Transcript FUTURECRETE

The Future of Concrete in
a Greener World
Ken W. Day
In 1981 my paper “Concrete in 1991” to OWICS was
easy to write
I even suggested it may be suitable for presentation in
1991 as “Concrete in 2001” due to excessive
conservatism in the industry and professions
Today it is much more difficult to look 10, or even 5,
years into the future
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A large variety of cement replacement
materials and chemical admixtures are already
in use;
Concrete can be self-consolidating or roller
compacted, have a strength of more than
200MPa, be pumped to 600 metres height and
claims are made of 100year durability.
Several organisations are either making or
investigating concrete made with no Portland
cement whatever;
Mix design is shifting from grading curves to
nano-technology and natural sand is becoming
unavailable in many areas.
Technical and economic considerations
are likely to be skewed by the imposition
of carbon taxes and the award of “green
points”,
Yet there is still no general agreement
on the best criterion of durability and
some countries, including USA, are still
using antique methods of specification.
We need to start our considerations with
cement
Cement production is a major source of CO2
But concrete is a desirable construction material,:
having low embodied energy, high thermal mass,
potentially high durability
Cement production is one of world’s major
businesses so there is:
Significant cost and inertia in any change
And the cement industry has a significant voice in
government policy
So the task confronting us is to minimise
the usage of cement without reducing the
production of concrete (and, if possible, with
minimum disruption of the cement industry)
Is Geopolymer a solution?
One current producer has said that it is easy
to make geopolymer and almost anyone can
do it.
However to make large quantities of GPC,
with acceptable properties, at a
competitive price, is a very different matter.
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Certainly potentially cost-competitive
GPC is possible, and indeed is already in
limited production,
in Europe by Joseph Davidovits (the
original discoverer of GPC)
in Australia by Jannie van Deventer and
Peter Duxon.
However such GPC needs to reach full
industrial scale to be fully costcompetitive
-see Duxon’s appendix to written paper.
So, if we cannot rely solely on GPC,
what can we replace cement with?
Fly Ash (PFA)
Slag (ggbfc)
Silica Fume
Metakaolin
Rice Hull Ash (RHA)
Superfine Calcium Carbonate (CaCO3)
Type
produced Used in C&C CO2 red.’10 CO2 red.’20
Fly Ash
490
120 (35%)
700
1200
GG BF Slag
102
90
55
75
Silica fume
0.9
0.5
0.3
0.3
Rice husk
15
0.3
6.5
6.5
Natural pozzolan
15 to 80? 12 to 30?
Calc. Carb. Fines
1000?
5 ? (0.5 %)
170-300?
200-500?
Metakaolin
0.2
0.1
-
-
60?
225?
Other ashes+wastes
Ex Boudewijn Piscaer
It is clear that there will in the future be very
little use of OPC as the sole cementing
material. Professor Swami of Sheffield
University, UK, has written: “No concrete should
be made without incorporating mineral
admixtures or other pozzolanic cement
replacement materials. Indeed, if one uses
Portland cement alone in the cementitious
system, then it should be (i.e. needs to be)
justified”
•Cement
replacement obviously important
•but need also to consider reduction
through better mix design and QC and
better admixtures.
•A major problem in USA is the continued
use of prescription specifications,
•These provide zero motivation for the
development of skill at mix design & QC
Means Prescription to Performance as a specification
basis
I like to think of it as Purchaser to Producer as to
responsibility for mix design and QC
This change is now imminent
Prescription must involve a substantial additional
margin which is wasteful and expensive
It also involves more knowledge of both technology and
material cost and availability than most specifiers have
And more responsibility than most are prepared to accept
•Recognition
that concrete mixes should be designed and
controlled by the concrete producer
•as has been done for 30 years in Australia
•will have huge consequences.
•It will mean that additional profit can be made by having a
high class laboratory and top class personnel.
•Many producers will be unable to compete so that, as in
Australia for many years, Most RMC plants will be owned
by a few major organisations
•The
number of independent producers in USA
currently several hundred, can be expected to
more than halve in the next few years!
ACI has concentrated on individual technician training
and certification, but this also will have to change to the
Australian model where each laboratory is responsible for
training its technicians and NATA (National Assn of
Testing Authorities) conducts periodic assessment visits
covering:
•Equipment
•Staff
•Procedures
•Record keeping and verification
•-
purchasers must be able to inspect
and rely on a Producer’s QC records
Modern QC software automatically assesses the
performance of each individual TO and producer’s labs
will be keen to weed out / re-train anyone sub-standard
Project supervisors need to ensure that sampling is at an
appropriate stage (e.g. Not prior to addition of water)
But in fact very few duplicate tests would be needed to
reveal any dishonest practices if MMCQC software is in
use.
(MMCQC = Multigrade, Multivariable, Cusum QC)
Repair and replacement of concrete is a huge problem
• Lack of durability is in fact a larger problem than initial
construction and must be solved, but we have been
slow to learn.
• One approach, still persisting amongst ignorant
specifiers, was to specify a minimum cement content.
• Then it became apparent that, at a given W/C ratio,
more water was more deleterious than less cement.
• So at a given W/C ratio = a given strength, the
concrete with the least cement content was the most
durable i.e. Least permeable and lowest shrinkage.
•But
strength, or W/C ratio, is now seen as by no
means a sufficient guarantee of durability
• A recent paper has shown that the introduction of
ggbfs (ground, granulated, blast furnace slag) can
have distinctly more effect on durability/permeability
than a 25% reduction in W/C
• So it is currently reasonable to specify concrete by
strength (=W/C ratio) but to require a stated % of a
nominated replacement material be used.
•A
definitive test for durability is urgently needed
so that we can return to full specification by
required properties.
•A
great deal of work is being done on this:
•RCP used in USA and elsewhere
•VPV ([email protected])
•Mark Alexander at Capetown Uni. South Africa
•James Aldred’s 260page PhD thesis
•Accepting such a criterion would allow the producer to
determine which replacement material and/or admixture
provides the best value in his area
• And to have long term test data available to support his
choice
Current specifications limit the amount of magnesium in
cement (due to delayed expansion tendency).
• But several years ago, Harrison in Australia has shown that
a large proportion of magnesia (magnesium oxide) gives a
substantial improvement in durability and other properties by
supplementing Portlandite with Brucite (Mg(OH)2) in otherwise
normal concrete, while reducing CO2 generation in production
and continuing to absorb it after placing.
• Imperial College UK have started working on “Novacem”,
entirely replacing Portland Cement with magnesia
•
•(Imperial
have just received extensive funding, unfortunately Harrison has not, and
substantial capital is needed to progress to initial commercial production)
•Supplies
of suitable natural sand are
becoming depleted - or made inaccessible
by expanding housing suburbs
•Nearly every Quarry is surrounded by
discarded mountains or filled pits of “dust”.
•It was assumed that such material will
affect workability and increase water, and
therefore cement, requirement to such an
extent as to have a negative value.
•“It
ain’t necessarily so” if produced by suitable crushers
(especially a recent Japanese development), crusher
fines can have “equidimensional” shape, rounded edges,
and almost any desired grading.
•So suitable crusher fines can be used as the sole fine
aggregate
•And coarser crusher fines can be used in conjunction
with an excessively fine sand
•And finer crusher fines with an excessively coarse sand
•The very “microfines” (<300 micron), if suitably produced,
can have very beneficial results, especially if of limestone.
•Obviously
better mix design can save cement, improve
durability and economy, and use available and local,
rather than scarce and imported, resources.
•For more than 30 years I have been designing cost
competitive mixes, overseas and “over the telephone”,
with no trial mixes and no more information than a sand
grading and verbal description of coarse aggregate.
•The likelihood that such a procedure would still yield an
optimum mix, capable of competing with a higher tech
assessment and investigation, is perhaps now remote.
•Mix design must now consider ALL available materials
and “grading” applies to material finer than cement as
well as to sand and coarse aggregate,
•Skilled
site labour is in shorter supply and more
expensive than ever - and likely to continue to be
even more so
•Surface finish requirements are becoming more
demanding and may require self-compacting
concrete in special formwork
•Such concrete can be obtained by oversanded
and over-cemented ordinary concrete heavily
dosed with admixture
•It takes a lot more skill to use suitable fines to give
the required cohesion without excessive cost.
•It
is not my intention to try to predict the
future of admixtures
•They
are being used to reduce water content; improve
workability; entrain air; accelerate or retard setting and strength
development; and to reduce permeability, shrinkage or bleeding
•It used to be that the purpose of admixtures was to save cost
by reducing cement content, but the new high range waterreducers cost distinctly more than the cement they replace
•So the cost justification was saving labour content
•But they took hold faster in cheap labour countries,
•Because the big justification was saving skill
•There is even an admixture which produces improved
properties while increasing water requirement but more
thoroughly activating cement
We are used to striving for maximum
impermeability in our concrete, but now
there is an increasing requirement for
pervious (=permeable) concrete paving
(both insitu and with blocks).
.Also RCC (Roller Compacted Concrete) is
another way of reducing labour content
So what will concrete be like in 10 years time?
•Certainly there will be endless variety
•RMC will be produced under strict QC (by Producers)
to meet limits of strength, workability, pumpability,
durability, and shrinkage, often using crusher fines
and always at least one cement replacement material.
•As much as 30 to 40% will be self compacting.
•Another
substantial % will be for RCC roads,
•Permeable
concrete will be more widely used
Strengths of up to at least 200MPa will be
available, but only in limited use.
•
The extent of change due to the GREENHOUSE GAS
situation will depend more on scientists and politicians
than on concrete technologists – how much regulation
and financial pressure/incentive will be provided?
• Geopolymer concrete likely to be quite widely used but
cannot amount to a substantial proportion of concrete in
the near future (also note that it uses fly ash and ggbfc).
• Fly ash, ggbfc, silica fume, rice hull ash, metakaolin and
especially superfine calcium carbonate, will be widely
used – all concrete will contain at least one of these and
there will be competition for available supplies of them.
• Cement may be produced from CO2 and seawater
•
(Calera in USA already working on a small scale demo plant)
What is certain is the demise of
“OLD FASHIONED CONCRETE”
This is concrete which is:
• specified by minimum OPC content
• required to use natural sand of specified grading
• produced (and specified) by a company having no
knowledge of concrete technology or QC and having no
laboratory facilities or effective control system
• Such concrete is often of low specified strength and high
variability and likely to have a limited lifespan.
•
Not all trumpeted new developments will live
up to the claims of their originators
and it will be beyond the knowledge and
ability of many specifiers, users, and even
producers, to distinguish between hype and
genuine advance.
There will be a need for genuine experts to
guide others in their choices,
and even then, not all self-professed
experts will be fully competent.
•However,
as ordinary mortals requiring
guidance, you will not be defenceless
• if you always require your selected
expert to back his recommendation with
actual test data from a recognized
laboratory
• and reference to at least one peerreviewed article supporting that expert’s
choice of test
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Your questions and discussion welcome if
permitted by the Chairman
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Please note an extensive discussion on the future of geopolymer
concrete by Peter Duxon as an appendix to the written paper