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Κυριακή, 19 Σεπτεμβρίου 2021

Παρουσίαση της νέας τεχνολογίας προστασίας κατασκευών από σκυρόδεμα με επικάλυψη από πλακίδια υπέρ-υψηλής αντίστασης (HECTs) στην ενανθράκωση και στην επίθεση απο χλωριόντα μέσω χρήσης πολυμερών κονιαμάτων με φωτο-εκκίνηση.

Fighting RC Degradation: The Technology of High Equivalent Concrete Thickness Tiles/HECTs Chris A Rodopoulos, Prof. of Structural Integrity, Centre for Expertise in Structural Mechanics University of Monash, Australia and Editor in Chief of the International Journal of Structural IntegrityThe cost is mainly due to the maintenance of bridges and other major structures like power plants, industrial plants, water treatment units and waste treatments plants. In 08/07/2011 a report from the ASCE was published in Forbes. According to the article, one in every four bridges in US has exceeded its intended lifespan and has serious structural problems.1. The problem in simple termsFrom the beginning of the 1990s, governments and engineers around the world, have to tackle the exponentially increasing cost of maintaining public infrastructure. Cost breakdown adds up to the staggering 2.2 trillion USD per year only in US [1] while Canada follows with a figure close to 889 million USD per year [2]. The above figures do not include indirect costs, i.e. increased transportation costs, lost manhours, cash flow, etc. Photographic documentation of the growth rate of corrosion on bridges pillars. In 2012, Senator Tom Carper (Delaware) from the U.S. Senate Environment and Public Works Subcommittee on Transportation and Infrastructure asked ASCE's President the following question, "what will be a cost estimate of replacing our over aged and dangerous infrastructure?" The reply was immediate "a good estimate is around 25-30 GDPs" (A simple multiplication gives us a top figure around 400 trillion dollars) State of sea water cooling tower internal beam.The highly sensitive (after the collapse of Anthony Falls Bridge, on I35W across the Mississippi River, 01/08/2007) Senator Roger Wicker asked Prof. C. French from the University of Minnesota what guarantees we have that catastrophic failures will not happen again? Prof. French replied, "On existing structures even with good monitoring it would be completely unprofessional to say that we have the capacity to know exactly when and how catastrophic failure will take place. On new structures however we have the technology to provide a very high reliability index on concrete corrosion for 120 years"Typical corrosion damage of the sea water cooling tower. 1The article was written for Αρχιμηδης Forum, http://e-archimedes.gr, Saturday, April 06, 2013 2. Concrete Strength and durabilityIn the last three decades there is a growing quest to produce a durable concrete. This was definitely not the case in the 1960s, 1970s and to some extends to mid 1980s. Concrete for over 150 years has been extensively used under the title "the immortal material". As a matter of fact, concrete was advertised as the solution to everything man can build.For the last 30 and more years, almost 30% of research in construction materials is around our hunt to produce a durable concrete. As a matter of fact, today we have over 30 ASTM, ASTHOO, EN, BS, DIN tests specifically designed to classify concrete's resistance to degradation mechanisms.Alkali Silica reaction (ASR) in a retaining wall. Early advertising of concrete "promising" immortality. Conceivably if we prevent deterioration mechanisms like alkali silica reaction and chemical leaching, concrete has an excellent performance to cost ratio. The moment however we introduce steel this immortal material begins to receive "cancer cells". According to Wikipedia [3], Concrete cancer in layman's terms, is referring to concrete degradation caused by the presence of contaminants or the action of weather combined with atmospheric properties.Perhaps the biggest disappointment in the translation of all these years of research, testing, committees, legislation etc, was to come up with a motto that a "strong concrete is a durable concrete" instead of saying that "a durable concrete will most likely be a strong concrete". Today's legislation, like EN 206-1 or ACI 201.2R-08, points towards increased compressive strength and cover thickness as a universal solution to increase durability [4, 5].The consequences from the above mechanisms are twofold. Those considered intrinsic, with most common the alkali silica reaction which leads to the production of expansive agents into concrete paste and hence extensive micro cracking with direct degradation of concrete's compressive strength and b) extrinsic mechanisms, like carbonation, chloride ion attack, chemical attack and freeze/thaw, leading to loss of reinforcement passivity and the beginning of corrosion. In term, reinforcement corrosion is known to produce a series of additional degradation mechanisms or structural integrity reduction mechanisms, like bond loss, reduction of steel's cross sectional area and changes of structural geometry due to spalling. 2The article was written for Αρχιμηδης Forum, http://e-archimedes.gr, Saturday, April 06, 2013 3. A strong concrete IS NOT a durable concreteBeforeentering into argumentation why a strong concrete is not a durable concrete there are 3 items that we need to remember,Concrete Porosity under a Scanning Electron Microscope (backscattered mode) Paul Stutzmann, Concrete Microscopy Library.Corrosion rehabilitation of a dry dock in US, 2007. a) Reinforcement operates as a continuous electrical cage. In other words, electrons being the result of corrosion can travel long distances prior to reaching a halt (electrical resistance of steel/concrete). As a result is not unlikely to find distances between anode and cathode as big as 60 cm. b) The rate that extrinsic degradation mechanisms are penetrating into concrete is not constant and is affected by a number of local parameters like humidity, cement paste porosity, wet/dry cycles, UV exposure from the sun, type of cement, type of aggregates, temperature, etc. c) The physical properties of surface or near surface differ from those in the bulk. Owing to various reasons, such as insufficient compaction, early drying, etc., the porosity of near surface concrete is usually higher than that of inner concrete made of the same initial concrete. Unfortunately, durability of reinforced concrete in terms of the corrosion of reinforcing bars is largely governed by the resistance of this surface concrete against the ingress of harmful substances. Typical results in terms of air permeability (controlling predominantly carbonation and chloride attack) have shown differences as high as 700% between the air permeability of the first 20mm from the surface compared to inner concrete [6-8].3Laboratory specimens been made of C16/20 and receiving a 28 days water curing have shown an identical near surface air permeability compared to air cured C40/50 [9]. Other parameters acknowledged to affect air permeability are the type of cement, for example CEM I is less sensitive to porosity compared to CEM III, the type of aggregate, the chemistry of high range water reducers, the ratio between the largest aggregate to cover thickness, other binders like silica fume, fly ash, slag, compaction, etc. The problem of controlling near surface air permeability is known for many years. Today there are several commercially available solutions. Controlled permeability formwork liners (CPFLs) manage hydration and hence surface humidity allowing for a smooth surface with minimum near surface porosity. Experimental results have shown that the application of the liners can reduce carbonation rate by as much as 400% [10].DuPont's Zemdrain CPFL.The article was written for Αρχιμηδης Forum, http://e-archimedes.gr, Saturday, April 06, 2013 Other types of CPFLs are based on a mixture of rubber and cork. Herein breathability and hydration are controlled via the percentage of cork in the mixture. Typical values are between 35-55% cork.Poor surface hydration and consolidation.A 42% cork/rubber CPFL.Despite the fact that plastic shrinkage cracks up to a certain width (normally 0.3mm) are not considered as structural, in terms of durability things are quite different.Rubber/cork sheets are quite common in US and Australia due to their high reusability capacity. Liquid curing compounds being sprayed over the surface of fresh concrete is another solution controlling near surface porosity. These are water soluble waxy emulsions creating a membrane able to control evaporation. However they can only be applied to free surfaces like decks and slabs.Typical plastic shrinkage cracking. In several research works it was found that a 0.1mm width crack can increase the carbonation rate by a factor 3 [11-13]. Similar reduction factor was found in the case of chloride attack [14, 15].Application of a water based curing compound over a bridge deck in Canada. Other common problems affecting near surface concrete durability is poor consolidation and plastic shrinkage cracking.4The article was written for Αρχιμηδης Forum, http://e-archimedes.gr, Saturday, April 06, 2013 The problem of plastic shrinkage cracking becomes even more pronounced the moment, cover thickness reaches values larger than 3/4 of the larger aggregate. Considering that the most typical aggregate is in the region of 32mm, the above limit translates to 24mm. As such cover thicknesses according to EN 206-1 will most likely necessitate the use of PP fibres to ensure that plastic shrinkage cracking is prevented. 4. The technology of HECTsHigh Equivalent Concrete Thickness Tiles or HECTs are trying to answer the following two questions,Cross section of carbonation profile across a 0.3mm crack. Major problems however are experienced in the case of liquid containing structures or aggressive environments. Herein cracking can significantly reduce the operational/design life of the structure to a mere 40%. In US alone, it was found that within 12 years of operation around 72% of sewage processing plants are suffering from serious corrosion of reinforcement despite the use of sulphate resisting concrete.a) "Why I have to pay for the cost, for example of PP fiber reinforced C40/45 concrete, if structurally is not needed, just to protect against durability?" b) "What can be done on existing structures where cover thickness is not enough to secure durability over time?" The story of HECTs started in 1988 when Prof. P.M. Orsel noticed that buildings dated back to the 1950s and having marble facades exhibited minimum carbonation (average value of 10mm after 35 years of exposure). In contrast bridges and buildings without marble facade, showed deep carbonation fronts with an average value of around 52mm. Polymer modified mortar (PMM) is not a new story. Several attempts and patents dated back to 1924 [16]. Throughout the 1920s and 1950s, polymer-modified mortars and concretes using natural rubber latexes were developed [16-18]. Research was predominantly driven by US, USSR, Japan, UK and West Germany. Throughout those 50 years of research several standards, standard specifications and guidelines have been developed [19].Sulphate attack exploits the shrinkage crack network to penetrate to a depth of 35mm within 8 years of operation in an oil refinery in Japan.5In collaboration with DuPont, Orsel started a small research program based on the idea of creating a "sacrificial" layer of PMM in the shape of tile. Orsel believed, based on the existing technology, that those tiles should be replaced in regular intervals and hence the term sacrificial. Both DuPont and Dow Chemicals invested a lot in the idea of PMM tiles and is safe to say that their endeavors resulted in numerous products being sold even today. ACI 548-3R-03 Polymer Modified Concrete contains several information and application examples.The article was written for Αρχιμηδης Forum, http://e-archimedes.gr, Saturday, April 06, 2013 Perhaps the reasoning why instead of tiles, PMMs appeared in the shape of spraying concrete has more to do with architects than structural engineers. At the time architectural trend was "béton brut" or exposed concrete. Things dramatically changed with the turn of the century and the introduction of enamel or metal facades.HECT mould ready for pressing.Brutalism in 'béton brut': Ministry for Road Building in Tiflis, Georgia, George Chakhava and Zurab Jalaghania, 1975. HECTs are made of special polymer modified cementitious mortar with the addition of silica fume or fly ash. The mixture contains low water to binder ratio (typical values are 0.32-0.38) and is being cured under a particular UV cycle. The polymer is mainly made of three compounds, i.e. a) N-isopropyl acrylamide (monomer, NIPAM), b) N-methylenebisacrylamide (BIS) as cross-linker and c) photoiniciators with most popular the Irgacure 2959 from Sigma-Aldrich. The slurry apart from the polymer, uses ultra fine cement (Blaine >6500 cm2/g), powdered quartz sand (10-15 µm) and water. The slurry is stirred for periods depending mainly on the polymer compounds from 30120 minutes. At the end of the stirring process, only 2.23% of water remains in the mixture. Water uptake represents the migration of water molecules into preformed gapes between polymer chains [20]. The slurry is then pressed into tile moulds using a high load press normally operating between 8-14 Kgr/cm2.6The tiles are then placed into a UV chamber operating at wavelengths between 220-350 nm. The UV cycle strongly depends on the polymer compound used, its concentration and water percentage. In most cases the actual cycle is proprietary information. However research reports indicate duration between 80-120 minutes at 250-320 nm and power density 1500-1600 W/m2. After completion of the curing cycle, HECTs are cut according to customer specifications. HECT thickness ranges from 10-20mm depending on application and durability requirements. The weight of HECTs ranges from 10.7-13.8 Kgr/m2.HECT UV curing chamber in Montreal, Canada. Image distortion is due to UV light. Latest research uses surplus concentration of crosslinker by 0.04-0.17% in order to allow for unpolymerized NIPAM without agglomeration.The article was written for Αρχιμηδης Forum, http://e-archimedes.gr, Saturday, April 06, 2013 Typical finished product in stone white. Typical scanning electron microscopy image of HECT showing un-polymerized NIPAM. This allows a significant reduction in the average pore size to 415-427 Ångström (1 Ångström = 10-10 m). As a result, HECTs are considered water tight (Grade 1 BS 8102). At the same time, chloride permeability ratings according to ASTM C1202 registered charge passed below 230 Coulombs depending on manufacturer. Chloride diffusion coefficient according to ASTM C1556 is below 2.45 x 10-13 m2/s. To better understand the above number is worth considering that a high strength concrete (>C50/60) without any cracks has a chloride diffusion coefficient between 1.9-7.5 x 10-12 m2/s. HECT with Stainless Steel like surface. The porosity system however allows HECTs to breathe and expel humidity. Average value of water vapor diffusion coefficient (breathability) SD, H2O=0.1920.354m, with limit value of 4 m according to EN 1504-3.Typical finished product in graphite grey.7HECT's compression strength has typical values from 78136MPa (cube), having a coefficient of linear thermal expansion similar to concrete 7.9-8.5 10-6/oC. HECTs exhibit high resistance to carbonation with average values of CO2 (Diffusion Coefficient) SD, CO2 between 1140-1840 m, with limit value SD CO2 being >50m. Unlike paint systems, suffering from UV exposure aging, HECTs retain their properties for periods over 75 years.The article was written for Αρχιμηδης Forum, http://e-archimedes.gr, Saturday, April 06, 2013 Application of HECTs in a Hospital in Germany.Typical carbonation rate of HECT. Comparison of HECT to C50/60 and C25/30. Exposure class XC2. Application of HECTs is similar to tilling. Different interlocking systems depending on manufacturer allow joints to be as small as 1mm. The joints are filled with polyurethane sealant.Application of HECTs in Australia.Rehabilitation of a bridge in US with HECTs. Application of HECTs in Greece.8The article was written for Αρχιμηδης Forum, http://e-archimedes.gr, Saturday, April 06, 2013 HECTs today are also used as spacers/moulds to concrete casting. This allows the monolithic operation of HECTS.[8] Meyer A. The importance of surface layer for durability of concrete structures, ACI Special Publication, 1987 SP 100 (1), American Concrete Institute, Farmington Hills, MI, USA. [9] C. A. Rodopoulos (2010) Parameters controlling the Air Permeability of concrete cover, Concrete Institute of Australia, R-587-12-2011. [10] Technical University of Munich (1996) Comparison of concrete surfaces made with formwork liner Zemdrain and Zemdrain MD, 3360-03-96.The use of HECTs in this sewage treatment plant in US is estimated to save 570,000 dollars over a period of 75 years. Saving is in terms of using lower class bulk concrete and future maintenance costs. 5. References [1] ASCE 2013 Report Card for America’s Infrastructure. [2] Gaudreault, V. and Lemire, P. (2006). The Age of Public Infrastructure in Canada. Statistics Canada. [3] D. DiSalvo (01/04/2012) Forbes Magazine: 2012 is the Year America Must Get Serious About its Infrastructure [4] Eurocodes 1999 [5] ACI 201.2R-08 Guide to a Durable Concrete, American Concrete Institute, 2008. [6] P A Claisse, H I Elsayad and I G Shaaban. Absorption and Sorptivity of Cover Concrete, ASCE materials Journal 9 (3) August 1997 pp.105-110. [7] Hong C Z and Parrott Leslie J. Air permeability of cover concrete and the effect of curing, C&CA Services, October 1989, British Cement Association, Crowthorne, Berkshire, UK.[11] S. Talukdar, N. Banthia and J.R. Grace, The effects of structural cracking on carbonation progress in reinforced concrete. Is climate change a concern? 3rd International Conference on the Durability of Concrete Structures, 2012, Queen’s University Belfast. [12] Iyoda, T. and Uomoto, T. (1998) “Effect of Existence of Cracks in Concrete on Depth of Carbonation”, Translated from Japanese. [13] Laura E. Sullivan-Green (2005) Effect of crack width on carbonation: implication for crack-dating, MSc Thesis, Northwestern University. [14] O. G. Rodriguez and R. D. Hooton (2003) Influence of Cracks on Chloride Ingress into Concrete, Materials Journal, 100(2), pp.120-126. [15] M. Ismail, A. Toumi, R. Francois, R. Gagne (2004) Effect of crack opening on the local diffusion of chloride in inert materials, Cement and Concrete Research 34, pp. 711–716 [16] L. Cresson, British Patent 191,474 (Jan. 12, 1923). [17] Jaenicke, J., Knoop, H., Miedel, H., and Schweitzer, O., U.S. Patent 2,311,233 (Feb. 16, 1943) [18] “Dow Latex 560 for Portland Cement Compositions,” Dow Chemical Co., Midland, Michigan (1959)[19] Y. Ohama (1995) Handbook of Polymer modified concrete and mortars, Noyes Publication, NJ, USA. [20] V. Can, S. Absurrahmanoglu, O. Okay, Polymer, 48 (2007), 5016–50239The article was written for Αρχιμηδης Forum, http://e-archimedes.gr, Saturday, April 06, 2013
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