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Κυριακή, 19 Σεπτεμβρίου 2021

Παρά το γεγονός ότι το θέμα της ανθεκτικότητας του σκυροδέματος βρίσκεται στο προσκήνιο από την δεκαετία του 1970, σήμερα οι μηχανικοί οφείλουν να ανταποκριθούν σε δύο συναφείς αλλά διαφορετικής προσέγγισης κανονιστικές διατάξεις.

Το Πρότυπο EN 206-1 δίνει έμφαση στην ποιότητα του σκυροδέματος και το πάχος επικάλυψης του οπλισμού, ενώ το EN 1504 καθορίζει ειδικές και μετρήσιμες απαιτήσεις επιδόσεων.

EN 206-1 vs EN 1504. Enemies or Friends? Chris RodopoulosProf. of Structural Integrity at Monash University, Editor in Chief International Journal of Structural IntegrityEven though the concept of durability goes way back (1970s), engineers today are having to cope with two similar but also distinctive regulations. EN 206-1 heavily relies on concrete quality and cover thickness while, EN 1504 relies on specific and measurable requirements. Perhaps the most indicative distinction between them is that EN 206-1 does not contain the principle of maintenance as an action taking place in predetermined intervals while EN 1504 recognizes that maintenance is important and that nothing last for ever without it. Maintenance intervals however are not new. From ACI to BS, DIN and IS its an important parameter known to everybody. As a matter of fact maintenance is compulsory especially when insurance coverage is part of the equation. With the introduction of EN 206-1 and EN 1504, the very first and perhaps strongest reaction was, " Why I need to use high class concrete if structurally not necessary? Similarly to the above, some others expressed a more comprehensive question " if I use both standards then automatically the operational life will go beyond my designed life." Perhaps a third question emanates from cost. Why shall I pay for both, it their dual use creates durability standards beyond my structural requirements?" I will try to put some flesh on the issue. EN 206-1 follows Prescriptive specifications. Some of the problems emanating from that are, a) Represent basic minimum specifications without specific bottom limits b) Restrictive and not able to accommodate developments or other regulations, i.e. the addition of fly ash, silica fume, slag, etc. c)Ineffective in many cases of combined actions or in-situ characteristics, i,e, XC+XS+XFd) Is not related to deterministic parameters of durability e) It cannot assess actual as-built quality f)It simplistically assumes as-built quality to be what is specifiedg) It fails to recognize the necessity of admixtures i)Suggests high cover thickness and hence increases the potential for cover cracking1/6 j)Creates an environment of responsibility doubt. Who is to blame? the engineer, the concrete producer, the contractor?k) It points towards cost based prescription. For example if a company produces a mix design having Performance based specifications, it will immediately been classified according to EN 206-1 and therefore will be extremely difficult to justify the extra cost. In other words someone can claim the following principles,a) EN 260-1 is a prescriptive standard b) EN 1504 is a performance based standard.The issue, was a headache for the last 5 years everywhere in the world and capitalized the 54th CBC Conference. Due to the Olympic games, the issue was a major headache for the Brazilian Government. http://www.ibracon.org.br/ingles/default.aspSome European Countries recognize the issue and force modifications to their National Annex A typical example is http://beton-profi.ru/standards/EN206-1.pdf and the paragraph additional requirements 6.2. Another critical issue can be found in Appendix 3 http://www.concrete.ie/downloads/concrete_standards.pdf Try to decipher the combination of the exposure classes which automatically create conditions for more than 3 different concrete types on the some structure. Problems will emerge the moment the owner requires specific guarantees related to maintenance cost and service life. Herein the engineering team will be obliged to undertake service life prediction modeling in order to provide justification. Perhaps the most common problem would be to evaluate the as-build quality. Herein, the RM plant will never provide specific performance justification apart from the limits contained into EN 206-1. Contractors on the other hand will never accept responsibility based on the fact that concrete pouring quality is a function of concrete mix design, reinforcement placement, member geometry and reinforcement density.Therefore the engineering team will find themselves in front of three problems,a) EN 206-1 apart from being prescriptive is not addressing responsibilities,b) The other two stakeholders (RM plant, contractor), will never accept contractual clauses outside those readily available in regulations,c) the Owner's durability requirements cannot be justified. Several National Annexes have tried to tackle the problem making things even worst, for example http://beton-profi.ru/standards/EN206-1.pdf In Appendix J we can find, Performance-related design methods with respect to durability J3. This approach may be appropriate where: – – –a working life significantly differing from 50 years is required; the structure is "special" requiring a lower probability of failure; the environmental actions are particularly aggressive, or are well defined;2/6 – – – – –standards of workmanship are expected to be high; a management and maintenance strategy is to be introduced, perhaps with planned upgrading; significant populations of similar structures, or elements, are to be built; new or different constituent materials are to be used; method according to 5.3.2 has been used in design, but there has been a failure to conform.J.4 Performance-related methods with respect to durability In applying the methods listed below, it is important to define in advance, at least the following: – – – –type of structure and its form; local environmental conditions; level of execution; required working life.Some assumptions and judgements on these issues will usually be necessary to reduce the chosen method to a pragmatic and practical level. The methods that may then be used include: a) The refinement of the method according to 5.3.2, based on long-term experience of local materials and practices, and on detailed knowledge of the local environment. b) Methods based on approved and proven tests that are representative of actual conditions and have approved performance criteria. c) Methods based on analytical models that have been calibrated against test data representative of actual conditions in practice. The concrete composition and the constituent materials should be closely defined to enable the level of performance to be maintained. It is easily understood that in a courtroom all the above are "Legally Vague". In other words EN 206-1 by itself, fails to provide a simple and well covered environment of application. Particular problems will be experienced in National Annexes, i.e. the ELOT where such Appendices are not included and therefore allow even more space for legal interpretations. I used a typical commercial mix design giving C25/30 as prescribed for XC2 in ELOT EN 206-1. I considered the following parameters a) cover thickness 25mm as per EN 206-1 b) Maximum aggregate 31mm as typically used c) W/C=0.63/6 d) Casting Temperature 25 degrees C e) 300 kg/m3 cement CEM II 42.5 f)Molded Hydration time =3 days quite typical in Greeceg) Conditions for Medium Quality Contractor i)Hydration cracking =0.3mmj)Operational Life =50 yearsk) Location = Ilioupoli-Athens Greece l)Plaster = NORunning the above we get Reliability index for carbonation reaching the 25mm: β max= -1.47, β min=-0.51 In other words carbonation will take place between 22 and 38 years. Let us try to decipher EN 206-1 keeping however the minimum characteristics prescribed and taking out as-build conditions a) cover thickness 25mm as per EN 206-1 b) Maximum aggregate 20 mm c) W/C=0.6 d) Casting Temperature 20 degrees C e) 300 kg/m3 cement CEM II 42.5 f)Molded Hydration time =7 daysg) Conditions for High Quality Contractor i)Hydration cracking =0mmj)Operational Life =50 yearsk) Location = Ilioupoli-Athens Greece l)Plaster = NOReliability index for carbonation reaching the 25mm: β max= 1.27, β min=0.21 In other words carbonation will take place between 52 and 74 years. It is therefore quite rational to conclude that EN 206-1 is quite sensitive to good practice concreting or the parameters – –level of execution standards of workmanship are expected to be high4/6 –significant populations of similar structures, or elements, are to be built are so important as to govern a reliability index ranging from negative (failure) to positive (safe).Of course EN 206-1 suggests minimum values. To identify those minimum conditions we can play the following game, a) cover thickness 30mm b) Maximum aggregate 31mm as typically used c) W/C=0.52 d) Casting Temperature 25 degrees C e) 350 kg/m3 cement CEM II 42.5 f)Molded Hydration time =3 days quite typical in Greeceg) Conditions for Medium Quality Contractor i)Hydration cracking =0.3mmj)Operational Life =50 yearsk) Location = Ilioupoli-Athens Greece l)Plaster = NOIt is easily understood that our concrete class is now C30/37 Reliability index for carbonation reaching the 30mm β max= 1.89, β min=0.71 In other words carbonation will take place between 59 and 92 years. From the above we can understood that even a higher class concrete and cover are still sensitive to the quality of the contractor (βmin<1!!!). In order to understand the basic difference between EN 206-1 and EN 1504 I am using some data describing a EN 1504-3 product Carbonation Depth Years 0.1 mm 10 0.1mm 18 0.2mm 25 0.3mm 43 0.5mm 71 Τhe product (mortar) should be applied by spraying at a minimum thickness of 10mm.5/6 Therefore by applying this mortar on our: a) cover thickness 25mm as per EN 206-1 b) Maximum aggregate 31mm as typically used c) W/C=0.6 d) Casting Temperature 25 degrees C e) 300 kg/m3 cement CEM II 42.5 f)Molded Hydration time =3 days quite typical in Greeceg) Conditions for Medium Quality Contractor i)Hydration cracking =0.3mmj)Operational Life =50 yearsk) Location = Ilioupoli-Athens Greece l)Plaster = NOwe have: Reliability index for carbonation reaching the 25mm β max= 6.41, β min=3.70 The obvious question would of course be whether the combination of EN 206-1 and EN 1504 is too much? or why shall I pay for both if the final product exceeds my life expectations? An exercise based on using EN206-1 concrete + mortar + plaster + hydrophobic impregnation+paint all according to EN 1504 would create a Reliability index over 10 or a probability of failure < 10 to -7. Hence down the line the emerging question would be structured as, is the dual use of the standards way too much? Τhe obvious answer is YES IT IS especially for structures with life span of 50 years. This contradicting answer is unfortunately the result of having different working groups working in isolation and perhaps in most cases in rivalry. In my opinion and is the main reason having started this thread, lays down to Human factor and the as-build quality which so desperately the EN 206-1 fails to incorporate or even managed in such way as to raise it as a vital part of durability. Most certainly, SCC could have been a partial answer to as-build quality and to some extend to our problems with EN 206-1. In vain. Market forces proved once again much stronger while the economy is not in its prime to absorb the extra cost. Perhaps our inability as a "building society" to adapt "new" technologies promoting the end user/owner requires much more education and a longer history line. Yet, I cannot see how such delays will ever catch up with the innovation. Perhaps the 21st century will prove itself as the century where the mighty beast of technology overthrows standards as obsolete the very moment they are printed.6/6
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28 Οκτωβρίου 2013

Παναγιώτη,

Mην ξεχνάς οτι εκτός απο το επίχρισμα έχουμε και σχεδόν πάντα σε συνεργασία την βαφή. Με λίγα λόγια εαν θέλεις έχουμε εφαρμογή στις παρακάτω μεθόδους:

M1.6 Structural shielding and cladding
M1.7 Surface protection with paint
M4.4 Application of mortar or concrete
M4.6 Filling of cracks, voids and interstices
M6.1a Increase of chemical resistance by wearing surface
M6.2 Increase of chemical resistance by sealing
M7.1 Increase of cover with mortar/concrete

Τα πιστοποιημένα επαγγέλματα χρειάζονται πιστοποιημένη τεχνική εκπαίδευση κατα βάση αλλα και πιστοποιημένα υλικά. Εαν θέλεις το ΕΝ 1504 παρέχει προϊόντα με αυξημένο βαθμό ασφαλείας σε τέτοιο επίπεδο που πλέον οι απαιτήσεις σε craftmanship έχουν μειωθεί στα βασικά.

Επίσης επέτρεψέ μου να σου πω ότι ενώ υπάρχει σίγουρα μια χρονο-καθυστέρηση με την χρήση επιχρισμάτων και ορθομαρμαρώσεων τα προβλήματα υπάρχουν και μάλιστα τις περισσότερες φορές δεν είναι εμφανή πριν να είναι πολύ αργά. Σοβάδες που συγκρατούν υγρασία και άρα αλλάζουν τοπικά πχ το XC2 σε XC3 είναι ένα ιδιαίτερο πρόβλημα. Εδω βέβαια παίζει ρόλο και το texture. Το ότι στο κέντρο πχ της Αθήνας δεν έχουμε σοβαρά προβλήματα έχει κατά κύριο λόγο να κάνει με την υγρασία του περιβάλλοντος και όχι με το σοφά, που σαφώς μετά από 60 χρόνια δεν παρέχει καμία προστασία. Το μέγιστο πρόβλημα που έχουμε πάντως είναι στις θεμελιώσεις και εκεί δύσκολα υπάρχει πρόσβαση.

Σήμερα τον ρόλο του σοφά βάση του EN 1504 το παίζουν τα polymer modified cements. Εδω πλέον μιλάμε για μέγιστη προστασία σε διεισδύσεις με ταυτόχρονη διαπνοή. Η διαδικασία είναι ακριβώς η ίδια και το κόστος σαφώς μικρότερο. Κατα την κλασσική νοοτροπία υπάρχουν τα χοντροκοκκα και λεπτόκοκκα. Πλέον το κόστος μπορεί να είναι και μικρότερο απο το κλασσικό σοφά.

Χρήστος Ροδόπουλος
---------------------------------

28 Οκτωβρίου 2013

Κατά κανόνα οι Φ.Ο. των κτιρίων στην Ελλάδα είναι επιχρισμένοι (βεβαίως κατά απόλυτα εμπειρικό τρόπο, αφού η σύνθεση και τα χαρακτηριστικά των κονιαμάτων ήταν πάντοτε αντικείμενο του "μάστορα").

Κατασκευές της δεκαετίας του 60 (έχουν υπερβεί την 50ετία) δείχνουν ότι βρίσκονται σε καλή κατάσταση. Δεν συμβαίνει όμως το ίδιο με τα δομήματα από εμφανές σκυρόδεμα (λ.χ. γέφυρες).

Εδώ νομίζω ότι πρέπει να αρχίσουν να συζητώνται ευρύτερα οι δυνατότητες που παρέχονται από την ηλεκτροχημική πρόληψη/αποκατάσταση, τομείς γενικώς "αγνωστοι" στην Ελλάδα.

Ενα εισαγωγικό άρθρο με στοιχεία από την CPA, UK έχει αναρτηθεί στο www.e-archimedes.gr : http://www.e-archimedes.gr/component/k2/item/3148-

Τόσο το ΕΝ 206-1 (αρκετοί έχουν "ξεχάσει" το ΕΝ 13670), όσο και το ΕΝ 1504 απαιτούν craftmanship (πιστοποιημένα επαγγέλματα)

Παναγιώτης Αναγνωστόπουλος
--------------------------------------------

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