Stress Relaxation Test
(Bar & Wire Relaxation Test)

Long-duration relaxation-loss assessment of prestressing wires, strands and high-tensile bars — verifying tendons retain their design prestress force over time, per IS 6003, IS 14268 and IS 2090.

Overview

What is the Stress Relaxation Test?

The Stress Relaxation Test measures the loss of stress (tension) in a prestressing tendon — wire, strand or bar — held at a constant length over an extended period. This is distinct from creep, which is deformation under constant stress; relaxation is the phenomenon prestressed concrete designers must account for, since the tendon's ability to hold its designed prestress force directly determines the long-term crack control and structural performance of the finished member.

The test applies across all three major categories of prestressing tendon used in Indian construction: indented wire under IS 6003, seven-ply (seven-wire) strand under IS 14268, and high-tensile bars under IS 2090 — the material specifications referenced by IS 1343, the governing Code of Practice for Prestressed Concrete. Global Lab conducts relaxation testing with precision load cells and constant-strain test rigs, verifying whether a given tendon batch qualifies as Low Relaxation (LR) grade.

Reviewed by Mahemood Ahmad, Lab Manager — NABL Accredited Laboratory, Global Lab India

Long-Duration Testing

1000-hour continuous relaxation monitoring, or a shorter validated period where extrapolation to 1000 hours is demonstrated.

Constant-Strain Method

Tendon held at a fixed gauge length while load-cell instrumentation tracks the resulting stress decay over time.

All Tendon Types

Covers indented wire (IS 6003), seven-ply strand (IS 14268) and high-tensile bar (IS 2090).

Precision Load Cells

Continuous, high-resolution force monitoring throughout the full test duration.

Low-Relaxation Verification

Confirms whether a tendon batch meets Low Relaxation (LR) classification for high-performance PSC structures.

NABL Certified Reports

Traceable, accredited results accepted for project QA/QC and statutory documentation.

Test Procedure

How the Stress Relaxation Test is Conducted

1

Specimen Preparation

A tendon sample is cut to the test gauge length — a minimum of 60 times the nominal diameter — and mounted in the relaxation-test rig.
2

Initial Load Application

Load is applied uniformly over 3 to 5 minutes up to the specified initial stress, commonly 70% of the tendon's specified breaking load, without over-stressing the specimen.
3

Constant Strain Hold

The gauge length is held fixed for the remainder of the test; load-relaxation readings commence 1 minute after the total load is reached.
4

Continuous Monitoring

Stress decay is recorded at defined intervals over 1000 hours, or a shorter period of at least 200 hours where extrapolation to 1000 hours can be reliably demonstrated.
5

Reporting & Classification

Total relaxation loss is calculated and the tendon is classified against Normal/Low Relaxation limits; an NABL-accredited report is issued with the full time-vs-stress data.

Standards & Specifications

Applicable Test Standards

Standard	Title	Scope	Type
IS 6003	Indented Wire for Prestressed Concrete — Specification	Wire Relaxation Requirements	Indian Standard
IS 14268	Uncoated Stress Relieved Low Relaxation Seven-Ply Strand for Prestressed Concrete — Specification	Strand Relaxation Requirements	Indian Standard
IS 2090	High Tensile Steel Bars used in Prestressed Concrete — Specification	Bar Relaxation Requirements	Indian Standard
IS 1343	Code of Practice for Prestressed Concrete	Design Use of Relaxation Data	Indian Standard

Applications

Where is the Stress Relaxation Test Used?

Quality verification of prestressing wire, strand and bar before use in bridges, flyovers and PSC girders

Confirming Low Relaxation (LR) classification for high-performance prestressed structures

Pre-construction material acceptance testing for PSC railway sleepers and poles

Long-term prestress-loss estimation for structural design and durability assessment

Batch verification testing for tendon manufacturers and suppliers

Investigation of in-service prestress loss on existing structures

Why Choose Global Lab

Our Advantages

NABL Accredited ReportsLegally valid certificates accepted by all government and statutory bodies.

Precision InstrumentationCalibrated load cells and constant-strain rigs for repeatable, defensible results.

All Three Standards CoveredWire, strand and bar relaxation testing under one roof.

Detailed Relaxation Curve ReportingFull time-vs-stress data supplied with every certificate, not just a single figure.

Pan-India CoverageField engineers and sample logistics across 7 lab locations.

Honest Turnaround GuidanceRelaxation testing takes real time — we tell you the true 1000-hour (or validated 200-hour minimum) schedule upfront.

Quick Answers

Frequently Asked Questions

What is the difference between stress relaxation and creep in prestressing steel?

Relaxation is the loss of stress in a tendon held at constant length — the relevant phenomenon for prestress-loss calculations. Creep is deformation under constant stress. They are related but distinct properties, and prestressed concrete design specifically relies on relaxation data.

How long does a stress relaxation test take?

The standard test duration is 1000 hours. A shorter validated period of at least 200 hours may be used where extrapolation to the full 1000-hour value can be reliably demonstrated.

Which materials are covered by IS 6003, IS 14268 and IS 2090?

IS 6003 covers indented wire, IS 14268 covers seven-ply (seven-wire) strand, and IS 2090 covers high-tensile bars — all used as prestressing tendons in prestressed concrete construction under IS 1343.

What is Low Relaxation (LR) steel?

Low Relaxation tendons are specially stabilized during manufacture to lose significantly less stress over time than Normal Relaxation tendons — the preferred choice for modern prestressed concrete structures where long-term prestress retention is critical.

Stress Relaxation Test(Bar & Wire Relaxation Test)