Materiali da costruzione avanzati - Parte 1

Advanced construction materials aims

Advanced construction materials aims the of level advanced an at understanding detailing providing composition, applications, microstructure, engineering structures the of rehabilitation to relation in materials construction various using how assess to able be to durability, building of serviceability, safety.

Composition and structure needs use property load external an to responds material how. Divided into:

• Mechanical.
• Physical.
• Chemical.

And traditional materials advanced consideration, environmental cost, workability... used be to materials.

Analysis and structure of materials

The macrostructure, the considered is it generally analysis the for macrostructure; the need we when only considered is materials the of microstructure a have we why reason the to related comprehension the and information microstructure materials specific a of behavior specific structure atomic/molecular.

Bond. Metal called boundary, specific the is there conductivity, electrical high very a have metals conductivity. High very a have we why reason the is this free, is that electron have we band special have we if freely material the in flow can that and free are that electron of presence the on depends conductivity.

The materials generally are classified in 4 categories.

• Metals.
• Polymers.
• Ceramics.
• Composites.

The composites materials are the combination of two or more materials, two metals, metal + ceramic ... ↳

• Reinforced Concrete -> Use ceramic material, concrete, with the steel bar.
• Aluminum that contain ceramic material -> To increase the resistance to wiring, because aluminum has a very slow resistance to wiring.

Main characteristics

Density

Metals have high density. Density of steel is 7500-8000 kg/m². Polymers, wood and composites materials generally have low density than metals. The density of wood is closed to the density of water 1 g/cm² or 1000 kg/m², generally used in the construction field.

Foam LD is an insulator used in the construction -> for a good insulation we need a very low density. For composites the density depends on the density of materials that composed.

Temperature

Also the temperature in which we can use the materials is different. Ceramic materials have a good resistance to high temperature.

Metals have a resistance to the temperature that depend on the metal, for steel we have to considered that there is a limit at 500 degrees, in which there is a very strong decrease in the mechanical properties.

This is the problem of steel when it is used in the construction, if we have problem related to fire, there is the collapse of structure, 500 degree is a temperature not very high.

To solve the collapse problem of steel we can use SSTEEL that has a more resistance to fire 800 degree.

Polymers have very low resistance to temperates but there are some polymers that can have problem also with environmental temperature.

Mechanical properties

We have to considered the resistance to tensile stress. Ceramic materials have a very low resistance to this load --> concrete has a very poor tensile stress, it has resistance to compression but not to tension.

Metals are isotropic materials, have the same resistance to compression and tension, they have a very high resistance to mechanical load. This is the reason why the steel is so used in the construction. Another advantage to the use of metals is linked to the Young Modulus.

E = 200 GPa.

Elastic Modulus is related to the rigidity of the materials, the capability of the materials to have a low strength and a low misapply.

Composites materials can have high resistance because we can use the fiber to help polymers in their mechanical properties. The logic of composites materials is to use the best properties of the singles materials to composite together.

Electrical resistivity and thermal conductivity

Electrical resistivity and thermal conductivity -> related to the structure of materials.

In metals there is the metallic bonds, very slow electrical resistivity and high thermal conductivity, due to the presence of the atoms. The transmission of heating is only linked to the vibration of atoms.

In polymers the molecules are connected by low energy bonds, in fact is not so easy the transmission of heat among the macromolecules.

Costs

Also costs are important, concrete, for example, is a material with very good mechanical properties and very low cost. Also steel has a low price, due to the fact that there are a lot of iron in the Earth.

For the polymer there are techno polymer that have a very high cost. The cost is related to 1 kg of material, for example 1 kg of polymer is a lot of materials due to the density, that is closed to the density of water.

Materials selection

Materials selection physical, thermal, electrical, acoustical, optical ... properties mechanical chemical, degradation ... economic consideration, environmental consideration, LCA.

All this properties can be evaluated looking the load that is apply to the materials: the thermal properties is the property that we can see when a real load is apply to a material.

Physical properties

Thermal properties

Thermal properties: describe the behavior of materials under thermal load. With a thermal load there is an increase of temperature in the system, and this increase the temperature that is transmitted to the materials and the temperature of this start to increase, in this way the material can transmit the heat.

Inside every material there are atoms that start to vibrate with the increase of temperature.

The increase of temperature has like consequences the increase of vibration of atoms and, at the end, there is the increase of the volume of the material.

There are three characteristics that express the thermal properties of materials:

Specific heat (cp) = amount of energy that is necessary to transfer to, or from, a unit mass of a material, 1 g or 1 kg, to produce a temperature rise of 1°C, P constant; atmospheric P.

Generally: specific heat is lower when density is higher.

Thermal conductivity (k) = characterizes the ability of a material to transfer heat by conduction.

q = - k · (dT/dx).

q = heat flow [W/m²].

dT/dx = temperature gradient [°C/m].

k = thermal conductivity [W/m · °C].

Valid only if dT/dx = constant, steady-state heat flow, - heat flow from hot to cold.

Thermal expansion (α) = indicates the variation of dimensions of a material when a variation of temperatures occurs.

ΔL = Lo · αl · ΔT ΔL= L α ΔΤ0.

ΔL = variation of length [m] ΔΤ/L = α ΔΤ0.

Lo = initial length [m] ε = ΔΤ/L0.

ΔT = variation of temperature [°C] ε = α ΔΤ strength.

αl = linear coeff. of thermal expansion [°C^-1] σ = ε E stress.

αl is lower when higher atomic bonding energy.

Thermal transmittance (U) = measure the heat flow, conduction, convection, radiation, through one square meter of a surface per difference of 1° in temperature across the material or materials.

U = q/ΔT [W/m² · °C].

U = 1/ΣRi, considering conductivity: R = d/k; d = thickness [m]. Is fundamental for the insulation of the building and it is related to the structure of material.

Metals have the metallic bonds, so it is very easy to have a flux of the electron and it is not a problem to transfer the heat. In metals thermal conductivity is high.

Aluminum and copper are generally materials used in the cooling system, they have a very good conduction.

Stainless steel has a very poor conductivity, but it is used for the pot, in fact on the bottom of pots there is a very thick material that give the conductivity, the s-steel is used in this case because it is easy to clean even if it has a terrible conductivity.

Ceramic and plastic materials have a very low conductivity, this is a reason why these materials are used in thermal insulation.

These value are related to the solid conditions, if the materials have the fiber or the foam form, the thermal conductivity decrease. For example the polyester have a thermal conductivity of 0,13, the expanded.

The specific heat also explain the polyester, with a gas inside, have a conductivity of 0,013. Capability of water to manage the external temperature, the specific heat of water is 4000 J/kg C.

Plastic and wood have a very high specific heat. The variation of dimension depends on the coefficient of thermal expansion that is related to the atomic bond energy that we have in the materials.

Water take the external energy to heat the material, different material has different capability of water and they need different time to increase their temperature.

Ceramic materials, with an high energy atomic bond, have a very low value of thermal expansion. Polymers have a very high thermal expansion and very low energy atomic bonds.

Thermal expansion can give a lot of problem, if we do not consider the thermal expansion there are the disasters, for example the joints are very important for the increased of dimension of steel.

Due to the characteristics of the materials, there are materials with high bond energy and materials with low bond energy. For example if we put a polymer with a ceramic or metallic material there are problem related to their differences.

Generally for the protection of steel is used the painting, this can be a problem because the paint has the higher thermal expansion than the steel, the phenomenon that we can observe is the Disboundame.

The same problem is in the windows between glass and frame in PVC.

Electrical properties

Electrical properties: describe the response of materials to an applied electrical field.

Ohm’s law:

ΔV = R I ([V]=[Ω] [A]) First law.

R = ρ (L / A) Second law.

ρ = electrical resistivity [Ω m].

L = distance between two point [m].

A = cross section area [m²].

(ρ is higher when T is higher; also impurities change ρ).

Electrical conductivity: c = 1/ρ [(Ω m)^-1].

The characteristics of materials are linked to the resistance that materials give for the circulation of the current.

R = ρ L/A where ρ is the resistivity, it is the characteristic of the materials. Change the capability of material to have the conduction of the current, resistivity depends also to the temperature.

High resistivity can cause big problem related to the circulation of the current, because there is the dissipation of the current that is in the system.

We apply a different voltage between these two points, v1 and v2.

220 volts is the difference of voltage in buildings.

ΔV = R I.

When we want to have a super conductor, a material with very low resistivity, we can considered the use of a very low temperature liquid, generally it is the liquid hydrogen, -200 degree.

Also the impurities give problem with resistivity, for the cable for electrical applications it can be use the copper that is pure copper, the material with low resistivity is gold, for example in the smartphone there is a small component made by gold.

The metals have very low resistivity because the metallic bonds give an easy flow of electrons, the resistivity of ceramic material is higher that the metallic ones.

Optical properties

Optical properties: describe the interaction of materials with the visible light. The visible light is something that is related to the double behavior, part related to the electromagnetic waves and part related to a transport mass.

If we considered the electromagnetic radiation the visible light have length that goes from 350 ηm to 700 ηm, in this range there are all the colors that the human eyes can see.

The electromagnetic radiation is also cosmic rays, x-rays and ultra violet.

Interaction with sunlight: if sunlight is incident with a material there is the incident radiation that can be reflected or passed through the material.

Surfaces are characterized by the presence of atoms. The angle depends on the characteristics of the materials. The incident angle is equal to the reflected angle of the incident light.

Opaque materials do not have the refraction.

The refraction can cause the collision, in this phenomenon there are vibration of atoms and the consequences production of energy.

Generally the characteristics that is used to analyze the capability of the materials to transmit the solar energy is related to the refracted index.

n = c/v.

c = speed of light in a vacuum, 300.000 km/s.

v = speed of light in the medium.

Transparent materials: light is transmitted, with relatively low reflection and absorption.

Opaque materials: light transmission is not allowed, radiation is reflected and partially absorbed.

Translucent materials: light is transmitted widespread.

If “n” is closed to 1 the material is transparent.

The polymers are transparent because they have the similar structure to the glass —> amorphous material.

The “Lead oxide” is known as crystal, the lead is added to glass to give the brilliance.

Diamond is not transparent, it need the reflection property to reflect the light, it has n = 2.41.

Percentage of transmitted light: the characteristics of the materials are used to move and transport light in the building using the optical fiber.

I = intensity of transmitted light.

T External part has an high reflectivity.

I = intensity of incident light.

0 that permit the reflection of light and R = reflectivity the transportation. This mechanism has α = coeff. of absorption an high optical advantage.

x = thickness.

Acoustic properties

Acoustic properties: describe the interaction of materials with the sound, audible, by the human ear; mechanical wave; produced by the vibration of a source.

Material: generates, absorbs, reflects and makes propagate the sound.

Transmission of the acoustic wave in a medium will depend on its elastic properties.

The acoustics properties is only related to the human capability to hear something, this is due to the structure of the ear and the capacity to receive the vibration of sound.

The sound has a mechanical waves, we need a media, like air or water, to hear something.

In the human the vocal cords make a vibration these are transmitted in the air, the air atoms start to vibrate, in this way the sound is transmitted and receive to a receiver.

The material create the vibration but also start vibrate, this vibration create vibration of atoms that create heat.

The source produce some waves that heat the material, due to the increase of temperature, the material start to vibrate and generates other waves, this can cause disaster.

The materials used in buildings have the goal of reducing refraction, for example the case of plasterboard.

Material: generates, absorbs, reflects and makes propagate the sound.

The human ear is able to perceive sounds with a frequency, number of oscillations produced per second, between about 20 Hz to about 20,000 Hz.

Generally we consider that the perception of sound is related to the pressure that is applied to a surface.

Acoustic intensity level: LI = 10⋅log10 (I/Io) [dB].

I = sound intensity [W/m²]; power supplied from the source.

Io = hearing threshold (10^-12 W/m² at a frequency of 1000 Hz).

Hearing threshold = 0 dB, at a frequency of 1000 Hz.

Pain threshold = 1 W/m², 120 dB; at a frequency of 1000 Hz, below this value human can’t hear anything.

Use the logarithm in order to reduce the problem related to the high number, we considered the acoustic intensity go to value that is 0 dB to 120 dB. hearing threshold = 0 dB; pain threshold = 120 dB.

Sound-absorbing material: a material that tends to mitigate the reflection of sound, sound-absorbing capacity is higher, higher the porosity and lower density and elastic modulus of the material.

Generally they have high porosity, low density and low elastic modulus, the best materials are fiber materials, for example fiber glass is a very good absorbing material.

Sound-insulating material: a material that reduces sound transmission, sound-insulating capacity higher, higher elastic modulus, under resonance frequencies, and higher mass, over resonance frequencies.

Generally we considered the mass, higher is the mass, higher is the insulation. For example lead is an incredible sound insulation, it has also a low elastic modulus.

The resonance is related to the characteristics of the material, the construction materials have a resonance frequencies that is very low, it is not easy to reach the resonance frequency of the material.

If we want to reach the resonance frequency we have to start to increase the vibration inside the materials, if we continuous to increase these there is the fracture of the material.

Mechanical properties

Describe the behavior of materials subjected to an applied load, or forces.

If we use an amount of forces very low there are no problem, if we increase the forces there is the fracture of the materials.

Also the direction of forces are important, if apply the force in a specific direction there are no problems, but if the force has an other direction the material can be fractured.

Depends on: amount of the force and its application, direction, time ... nature of material geometry of the element.

The geometry of the material is also important, for example the diameter, same material but different diameter has different deformation due to the load that is apply.

Stress strain elastic and plastic deformation fracture.

When we apply a stress in same material, initially there is the elastic behavior of the material and the material can come back in the initial shape, when we r