Cementitious and Ceramic Material Engineering

Transport processes into concrete

lunedì 5 dicembre 2016 09:01

Concrete: porous material. Reinforced with CS -> increase the heterogeneity -> problems in the environment. Concrete: porosity -> it doesn't hydrate by itself. No problem with

mechanical property but with porosity. Damage directly to the concrete.

Sulphates: very dangerous. Dependance on composition, lead to severe problems.

Carbonation: reaction between concrete and CO2, very dangerous too.

Problems between concrete and ions and concrete and gases. Think about concrete as sponge with a lot of holes -> possibility to gases to diffuse; or filled with water: no gas but ions.

We can also have partial permeation.

Transport: something outside concrete that goes inside it, diffusing. Cl dangerous for rebars that can be transported inside concrete for example.

Concrete is always in equilibrium with environment! Dry environment = dry concrete. Always. The

relation between RH outside and RH inside concrete is not linear generally. Concrete completely dry or

completely wet: rebars are not supposed to corrode. The problem is that we never use concrete in these

conditions.

PERMEABILITY of concrete: general entrance of substances, also by chemical affinity (capillary

absorption, chemical migration, electrophoretic mechanisms). It is not permeation!

PERMEATION related to pressure, when things are pushed inside concrete.

Aggregates are generally not reactive, so we don't care about their

porosity. The porosity of the cement phase is the one that can cause

damage.

Less porous material: low degradation -> water to cement ratio:

proportion between water and cement. More water: easy to work but

material less resistant from the mechanical and the durability point of

views.

PERMEABILTIY: Calcium hydroxide: cement + water byproduct. Very alkaline solution in the pores therefore. pH = 13 ca. Good for

rebars!

Transport: entrance of substances but if they have to react the transport is slow. Problem of permeability and

chemical reaction.

Distance between outer surface of concrete and rebars is

a design parameter: the concrete cover. The thin

concrete cover is good, but mechanical problem. We

need rebars and relatively closer to the surface, no more

than 5-6 cm.

Wet environment: easy transport of ions; dry

environment: easy transport of gases. Indoor: generally

dry, 30-50%, reasonable for humans, low amount for

water related transport mechanism. Outside, foggy or

rainy day: high humidity, water transport mechanism.

High floors different from foundations! Foundations are

in contact with soil: more humid than air. immobilized 80-90% problems of degradation generally.

Environment dry: concrete even more dry. Under 20-25% evaporation of water even in

the pores: dangerous situation but it can be reached only in a fire.

Segmentation process: make

the segments the thinner

possible.

No transport, no reactions.

Cementitious Pagina 1 Rain water captures the pollution in air and carries it in the concrete: sulphates, nitrates, … that

enter concrete. On the other hand the calcium hydroxide diffuse towards water.

Diffusion mechanism depends again on the structure of concrete. Generally NON-stationary

conditions. Higher distance from the cover: lower

concentration of chlorides.

Higher D: higher concentration of chlorides

(higher transport possibilities).

Look just at the solution. The concentration of chlorides change in time and space. The

properties of the material are assumed as constant in time. The surface concentration of

chlorides is also assumed as a constant in time (actually this is true just in the seawater).

Profile of chlorides concentration

Estimation of it: allows to undestrand

how long the material lives without

degradation, before corrosion. Cl tend to react also with aluminates not only with rebars. Aluminates retain Cl, this is good for

corrosion because they are chemically bound. Apparent diffusion coefficient includes both chlorides free

to move and bounded. The profile so overestimate the content of dangerous chlorides.

CO2 diffuses as well, migrating through pores, reacting with concrete (layer by layer) so it is gradually

consumed, it never reacts in deep zones.

Still chemical mechanism. Concrete is hydrophilic. Water enter the pores and fill them. The more

viscous is the fluid, the more difficult is its entrance.

Small pores + hydrophilic material = water will be (capillary) absorbed very easily and goes deeper. But

pay attention: the big pores tend to favour permeability, by diffusion. This consideration is only made

by considering capillary absorption alone.

Concrete is hydrophilic by nature, we can only make hydrophobic the surface.

Compact concrete: less but smaller pores: more capillary absorption.

Cementitious Pagina 2 Compact concrete: less but smaller pores: more capillary absorption.

Porous concrete: high amount of pores, but larger: more water.

Mechanical phenomenon.

Geometry of pores

The thicker layer of water on the top, the easier is permeation (gravity).

Permeation is relatively easy!

permeation Entrance of gases: diffusion only.

Water: ion migration, diffusion, permeation, capillary absorption.

Resistivity of concrete: very high -> ionic transport is reduced. What is the resistivity of

concrete? It depends on porosity (less porous = more resistive; large porosity IT MAY BE

conductive only if we have water inside). Dry environment: high resistivity. Resistivity

depends on the humidity.

Need of water In italy line cement is generally used (bad kind of blended cement).

Ordinary Portland Cement.

Ground Granulated Blast Slag: blended cement containing pozzolanic mixture. Fine,

reactive = lower porosity than OPC.

Silica Fumed: even smaller -> fine particle -> lower porosity -> higher resistivity.

Which type of transport mechanism do processes depend on?

(capillary only in foundations)

(only gases = only diffusion) Matter of reaction between

concrete and ions in water

Cementitious Pagina 3

Scenario 1: diffusion of CO2. Corrosion of rebars -> degradation of concrete. Very saturated concrete -> in contact with dry atmosphere: diffusion + increasing concentration

Scenario 2: capillary absorption. of salts (water evaporate, salts remain). Highest damage zone!

Cementitious Pagina 4

Degradation

lunedì 5 dicembre 2016 10:44

Not all concrete degrade in the same way. We can produce a kind of concrete able to survive in a particular environment. Match between environment and material establish

degradation!

Chlorides are not so dangerous for concrete, but extremely for rebars. Freeze-thaw: icing problem typical of mountain environment (cycles).

At the beginning of the service

life: large volumes = large heat of

hydration. Increase in T coming

produced by the material itself

(cement + water -> exotermic

reaction).

Meters of concrete: it is thermally

resistive, does not diffuse heat in

a good and efficient way. The

reaction occurs, in the heart of

the material it is difficult to

transport this heat wrt the skin.

Water may evaporate inside! The

heart of the material cannot react

in the correct way. Or at the end of the reaction the heart will cool down and shrink, while the

surface has already solidified: cracking. Concrete is terrible at tensile stresses (that's why we use

rebars).

More time to react = mechanical properties take more time to develop = heat of hydration is

more gradual and can be eliminated gradually = no problems of heating localized and rapidly

developed. Cements that react slowly are blendend cements (portland + something else).

Concrete unfortunately is based on water. Bad conduction of heat: theoretically good in fire, but

actually it is not like this. Fire starts: water in the pores evaporates. Problem if it is the water that

keeps the CSH gel together (the chemical absorbed water) -> the pore collapse, the material

shrinks. Chemically absorbed water is bound and requires much energy.

Aggregates are made of silicates and carbonates: they may change composition (?).

CSH gel: calcium and silicate hydroxide.

Portlandite (CaOH): could release the OH part and bind to CO2 creating a carbonate. All the

water that you release create vapour, gases -> creation of tensions. The same applies to silica.

Also release of oxygen is possible. Limestone: CaCO3. First expansion (formation) and then

contraction because the carbonate decomposes.

Cementitious Pagina 5 Not the freezing point of pure water because of salts (few degrees under zero). Water freezes =

expansion of 9%. This happens to all pores: no space in the concrete to accomodate extra volume when

RH is about 80-90% -> tensile stresses.

Way to avoid this problem: reduce water, segregate porosity, reduce porosity, extra

porosity (to accomodate extra volume), …

micrometers Very low w/c starting point is

necessary, together with air

entraining agents.

Salts not aggressive: problem of evaporation of water and high concentration of salts.

Fluorescences (?). Decrease in pH

Loss of mass (losing components)

Avoid contact with water, …

Cementitious Pagina 6 Ettringite forms when the material is already solidified. (recuperare questo pezzo)

ASR: alkaly-silica reaction.

Silica can be crystalline or

General amorphous. Wrong type of

working aggregate: reactive! The

condition crystalline is more

chemically stable while the

amorphous is reactive.

Concrete: environmentally

based technology.

Aggregates are simply taken

from the place.

Opal silica: amorphous ->

chemical degradation

attack related to pH (acid

material in an alkaline

environment). Swelling of

silica -> lot of internal

stresses -> cracking.

Random cracks: ASR. Different kind of cracks indicate different reactions.

Pop out: release a portion of concrete ("like a popcorn piece"). Low alkaline concrete exists.

Blended cements not for the porosity but for the pH. Pozzolanic mixture consumes part of the

alkalinity (pH = 12.5). Less aggressive reaction.

Higher T = higher reactivity.

POP OUT

Carbonation: chemical modification of concrete but not harmful towards concrete. It creates problems with rebars.

Cementitious Pagina 7 Leaching out: rain water has partially dissolved the material. Some acid attack also here because this is

via Mancinelli and in Milan we have acid rains (slightly, pH = 5).

Pop out: ASR. (also web of cracks). Follow the profile of rebars

Cementitious Pagina 8

Corrosion of reinforcements

martedì 6 dicembre 2016 14.21

Carbonation takes place on concrete -> corrosion of rebars.

Chlorides enter concrete and may cause fluorescence (not an hazard) because of water evaporation and salts deposition.

Stray current: induced rebars corrosion. No rebars: no current conduction, thus no hazardous to concrete itself.

Concrete: pores solution is alkaline -> rebars are passive. Generally CS rebars are used. pH: 13 (Portland cement) or less (blended cement).

Portland cement creates CSH+portlandite (calcium carbonate), the latter reacts in part with pozzolanic blend.

The lower the pH, the weaker the passive film on rebars (min pH = 9). Cathodic reactant is oxygen. Concrete is a oxygen shiel d itself. If

concrete is saturated = no corrosion. Alkaline environments give negligible CR.

Main causes of depassivation:

• Concrete is no longer alkaline -> carbonation (general corrosion)

• Depassivation by chlorides, local removal of film (localized corrosion)

• Stray currents (localized corrosion). Beware of macrocouples in rebars, H embrittlement.

TUUTTI DIAGRAM: penetration of corrosion as function of time. Interval of time with no corrosion: rebars are passive. Initiation: activation of

rebars, at a certain speed proportional to [O2], water, chlorides, etc.

Max acceptable penetration: limit state. We can allow some corrosion but the structure must be still safe and perform well. Limit state is only

valid for carbonation (induced corrosion), because it is easy to predict and control. For Cl - induced, service life ends when initiation happens

because it is autocatalytic and we cannot take the risk.

Max acceptable penetration 50-100 then we need to repair. Chosen by the designer.

µm,

Rust decreases adherence between rebars and concrete; it swells and creates internal tension.

Carbonation = generalized corrosion. Production of rust that can diffuse to concrete surface, so easy to spot. Chlorides -induced = localized

corrosion. Less rust, so better for stability of concrete.

Time for air to reach the rebar = initiation time. Vrust = 2-6 Vsteel.

Beware of spalling: cracking of concrete due to internal stress. Spalling is already too late -> failure. Elimination of a part of concrete cover.

Chlorides-induced may happen even in urban environment due to anti-freeze salts.

In pristine concrete rebars passive: we stop the anodic process. If the environment changes, it occurs anyway: we stop cathodic process by

saturating concrete or by repassivating rebars. Completely dry environment on the other hand means no corrosion.

CR in concrete negligible if <1 µm/y

Carbonation is slower: max speed is 100 because we can't make oxygen go faster than that. In Cl-induced: big cathode, small anode ->

µm/y

very fast CR.

Evans diagram: if rebars are depassivated for some reason, H evolution may cause corrosion. If the material is active, the speed is

proportional to RH. RH < 95% = ohmic drop increases until we reach negligible CR; RH > 95% = negligible ohmic drop = max CR.

CARBONATION

CO2 + Ca(OH)2 --> CaCO3 + H20 (when water or NaOH are present this reaction occurs).

pH drops to neutral: rebars becomes active. This process can take years so it is very slow, but once all calcium hydroxide is converted and we

consume that attached to rebars, they have been in contact with a CaCO3 solution and they activate. It doesn't happen in dry environment

(RH < 40%). We need a lot of CO2 so we need some unsaturated space for CO2 to flow.

Phenophtalein test: dye changes colour to pink if there is alkaline solution. We need to analyze concrete next to the rebars extraction of a

:

core and spray of its lateral surface with phenophtalein. Quick test: 1 day to avoid carbonation to air. Line that separates pink from other

colors tells the carbonation depth.

It starts faster because air has a lot of CO2 but then it slows because CO2 has to diffuse. Carbonation depth increases relat ively fast in the 1st

year (some cm in 10 years). A good concrete has a carbonation coefficient (proportional to t^1/2) of 10 -15. The diagram carbonation depth vs

time tells us that errors in construction are very dangerous. If rebars aren't kept in the same position, and cover ends up 2 0 instead of 40,

service life will be 1/4 because it goes with the square root.

Carbonation depth = Carbonation coefficient * square root of time

Cracks are highways for entrance of gases. Speed varies all the time because of many factors.

Formworks: builders tend to remove formworks because they build something else with them. Practical curing time is 2 days instead of 28

days. This impacts on mechanical resistance, porosity, … Extending curing time can reduce carbonation depth by even 100%.

Carbonation depends on RH: negligible if < 40% (no water) and > 95% (no air). The max peak is at 70-80%.

Importance of microclimate: position and exposure to some agents change carbonation, e.g. a balcony -> lot of rain at the top = no

carbonation. But the bottom of the balcony is never hit by rain = carbonation depth high.

Most subjected areas: indoor, areas shielded by rain.

Corrosion happens when RH is very high. Carbonation and corrosion are mutually exclusive. We need carbonation and then period s when the

structure is wet: cycles.

Corners are most dangerous points because rain follows them. Also rust pushes in all directions.

Cementitious Pagina 9

Carbonation

lunedì 12 dicembre 2016 09:06 Saturation condition or dry environment: both the phenomena are negligible. Effects of chlorides on

carbonation: hygroscopic nature -> increase the relative humidity. No chlorides: corrosion - linear

relationship between RH and CR - no carbonation - until 70% negligible CR.

Cl presence: threshold is already risky at 70% humidity or less. The more chlorides you have, the more

the RH, the higher the risk of having a risky CR.

Threshold for initiation of Cl-induced corrosion: the two phenomenon may risk. Blue line: CR not only

due to carbonation but also to Cl-induced corrosion.

% here are by weight of cement (usually).

Red line (and lower) only carbonation and more aggressive situation in term of RH.

Accumulation of water at the edges: high CR with the contact of water + pushing of rebars in all

direction.

Cementitious Pagina 10

Cl-induced corrosion

lunedì 12 dicembre 2016 09:15 Water brings chlorides inside the material; the cathodic reaction is oxygen reduction.

We need a certain amount of chlorides until the

rebars are depassivated.

Localized corrosion. Inside concrete is that we have an

alkaline material, that can bind chlorides to the material itself:

reaction of a part of chlorides with aluminates. Safety

coefficient, because we don't have to understand how many

Cl are dangerous. We consider all chlorides are free to move

and dangerous (worst scenario possible). Some time needed

to water to be absorbed. Skin (first layer of concrete) in

equilibrium of the environment: less Cl near to the rebars.

The important parameter is properly the concentration of

chlorides at the rebars!

Carbonation: less Cl are necessary to initiate pitting corrosion. 0,4% of Cl by weight of cement (not an exact value): generally pitting starts. But pitting is a stochastic mechanism and

can start even if Cl is 1%. Below 0,4% we are reasonably sure that we have no corrosion. Above 1% we are sure that we have corrosion. Between 0,4% and 1% we know that it can

start. So we need the lowest value to be in the safe condition! The threshold value is 0,4%.

Porosity, alkalinity, thickness of the cover, …

Propagation time < 1 mm/y. We never consider propagation time. Higher pH, higher threshold. Speed of reach the critical Cl threshold is reached according to RH (higher

humidity, higher diffusion, they're not able to diffuse in air). Structure on the beach: concrete with

microvoids, you may have a very