The Effect of Wire Type on Cracking Propensity in Prestressed Concrete Prisms

Abstract

This paper is a continuation of a previous study conducted at Kansas State University [8]. This paper demonstrates the influence of the thickness of concrete cover, compressive strength of concrete and the type of wire indentation on bond performance between steel and concrete in pre-stressed concrete ties using a consistent concrete mixture. A key objective of this research is to find the best parameters for pre-stressed concrete ties to prevent them from splitting/cracking in the field. This is very important for pre-stressed manufacturers, and especially for the railroad crosstie industry, so as to avoid failures in the field. The goal is to develop a qualification test with the capability to identify the compatible combinations of wire type and concrete mix before the ties are manufactured. A study took place at Kansas State University to understand and quantify the influence of variables such as the thickness of concrete cover, type of indents, and the compressive release strength on the bond behavior between steel and concrete. For the experimental testing three prisms with different cross sections were cast at the same time in series. Four pre-stressing wires were symmetrically embedded into each concrete prism and the spacing between wires was 2.0 inches. All prisms had the same length of 59.5in with square cross section. With the thickness of concrete cover of 3/4″ the first prism had a 3.5×3.5in square cross section, the second prism had a 5/8″ thickness of concrete cover and 3.25×3.25in square cross section and the third prism had a 1/2″ thickness of concrete cover and a 3.0×3.0in square cross section. All pre-stressing wires which were used in these tests had a 5.32mm diameter and were of different wire types. The indent pattern variations of the wire types included spiral, classical chevron shape, and the extreme case of smooth wire with no indentations. 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 4500 psi (31.03 MPa) and 6000 psi (41.37 MPa). For this study, a consistent concrete mixture with 0.32 water-cement ratio was used for all prisms, except for prisms casted with WE wire. For these prisms a water-cement ratio of 0.38 was used. Prisms had almost identical geometrical and mechanical properties as pre-stressed concrete ties which are manufactured in the railroad industry. Each prism provided a sample of eight different independent splitting tests of concrete cover (four wire cover tests on each end) for a given release strength. All cracks which appeared after de-tensioning were observed and measured to identify the cracking field, and all sides of the prisms on the live and dead end were marked for identification. For all prisms, longitudinal strain profiles on the live end and dead end were measured along with the values of transfer lengths. The strain profiles were taken using an automated Laser-Speckle Imaging (LSI) system. All results, representing quantitative and qualitative assessment of cracking behavior, are given in this paper as a function of thickness of concrete cover and release strength of concrete. For each sample prism, crack length and crack width were measured, and crack area was calculated as a simple function of crack length and crack width. In the case where spalling occurred, the crack width used was arbitrary set at 0.2in. These tests reveal the influence of thickness of concrete cover, the indented wire type and the release strength of concrete on the bond between steel and concrete. This work represents a successful first step in the development of a qualification test to ensure adequate splitting resistance in pre-tensioned concrete railroad ties.