Low relaxation strands are used for pre-stressed concrete as they lose stress more slowly over time compared to normal strands. This provides benefits like reduced steel and concrete usage, improved performance, and more predictable loading capabilities. Low relaxation strands have properties like less stress relaxation, higher breaking and proof loads, and better corrosion resistance. Quality requirements for strands include standards like ASTM A416 and testing frequencies specified by project specifications and quality control standards. Testing must be done by approved third parties regardless of manufacturer testing and the most stringent requirements must be followed.

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Quality Bulletin # 50
Low Relaxation PT Strands for
Pre-stressed Concrete
Why Low Relaxation Strands?
Low Relaxation Strands that are pre-stressed and
embedded in concrete lose the initially applied stress
exponentially with the passage of time. The utmost
important factor attributing to this loss in stress is the stress
relaxation property of the steel itself. By treating the steel
through a thermomechanical process known as stabilizing,
the propensity of the steel to “relax” under a stressed
condition is controlled to a great extent.
Some of the advantage are :
· Up to 10% reduction in steel reinforcement is possible,
· Savings in No.of strands being used,
· Reduction in concrete requirement due to reduced size
of structural members,
· Less loss of initial tension, resulting improved & more
predictable service performance, and in many cases
allow a higher load carrying capability,
· Provide improvements in deflection and crack control.
Benefits of Low-relaxation strands Vs Normal
relaxation strands :
· Less relaxation loss,
· Higher breaking loads,
· Higher proof loads,
· Higher fatigue and corrosion resistance,
· Better performance at elevated temperature.
Quality requirements of PT Strands :
· Applicable code of practices :
o ASTM A416
o BS 5896
o BS EN ISO 15630-3
The scope of BS 5896 standard applies only to products as
supplied by the producer.
Test Frequency requirements :
Project specification :
· Relaxation: 3 samples on random reels+ other
requirements
· Tensile : 1 sample from each reel
QCS 2014, Section 5, Part 18 – Prestressed Concrete
· Relaxation : 3 samples per each reel
· Tensile : 1 sample per every 1000m of length
QCS 2014, Section 2, Part 14 – Quality Assurance and
Quality Control
· Relaxation : 1 test per each batch
· Tensile : 1 test per each reel
It should be noted that ASTM A416 and BS 5896 are silent
on the required testing frequency for the relaxation testing.
Testing – Other requirements :
As per project specification & QCS 2014, All required
testing shall be performed by approved 3rd party
laboratories irrespective of whether the material have been
tested by the manufacturer of by an intermediate party or
the availability of conformity certificates, also the Contractor
shall carry out the maximum number of tests, i.e. whichever
condition comes first (i.e. whichever is more frequent).
In case of conflict between the requirements mentioned in
the project specification & QCS, the most stringent
requirements shall take precedence, or as approved by the
Engineer.
ASTM A416 Requirements :
BS 5896:2012 Requirements :
Duration of the testing :
As per Cl. 6.5 ASTM A416m the relaxation testing shall be
carried out for 1000 hours.
As per Cl. 8.4.9 of BS EN ISO 15630-3:2010, a common
duration of the testing is 120 or 1000 hrs, however the value
of stress relaxation at 1000 hrs (or more) may be
extrapolated from tests terminating at not less than 120 hrs,
where adequate evidence is provided that the extrapolated
1000 hrs (or more) value is equivalent to the actual 1000
hrs (or more) value. In this case, the extrapolation method
should be clearly described in the test report.
Where the projects allow testing to be carried out to this minimum
requirement, a cost saving will be offered to the Client.
Approved: Date :
This Quality Bulletin is for information to all Supervision Consultant and Construction
Contractors working on the Expressway Programme.
This Quality Bulletin is not to be released outside of ASHGHAL Expressway
Programme without prior authorisation by the HPD Quality Manager. Issued by
Expressway HPD Quality Department Contact Tel. +974 5520 6346