Pretensioned Concrete Elements Research Articles

Experimental Research of Bond Strength of Lightweight Aggregate Concrete to 15.7 mm Non-Pretensioned Steel Strands.

This paper deals with the issue of the bond of concrete with the new artificial aggregate Certyd to prestressing steel strands. The solution of the problem is of great importance in the development of the use of lightweight aggregate concrete for prestressed concrete elements. Experimental research on the bond stress-slip relationship of concrete to 15.7 mm non-pretensioned steel strand was carried out. The results of bond stress-slip tests for various embedment lengths (40, 80, 120, 240, 330 and 460 mm) for test specimens made of the same lightweight aggregate concrete mixture, in which the transfer of prestressing force took place at different levels of concrete maturity (after 3, 7 and 28 days of concrete maturing), are presented. Based on the obtained results, an analytical model of the bond stress-slip relationship of lightweight aggregate Certyd concrete to 15.7 mm non-pretensioned steel strand was proposed. The tests presented demonstrated that the lightweight aggregate (Certyd) concrete is suitable for the production of pretensioned concrete elements.

Effects of exposure to elevated temperatures on the transfer length of pretensioned concrete elements

Pretensioned concrete elements are widely used in bridges and precast buildings, which may be exposed to fire. The residual performance of a pretensioned element that has survived a fire event is dependent on the effects of elevated temperatures on steel, concrete, and the bond between these materials. This paper investigates changes in the transfer length of pretensioned beams due to exposure to elevated temperatures. Ten large-scale pretensioned concrete beams with different cross-sectional dimensions and strand layouts were fabricated. The transfer length of these elements was measured using demountable mechanical strain gages. The specimens were subjected to elevated temperature cycles with maximum temperatures of 500 and 700 degrees for two hours. After natural cool down, changes in the transfer length of specimens were determined. The results showed that heat exposure leads to an increase in the transfer length by up to 35% and 42% for maximum temperatures of 500 and 700 degrees Celsius, respectively. Strand arrangement, compressive strength of concrete, cross-sectional dimensions, and clear cover were found to affect the sensitivity of elements to heat exposure. A simplified equation was proposed for estimating the transfer length of pretensioned concrete beams that survive elevated temperatures, which may be used for structural assessment applications.

Composite Slab Made from Precast, Pre-Tensioned Concrete Planks and Lightweight Concrete

New architecture trends consisting of designing large, space-free spaces, force structural engineers to develop new solutions that allow them to bridge large spans with relatively small thicknesses. Therefore, structural engineers are looking for new solutions and improvements to existing solutions both in the field of post-tensioned slabs as well as pre-tensioned, precast elements. In particular, precast concrete manufacturers offer many solutions of partially or fully precast slabs, each of which performs better in some situations and worse in others. The advantages of precast structures are increasingly noticed by all participants of the construction process, including producers who want to meet the requirements of the construction market for developing newer, better and more hybrid solutions, so that competition with traditional concrete technology is possible. In the field of the optimisation of existing precast slab solutions in Poland, work has been undertaken in recent years on the use of pre-tensioned concrete planks proposed a few decades ago as shuttering elements, which include a tension reinforcement zone in the slab. The author of this work proposes making a slab made of pre-tensioned concrete elements of low thickness for use as shuttering for reinforced concrete made of lightweight concrete, which is lighter than conventional, traditional concrete by around 30%. The resulting composite slab of precast elements would be relatively cheap with regard to transportation and assembly, which makes them competitive for the popular ribbed floor structures usually used in houses, whilst at the same time having the advantages of solid slabs, such as good mechanical and acoustic properties. In addition, they also compete with each other with regard to pricing, which is why they could be a good alternative for investors to choose. The work presents the concept of such a slab and an example of analysis.

A Literature Review on Post-Tensioned and Pre-Tensioned Concrete Elements

A Literature Review on Post-Tensioned and Pre-Tensioned Concrete Elements

Behaviour of End Zone of Pre-tensioned Concrete Elements

Application of high performance concrete or self-compacting concrete is verycommon in prefabrication plants. For pre-tensioned concrete structures prestressing reinforcement may be prepared in the form of smooth or profiled steel wires or multi-wire strands.The transfer of the prestressing force onto pre-tensioned element takes place through the bond between concrete and steel. Proper recognition of bond phenomenon directly influences the safety and durability of structure. It is especially important for pre-tensioned concrete elements without the normal reinforcement, such as railway sleepers or hollow core floor slabs.The value of bond stress in pre-tensioned structures depends on the value of tendon's slippage and many material and technological aspects. Complex stress state in end zone of the element, caused by transferring the prestressing force to concrete, may result in different damages. In the elements without transverse reinforcement cracking of end zone can endanger its safety use.The paper presents also the calculations of the transmission length of prestressing force according to the latest regulations – Eurocode 2 and Model Code 2010.

Predicting bond formulations for prestressed concrete elements

One of the most important features in design of pretensioned concrete elements is the determination of correct transmission (transfer) length. A number of formulations have been proposed by some authors, codes and guidelines but, although a huge experimental literature is available, the problem does not seem completely solved. In this work an experimental database of measured values of transmission length in prestressed concrete specimens is gathered and an assessment of some current formulations is developed. A statistical analysis is also performed in order to identify the effect of the main variables influencing transmission length. Some useful information is obtained and can be eventually used for a possible validation/improvement of actual relations; a new formulation for transmission length evaluation is also proposed.

Stress transfer characteristics of sheathed strand in prestressed concrete beams: Experimental study

Strand debonding is a common approach used to control stress and reduce cracking at the ends of pretensioned concrete beams. However, damage in pre- tensioned beams with debonded strands during manu- facture has raised concerns about the use of debonding as currently specified. Numerical simulations were conducted in this study to improve understanding of the stress transfer characteristics of sheathed strand in pretensioned concrete elements. Nonlinear finite element models of small-scale prestressed concrete beam units in which concrete and strand were mod- eled with three-dimensional continuous elements were established with commercial software. The numeri- cal simulations were calibrated with experimental data, and results show that the lack of bond resistance along the debonded region maximizes the dilation of the strand after release. This effect may damage the concrete if there is tight contact between concrete and strand, which may result when using flexible, tight- fitting debonding material. This problem is eliminated if enough room is provided for strand dilation.

Modelling the prestress transfer in pre-tensioned concrete elements

Three models were developed to simulate the transfer of prestress force from steel to concrete in pre-tensioned concrete elements. The first is an analytical model based on the thick-walled cylinder theory and considers linear material properties for both steel and concrete. The second is an axi-symmetric finite element (FE) model with linear material properties; it is used to verify the analytical model. The third model is a three dimensional nonlinear FE model. This model considers the post-cracking behaviour of concrete as well as concrete shrinkage and the time of prestress releasing. A new expression from the analytical model is developed to estimate the transmission length as well as the stress distribution along the tendon. The paper also presents a parametric study to illustrate the impact of diameter of prestressing steel, concrete cover, concrete strength, initial prestress, section size, surface roughness of prestressing steel, time of prestress release, and the member length on the transfer of stress in pre-tensioned concrete elements.

Bond-slip-strain relationship in transfer zone of pretensioned concrete elements

In this paper, a novel bond-slip-strain relationship for a strand in the transfer zone of a pretensioned concrete member is presented. To draw a relationship among the bond stress, strand strain, and slip along the transfer zone, pretensioned, prestressed concrete members were fabricated, and strand strains were measured from electrical resistance steel gauges attached on the strand during prestressing force release. The test variables were the compressive strength of the concrete, the size of the cross section, the cover thickness, the diameter of the strand, the curing method, and the debonded region. Estimates obtained from the proposed model were in good agreement with the test results from other studies, as well as those from this work. Using the advantage of the strain term in the proposed formulation, the applicability of the derived model to fiber-reinforced polymer tendons was also evaluated.

Control of the transmission length in the prestressed hollow-core slabs using values of the strand end slip

It is shown [1] that even though the production of precast pre-tensioned concrete elements started more than 50 years ago, no one has ever succeeded in determining the real transmission length or in obtaining adequate measures to control it, due to the quality of the production. These arrears are confirmed by the study of existing standards. In general, it is assumed that the measurement of the magnitude of the slippage of strand at the end of hollow core prestressed slab is an indirect method of estimation and control of the value of the transmission length. One should mention that EN1168 and EN13369 that regulate requirements of designing and production of hollow-core slabs are applied as the method of control. Based on a particular bond-slip relation for strands, a non-linear equation is developed for the transmission length and for the slippage of prestressed strand, taking into account such factors as the concrete strength at transfer, the degree of the initial prestress type and diameter of tendons. These equations have been used for the control of transmission length of the strands calculated by different methods available recently, including a code provision. Then they were compared to measured data, obtained in production control of the hollow-core slabs.

Evaluation of strand bond properties along the transfer length of prestressed lightweight concrete members

This paper presents the results of an experimental program developed to study the feasibility of producing prestressed concrete girders using lightweight concrete. During the experimental campaign, bond tests were performed in push-in (TL bond test) and pull-out (FB bond test) types of specimens. To-scale prestressed concrete elements were produced, implemented, and tested. The variables studied were the type of concrete, the test age, and the delayed effects of bond. The TL bond test allowed the behavior of the transfer length to be characterized, governed by the Hoyer effect. Equations have been calibrated to identify the upper and lower transfer lengths using the experimental results of this trial. The proposed equations delimit the safety boundaries of the experimental transfer length values obtained in the experiments performed on the beams. The results obtained in the experimental phase discourage the use of the studied lightweight concrete for producing pre-tensioned concrete elements.

Numerical Optimization of a Compact and Reusable Pretensioning Anchorage System for CFRP Tendons

Efficient use of pultruded carbon fiber–reinforced plastic profiles (CFRP tendons) to prestress high-performance concrete (HPC) highly depends on the performance of the anchorage system and on material choice. For current applications, a prestressing degree of approximately 40% of the CFRP material strength is utilized in pretensioned concrete elements. A higher prestress implicates lower costs of fully prestressed concrete elements. The present project aimed to optimize the design of a removable and reusable pretensioning anchorage system for sand-coated CFRP rods. The optimized design was achieved by means of finite-element calculations in which parametric studies were complemented with extensive experimental work for validation. Analytical results demonstrated a reduction up to 25% for the relevant stress peaks in the tendons. The static rupture load under laboratory conditions increased by 25%, and the pretensioning level on-site could be increased by 50%. This improvement in production efficiency can be explained by easier applicability of the new system, i.e., failure tolerant assembly and prestressing process.

Transfer Length Of High Performance Pretensioned Concrete Elements

Transfer Length Of High Performance Pretensioned Concrete Elements

Closure to “Measurement of Transfer Lengths on Pretensioned Concrete Elements” by Bruce W. Russell and Ned H. Burns

Closure to “Measurement of Transfer Lengths on Pretensioned Concrete Elements” by Bruce W. Russell and Ned H. Burns

Discussion: Measurement of Transfer Lengths on Pretensioned Concrete Elements

Discussion: Measurement of Transfer Lengths on Pretensioned Concrete Elements

Measurement of Transfer Lengths on Pretensioned Concrete Elements

This paper reports an experimental program conducted to measure the transfer lengths (or transmission lengths) of pretensioned seven-wire strands in pretensioned concrete specimens. The primary goals of the research were to investigate the ability of the larger 15.2 mm (0.6 in.) strand to reliably transfer its prestressing forces within a reasonable transfer length, and to evaluate the current design provisions for transfer lengths. Eight specimens were pretensioned with 12.7 mm (0.5 in.) diameter strands and 10 specimens were pretensioned with 15.2 mm (0.6 in.) strands. Experimental results indicate that the larger 15.2 mm (0.6 in.) diameter strands require longer transfer lengths and cause more distress than the smaller 12.7 mm (0.5 in.) strands. However, the tests indicate that 15.2 mm (0.6 in.) strands can be readily employed for pretensioned concrete structures. Additionally, the results demonstrate that current design practice estimating transfer lengths can lead to unsafe designs and that a more reliable expression for transfer lengths should be adopted.