Cementitious and Ceramic Material Engineering

Transport processes into concrete

Concrete is a porous material. Reinforced with CS, it increases the heterogeneity, leading to problems in the environment. Concrete has porosity, meaning it doesn't hydrate by itself, creating no problem with mechanical properties but posing challenges with porosity. This can cause direct damage to the concrete.

Influence of substances

Sulphates are very dangerous and their impact depends on the composition, leading to severe problems. Carbonation is the reaction between concrete and CO₂, which is also very dangerous. Problems can occur between concrete and ions, as well as concrete and gases. Think of concrete as a sponge with a lot of holes, allowing gases to diffuse; or filled with water, where there are no gases but ions.

Transport and equilibrium

Transport refers to something outside concrete that goes inside it, diffusing. Cl is dangerous for rebars and can be transported inside concrete, for example. Concrete is always in equilibrium with the environment. A dry environment equals dry concrete. The relationship between relative humidity (RH) outside and inside concrete is generally not linear. Concrete that is completely dry or completely wet prevents rebars from corroding. However, concrete is never used under these conditions.

Permeability and permeation

Permeability of concrete refers to the general entrance of substances, also by chemical affinity (capillary absorption, chemical migration, electrophoretic mechanisms), and it is not permeation. Permeation is related to pressure, when things are pushed inside the concrete.

Aggregates are generally not reactive, so their porosity is not a concern. The porosity of the cement phase is the one that can cause damage. Less porous material means low degradation, influenced by the water-to-cement ratio: the proportion between water and cement. More water makes it easier to work with but less resistant from a mechanical and durability perspective.

Calcium hydroxide and chemical reactions

Calcium hydroxide is a cement and water byproduct, resulting in a very alkaline solution in the pores, with a pH of about 13, which is good for rebars. Transport involves the entrance of substances, but if they need to react, the transport is slow. The problem lies in permeability and chemical reaction.

The distance between the outer surface of concrete and rebars is a design parameter known as the concrete cover. A thin concrete cover is good but poses mechanical problems. Rebars need to be relatively close to the surface, no more than 5-6 cm.

Environmental and material considerations

Wet environments allow for easy transport of ions, while dry environments ease gas transport. Indoors are generally dry, at 30-50%, which is reasonable for humans but low for water-related transport mechanisms. Outside, on foggy or rainy days, high humidity promotes water transport mechanisms. High floors differ from foundations; foundations are in contact with the soil, which is more humid than air.

In a dry environment, concrete becomes even drier. Under 20-25% evaporation of water occurs even in the pores, creating a dangerous situation that can only be reached during a fire.

Rainwater captures pollution in the air and carries it into the concrete, such as sulphates and nitrates that enter. On the other hand, calcium hydroxide diffuses towards water.

Diffusion mechanisms depend on the structure of concrete. Generally, non-stationary conditions exist. The higher the distance from the cover, the lower the concentration of chlorides. A higher diffusion coefficient indicates a higher concentration of chlorides and transport possibilities.

The concentration of chlorides changes over time and space, with the material's properties assumed constant over time. The surface concentration of chlorides is also assumed constant in time, which is true only in seawater.

Chlorides, carbonation, and diffusion

Chlorides tend to react with aluminates, not only with rebars. Aluminates retain chlorides, which is good for corrosion because they are chemically bound. The apparent diffusion coefficient includes both free-to-move chlorides and those that are bounded. This overestimates the content of dangerous chlorides.

CO₂ diffuses as well, migrating through pores and reacting with concrete layer by layer, so it is gradually consumed and never reacts in deep zones. This is still a chemical mechanism. Concrete is hydrophilic, meaning water enters the pores and fills them. The more viscous the fluid, the more difficult its entrance.

Small pores combined with hydrophilic material allow water to be capillary absorbed very easily and deeper. However, pay attention: large pores favor permeability through diffusion. This consideration is only made by considering capillary absorption alone.

Concrete is hydrophilic by nature, and we can only make the surface hydrophobic. Compact concrete has fewer but smaller pores, leading to more capillary absorption, whereas porous concrete has a high number of larger pores, allowing more water. This involves a mechanical phenomenon related to the geometry of pores.

Permeation and resistivity

The thicker the layer of water on the top, the easier the permeation due to gravity. Permeation is relatively easy! The entrance of gases occurs only through diffusion. Water allows for ion migration, diffusion, permeation, and capillary absorption. Concrete resistivity is very high, which reduces ionic transport. The resistivity of concrete depends on porosity and humidity (less porous equals more resistive, large porosity may be conductive only if water is inside). In a dry environment, resistivity is high.

In Italy, line cement is generally used, which is not ideal (a bad kind of blended cement). Ordinary Portland Cement and Ground Granulated Blast Slag are blended cements containing pozzolanic mixtures. Fine and reactive, they have lower porosity than OPC. Silica Fumed is even smaller, with fine particles leading to lower porosity and higher resistivity.

Transport mechanisms and scenarios

Transport mechanisms depend on the processes, such as capillary absorption only in foundations and gas diffusion. This is a matter of reaction between concrete and ions in water.

Scenario 1: Diffusion of CO₂ causes corrosion of rebars, leading to the degradation of concrete. In very saturated concrete, when in contact with a dry atmosphere, diffusion occurs with an increasing concentration.

Scenario 2: Capillary absorption of salts leads to water evaporation, where salts remain, causing the highest damage zone.

Degradation

Not all concrete degrades in the same way. We can produce a kind of concrete able to survive in a particular environment. The match between environment and material establishes degradation! Chlorides are not so dangerous for concrete but extremely for rebars.

Freeze-thaw is an icing problem typical of mountain environments, occurring in cycles. At the beginning of the service life, large volumes lead to large heat of hydration, with increased T produced by the material itself (cement + water equals an exothermic reaction). Meters of concrete are thermally resistive, not diffusing heat in a good and efficient way. The reaction occurs in the heart of the material, where it is difficult to transport this heat with respect to the skin.

Water may evaporate inside, and the heart of the material cannot react correctly. Or, at the end of the reaction, the heart will cool down and shrink, while the surface has already solidified, causing cracking. Concrete is unsuitable for tensile stresses, which is why rebars are used. More time to react allows mechanical properties to take more time to develop, with gradual heat of hydration being eliminated gradually, preventing heating problems that are localized and rapidly developed. Cements that react slowly are blended cements (Portland plus something else).

Concrete, unfortunately, is based on water, which is a bad conductor of heat. Theoretically good in fire, but actually, it is not like this. When a fire starts, water in the pores evaporates. The problem arises if it is the water that keeps the CSH gel together (the chemically absorbed water) leading to pore collapse, and material shrinking. 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) and Portlandite (CaOH) could release the OH part and bind to CO₂ creating a carbonate. All the water released creates vapour and gases, leading to tensions. The same applies to silica, and the release of oxygen is possible. Limestone (CaCO₃) first expands and then contracts because the carbonate decomposes.

It is not the freezing point of pure water because of salts (a few degrees below zero). Water freezes and expands by 9%. This happens to all pores, with no space in the concrete to accommodate extra volume when RH is about 80-90%, leading to tensile stresses.

Ways to avoid this problem include reducing water, segregating porosity, reducing porosity, and extra porosity (to accommodate extra volume). A very low water-to-cement starting point is necessary, together with air-entraining agents.

Salts are not aggressive; the problem is the evaporation of water and high concentration of salts, leading to fluorescences and a decrease in pH, resulting in a loss of mass (losing components). Avoid contact with water.

Ettringite, ASR, and aggregates

Ettringite forms when the material is already solidified. ASR (alkali-silica reaction) can occur with silica being crystalline or amorphous. Wrong type of working aggregate is reactive. The crystalline condition is more chemically stable while the amorphous is reactive. Concrete is environmentally based technology. Aggregates are simply taken from the place.

Opal silica is amorphous, leading to chemical degradation attack related to pH (acid material in an alkaline environment). Swelling of silica results in a lot of internal stresses and cracking. Random cracks indicate ASR. Different kinds of cracks indicate different reactions.

Pop out: release a portion of concrete (like a popcorn piece). Low-alkaline concrete exists. Blended cements are used not for porosity but for pH. A pozzolanic mixture consumes part of the alkalinity, reducing pH to 12.5. This results in a less aggressive reaction. Higher temperature increases reactivity.

Carbonation is a chemical modification of concrete, not harmful towards concrete but creating problems with rebars.

Leaching, ASR, and carbonation issues

Leaching out occurs when rainwater partially dissolves the material. Some acid attack happens here because this is via Mancinelli, and in Milan, we have slightly acidic rains (pH = 5). Pop out is related to ASR (also web of cracks), following the profile of rebars.

Corrosion of reinforcements

Carbonation takes place on concrete, leading to the corrosion of rebars. Chlorides enter concrete, causing fluorescence (not a hazard) due to water evaporation and salts deposition.

Stray current induces rebars corrosion. No rebars mean no current conduction, thus no hazard to concrete itself. Concrete has a pores solution that is alkaline, keeping rebars passive. Generally, CS rebars are used. The pH of 13 (Portland cement) or less (blended cement) creates CSH plus portlandite (calcium carbonate), the latter reacting in part with the pozzolanic blend.

The lower the pH, the weaker the passive film on rebars (min pH = 9). The cathodic reactant is oxygen. Concrete is an oxygen shield itself. If concrete is saturated, there is no corrosion. Alkaline environments give negligible corrosion rates (CR).

Causes of depassivation

• Concrete is no longer alkaline leading to carbonation (general corrosion).
• Depassivation by chlorides causes local removal of film (localized corrosion).
• Stray currents cause localized corrosion. Beware of macrocouples in rebars, causing hydrogen embrittlement.

Tuutti Diagram: penetration of corrosion as a function of time shows an interval of time with no corrosion where rebars are passive. Initiation marks the activation of rebars, at a speed proportional to [O₂], water, chlorides, etc. The max acceptable penetration is the limit state. Some corrosion is allowed, but the structure must still be safe and perform well. The limit state is only valid for carbonation-induced corrosion because it is easy to predict and control. For Cl-induced corrosion, service life ends when initiation happens because it is autocatalytic, and we cannot take the risk.

Max acceptable penetration is 50-100 micrometers, after which we need to repair. This is chosen by the designer. Rust decreases adherence between rebars and concrete, swelling and creating internal tension. Carbonation is generalized corrosion, producing rust that can diffuse to the concrete surface, making it easy to spot. Chlorides-induced corrosion is localized, with less rust, which is better for the stability of concrete.

Time for air to reach the rebar equals initiation time. V_rust is 2-6 times V_steel. Beware of spalling: cracking of concrete due to internal stress. Spalling indicates failure, and it's too late for elimination of a part of the concrete cover. Chlorides-induced corrosion may even happen in urban environments due to anti-freeze salts.

In pristine concrete, rebars are passive, stopping the anodic process. If the environment changes, corrosion occurs anyway, stopping the cathodic process by saturating concrete or repassivating rebars. A completely dry environment means no corrosion. CR in concrete is negligible if less than 1 micrometer/year.

Carbonation is slower with a max speed of 100 micrometers per year because oxygen cannot be made to go faster than that. In Cl-induced corrosion, a big cathode and small anode results in a very fast CR. Evans Diagram shows that if rebars are depassivated, H evolution may cause corrosion. If material is active, the speed is proportional to RH. RH less than 95% causes an increase in ohmic drop until CR is negligible; RH greater than 95% results in negligible ohmic drop with max CR.

Carbonation process

CO₂ plus Ca(OH)₂ results in CaCO₃ plus H₂O when water or NaOH is present. pH drops to neutral, making rebars active. This process can take years, but once all calcium hydroxide is converted and attached to rebars, they have been in contact with a CaCO₃ solution and activate. It does not happen in a dry environment (RH less than 40%). A lot of CO₂ and some unsaturated space is needed for CO₂ flow.

Phe^nophtalein test: dye changes color to pink if there is an alkaline solution. Concrete next to the rebars needs to be analyzed by extracting a core and spraying its lateral surface with phe^nophtalein. The quick test takes 1 day to avoid carbonation to air. The line that separates pink from other colors indicates carbonation depth.

Carbonation starts faster because air has a lot of CO₂, but it slows as CO₂ has to diffuse. Carbonation depth increases relatively fast in the first year (some cm in 10 years). Good concrete has a carbonation coefficient (proportional to t^1/2) of 10-15. The diagram of carbonation depth vs time shows that errors in construction are very dangerous. If rebars aren't kept in the same position, and the cover ends up 20 instead of 40, service life will be reduced by a quarter because it goes with the square root.

Carbonation depth is the carbonation coefficient times the square root of time. Cracks are highways for the entrance of gases. Speed varies all the time due to many factors. Formworks tend to be removed early, impacting mechanical resistance and porosity. Extending curing time can reduce carbonation depth by 100%.

Carbonation depends on RH: negligible if less than 40% (no water) and greater than 95% (no air). The max peak is at 70-80%. Microclimate is important: position and exposure to some agents change carbonation, e.g. the top of a balcony gets a lot of rain = no carbonation, but the bottom of the balcony never gets hit by rain = high carbonation depth.

Indoor and areas shielded by rain are most subjected. Corrosion happens when RH is very high. Carbonation and corrosion are mutually exclusive. We need carbonation and cycles when the structure is wet. Corners are the most dangerous points because rain follows them, and rust pushes in all directions.

Cl-induced corrosion

Water brings chlorides inside the material, where the cathodic reaction is oxygen reduction. A certain amount of chlorides is needed for rebars to depassivate. Localized corrosion inside concrete occurs because it is an alkaline material that can bind chlorides to itself: a reaction of a part of chlorides with aluminates. This acts as a safety coefficient because understanding how many chlorides are dangerous isn't necessary. All chlorides are considered free to move and dangerous in the worst-case scenario. Some time is needed for water to be absorbed. The first layer of concrete is in equilibrium with the environment, meaning fewer chlorides near the rebars.

The critical parameter is the concentration of chlorides at the rebars! With carbonation, fewer chlorides are necessary to initiate pitting corrosion. Generally, pitting starts at 0.4% of chlorides by weight of cement, but it is stochastic and can start with 1%. Below 0.4%, we are reasonably sure that no corrosion occurs. Above 1%, corrosion is almost certain. Between 0.4% and 1%, pitting can start, so the lowest value is needed for safety. The threshold value is 0.4%.

Porosity, alkalinity, thickness of the cover, and other factors influence propagation time, which is usually under 1 mm/year. Propagation time is rarely considered. Higher pH results in a higher threshold. The speed of reaching the critical chloride threshold is determined by RH, with higher humidity leading to higher diffusion. Structures on the beach may have issues due to concrete with microvoids.