Concrete

The main constituents of concrete are cement, sand, aggregate&water.As the time changed and due to demand of higher grade, the above constituents were mixed in different proportions with addition of admixtures to achive the required strength, there are various grades of concrete eg.M5, M10,M15…etc Strength, durabilityand workability may be considered as the main properties of concrete, in addidtion good concrete should be able to resist wear and corrosion and it should be water tight, compact and economical.

Types of Concrete

1. Light  weight Concrete
2. Aerated concrete
3. No-fines Concrete

Precautions while laying

1. Concrete cover for steel bar is much necessary to protect the steel against corrosion (rusting) and to provide resistance against fire. 
2. If fire occurs in the building,  the steel might lose so much strength to impair its reinforcing effect and the beams might collapse. 
3. For R.C.C. Slab, the cover should be 15 mm. 
4. For RCC column the cover should be 25 mm to 40 mm depending on the size of steel bar and on thickness of the column also. 
5. In case of footing of column or walls, the cover shall be 50 mm to 75 mm . 
6. In case of beams the cover should be 25 mm to 40 mm (1 to 1.5) depending on the size of  steel bars.
7. When the numbers of steel bars are more than spacer between the bars  must be used.
8. To keep crank bar in position, support of required height may be used under crank bar in the RCC slabs. 

Effects on Quality of concrete due to

Water cement ratio

1. Creep increases with increase in W/C ratio
2. High W/C ratio causes plastic shrinkage
3. High W/C ratio leads to higher permeability which inturn affects the durability of concrete

Sand Quality

1. If sand is measured by volume and no allowance is made for bulking, the mix will be richer than that speceified, because for given mass, moist sand occupies a considerable larger volume than the same mass of the dry sand .this results in mix deficient in sand increasing the chance of segregation and honeycombing of the concrete.
2. If no allowance is made for bulking the concrete strength may vary as much as 25%.

Technical Details of Concrete

1. Wet Density of Concrete = 2420 kg/Cum
2. Conductivity=1.4 to 3.4 j/sm^2 degree C/m        
3. Diffusivity= 0.002 to .006 Sqm/Hr
4. Specific heat = 840 to 1170 j/kg per degree C
5. Poisson's ratio= Varies from 0.15 to 0.20
6. 26% of the 20 Yrs creep occurs in 2 week, 55% of 20 Yrs creep occurs in 3 months and 76% of 20Yrs creep occurs in one year
7. The rate of shrinkage decreases rapidly with time it is observed that 14-34 % of the 20 Yrs shrinkage occurs in 2 week, 40-80% of the 20 Yrs shrinkage occurs in 3 months and 66-85% of the 20 Yrs shrinkage occurs in one year.
8. Tensile Strength = 3.5 N/mm^2
9. One or more Expansion joint to be provided for building longer than 45 mts.
10. Contraction joint 4.5 to 5 mts intervals in unreinforced slabs and up to 15 mts in reinforced slabs.
11. Fire resistance = 300 C
12. Co efficient of thermal Expansion= Varies from 0.7 to 1.2 X10^-5

Sl No	Type of Aggregate	Coefficient of expansion
1	Quartzite	1.2 -1.3 X10^-5
2	Sandstone	0.9-1.2 X10^-5
3	Granite	0.7-0.95 X10^-5
4	Basalt	0.8-0.95 X10^-5
5	Lime Stone	0.6-0.9 X 10^-5

13. Mix proportions.

Sl No	Grade	Approx proportion of Components
1	M10	1 : 3 : 6
2	M15	1 : 2 : 3
3	M20	1 : 1.5 : 3
4	M25	1 :  1 : 2
5	Other grades	Mix design

Mix Proportions

This below mentioned proportion are just approximate quantities for 1 cum of concrete.

Sl No	Grade	Cement	Sand	Jelly		Water
				12.5mm	20 MM
1	M-20	320 kg	804 kg	435 kg	624 kg	173 ltr
2	M-25	340 Kg	782 kg	452 kg	621 kg	170 ltr
3	M-30	375 kg	735 kg	445 kg	649 kg	169 ltr
4	M-35	400 Kg - 425 kg	655 kg	570 kg	570 kg	160 ltr

Lab Test

Slump Cone test

Aim:To determine the workability of concrete by slump test.

Apparatus:Slump Cone, Tamping Rod, scale etc.

Theory:Unsupported fresh concrete, flows to the sides and a sinking in height takes place .The vertical settlement is known as slump. Slump increases as water content is increased. Slump is a measure indicating the consistency or workability of cement concrete .A concrete is said to be workable if it can easily mixed and placed, compacted and finished .A workable should not show any segregation or bleeding.

Procedure:

1. Prepare 1:2:4 Concrete mix using different water cement ratio.
2. Place the mixed concrete in the slump cone in 3 layers each, each layer being tampered 25 strokes using Std tampered rod.
3. Level the top surface with trowel.
4. Remove the mould immediately, raising it slowly and carefully in the vertical direction.
5. Measure the subsidence in concrete cone which gives the value of slump.

Observation:

Sl No	Description	Remarks
1	Height of the cone h1	30 Cms
2	Final reading h2=(23-21)/2	22 Cms
3	Slump cone value=h1-h2=30-22	8 Cms
4	Water -cement ratio	0.7
5	Concrete mix proportion	1 : 2 : 4
6	Water Added=0.7XWt of cement=0.7x6000 gms	4200 cc

Results:

The value of slump for water cement ratio 0.7= 8 Cm

Note:

The recommended slump values for concrete for various jobs.

Sl No	Name of the works	Slump in MM	Water-Cement ratio
1	Concrete for roads and mass concrete.	25 to 50	0.7
2	Concrete for R.C.C, beams and Slabs.	50 to 100	0.55
3	Column and retaining wall	75 to 125	0.45
4	Mass concrete in foundation	25 to 50	0.7

Compaction Factor Test

Aim:To determine the compaction factor of a workability of concrete.

Apparatus:Compaction factor apparatus measurement jar, travellor, scale.

Theory:It is based on the definition, that workability is that property of the concrete which determines the amount of work required to produce full compaction.

Procedure:

1. Keep the apparatus on a level surface, oil it properly.
2. Weigh the empty cylinder say W1 gms
3. Prepare a concrete mix of proportion 1:2:4.
4. Fill the fresh mixed concrete into the upper hopper gently and carefully with out compaction.
5. Release the clamp the concrete falls into the second hopper, release the lid of second hopper and allow the concrete into the bottom cylinder of weight Say W2 gms.
6. Trim the surface and weigh the cylinder.
7. Weigh the cylinder after compacting manually in 3 layers say W3 gms
8. Determine the compaction factor by using the ratio of partially compacted to the fully compacted concrete.
9. Water Cement ratio=0.7

Observation:

Sl No	Description	Remarks
1	Concrete mix proportion	1 : 2 : 4
2	Weight of cylinder mould   W1	11.5 Kgs
3	Weight of cylinder mould + concrete at loose state  W2	22.70 Kgs
4	Weight concrete at loose state W2-W1=22.70 - 11.5	11.2 Kgs
5	Weight of  mould + concrete at dense state  W3	24.15 Kgs
6	Weight concrete at dense state W3-W1=24.15 - 11.5	12.65 Kgs

Calculation:

Compaction Factor= (W2-W1)/ (W3-W1)=11.2/12.65 = 0.88<1

Results:

The compaction factor for W/C ratio of 0.6 is …….Kg

The compaction factor for W/C ration of 0.7 is 0.88 Kg

Conclusion:-

As per IS Code 0.5 for low workability & 0.95 better workability.

Vee-bee Consistometer Test

Aim:To determine the workability of given concrete mix.

Apparatus:Vibrating table, Std iron rod, travellor, scale.

Theory:-The workability of fresh concrete is a composite property which includes the diverse requirement of stability, mobility, compactibilty, and placeabilty and finish ability. This test gives an indication of mobility and to some extent of compactibility of freshly mixed concrete.   The test measure the relative effort required to change a mass of concrete from one definite shape to another (i, e Conical to Cylindrical) by means of vibration. The results of this test are of values in studying the mobility of the masses of concrete made with varying amount of water, cement and with various type of grading of aggregate.

Procedure:

1. In this test, first the slump cone is formed as in slump cone test.
2. The glass plate is then brought into the position so that it is stretching the top of the concrete.
3. The sub-sided concrete is then subjected to vibration action by starting the vibrator and simultaneously the stop watch.
4. Allow the concrete to re mould by spreading out in the cylinder the vibrations are continued until the concrete is complete ly remolded I, e the surface becomes horizontal.
5. This can be judge by observing through the glass plate immediately when concrete assumes the cylindrical shape the stop watch is stopped.
6. The time required for the shape of the concrete to change from slump test cone shape to cylindercal shape of the concrete in second is known as VEE-Bee-Degree.

Observation:

Sl No	Description	Remarks
1	Concrete mix proportion	1 : 2 : 4
2	Water Cement ratio	0.65
3	Initial reading on graduated scale    a
4	Final reading on graduated scale    b
5	Slump value  b-a
6	Vee -bee seconds	11 sec
	Slump cone                                                         Top dia=d1	10 Cm
8	Bottom dia=d2	20Cm
9	Height=h	30 Cm

Results:

Vee-bee degree of Concrete mix= 11 Sec

Conclusion:

For concrete of slump in excess of 50 mm, the remolding is so quick that the time cannot be measured.