Time-dependent Prestress Losses Research Articles

Nonlinear behavior of existing pre-tensioned concrete beams: Experimental study and finite element modeling with the constitutive Serial-Parallel rule of mixtures

The performance of existing prestressed concrete (PC) beams and their time-dependent behavior have been concerns for bridge maintenance services. In this paper, laboratory tests of three PC void slab beams taken away from an obsolete bridge were conducted and an alternative more efficient finite element (FE) method was developed. In preparing the three beam specimens, two retained their original reinforced concrete (RC) overlay and one chiseled off its RC overlay. Through the four-point loading tests, the load response plots of the beams from elastic to plastic failure were carefully documented, including the strains along the mid-span section, the strains of longitudinal rebars and strands, and the mid-span deflection. The test results showed that no interface slip was observed between the RC overlay and the top of void slab during the whole loading process, and the yield load of the test beams with the overlay was approximately 45 % higher than that without the overlay. Regarding the proposed nonlinear numerical formulation, the modelling process avoided the treatment of complex reinforcement and prestressing tendon layouts. The prestressed concrete was simulated as a composite material whose behavior is depicted by a constitutive serial-parallel rule of mixtures (SP-RoM). The generalized Maxwell model and generalized Kelvin model were applied to calculate the time-dependent losses of prestress. All the developments have been implemented within the open-source FE code Kratos-Multiphysics environment and are fully available. Compared with the test outcomes, it shows that the numerical results are in good agreement with the solutions in terms of stiffness, load-deflection curves, and damage evolution. The differences between the experimental and numerical values of ultimate load for the specimens with and without RC overlay were all within 10 %. Furthermore, the proposed nonlinear models can also predict the time-dependent prestress losses of existing PC beams.

Two-Way Time-Dependent Prestress Losses of Prestressed Concrete Containment with Bonded Prestressing Strands

Prestressing plays a pivotal role in ensuring the tightness and integrity of prestressed concrete containment in nuclear power plants. The prestress loss reduces the compressive stress in concrete resulting from the prestressing strands and increases the risk of containment leakage under severe accident conditions. Therefore, the accurate prediction of prestress loss is essential for the design and in-service management of prestressed concrete containment. Unlike one-way beams or girders in building structures and bridges, two-way prestressing systems are used in prestressed concrete containment. In the current simplified method for evaluating time-dependent prestress loss, the interaction of concrete creep in two directions resulting from the two-way prestressing strands and the influence of the steel liner and mild steel rebars in two directions are neglected. In this study, based on the principle of creep superposition, the age-adjusted effective method for the creep estimation of concrete, and considering concrete shrinkage, concrete creep, and the relaxation of prestressing strands, as well as the influence of the steel liner and mild steel rebars in two directions, a sectional analysis is performed for prestressed concrete containment with bonded prestressing strands, and equations for calculating the two-way time-dependent prestress losses are derived. The results of the two-way time-dependent prestress losses predicted by the derived equations are compared with those of tests in the literature, and great agreement is achieved. Finally, a case study is given to show the application of the proposed method for the prediction of prestress loss in prestressed concrete containment in the nuclear power plant.

Effect of wetting/drying cycling on flexural behavior of RC beams strengthened by prestressed CFRP plate: Experimental and analytical investigations

Using prestressed carbon fiber reinforced polymers (CFRPs) is a promising technique for strengthening reinforced concrete (RC) structures. This study aimed to investigate the prestress losses and flexural behavior of strengthened RC beams under wetting/drying cycling (WDC) in a chloride-containing environment. Two prestress levels (20% and 40%) and two WDC durations (90 and 180 cycles) were considered. The time-dependent prestress loss under WDC was first monitored by fiber bragg grating (FBG) for 90 days, and the composition of the prestress loss was theoretically analyzed. A four-point bending test was conducted on the strengthened beams after the WDC exposure, and the intermediate crack (IC) debonding strain and the ultimate moment of the strengthened beams were analytically evaluated. The results show that although WDC exposure aggravated the time-dependent prestress losses, the maximum time-dependent prestress losses after 90 WDCs were only 1.75%, indicating the effectiveness of the end anchorage system. After WDC exposure, the failure mode of the strengthened beams shifted from CFRP rupture to IC debonding, accompanied by the reduction of cracking and the ultimate load, CFRP utilization, and the increase of the maximum crack width. Besides, the model of Lu et al. and Said and Wu could be applied to the IC debonding strain prediction after the WDC. When the environmental reduction coefficient was incorporated, ACI 440.2 R and CECS-146 resulted in a precise IC debonding strain. The proposed method could accurately predict the ultimate moment of the prestressed FRP-strengthened RC beams with different failure modes after WDC.

Transfer Length and Prestress Losses of a Prestressed Concrete Box Girder with 18 mm Straight Strands

Despite the potential advantages of 18 mm strands, the limited research on the behavior of girders with larger-diameter strands hinders the application in bridges. Transfer length and prestress losses are two important indicators. In this research, a 32.6 m long prestressed concrete box girder with 18 mm straight strands and 15 mm harped strands was produced, and the transfer length and the prestress losses were studied. The transfer length was calculated based on the existing equations in codes and previous research. Three beam specimens were fabricated, and strain gauges were pasted on the concrete surface to measure the transfer length of 18 mm strands. It indicated that the average measured transfer length was 700 mm. This value was smaller than the transfer lengths predicted by AASHTO LRFD 2017 and ACI 318-19, while Mitchell’s equation offered the closest prediction to the average measured transfer length. Additionally, the prestress losses at different stages were evaluated. A one-end stressing test was conducted to analyze the effect of strand harping on the loss of tensile force. In comparison with the actual measured loss based on the concrete strain and the longitudinal shortening, the instantaneous prestress loss calculated using the AASTHO LRFD 2017 alternative equation was appropriate. The time-dependent prestress losses due to shrinkage, creep, and relaxation were predicted using two different methods addressed in AASHTO LRFD 2017. The time-dependent predicted losses of 69.2 MPa at 28 d using the refined method were 37% higher than the measured losses 47.4 MPa at 28 d, indicating an overestimation of AASHTO LRFD 2017. The accumulation of the total losses over time revealed that the prestress losses developed in the first two months occupied the majority of the total losses in the long term. The research may provide guidelines for the design of a pretensioned concrete box girder with 18 mm strands.

A general equation for estimation of time-dependent prestress losses in prestressed concrete members

Due to the inherent nature of concrete shrinkage, creep and relaxation of prestressing strands, the prestress in prestressed concrete structures decreases continuously with time, i.e., there is an apparent time-dependent prestress loss phenomenon for prestressed concrete structures. Therefore, it is an important issue for the estimation of time-dependent prestress losses in the design of prestressed concrete structures. Many current design codes provide relevant equations for the evaluation of prestress losses. For the prestressed concrete structure, in addition to the prestressing strands, a great amount of ordinary reinforcing steel is configured to enhance its load-carrying capacity. It is generally considered that no stress relaxation occurs for ordinary reinforcing steel due to its lower stress. However, the presence of ordinary reinforcing steel affects the stress distribution in the structure and thus has an obvious impact on the prestress losses, which is not considered in the prediction equations in most current codes for the design of concrete structures up to date. In this study, based on the cross-sectional analysis of prestressed concrete members that combines the age-adjusted effective modulus method of concrete creep analysis, a general equation for estimation of the time-dependent prestress losses in prestressed concrete members considering the influence of ordinary reinforcing steel is derived and simplified. When the influence of ordinary reinforcing steel is ignored, i.e., the percentage of ordinary reinforcing steel is taken as zero, the proposed equation reduces to those in Eurocode EN 1992-1-1:2004 and AASHTO LRFD 2020. In the end, an example of a T-shaped prestressed concrete girder is provided to show the application of the proposed equation for the evaluation of time-dependent prestress losses and the error resulting from the simplification of the equation. The results show that ordinary reinforcing steel has a significant effect on the time-dependent prestress losses in prestressed concrete members for the commonly used percentage of ordinary reinforcing steel, and the error arising from the simplification of the first derived equation is quite small.

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.

Novel method for identifying residual prestress force in simply supported concrete girder-bridges

Testing methods are required for estimating prestress losses in Prestressed Concrete (PC) girder-bridges. They mainly include destructive approaches which cause significant damages. Conversely, dynamic nondestructive methods are unsuitable. Given these findings, a novel method for identifying residual prestress force in simply supported PC girder-bridges was implemented. Following the vertical load application in a three-point bending, the method estimates the prestress force by measuring the vertical deflection at a quarter or, alternatively, at the midspan of the PC girder-bridge. The method also requires information regarding its flexural rigidity. Particularly, the initial tangent Young’s modulus must be evaluated by compression tests on cores drilled at its quarter and midspan cross-sections after three-point bending. In absence of the geometric and/or material properties, the flexural rigidity can be estimated according to free vibrations. Secondly, the method comprises a reference solution, or a finite element model of the PC girder-bridge, in which the prestress force is unknown. Thirdly, the measured deflection becomes a parameter of the prestress force identification process. Accurate identifications are obtained when the deflection, under a higher vertical load, was precisely measured and the flexural rigidity was determined using reference solution and initial tangent Young’s modulus. In this article, the novel method was simulated on a simply supported PC beam-bridge subjected to time-dependent prestress losses for ≈9.5 months in the laboratory.

Effect of the chloride environmental exposure on the flexural performance of strengthened RC beams with self-anchored prestressed CFRP plates

Carbon fibre-reinforced polymer (CFRP) plates are widely used to strengthen reinforced-concrete (RC) structures. At present, most studies focused on the short-term performance of strengthened RC beams with a prestressed CFRP plate, and very limited information is available on their durability, which is of importance in certain cases, such as high-piled wharfs and bridges in marine environments. This study investigates the effects of chloride environment exposure on the flexural performance of strengthened RC beams with self-anchored prestressed CFRP plates. In total, seven RC beams, four of which were strengthened by a self-anchored prestressed CFRP plate, were fabricated and tested under three-point bending. The testing parameters included 25% and 40% prestress level of the ultimate tensile strength of the CFRP plate, 5% and 10% tensile steel rebar corrosion ratio of mass loss and 10% and 20% anchor corrosion ratio in the chloride environment. The evolution of time-dependent prestress losses in the strengthened beam was determined, and the flexural behaviour of the strengthened beams was analysed. A theoretical model is proposed to predict the flexural strength of prestressed CFRP strengthened RC beams in a chloride environment. The experimental results indicated that the coupling of the corrosion expansion of the metal anchor and rebar in the chloride environment had a large negative effect on the flexural performance of the beams strengthened by the self-anchored prestressed CFRP plates. After exposure to the chloride ion solution, the failure mode of the RC beams strengthened by the prestressed CFRP changed from concrete crushing in the compression section to anchor pullout. The proposed model, which considered the effect of a chloride environment to predict the flexural strength of the strengthened beams, agreed well with the test results.

RC beams strengthened by prestressed CFRP plate subjected to sustained loading and continuous wetting condition: Time-dependent prestress loss

Prestress loss of CFRP during prestressing and service stage is one of the critical technical issues in concrete structures rehabilitation. In this work, the prestress loss of reinforced concrete (RC) beams strengthened by a prestressed CFRP plate subjected to sustained loading and continuous wetting condition were experimentally studied. Eight RC beams strengthened by prestressed CFRP plate were prepared with different prestress levels of 20% and 40% respectively. The initial prestress loss was monitored up to 28 days and the loss ratios of all specimens were around 5%, which was dominated by slippage of the end anchorage system. Then, the specimens were sustainedly loaded at two levels under wet or dried environments for 170 days. The average prestress loss ratios during this period were all less than 2%, which indicates that the CFRP plate end anchorage system had good long-term performance. Moreover, the adhesive bonding was weakened by the moisture penetration, and the specimens with lower prestress levels were more susceptible to the wet condition. Nevertheless, regardless of the environmental exposure, the maximum crack widths of 40% prestressed specimens remained less than 0.2 mm even subjected to high sustained load level, which is within the limitation of the prestressed concrete structures.

Durability of flexurally strengthened RC beams with prestressed CFRP sheet under wet–dry cycling in a chloride-containing environment

Using prestressed carbon fibre reinforced polymer (CFRP) to strengthen reinforced concrete (RC) members is currently considered a promising technology. This study aims to investigate the durability of RC beams externally strengthened with prestressed CFRP sheet in a chloride-containing environment. The prestressed load levels were set at 7.5%, and 15% of the ultimate tensile strength of the CFRP sheet at room temperature (~26 °C), and the test parameters included an inclined U-jacket (or not) and 90-day exposure to a wet–dry cyclic environment. The wet–dry cyclic condition was set as 8-h immersion in 3.5% NaCl solution at 40 °C and 16-h drying at 25 °C and 60% RH (relative humidity). After exposure to the wet–dry environment, the prestressed CFRP sheet strengthened RC beams were tested under four-point bending test. The evolution of time-dependent prestress losses in the strengthened beam was discussed, and then the flexural behaviour of the strengthened beams was analyzed. Finally, a theoretical model based on the classical beam theory was proposed to predict the bearing capacity of prestressed-CFRP strengthened RC beams. The results show that the prestressed CFRP sheet can significantly improve the flexural performance of RC beams, and the cracking and ultimate loads of the RC beam strengthened with a prestressing level of 15% could increase by approximately 50% and 40%, respectively. Moreover, the inclined U-jacket enhanced the flexural performance of the strengthened RC beams and changed the failure mode from CFRP debonding to CFRP fracture. Although the exposure in chloride-containing environments increased the prestress losses, a 90-day exposure in the simulated subtropical marine climate environment did not cause a significant detrimental influence on the flexural performance of the strengthened RC beams. The proposed model agrees well with the experimental results, indicating it could conservatively predict the bearing capacity of strengthened RC beams failed by CFRP fracture.

Experimental investigation on long-term behavior of prestressed concrete beams under coupled effect of sustained load and corrosion

Prestressed concrete beams are widely used in construction, while they may be attacked by the corrosion medium during the service. Past research works show that coupled effect of high stress and corrosion can significantly deteriorate the performance of prestressed concrete structures. This article presents an experimental investigation on long-term behavior of prestressed concrete beams under the coupled effect of sustained load and corrosion. During the accelerated corrosion process, six prestressed concrete beams were subjected to different levels of sustained loads, and time-dependent prestress loss and concrete stains were recorded and analyzed. It is observed that the local corrosion (i.e. pitting corrosion) of the steel strands was significantly affected by the sustained load level, and the coupled effect led to more serious damage on the beams than individual sustained load or corrosion. Bending loads were finally applied to investigate the influence of corrosion on flexural capacities of these beams. It was found that the flexural capacities and failure mode of the beams were significantly influenced by corrosion. Meanwhile, the corrosion has more significant effect on tension strength of strands rather than bond strength.

Analysis of flexural concrete beams prestressed with basalt composite bars. Analytical-experimental approach

This present work describes the complex analysis of flexural concrete members prestressed with basalt fiber reinforced polymer (BFRP) bars considering consecutive phases of the loading applied. The aim of this research was to propose analytical approach for the prediction of effective prestress force that directly influence the serviceability limit state of prestressed concrete members, in particular to deflections. Proposed method considering prestress losses for deflection estimation is based on the concept of prestressed concrete laws within particular assumptions. The method is based on strain and curvature analysis, effective moment of inertia including decompression effect and early aging effects of both the concrete and BFRP. The instantaneous and time-dependent prestress losses, cross-sectional stress-strain, deflection and cracking behavior of concrete members reinforced with BFRP bars are described as an interrelated process. This complex approach allows to avoid disappearing of prestressing effect, such as camber or strain of decompression, as a base point and the principle of serviceability analysis of prestressed concrete members. Also, an experimental program was expanded with the purpose to evaluate the influence of prestress level for flexural stiffness of the real scale concrete beams prestressed with BFRP bars.

Evaluating prestress losses in a prestressed concrete girder railway bridge using distributed and discrete fibre optic sensors

This paper evaluates the data collected during a comprehensive monitoring campaign aimed at capturing the prestress loss behaviour of four 11.9 m prestressed concrete beams. These beams formed part of the superstructure of a newly-constructed railway bridge in Staffordshire, U.K. Two types of prestressed concrete beams were monitored, two TY7 internal beams and two TYE7 edge beams. Both distributed and discrete fibre optic sensor (FOS) systems were used to measure strain and temperature for the first two and a half years since the beams were cast. Prestress loss mechanisms were investigated in detail including immediate prestress losses due to elastic shortening of concrete and time-dependent prestress losses due to steel relaxation, concrete shrinkage and creep. Prestress loss predictions were calculated using both European and American standards, which were then compared with measured prestress losses. Both simplified and advanced time-step methods were used to provide more refined loss predictions by taking into account the interrelationships between various prestress loss mechanisms and the total prestress force at the time of interest. To provide better interpretation of the monitoring measurements, a sensitivity analysis was performed to evaluate the effects of various input parameter uncertainties on prestress loss predictions. It was found that (i) time-step methods produced prestress loss estimates that were lower compared with the simplified method; and (ii) code estimates of prestress losses using measured material properties gave reasonable agreement with the field measurements. As structurally-integrated FOS systems are becoming more commonplace and hold great potential, it is envisaged that they enable better understanding of field performance and thus facilitate data-informed asset management.

Refined calculation of time-dependent prestress losses in prestressed concrete girders

Considering the interaction between concrete shrinkage and concrete creep and between the relaxation of prestressing steel and concrete creep in the prestressed concrete member, a refined method for estimating the time-dependent prestress losses is developed. Using the integral mean value theorem, the integrations regarding the coupling terms in the derived equation are transformed into a one-step calculation. Through step-by-step numerical analysis, the median coefficients and their simplified expressions as a function of loaded age and age of observation are established based on the models in fib MC 2010 and AASHTO-LRFD 2014. A case study of the time-dependent prestress losses in a simply supported girder with a T-shaped cross-section is conducted. The effectiveness of the proposed method is verified by performing a comparison with the numerical results obtained by the step-by-step method which is regarded as a relatively exact solution and compared with the results obtained by the methods recommended in EN 1992-1-1:2004 and AASHTO-LRFD 2014. It is demonstrated that the proposed method achieves good accuracy compared to the results of the step-by-step numerical method.

EVALUATION OF TIME DEPENDENT PRESTRESS LOSSES IN PRETENSIONED CONCRETE MEMBER WITH TOP AND BOTTOM TENDONS

Abstract Prestress losses are usually calculated by bringing the tendons in the cross-section to the single resultant tendon. This solution is correct only for tendons concentrated in one part of the cross-section, while with their wider distribution, for example after the use of upper tendons, it may result in underestimation of losses in the lower tendons and overestimation in the upper tendons. The paper presents formulae to determine time dependent prestress losses separately for the top and bottom tendons of a pretensioned concrete member. Furthermore, variations of stress in the prestressing steel are analysed for two pretensioned concrete members to evaluate the possibility of using the equation (5.46) from the Eurocode 2 for the resultant tendon instead of the deduced formulae. The aim of this study is to show how great the error of estimated loss of prestressing force on the basis of several typical prestressed concrete sections can be.

Simple model for time-dependent bond transfer in pretensioned concrete using draw-in data

Time evolution of prestress loss and bond transfer length holds vital information concerning long-term performance of pretensioned prestressed concrete. In this paper, it is proposed to utilize long-term measurements of strand draw-in to extract this time-varying information, which could be more effectively obtained than concrete or strand surface strain measurements. Theoretical investigations combined with numerical studies of experimental data are carried out for this purpose. This study builds on Guyon’s boundary value problem (BVP) model which quantifies draw-in for instantaneous elastic response. A one-dimensional linear viscoelastic standard solid model is employed to model the creep of concrete, which is a simple but effective model of the time-dependent response of concrete. Guyon’s BVP model is generalized to include time dependence and then combined with an existing initial value problem (IVP) model for post-tensioned concrete leading to a new mixed model for time-dependent prestress loss and bond transfer in pretensioned concrete.Analysis is undertaken in this study to (i) quantify long-term prestress loss and bond-transfer behaviors by applying the proposed model and directly utilizing measured draw-in time history data, (ii) validate the proposed quantitative analysis by using scaled pretensioned concrete beams with different types of aggregates and strengths of concrete - among other factors, and (iii) offer discussion of future work. For example, introducing concrete drying shrinkage into the proposed simple model seems promising and is necessary to provide an improved representation of the time-dependent behavior of prestressed concrete.

Monitoring and analysis of long-term prestress losses in post-tensioned concrete beams

In order to investigate the long-term prestress losses in post-tensioned concrete beams, time-dependent responses of eight post-tensioned concrete beams were monitored for more than one year. Load cells, vibrating wire strain gauges (VWSGs) and elasto-magnetic (EM) sensors were used jointly to measure the tendon forces at beam ends, the prestress losses due to concrete creep and shrinkage and the total prestress losses along tendons, respectively. In this way, the prestress losses due to different factors and at different locations were observed more accurately. Based on the monitored data, the influence of tendon layout, reinforcement ratio, duct grouting, stress level and jacking sequence, etc., on long-term prestress losses were analyzed. An improved model for predicting the time-dependent prestress losses was thus proposed, in which non-prestressed steel and the interaction among the shrinkage, creep of concrete and steel stress relaxation were taken into account. The proposed model has higher accuracy as compared with existing models in specifications, and the maximum difference between test and predicted results is within 10%.

Time-Dependent Prestress Loss Behavior of Girders in Missouri Bridge A7957 Compared with a U.S. Data Set of High Performance Concrete Bridge Girders

Time-Dependent Prestress Loss Behavior of Girders in Missouri Bridge A7957 Compared with a U.S. Data Set of High Performance Concrete Bridge Girders

Prestress Loss of a New Vertical Prestressing Anchorage System on Concrete Box-Girder Webs

A new, double-tensioned prestressing steel strands anchorage system (DTPSSAS), characterized by low prestress loss caused by prestressing tendon retraction, is described. When it is used for short prestressing tendons in concrete box-girder webs, this anchorage system significantly reduces prestress loss. To investigate its efficiency in reducing prestress loss, a rectangular thin plate using the DTPSSAS was devised and tested; the results demonstrated that the average rate of instantaneous prestress loss decreased from 32.35% after the first tension to 2.68% after the second tension. Further, long-term observations of prestress loss with the DTPSSAS were conducted in the laboratory and in a field test. Based on the field test of a continuous concrete box-girder bridge, the average loss rate of instantaneous prestress loss to the control tensile stress of two strands in the web is 7.61%, and the average rate of prestress loss at 150 days is 10.53%. Concurrently, the time-dependent prestress loss was calculated with Models GL2000 and CEB-FIP90 for different concrete shrinkage and creep prediction by the step-by-step calculation method. In comparison with the laboratory test results, the calculated results with the two models were greater than the test values. In addition, the effect of stress redistribution caused by the constraint of reinforcing bars to concrete shrinkage and creep was evaluated. The results revealed that the stress redistribution effect increased the vertical compressive prestress loss of concrete, indicating that the time-dependent compressive prestress loss rate of concrete was greater than the tensile prestress loss rate of prestressed tendons. In this regard, bridge engineers may need to pay attention to the stress redistribution phenomenon in vertical prestressing design.

Time-Dependent Prestress Losses in Historic Clay Brick Masonry Walls Seismically Strengthened Using Unbonded Posttensioning

AbstractTime-dependent prestress losses in historic unreinforced clay brick masonry (URM) walls strengthened using unbonded posttensioning were investigated, with a particular emphasis on masonry shortening resulting from creep and shrinkage. An experimental program was undertaken that involved continuous monitoring of masonry shortening occurring in prestressed URM wallettes over a period of 180 days. The test wallettes were extracted from a real historic URM building and were subjected to varying magnitudes of prestress, representing axial stresses that would be developed at the wall base when strengthened using unbonded posttensioning. A rheological model is proposed for predicting masonry creep shortening, which can subsequently be used to predict posttensioning losses. It was established that a prestress loss of up to 16.4% for normal threaded steel bars and up to 5.4% for sheathed greased seven wire strands can be expected in posttensioned historic URM walls when the tendons are posttensioned to a s...