Pretensioned Concrete Members Research Articles

Verification of Composite Beam Theory with Finite Element Model for Pretensioned Concrete Members with Prestressing FRP Tendons.

Composite beam theory was previously developed to establish an analytical solution for determining the transfer length of prestressed fiber-reinforced polymers (FRP) tendons in pretensioned concrete members. In the present study, a novel finite element (FE) modeling approach is proposed to provide further verification of the developed analytical method. The present FE model takes into account the friction coefficients obtained from pull-out tests on the FRP tendons and prestressed concrete members. Convergence analysis of two numerical simulations with different mesh densities is carried out as well. The results demonstrated that the transfer length predicted by the fine FE model with a friction coefficient of α = 0.3 for high pretension is in good agreement with the measured values and the analytical solutions. The consistency between the analytical solution and FE simulation not only further proves the reliability of composite beam theory but also demonstrates the importance of the bond-slip relationship in predicting the transfer length of pretensioned concrete members prestressed with FRP tendons.

Probabilistic investigation of transmission length formulations considering model uncertainties

Transmission length is a key parameter for detailing and assessing shear and anchorage resistance of pre-tensioned concrete members, and according to the main design codes, semi-empirical models are adopted to estimate its value. In these approaches, model uncertainty may affect the results and often plays an important role in achieving the desired reliability. The purpose of this study is to investigate the current code design formulations for the transmission length described in the upcoming Eurocode2:2020 and fib Model Code 2020, and to propose new coefficients to consider the model uncertainties. To this end, a database with 951 experimental measures of transmission length was collected from the existing literature and properly filtered to obtain a homogeneous sample of 561 experimental observations on which model formulations were applied. The uncertainties of the two models were estimated and suitable probability models were derived. Results show that the prestress force release methodology strongly affects the uncertainties, both in terms of mean value and dispersion of the prediction. Finally, new probabilistic coefficients were proposed to adjust the formulations and ensure the achievement of the target reliability length to be adopted in each design situation, both at the serviceability and ultimate limit states.

Effect of Water Condensate on Corrosion of Wires in Ungrouted Ducts.

In the case of existing prestressed concrete structures, information about the actual state of prestressing is an important basis for determining their load-carrying capacity, as well as remaining service lifetime. This is even more important in the case of existing prestressed concrete bridges, which are exposed to a more aggressive environment than the other prestressed concrete structures. The level of prestressing is affected and reduced by prestress losses at a given time. In calculating the internal forces and stresses, required for the assessment of the Ultimate Limit State and the Serviceability Limit State, it is necessary to know not only the prestressing level but also the cross-sectional area of the prestressing steel (wire, strand or cable), which can change in time due to corrosion. In practice, in the case of the pre-tensioned concrete members, it has often happened in the past that cable ducts have been grouted only partially, or not at all, due to poor grouting technology. Experts did not realize what this could cause in the future—the penetration of water with aggressive agents directly into the cable duct and consequently corrosion of the prestressing steel, which means not increased protection of the steel, but rather acceleration of degradation. On the other hand, in many cases, corrosion also occurs in ducts that are not grouted and no water has entered them. This paper deals with this phenomenon—the formation of corrosion of prestressing steel in cable ducts in ungrouted ducts due to moisture. This problem was investigated experimentally and numerically in the simulation program ESP-r. Experimental measurements and numerical simulations have shown that the water vapor condenses in the cable ducts, which can subsequently cause corrosion of the prestressing steel.

Innovative additional aluminum alloy ribs anchorage for improving the bond reliability of pretensioned CFRP bar: A feasibility study

An innovative anchorage method for CFRP bars through providing additional aluminium alloy ribs (ARs for brevity) is proposed in this paper. The results of the pull-out tests are firstly summarized to comprehensively evaluate the bond performance of ARs anchored CFRP bars in concrete and the arrangement of the ARs along the bond length of the CFRP bars was meanwhile optimized. The transfer length test for the pretensioned CFRP bar was then performed to investigate the effect of the ARs on the bond transfer at prestress releasing. It was found that ARs anchored bar had resulted in a shorter transfer length in comparison to that of the unanchored bar due to an improved bond mechanism. Finally, the feasibility of using such ARs anchored CFRP bars in pretensioned prestressed concrete (PPC) members was explored by performing short- and long-term tests, respectively. The results showed that the ARs presence could lead to benefits of enhancing the shear anti-cracking capacity, reducing the end slips, and delaying the long-term deflection increase of the CFRP reinforced PPC beams, proving that the ARs can act as a reliable end anchorage system for pretensioned CFRP bars in PPC members.

The Influence of Wire Type Indentation on Longitudinal Splitting in Pre-Stressed Concrete

The important characteristic in the creation of longitudinal splitting cracks in pretensioned concrete members has found to be the geometry of the pre-stressing wire indents. Longitudinal splitting along prestressing tendons can result in severe splitting of prestressed member in the field under loading over time. The research evaluated the influence of wire type indentation on the longitudinal splitting in prestressed concrete members fabricated with different concrete mixtures and different compressive strength of concrete. A key objective was to find the best type of wire to avoid failures in the field. A study was conducted at Kansas State University to understand the effect of wire type on the longitudinal splitting between prestressing steel and prestressed concrete. Three different types of wires will be presented in this paper denoted as “WB”, “WF” and “WQ”. The wires have different parameters which include indent depth, indent width, indent sidewall angle, indent pitch and indent volume.

Suggested Qualification Test Procedure to Secure Splitting Resistance in Pretensioned Concrete Members

Prestressed concrete ties could develop end-splitting cracks along tendons due to lateral bursting stresses. The lateral bursting stresses can form due to Hoyer effect (change in diameter of the prestressing tendons due to Poisson’s ratio), the jacking force in the tendons, geometrical features and indent characteristics of the prestressing tendons. End-splitting cracks can occur immediately after de-tensioning procedure in some cases, but they also can be developed during the first weeks after de-tensioning procedure due to sustained lateral stresses exerted by the prestressing tendons. The ability of concrete to resist these bursting stresses without cracking is primarily the function of the thickness of concrete cover, the type of concrete mixture used and the maximum compressive strength of the concrete. Qualification test will be great tool for prestressed concrete tie manufacturers to identify tie designs that may be susceptible to end-splitting cracks. This test was formally adopted as section 4.2.4 in Chapter 30 of the 2021 AREMA Manual for Railway Engineering.

Cracking behavior of full-scale pre-tensioned prestressed concrete double-Tee members with steel-wire meshes

Prestressed concrete double-Tee (PCDT) members are widely used in short-to-medium span structures, such as bridges, parking structures, industrial plants and warehouses. To closely investigate the cracking behavior of PCDT members with steel-wire meshes, four full-scale specimens with different spans and amount of prestress tendons were tested under monotonic loading in this paper. The distribution, number and width of cracks were presented in detail. It could be found that distribution of cracks was dense and uniform, and crack widths of PCDT members were more scatter than ordinary reinforcement concrete (RC) members. Moreover, a comparative study of maximum crack widths predicted respectively by Eurocode 2 and Chinese codes was carried out. Results showed that maximum crack widths calculated by Eurocode 2 were a little bit conservative and relatively agreed well with test results. In addition, the Chinese codes underestimated the test results, especially when the stress of non-prestress steel was lower than 200 MPa, which would be unsafe for the design.

Measured transfer length of 15.2-mm (0.6-in.) duplex high-strength stainless steel strands in pretensioned girders

Duplex High-Strength Stainless Steel (HSSS) corrosion-resistant strands are desirable in concrete members in extremely aggressive environments to mitigate corrosion problems. Recently, HSSS strand was introduced to the construction industry in 15.2-mm- (0.6-in.-) diameter size. To practically use the 15.2-mm- (0.6-in.-) diameter HSSS strand in pretensioned concrete members, its transfer length needs to be evaluated using full-scale members, and the applicability of the prediction models in design provisions requires examination. A total of eight 12.8-m- (42-ft-) long AASHTO Type II girders were fabricated using the long-line method. Five girders were prestressed with HSSS strands while the other three were prestressed with carbon steel strands. The initial prestressing stress was 1102 MPa (159.8 ksi) for HSSS strand and 1396 MPa (202.5 ksi) for carbon steel strand. Three different types of confinement reinforcement were used: carbon steel, glass fiber reinforced polymer (GFRP) and duplex stainless steel. The effects of type and spacing of confinement reinforcement on transfer length were evaluated. The effect of the strand release sequence among multiple girders’ ends in the casting bed on transfer length was investigated. Predictive models from all evaluated design provisions were conservative, with different accuracy, in estimating the transfer lengths of HSSS and carbon steel strands. Eurocode 2 was relatively the most accurate provision among the evaluated design provisions, while AASHTO LRFD, which had the simplest equation (60db), was the most conservative provision. Because HSSS strands are relatively new, there are no design guidelines available for their use in pretensioned concrete members. Therefore, findings from this experimental study will be helpful in the development of design guidelines.

Genetic programming approach and data generation for transfer lengths in pretensioned concrete members

This study aims to derive a practical equation that can predict the transfer length of prestressing strands with the use of genetic programming. Towards this end, a total of 260 transfer length test results were collected from previous studies, and a feature selection procedure was applied to the collected database to extract the key features influencing the transfer length. Based on the five most important features, a practical equation was derived using a genetic programming approach, and the rationality of the proposed equation was verified by comparing it with design codes, existing models, and machine learning models (random forest and artificial neural network). In addition, 1.0 × 104 fake transfer length data that follow the probability distribution of the real data were generated using a generative adversarial network, based on which the prediction performances were visualized and compared in detail. The results showed that the proposed equation exhibited a higher level of accuracy than other existing equations.

Bond model of 15·2 mm strand with consideration of concrete creep and shrinkage

The bond between prestressing strands and concrete in the transfer zone of pretensioned concrete members is a complicated mechanism. Concrete creep and shrinkage are dominant time-dependent factors that affect the strand bond. In this study, a semi-analytical bond stress–slip model was developed, based on test results of the standard test for strand bond for 15·2 mm strands. Effects of concrete creep and shrinkage are incorporated in the model. Measured transfer lengths collected from the literature are compared with predicted values, to validate the accuracy of the bond stress–slip model. Analytical results indicate that the strand bond decreases over time, and is almost constant after 360 d of age. An equation is proposed to predict the transfer length of the prestressing strand at 28 d, with an incorporation of concrete creep and shrinkage effects.

Prediction of splitting crack growth in prestressed concrete members using fracture toughness and concrete mix design

In this study, the effect of concrete mix design on end-splitting cracks in prestressed concrete members was examined. The concrete material needs to be resistant to splitting-crack growth at the strengths at which the pretensioned member is detensioned. Mixtures with variable coarse aggregate volume, angularity, and water-to-cementitious ratio were designed and Fracture toughness tests on 4 three-point bending concrete specimens for each mix were carried out. Next, pretensioned prisms with three different reinforcement edge distances 0.750, 0.625 and 0.500 in. (19 mm, 16 mm and 13 mm) were fabricated and detensioned when the concrete compressive strength reached 4500 psi (31 MPa). Splitting cracks were quantitatively evaluated by direct measurement. The results indicated a strong correlation between crack length measurements and fracture toughness. Additionally, a linear regression model was developed by which splitting-crack growth in prestressed cross-sections with varied edge-distance thickness can be predicted. The findings from this study could lead to a significant improvement in splitting crack resistance in pretensioned concrete members where the high amount of transverse stress is often introduced at low compressive strengths.

Effect of compressive strength of concrete on transmission length of pre-tensioned concrete systems

This paper presents an experimental study on Lt in pre-tensioned concrete (PTC) members made of four values of compressive strength of concrete. Also, it presents a comparison between the formulations of transmission length (Lt) given in various codes and other literature. A test specimen consisted of a concrete prism (100 × 100 × 2000) mm with a prestressed seven–wire strand (12.7 mm diameter) at the center. The values of average compressive strength for concrete at transfer (fci) were 23, 28, 36, and 43 MPa. The challenges involved with the measurements of DEMEC readings, difference between the readings from surface–mounted discs and the inserts are discussed. The results indicate that Lt could decrease by 33% when the fci increases from about 23 to 43 MPa. Based on this data, a new bi-linear formulation to determine the Lt as a function of fci is proposed. Further, it is shown that higher estimates of Lt as per the available formulations, will lead to lower estimate of bursting tensile stress in concrete generated during transfer. Hence, a precise estimate of Lt as a function of the strength of concrete at transfer is expected to provide more rational design of transmission zone reinforcement.

Transmission length of pretensioned concrete systems – comparison of codes and test data

This paper presents a comparative study of the transmission length (Lt) of pretensioned concrete (PTC) members obtained using experiments and AASHTO, ACI, BIS, Eurocode 2 and fib Model Code (fib-MC) codes. Tests on six PTC specimens (with 12·7 mm dia. strands (ds) and effective prestress (fpe) of 1214 MPa) indicated that Lt can reduce by about 20% when the concrete strength (fci) increases from about 23 to 36 MPa. A case study of the influence of Lt on the shear resistance of a typical precast hollow-core slab showed that the ACI and fib-MC codes provide conservative shear designs, whereas the IS code considers only ds and provides a less conservative design. This paper proposes a revised tentative formula of Lt (as a function of ds, fpe and fci) to be incorporated in the IS 1343 and other codes. It is anticipated that this will lead to more rational and conservative shear designs.

Effect of concrete mixture components, proportioning, and compressive strength on fracture parameters

Splitting cracks in pre-tensioned concrete members often occur at the early stages after detensioning. To prevent these splitting failures, concrete members need to be resistant to crack growth at the strengths in which the beam is detensioned. In this paper, the effect of different concrete parameters including aggregate shape and content, water-to-cementitious (w/c) ratio, fly ash, paste and air void content on the crack resistance of concrete used in concrete railroad ties were investigated using Two-Parameter Model (TPM). For each mixture evaluated, twelve three-point prisms were tested at 27.5, 41.3 and 55.1 MPa concrete compressive strengths (4000, 6000 and 8000 psi) to determine the effect of concrete compressive strength on crack growth potential. In addition, splitting tensile tests were conducted on three samples at each compressive strength for all mixtures. Finally, the results were analyzed and the function of utilized method was discussed. The results show that increasing angularity, aggregate size and volume, and decreasing w/c ratio improve fracture toughness by 30% whereas changing paste, fly ash and air void content negligibly influence fracture toughness. However, all improving factors were seen to be most effective at low strengths. The obtained fracture toughness values from this study could lead to a significant improvement of splitting crack resistance in pre-stressed concrete members where high amount of transverse stress is often introduced at low compressive strengths.

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.

Quantification of bond performance of 18-mm prestressing steel

The use of 18-mm prestressing strands has extensive advantages in building and bridge construction. Strand bond is a critical factor that constitutes the strand behavior in pretensioned concrete girders and directly affects the applicability of existing bonding equations. This study systematically examines the bond performance of 18-mm strands. The bond strength of untensioned strands was evaluated using simple bond tests. Twelve pretensioned concrete beams were additionally cast using self-consolidating concrete to assess the bond-related parameters of transfer and development lengths. The experimental results indicate the bond performance of 18-mm strands complies with the current design codes of pretensioned concrete members.

Prediction of development length from free-end slip in pretensioned concrete members

Development length of prestressing strands is a significant parameter in the design of pretensioned concrete members. The adequacy of the code equations used to predict the development length needs to be verified when a new material or design parameter is introduced for precast, prestressed concrete applications. This study develops a simple and reliable technique to quantify the development length based on the premise of the slip theory, which proposes the free-end slip (FES) as an indicator of the development length. Twenty-five pretensioned concrete beams were cast with 15 mm diameter strands. FES was measured at both beam ends at prestress transfer. Development length was determined by performing bending tests at different embedment lengths. The experimental results were synthesised to revise the slip theory. The applicability of the revised-slip theory was validated by collected experimental data of pretensioned concrete members using various types of concrete, strand diameters and release techniques.

Assessment of transmission length of prestressing strands according to fib Model Code 2010

Abstract The use of 18-mm prestressing strands is advantageous in the construction of long-span bridges. Transmission length is a significant parameter in the design of pretensioned concrete members. This study provides a unique set of experimental data to validate the applicability of using the fib Model Code 2010 to predict the transmission length and strand draw-in of 18-mm strands. Twelve pretensioned concrete beams were cast with high-strength concrete. Transmission lengths were measured at prestress transfer and at 28-day of age. The experimental results indicated the lower and upper bounds of transmission length specified by the code adequately predict the measured values.

Transfer lengths of pretensioned concrete members reinforced with 2400 MPa high-strength prestressing tendons

Transfer lengths of pretensioned concrete members reinforced with 2400 MPa high-strength prestressing tendons

A higher-order equation for modeling strand bond in pretensioned concrete beams

In pretensioned concrete members, the bond between prestressing strands and concrete in the transfer zone is necessary to ensure the two materials can work as a composite material. This study develops a computer program based on the Thick-Walled Cylinder theory to predict the bond behavior within the transfer zone. The bond was modeled as the shearing stress acting at the strand-concrete interface, and this generated a normal stress to the surrounding concrete. The stresses developed in the concrete often exceeded its tensile strength, which resulted in radial cracks at the strand-concrete interface. These cracks reduced the concrete stiffness and redistributed the bond strength along the transfer zone. The developed program was able to determine the bond stress distribution, degree of cracking, and transfer length of the prestressing strands. The program was validated using a data set of transfer lengths measured at the University of Arkansas and a data set collected from the literature.