Effects of Cover, Compressive Strength, and Wire Type on Bond Performance in Prismatic Prestressed Concrete Members

Abstract

The bond between wire and concrete is crucial for transferring the stresses between the two materials in a prestressed concrete member. Furthermore, bond can be affected by such variables as thickness of concrete cover, type of pre-stressing (typically indented) wire used, compressive (release) strength of the concrete, and concrete mix. This work presents current progress toward the development of a testing procedure to get a clear picture of how all these parameters can ruin the bond and result in splitting. The objective is to develop a qualification test procedure to proof-test new or existing combinations of pre-stressing wire and concrete mix to ensure a reliable result. This is particularly crucial in the concrete railroad crosstie industry, where incompatible conditions can result in cracking and even tie failure. The goal is to develop the capability to readily identify compatible wire/concrete designs “in-plant” before the ties are manufactured, thereby eliminating the likelihood that defectively manufactured ties will lead to in-track tie failures due to splitting. The tests presented here were conducted on pre-tensioned concrete prisms cast in metal frames. Three beams (prismatic members) with different cross sections were cast simultaneously in series. Four pre-stressing wires were symmetrically embedded into each concrete prism, resulting in a common wire spacing of 2.0 inches. The prisms were 59.5in long with square cross sections. The first prism was 3.5 × 3.5in with cover 0.75in, the second was 3.25 × 3.25in with cover 0.625in and the third prism in series was 3.0 × 3.0 in with cover 0.50in. All pre-stressing wires used in these initial tests were of 5.32 mm diameter and were of the same wire type (indent pattern) denoted by “WE”, which had a spiral-shaped geometry. This is one of several wire types that are the subject of the current splitting propensity investigation. Others wire types include variations of the classical chevron shape, and the extreme case of smooth wire with no indentions. The wires were initially tensioned to 7000 pounds (31.14 KN) and then gradually de-tensioned after reaching the desired compressive strength. The different compressive (release strength) strength levels tested included 3500 psi (24.13 MPa), 4500psi (31.03 MPa), 6000 psi (41.37 MPa) and 12000psi (82.74MPa). A consistent concrete mix with water-cement ratio 0.38 was used for all castings. Geometrical and mechanical properties of test prisms were representative of actual prestressed concrete crossties used in the railroad industry. Each prism provided a sample of eight different and approximately independent splitting tests of concrete cover (four wire cover tests on each end) for a given release strength. After de-tensioning, all cracks that appeared on the prisms were marked, and photographs of all prism end surfaces were taken to identify the cracking field. During the test procedure longitudinal surface strain profiles, along with live-end and dead-end transfer lengths, were also measured using an automated Laser-Speckle Imaging (LSI) system developed by the authors. Both quantitative and qualitative assessment of cracking behavior is presented as a function of cover and release strength. In addition to the identification of whether cracking took place at each wire end location, measurements of crack length and crack area are also presented for the given WE wire type. The influence of concrete cover and release strength are clearly indicated from these initial tests. The influence of indented wire type (indent geometry) will also be discussed in this paper, along with a presentation of some preliminary test results. This work represents a successful first step in the development of a qualification test for validating a given combination of wire type, concrete cover, and release strength to improve the reliability of concrete railroad crosstie manufacturing.

The influence of different type of aggregate, fly ash and fiber-reinforced polymer on splitting resistance in pretensioned concrete railroad ties

“…• The second significant parameter is the wire indent characteristics. According to research conducted by A. Savic [3], [4], [5] the shallow chevron type of wires did not split on the prisms having 3/4in edge distance.…”

“…The research which was conducted at Kansas State University used this wire type having 1in edge distance and experiments with this wire were successful without longitudinal splitting in prestressed concrete members. The influence of variables was presented in the research of Bodapati [1], Savic et al [2], [3], [4], [5], [6]. They investigated the influence of edge distance and wire type indentation on longitudinal splitting.…”

The Influence of Concrete Cover, Type of Wire Indentation and Concrete Mix on Bond between Steel and Concrete in Prismatic Prestressed Concrete Members

“…The length required to transfer the pre-stress force into the concrete member is well-known as the "transfer length". In this testing a Laser Speckle Imaging (LSI) system [5] was used to measure the transfer length and the shape of longitudinal strain profiles. The top surface of each concrete prism was scanned by this system before and after de-tensioning.…”

“…The top surface of each concrete prism was scanned by this system before and after de-tensioning. The resulting strain profiles were then automatically plotted, and the transfer length was subsequently determined using a standard leastsquares algorithm developed for the prismatic shape [5]. Photographs of all the prism surfaces were also investigated for quantitative and qualitative evaluation of cracking behavior.…”

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

“…These bursting stresses can be formed due to change in diameter of prestressing wires/tendons due to Hoyer's effect [1], the jacking force in the tendons, geometrical characteristics of the prestressing tendons the type of concrete mixture and compressive strength of concrete [2][3]. According to experimental testing some of the splitting cracks occurred immediately after the process of de-tensioning, but they also developed during the first weeks after de-tensioning procedure due to sustained lateral stresses exerted by the prestressing tendons [4][5].…”