CONCRETING BY VOLUME BATCHING AT SITE ....

HOW  WE   DO   CONCRETE   BY   VOLUME   BATCHING  AT SITE ....

There are two methods of batching of concrete

1) By weigh  batching- In which all the ingredients of concrete like cementitious , coarse, fine aggregate, water and admixture when discharge ( inlined bins to bucket / belt, silos to cement hopper,admixture from tank to admixture jar and water from water tank to water hopper ) is weight by suitable equipment generally known as Load cell is known as weigh batching, which is very accurate method with permissible variation is known as weigh batching.


2) By volume batching - It is a type of batching in which all the material is measure  in volume , generally use in the area where it is impossible to go more manpower and where quantity of concrete is less ,Volume batching is generally done by a farma volume of this farma must be kept equal to volume of one cement bag .


Let start with one example :

we  want to made M15 grade of concrete at site    

We know  ratio of ingredients in M15 concrete is   1:2:4

where   1 is    cement
             2 is    Fine aggregate
             4 is    Coarse aggregate. 




First  of  all we  make a  farma   having  volume same as volume of  one   bag   of   cement


Let we assume  length of  farma  0.30 m, breath of farma also  0.30 m .

Than height of pharma 
                           
                  0.3*0.3*H = 0.035 ( 0.035 is volume of one bag of cement)
                  0.09 H = 0.035
                          H = 0.035 /0.09
                              = 0.38 m

so we got Length breath height of farma  after this we make farma which volume is same as one bag of cement.

As volume of farma same as cement so it is full by one bag of cement

We want to make M 15 grade of concrete by volume batching at site

Than cement quantity for one measurement is  1 bag   (as volume of pharma is 0.035 m3 i.e 35 ltr meaning is if we fill farma  with cement it will be fully filled  by 1 bag of cement)


Fine aggregate is  twice of cement volume  i.e   35*2 = 70 ltr ( i.e two bucket of farma)
corase agrregate is 4th time of cement volume i.e 35*4 = 140 ltr ( i.e Four bucket of farma)


35 ltr+70 ltr +140 ltr   = 245 ltrs

i.e in one measurement we make 245 ltr = 0.245 m3  concrete so we make four more measurement for making 1 m3 of concrete.


we can also make calculation for admixture and water also by measuring jar .

let water cement ratio is  0.60

Than water quantity is = 30 kg i.e 30 ltr for 1 measurement of farma .( specific gravity of water 1)

let admixture dose is 1%   cement weight in kg 50*1/100 = 0.5 kg = 500gm

500gm is divide by  admixture specific gravity( 1.1) for volume batching = 454 ml


so for 1 measurement quantity of material

cement = 0.035 m3= one bag of cement
sand   =70 ltr
coarse aggregate = 140 ltr
water  =  30 ltr

adx  = 454 ml

HYDRATION OF CEMENT....

HYDRATION    OF    CEMENT

Reaction   between  cement compounds  and  water  due to which  different structures ( C-S-H, ca(oH)2, ettrigite,  monosulphate ) are formed in the cement / concrete and  heat is extract from it is known as heat of hydration .This is extothermic reaction.

Here we discuss  about  different  bogues   compouds  and  reaction of  these  products  with water .

In   cement  bogues   comounds  are :

1. C3S   -  Tri calcium silicate
2. C2S   -  Di calcium silicate
3. C3A   -  Tri calcium Alumiate
4. C4AF -  Tetra calcium Alumino ferrite

When water  intracts with cement  it first reacts with Aluminate compound which reaction is give below:

Tricalcium aluminate reacts with water in presense of gypsum and make calcium aluminate trisulphate hydrate which is known as ettrigite and librate heat, ettrigite  which is form above is stable only in solution of gypsum but due to deficiency of gypsum its reacts( ettrigite) reacts with balance C3A ad make calcium alumiate monosulphate hydrate, this monosulpahe is stable only in presence of deficiency of sulphate but monosulphate reacts with sulphate present in water and soil and again convert in to trisulphate hydrate which moleucule size is 2.5 to 3 times more than monosulphate structure due to which cracks are appears in concrete.

After this ferrite reacts with water in presence of gypsum and make calcium aluminate trisulphate hydrate( ettrigite) , aluminium hydroxide and lime in concrete.

Ettrigite futher reacts with ferrite and make garnets in concrete which do not contribute any strength in concrete.

After this water reacts with C3S in cement and make calcium silicate hydrate and  calcium hydroxide and liberate more amount of heat.

After reaction with C3S water reacts with C2S and make same calcium silicate hydrate and calcium hydroxide and librate heat. 

In above reactions C-S-H is only responsible for strength in concrete/cement which is formed in the case of C3S and C2S .

In the case of C3S reaction more amount of calcium hydroxide form in comparison of C2S , calcium hydroxide is harmful for cement which reacts with sulphate from water and soil and make calcium sulphate which cause deterioration of concrete and cause lack of  durability of concrete.

Only one benefit of calcium hydroxide is it maintain PH VALUE of concrete  alkaline and reduce the corrosion of reinforcement.

C3A and C4AF is responsible for initial set of concrete.

C3S is responsible for high early strength in concrete.

C2S is responsible for strength in concrete in later stage.

C-S-H  which is produce during reaction of C2S with water is more than the quantity of C-S-H produce by C3S and also quantity of calcium hydroxide is low in case of C2S . C-S-H which is produce by C2S is good quality than C-S-H produce by C3S  and also density of C-S-H produce by C2S is high.

CRACKS IN CONCRETE DUE TO DRY SHRINKAGE......

CRACKS   IN  CONCRETE......


WHY   CRACKS   DEVELOPS   IN    CONCRETE:  A  building components develops cracks whenever stress in the component  exceeds its strength.

These cracks develops due to following factors: 1) Externally applied load such as dead load, live load, wind load, seismic load  and due to thermal movement ,foundation settlement, moisture changes as well as due to chemical reaction.


CRACKS  ARE  GENERALLY  CLASSIFIED  AS :

1) STRUCTURAL  CRACKS:  Cracks which appears due to faulty design , overloading as well as due to incorrect design which causing danger of life is known as structural cracks.

2) NON   STRUCTURAL  CRACKS: Are develops  due to internally induces  stress in building material is known as non structural cracks.

CLASSIFICATION   OF   CRACKS  BASED   ON  WIDTH :

THIN  CRACKS  : Width of cracks generally less than 1 mm.
MEDIUM CRACKS :  Width of cracks generally between 1 mm to 2 mm.
WIDE CRACKS : Width of cracks more than 2 mm.

CAUSING OF NON STRUCTURAL CRACKS  IN BUILDING...

1) MOISTURE MOVEMENT
2) THERMAL VARIATION
3) ELASTIC DEFORMATION
4) CREEP
5) CHEMICAL REACTION
6) FOUNDATION MOVEMENT
7) VEGETATION

HERE WE CONCENTRATE ONLY ON MOISTURE MOVEMENT ...

As we know that  most of building material having pores in their structure in the form of inter molecular space due to which material expands on absorbing moisture and shrink when dry, this process is cyclic in nature (i.e reversible movement occur) which is depends  upon  increase and decrease in  moisture and causing shrinkage in concrete.

DRY SHRINKAGE    

Shrinkage is generally occurs due to loss of adsorbed water from hydrated cement paste. when concrete is initially exposed to environment their is difference between humidity of concrete and environment due to which first free water in concrete loses from large capillary pores this result is small but in small capillary pores curved menisci are formed due to loss of capillary water and the surface tension of water pull the wall of pores, thus internal negative pressure develops  in the capillary pores this pressure is compressive in nature and due to which shrinkage occur in concrete.

If the concrete is continued dry than in this case volume changes occur in cement paste and causing increasing in attraction force between C-S-H  hydrated products that leads to shinkage.

HOW TO MINIMISE  SHRINKAGE IN CONCRETE

By using shrinkage reducing admixture.

It reduce the surface tension of water with in the pores of concrete which leads to reduction in capillary tension and pulls on the wall of the pores and reduce the magnitude of shrinkage.

By using low water cement ratio.

By increasing aggregate content in concrete.

By reducing fines content in concrete.


SOME   TECHNICAL  TERMS   ARE   GIVEN   BELOW:

Capillary water :  Present in voids larger than 10 nm further classified as:

free water : Removal of which is not cause shrinkage crack and other is water in small capillary pores caused shrinkage cracks.

Adsorbed water : water adsorbed on the surface of hydrated product drying of this responsible for lot of shrinkage in concrete.

Interlayer water: water held in between layer of C-S-H structure made in concrete is known as interlayer concrete.

Bound water : water held between the structure of C-S-H or calcium hydroxide or ettrigite as well as monosulphate is known as bound water.


When cement reacts with water the resultant product that forms has a lower volume about 25.4% of bound water.

Both adsorbed as well as interlayer layer water is known as gel water  which is present in gel pore, size of pores is less than 10 nm.

And size of capillary pore is more than 10 nm.