Long-term Prestress Loss Research Articles

Probabilistic Analysis of Prestress Loss due to Creep in Concrete Box Girders

The cross-section warping effect and warping-induced long-term prestress loss due to creep are investigated in this paper. Both phenomena were analyzed deterministically and stochastically. The random character of concrete creep and environmental factors have substantial effects on the statistical scatter of the total prestress loss. To obtain the real development of concrete long-term performance, sophisticated models for concrete creep and shrinkage prediction were used. The Monte Carlo–Latin hypercube sampling method was applied to capture the statistical variability of boundary and environmental conditions. It is shown that the severity of cross-section warping can be important, particularly in the case of short spans and unevenly distributed and isolated tendons in a cross section. The three-dimensional effects and the statistical variability of creep are considered to enable reliable design of box girder bridges.

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%.

Experimental investigation on long term flexural performance of expansive concrete beams eccentrically reinforced by CFRP

Previous research has shown that unsymmetrical carbon fiber-reinforced polymer (CFRP) wrapped expansive concrete composite beams performed efficiently and effectively on delaying cracking and eliminating corrosion. However, it is crucial to explore the effect of self-prestressing on beams flexural performance after long-term prestress loss. In this research, fourteen composite beam specimens and four control specimens were designed. A series of tests, including material test, long-term static strain monitoring of beam specimens and four-point bending test, were conducted. Test results indicate that, in long-term, at the same layout of CFRP, SHCC specimens presented better performance than PCC on delaying cracking, controlling crack width of concrete and enhancing ultimate load capacity. The flexural capacity of the beam specimens increased with the increase of reinforcement ratio of CFRP. Furthermore, eccentric layout beam specimens presented superior performance than symmetric layout on the flexural behavior. It also indicated that the calculated results agree well with the experimental data, which can provide references for engineering practice.

Prediction model of long-term prestress loss interaction for prestressed concrete containment vessels

Conventional methods disregard the interaction effects between concrete creep and tendon relaxation to evaluate the long-term prestressed losses. The main contribution of this paper is to present a new method to compute this interaction for prestressed concrete containment vessels. A time-dependent numerical study is conducted to compare long-term prestress losses in different prediction models. A prestress loss test program is conducted to validate the numerical analysis method for a concrete containment vessel. To perform a more comprehensive numerical study, various variables such as radius to water height ratios (R/H), tendon material (improved or normal relaxation), and different constant temperature conditions are considered in the analysis models. The obtained analytical results are categorized into three classes: creep and shrinkage losses, stress relaxation losses and simultaneous long-term losses. The analytical results show that the (R/H) ratios have no significant effects on long-term prestress loss. Nevertheless, the other investigated variables have impressive effects on the long-term loss evaluation. The analytical results are used to calculate the interaction coefficients, which demonstrate the interaction between concrete creep losses and tendon relaxation. Eventually, the interaction coefficient data are expressed as a function of stress relaxation loss to creep and shrinkage loss ratio.

Time-dependent behavior of concrete beams prestressed with bonded AFRP tendons

Abstract This paper presents a numerical study on the long-term performance of bonded aramid fiber reinforced polymer (AFRP) prestressed concrete beams. A computer model capable of predicting the service load long-term response of prestressed concrete beams with AFRP and steel tendons is developed. Model predictions agree well with the results of long-term tests on prestressed concrete beams. The time-dependent behavior of AFRP prestressed concrete beams is evaluated and the results are compared with those of steel ones. It is indicated that, when tendon relaxation is considered, AFRP tendons register a higher long-term prestress loss than steel tendons although the prestress loss contributed by concrete creep and shrinkage is significantly lower in AFRP tendons than in steel ones. As a consequence, AFRP tendons mobilize a lower prestressing camber and a higher downward load deflection compared to steel tendons. The analysis also shows that the influence of bottom nonprestressed steel on the time-dependent performance can be important or not so important, depending on the level of loading.

Experimental Investigation of Prestress Losses in Full-Scale Bridge Girders

An experimental study was conducted in which 30 full-scale, precast, pretensioned bridge girders were constructed and instrumented with the intention of investigating prestress loss. Several different precast beam fabrication plants were used to investigate the influence of different concrete materials and construction techniques. The constructed girders were conditioned in several different climates for up to 3 years. During this period, prestress loss was measured by using vibrating wire gauges (VWG) embedded in test specimens. Following the conditioning period, the girders were flexural service load-tested to quantify the prestress loss at the time of testing and in turn verify the losses measured using VWGs. Prestress losses were found to be heavily influenced by concrete stiffness, which was dependent on coarse aggregate type and quantity. The measured short- And long-term prestress losses were compared to those determined using several different estimation procedures, suggested by ACI Committee 423.

Long-Term Prestress Loss and Camber of Box Girder Bridge

This paper presents results regarding the effect of elevated temperature (ET) curing on the compressive strength and electrical resistivity of concrete with slow-reacting supplementary cementitious materials (SCMs). Concrete mixtures with Class F fly ash (FA), ground-granulated blast furnace slag (GGBS), or both were tested. Elevated temperature (ET) curing (in a 36°C [97°F] lime water bath) and room temperature (RT) curing (in a 21°C [70°F] lime water bath) with different durations were investigated. Test results show that the ET curing method can be employed to achieve the specified 28-day compressive strength and resistivity values on concrete with slow-reacting SCMs without compromising the long-term strength and resistivity properties. It is also shown that the use of granite as coarse aggregate as an alternative to limestone can increase concrete’s electrical resistivity but has a modest adverse effect on the compressive strength of not fully hydrated concrete.

Long-Term Prestress Loss and Camber of Box-Girder Bridge

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Estimation of Long-Term Prestress Losses in Post-Tensioned Girders

Post-tensioned concrete girders are commonly used in bridge structures. Long-term prestress losses due to the creep and shrinkage of concrete in post-tensioned girders are smaller than those in pretensioned girders because of the higher amount of non-prestressed steel present in the former, and also the fact that post-tensioning can take place a long while after a girder has been cast and when the concrete has reached a mature age. Two methods that can be used to estimate long-term prestressed losses in post-tensioned girders are presented in this paper. One is a refined method and the other is a simplified method. The accuracy of the methods has been verified by field data collected from two bridge structures monitored for long-term losses. A numerical parametric study conducted with these methods has shown that the concrete strength, the amount of non-prestressed steel, the relative humidity, and the age of concrete at post-tensioning are equally important for the assessment of prestress losses. Because the post-tensioning of a bridge girder in a long multi-span bridge can take place at a concrete age between 100 and 200 days, when an appreciable amount of shrinkage has already occurred, an accurate account of the incremental shrinkage strain from the time of post-tensioning is as important as that of the ultimate shrinkage strain. The calculation of this incremental strain relies on an accurate equation to predict the increase of shrinkage strain with time over the entire shrinkage period.

Prediction of prestress losses in RC beams externally strengthened with prestressed CFRP sheets/plates

This paper presents an analytical model for predicting the short and long-term prestress losses in reinforced concrete (RC) beams strengthened with prestressed carbon fiber reinforced polymer (CFRP) sheets/plates. The long-term prestress loss model is developed by treating the strengthened beam as two sub-elements: an RC beam and a prestressed CFRP sheet/plate, connected through an adhesive bonding layer. Through force equilibrium and displacement compatibility of the two sub-elements, the influences of concrete shrinkage, concrete creep as well the interlayer slip between the RC beam and the CFRP sheet/plate on the prestress losses of CFRP are modeled using an incremental model. For a post-tensioned system with strong end anchorages, closed-form solutions for the prestress losses due to creep and shrinkage of concrete are obtained. The reliability of the proposed analytical model is validated through comparisons with previous test results reported by the authors and others research groups. The maximum difference between the experimentally observed values and the predicted values for the long-term prestress losses was found to be only 2.2% of the initial prestress of the CFRP.

High stress monitoring of prestressing tendons in nuclear concrete vessels using fibre-optic sensors

Maintaining the structural health of prestressed concrete nuclear containments is a key element in ensuring nuclear reactors are capable of meeting their safety requirements. This paper discusses the attachment, fabrication and characterisation of optical fibre strain sensors suitable for the prestress monitoring of irradiated steel prestressing tendons. The all-metal fabrication and welding process allowed the instrumented strand to simultaneously monitor and apply stresses up to 1300MPa (80% of steel's ultimate tensile strength). There were no adverse effects to the strand's mechanical properties or integrity. After sensor relaxation through cyclic stress treatment, strain transfer between the optical fibre sensors and the strand remained at 69%. The fibre strain sensors could also withstand the non-axial forces induced as the strand was deflected around a 4.5m bend radius. Further development of this technology has the potential to augment current prestress monitoring practices, allowing distributed measurements of short- and long-term prestress losses in nuclear prestressed-concrete vessels.

Prestress Losses and Flexural Behavior of Reinforced Concrete Beams Strengthened with Posttensioned CFRP Sheets

An experimental study was conducted to investigate the flexural behavior and long-term prestress losses of reinforced concrete (RC) beams strengthened with posttensioned carbon fiber-reinforced polymer (CFRP) sheets. The experimental program consisted of tensile tests of flat CFRP coupons under sustained loads and flexural tests of a total of eight RC beams: six strengthened with posttensioned CFRP sheets, one strengthened with nonprestressed CFRP sheets, and one control beam. The main objective of the tests was to gain a better understanding of the long-term prestress losses of CFRP sheets in the posttensioned system under different prestress levels and strengthening ratios. It is shown that the prestress losses of CFRP sheets in the posttension system are mainly attributable to anchorage set (approximately 12.6 to 18.2% of the initial prestress), whereas the time-dependent losses caused by creep and shrinkage of concrete and relaxation of CFRP sheets are relatively small (approximately 2.3 to 3.9% of the initial prestress).

Estimating prestress loss in pretensioned, high-strength concrete members

This paper presents research conducted under the National Cooperative Highway Research Program (NCHRP) project 18-07, Prestress Losses in Pretensioned High-Strength Concrete Bridge Girders. The purpose of this project was to extend the American Association of State and Highway Transportation Officials' AASHTO LRFD Bridge Design Specification provisions for estimating prestress losses to cover concrete strengths up to 15 ksi (104 MPa). This paper presents the portion of the work that deals with methods of estimating long-term prestress loss. The results reported in this paper were adopted by AASHTO and included in the 2005 and 2006 interim revisions and in the fourth edition of the LFRD specifications, which was published in 2007. This paper explains the theory of time-dependent analysis. It follows the pseudoelastic, age-adjusted, effective-modulus method. The experimental component consisted of materials properties, which are covered in a companion paper, and prestress loss measurements, which are highlighted in this paper. The measurements were taken in seven girders in bridges in Nebraska, New Hampshire, Texas, and Washington to encompass the regional diversity of environmental and materials properties throughout the country. Theory was also compared with experimental data reported in the literature on 31 pretensioned girders in 7 states.

Prediction of Long-Term Prestress Losses

This paper presents an analytical method to predict long-term prestress losses in precast, pretensioned, or posttensioned concrete members. Design aids are presented for cast-in-place, posttensioned concrete bridges. The proposed method is based on equilibrium and compatibility principles of solid mechanics and can be used for multistage loading and prestressing. In deriving some of the equations, however, it is assumed that prestressing and dead loads are simultaneously applied and that long-term effects of concrete creep, shrinkage, and relaxation of prestressing steel occur gradually thereafter. Empirical equations in current bridge codes either underestimate or overestimate long-term prestress losses in concrete members, depending on the concrete creep and shrinkage properties and on the nonprestressed steel ratios. Examples show that the presence of nonprestressed steel has significant effects on the long-term deformation and compressive stress remaining in the concrete after prestress losses.

An approach to determine long-term behavior of concrete members prestressed with FRP tendons

The combined effects of creep and shrinkage of concrete and relaxation of prestressing tendons cause gradual changes in the stresses in both concrete and prestressing tendons. A simple method is presented to calculate the long-term prestress loss and the long-term change in concrete stresses in continuous prestressed concrete members with either carbon fiber reinforced polymer (CFRP) or aramid fiber reinforced polymer (AFRP) tendons. The method satisfies the requirements of equilibrium and compatibility and avoids the use of any empirical multipliers. A simple graph is proposed to evaluate the reduced relaxation in AFRP tendons. It is shown that the prestress loss in FRP tendons is significantly less than that when using prestressing steel, mainly because of the lower moduli of elasticity of FRP tendons. The long-term changes in concrete stresses and deflection can be either smaller or greater than those of comparable girders prestressed with steel tendons, depending on the type of FRP tendons and the initial stress profile of the cross-section under consideration.

Structural Modeling of Posttensioned Members

This state-of-the-art paper offers a comprehensive overview of the modeling techniques used for the analysis of posttensioned structures. The merits and limitations of each of the modeling schemes are discussed within a consistent and comparative framework. Several numerical examples are used to illustrate the tacit features of the models. The balanced loading, primary actions, hyperstatic (secondary) actions, and prestressing moment concepts commonly used in the analysis of prestressed structures are revisited and clarified. The work concludes with an example of the most recent modeling technology—the discrete modeling of tendons. The example illustrates the calculation of long-term prestress losses as an integral part of the analysis as opposed to the traditional approach where long-term losses are computed independently from the solution.