In the static tensile test to ISO 15630-3, ASTM A416 and ASTM A1061 the steel strand is elongated until it breaks. Steel strands consist of several wires that are twisted together, which try to untwist when subjected to tensile loads. During the tensile test, the force and elongation of the specimen are measured. Since prestressing steel is a high-strength steel, primarily testing machines for high loads are used.
Tensile tests can be performed on steel strands with a diameter of 3 to 20 mm. Depending on the product or test standard, the gauge length L0 can, for example be 500 mm or 610 mm. The overall elongation can reach values up to 20%.
Deflected tensile tests, e.g to EN ISO 15630-3, can also be performed. For these tests, the moving crosshead of the testing machine is equipped with a through-hole.
- The yield point as offset yield (Rp0.2); generally determined as replacement yield point at 0.2 % plastic elongation
- The yield point elongation; more accurately yield point extensometer elongation, because it can only be determined with the use of an extensometer (Ae)
- Tensile strength (Rm)
- Uniform elongation (Ag)
- Elongation after break (A), whereby the normative specifications with regard to the gauge length are of significant importance
Due to the high tensile strength of the individual steel strand wires—up to 2000 Mpa—and their smooth surface, it is particularly important to ensure that the specimen is not damaged by clamping (for example by a pyramid pattern on the jaw inserts). Therefore hydraulic, parallel-closing specimen grips with adjustable clamping pressure are required. In addition, the geometry of the jaws should be designed in such a way that reliable and homogeneous gripping of the specimen is possible, so that the specimen does not start slipping during the test.
To avoid specimen break outside the gauge length L0, the product quality of the strand sample itself is also important. Only specimen tearing within the gauge length can be accepted as a reliable test result. A damaged specimen surface or strongly inhomogeneous tensile strength of the individual wires often lead to specimen break outside L0, near the jaws.
Prestressing steel is a high-strength steel that is primarily used in prestressed concrete construction. The use of prestressing steel makes it possible to significantly increase the properties of concrete components for static and dynamic loads.
Prestressing steel is manufactured from a long product and its surface shape varies according to the manufacturing standard. An individual wire, for example, is designed according to the manufacturer standard DIN EN 10138-1, while a strand wire can be designed according to DIN EN 10138-3. The individual wire has strength values up to 2000 N/mm² and is designed in a round shape, whereby there are also trapezoidal cross-sections, which are then referred to as compact strands.
Under the general heading of prestressing steel, there are three forms that it may take depending on the product standard.
- Bar steel (diameter 15 to 36 mm)
- Wire (diameter 5 to 16 mm)
- Steel strands made up of 3, 7 or more twisted individual wires (three-wire strand: 5.2 to 7.5 mm, seven-wire strand: 7 to 18 mm)
These different forms impose high demands on materials testing and therefore a wide variety of test standards (ex. ISO, ASTM, etc.) are used to ensure that the required standard of quality is met.