paper tseng thaumasite prevention
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7/28/2019 Paper TSEng Thaumasite Prevention
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6 May 2003 – The Structural Engineer|15
technical update: thaumasite
Following the identification of the
thaumasite form of sulfate attack
(TSA) on the concrete foundations
to a number of bridges on the M5
motorway in 1998, the then Minister
for Construction, Nick Raynsford,
established the Thaumasite Expert
Group (TEG) under the chairmanship
of Les Clark, Professor of Structural
Engineering, University of
Birmingham. It was asked to report on
the nature and threat of this phenome-
non and to provide interim guidance on
its avoidance.
The Report of the TEG1 was
published by the Department of
Environment, Transport and the
Regions (DETR) in January 1999, and
a summary of the structural implica-
tions of TSA was published in The
Structural Engineer the following month2. At the Minister’s request, the
Report was followed up in Spring 2000
by a One-Year Review3 of the Report’s
factual statements and guidance in the
light of new developments on TSA.
A second review4 of the Thaumasite
Expert Group Report has now been
undertaken following 3 years experi-
ence up to March 2002. Following the
demise of DETR, this review was
accepted by the Office of the Deputy
Prime Minister (ODPM) in January
2003. The principal sections of the
Review are summarised in a paper in
the journal Concrete prepared by theGroup5. This article gives a brief
summary of the main conclusions of
the Review and highlights a structural
issue.
Current impacts and awarenessThe TEG Report continues to be well
received by the construction industry
and is broadly viewed as well balanced,
safe and robust. Its publication has not
had a significant adverse impact on the
various stakeholder sectors.
In respect of existing buried concrete
construction, the greatest impact of
TSA to date has been on the Highways Agency (HA). The HA is undertaking a
strategic assessment of structures in
its care to check for TSA. Its experience
has shown that where the risk factors
identified in the TEG Report are all
present, the prevalence of TSA is high,
although the severity varies.Where
necessary, remedial measures involv-
ing concrete repair and protection have
been undertaken, providing assurance
of serviceability and safety.
The TEG is concerned that no other
owners of existing structures with
buried concrete elements have under-
taken a similar assessment. The expe-
rience of the HA indicates that there
could be a large number of structures
with some degree of attack to buried
concrete. It is therefore important that
owners of key structures do assess the
risks and take necessary measures.
New guidance A substantial body of new guidance
has been published in 2000 – 2002.
This has evolved from the TEG Reportbut has taken into account industry
consultations and new research find-
ings. Publications include a new BRE
Special Digest 1: Concrete in aggressive
ground6, Amendments to BS 5328:
Concrete7, Amendments to BS 882:
Specification for aggregates from
natural sources for concrete8, new
BS 8500: Concrete – complementary
British Standard to BS EN 206-19, and
a new BCA Guide: Concrete resistant to
chemical attack10.
There have been a few problems in
interpreting and/or implementing the
intentions of the guidance given in theTEG Report in some situations. These
problems have been addressed through
publication of the new guidance listed
above and through discussions with
the relevant industry bodies. In partic-
ular, problems were encountered by the
manufacturers of precast concrete
construction products, including those
for precast concrete pipes and tunnel
linings, in complying with the TEG
Recommendations. These have been
resolved by the inclusion in the new
BRE SD16 of specific design guidance
for these products.
Guidance in preparationKey items of guidance which have been
prepared for publication in 2003 are:
• A second edition of BRE SD1 to align
included cements or combination
groups with the new BS EN 197-
11111 classification.
• A revision of the NHBC Standards,
Chapter 2.1, Concrete and its rein-
forcement12.
New field cases of TSA About 50 new cases of TSA have been
identified since the publication of the
TEG Report and 30 since the publica-
tion of the first review in 2000. Most of
these were in the foundations of
bridges founded on Lower Lias Clay
(Charmouth Mudstone Formation),
found as a result of HA investigations
in Gloucestershire, Somerset and
Wiltshire. The remaining cases have
occurred in a wide range of structures,
buildings and environmental condi-
tions. Most of these new cases have
taken place in conditions that were
anticipated in the TEG Report.
Significant features which have
emerged from inspection of these new
cases of TSA are:
• Three further sources of sulfate in
the ground have been identified
leading to TSA: Kimmeridge Clay,
Rheatic Mudstone and railway ash.
• Two new field cases of TSA in buried
concrete containing siliceous aggre-
gates, reinforcing a concern
expressed in the TEG Report that
concretes containing little or no
carbonate in aggregates can be
affected by TSA if an external source
of carbonate ions is available, for
example from groundwater. This is of
significance since currently the guid-ance for specification of concrete
presumes that restriction of the
carbonate content of aggregate is an
effective measure for combating
TSA. Present understanding of the
problem is insufficient to underpin
amendment of the guidance, but
further research is underway.
• Investigations have concluded that
bituminous waterproofing applied to
some of the buried concrete exposed
to sulfates may have prevented the
onset of TSA.
Research on occurrence andmitigation of TSARecommendations for future research
given in the TEG Report are being
largely met by projects initiated by the
BRE, the universities and industry.
Initial findings of particular signifi-
cance are:
• The mix designs recommended in
the TEG Report are robust.
• Significant deterioration in some
concretes not meeting the recom-
mendations in the TEG Report have
been observed in both laboratory and
field trials after as little as 3 years
exposure to sulfate Class AC-3conditions.
• Mixes containing ground granulated
blastfurnace slag (ggbs), in sufficient
proportions, continue to demonstrate
excellent resistance to TSA.
New guidance andprogress on
thaumasite control A summary the findings of the Thaumasite Expert GroupReport: ‘Review of structural aspects after 3 years experience’
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