results show that the load-deformation curves of unprestressed cables are bilinear, which is similar to that of a simply supported beam. The physical stiffness of the prestressed cables is not obvious. The geometric stiffness produced by the prestress dominates, and the effective stiffness coefficient can exceed 1000. Based on the Papailiou theory and beam theory, a predictive model is proposed for predicting the bending stiffness of cables under high prestress conditions. This model provides a reference for bending stiffness predictions of cables.

Development and evaluation of novel modules for deployable tension-strut structures

There is a growing need for new alternatives of long-span roof structures with high level of transformability and structural robustness. This led to the development of deployable cable-strut structures, which are composed of a continuous net of struts and another continuous net of cables. Subsequently, a special type of these systems was pioneered and given the term deployable tension-strut structures (DTSSs). The motivation beyond this new concept was the lack of structural efficiency and form flexibility of conventional space trusses that are usually employed for covering large spaces. Typically, DTSSs are roof structures consisting of multiple modules put together to form the roof system. This paper is mainly concerned with developing new robust modules for DTSSs. The technique that was adopted for this purpose is a form-creation methodology previously introduced in the literature. A few modules were already developed based on this shape grammar. However, its potential to develop multiple efficient modules has not been sufficiently investigated. In this current work, the afore-mentioned algorithm was utilized to form 16 new modules. A comparative study based on a nonlinear finite element technique was conducted to investigate the efficiency of the novel modules as compared to that of the previously proposed in the literature. The results show that some of the new proposed modules are far more efficient than those presented in previous researches. Based on this comparative study, the most two efficient modules among the novel ones were picked for further study. Parametric studies were conducted on these two systems under gravity loads and wind loads considering the following parameters: no. of modules, span/depth ratio, and cables’ pre-stress level. For each parameter, the optimal range of values were determined to be used as a guide for the design of such systems.

Stressing state analysis of multi-span continuous steel-concrete composite box girder

This paper uses the theory of structural stressing state to study the working characteristics of three multi-span continuous steel-concrete composite box girder (SCCBG) models with different prestress levels. Firstly, the generalized strain energy density (GSED) is constructed to reveal the working state of the structure based on the experimental strain data, the Mann-Kendall (M-K) criterion is applied to distinguish the leap characteristics load points (P and Q) of the structural stressing state and defined as the “failure load”. It is used to update the existing structural “destruction” load to more clearly reflect the entire structural failure process. Then, the working state parameters of the cross-section are described (strain distribution mode, midspan deflection, crack width, etc.) to analyze the working behavior of the box girder model. Finally, the non-sample point interpolation method (NPI) is used to reasonably expand the experimental strain data, to verify the rationality of the characteristic load points, and the stressing state characteristics of box girders with different prestressed levels are further discussed.

Efficient shear retrofitting of reinforced concrete beams using prestressed deep embedded bars

The Deep Embedment (DE) technique is a promising reinforced concrete (RC) shear strengthening scheme. When compared with the other retrofitting systems, the DE approach is impressive in many ways. Particularly, since the DE element is installed to the beam core, the truss mechanism is enhanced and that enables achieving high shear enhancements. Due to the internal application of the DE reinforcement, steel can also be utilized as the retrofitting material without reservations on corrosion. It is however identified that the tensile capacity of the DE bar is partially utilized. In this context, the potential of using prestress in the DE system as an improvement was explored through a non-linear numerical study and an experimental study. Two DE reinforcement types of normal steel and high-tensile steel were considered and the prestress level was set to 40%. Both approaches showed that the use of prestress in the DE system was capable of enhancing the beam shear capacity significantly, where the extra strength gain in one case was almost 26%. The serviceability performance of the beams was also improved due to the prestress application. Meanwhile, the numerical predictions showed good correlations with the global and local behaviours of the experimental beams.

PROGRAMMABLE INSTABILITY OF SPATIAL STRUCTURES BASED ON KRESLING ORIGAMI

A spatial structure based on the Kresling origami has multiple stability states under external compression. To realize an intelligent structure with a programmable failure mode, the finite particle method (FPM) is used to study the instability of spatial structures based on the Kresling origami. We propose a method of controlling the structural stiffness by adjusting the material, prestress, and structure arrangement. The calculation of the cable, rod, and membrane structures using the FPM is introduced. The method is verified by a bi-stability analysis of the Kresling pattern. The instability modes of the cable-rod model, the cable-rod-membrane model and the nested model based on the Kresling origami are analyzed. The influences of the elastic modulus, prestress level and structural arrangement on the structural instability mode and strain energy conversion are investigated. At last, the programmable failure of a double-layer Kresling structure is realized by adjusting the Young’s modulus of the structure, providing a new method for the design of intelligent structures in the future.

Tendon stress evaluation of unbonded post-tensioned concrete segmental bridges with two-variable response surfaces

This paper comprehensively studied the combined effects of prestress change and aspect ratios on the nonlinear structural performances of externally prestressed precast concrete segmental bridges (PCSB). An experimentally validated discrete-finite element model was adopted and various analytical cases were generated with variable span lengths. Furthermore, a simulation study is performed considering the change in prestress level to understand its effect on structural response, failure behaviour, and tendon stress at the ultimate limit state (ULS). The result showed that the stress in the unbonded tendon before the failure stage varies from 0.79 to 1.03fpy for the shorter tendon (T6L) and 0.66 to 0.94fpy for the longer tendon (T5L), and on comparing with the prediction of the existing codes, the stresses are highly underestimated. However, for the typical prestress level of around0.6-0.7fpu, the ACI318-19 code could quite well predict the ultimate tendon stress change. To establish the dependency of stress in the unbonded tendon at ULS to the normalized prestress factor (α) associated with the aspect ratio (L/d), the response surface methodology (RSM) was implemented. The mediocre prediction of one-variable linear regression analysis concludes the dependency of the combined effects of the two variables on the response variables. The 3rd order two-variable response surfaces were able to predict an increase in stress and total stress of tendon at the ULS with high goodness-of-fit values of 0.97 and 0.92 respectively.

Investigating the relationship between radial pre-stress magnitude and ballistic projectile dwell in heavy confined ceramic targets

A series of bare silicon carbide ceramic armour disks were manufactured with high-strength steel containment to induce varying magnitudes of radial pre-stress. The induced radial stresses, ranging from 0 to 900 MPa, were measured via neutron diffraction and verified by comparison with numerical calculations. Four targets of each configuration, referred to as slip-fit, moderate pre-stress, and high pre-stress, were subject to ballistic testing with hemispherical-nose, tungsten heavy alloy long rod projectiles to determine the interface defeat transition velocity. In-situ diagnostics were unsuccessful in aiding the identification of interface defeat, necessitating a reliance on post-mortem assessment. A transition velocity of approx. 1000 m/s was identified for the unstressed target, increasing to approx. 1200 m/s for the pre-stressed configurations. No performance effect was discernible between the moderate (372 MPa) and high (899 MPa) pre-stress configurations, suggesting that an optimal performance may be achieved for lower pre-stress levels (i.e., <372 MPa). The test results were compared with four semi-analytical predictions of interface defeat performance and good agreement was found, albeit with significant range in the model predictions.

Fire-Resistance Mechanism and Residual Bearing Capacity of Prestressed Concrete Beams after Fire Exposure

The fire resistance and residual bearing capacity of prestressed concrete (PC) beams after fire exposure are very important to the safety of engineering applications. This study performed experimental investigations and theoretical calculations of the fire resistance of PC T-beams subjected to a standard fire. Effects of fire exposure time, steel strand location, and prestress level on failure mode, ductility, residual bending capacity, and residual stiffness of PC T-beams were investigated. The bending working mechanism of PC T-beams after exposure to the standard fire was determined. With increased fire exposure time, the ductility and bending stiffness decreased significantly and the failure mode changed. However, because of catenary action, extra vertical force between the end support of specimens and steel strands increased the ultimate bending capacity of fire-damaged PC T-beams. Multilayer steel strands with high prestress could delay initial crack occurrence and increase residual bending capacity effectively. However, they have little influence on residual stiffness.

PARAMETRIC STUDY ON COMPOSITE BEAMS STRENGTHENED WITH PRESTRESSED CFRP PLATES AND TENDONS

Advanced carbon fiber reinforced polymer material (CFRP) has been widely used for strengthening structures. This paper aims to improve the ultimate capacity and stiffness of the composite beam externally strengthened with pre-stressed carbon fiber reinforced polymer plate by using CFRP tendons. The models were presented by 3-D FEM (finite element model) using ANSYS program to investigate the performance of composite beam which validated with experimental results presented. Various parametric studies achieved as degree of pre-stress level of CFRP tendons, tendon profile, degree of shear connection, the most effective beam load level to add CFRP tendons. Finally, the optimum strengthening conditions for the composite beam were studied.