The steel strand for prestressed concrete is an ordinary steel strand made of high-carbon steel wire rod, which is cold-drawn into steel wire after surface treatment, and then a certain number of steel wires are twisted into strands, and then subjected to a stable process of stress relief. The steel strand for prestressed concrete has high strength, and has the characteristics of good flexibility, stable quality, simple construction, etc., and can also be cut off according to the required length when used, and is suitable for prestressed concrete structures with large load, large span and curve reinforcement.
Test items and methods
Surface quality
The surface quality is checked visually. It is required that there shall be no oil, grease and other substances on the surface of the steel strand;There shall be no harmful defects that affect the performance of use, and axial surface defects are allowed, but the depth should be less than 4% of the diameter of a single steel wire;Slight rust is allowed on the surface, but no visually visible rust pitsTemper color is allowed to exist on the surface.
Dimensions
The diameter of the steel strand should be indexed with a value not greater than 002mm measuring tool, the measuring position is not less than 300mm away from the end. The diameter measurement method and position of steel strand of different structures are different, and they are measured according to the actual situation.
Tensile test
The test equipment should meet the requirements and be calibrated and calibrated to ensure the accuracy of the test data. The test should choose a suitable fixture to avoid the specimen breaking in or near the fixture. The prepared specimen is clamped on the testing machine to ensure that the specimen is aligned with the chuck, so that the clamped specimen is subjected to axial tension and bending is minimized. It is then stretched at a defined rate until the strand specimen breaks. During the test, the data was recorded, and the elastic modulus was measured with an extensometer, 01% yield force and 02% yield force and maximum force total elongation, the gauge distance of the extensometer should be determined according to the requirements of the relevant product standards.
Stress relaxation test
The relaxation test is to measure the change in the force on the specimen starting from the initial force by holding the specimen for a certain length at a given temperature. The specimen for the relaxation test should be kept straight. There should be no mechanical damage or handling of the specimen at the free end between the grips. Before the test, the specimen should be left in the relaxation test chamber for at least 24 hours. The specimen is clamped with grips to ensure that the specimen does not slip during loading and testing. During the whole test, the force should be applied smoothly and without oscillation, and the force should not be allowed to exceed the deviation range of the initial force at any time. Keep data records during the test.
Fatigue test
The fatigue testing machine used in the test should be calibrated regularly to an accuracy level of at least 1%. The specimen is clamped in the testing machine in the direction of the axial force, so that the specimen is not bent and all wires are evenly stressed. The specimen should be performed at a stable maximum force, a range of forces, and a frequency. There should be no pause in the cyclic force during the whole test, and the cyclic frequency should not exceed 20Hz. The test shall be terminated when one or more wires in the strand are broken, or when the damage has not occurred to the specified number of cycles.
Deflection tensile test
The deflection tensile test of the strand should be carried out by fixing the required strand specimen on a deflection device at 20° to the mandrel to determine the reduction factor of the maximum force. The surface of the mandrel groove should be carefully cleaned before the test, and if there is a slight bending of the strand, the curvature should be consistent with the direction of skew. During the loading period, there should be no slippage between the strand and the clamp to ensure the anchoring effect, and the loading speed should meet the requirements. Note: The test is invalid when the fracture of one or more of the strands is not in contact with the mandrel at the point where it is in contact. Record the valid test data, and the final test results are calculated according to the corresponding formula.
Stress corrosion test
The specimen should be wiped with a soft cloth and degreased with acetone, and allowed to dry in air. The part of the specimen that is at least 50 mm long into the container should be protected by a method to prevent corrosion, such as painting. The specimen is passed through the container, placed in the rack, and loaded with the specimen, taking care that the change in force shown throughout the test period should be kept within 2%. Once loaded, the container should be sealed to prevent leakage, and the solution should be replaced again each time the test is conducted. The sample solution should be preheated to the specified temperature before being injected into the container. The filling of the solution should be completed within 1min, and in the test, the solution cannot be circulated. The test is terminated when the wire of the strand breaks.
knotsTail
Steel strand for prestressed concrete is an important part of prestressed concrete, which can improve the bearing capacity, seismic performance and durability of concrete, and is an indispensable and important material in modern construction engineering. It is tested before use or before construction to ensure the quality and performance of prestressed concrete.