my first post ( just sharing my knowledge )
Aggregates occupy ¾ space in the concrete mass and have following functions
( a ) provide a filler for concreting material
( b) provides mass to the concrete and reduce volume changes while setting and hardening of concrete.
( c ) the rest ¼ is occupied by the paste ( water + cement ) binder which is an active component of concrete and having following functions:
( a) it fills the voids between aggregates
( b ) it gives strength to hardened concrete
The Reinforced Concrete ( R.C.C.) , the steel and concrete forms a very good combination because the co-eff of expansion of concrete and steel are nearly equal.
Concrete has low tensile strength & hence cracks easily .Therefore concrete is to be reinforced with mild steel bars, high tensile steel bars or mesh .
Concrete expands and contracts with change in temp.
Constituents of cement
Lime ( Ca O ) calcium oxide 60 – 67 %
Silica ( Si O2) Silicon Oxide 17 – 25 %
Alumina (Al2 O3 ) 3 – 8 %
Iron Oxide ( Fe2 O3) Ferric Oxide 0.5 – 6 %
Magnesia ( Mg O) 0.1 – 4 %
Soda or Potash ( Na2 O + K2O ) 0.5 – 1.3 %
Sulphate Trioxide (SO3) 1 – 3 %
The initial set is a stage in the process of hardening after which any cracks that may appear will not reunite. Every cement is supposed to retain its plasticity for at least 30 min from the time water is mixed to it.
The quantity of cement for its strength in compression is judged by its compressive strength of cement sand mortar
For this purpose 1 part of cement and 3 parts of standard graded sand is used for preparation of mortar.
Eg
A container of 10 lt capacity has been filled with aggregate the water require to fill the voids of aggregates is 3.6 lt find % age of voids ?
Vol of aggregate ( aggregate + voids ) = 10 lt
Vol of water reqired = 3.6 lt
Vol of voids = Vol of water = 3.6 lt
Percentage of voids = 3.6/10 x 100 = 36 % voids
Total water reqired for a constant w/c ratio to be added will depend upon the type of moisture content and calculated as under
( A ) Dry aggregates
Total water added = effective water + water absorbed in 24 hrs
( B) In air dry
Total water added = effective water + water absorbed in 10 to 15 min
( C ) In Saturated Surface Dry
Total water added = effective water
( D) In moist aggregate
Total water added = effective water – water available from surface
Note : Effective water is calculated from designed w/c ratio
An aggregate mass occuping a gross volume of 3 lts and weight 5.2 kg Its specific gravity is 2.6 then find bulk density & percentage of voids
Gross volume of aggregate = 3 lts
Bulk Density = 5.2/3 = 1.73kg/lt
Absolute volume of agg particles = wt/sp. gr
= 5.2/2.6 = 2 lts
% age of voids = gross vol - absolute vol / gross volume x 100
= (3-2)/3 x 100
= 33.3%
Concreting should not be done in water below temperature 5 oC
While concreting the temp of concrete should not be less than 16oC
And climate temp should be not more than 40 oC
Some important conclusions while designing the mix design of concrete
As the coarser material is on higher side the slump is more
As the sand content is lower side the slump is more and vice versa
The more is rounded particles in aggregate the workability is more and slump is higher side
If the fineness modulus of cement is more it will consume more water
As the finer particles increases the water consumption increases
Always perform the actual moisture content of sand while appling the correction factor of sand on plant
The incorrect moisture content can decrease the quantity of sand in concrete hence the
volume of concrete can be decreased as it will disturb the water in concrete as results the
quantity of concrete will effected
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