Presentation on theme: "The Future of Concrete in a Greener World Ken W. Day."— Presentation transcript:
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
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.
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 cost- competitive -see Duxon’s appendix to written paper.
So, if we cannot rely solely on GPC, what can we replace cement with?
Ex Boudewijn Piscaer TypeproducedUsed in C&CCO2 red.’10CO2 red.’20 Fly Ash (35%) GG BF Slag Silica fume Rice husk Natural pozzolan15 to 80?12 to 30? Calc. Carb. Fines1000?5 ? (0.5 %) ? ? Metakaolin Other ashes+wastes60?225?
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 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 water- reducers 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 peer- reviewed article supporting that expert’s choice of test
Your questions and discussion welcome if permitted by the Chairman Please note an extensive discussion on the future of geopolymer concrete by Peter Duxon as an appendix to the written paper