risposte cementitious and ceramic material engineering

Potential of Corrosion and Destructive Techniques

The potential of corrosion of the rebar can be determined by measuring the polarization resistance, ∆i. However, this value is only significant in the case of generalized corrosion and not for localized corrosion, as it represents an average value.

Destructive techniques for assessing corrosion damage involve core extraction, which entails removing a cylinder of cement with a diameter of about 2-3 cm. This core can then be used for various tests, such as the phenolftalein test to measure carbonation or to measure the penetration of chlorides. In the latter case, the core is divided into slices, which are analyzed in a laboratory. From each slice, an average value of chloride concentration can be obtained and plotted to create a penetration profile.

In order to implement an appropriate prevention method, it is necessary to restore the safety conditions, halt corrosion, and prevent further degradation. After a thorough inspection, diagnosis, and evaluation of future damage evolution, the most suitable type of repair intervention should be chosen from various options, such as replacement or rebuilding of damaged elements.

chloride extraction, coating application, cathodic protection, reinforcement, and so on) depending on the cost, the extension of the damage and the cause of the damage. We can rebuild partially the elements, by replacing the material, reduce the corrosion rate to complete the service life or repassivate the rebars. For sure, the correct intervention is chosen depending on the type of corrosion, i.e. for Cl we must intervene during initiation, while for carbonation induced corrosion we can allow a certain propagation below a threshold. Moreover, also the costs should be considered, as well as the residual service life. Carbonated structure:

1. Remove all concrete that is carbonated and replace these parts with mortar
2. Apply to the surface of the concrete a thick layer of new high alkaline concrete, this allows us to have the diffusion of alkalinity through the carbonated layer gradually, so return to the alkaline conditions.
3. Clean rebars from corrosion products and apply a coating to the

rebars.Chloride structure:

1. Remove all portions of concrete with chlorides content greater than critical threshold and replace these parts with mortar.
2. Clean the rebars from corrosion products even in the case of pits, because if I do not do this step corrosion can continue to proceed, even if the external environment has been refreshed.
3. Apply a coating on the surface of the rebars; if I want to do this I always have to clean the rebars from corrosion products first.

It is mandatory to remove all the concrete because it can contain chlorides that can lead to a new zone of corrosion.

Galvanic anode system can repair only a limited area since concrete is very resistive, while impressed current can repair a larger zone since the voltage can be a lot higher.

Cathodic prevention is applied when the rebas are still passive, it uses a current of 8-20 mA/m in presence of chlorides or 4-8 mA/m in case of carbonation. Cathodic protection is used when the structure is already undergoing corrosion.

it will be used for the entire service life of the structure, with a current density of 1-3 mA/m. Temporary techniques use higher current for realkalization and chlorides removal. In realkalization current density are 0.7-1A/m and time is 7-21 days, while in case of chlorides removal current is higher and time is 1-3 months.

Mortar is cement paste with addition of aggregates with small dimension, between 20 and 40 mm, which allow high quality with respect to standard concrete (lower maximum diameter of aggregates, low W/C ratio, low porosity). Mortar should be tixotropic, so it should have a low viscosity when in motion and a high one when stationary, it should provide good adhesion to concrete and rebars, low shrinkage (add calcium sulphate that expands) to prevent excessive internal stresses. It should have significant mechanical strength, resistance to the penetration of aggressive agents (low w/c) and sufficient thickness to ensure a good cover thickness to the concrete to remain in.

servicelife.

Ceramic materials and glass

Ceramics are inorganic non-metallic solid materials, non-carbon based. A special case is carbon as a solid, in the form of graphite or diamond, that is ceramic even if it contains C. In general, they are metal oxides (M+O) but can also be formed with metal bonded to carbon, nitrogen, or sulfur, forming carbides, nitrides, or sulfides.

They have a crystalline structure submerged in a vitreous phase (amorphous phase). They have poor thermal and electrical conductivity, this is not true for the special case of diamond, and they can behave as superconductors at very low temperatures. They may undergo ionic conduction at very high T (movement of ions instead of electrons). They are brittle except for some composites or for zirconia. The traditional ceramics are cheap, and they are produced in high volume, while fine ceramics are more expensive.

More resistant to compression respect to traction, one order of magnitude.

E = 200GPa

Ceramics have a crystalline structure, with

ionic and covalent bonds: they have lattices with an alternation of positive (+) and negative (–) ions forming a compact crystalline lattice. When we try to move dislocations, it is highly expensive from an energetic point of view because we have to overcome the electrostatic attraction and we cause electrostatic repulsion by putting too near ions with the same charge. From the point of view of free energy, in stable conditions, we have a minimum. On the other hand, if we try to move dislocations, some ions of the same charge will be near to each other and we reach a maximum in energy. The energy required to overcome this maximum in energy is generally larger than the binding energy of the material, so the material breaks before the dislocation movement (brittle failure). Moreover, we have a very little number of dislocations due to very precise crystallinity, differently from metals. Atoms are bound with covalent or ionic bonds, so electrons aren't free to move inside the material, but they.

are strongly bound to the atoms, so the conductivity is very low. With ionic bonds at high temperature we can transport current due to the presence of defects where we have mobility of anions and cations (not electrons). At very low temperature they can become superconductors.

Clay is used to give plasticity because it is very hydrophilic and retains water adding workability. Silica gives refractory properties giving stability at high temperature. Feldspates have low melting temperature (T): by heating them ceramic during the firing, feldspates will melt and create a glassy matrix that keeps together the material and the decomposed clays, filling the voids and reducing the porosity.

Thermal shock occurs when the thermal stresses (due to a quick change in temperature) exceed a critical stress value. In fact, during cooling the material undergoes expansion depending on its thermal expansion coefficient. If porosity is not enough (<12%) to counterbalance this effect, internal stresses arise.

1. inducing rupture. It depends on Young modulus, expansion coefficient, thermal conductivity and fracture toughness of the material.
2. In general good thermal stability is fundamental to avoid thermal shocks.
3. Ceramic materials are in general made of metallic oxides. Traditional ceramics are made of clay, giving workability, silica, giving high melting temperature, and feldspates, which are melted during firing in order to obtain a vitreous phase.
4. Traditional ceramics can't be used in aggressive environments and very high temperatures, due to the many defects present in the crystalline structure.
5. The steps required for their production are:
   1. Raw materials preparation
   2. Mixture
   3. Forming (pressing, extruding or casting)
   4. Drying
   5. Firing (glass formation, sintering)
   6. Finishing (coating and decoration)
6. Advanced ceramics have a highly controlled composition, containing low impurities and low porosity; this is why they require very pure raw materials chemically synthesized (differently from raw materials).

concentrations of alkali metals and have good thermal conductivity. Oxides like Al2O3 have high hardness and wear resistance. Carbides like SiC are brittle and have good performance at high temperatures. The main production steps for advanced ceramics are as follows: 1. Powder synthesis 2. Forming process (pressing, extrusion, casting) 3. Sintering (HUP, HIP) 4. Coating (CVD, plasma spray, sol-gel) Sintering is the process of compacting and forming a solid mass of material by heat or pressure, without melting it to the point of liquefaction. Two particles in contact will form a neck and deform, then by solid state diffusion the material fills the neck region, removing porosity. Sintering is performed below the melting point, exactly at 2/3 T, to maintain the crystalline structure. During firing, the material is heated to temperatures ranging from 900-1400°C. This process creates a vitreous phase that lowers the resistance to temperature, but closes porosity, thanks to the melting of feldspates. In the case of advanced ceramics, there is no glassy phase, only sintering. SiC, nitrides like Si N, and oxides like Al2O3 are commonly used in advanced ceramics due to their specific properties and performance at high temperatures.

presence of alkali,3 4but CR is lower respect to SiC, in presence of cyclic oxidation crystalline SiO 2detaches more easily respect to SiC due to higher difference in the expansioncoefficient.

Oxide compounds for fine ceramic matrixes are alumina (for anti-wear propose)andzirconia (for toughness). They don’t suffer oxidation.

Alumina has the following properties: hardness, thermal stability (up to1650°C), durability (corrosion and oxidation resistance both in acid and alkalineenvironments), insulating properties and biocompatibility. Its cons are limitedthermal shock and resistance to tensile stresses, brittle. It is mostly used as arefractory (bricks for fournaces), for cutting tools due to the good performancesat very high temperature (better than metals) or as an abrasive. It can also beexploited for its optical properties at high levels of purity (99.9%).

Sintering can be done by HUP (high uniaxial pressure) or HIP (hot isostaticpressure). The powder mixed with a binder

curving: The crack is deviated around the particle shape, creating a non-perpendicular front. - Crack bridging: The reinforcement fibers or particles act as bridges, preventing the crack from propagating further. In addition to these mechanisms, the ceramic matrix can also undergo transformation toughening, where a phase transformation occurs in the matrix material, absorbing energy and preventing crack propagation. Overall, the use of ceramic matrix composites allows for improved toughness and resistance, making them suitable for applications in high-stress environments such as aerospace and automotive industries.