13 - 07 - 2020 NOTES
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BDT PRE-TECH
NOTE 1
TERMS INVOLVED IN MEDIUM CARBON STEEL.
Annealing: is a heat-treatment process that is used to change the properties of metal. It also induces greater ductility. The process involves heating the metal to specified temperature /colour and then allowed to cool slowly. The quenching medium is ash.
PHYSICAL PROPERTIES OF ANNEALED METALS
Annealed metals are relatively soft and can be cut and shaped more
easily. They bend easily when pressure is applied.
THE PURPOSE OF ANNEALING
1.To soften the steel and improve machinability.
2.To relieve some internal stresses induced by some previous treatments like rolling and forging.
HARDENING, if steel heated until it grows red and is quenched in clean water immediately, it becomes very hard also brittle. This means it is likely to break or snap if put under great pressure.
PHYSICAL PROPERTIES OF HARDENED
METALS.
1.Makes steel very hard.
2.Metals resist wear, penetration and wear.
NORMALIZING this is a heat - treatment process that the object of relieving internal stresses refining the grain size and improving the mechanical properties. The still is heated to 800-900oC. It is quenched in air.
TEMPERING A heat treatment applied to ferrous products after hardening. The cooling is done at suitable rate, preferably in oil.
PURPOSE OF TEMPERING
It is done to decrease the hardness and increase the toughness to produce the required combination of mechanical properties.
BDT PRE -TECH
NOTE 3.
AGGREGATES
They are materials in particle or granular form. This term therefore refers to crushed stones ,gravels and sand. There are two kinds of aggregates. These are fine aggregate and coarse aggregate.
FINE AGGREGATE (SAND). Sand is referred to as fine aggregate. It is made of small particles, which can pass through a British standard square mesh sieve of 5mm size. Sand can be obtained from several sources. These are :
- River sand (from the bank of the river).
- Pit sand (excavated from ground which is Sandy).
- Sea sand (from the sea shore ).It contains salt and therefore must be washed under rain before use. The process is termed seasoning.
Method :I) washing with fresh water II) dry in an open area.
- Flood truck sand (gathers in pits and gutters).
PROPERTIES OF GOOD SAND
Sand used in building construction should :
a) Be clean or free from impurities.
b) Not to be too fine for block moulding.
c) Be fine for mortar for plastering purposes.
d) It should not stain the skin.
e) It should not be dusty.
USES OF FINE AGGREGATE
Aggregate are mixed with cement and water (mortar) for :
- Pestering/Rendering
- Moulding blocks
- Screeding
- Laying Blocks/Bricks
FUNCTION OF FINE AGGREGATE IN A MORTAR MIX
- It increases the volume of the mixture.
- It reduces shrinkage of the mix(cracks).
- It prevents the sticking of the shovel /spade or trowel.
COARSE AGGREGATES(STONES )
Stones are called coarse aggregates. It is obtained from hard broken rocks into smaller units. The particles retain on a 5mm standard square mesh.
SOURCES OF
STONES
Stones can be obtained from the following places
I. Rocky area(earth)
II. Mountainous areas.
Trips of stones are usually obtained from quarries. A quarry is a place where large rocks are dug out (excavated) and reduced to different sizes for various building purposes. The rocks are either crushed by machines or manually with hammers.
QUALITIES OF GOOD STONES
Good quality stone should :
I. Be very hard.
II. Not be crispy(like biscuits)
III. Be free from silt and impurities.
IV. Be about 10mm to 50mm in size.
USES
- It is use in making concrete.
- It is used as surface gravels in road construction.
- It is used for filing trenches.
TYPES OF STONES
1.SAND STONE:Is obtained after a heavy down-pour.It is not too hard,it can be used for mixing.a) Mass Concrete b) Reinforced Concrete
2.Granite :Is obtained by breaking hard rocks. It is used for a) Road Construction b) Mixing concrete
TYPES OF CONCRETE
Concrete is a mixture of cement ,sand and stones. These are two types of concrete.These are:
a)Mass concrete:It has no reinforcement (steel) rods in it.The mixing ratio is 1:3:6.
b) Reinforced Concrete:This type of concrete has strong steel rods in it. It is used for pillar, beam or columns. The mixing ratio 1:2:4.
