What is low-relaxation PC strand?
Low relaxation is the property that stops a prestressed structure from quietly losing its prestress. This guide explains what stress-relieving does, what the 1000-hour limit actually controls, and how to verify the claim instead of trusting the label.
Low relaxation means the strand has been stress-relieved so it loses less force over time.
Prestressing strand is held under permanent tension for the life of the structure. Over time, some of that tension is lost to relaxation — a slow, permanent deformation of the steel under sustained load. Low-relaxation strand is stress-relieved during production so that this loss is small. For ASTM A416 Grade 270 strand, a low-relaxation product is required to lose no more than about 2.5 percent of its initial load after 1000 hours at 70 percent of the specified breaking load. Normal-relaxation strand loses several times that amount, which the designer would have to compensate for by using more strand or a higher initial force.
Why relaxation matters more than it looks
A bridge girder or a slab is designed around a specific effective prestressing force. That force is what keeps the concrete in compression and stops cracks opening under service load. If the strand relaxes more than assumed, the effective force drops, the compression drops with it, and the structure behaves differently from the design — often without any visible warning.
Relaxation is not the same as creep of the concrete, and it is not the same as elastic shortening. It is a property of the steel itself, driven by the metallurgy of the wire and by the stress-relieving treatment applied after stranding. That is why it is verified by test rather than by calculation.
What stress-relieving does
Strand is made by drawing high-carbon wire rod down to wire diameter, then helically winding six outer wires around a straight core wire at a controlled lay length. In the as-stranded condition, the wires retain internal stresses from drawing and twisting. Those stresses are what drive relaxation under load.
Stress-relieving — a continuous controlled heat treatment after stranding — relieves a large part of that internal stress. The result is a strand that holds its tension far better under permanent load. The process is one of the steps that separates a genuine low-relaxation product from ordinary strand, and it cannot be replicated by inspection or by re-testing after the fact.
The 1000-hour test, and what it actually measures
Relaxation is measured by loading a strand sample to a fixed percentage of its specified breaking load — commonly 70 percent — holding it at constant length, and recording how much load is lost over time. The standard reporting interval is 1000 hours, which is roughly six weeks. Because that is impractical for every coil, relaxation is normally verified per production batch rather than per coil.
Two numbers matter when you read the result:
- Initial load — the percentage of breaking load the sample was held at. A relaxation figure quoted without this is meaningless.
- Measured relaxation — the percentage of load lost at the stated duration, compared against the limit in the standard named in your specification.
ASTM A416, BS 5896, EN 10138, AS/NZS 4672 and IS 14268 all define relaxation classes, and their limits differ slightly. The applicable limit is the one in the standard written into the approved project specification, not the one printed on a supplier's brochure.
How to verify a low-relaxation claim
A data sheet stating "low relaxation" is a claim, not evidence. To verify it:
- Ask for the relaxation test report for the production batch, not a generic certificate.
- Check that the report names the same diameter and grade as the material being supplied — a 12.7 mm result does not certify 15.24 mm.
- Confirm the initial load percentage and the test duration are stated.
- Compare the measured value against the limit in your specification's standard.
- If the order requires it, arrange third-party verification by SGS, BV or Intertek.
What low relaxation does not cover
Low relaxation says nothing about tensile strength, elongation, fatigue performance, corrosion protection or dimensional tolerance — each of those is a separate requirement in the same standard. A strand can be fully low-relaxation and still fail a project for another reason. Treat the relaxation report as one line of evidence in a document set, not as a complete quality statement.
Common questions on relaxation
What does low relaxation mean?
Low relaxation means the finished strand has been stress-relieved so that it loses less prestressing force over time under permanent load. For ASTM A416 Grade 270 strand, a low-relaxation product is required to lose no more than about 2.5 percent of its initial load after 1000 hours at 70 percent of the specified breaking load, compared with a much higher loss for normal-relaxation strand.
How do I verify the claim?
Ask for the relaxation test report for the production batch, not just a statement on the data sheet. The report should state the test duration, the initial load as a percentage of breaking load, and the measured relaxation. Check that the value is at or below the limit set by the standard named in your specification, and that the report references the same diameter and grade as the material being supplied.
Which standards define it?
ASTM A416, BS 5896, EN 10138, AS/NZS 4672 and IS 14268 all define relaxation classes for prestressing strand, with limits that differ slightly between them. The applicable standard is the one written in the approved project specification; the relaxation limit must be taken from that standard rather than assumed.
Need a relaxation report for your order?
Send the standard, diameter and quantity with your inquiry. The relaxation report is issued with the mill test certificate, and third-party verification can be arranged before shipment.
Request documentationSpecifying low-relaxation strand for a project?
Send the standard, diameter and quantity — we will confirm the relaxation limit and the document set.