Ενανθράκωση με διάβρωση

[Ροδοπουλος]

Εγω μεσα απο τωρα.... Μπορουμε να βαλλουμε και μια ψηφοφορια....

 

Σε παρακαλώ βαλε την ψηφοφορία

 

Διαβασμα

 

Durability of concrete structures: investigation, repair, protection

 

Για μένα ενα βιβλιο που πρέπει καθε Πολ. Μηχ. να διδαχθει στο 4ο έτος.

 

Απο ACI

Παιδια σαν μελος του ACI βάζω καποιες τεχνικές απαντήσεις. Με τον τρόπο αυτό θελω να δωσω μια διασταση στο θεμα που να μην ειναι σκέτο Ροδοπουλος.

 

Q. A reinforced concrete façade has been investigated and the concrete has been found to be carbonated to an average depth of 1.5 in. (38 mm). Given a concrete cover of 2 in. (51 mm), should remedial measures be considered to arrest further carbonation in the concrete? If not, what are the implications for the future performance of the façade?

 

SIGNIFICANCE

 

When concrete or any other cement-based material in contact with the embedded reinforcing steel is carbonated, the steel surface is depassivated. Therefore, the reinforcing steel is no longer protected from corrosion. Corrosion may then commence when moisture and oxygen gain access to the steel surface.

 

A. The decision to protect the concrete façade should be based on the expected life of the building. Remedial measures to protect the reinforcing steel should be considered if the intent is to prolong the remaining service life of the façade. At early ages, steel reinforcement is protected from corrosion by the high alkalinity of the surrounding cement paste. The protective passive layer is stable and adherent in this range of alkalinity. However, alkalis in concrete eventually react with acidic components of the atmosphere, particularly carbon dioxide (CO2). As a result, the alkalinity of the concrete is progressively reduced by converting the calcium hydroxide to calcium carbonate. This conversion reduces the pH value of the concrete below 10, thus reducing the concrete’s protective ability. This reaction of carbon dioxide with the products of cement hydration is defined as carbonation.

 

The carbonation is not a linear function, the rate changes with time and depth. The natural process of carbonation in good quality concrete is very slow—on average about 0.04 in. (1 mm) a year.1 Thus, after 35 years, the depth of carbonation in the concrete façade can be estimated at approximately 1.5 in. (38 mm). Given this rate, carbonation in the remaining 0.5 in. (13 mm) of concrete cover will take about 12 years.

 

The rate of carbonation is mainly influenced by the permeability and the calcium content of the concrete as well as the ambient atmospheric conditions: amount of carbon dioxide, relative humidity, and temperature. Concrete carbonates more rapidly in a hot climate than in a moderate climate.

 

Carbonation has an adverse effect on the degree of concrete alkalinity and its ability to protect the reinforcement. Physical effects of carbonation within the concrete are usually positive—carbonation of mature concrete densifies its structure, increases strength, and reduces permeability. However, carbonation increases shrinkage of concrete that is fully matured, which can cause additional cracking.

 

If cracking is present in the concrete façade, carbonation may be substantially deeper in localized areas. Cracks in the concrete allow carbon dioxide easy access through the concrete cover, and the carbonation occurs. The active coefficient of carbon dioxide diffusion in a concrete crack 0.008 in. (0.2 mm) wide is about three orders of magnitude (1000 times) higher than in average-quality crack-free concrete.2 Cracked areas should be included in tests to determine the depth of carbonation.

 

Methods such as coatings or sealers will help to protect reinforcing steel by reducing the ingress of moisture into the concrete. The corrosion process can be stopped by incorporating cathodic protection.

 

Selection of carbonation remediation measures for the building façade is complicated by the fact that long-term performance data are lacking for most commercially available systems. Nevertheless, there are benefits for using one of the commercially available anti-carbonation systems. Most of these systems are elastomeric and, if detailed properly, have the capacity to bridge small moving cracks. The following factors should be considered:

 

- The rate of vapor transmission through the exterior wall;

- The amount of moisture in the façade;

- The breathability of the protective system; and

- The temperature gradient between the concrete surface and ambient air while the protective coating is curing. If the concrete temperature is below that of the dewpoint of the surrounding air, moisture will condense on the concrete surface.

 

If the concrete cover adjacent to cracks is completely carbonated, then it is too late to protect against carbonation. Therefore, access of moisture and oxygen to the reinforcement should be minimized. This access can be minimized by sealing or injecting cracks, provided corrosion of the reinforcing is not present. Additional guidance on carbonation of concrete is available in ACI 201.2R-01.3

 

REFERENCES

 

1. Vaysburd, A. M.; Sabnis, G. M.; and Emmons, P. H., “Concrete Carbonation—A Fresh Look,” Indian Concrete Journal, V. 67, No. 5, May 1997, pp. 215-220.

2. Alekseev, S. N., and Rosenthal, N. K., Resistance of Reinforced Concrete in Industrial Environment, Moscow, Stroyisdat, 1976.

3. ACI Committee 201, “Guide to Durable Concrete (ACI 201.2R-01),” American Concrete Institute, Farmington Hills, Mich., 41 pp.

 

CORTEC

Η CORTEC Θεωρείται μια απο τις πιο γνωστές εταιρείες στον χώρο. Εχουνε βάλει πολύ υλικό για διαβασμα.

 

http://www.cortecvci.com/Products/products.php?showonly=ConcProt

 

διαβασμα

 

Repair and strengthening of concrete structures

 

Παιδια βαζω βιβλια που μπορούν να βρεθούν στην Ελλαδα.

[Ροδοπουλος]

Επισκευή με ρυτίνη

 

Resin Injection: A Permanent Fix

by John Trout

 

 

 

Epoxy injection has the reputation of being a structural repair procedure, and it is. However the vast majority of concrete crack injection applications (over 70% by the author's reckoning) are structural preservation rather than repair; that is, instances where resin is injected into faults in order to keep good concrete good rather than to restore design strengths. The sealing of cracks to protect exposed reinforcement is the most common example.

 

A brief look at the applications and the technology explains this wide and increasing popularity of injection for concrete preservation.

 

To begin with, most crack repair techniques other than injection have proven to be temporary fixes. They have included scrubbing with cementitious grouts, coating with various epoxy and textured formulations, the application of liquid membranes, the sandwiching of fiberglass membranes between coatings and even between liquid membranes themselves, sealants of all sorts installed over the fault or in a routed groove, the use of mastic and epoxy pastes to cap the faults, and the list continues.

 

After one to twenty years, failure of all of the above has been virtually assured. In many cases the surface repair materials embrittled with aging; in others, they curled away from the surface due to exposure to ultraviolet. Often the cracks elongated, especially in instances of live loads or significant temperature changes. Traffic and vandalism took a toll as well, but the most common failure has been due to fatigue of the repair material. This fatigue factor needs to be understood to fully appreciate the probability of failure of those repair procedures which merely bridge a void.

 

Cracks in concrete which are exposed to seasonal temperature changes open and close in their entirety as the volume of the concrete annually changes in its entire mass. However cracks open and close at their mouth with much greater frequency as the surface of the concrete creeps back and forth in response to daily changes in temperature; and in many areas with greater frequency due to alternating exposure to warming sunlight and cooling shade throughout the day.

 

Though the amount of movement is very slight and the crack quite narrow, failure is nevertheless inevitable as the aging repair material is repeatedly strained by this yawning at the mouth of the fault. It may take years, but as many injection contractors can attest, this phenomenon is often observed overnight when a high modulus epoxy capping material is applied over a crack and temperatures drop significantly. Their high modulus cap seal is cracked along the line of the fault by morning.

 

By comparison, an injected epoxy is a permanent fix for these reasons: the resin fills the void rather than bridging it, preventing the entry of atmospheric elements, and coating exposed reinforcing steel to starve the corrosion process; the high bond and tensile strengths of the epoxies prevent yawning and elongation of the crack; and injected resin is not vulnerable to ultra violet rays, weathering, traffic or vandalism.

 

Though recognizing that injection is probably a better fix, many specifiers are nevertheless reluctant to inject a rigid epoxy into faults where there is evidence of movement (and move they all do since they yawn) for fear the crack will simply reappear nearby. The use of a sealant or flexible membrane seems more sensible. However, whether or not there is movement at a fault is not important; but rather whether the movement must be accommodated is the consideration. More often than not movement occurs at a crack simply because it is permitted to, not because it must be accommodated there. In instances where restraint of movement at a random crack is likely to result in fracturing of a member, design or construction is obviously flawed. If provision for movement was required at the location, it should have been specified by the engineer and put into place by the builder.

 

If the design or construction is not flawed, inject the moving crack; the structure will then simply behave as designed, resisting the stresses or relieving them elsewhere.

 

The permanent fix is the most economical repair for permanent structures not only because the process does not have to be repeated, but also because as temporary repairs fail, they are seldom restored immediately since inspections and budgets and priorities must first fall into place. Such delays prove costly as further corrosion of reinforcement occurs, autos are damaged by calcium deposits, interior finishes are marred, etc.

 

When the cost and disruption of spall repair generated by corrosion is considered, the case for the permanent injection fix of cracks is overwhelming.

 

Another advantage of the injection selection is appearance. No repair has a lower profile. This was not always so when it was once necessary to drill holes and use an epoxy paste to cap the crack to contain the injected resins. But today holes are seldom needed and low pressure injection technology has made it possible to seal cracks with strippable, nonstaining silicone sealants leaving only an amber glue line.

 

Though cracks less than .30 mm are normally not a threat to reinforcement unless in a severe environment such as a water retaining structure, cracks of only .05 mm commonly cause coating failures as a result of the yawning phenomenon. In California for example, where the use of coatings on exterior concrete is common, the appearance of many otherwise elegant structures is severely marred by coating failures along very fine cracks. Once the crack has reflected through a coating, the coating has a tendency to curl away from the surface, greatly exaggerating the width of the fault in the coating and accumulating dirt.

 

As the combination of high tensile and bond strengths of an injected epoxy resin discourage elongation of a crack, it also stops the yawning at its mouth. A surface can then be painted without fear of the crack reflecting through. Elastomeric coatings anticipate the movement of yawning and are less vulnerable for the first few years, but aging embrittles all coatings. If the cracks are injected, relatively inexpensive coatings without elastomeric properties can be confidently specified.

 

An often overlooked application of injection is the resurfacing of floor slabs, whether on grade or suspended. While no expense is spared in the preparation of the surface for an epoxy overlay for example, cracks and cold joints in slabs are typically treated with layers of sandwiched fiberglass: a treatment which is not permanent and is usually unsightly. Only the injection of a fault can assure the permanent integrity of the overlay. Though a bond may not be available due to contamination from oil or other substances, injection is nevertheless reduced to an impenetrable seam.

 

Further regarding slabs, costly flexible membranes are often specified for parking structures in order to bridge cracks and seal the concrete against the penetration of chlorides or other detrimental elements. The injection of the cracks followed by resurfacing with a relatively inexpensive product may yield a less costly yet permanent solution.

 

We receive inquiries from contractors almost daily regarding injection equipment. The most common reason we hear for getting into injection is: "Well, I ran into this spec..." There are also better reasons.

 

American Concrete Institute Committee 224-R-72 guideline for concern with cracks in concrete:

Exposure Condition Maximum Allowable Crack Width

Dry Air

.41 min

Humidity, moist air, soil

.30 mm

Deicing chemicals

.18 mm

Seawater or spray

.15 mm

Water retaining structures

.10 mm

 

Επισκευή με ρυτίνη

 

Ενα απο τα μεγαλυτερα προβληματα που αντιμετωπίζουν οι μηχανικοί ειναι η επίσκευη ρυγματωσης. Ειναι πολύ δύσκολο να γνωρίζουμε εαν η ρωγμή ειναι ενεργή ή αποτελεσμα αλλον παραγόντων (διαβρωση, κλπ.). Σαφως μια μετρηση διαφοράς δυναμικού θα μας πει εαν εχουμε διαβρωση. Εαν η απάντηση ειναι αρνητική τότε παραθέτω ενα τρόπο επισκευής που εγω πιστευω οτι ειναι χρήσιμος. Πριν βάλουμε την ρυτίνη δημιουργούμε μικρές οπες με Φ = 8 χ πλάτος ρωγμής ανα 50 χιλιοστά και παντα στα δυο ακρα της ρωγμής (crack arresters). Μετά με ενα μικρό καλέμι κανουμε επιφανειακές τομές κάθετες στην ρωγμή crack bridges or crack opening preventer που περιορίζουν την περαιτέρω αυξηση του πλάτους. Το μήκος των τομών ειναι τουλάχιστον 70 χιλιοστά. Τόσο οι crack arresters, crack bridges και η ρωγμή γεμίζονται με ρυτίνη. Προσοχή εαν εχουμε ενανθρακωση τοτε κάνουμε χρήση ειδικών ρυτίνων.

 

Επισκευή πλάκας με ρυτίνη

 

http://www.concrete.org/general/RAP-2.pdf

 

διαβαστε προσεκτικά για να καταλαβεται τους περιορισμούς. Οπως εχω πει ενας σωστός μηχανικός θα πρέπει να μπορεί να πείσει τον πελατη βάση στανταρτς.

 

REINFORCEMENT FOR CONCRETE— MATERIALS AND APPLICATIONS

http://www.concrete.org/general/fE2-00.pdf

 

Επισκευή

http://www.concrete.org/Technical/CKC/Repair_Application_Procedures.htm

 

Περιέχει πολλα ενδιαφεροντα πραγματα. Εχουμε καταληψη αρα θα σας ταράξω σημερα.

 

Ενανθρακωση και χλωριόντα

 

Πολλοί θα ρωτήσουν τι ειναι χειρότερο η ενανθρακωση ή τα χλωριόντα?

 

Η απαντηση ειναι ΤΟ ΙΔΙΟ. βεβαια το χαρμανι των δυο ειναι πολυ χειρότερο. Βαζω και φωτό

 

Βρειτε τον λόγο νερου/τσιμέντο

 

http://www.ndtjames.com/catalog/moistureTesting/cementometer.html

 

Δεν θα σας κοροιδεψει κανείς.

[terry]

Αυτο το εγραλειο στο νωπο σκυροδεμα τι ακριβως μετραει???

[Ροδοπουλος]

το W/C ratio

[terry]

λογος νερου - τσιμεντου εννοειται???

 

Εχει διαφορετικες κεφαλες ομως...γιατι??

[Ροδοπουλος]

με χάνεις. Κάνε καλυτερη την ερωτηση.

Μαθημα 3

 

Ενας μηχανικός και οχι χημικός θα πρέπει να έχει παντα στο μυαλό του το

Pourbaix diagrams (φωτό). οτιδήποτε βαζουμε στο σκυρόδεμα, η εκθεση του στο περιβάλλον επηρεάζει το πεχα.

Τωρα σαφως δεν θελω να σας κανω μαθημα επιστημονικό αλλα τεχνικό οπως αρμοζει σε μηχανικούς και οχι φοιτητες. προσπαθω να απλοποιήσω τα πραγματα οσο μπορώ χωρίς να χασουμε το αποτελεσμα. Σιγουρα θα χασουμε την θεωρία και την εξηγηση των φαινομένων.

 

Το διαγραμμα μας δίνει την συμπεριφορα του δυναμικού (κινηση ηλεκτρονίων) σε σχεση με το πεχα και την κατάσταση διάβρωσης. Κοιτώντας το θα καταλάβεται γιατι ο ψεκασμός με δεικτη (<9) μονο μας δινει αποτελέσματα οταν ειναι αργά πλέον.

 

Ευλογα θα μου πείτε μπορεί το δυναμικό να ειναι θετικό και να έχουμε διαβρωση? Βεβαια αλλα σε αερια κατάσταση (οπλισμός στην μαντρα)

[terry]

w/c ειναι ο λογος νερου προς τσιμεντο????

 

και απανταω ...ναι αυτο ειναι....Διαβασα καλυτερα την περιγραφη..

[Ροδοπουλος]

Two units are available to encompass the full range of water

cement ratio’s found in wet concrete. The Cementometer™ Type R

handles normal water cement ratio’s with its two prong probe.

The range of this instrument is approximately 0.35 to 0.65

water/cement. Cementometer™ Type L handles low water/cement

ratio’s with its five prong probe. The range of this instrument is

approximately 0.25 to 0.5 water cements.

 

-------------------------------------------------------------------

 

Μαθημα 4

Ειπα για το χαρμάνι χλωριόντων και ενανθρακωσης. Ας πούμε ενανθρακωση <9 κοιταξτε την φωτο.

 

διαβασμα

 

http://www.episkeves.civil.upatras.gr/English/ergasies%202010/2.%20%CE%9A%CE%9F%CE%A5%CE%A1%CE%9D%CE%95%CE%A4%CE%91%CE%A3%20%CE%94..pdf

[agior]

κ. καθηγητά αν είχα αυτό το μαραφέτι το τσιμεντόμετρο πίσω στο 1987 θα έκανα άγαλμα σ' αυτόν που το έφτιαξε (πράγμα αδύνατο, τότε ήταν αγέννητος)

[Ροδοπουλος]

Υπάρχει απο το 1972. Παιδια ειμαστε πολύ πίσω. Εχουμε θεωρία στην θεωρία και δεν εκπαιδευόμαστε στην τεχνολογία. παντα πιστευα οτι ο μηχανικός ειναι σαν τον γιατρό. Εαν δεν του δωσεις την πληροφορία τον εκαψες ή τον κάνεις χρήστη λογισμικού.