Bond Characterization of 17.8-mm (0.7-in.) Diameter Prestressing Strand
Publication: Journal of Bridge Engineering
Volume 29, Issue 4
Abstract
Seven-wire prestressing strands are commonly used to pretension concrete bridge members. While the industry standard is 12.7-mm (0.5-in.) or 15.2-mm (0.6-in.) diameter strand, interest in developing greater pretensioning forces has resulted in the proposed use of 17.8-mm (0.7-in.) strands. This paper reports an experimental program aimed at characterizing the bond properties of 17.8-mm seven-wire prestressing strand and contrasting these with 9.5-mm (3/8-in.), 12.7-mm (1/2-in.), and 15.2-mm (−0.6-in) strands. The geometric and material properties of the strands are reported, and the evaluation of bond performance is assessed by standard test methods. Beam-end bond tests of both straight and 90° hooked strands are presented. The results of in situ transfer length determined from full-scale girders are also presented. Finally, finite-element analyses aimed at evaluating the influence of the Hoyer effect and strand spacing are presented. In all the tests, bond performance of 17.8-mm strand was found to be adequate and appropriately represented by existing AASHTO transfer and development length equations. The potential for utilizing the embedment of hooked strands into cast-in-place end diaphragms to increase the strand stress that may be developed near girder ends is proposed. From the in situ determination of transfer length, the mean and fifth percentile transfer lengths (determined with 95% confidence) of 0.7-in. strands were determined to be 30.4db (db is the strand diameter) and 53.4db, respectively, both less than the 60db prescribed by AASHTO. Additionally, the conventional 51-mm (2-in.) minimum center-to-center strand spacing appears to be adequate for 17.8-mm strands in terms of release stresses. No local cracking or other deleterious effects associated with strand spacing were observed. It is shown numerically that the lower Hoyer effect strand dilation and larger circumference result in marginally lower crack-inducing circumferential stresses. Although the current AASHTO determination of transfer and development length appears to overestimate these values for all strand diameters including 17.8-mm strands, the overestimation of transfer length underestimates concrete tensile stresses at prestress transfer in the area affected by the transfer length. This may result in unanticipated cracking. To address this, a two-tier approach is proposed: (1) using a reduced transfer length—40db is proposed—to check tensile stresses at prestress release; and (2) using the longer development length—using a transfer length component of 60db—to determine the load-carrying capacity of a prestressed concrete member.
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Data Availability Statement
Data are available from the authors upon reasonable request.
Acknowledgments
Notation
The following symbols are used in this paper:
- A
- length of 90° hook extension;
- A, B, and C
- Ramberg–Osgood parameters [Eq. (4)];
- Aps
- area of prestressing strand;
- As
- area of nonprestressed reinforcing steel;
- c
- concrete cover dimension measured from center of prestressing strand;
- d
- effective girder depth;
- d
- outer diameter of deformations;
- db
- diameter of prestressing strand;
- dc
- diameter of straight central wire in seven-wire prestressing strand;
- dh
- diameter of helical wires in seven-wire prestressing strand;
- di
- inner diameter of deformations;
- dr
- partial debonding ratio;
- dv
- effective shear depth;
- Ec
- elastic modulus of concrete;
- Ep
- tensile modulus of prestressing strand;
- specified 28-day concrete strength;
- concrete strength at prestress transfer;
- fcr
- cracking strength of concrete;
- fcrack
- strand stress at Pcrack;
- fmax
- strand stress at Pmax;
- fpe
- effective pretensioning stress;
- fps
- strand stress to be developed;
- fpu
- ultimate strength of prestressing strand;
- fR
- relative rib area;
- fslip
- strand stress at Pslip;
- fsp
- 28-day splitting cylinder strength of concrete;
- fsplit
- strand stress at Psplit;
- fy
- yield strength of nonprestressed reinforcement;
- k
- number of helical elements in prestressing strand;
- L
- overall length of girder;
- Ld
- development length;
- Lt
- transfer length;
- ldh
- embedment length of 90° hooked strand;
- le
- embedment length of straight strand;
- Mu
- factored moment;
- Nu
- factored axial force;
- P
- applied load;
- Pcrack
- applied load at the occurrence of transverse crack;
- Pmax
- maximum applied load observed;
- Pslip
- applied load corresponding to slip > 0.0025 mm;
- Psplit
- applied load corresponding to initial splitting;
- r
- radial dimension measured from the center of the prestressing strand;
- rs
- radius of prestressing strand;
- s
- center-to-center spacing of prestressing strand;
- sh
- helical wire pitch in seven-wire prestressing strand;
- T
- required tension force at critical section [Eq. (7)];
- Vp
- vertical component of the effective prestressing force from harped strands, if present;
- Vs
- shear resistance provided by transverse reinforcement;
- Vu
- factored shear;
- z
- distance measured from the beginning of the prestressed strand embedment;
- β
- twist angle of seven-wire prestressing strand;
- ɛcr
- modeled concrete cracking strain;
- ɛcu
- modeled concrete ultimate compression strain;
- ɛi
- measured strain at i;
- ɛi,smooth
- smoothed strain at i [Eq. (8)];
- ɛpr
- modeled dilation strain of prestressing strand;
- θ
- angle of inclination of diagonal compressive stresses;
- κ
- empirical parameter affecting development length;
- σct
- modeled concrete cracking stress;
- σcu
- modeled concrete ultimate compression stress;
- σr
- radial stress in concrete around s prestressing strand;
- σθ
- circumferential stress in concrete around prestressing strand;
- τ
- average bond stress;
- τmax
- average strand bond stress at Pmax;
- τslip
- average strand bond stress at Pslip;
- τsplit
- average strand bond stress at Psplit;
- υc
- Poisson’s ratio for concrete;
- υp
- dilation ratio of the prestressing strand; and
- , ϕc, and ϕv
- material resistance factors for flexure, axial load, and shear, respectively.
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Published In
Journal of Bridge Engineering
Volume 29 • Issue 4 • April 2024
Copyright
© 2024 American Society of Civil Engineers.
History
Received: Jul 31, 2023
Accepted: Nov 16, 2023
Published online: Feb 2, 2024
Published in print: Apr 1, 2024
Discussion open until: Jul 2, 2024
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