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The
Concrete Institute of Australia
introduced the new guideline document
“Performance Criteria for Concrete
in Marine Environments” at a well-attended
seminar in Sydney recently.
Industry
interest in a viable performance-based
specification for design-life durability
is obviously very strong, however
the document does not provide an
actual specification.
Drafting
Committee member Mr. Daksh Bawega
cited two major challenges as;
1)
The divergence of views on which
criteria are most appropriate (e.g.
absorption, diffusion, resistivity,
strength, etc.).
2) The difficulty
of correlating results from “early-age”
or accelerated tests, with long-term
durability under real conditions.
Formal agreement on the
criteria & test method that
satisfy all views may well be some
time away. Adam Fitzhenry of Standards
Australia suggested that an Australian
Standard for marine concrete might
be produced within two years at
best, calling for industry-wide
input.
In the meantime, important
marine project specifications are
limiting the absorptiveness or Absorption
of air-exposed concrete elements.
Researchers
from authorities like the CSIRO,
Transport Research Laboratory (UK),
the NSW RTA and VicRoads confirm
that capillary absorption is the
primary mechanism by which water
& chlorides infiltrate the air-exposed
elements of marine concrete. This
means that here diffusion is a secondary
“symptom” of absorption, & that
if concrete did not absorb water
or get “wet” inside, corrosion would
not occur.
In air-exposed
marine concrete, all corrosion processes
require the presence of absorbed
water, so this makes logical sense
too.
Work by international
concrete technologies group Cementaid
has validated this research. Non-Absorptive
concrete structures exposed in salt
pans over 40 years, remain corrosion-free.
This
has given Cementaid the unique ability
to correlate results from simple
to perform “early-age” tests, with
successful long-term performance
under varied conditions, over periods
of up to 40 years.
Absorption
testing & chloride analysis
of samples from these corrosion-free,
ultra-low absorption concretes,
allowed the development of a “time-proven”
Performance Specification, based
closely on the BS Absorption test.
For
all air-exposed elements, a maximum
7-Day BS Absorption value of 1%
(one percent) is specified. This
is based on the proven corrosion-free
durability of ultra-low absorption
concretes over periods of up to
40 years. If required, compliance
testing of production concrete and
non-compliance remediation procedures
are also provided.
The BS
Absorption test does not directly
“measure” durability – No test does.
However it is directly related to
the primary water & chloride
transport mechanism, and has a long
history of use elsewhere in the
(British) Standards for durability
of concrete paving (BS368:1956).
It also has the important advantages
of being accurate & repeatable,
as well as quick and low cost to
perform.
The
Specification further requires that
any admixtures or ingredient used
to achieve the absorption limit
should have a successful history
of use under severe marine conditions,
without any diminution of performance,
or adverse effect on other concrete
properties. This protects the owner,
specifier & builder against
unproven, “short-lived” and / or
unsafe materials.
Commenting on the need for low absorption at early-age (7 days), concrete consultant Dr. Adam Neville (UK) said; “While PFA or ggbfs mixes show good behavior at 28 days, they are very vulnerable at the age of a few days. Testing at an early age is useful both because the quality is assessed when there is still time to manoeuvre and also because it is important that the concrete is satisfactory at the time of its first exposure to an aggressive environment.” Noting cases of past admixtures where their effect did not persist over the years, Dr. Neville said “It is clear that a material with a long history of successful behavior justifiably inspires confidence”.
The Absorption-based Performance Specification has been in successful use for over 15 years now. “CALTITE” concretes are compliant with this Specification, and continue in-service now over 40 years under extremely severe chloride and sulphate conditions – without any incidence of corrosion, or maintenance down-time. |
40-YEARS
EXPOSURE IN TIDAL SALT-PAN; BHP
Steel. S. Australia
Above; Plain
concrete of normal absorption (3-5%)
after 40 years.
Below; “CALTITE”
Ultra-low Absorption concrete (<1%)
after 40 Years
NON-ABSORPTIVE
“CALTITE” concrete. Caltite:
Hydrophobic
Pore-blocking
Ingredient, or HPI, that reduces
concrete absorption to a fraction
of normal high quality
concrete.
HMAS
WATERHEN Naval Jetty, Sydney; Sinclair
Knight Merz specified maximum 7-Day
BS Absorption of 1% for jetty structure
concrete.
HMS TAMAR Multi-Level Submarine Maintenance Jetty; Mott
McDonald (HK) specified max. BS
Absorption of 1%, allowed favorable
modification of section thickness,
heat, & weight of structure.
SEABIRD Naval Base , India . Shiplift & Pier 1. (Redecon SKANSKA JV); All pile caps, precast and in-situ concrete specified low-absorption HPI concrete, with BS Absorption limited to max. 1% at 7 days-age. The extensive use of precast elements and practical QA procedures promoted excellent production quality outcomes. All test samples from 13,000m3 of HPI concrete complied with the Absorption limit, achieving an average BS Absorption of 0.69%.
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A
copy of the Specification follows,
and more information can be reviewed
/ downloaded at:http://www.cementaid.com/caltite2.htm
Performance
Specification (Absorption) for Marine
Concrete
GUIDANCE
NOTE: The performance and the effective
life of integral waterproofing compounds
can vary greatly within the industry.
Therefore, an ingredient with an
established, long-term history of
successful use, such as that stated
in clause (d) below, is ane ssential
component in aperformance-based
specification of this type. This
specification is based on the long-term
performance of one such ingredient,
Everdure Caltite, continuously performing
in acutal field structures up to
40 years old.
Ultra-Low Absorption
concrete;
“All concrete (specify
sections) shall conform with
all specified requirements and also
the following;
The minimum
contents of Ordinary Portland
Cement (OPC) and / or blended cementitious
materials (in Kg / m3) shall be:
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Mix
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Min.Total
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Min.OPC
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OPC
only
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335
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335
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OPC
/ slag
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400
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160
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OPC
/ PFA
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380
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300
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1)
The concrete shall contain an approved
High Range Water Reducing admixture
such that the free water:cement
ratio shall not exceed 0.40 and
the concrete will be of adequate
workability for placement.
2)
The concrete shall contain
a time-proven effective Hydrophobic
Pore-blocking Ingredient (HPI) system
(such as “Everdure Caltite”, or
proven equal and approved in writing),
used strictly in accordance with
manufacturers’ instructions and
providing a hydrophobic / non-absorptive
cement matrix throughout and dispersed
polymer particles suitable for use
as a pore-blocking agent.
3)
The HPI system must produce concrete
conforming with all specified requirements
and shall further be shown to produce
concrete with a corrected 30 minute
absorption of not greater than 1%
as measured by BS 1881: Part 122:
1983, except that the age at test
shall be 7 days.
3.1) Absorption
testing is to be conducted on cores
taken from cubes or cylinders as
specified in the standard, unless
the use of cast specimens is approved
by the Engineer.
3.2) Where
an existing or pre-approved mix
design shown to comply with the
above absorption requirement is
not available then, prior to construction
and at least 30 days before
the actual Ultra-Low Absorption
concrete is to be cast, the Main
Contractor shall arrange for trial
mixes to be conducted, under the
supervision of the Engineer or his
representative, and with the supplier
of the Hydrophobic Pore-blocking
Ingredient present, to confirm that
the proposed mix conforms with the
absorption limit and other specified
requirements.
4) Any admixture
or ingredient, other than cement,
aggregate or approved water-reducing
admixture, used by the Contractor
to comply with the absorption requirement
must be shown by an independent
authority to have been successfully
used in concrete structures, including
those exposed to a severe marine
environment, for a minimum of 15
years without significant deterioration
of the cement matrix, embedded reinforcing
steel, or measurable reduction in
performance of the product on absorption.
5) Admixtures must not contain
chlorides. The Supplier shall satisfy
the Engineer that any admixture
or ingredient used, or their combination,
does not detrimentally affect the
strength or other properties of
the concrete.
Production,
Testing and Quality Assurance
6)
During construction, two absorption
test samples shall be taken at the
same time as samples for compressive
strength testing. These samples
shall be tested as above and shall
conform with the specified absorption
requirement. In the event of non-compliance,
the Main Contractor will follow
the procedures set out in Section
II.
7) The concrete as placed
and cured in the actual structure
is required to comply with the water
absorption limit within 7 days of
placement. The Engineer reserves
the right to take cores to confirm
compliance.
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