Basic Principle of Concrete
Concrete is defined as in any product or mass made by the use of a cementing medium .The cementing medium is the product of reaction between hydraulic cement and water, which is also known as cement paste. But in this definition would cover a wide range of products , as concrete may be with wide range of products, several types of cement and also containing pozzolonic material ,GGBFS, ploymer fibre etc. But concrete , in concrete technology is defined in a restricted manner to mean a mixture of cement, water, aggregates(fine and coarse) and admixtures.
CONCRETE TECHNOLOGY
Concrete technology is , therefore the study about concrete to understand concrete ,i.e how concrete behaves under load ,under temperature and in humidity changes and under relevant conditions of environmental and industrial exposure and according to ensure the selection of correct ingredients for concrete making, to design and achieve a suitable mix and to obtain a technically sound execution of concrete work.
CEMENT
Raw material mixed.
Temperature – 1200℃ to 1500℃.
Gypsum – 2 to 3 %.
Rotary kiln is used for burning.
Weight of cement = 50 kg
Volume of cement = 0.035 m3
TWO TYPES OF RAW MATERIAL
- Calcareous Material
Lime stone
Shells
Chalk
- Argillaceous Material
Clay
Shale
Clinker is formed at temperature 1500℃ to 1700℃.
Size of clinker = 3 mm to 20 mm.
- ORDINARY PORTLAND CEMENT COMPOSITION
Lime
60% to 67%.
Provide strength & make cement sound.
In excess quantity makes the cement unsound & causes the cement to expand & disintegrate.
- Silica
17% to 25%.
Provide strength due to formation of C3S & C2 S.
In excess quantity causes the cement to set slowly.
- Alumina
3 % to 8%.
Provides quick setting property & lowers the clinkering temperature.
- Iron Oxide
0.5% to 6%.
Provide colour, hardness & strength.
- Magnesium Oxide
0.5% to 4%.
Provide hardness & colour when present in small quantity.
In excess quantity makes the cement unsound.
- Sulphur Trioxide
2.0% to 3.5%.
Makes cement sound when present in small quantity.
In excess quantity makes cement unsound.
- Alkalis
0.3% to 12%.
In excess quantity cause Efflorescence.
BOUGE’S COMPONENT
- C3S ( Tricalcium Silicate)
Hydrates Rapidly.
More Heat of Hydration.
25 – 50% in OPC (50%).
Related to the Early strength (7 days).
Used in Cold area.
- C2S (Diacalcium Silicate)
Hydrates Slowly.
Less Heat of Hydration.
20 – 45% in OPC (32%).
Related to Strength (After 7 days).
Good resistance to Sulphate attack.
- C3A (Tricalcium Aluminate)
More Heat of Hydration.
Flash Set First Compound react with water & get harden.
8 – 12% in OPC (10%).
Related to setting time.
Not related to Strength.
- C4AF (Tetra calcium Alumino Ferrite)
Less Heat of hydration.
8 – 12% in OPC (9%).
Related to Setting time.
Not related to strength.
Resistance to Sulphate attack.
Hydration of cement is continue until 1 year.
Total 38% of water is required for complete Hydration.
Water < 38% , then reduce strength of concrete.
Water > 38% , then concrete become porous.
1 gm cement produce 120 Cal/gm heat during Hydration.
- SETTING TIME OF CEMENT
Initial setting time of cement = 30 min (for OPC & RHC).
Initial setting time of cement = 60 min (for LHC).
Final setting time of cement = 10 Hour (for OPC, RHC & LHC).
REASONS OF FALSE SET
Gypsum
C3S
Alkalis
RHC strength in 3 days = OPC strength in 7 days
QUICK SETTING CEMENT
Reduce Gypsum content.
Add Aluminium Sulphate.
PORTLAND POZZOLANA CEMENT
Pozzolana content = > 10% to < 25%.
Moisture content up to 1% to 2% does not affect the strength of cement.
Cement with 5% moisture is not useful.
FINENESS
90 micron sieve used.
Retained cement = 10% for OPC
5% for RHC
Specific Surface area = 2250 cm2/gm for OPC
3250 cm2/gm for RHC
- CONSISTENCY
For finding Initial setting time, Final setting time & Soundness , Consistency is needed.
Vicat Apparatus is used to measure Consistency.
Plunger – 10 mm Dia., 50mm Length.
Penetration – 33-35 mm.
Water percentage = P% = 25% to 35%.
- SETTING TIME TEST
Water percentage = 0.85 P%.
- COMPRESSIVE STRENGTH TEST (for Cement)
Cement : Sand = 1 : 3 (for cube).
Cube size = 7.06 cm x 7.06 cm x 7.06 cm.
Water percentage = ( 𝑃4 + 30%).
Cube kept at Temp. = 27° + 2℃ ( 90% Humidity).
Compressive Strength for Diff. Grade :-
|
Grade |
3 days |
7 days |
28 days |
|
33 |
16 |
22 |
33 |
|
43 |
23 |
33 |
43 |
|
53 |
27 |
37 |
53 |
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3 days (N/mm2) |
7days ( N/mm2) |
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Tensile Strength |
2 |
2.5 |
|
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Compressive Strength |
11.5 |
17.5 |
|
- SOUNDNESS TEST
Unsoundness is more because of excess lime, magnesia & sulphate.
To reduce unsoundness, Magnesia content should not exceed 6% & Alkalis content should not exceed 0.6%.
Le-chatelier is used measure soundness.
Water percentage = 0.78 P%.
For soundness, Expansion of cement should be ≤ 10 mm.
For unsoundness, Expansion of cement should be > 10mm.
- AGGREGATE
75% of volume of total concrete.
Types of Aggregate
According to Weight
- Light Weight Aggregate
Bulk Density = 1200 kg/m3.
Used as Non-load bearing wall, Thermal insulation & fire resistance.
Compressive strength = 0.5 to 35 N/mm2.
Tensile strength = 15 to 20% of compressive strength.
- 2. Normal Weight Aggregate
Specific Gravity = 2.5 to 2.7.
Bulk Density = 2300 kg/m3 to 2600 kg/m3.
- Heavy Weight Aggregate
Specific Gravity = 2.8 to 2.9.
Bulk Density = 5000 kg/m3.
Used in nuclear reactor.
According to Size
- Fine Aggregate
Size of agg. is ≤ 4.75 mm.
More Bulking.
- Coarse Aggregate
Size of agg. is > 4.75 mm.
Less Bulking.
According to Shape
- Rounded
Less cement needed.
Workability more.
Porosity = 32% to 33%.
- Irregular
Porosity = 35% to 38%.
- Angular
Porosity = 38% to 40%.
Good Strength.
- Flaky
Least dimension is < 3/5 (0.6).
Give Less Workability.
Durability is less.
Percentage of Flaky in concrete is not more than 15%.
- Elongated
Least dimension is > 9/5 (1.8).
Give Less Workability.
Durability is less.
Percentage of Elongated in concrete is not more than 15%.
- PLUM CONCRETE
Aggregate size is 160 mm or more is used.
This concrete is used in Maharastra Koyna Dam.
In Normal Concrete Max. Size aggregate used is 80 mm.
Size of Silt = 0.002 to 0.06 mm
Size of Clay = < 0.002 mm
- AGGREGATE CRUSHING VALUE TEST
Cylindrical Measure = 11.5 cm Dia., 18 cm Height.
Mould = 15.2 cm Dia., 13 cm Height.
16 mm Dia. & 60 mm Length rod is used for 25 stroke.
Crushing value = 30% for road work & 45% for Building.
- AGGREGATE IMPACT VALUE TEST
Weight of Hammer used for blow is 13.5 kg to 14 kg.
380 mm height for falling hammer.
15 Blows.
Impact value = 30% for road work & 45% for Building.
- AGGREGATE ABRASION VALUE TEST
Temperature of Drum = 100℃ to 105℃.
Size of Drum = 70 cm Dia. & 50 cm Length.
Rotating speed of Drum = 30 to 33 r.p.m.
Num of Rotation = 500 rotation.
Abrasion value = 30% for road works & 50% for Building works.
|
Specific Gravity |
Bulk Density(KN/m3) |
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|
Cement |
3.15 |
1.428 |
|
|
F.A. |
2.65 |
1.5 |
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C.A. |
2.68 |
1.6 |
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Water |
1 |
1 |
|
Water Absorption in coarse aggregate is 0.5 to 1%.
- BULKING
Process of increase in volume of fine aggregate because of water.
Bulking in Normal sand = 15% to 30% & in Fine sand = 40%.
If bulking is not consider before making concrete than segregation & honey-combing occur.
- FINENESS MODULUS
|
Sand |
Fineness modulus |
|
Fine Sand |
2.2 to 2.6 |
|
Medium Sand |
2.6 to 2.9 |
|
Coarse Sand |
2.9 to 3.2 |
If F.M. is more than 3.2 than that sand is not used in concrete.
FLAKINESS
|
Aggregate |
Fineness modulus |
|
F.A. |
2 to 3.5 |
|
All in concrete |
3.5 to 6.5 |
|
C.A. |
5.5 to 8 |
|
Aggregate |
Fineness modulus |
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F.A. |
2 to 3.5 |
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All in concrete |
3.5 to 6.5 |
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C.A. |
5.5 to 8
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- FLAKINESS INDEX
Related to thickness (3/5=0.6).
Thickness Gauge is used.
Not applicable if size of aggregate is less than 6.3 mm.
- ELONGNATION INDEX
Related to length (9/5=1.8)
Length Gauge is used.
Not applicable if size of aggregate is less than 6.3 m.
- WATER FOR CONCRETE
If dissolved salts is more, than compressive strength of concrete reduce 10% to 30%.
Excess chlorides causes Dampness & Efflorescence.
Sodium & Potassium carbonate increase setting & decrease strength.
Calcium Chloride increase setting.
Algae reduce bond strength of cement & aggregate.
If sugar increase above 0.20% than increase setting & decrease strength.
- ADMIXTURE
- Accelerators
Calcium Chloride
Soluble Carbonate
- Silicates
Used for Rapid hardening, For remove formwork in short time & For proper use of concrete in cold reign.
- Retarders
Gypsum(calcium sulphate)
Sugar
Cellulose
Used for Reduce setting time, Increase strength by reducing w/c ratio & For proper use of concrete in hot reign.
Pozzolana
Fly ash
Surkhi
Clay
Shale
- Plasticizer
Plasticizer increase workability of concrete.
In concrete plasticizer content is 0.1 to 0.4%.
- WORKABILITY
If workability of concrete is good then it does not have property like Segregation, Bleeding, Permeability & Porosity.
Water content in concrete increase then workability increase & strength decrease.
If aggregate/cement ratio increase then workability decrease.
Big size aggregate give good workability then small size aggregate.
Rounded aggregate gives better workability then angular & flaky aggregate.
Smooth textured aggregate gives good workability than rough textured aggregate.
- SLUMP TEST
Used to measure Consistency.
Dimension of slump cone = 10 cm top dia., 20 cm bottom dia., 30 cm Height.
Tamping rod = 16 mm dia., 0.6 m length.
4 layer , 25 stroke.
True Slump
Same in all direction.
Shear Slump
One side of concrete fall.
Collapse Slump
All concrete fall and become rounded.
Slump test is applicable if size of aggregate is < or = 38 mm.
For stiff mix, slump value is zero.
|
Slump |
Workability |
|
0-25 |
Very low |
|
25-50 |
Low |
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50-75 |
Medium |
|
75-150 |
High |
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Type of construction |
Min. Slump |
Max. Slump |
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Mass concrete structure |
25 |
50 |
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R.C.C Foundation, footing & wall |
50 |
100 |
|
R.C.C slab & beam |
30 |
125 |
|
Column |
75 |
125 |
- COMPACTING FACTOR TEST
Used to measure Workability.
|
Compaction factor |
Workability |
Vee-Bee time in second |
|
0.78 to 0.80 |
Very low |
20-10 |
|
0.85 to 0.87 |
Low |
10-5 |
|
0.92 to 0.935 |
Medium |
5-2 |
|
0.95 to 0.96 |
High |
2-0 |
- FLOW TEST
Used to measure Consistency.
Flow value is between 0% to 150%.
Workability is directly proportional to w/c ratio.
Strength is inversely proportional to workability.
In Normal condition w/c ratio is taken 0.40 to 0.60.
DUFF ABRAMS LAW of w/c ratio
“ In perfectly compacted concrete, strength is inversely proportional to the w/c ratio.”
Hydration of 1 ml cement approximately formed 2.06 ml Gel.
In volume batching, volume of gauge box is equal to 1 bag of cement = 35 litre (0.035 m3). Gauge box dimension = 33.3 cm x 30 cm x 35 cm.
Cement batching is done by Weight.
Water batching is measure in kg or litre.
|
C : S : A |
|
|
M 5 |
1 : 5 : 10 |
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M 7.5 |
1 : 4 : 8 |
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M 10 |
1 : 3 : 6 |
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M 15 |
1 : 2 : 4 |
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M 20 |
1 : 1.5 : 3 |
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M 25 |
1 : 1 : 2 |
Concrete Mixer is rotated 15 to 20 rotation/min.
In R.C.C, thickness of concrete layer is 15 cm to 30 cm.
In Mass Concrete, thickness of concrete layer is 35 cm to 45 cm.
For placing concrete under water instrument used is Bottom dump bucket, Tremie pipe & Bagged concrete.
Dia. Of Tremie pipe is 20 cm or 200 mm.
|
Air in concrete |
Reduction in Strength |
|
5% |
30% |
|
10% |
50% |
Needle vibrator is used in Mass concrete.
Form vibrator is used in Column or thin wall.
Table vibrator is used in Laboratory for cube, cylinder.
Surface vibrator is used for Low thickness concrete like slab, road etc.
Membrane curing is used where water is not easily available.
- CONSTRUCTION JOINT
Construction joint is temporary joint.
It is provided at floor, sill & lintel level.
It is provided in beam-column junction below 3 to 4 inch.
It is provided where S.F & B.M is minimum.
- EXPANSION JOINT
If length of building is more than 45 m than expansion joint is provided.
- CONTRACTION JOINT
It is provided at interval of 5 m to 10 m.
- COMPRESSION TEST
Used to measure compressive strength & quality of concrete.
Cube size used for test 15 x 15 x 15 cm & 10 x 10 x 10 cm.
Cylinder size used for test 15 cm dia. & 30 cm Height.
H/d ratio is less, strength increase.
Correction factor < 1
15 x 15 x 15 cm size cube strength is standard strength.
10 x 10 x 10 cm size cube strength x 0.90 = standard strength (10% less).
15 cm dia., 30 cm height cylinder strength / 0.80 = standard strength (20% more).
- TENSILE TEST
Used to measure tensile strength.
Split cylinder test is tensile test.
FLEXURAL STRENGTH TEST OF BEAM
Used to measure Bending stress in fibre.
For Concrete, ideal temperature for strength is 10℃ to 15℃.
For Mass concrete, ideal temperature for strength is 5℃ to 10℃.
7 days strength of concrete at 4℃ temp. = ½ x 7 days strength of concrete at 17℃ temp.
Below 4℃ temp., Increase in strength of concrete is stopped.
Modulus of elasticity of aggregate is high, then creep is low.
w/c ratio is more, creep is more.
NORMAL DISTRIBUTION CURVE
Shape of curve Bell.
Used to measure changes due to mean strength.
|
Grade |
Standard Deviation (N/mm2) |
|
M-10, M-15 |
3.5 N/mm2 |
|
M-20, M-25 |
4.0 N/mm2 |
|
M-30, M-50 |
5.0 N/mm2 |
Mean group test result for M-15 = fm > or = fck + 3 N/mm2 &
for M-20 or above = fm > or = fck + 4 N/mm2.
- REBOUND HAMMER TEST
Also known as Schmidt hammer test or Sclerometer test.
Hammer is used on glossy side of the concrete.
Hammer test is done in Horizontal, Vertical & Inclined direction.
Test is used to measure resistance of concrete against wear & tear.
For good Durability, Less cement & more w/c ratio is required.
M-20
M – Mix
20 – Strength after 28 days.
Code used for concrete mix design = IS : 10262 – 1982.
- CYCLOPEAN CONCRETE
Size of aggregate used is 300 mm.
- FLY ASH CONCRETE
In India, content of fly ash in concrete is 15% to 50%.
Specific Gravity of fly ash is 1.9 to 2.4.
- PLAIN CEMENT & LIME CONCRETE
Strong in taking compressive stress.
REINFORCED CEMENT CONCRETE
Strong in taking tensile, compressive & shear stress.
PRE-STRESSED CEMENT CONCRETE
Strong in taking tensile & compressive stress.
|
Cement |
Uses |
|
Ordinary Portland Cement |
Common type of building, not subjected to chemical attack |
|
Rapid Hardening Cement |
Road pavement, cold weather, formwork is to be removed early |
|
Low Heat Cement |
Used in retaining wall |
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Blast Furnace Slag Cement |
Used in dam, bridge abutment & retaining wall |
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Pozzolana Portland Cement |
Used in dam, weirs |
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Sulphate Resisting Portland Cement |
Canal lining, culverts, sewer (Used in Bhakhra dam) |
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High Alumina Cement |
Structure subjected to sea water, chemical plants & furnaces |
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Coloured or Colocrete Cement |
Finishing floor & external surface |
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High Strength Portland Cement |
Prestressed, precast & airfiled work |
Maximum size of aggregate = 1/4 x Min. Dimension of P.C.C Member
Maximum size of aggregate = 1/5 x Min. Dimension of R.C.C Member
In case of Mass concrete in pier & abutment = M-10 (1:3:6) is used.
In case of Highly loaded column = M-25 (1:1:2) is used.
Higher w/c ratio require in coarse sand.
Portland cement density is 3.15 times heavier than water.
|
Ratio of |
|
|
28 days to 1 year |
0.80 |
|
1 year to 3 year |
0.98 |
|
3 months to 28 days |
1.15 |
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7 days to 28 days |
0.66 (2/3) |
Loss of prestress due to the shrinkage of concrete is equal to Product of Modulus of elasticity of steel & Shrinkage of concrete.
Harshness of the concrete due to the excess of middle-sized particle.
In case of heat & sound insulation purpose, Air-entrained concrete & Saw dust concrete is used.
Concrete prepared by the adding of aluminium is aerated, air-entrained or cellular concrete.