risposte cementitious and ceramic material engineering

Cement

The main transport mechanisms taking place in concrete are the following four:

Diffusion, due to concentration gradients between the external surface

1) and the bulk. It can be described using Fick’s laws: the first one for

stationary diffusion (constant and unidirectional mass flow) and the

second one for the non-stationary one (influence of time). It requires the

presence of water which allows the movement of ions.

2) Capillary adsorption, due to capillary action inside capillary pores of

cement paste, which is highly hydrophilic. The smaller the pores, the

greater the penetration even if friction slows the process, so the

adsorption rate increases with the diameter of the pores. We have to

consider also that capillary adsorption increases with the decreasing of

the contact angle (hydrophobic treatments can be done), with the

increasing of the surface tension of cement and with decreasing of the

It is typical of basements structures.

fluid viscosity\density.

Ion migration, in the case of applied fields, due to a potential difference.

3) It depends on the resistivity of the material, the cross section and the

length of the conductive path.

Permeation, due to pressure gradients. It is described by Darcy’s law, so

4) it is proportional directly to the pressure gradient and the cross section,

and inversely to the viscosity and the pore length through a coefficient K

called permeability coefficient.

The second Fick’s law is used in order to describe non-stationary diffusion, that

is, a flux depending on time. It is usually integrated under the assumption of a

constant concentration of the diffusing ion on the surface equal to C (for x=0

s

and any time t) and a diffusion coefficient D that is constant in time and space.

We also assume negligible binding effect, that is a constant diffusion condition.

The solution obtained is:

( )

( )

C x,t x

=1−erf √

C 2 Dt

s

For a high value of x, erf(x) tends to one, so the concentration goes to zero.

This is typical on the beginning if the service life. On the other hand, the small

x, the more erf(x) tends to zero, so the concentration gets closer and closer to

the superficial one and so to the end of the service life.

Permeation is a transport mechanism consisting of the penetration into

concrete of an incompressible fluid due to a pressure difference and it is

described by Darcy’s law. The flux is proportional to the cross section, to the

pressure gradient and to the permeability coefficient K, while it is inversely

( )

dq K ∆ PA

=

proportional to the length and the viscosity .

dt Lμ

Permeability is the property of concrete to allow the entrance of water with

dissolved ions like chlorides and sulphates or air and carbon dioxide from

atmosphere, it is a fundamental factor in the durability of reinforced concrete,

and it is closely related to the transport phenomena and the porosity: if

porosity increases, permeability increases as well, and it is mostly related to

macropores, so it can be hindered through pores segmentation. It is expressed

in m .

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The main factors affecting concrete penetration are:

Porosity and pores size, depending on curing time, which induce the

 mobility and entrance of water. It can be hindered through pores

segmentation.

Capillarity, leading to an interconnected structure, allowing the

 penetration of gases and liquids in the bulk of concrete.

Water to cement ratio and curing time (the higher W/C and the lower the

 curing time, the higher is the porosity of concrete and therefore the

penetration).

Aggregates: they must have a variable size in order to fill the pores as

 much as possible.

Permeability, which is the ability of concrete of allowing the entrance of

 water. It is affected by the parametest listed above

The higher the W/C ration, the higher the permeability because the evaporation

of water leads to the formation of capillarity pores, responsible for the entrance

of contaminants from the outside environment. Blended cement has a very low

porosity since it produces very fine hydration products, but it requires a longer

curing time in order to undergo a correct hydration process.

Freeze-thaw attack is a physical degradation process induced by

freezing/drying cycles. In fact, the solidification of water is an expansive

process. This means that, if porosity is too low, water cannot redistribute

correctly inside the pores, which will be entirely filled by water. When it freezes,

its volume increases and this leads to internal stresses that, with time, can

induce the formation and propagation of cracks, eventually leading to failure.

So the main aspect to consider are:

Porosity: the more extended, the more effective the redistribution of

 water (this is the reason why air-entraining agents are exploited to

prevent freeze-thaw degradation).

W/C ratio: the lower, the higher the resistance to internal stresses.

 Freezing speed: the higher, the lower the redistribution of water.

 RH: if it is below 80/90%, pores won’t be completely filled and so water

 has some space in which it can expand.

Presence of salts: they reduce the freezing point, so they have a positive

 effect, retarding freezing.

Type of cement: blended cements have a finer and more segmented

 porosity.

Nature of the surface: if the surface is treated with hydrophobic coatings,

 it does not allow the penetration of water, therefore freeze-thaw does not

take place.

Air-entraining agents induce the formation of a porosity network in the system

by creating tiny and uniformly bubbles with diameter of about 300 μm, that is

more space for water redistribution during freezing (volume of entrapped air is

increased of 4-7%). The reduced dimension of the pores makes it difficult to fill

them completely, preventing the formation of internal stresses. The drawback

is the reduction in mechanical resistance, which must be balanced by a lower

W/C ratio.

Alkali aggregates (silica) reduction (AAR) is a chemical degradation mechanism

caused by the interaction of alkali compounds with amorphous silica present in

the aggregates. This leads to the formation of a highly hydrophilic gel

containing OH groups, able to swell and expand, creating internal stresses and

eventually the nucleation and propagation of cracks. It can be detected easily

as it leads to the formation of a sort of spiderweb like crack network on the

external surface of concrete, or we can notice the presence of gel pop-outs.

The main parameters to be considered are:

alkali content in concrete (critical above 3-4 kg/m , where the strain due

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 to expansive reactions is excessive)

Reactivity of silica minerals (opal, a highly disordered structure, is the

 most reactive form of silica as this reaction requires an amorphous

structure)

Humidity (no damage if RH<80-90% because water is required for

 expansion and swelling)

Temperature (an increased temperature promotes the reaction)

 Blended cements reduce the alkali content and the porosity. Moreover,

 they have a lower PH, meaning a lower aggressiveness of alkali

compounds.

Diffusion of sulfates into concrete is very dangerous as it induces the formation

of gypsum from calcium and of secondary ettringite from aluminates (this

phenomenon is called DEF, delayed ettringite formation). The reaction Is

expansive, so it induces internal stresses.

It can be prevented by:

Controlling the capillary porosity (low W/C ratio and long curing or use

 pozzolanic additions to segment the pores) to avoid the sulphates

penetration.

Using cement with low content of C A (calcium aluminates) to avoid the

 3

formation of expansive products like ettringite.

Blended cements are used because they assure a lower porosity (higher

compactness). In fact, Pozzolana is not a binder itself, so it consumes the

calcium hydroxide produced by cement hydration to form the CSH gel. The

hydration products are finer, so they lead to a less porous structure. This higher

compactness reduces the penetration of sulfates and chlorides carried by

water.

Regarding ASR, blended cements consume the alkali during curing they have a

dilution effect reducing the content of OH ions in the pores solution of the

-

cement paste, so alkali transport is hindered and hydroxyl ions are consumed

by pozzolanic reaction during curing (hydration). Moreover, the lower PH typical

of blended cements reduces the aggressiveness of alkali.

During cement hydration, an alkaline pore solution is obtained (made of sodium

and potassium

hydroxides), leading to an increase of the PH to 13 (alkaline environment); iron

oxides are the thermodynamically stable compounds in this environment. As a

result, on reinforcing steel embedded in alkaline concrete there’s the

spontaneous formation of a thin protective oxide passive film.

Corrosion is an electrochemical process involving an electrolyte, that is water

contained in concrete, and an active material, that is the depassivated rebars.

The main processes that can lead to corrosion are:

Carbonation induced corrosion CO ): it decreases the pH, hindering the

 2

passive layer formation and therefore activating the rebars. In particular,

calcium hydroxide reacts forming CaCO which increases the hardness,

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but decreases the alkalinity of the environment. This can lead to the

depassivation of rebars, so to uniform corrosion on the whole surface of

the metal and to localized corrosion (pitting) in presence of chlorides that

break the passive layer.

Sulphate attack: it leads to the formation of gypsum and of secondary

 ettringite. This causes an increase in volume, so to internal stresses that

may induce the formation of cracks. This type of corrosion is more critical

than carbonation and it becomes even more important in presence of

chlorides.

AAR (alkali aggregates reaction): in this case the alkali react with

 amorphous silica contained in aggregates, leading to the formation of OH

bonds, highly hydrophilic. So, a gel il formed and it induces swelling and

internal stresses. This type of corrosion can be detected because it

causes the formation of a spider web like network of cracks or of pop outs

caused by the gel that pops up.

Chlorides: they can penetrate as well in concrete reaching the rebars

 and, if they accumulate in a concentration above the threshold, they can

locally destroy the passive film causing localized corrosion.

Since the corrosion rate is ruled by the slowest of the steps of the corrosion

electrochemical process is sufficient to act on one of the following:

anodic process

Slowing the (by passivating the rebars, SS rebars or

 pristine fresh alkaline concrete)

cathodic process

Slowing the (by removing oxygen, submersed

 structures)

Increasing the concrete resistivity (dry conditions, RH<70%)



So it is possible to avoid or slow down the penetration of contaminants by using

suitable coatings (like the hydrophobic one), by controlling the concrete quality

(low W/C, high curing to get a low porosity and therefore a slower penetration)

or by adding inhibitors (they reduce the initiation time but also the penetration

one in the case of chlorides). Otherwise, it is possible to use materials of higher

quality for rebars (SS, galvanized steel, zinc or epoxy coatings). It is also

possible to apply protection systems like cathodic protection or prevention,

electrochemical realkalization or electrochemical chlorides removal.

Carbonation is the reaction between the CO present in the atmosphere, that

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diffuse into concrete, and the alkaline constituents (CO 2 + Ca(OH) 2 -> CaCO

3 + H O)

2

CO dissolute into water, that is present in the pores, creating an acid solution

2

(pH lowered to 8-9) where it can react with alkaline products present in the

liquid phase, like NaOH or KOH, or with solid alkaline products, like Ca(OH) or

2

C-S-H gel, but always in contact with the acidic aqueous solution. In principle,

carbonation is not a big issue for concrete as CaCO3 increases the hardness of

the material.

Carbonation is a problem in presence of rebars which can undergo

depassivation in a PH=9 environment, so they may be subjected to generalized

corrosion. Moreover, the reduction of PH weakens the bond between

aluminates and chlorides, which may be released in the material. Salts are

dangerous also because they are hygroscopic, so they locally increase the

humidity. √

The penetration of carbon dioxide can be computed using the law ,

s=K t

where t is the time, s the thickness of penetration and K is the carbonation

constant, which depends on the quality of the environment (it increases with a

higher W/C and porosity, it is lower for blended cements).

Carbonation rate is maximum at RH = 100% while carbonation induced

corrosion is maximum at RH = 95-98% meaning that the most sever condition

is the wet/dry cycle. In a completely wet environment, corrosion does not take

place.

Corrosion is negligible in immersed concrete structure because the content of

CO as well as of oxygen is too low in soil (no diffusion) so the carbonation is

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not possible.

The threshold changes depending on the material used for the rebars: for

carbon steel it is in the range 0.5-1% with respect to cement weight, for

galvanized steel 1-1.5% while SS can arrive also to 5-8% (maximum for duplex

stainless steels), but the effect of the pH influences the maximum value. Once

this threshold has been reached, the service life ends as propagation is very

fast, so it is not considered (the only exception is in presence of inhibitors

which decrease propagation as well). These ranges are valid only in case of

high PH, otherwise they get lower .

For carbonation induced corrosion, the corrosion rate is negligible when

RH<80%, while in case of pitting, the relative humidity must be below 70%

(<1-2 micron/year). In presence of saturated concrete no oxygen can

penetrate, so no corrosion can take place in absence of a cathodic reactant.

In presence of expansive products, corrosion leads to internal stresses that may

induce the formation and propagation of cracks. They can also cause a loss in

adherence, and therefore spalling of the concrete, that is fall of portions of

concrete and as e consequence exposure of the rebar to the environment.

Another consequence of corrosion is the loss of mechanical properties due to

reduction of the cross section of the rebars.

The service life is the time a structure is expected to work safely, and it is

obtained by considering the initiation and propagation time. It may depend on

both the initiation and propagati