UK CARES

Report
Certification for the Installation of
Post-tensioning Systems
by Richard Digman
PT Certification
Overview
• CARES is a certification body which is accredited by the
UK Government via UKAS (United Kingdom
Accreditation Service).
• The Crown and Tick in the CARES logo indicates
Government accreditation and the registration number
0002 indicates that CARES was the second certification
body to be accredited.
• CARES is a specialist certification body operating in the
construction industry covering:
– Reinforcing and prestressing steels
– Ancillary products: couplers, continuity systems, punching shear
systems.
– Post-tensioning contractors, post-tensioning kits.
Post-tensioning
Contractor Certification
• CARES has two main schemes for PT
contractors:
• CARES PT1. for highway structures (bridges).
• PT2. for flat slab construction.
• Initial CARES certification requires an audit of
the head office QMS and audits of all PT site
activities.
• Maintenance of certification requires one audit of
head office and a minimum of three site audits
per year.
PT Office Requirements
• Quality management system to ISO 9001
• Contract review – to ensure contract
requirements are met.
• Purchasing – to ensure products comply with
contract specification and relevant standards.
• PT operative training – to ensure that only skilled
operatives undertake key PT operations.
• Qualified Post-tensioning system (kit).
• Site quality plan and method statements
covering all PT activities.
Post-tensioning Operative
Training
• Post-tensioning operations must be carried out
by operatives with appropriate knowledge,
training and proven experience.
• The PT contractor must train and certify
supervisors and operators to meet the
requirements given in CARES Appendix PT9.
• The number of inexperienced personnel
(labourers for example) should be limited and be
balanced according to circumstances but
normally not exceed 50% of the post-tensioning
personnel.
Site Requirements
• Compliance with contract specification,
quality plan and best practice.
• Receipt and storage of materials/products.
• Installation processes compliant with
method statements.
• Inspection and testing.
• Site records.
Installation: Storage of
Materials on Site
• Materials must be stored on site in a way
that prevents damage or deterioration.
• Bagged materials have a specified shelf
life and the bags are normally date
marked. The material must be used by the
specified date.
• Strand must not be corroded!
PT System Qualification
• Whole PT kit should comply with BS EN 13391 or
ETAG013 which ensures:
– Load efficiency of >= 95%.
– Fatigue resistance >= 2 million cycles.
– Anchorage can carry a load of 1.1 fpu.
• Evidence of PT kit qualification should be obtained such
as a product conformity certificate or test report.
• The evidence should also detail the PT kit specification,
it is not acceptable to mix components from different kits
particularly strand types.
Strand Requirements
• Strand to comply with BS 5896 and be relaxation
class 2.
• BS 5896 has three strand types
– (a) seven-wire standard strand,
– (b) seven-wire super strand,
– (c) seven-wire drawn strand.
• Strand should be designated in accordance with
BS 5896 for example seven-wire super strand,
nominal diameter 12.9 mm and nominal tensile
strength 1770, class 2 relaxation is designated:
– BS 5896/3 super strand - 1770 - 12.9 - relax 2.
Unbonded Tendons
• Unbonded tendons are enclosed in a plastic
sleeve which must be at least 1.0mm thick and
made from high density polyethylene or
polypropylene. PVC should not be used.
• High density polyethylene is more flexible and
less liable to embrittlement at extremely low
temperatures, while polypropylene is more
stable at high temperatures.
• The friction between the coating and the strand
should be no more than 60N/metre.
Duct and Vent
Requirements
• Duct, vent and connection material should be:
• Smooth galvanised steel with a minimum wall
thickness of 0.35mm.
• Corrugated galvanised steel with a minimum
wall thickness of 0.30mm.
• High density polyethylene or polypropylene with
a minimum wall thickness of 2.0mm.
• PVC should not be used.
Installation: Duct, strand,
anchorages ….
• The duct system/tendon must be adequately fixed to
resist movement and floatation during concrete
placement.
• Tendons should be installed to an accuracy of 5mm
vertically (t/40 for slabs less than 200mm thick) and
50mm horizontally in beams and 150mm horizontally in
floor slabs.
• Unbonded tendons can be deviated to avoid obstructions
with the agreement of the Contract Administrator (CA).
• Tendons with flat ducts are relatively stiff in the
transverse direction and cannot usually be deviated
around obstructions.
• Tendon profiles should be smooth.
• Bursting Reinforcement must be concentric with the
tendon.
Bursting Reinforcement
(Photo by Balvac)
Bursting reinforcement must be concentric with the tendon in order to
be effective (the above is awaiting fixing).
Concreting
(Photo by Balvac)
Tendons need to be fixed to avoid displacement or flotation during concrete
placement.
Location of Vents
• Vents should be fixed at injection and exit
points and should extend approximately
500mm above the slab surface.
• Where tendon drape exceeds 500mm,
intermediate vents should be fixed at
tendon high points.
• Intermediate vents should normally be
used on tendons over 15m-20m in length.
Installation: Stressing
Equipment
• Stressing jacks and their load measuring system
should have an appropriate and current
calibration certificate, which is traceable to
national standards, and no more than 6 months
old at the time of stressing.
• The calibration certificate should be provided by
a qualified laboratory and should include a
calibration curve.
• The stressing equipment should be capable of
establishing a tendon load to an accuracy of +/2%.
Installation: Stressing
• The post-tensioning contractor should calculate the
theoretical tendon extension.
• Stressing should not commence without prior agreement
on theoretical extensions nor before the concrete has
achieved the specified transfer strength.
• The jacking force should not normally exceed 75% of the
tendon’s characteristic strength
• At transfer, initial prestress should not normally exceed
70% of the tendon’s characteristic strength, and in no
case should exceed 75%.
• Tendons should be stressed in the specified sequence
and load increments.
Installation: Stressing
Tendon Elongation
• The prime measurement of tendon load is through the
stressing jack and recorded on the stressing record.
• Tendon extensions are a secondary indicative measure.
• For slabs load/extensions are normally measured prior to
stressing, and after stressing and locking off to an
accuracy of 2% or 2mm and take into consideration the
possible strand movement at the dead end anchor.
• For long complex profiles such as bridge decks, multiple
measurements are usually taken to allow extrapolation to
determine both the elastic and non-elastic extension.
• The actual extensions should normally be within +/-6%
of the theoretical extensions for groups of tendons and
+/-15% for individual tendons.
Extension Measurement
Measurement of tendon extension at the stressing jack.
Extrapolation of
Extensions
250
Extension (mm)
200
150
Measured
Extension
100
50
Extrapolated
Extension
0
0
10 20 30 40 50 60 70 80 90 100
% Jacking Force
Tendon extensions are usually measured at two loads points and plotted
on a graph and the extension extrapolated back to the zero load point.
The above graph shows approximately 25mm displacement at zero load
which represents the initial slack with the system.
Installation: Grout
• Steel in a stressed state is subject to accelerated
corrosion, it is therefore important that tendons are
quickly and correctly protected from corrosion.
• It is recommended that grout is made from pre-bagged
material requiring only the addition of a measured
amount of water.
• Grout should be batched to an accuracy of +/-2% for dry
materials and +/- 1% for mixing water.
• Grout made from bagged cement is non preferred
because of the variability of material properties and bag
weight.
Installation: Grout
Properties
• Grout should have the following properties, determined
in accordance with the test procedures of ETAG 013
Annex C clause C.4.3.3.2.
Extract from CARES Model Specification
Installation: Grout w/c
Ratio
• The w/c ratio should be kept as low as
possible as bleed water is excess water in
the grout that is not chemically bound and
can lead to low density grout, grout
cracking, shrinkage and porosity.
• The necessary grout properties can be
achieved with a properly formulated grout
and the addition of minimum quantities of
water.
Installation: Grouting
• It is recommended that ducts are blown
through with oil free compressed air prior
to grouting.
• Grout injection should be continuous at an
agreed rate and be slow enough to avoid
segregation of the grout.
• The method of injecting grout should
ensure filling of the ducts and complete
surrounding of the steel.
Grouted Tendon
(Photo Balvac)
Section through grouted duct
Installation: Grout Venting
• Grout should be vented from each
intermediate and exit vent until it is of the
same consistency as that of the injected
grout.
• All vented grout should be discarded.
• All vents should be closed one after
another in the direction of grout flow and
the injection sealed off under a nominal
pressure of 0.1MPa (14psi).
Installation: Post Grout
Inspection
• The level of grout in the injection and the vent tubes should be
inspected after the grout has set. If the level of grout has fallen
below the required final level or the material is degraded, all
defective grout should be removed from the vent and topped up.
• Grouted ducts should be protected to ensure that there is no
damage to the grout due to shock or vibration for 24 hours after
injection of the grout and that the temperature in the ducts does not
fall below 5°C for 24 hours after injection of the grout.
• Two days after grouting, the level of grout in the injection and vent
tubes should be inspected and if the grout level has fallen below the
top of the slab, remedial measures should be agreed with the
Contract Administrator (CA).
• On completion of grouting, grout vents should be cut off to slab level
and positively sealed to ensure the encapsulation to the tendons is
complete.
Installation: Grout Testing
• Grout suitability tests should be undertaken prior to
contract.
• Grout acceptance tests should be undertaken during
contract at the following frequencies:
Extract from CARES Model Specification
Installation: Grout Testing
(Photo Balvac)
Fluidity testing of grout
Installation: Sealing of
Anchorages
• Anchorage components must be sealed against the
ingress of water or aggressive agents likely to cause
corrosion of the steel or anchorage.
• In the case of grouted tendons, special mortars/renders
and bonding agents are recommended, as mortar/render
can be permeable and subject to shrinkage.
• The chosen method of sealing shall be capable of
resisting the specified grout pressure.
• For unbonded tendons it is recommend that anchorage
components are coated with grease of similar
specification to that used in the tendon and that a
watertight cap be applied over the coated area. The
minimum concrete end cover to the cap should be
25mm.
Installation: Sealing of
Anchorages
(Photo Balvac)
Sealing of PT anchorages against corrosion
Specification
• The key to a good PT installation is a good PT
specification.
• The lack of guidance to specifiers has lead to
some inappropriate and out of date PT
specifications.
• For this reason CARES has produced “Model
specification for bonded and unbonded posttensioned floors” to give guidance to specifiers.
• The model spec is available for free download
from www.ukcares.com
CARES Model
Specification

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