Prestress Level Research Articles

Flexural behavior of RC beams strengthened with fully or partially prestressed near-surface mounted FRP strips: An experimental investigation

The strengthening technique based on prestressed near-surface mounted (NSM) strips made of fiber-reinforced polymers (FRPs) is well known as an effective means to restore or enhance the flexural performance of reinforced concrete (RC) beams, notably in bridges. To better understand the mechanical behavior of RC beams strengthened with prestressed NSM FRP strips, 18 beams were tested under either static loading (15 specimens) or fatigue loading (3 specimens), with the focus on the failure mode, load-displacement response, stiffness, ductility, and fatigue life. The effects of the prestress level and bond length of the FRP strips were studied as well. The test results show that the bond length plays a major role in the flexural performance of the strengthened beams, since the greater the bond length, the higher the ultimate load, stiffness, and ductility. A strengthening method based on the partial prestressing of NSM FRP strips is proposed as well to reduce the bond stresses around the extremities of the strips and to increase the ultimate load of the beams by delaying the failure due to end debonding. According to the proposed method, the strengthening strips are prestressed along most of their length, but the segments close to the end sections are nonprestressed, because they are bonded to the concrete after prestressing has been released. Compared to the beams with fully prestressed NSM-FRP strips, partially prestressed strips improve the bearing capacity, ductility, and fatigue life of the strengthened beams. The role of the length of the nonprestressed segments is also discussed. Finally, a few strength models available in the literature, aimed at predicting the bearing capacity at the onset of beam failure (because of the separation of the concrete cover at the extremities of the strengthening strips), are applied to the beams tested in this project to check the reliability of the models vs. the test results.

EXPERIMENTAL STUDIES OF SELF-STRESSED CONCRETE

The paper considers the test results self-stressed concrete.In previous studies, it was noted that if the required level of pre-stress of concrete is provided,that is, at the level of a certain effective value, which depends on the structural and strengthparameters of the structural element, then in the limit state it is possible to fully use the strengthproperties of steel, and the concrete core operates in a three-axis stress state under compression withextreme values of longitudinal and radial stress. Therefore, it is necessary to study differentcompositions of self-stressed concrete. To achieve this goal, five series of prisms with differentcompositions were manufactured and tested. According to the results of experimental studies, it wasfound that the first, fourth and fifth series of prisms corresponds to the self-stress brand Sp 1.2 andis self-stressed concrete the second series of prisms does not correspond to any of the self-stressbrands due to the high content of Portland cement, the third series of prisms corresponds to the selfstress brand Sp 0.6 and is non-shrinkable concrete. Since the selected concrete is planned to be usedas the core of a steel concrete column it is advisable to check the ph metric of the given components.At the next stage of the study, the cement of the selected components was tested for ph-metry.For this purpose, aqueous solutions of cement additives and suspensions of selected components weretested for pH and EMF. As a result, possible inhibition of metal corrosion is also confirmed by highpH values of the liquid phase at the level of 8.25 1 13.85 (which is significantly higher than neutralpH = 7) in the presence of both individual additives and cement suspensions containing additives.Therefore, the use of self-stressed concrete with these additives is advisable.

Flexural behaviour of seawater–sea sand concrete beams reinforced with GFRP bars: Effects of the reinforcement ratio, stirrup ratio, shear span ratio and prestress level

Glass fibre-reinforced polymer (GFRP) bar-reinforced seawater–sea sand concrete (SWSSC) has been widely regarded as a new structural material with great potential due to its environmental friendliness. To safely and economically utilize seawater and sea sand to cast GFRP bar-reinforced flexural members in coastal areas, an experimental study was conducted to investigate the flexural behaviour of 12 GFRP bar-reinforced SWSSC beams. The various considered parameters included the (1) reinforcement ratio, (2) stirrup ratio, (3) prestress level, and (4) shear span ratio. The experimental results for the GFRP bar-reinforced SWSSC beams were verified via a comparison to the calculation models of GB 50608-20 and ACI 440.1R-15 for FRP bar-reinforced normal concrete (NC) beams. The results indicated that the prestress level is the main factor controlling the cracking load of the GFRP bar-reinforced SWSSC beams, and the cracking load of prestressed beams was 81% higher than that of nonprestressed beams. The ultimate load of the GFRP bar-reinforced SWSSC beams was strongly correlated with the reinforcement and shear span ratios (the higher the reinforcement ratio or the lower the shear span ratio was, the higher the ultimate load). Under the influence of the reinforcement and stirrup ratios, the failure mode of the GFRP bar-reinforced SWSSC beams could be transformed from GFRP bar rupturing into concrete crushing. Moreover, the calculation model of ACI 440.1R-15 overestimated the ultimate flexural capacity of the nonprestressed beams, while the calculation model of GB 50608-20 yielded conservative estimation values for the nonprestressed specimens and overestimated values for the prestressed beams. To ensure safety, the calculation model of GB 50608-20 for FRP bar-reinforced NC beams can be adopted when designing nonprestressed GFRP bar-reinforced SWSSC beams. In contrast, a prediction model for prestressed GFRP bar-reinforced SWSSC beams should be further investigated.

Buckling of cylindrical concrete tanks and silos due to prestressing — nonlinear approach

The buckling of concrete cylindrical tanks and silos is associated mainly with prestressing, which influences the meridional critical force. Higher compression produces a lower meridional critical force. For three-dimensional structures subjected to biaxial or triaxial loading, the simplified methods of stability analysis proposed in Eurocode standard for concrete structures are inappropriate. The authors try to answer which procedure may be helpful for the assessment of buckling possibility, which results from prestressing of cylindrical structures. The numerical analyses are performed with the use of “Simulia Abaqus system”, which allows the nonlinear stability analysis adopting the plasticity-based damage material model and assuming initial imperfections. Two variants of load distribution (constant and hydrostatic external prestress) and two variants of wall-foundation connection are considered. The results are presented with the use of two parameters. The first one constitutes the ratio of lateral pressure to vertical pressure on the upper edge. The second one describes the ratio of current vertical force – corresponding to real load – to the initial test value of the iterative process. The obtained relationships of critical vertical force and the level of external prestress for cylindrical tank of height twice the diameter can be the guideline for engineers in designing process.

Long-term mechanical behaviors of uncracked concrete beams prestressed with external basalt fiber-reinforced polymer tendons

In this study, an experiment was conducted on four uncracked concrete beams prestressed with external basalt fiber-reinforced polymer (BFRP) tendons or stress-relieved strands subjected to 300-day sustained loads. The experimental variables included tendon type, prestress level (40 kN and 50 kN), and concrete strength grade (C40 and C60). The effects of these variables on the long-term deformation and prestress loss of the beams were discussed and analyzed. Based on an existing approach using an age-adjusted effective modulus method (AEMM), a modified model was proposed for predicting the long-term deformation and prestress loss of concrete beams prestressed with external BFRP tendons. The results indicated that the prestress level had significant effects on the long-term deformation and prestress loss. The concrete strength grade significantly affected the long-term deformation, but had no effect on the prestress loss. The stress-relieved strands exhibited a prestress loss that was 16% greater than that of the BFRP tendons. The theoretical results based on the modified model were consistent with the experimental results. The results of this study contribute to the design and prediction of the long-term behaviors of concrete beams prestressed with external BFRP tendons.

Fatigue behavior of RC slabs flexurally strengthened with prestressed NSM CFRP laminates

AbstractSome reinforced concrete (RC) structures requiring strengthening intervention are subjected to cyclic loads during their lifetime. To evaluate the effect of this kind of loading on the behavior of flexurally strengthened RC slabs with prestressed near‐surface mounted (NSM) carbon fiber reinforced polymer (CFRP) laminates, an experimental program formed by 10 RC slabs was carried out. Four specimens were submitted to monotonic loading and the remaining were submitted to 1 million cycles at a frequency of 1 Hz. Two levels of fatigue loading were applied and, for each one, three prestress levels for the CFRP were tested (0%, 20%, and 40%). Failure during the fatigue loading was occurred only in one specimen (at 752,000 cycles) with a maximum fatigue stress level of 92% (percentage of the maximum load carrying capacity of the equivalent slab tested up the failure under monotonic static loading). The other specimens, submitted to a maximum fatigue stress level between 46% and 82%, did not fail during the fatigue loading process. After 1 million cycles, the NSM strengthened slabs with 0%, 20%, and 40% of prestress presented an increase of 20%, 18%, and 25%, respectively in mid‐span deflection. In general, the strains in the CFRP laminates marginally increased during the fatigue loading. The results of the postfatigue monotonic tests until failure showed that the damage accumulation due the fatigue cycles neither affected the ultimate load capacity of the strengthened NSM CFRP slabs nor the failure mode in which the CFRP rupture.

Monitoring fish welfare using heart rate bio-loggers in farmed Atlantic salmon (Salmo salar L.): An insight into the surgical recovery

Atlantic salmon are one of most studied cultured species, still continuous fish welfare monitoring is a challenge during commercial farming operations. Although, mortality is the terminal indicator of fish welfare, but without further explanation, this parameter is of limited use. Fish welfare on-site can be monitored by using implantable DST milli-HRT bio-loggers that record heart rate, electrocardiography and temperature. These heart rate loggers gave us the opportunity to monitor fish welfare under common farm handlings in free-swimming farmed Atlantic salmon (~1 kg). Three different stress tests such as crowding (test 1), grading (test 2) & vaccination (test 3) were performed on three separate fish groups three weeks post-surgery. Heart rate and biochemical parameters (blood plasma and mucous) including cortisol were measured and compared in pre-stressed (T0), stressed (T1) and post-stressed (T48) fish. A modified loggers anchoring technique was used where two separate thin sutures were threaded at front hole of the logger and knot them through the skin/muscle for anchoring as it improved the fish welfare during surgery. Surgical wound showed complete skin/peritoneal apposition with regenerated skeletal muscle, fibrosis, lymphocyte-like cells and vascularization at the site three weeks post-surgery. Following surgery, heart rate (fH) was elevated (˃70 beats min−1) which stabilized 3 days post-surgery in tests 1 & 3. However, it took at least 10 days to stabilize fH in case of test 2. All three stress tests inflicted tachycardia suggesting aquaculture practices incur stress in farmed Atlantic salmon. Heart rate was raised at stress (range 62–78 beats min−1) as compared to pre-stress levels (range 44–48 beats min−1) that corresponds to 42–77% elevation in all three tests. Cortisol levels were upregulated following tests but only significantly different in test 3. This study reveals that physiological responses (heart rate) should be combined with histological evaluation of surgical healing before utilizing the heart rate (fH) data. Upon handling, fish should be given enough time (˃48 h) to stabilize fH levels before commencing to other tests/handlings. In conclusion, heart rate can be used for continuous fish welfare monitoring under farming conditions.

Progressive damage in pretensioned and reinforced concrete plates against repeated impacts

This study is planned to explore the performance of pretensioned concrete (PC) plates under multiple impacts. A detailed investigation has been carried out on pretensioned concrete plates (0.8 × 0.8 m2) against drop impact. The plates prepared using Mix-40 and Mix-60 grade concrete have been induced with two different levels of initial prestress, that is, 1/10 and 1/5 (i.e. level-1 and level-2) times the strength of the concrete. The PC plates have been impacted by a falling impactor (2382 N) dropped from 0.5 m height. The response of those plates has been obtained and compared with the reference RC plates. The post-impact performance of the damaged plates has been further discovered by subsequently dropping the impactor multiple times from the identical height. The FE simulations of the problem have been carried out using Johnson-Holmquist-2 and metal-plasticity constitutive models for concrete and steel, respectively. The models have been initially verified with the experimental results available in literature, and subsequently the simulations for drop impact have been carried out. The simulation results are also compared with the results of drop impact experimentations performed. In general, both the pretensioned and reinforced concrete have witnessed flexural cracks at the beginning, such that pretensioned concrete witnessed lesser cracks compared to reinforced concrete. As the number of drops increased, one major splitting crack developed only in pretensioned concrete, whereas the reinforced concrete exhibited additional punching cracks. For a given concrete grade, the pretensioned concrete level-2 witnessed the smallest damage, minimal cracks, and also minimal spalling followed by the pretensioned concrete level-1 and reinforced concrete. The reinforced concrete absorbed the minimal impact energy followed by the pretensioned concrete level-1 and level-2 under the multiple impacts. The FE simulations predicted the impact force and reaction within 11.9 and 9.9% variation, respectively, with the corresponding experimental results.

An alternative method to reduce process-induced deformation of CFRP by introducing prestresses

Process-induced deformation is an important limiting factor to the application of high performance composite structures. An alternative method to reduce process-induced deformation is proposed by introducing prestress to a single layer or part of the layers instead of all the applicable layers. In this method, the necessary prestress level that applied to the single layer was in the reasonable range. Two kinds of unsymmetric laminates are manufactured at varying prestress levels. The experimental results described that the curing deformation is changed rapidly with the growing prestress level and flipped after a certain prestress level. Moreover, there is a good linear relationship between prestress level and final curing deformation, which gives a convenient way to calculate the prestress level that can fully counteract the curing deformation. The numerical model to predict the curing deformation was built. A good agreement between the numerical and the experimental results shows the effectiveness of the numerical model. To predict the prestress level that can fully counteract the curing deformation, an analytical model is also proposed. Theoretically, the prestress method can fully counteract the curing deformation of the specimens, while the experimental results show that the prestressing layer reduces more than 80% of the curing deformation.

The in-plane expansion of fractured thermally pre-stressed glass panes: -An equivalent temperature difference model for engineering glass design

The present paper is concerned with deriving simplified design equations and charts for modelling in-plane expansion of fractured thermally pre-stressed soda-lime-silica glass panes using the method of equivalent temperature differences (ETD) together with a thermal expansion analogy for strains. The starting point is a theoretical method based on linear elastic fracture mechanics merged with approaches from stochastic geometry to predict the 2D-macro-scale fragmentation of glass. The approach is based on two influencing parameters of glass: (i) fragment particle size, δ, and (ii) fracture particle intensity, λ, which are related to the pre-stress induced strain energy density, UD, before fracture. Further Finite Element (FE) analysis of single cylindrical glass particles allow for establishing functional relations of the glass fragment particle dimensions, the pre-stress level and the resulting maximum in-plane deformation. Combining the two parts of two-parameter fracture pattern modelling and FE results on fragment expansion, formulas and engineering design charts for quantifying the in-plane expansion of thermally pre-stressed glass panes due to fracturing via an ETD approach is derived and provided within this paper. Two examples from engineering practice serve as demonstrators on how to use our ETD approach to compute the equivalent temperature difference and resulting internal forces as well as deformations. This approach serves furthermore as a basis to estimate secondary effects (such as fracture-expansion-induced deformations or stresses) on support structures or remaining parts of glass laminates in form of handy ETD load cases within analytical as well as FE analysis.

Durability of GFRP bars embedded in seawater sea-sand concrete: A coupling effect of prestress and immersion in seawater

Using prestressed glass fibre-reinforced polymer (GFRP) bars for seawater sea-sand concrete (SSC) structures in offshore engineering is of particular interest for improving the utilisation of GFRP and natural resources. However, limited information is available on the effects of prestressing on the durability of GFRP. Therefore, the durability of SSC-coated GFRP bars was investigated under the coupling effect of prestressing and immersion in seawater at room temperature, 40 °C, and 60 °C, with prestress levels set at 20% and 40% of the ultimate tensile strength (%fu) of GFRP. The tensile properties, interlaminar shear strength, transverse shear strength degradation after 0 d, 30 d, 60 d and 120 d, and the prestress loss of GFRP within 70 d were studied. The results showed that SSC wrapping was not conducive to the long-term performance of GFRP bars due to the alkaline environment, and the prestress further accelerated the tensile strength degradation, with the tensile strength decreasing almost to zero for the specimens (with 40% fu prestress level) immersed in seawater after 120 days at 40 °C and 60 °C. However, the interlaminar shear strength and transverse shear strength retention were 37.1% and 52.2% for 60 °C immersion under the same conditions, respectively. These findings suggest that the shear properties degenerated more slowly than the tensile properties of the prestressed GFRP bars embedded in SCC under seawater environment, which also evidences that the glass fibres were damaged more heavily than the epoxy vinyl resin under the prestressing.

Long-term behavior of CFRP plates under sustained loads

This paper studies the long-term behavior of carbon fiber–reinforced polymer (CFRP) plates under sustained loads. Two typical loading conditions, CFRP plate loading controlled by displacement constraints and force constraints, are studied. The first section investigates the prestress loss of the CFRP plate when it is loaded via displacement constraints, which serves as a prestressed system applied for prestressed structures. Short- and long-term measurements show that the prestress loss of CFRP is caused by material creep, temperature change, and CFRP and anchorage slippage. When the CFRP prestress level is 5.0%–12.3%, the average prestress loss after 42 days is 5.1%. The second section investigates the permanent deformation of the CFRP plate with curved anchorages caused by creep under a certain sustained load (i.e., controlled by force constraints), which serves as a suspension bar or cable applied for determinate structures. The creep strain-to-time relationship, creep rate, and residual strength were obtained. The creep strain-to-time relationship strongly depends on the stress level applied; the creep coefficient is less than 10.94% and the residual strength is greater than 80% of the initial tensile strength after 1000 h. The findings show the effectiveness and reliability of these two typical cases of CFRP plates under long-term sustained loads.

Experimental study on the cutting compensation of PTFE-coated fabric membrane structures

As a flexible structure, the membrane structure is tensioned to generate prestress and produce corresponding stiffness. The prestress level is controlled by scaling the cutting pattern according to the cutting compensation. In this paper, the cutting compensation of the PTFE-coated fabric membrane is taken as the research object, and different test schemes are carried out to simulate the actual stress of the membrane structure. The test scheme is divided into four stages according to the construction process and external loads condition of membrane structures: initial tension, load-bearing, creep, and elastic recovery. Then the combination schemes of different loading modes are conducted to explore the influence of loading factors on the cutting compensation. Through the simulation analysis of the construction process and stress relaxation of an HP-type example, the feasibility study of the cutting compensation under different test schemes is performed. Finally, a proper test scheme of cutting compensation is put forward, which provides the theoretical basis and scientific demonstration for guiding engineering practice.

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.

Theoretical and Numerical Study on the Three-Edge Bearing Capacity of Prestressed Concrete Cylinder Pipe

Prestressed Concrete Cylinder Pipe (PCCP) is a composite pipe structure commonly subjected to earth pressure, internal pressure, and ground loads. To confirm the bearing capacity of PCCP by using the in-direct design method, one of the most important experiments is the Three-Edge Bearing (TEB) test. A rational theoretical analysis method and Finite Element Analysis (FEA) are developed in this study because the TEB test is costly and difficult to conduct. The proposed theoretical analysis method and Finite Element (FE) method can be used to calculate the internal force, the displacement, the cracking load, and the bearing capacity of PCCP under the TEB load. In addition, a parametric study is carried out by using the validated theoretical analysis and FEA method. It is found that the concrete strength, the prestressing level, and the radius-to-thickness ratio of the steel cylinder affects the bearing capacity of PCCP under the TEB load. Furthermore, the proposed theoretical analysis method can be used to calculate the bearing capacity of other kinds of rigid pipes under the three-edge bearing load.

Fatigue behavior of notched steel beams strengthened by a prestressed CFRP plate subjected to wetting/drying cycles

Prestressed carbon fiber reinforced polymer (CFRP) strengthening is effective in suppressing crack propagation of steel beams. However, the fatigue behavior of notched steel beams with prestressed CFRP under a hygrothermal environment is not fully understood. Generally, notch machining on the steel beam is an ordinary method to simulate a fatigue defect for laboratory tests. In this study, the interfacial energy release rate at the notch location was first derived for strengthened steel beams. The theoretical results show that the interfacial energy release rate can be significantly reduced by prestressed CFRP strengthening, contributing to improving the interfacial fatigue performance. In addition, notched steel beams strengthened by prestressed CFRP (with a prestress level of 25% of tensile strength of CFRP plate) were exposed to wetting/drying cycles (WDCs) and then tested under fatigue loading. The experimental results show that prestressed CFRP strengthening increases the fatigue resistance of interfacial debonding initiation and failure 3.47 times and 7.83 times, respectively, which agrees with the tendency revealed by the theoretical results. In contrast, WDC exposure reduced the fatigue lives of interfacial debonding initiation and failure by 34.6% and 27.2%, respectively. An S – N curve based on the maximum principal interfacial stress at the notch location was proposed to predict the tendency of fatigue life of debonding initiation for strengthened steel beams. Moreover, the tendency prediction for fatigue life of interfacial debonding development for the strengthened steel beams with/without WDC exposure was developed based on Paris’ law. • Theoretically reveals that the prestressed CFRP reduces the interfacial energy release rate. • Prestressed CFRP strengthening significantly improves interfacial fatigue life. • Hygrothermal exposure leads to a significant reduction of interfacial fatigue life.

Scaling Effect on the Behaviour and Design of Prestressed Stayed Steel Columns

This study presents the results of a small-scale experimental campaign on prestressed stayed steel columns, subsequent numerical model validation and design guideline development. The majority of previous experimental studies have focused on large-scale systems, which are expensive and can be difficult to perform due to the required specialised experimental set-up, whereas small-scale experiments are less restrictive with both space and experimental set-up requirements. Also, existing design guidelines were developed from a single system scale, so have not been shown to be applicable to changes in geometric scale. Thus, the scaling effect on prestressed stayed steel columns was investigated to promote the use of small-scale experiments in the study of large-scale prestressed stayed steel column systems and update design guidelines for change in geometric scale. A total of 17 prestressed stayed steel columns and a control column with no cross-arms were tested. These tests investigated the symmetric and antisymmetric buckling behaviour as well as the interactive post-buckling phenomenon. These tests were designed to investigate the scaling effect on the behaviour of the system and to determine the optimal prestress level of prestressed stayed steel columns close to the transition point. A numerical model was also validated by the experimental results to perform a full geometric scaling comparison study and update existing design guidelines. It was found that small-scale structures are capable of representing the behaviour of large-scale prestressed stayed steel columns. Furthermore, it was shown that the highest efficiency in terms of the weight of materials is found close to the transition point between symmetric and antisymmetric buckling behaviour. Various scale systems were modelled numerically, and the results compared with existing guidelines, which resulted in low levels of accuracy. Therefore, existing design guidelines were updated using the validated numerical model and shown to yield more accurate results for the L/400 and L/200 imperfection levels.

No Evidence for Light-Induced Embolism Repair in Cut Stems of Drought-Resistant Mediterranean Species under Soaking.

(1) Recent studies suggested that stem photosynthesis could favor bark water uptake and embolism recovery when stem segments are soaked in water under light conditions, but evidence for this phenomenon in drought-resistant Mediterranean species with photosynthetic stems is missing. (2) Embolism recovery upon immersion in water for 2 h–4 h under light was assessed (i) via a classical hydraulic method in leafless Fraxinus ornus and Olea europaea branch segments stressed to xylem water potentials (Ψxyl) inducing ca. 50% loss of hydraulic conductivity (PLC) and (ii) via X-ray micro-CT imaging of the stem segments of drought-stressed potted F. ornus saplings. Hydraulic recovery was also assessed in vivo in intact drought-stressed F. ornus saplings upon soil re-irrigation. (3) Intact F. ornus plants recovered hydraulic function through root water uptake. Conversely, the soaked stem segments of both species did not refill embolized conduits, although Ψxyl recovered to pre-stress levels (between −0.5 MPa and −0.2 MPa). (4) We hypothesize that xylem embolism recovery through bark water uptake, even in light conditions, may not be a common phenomenon in woody plants and/or that wounds caused by cutting short stem segments might inhibit the refilling process upon soaking.

Experimental study on flexural behaviour of reinforced concrete beams strengthened with passive and active CFRP strips using a novel anchorage system

The paper presents the research on reinforced concrete (RC) beams strengthened with carbon fibre reinforced polymer (CFRP) strips with various configurations in terms of anchoring and tensioning. The five full-scale RC beams with the total length of 6.0 m were strengthened with passive strips, without and with mechanical anchorages at their ends, as well as with strips tensioned by the novel prestressing system with three various prestressing levels ranging from 30 to 50% of the CFRP tensile strength. All RC beams were tested under static flexural load up to failure and they were investigated in a full range of flexural behaviour, including the post-debonding phase. The main parameters considered in this study include the use of mechanical anchorages, the effect of tensioning the strips and the influence of the various prestressing levels. Several performance indicators have been established to evaluate the beams’ behaviour. The study revealed that the RC beams strengthened using tensioned CFRP strips exhibited a higher cracking, yielding and ultimate moments as compared to the beams with passively bonded CFRP strips. Moreover, increasing the beams’ prestressing level has a significant positive influence on the performance of strengthened beams. However, it did not affect the ultimate load-bearing capacity of the beams. The optimal prestressing level for the novel system has been determined as 60% of CFRP tensile strength.

Bayesian inference for predicting the long-term deflection of prestressed concrete bridges by on-site measurements

Predicting the long-term deflection of prestressed concrete bridges (PSCB) by the deterministic models is subject to significant epistemic uncertainties, and the Bayesian statistics provide a practical way to make more reliable prediction by use of the long-term measurements. In this paper, a comprehensive Bayesian inference frame was built, in which the model parameters associated with creep, shrinkage, dead load and prestress level were updated by the Bayesian inference of the deflection measurements. A 3-D time-dependent finite element model was constructed, based on which a surrogate model was built to enhance computation efficiency. The Markov Chain Monte Carlo (MCMC) algorithm was adopted to approximate the manifold of the posterior distribution. In the case study, two sets of deflection of the case bridge were utilized to perform the Bayesian inference in a sequential manner. The results revealed that the updated models could produced better fit to the measurements than the deterministic model, while the variance of the prediction was significantly reduced. The updated model revealed changing trends of the time-dependent deflection, which characterizes a later-age acceleration from the initial moderate trend. Based on the updated models, the time-dependent reliability associated with the limit states defined by deflection limits was computed. For such a task, the MCMC-based pre-sampling and the quasi-optimal importance sampling were adopted to reduce the variance of the computation.