Cementitious and Ceramic Material Engineering

CR

If we have chlorides: No propagation time here. The service life is just equal to the initiation time. More complicated to make

calculation here, because the depth where we reach the critical Cl threshold contains the error function.

Generally the exercise is risolved graphically.

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Probability

lunedì 12 dicembre 2016 08:30

Associate the different percent of chlorides to the probability.

Distribution of values. Gaussian distribution with the central value that is the most likely.

Probabilistic distributions. Certain peak value in the lognormal distribution but below it is very

unlikely situation. 100% probability when Cl are 1%.

We can choose an average value, or the most dangerous condition. We generally choose 0,4% even

if it is not the most frequent value.

Very complicated to use the range of value but it is a more correct procedure. There are softwares

that calculate these things (by iteration).

Montecarlo approach. Cementitious Pagina 15

Other forms of corrosion

martedì 13 dicembre 2016 13:28 Heterogeneous material in contact with heterogeneous environment. Some rebars are in a condition;

other ones in another condition. Think for example to carbonation:

Rebars are deposited as a cage, one connected

to each other, for working reasons.

Still passive rebars

Macrocouple is formed between a layer of rebar active (carbonation) and the inner rebars that are

still passive.

This doesn't occur so much because of ohmic drop RI. Only 10% is exploited to accelerate

corrosion of the rebars, the other is consumed in ohmic drop.

If we use SS or galvanized rebars we still have this driving force? Yes. Epoxy coated

rebars: somehow localized corrosion in the defects of the painting. During time:

This kind of corrosion is added to a corrosion that is going on: creation of rust on the spot, that will gradually cause the detatch of the painting.

enhancement of corrosion rate! The surface area available for macrocoupling effect will be very small: cathodic

Current circulation between the less noble metal that corrodes faster (1st layer) and the more surface area available for the connection is very small, and distance and resistance

noble metal (2nd layer of rebars). Chlorides are driven towards the first layer (negatively charged are very large, so the phenomenon is limited.

ions present in the concrete go opposite the direction of the current).

Different exposure to air and oxygen causes great damage (buried and immersed structure): low

resistivity, most of the driving force is not wasted in ohmic drop but is exploited in corrosion.

Seawater: very high humidity or saturation condition. Many chlorides.

Consider the two different sides that are different but electrically connected because rebars are

connected. If we don't have the pocket of air nothing happens (saturation = no corrosion). But here we

have anodic reaction where we have chlorides, and cathodic reaction where we have oxygen. This is

similar to the differential aeration condition. No chlorides: driving voltage too small because rebars with

high oxygen content and rebars with low oxygen content.

Ions coming from the soil, rebars can be activated easily.

Very large cathodic areas (still passive) electrically connected with the other activated rebars.

Carbonation is slower than transport of chlorides. That's why are still passive; not only,

carbonation is retarded because of the production of alkalinity due to charge transport, diffusion and

corrosion of the other rebars.

Humidity condition of concrete: liquid separated by gas -> liquid environment very high

saturation very good conductivity enhance the macrocouple effect.

Large ohmic drops, environment changing in time according to the weather for example. (?)

Oxygen rich part in contact with oxygen poor = biggest macrocouple effect.

Current comes from traction, CP, … systems. No metal directly exposed because we start from a passive

Cementitious Pagina 16 Current comes from traction, CP, … systems. No metal directly exposed because we start from a passive

material. CS in concrete is passive. The first thing that interference will do is nothing.

Simply current circulates.

The anodic process is oxygen evolution! Exactly the

opposite direction. Byproduct: H+ = acidity so

gradually the metal will be depassivated.

Distribution of H+ away from the spot of production, over the larger

volume, the effect will be buffer. We need a lot of acidity to start

(needed a threshold value).

VEDI ANNA.

More interference more corrosion Overall effect: interference corrosion starts more

easily because the passive film is dissolved more

easily in the presence of chlorides. On the other hand

we can have Cl-induced corrosion before so the two

corrosions sum up.

Cathodic protection is the only one solution to interference corrosion. CP cannot be used with all the

kind of rebars. Viaduct generally built with prestressed concrete: rebars with higher strenght ->

hydrogen embrittlement. Be careful to the correct value of CP current.

1 A/m^2 is needed actually

Few hundreds according to %Cl Other forms

of corrosion

Registrazione audio avviata: 14:18 martedì 13 dicembre 2016

Sometimes conditions for this already happen during stucture buildings not completely isolated during

Cementitious Pagina 17 Sometimes conditions for this already happen during stucture buildings not completely isolated during

concrete casting. Corrosion attack on the rebars before filling the mortar = stress corrosion cracking.

The mortar have to be filled quickly or very perfectly. Starting of the corrosion before the complete of

the cast = continuosly corroded even without oxygen because we may have hydrogen evolution.

Metal dissolution happen generally in stables where there is ammonia. Ammonia can penetrate

concrete and be in contact with CS and induce stress corrosion cracking.

H embrittlement only on high strength steel. Cold drawn the most sensible.

Prestressed concrete: the rebars want to return to the equilibrium

condition (they are under tensile stress). Buffer of compressive

stress that increases the resistance of concrete. Rebars remains a

little bit in tensile stress (problem with H evolution).

HYDROGEN EMBRITTLEMENT H embrittlement affects only steels used in prestressed concrete. Dislocation are not able to move

anymore because of H presence: they become fragile.

Less defects, more open structure, possibility to accomodate more H before embrittlement occurs.

Localized attack that creates a defect on the rebar -> fracture mechanics until brittle failure. Subcritical propagation is relatively slow. Beta goes from 0,5 to 2

Cementitious Pagina 18 Generally 800 MPa but some

steels starts at 750 MPa.

cathodic overprotection

Prestressed concrete is very interesting because we can have shapes and use it with tensile stress but you have to avoid an incorrect design.

Starting of the corrosion + mechanical failure.

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Prevention

martedì 13 dicembre 2016 14:45 Resistance from a mechanical point of view, to fire, it has to keep the same behaviour in terms

of heat absorption and release, etcetera. Change of environment: change of precautions.

Service life ends when the limit state is reached, example 100 micron in carbonation or when

pitting starts. Ultimate limit state exists but generally it is not used.

infrastructures

Personal use Cracking: more availability of water and oxygen -> the corrosion penetration increases up

to spalling.

Thicker cover -> longer initiation time. Compaction: less defects possible. Environment: which kind of corrosion? Even the microclimate is v ery important. Coatings do not last

forever: typical scheduled intervention (5-10 years). The more you wait, the bigger the damage.

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Prevention and Preventative Techniques

mercoledì 14 dicembre 2016 15:29

Brief summary: prevention is a very important phase of the development of concrete. Save money, according to the environment! Good match between

the material and environment. W/C, composition of the cement (type of cement, portland or blended), rebars are the most important parameters to

design. Mechanical resistance given as a class of resistance. Change W/C changes the class of resistance. The class of environmental exposure is an

important parameter to decide which kind of material is needed. EN206 important european standard, and also EUROCODE2: important requirements in

terms of numerical and things you have to take care of, as

• W/C. To decide porosity and mechanical resistance; given always by weight. Maximum value is indicated.

• Cement content. We have also aggregates! Small amount of cement, and small amount of water, low W/C so it is good, but with many aggregates

the mechanical resistance is not good at all! The glue that keeps all together is not sufficient. Minimum value is indicated.

• Cover. Environmental performance; sufficient space between the outer environment and the rebars. Minimum value is indicated.

• Rck. Class of resistances (C20/25; C35/45; … the second number is the resistance of a cube of concrete (see introduction)).

• Curing. Don't remove the formworks before the correct time.

• Air. Important in the case where there is freeze and thaw cycles. Minimum value is indicated.

XO: no risk of corrosion, no degradation of any type. Impossible class, because rare: as concrete not reinforced in an environment without sulfates.

XC: carbonation.

XA: chemical attack.

XF: freeze and thaw.

XS: seawater.

XD: all environments with chlorides that are not seawater (airbone chlorides, antifreeze salts, …)

Just do concrete: XO. All the other classes require all the 6 requirements.

We also have subclasses: microenvironment. Dry, humid, cycles, … Humid intermediate requirements; toughest requirements for durability.

Carbonation in very dry environment W/C = 0,65 maximum (very high value); cycles of wet and dry (very aggressive) W/C = 0,5 maximum.

Chlorides: max W/C = 0,56 more conservative than carbonation because chlorides make things worse. Dry and wet cycles require W/C = 0,45.

NB: there are tables to be used, we need only to know how to use the classes. The labels (X..) are to be known.

Min strenght in opposite direction than W/C.

Dry carbonation: class C20/25; cycles: class C30/37.

Concrete cover: more or less the same.

Quantity of cement: min content for all classes of concrete. Low requirement of mechanical resistance: low requirement of cement.

European std different from italian std for the carbonation class (Italian higher): the reasons are money.

These regulations are dedicated to structures with SL = 50 y with OPC: some limitations. Portland cement is not used in Italy, and not so much in Europe.

We use always blended cement. Finer pores in the blended cements.

Durability less restrictive. Correct concrete cover and correct distribution of the rebars are to be decided. Don't put too many rebars, not to close (mixture

can't fill the spaces between rebars -> not enough cement phase a