Strand Bond Research Articles

Theoretical and Experimental Evaluation of the Electronic Relaxation Mechanisms of 2‐Pyrimidinone: The Primary UVA Absorbing Moiety of the DNA and RNA (6‐4) Photolesion

The (6‐4) photolesion is a key photodamage that occurs when two adjacent pyrimidine bases in a DNA strand bond together. To better understand how the absorption of UVB and UVA radiation by the 2‐pyrimidinone moiety in a (6‐4) lesion can damage DNA, it is important to study the electronic deactivation mechanism of its 2‐pyrimidinone chromophore. This study employs theoretical (MS‐CASPT2/cc‐pVDZ level) and experimental (steady state and femtosecond broadband spectroscopic) methods to elucidate the photochemical relaxation mechanisms of 2‐(1H)‐pyrimidinone and 1‐methyl‐2‐(1H)‐pyrimidinone in aqueous solution (pH 7.4). In short, excitation at 320 nm leads to the population of the S11(pp*) state with excess vibrational energy, which relaxes to the S11(pp*) minimum in one picosecond or less. A trifurcation event in the S11(pp*) minimum ensued, leading to radiative and nonradiative decay of the population to the ground state or the population of the long‐lived and reactive T13(pp*) state in hundreds of picoseconds. Collectively, the theoretical and experimental results the idea that in DNA and RNA, the T13(pp*) state of the 2‐pyrimidinone moiety in the (6‐4) lesion can further participate in photosensitized chemical reactions increasing DNA and RNA damage.

Effect of magnesium phosphate cement mortar coating on strand bond behaviour

Surface coating on prestressing strand is an effective and economic measure to prevent pre-tensioned prestressed concrete structures from corrosion. The effects of magnesium phosphate cement (MPC) mortar coating on the strand bond behaviour are investigated experimentally in the present paper. Ten pull-out specimens with different MPC mortar coating thickness are designed to study the effect of coating thickness or thickness ratio on the bond failure mode, bond strength and force-slip response. Experimental results demonstrate that the bond failure mode changes from pull-out failure accompanied with strand rotation to cover splitting failure when coating thickness and thickness ratio exceed 10 mm and 0.15, respectively. Compared with the uncoated and epoxy coated control specimens, coating thickness ranging from 25 mm to 35 mm or thickness ratio ranging from 0.37 to 0.52 contributes the most to the increase of bond strength and stiffness. Beyond that range, the facilitation in both bond strength and stiffness gradually decreases. Besides, an analytical model, which considers the composite cover confinement provided by the coating and concrete, is proposed to reproduce the bond strength evolution. Comparison of experimental and prediction results show that the proposed model can be used to reasonably evaluate the bond strength.

Influence of strands corrosion on the flexural behavior of prestressed concrete beams

In this paper, the research goal was the determination of the flexural response of corroded pre-stressed reinforced concrete beams (PC). A simplified flexural model is bases on the calcualus of the moment and curvature/deflection curves for simply supported beam under uniformly distributed and concentrated loads. Primary research outcomes was the introduction in flexural model of the effects of pitting corrosion though the reduction of area, of the mechanical properties of steel strands and of the loss of bond of steel strands of precast pre-tensioned beams.A comparison with available experimental data of literature and with numerical simulation carried out with nonlinear finite element code (ABAQUS) was made. Results obtaiend showed that the corrosion of strands reduce the bearing capacity of the viaducts with prestressed beams especially and because of the reduction of strain capacity of steel strand, arises a risk of brittle collapse under service conditions due to the strand failure.

Experimental Research on Bonded Anchorage of Carbon Fiber Reinforced Polymer Prestressed Strands.

Aiming at the problems of a large number of corrosion and fatigue damage of the current prestressed steel strands, this paper adopts carbon fiber-reinforced composite (CFRP) strand with better corrosion resistance and fatigue resistance and uses it in concrete structures. The bond anchorage is usually used to anchor CFRP tension members, which bonds the CFRP through the binding medium. Through experimental research on the CFRP strand bond anchorage, the inner taper of the CFRP prestressed strand cone was anchored and the influence of different anchor lengths and bonding media on the anchorage performance was determined. The test results demonstrate that the taper of the conical anchorage described in this paper is a key factor affecting its anchorage performance and increasing the inner taper within a certain range is beneficial to improving the anchorage performance of the conical anchorage. The bonded anchorage of the CFRP prestressed strand with a 200 mm anchor is the most reliable and efficient, as the taper of the 200 mm anchor is the largest. The average anchoring efficiency coefficient of the 200 mm anchor was 96.4%, which is 3.7% and 2.6% higher than the average anchoring efficiency coefficient of 220 mm and 250 mm anchors, respectively. The anchoring efficiency of the anchor is also high (94.1%) when the epoxy resin mortar is used as the bonding medium. Moreover, after an appropriate amount of quartz sand is added to the epoxy resin, the overall comprehensive performance of the anchor can be improved to a certain extent and the stress of the CFRP strand can be improved. The coupling between ultra-high-performance concrete dry mix (UHPC-GJL) and CFRP strand materials is not suitable for UHPC-GJL being used, as its binding medium as the average anchoring efficiency coefficient is only 44.5% when UHPC-GJL is used as the anchor bonding medium.

Background for the new PCI recommended practice on strand bond

The new PCI “Recommended Practice to Assess and Control Strand/Concrete Bonding Properties of ASTM A416 Prestressing Strand” specifies two new threshold limits for pullout tests conforming to ASTM A416 and new equations for the transfer and development length of prestressing strand. This article provides a summary of more than 30 years of research and knowledge advancement on the bond between concrete and prestressing strand related to the development of the new recommended practice. Discussions regarding the adoption and incorporation of the new recommended practice into structural design, strand production, and precast concrete fabrication and quality control practices are ongoing.

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.

Effects of Grouting Defects in a Duct on the Bonding of Prestressing Strands

Insufficient grouting has a great effect on the bonding performance of pressing stand for a post-tensioned prestressed concrete beam, which leads to a decrease of beam's bending capacity. This study focused on the influence of defects of local grouting length and grouting fullness on the prestressing strand's bond performance. The prestressing strand pull-out test was used to evaluate the bonding performance of pressing stand with different defects of local grouting length and grouting fullness. The results indicate that two forms of grouting defects reduce the bonding properties of a prestressing strand, when the slip of the free end of prestressing strand is 0.12 mm (s = 0.12 mm), the bond stress decreases briefly. To analyze the bonding mechanism of the prepressing strand in duct, the bond stresses τ0 and τu at s = 0.12 mm and s = 25 mm were adopted as the characteristic parameters of bond stress to deduce the calculation formula of the bond strength by fitting the test data. A bond-slip constitutive model of prestressing strand in duct under two classes defects was proposed by improving the modified Eligehausen, Popov and Bertero (BPE) model of fiber reinforced polymer (FRP) rebars.

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.

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.

Use of Recycled Concrete Aggregates in Precast/Prestressed Concrete

This paper describes a preliminary investigation on factors related to the use of recycled concrete aggregates (RCA) in precast/prestressed concrete applications. In particular, the effects of RCA are experimentally investigated for: 1) the bond between seven-wire prestressing steel strand and concrete; 2) the compression strength gain of concrete with time; and (3) the compression stiffness gain of concrete with time. The test specimens were made by replacing the natural course aggregates in the concrete mixtures with RCA from: 1) discarded/rejected precast concrete; and 2) returned ready-mix concrete. The data to-date show that the introduction of RCA results in an effect (decrease or increase) in the strand bond strength that is consistent with the effect of RCA on the compression strength of concrete. For the materials tested, RCA from precast concrete performed better than RCA from returned ready-mix concrete. The use of RCA did not have a significant effect on the rate of concrete compression strength gain with time. The results for the stiffness gain of concrete with time were somewhat more varied than the results for the strength gain.

Establishment of Minimum Acceptance Criterion for Strand Bond as Measured by ASTM A1081

Establishment of Minimum Acceptance Criterion for Strand Bond as Measured by ASTM A1081

Spacing requirements of 0.7 in. (18 mm) diameter prestressing strands

The authors thank Insteel Industries Inc. for providing the strands and RJ Peterman and Associates Inc. for conducting the ASTM A1081 test method for strand bond for this research. The authors thank Don Logan for providing the financial support to conduct the ASTM A1081 test method for strand bond. The authors are also thankful to Richard Deschenes Jr., Cameron Murray, Joseph Daniels III, William Phillips, Doddridge Davis, Alberto Ramirez, and Ryan Hagedorn for helping fabricate the beams at the Engineering Research Center at the University of Arkansas.

Experimental Investigation of Prestressing Strand Bond on Behavior of Concrete Members

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A method for quality assurance of seven-wire strand bond in portland cement concrete

A method for quality assurance of seven-wire strand bond in portland cement concrete

Strand bond stress–slip relationship for prestressed concrete members at prestress release

The transfer of prestress force from prestressing strands to the surrounding concrete is dependent on the bond between the two materials. Understanding the actual bond stress distribution along the transfer length results in optimized design of the transfer zone of prestressed concrete members. Equations of estimating the transfer length in ACI 318 code and AASHTO LRFD bridge design specifications simply take into account the effect of the strand diameter only. The objective of this study is to provide a generalized procedure for determining the bond stress–slip relationship accurately by incorporating the effects of additional parameters, such as concrete compressive strength at prestress release, center-to-center strand spacing, and concrete bottom cover. First, the bond stress distribution along the transfer length of a prestressed concrete member is formulated based on longitudinal slip–strain compatibility, force equilibrium and invariable bond stress–slip relationship along the transfer length. Second, a generalized Inverse Problem-Solving approach is introduced to determine best parameter coefficients through minimizing the discrepancy between the calculated and measured results. Two types of measurements (i.e., transfer length and end slip) reported in the literature are utilized to demonstrate the proposed approach. Predicted transfer length and end slip values using the calibrated bond stress–slip relationship show better agreement with the test data compared to those predicted by ACI 318 code and AASHTO LRFD bridge design specifications. Third, a computational procedure is developed and an example is presented to assist engineers using the developed formulae for determining the bond stress distribution along the transfer length of prestressed concrete members.

Analysis of bond stress distribution for prestressing strand by Standard Test for Strand Bond

Strand bond is the unique interaction between prestressing strand and adjacent concrete, and directly affects the performance of the strand in the anchor region. However, strand bond depends upon numerous factors, such as concrete properties and strand surface condition. The Standard Test for Strand Bond (STSB) has been developed to assess the bonding capacity of prestressing strands. Test data are used to propose a bond stress–slip model and investigate the bond stress distribution. The study outcomes are verified by STSB of 0.6in. (15.2mm) and 0.7in. (17.8mm) strand conducted at the University of Arkansas and collected in the literature. Experimental and analytical results show that the proposed model can predict bond stress versus slippage of prestressing strand within a 2% error. The results also show that the bond stress distribution along the embedment length is not linear or uniform as was assumed by previous researchers.

Size Effect on Strand Bond and Concrete Strains at Prestress Transfer

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Strand bond performance in prestressed concrete accounting for bond slip

This paper presents the results of an experimental research program addressing the bond behavior of prestressing strands in pretensioned prestressed concrete members after anchorage failure has occurred. A test methodology based on measuring the prestressing strand force and strand end slip at the specimens’ free end was employed. Transmission- and anchorage-length tests were performed on several series of prestressed specimens with different embedment lengths using twelve concrete mixes. Average bond stresses along the transmission length and the anchorage length were obtained for specimens with release strengths ranging from 24MPa to 55MPa. For the anchorage analysis, a parameter was developed that includes strand slip to be used in determining anchorage length. Based on the test results, an analysis to experimentally substantiate the Stress Waves Theory of Janney has been proposed. Additionally, the potential bond performance of prestressing strands after anchorage failure at the end regions has been suggested.

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.

Effects of corrosion on bond behavior between steel strand and concrete

An experimental program involving 12 pull-out specimens investigated the effects of corrosion on bond behavior between steel strand and concrete. The specimens were mixed with chloride and exposed to both indoor and outdoor natural climate to approximate the actual effects of corrosion. The specimens were corroded for 17months until the widest corrosion crack reached 0.7mm. The pull-out test results show that the bond–slip curve of the uncorroded strand consists of three segments, namely, ascending, approximate horizontal, and descending, whereas the curve of the corroded strand consists of only two segments, namely, ascending and descending. Additionally, the gradient of the descending segment of the curve of the corroded strand increases with the severity of corrosion. The bond strength of the corroded strand does not degenerate within the corrosion level. The pull-out process of the strand may be divided into four stages: elastic, strengthening, softening, and residual. Two comparisons revealed the effects of corrosion on strand bond behavior. Compared with ordinary corroded bars in terms of bond performance, the strand exhibited the largest critical crack width corresponding to the degradation of bond strength, the deformed bar exhibited the second-largest value, and the plain bar had the lowest value of 0mm. Corrosion of the strand in a pre-tensioned prestressed concrete structure was found to degrade the tensile strength of the strand instead of its bond strength.