Tensile Testing of Multi-Wire Strand for Prestressed Concrete to ASTM A370, A416, and ISO 15630
The mechanical performance of 7-wire strand used in prestressed concrete structures is subject to strict tensile testing requirements under ASTM A370, ASTM A416, and ISO 15630. Testing wire strand presents a unique set of challenges: specimens are long, clamping forces are high, and obtaining accurate strain measurements on a twisted, multi-wire specimen requires specialized extensometry. Instron offers a fully integrated testing solution designed to meet these demands reliably and efficiently.
Testing System
For 7-wire strand testing, Instron recommends the Industrial Series Model 600KPX. Most wire strand applications require specimen lengths of 500 mm or greater, and the 600KPX was selected for this application primarily because of its large test space and long-stroke actuator. This configuration allows operators to install and remove specimens comfortably at floor level — without the need for a ladder or pit-mounted installation — improving both workflow efficiency and operator safety.
Gripping
A set of hydraulic side-action grips with wire strand inserts is recommended for this application. The side-acting design overcomes several limitations associated with traditional wedge grips, including grip failures, time-consuming changeovers, and inaccuracy in total elongation measurements.
Clamping forces of up to 900 kN (202,300 lbf) are available, providing positive, secure gripping of even the largest wire strand specimens. The wire strand insert arrangement allows for quick changeovers between different specimen diameters, minimizing downtime between tests. Semi-cylindrical inserts distribute the clamping force evenly across all wires in the stranded specimen, reducing the risk of grip failures and ensuring consistent, repeatable gripping performance.
Strain Measurement
Accurate strain measurement on wire strand requires a purpose-designed extensometer. For this application, an extra-long clip-on extensometer with a 24-inch (610 mm) gauge length and 15-degree twist capability is used to accommodate the helical geometry of the stranded specimen. This ensures the extensometer maintains contact with the specimen throughout the test, providing accurate strain data across the full gauge length.
Accurate strain measurement allows the system to automatically perform the calculations required by the standards, including 1% yield strength and elongation at maximum load.
Software
Bluehill® Universal software manages test setup, execution, and reporting. The software automatically zeroes the strain measurement at the preload values specified in the test standards, ensuring consistent starting conditions across all tests. Stress-strain curves are clearly displayed during and after testing, with calculation results indicated directly on the plot for easy interpretation and reporting.
Talk to an Instron Expert
Whether you are setting up a new wire strand testing program or looking to improve the accuracy and efficiency of your existing process, Instron has the systems, grips, and extensometers to meet the requirements of ASTM A370, A416, and ISO 15630.
Contact Sales to discuss your wire strand testing requirements with our team.
