Development of a Standard Bond Test for Indented Prestressing Wires

An experimental testing program was conducted at Kansas State University (KSU) to test the bond characteristics of various 5.32 mm-diameter, Grade 270 low-relaxation steel wires used in prestressed concrete railroad ties. This un-tensioned pullout test could serve as a quality control test similar to the NASP (North American Strand Producers) Strand Bond Test that has been developed for pre-tensioned strands. A total of twelve (12) wires produced by six different steel manufacturers were used to develop the wire pullout test. All of the wires were tested in their “as-received” condition and have different indent geometries. It is generally accepted that indentations in the wire improve the bond between the steel and concrete. However, there are currently no commonly accepted quality control tests that accurately predict a wire’s bond characteristics in a pre-tensioned application. The un-tensioned pullout test developed is comparable to the NASP [Strand] Bond Test. The specimens consist of a 4 in. (100 mm) outer-diameter tube with a total length of 8 in. (200 mm) and a steel plate welded to the bottom. The 5.32 mm-diameter wire was centered in the tube and the sand-cement mortar was placed and allowed to cure. The flow of the mortar was measured for consistency and 2” × 2” (50 mm × 50 mm) mortar cubes were used to determine the compressive strength of the mortar. The specimens were tested when the compressive strength of the mortar was between 4500 and 5000 psi (31.0 MPa and 34.5 MPa). Each batch of mortar contained 12 pullout specimens; one with each wire type. Each wire was tested six times leading to a total of six batches and a total of 72 mortar specimens. During testing, the wires were loaded in force control at the bottom, while continuously monitoring and recording the movement (slip) of the wire with respect to the mortar at the opposite (top) end. The force verses end-slip data of the six tests for each wire type were numerically combined to obtain the average bond performance. These average results from the un-tensioned pullout tests were then compared to transfer length measurements from accompanying pre-tensioned concrete prisms. In general, the wire end slip measurements from the pullout tests were found to have good correlation with the measured transfer length. For all 12 wires, a coefficient of determination (R2) of 0.872 was found between the average pullout force (at 0.10-inch (2.54 mm) of wire free-end slip) and average transfer length measurements from the accompanying concrete prism tests. However, when only the indented wires were considered, the R2 increased to 0.913.