COMPOSITES IN CONSTRUCTION
Composite is a mixture of two or more materials that differ in form, chemical composition and essentially insoluble.
There are two categories of constituent materials: matrix and reinforcement. The reinforcement imparts its special mechanical and physical properties to enhance the matrix properties. Composite produces properties unavailable from the individual constituent materials.
Due to the wide variety of matrix and reinforcement materials available, the design potentials are incredible.
The most primitive composite materials comprised straw and mud in the form of bricks for building construction.
Composites are widely used in aerospace industry.
The most visible applications are our roadways of concrete i.e. a composite of sand, cement (matrix), gravel, water, steel bars (fibers, reinforcement).
There are the so-called natural composites like bone and wood.
At atomic scale, composites are usually:
Metal alloys, polymers, Al-Cu alloys, Al-Si alloys, Al-Zn alloys and iron alloys.
Iron accommodating 0.2% carbon is a composite of ferrite and pearlite.
GFRP: Glass Fiber Reinforce Plastics (used in the manufacturing of boats, automotive cylinders)
GFRPMC: Glass Fiber Reinforce Polymeric Matrix Composites.
These are structural composites.
There two types of composites:
Most common matrix composites are:
MMC: Metal Matrix Composite.
PMC: Polymer Matrix Composite.
CMC: Ceramic Matrix Composite.
Metal matrix composites are usually made up of aluminium to give it enough strength as it is less dense than iron so used in aerospace.
Reinforcement composites are usually fibers.
Fibers are usually strengthy, low density, resistant to heat, corrosion, electricity, cheap, easy to handle and easily available.
These are mainly continuous fibers and made by drawing or extrusion method. They are structural composites. Glass fiber is cheaper than others but has high density. It is widely used. For example: CNG cylinders and bus bodies.
They have greater tensile strength and less compressive strength.
There are two types of glass fibers:
S-type: they have high strength. Example: Al2O3 (50-45%), SiO2 (10-15%), MgO (10%). Here Al2O3 and MgO are expensive. It is 650kps.
E-type: These are electrical structural composites usually steel bars. Example: lime-aluminum borosilicate glass (CaO, Al2O3, B2O3, SiO2). E-type is cheaper than S-type because of SiO2. It is 500kps. (103 pounds = 1 kips)
(B)-Carbon & Aramid fibers:-
Carbon and aramid are both expensive and of low density. They are used mainly in aerospace industry.
Carbon fibers are of two types: PAN (PolyAcryloNitrile) and Pitch. Pan is of high quality. Pitch is almost pure form of carbon i.e. percentage of carbon is a little bit less in it than PAN.
Carbon/Aramid Reinforcement Plastic is used in aerospace or military applications.
Carbon fiber production:-
Production process consists of three steps:
In this step PAN is stretched and oxidized at 200-2200 C under tension.
Since the amount of Nitrogen, Oxygen and Hydrogen is increased at this stage so they react with carbon at 1000-15000C and then removed to increase carbon percentage. In this step PAN attains stiffness (i.e. ability to resist deflection), high strength and low density.
Now the temperature is increased to 18000C and carbon comes out in the form of graphite flakes. Strength of Pan is decreased and its stiffness is increased.
Aramids are aromatic polyamides and used in aerospace as well as bullet proof jackets because it has greater impact strength.
Plastics are C-C bonded, long chained molecules. These are organic polymers.
High compressive strength
Low melting point
· Concrete, metal and wood are replaced by composites now days.
Hand lay-up process:-
· Composites are made by this simplest method.
· Reinforcement phase: fibers in woven form or chopped fibers held together loosely by a binder. Unidirectional fabric or biaxial fabric is also used as a reinforcement phase.
· By the help of roller or spray matrix i.e. epoxy resin is painted on the reinforcement fiber.
· Mould used in composite production is chemical resistant and also a composite.
· Some catalyst or hardener is added in resin and color is also added in the form of pigments.
composites in construction
Filament winding method:-
This method is used for tabular structures.
It consists of winding continuous roving of fiber onto a rotating mandrel in predetermined patterns after passing fiber through resin bath.
After several layers are wound, the component is cured and removed from the mandrel.
composites in construction
· In unidirectional fibers, strength is greater. In transverse direction it will be less.
· In chopped form (random direction), overall strength will be same but less than that in unidirectional fiber.
Concrete is an example of ceramic composite which consists of granular aggregate embedded in Portland cement.
In general concrete is a mixture of mineral aggregate, water and cement forming a workable mass.
Aggregate consists of sand (fine particles: diameter less than 6mm) and gravel (coarse particles: diameter more than 1.8mm).
Overall production of concrete is more than steel.
It has 10 times more compressive strength than tensile strength.
Its density is much higher than structural composites.
It is economically available material.
It is resistant to heat.
It’s easy to fabricate on required site.
It is easy to cast.
Matrix in concrete:-
Cement acts as a matrix phase in it as well as a binder to hold granular aggregate together.
Raw material for cement:-
These raw materials are crushed, ground and then heated at 14000C to 16500C in rotatory kiln.
Functions of cement raw materials:-
The only chemical reaction in between water and cement is “Reaction of Hydration”. Heat of hydration of 3CaOSiO2 is greater than others, so it is the early strength giver to concrete.
It gives strength to concrete after 1 week of setting in specific environment.
3CaOAl2O3 & 4CaOAl2O3Fe2O3:-
These are used in sulphate environment. Their M.P is high, so they lower the temperature of cement in rotatory kiln.
Grades of cement:-
There are 5 grades of cement for different applications obtained by adjusting the percentages of raw materials.
Ordinary strength cement
Moderate strength cement
High strength cement
Sulphate environment cement
· Concrete gains maximum strength in 90 days. Its rate of setting is much lower.
The mineral aggregate is taken of proper size and a paste of cement and water is added in it. The chemical reaction between cement and water is simple hydration while there is no chemical reaction between cement and gravel. The paste of cement and water just binds the mineral aggregate strongly. After setting and drying, the mass structure becomes very strong.
Overlapping of diameters of particles of mineral aggregate results in overlapping of grain size.
Cement Sand Gravel
1 2 4
7 to 15 % 15 to 31 %
Here cement is the most expensive one while gravel is less expensive than cement. Sand is the cheapest one among them all.
Air is also present: 0.5 to 8 %.
Air entrapped concrete is also used for special purposes. It accommodates air 4 to 8 %. Its strength is lower (less impact strength) and freezing point is higher. Control freezing acts at 0K to -273oC.
It is made by casting concrete on steel rods. Since the surface of steel bars is usually rough so concrete sets on it with good mechanical strength.
Compressive strength of concrete is 55000 psi while tensile strength of steel is 40000 to 80000 psi so these two materials give much better properties when combined to form a composite.
When steel bars are put in tension, properties become much better. Thos property is called work hardening.
It is a natural composite and is composed of cellulose chain. These longitudinal chains are bound together by amorphous lignin. Wood has fibrous structure and hence has greater strength in longitudinal direction than in the transverse.
Soft wood: It is obtained from ever green trees.
Hard wood: It is obtained from seasonal trees.
Fiber direction of wood can not be controlled.
· Annual production of wood is greater than steel and concrete.
· A composite having same properties in different directions has isotropic properties and the other having different properties in different directions has anisotropic properties.
· The composites which are easiest to make are Fiber Reinforced Plastics and in them the easiest one is GFRP i.e. Glass Fiber Reinforced Plastic.
Ceramic Matrix Composite:-
These are materials consisting of a ceramic or carbon fiber surrounded by a ceramic matrix; usually SiC (silicon carbide).
Slip casting is perhaps the best known of the ceramic matrix composite.
First we start out with a slip, which is composed of ceramic particles (usually clay) dispersed in water.
We then pour the slip into a clay or plaster of Paris mold of the desired shape.
The porous mold sucks up the water in the slip through capillary action, thus depositing the slip on the walls of the mold.
Depending on how long one casts, one can create solid or hollow objects.
The resultant body is hard and dry to the touch but is very brittle and bears little or no resemblance to the finished article but it is now ready to be transformed in the kiln.
Metal Matrix Composite:-
“It is a material in which continuous carbon, silicon carbide, or ceramic fibers are embedded in a metallic matrix material.”
Most common MMC’s are Aluminium Matrix Composites.
In general, the major advantages of Aluminium Matrix Composites (AMC’s) compared to unreinforced materials, such as steel and other common metals, are as follows:
Increased specific strength
Increased specific stiffness
Increased elevated temperature strength
Improved wear resistance
Good corrosion resistance
MMC’s are usually made by powder metallurgy or casting.
Reinforcements in MMC’s & CMC’s:-
Reinforcements in MMC’s and CMC’s are usually in particulate form and are boron (B), Silicon Carbide (SiC) and Aluminium Nitride (AlN). These are continuous fibers. The diameters of these fibers range from 2 to 200 µm.
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