CONCRETE
concrete is a composite product produced by mixing cement fine aggregate, coarse aggregate and water in a suitable proportion.CONCRETE MIXES
Nominal Mix:- Mixes of fixed proportions generally by volume which ensure adequate strength. As per IS:456-2000, up to M20 can be designed by nominal mix method.
Standard Mix:- As published in IS:456 set of mixes specified in terms of dry weights of ingredients.
For eg: M10-1:3:6M15-1:2:4
M20-1:1.5:3
Design Mix:- The proportions are by weight. No Fixed Proportions For a specified strength For mixes > M25 design mix is a must As per IS:456-2000 up to M55 - ordinary concrete > M60 high strength concrete
Factors To Be Considered For Mix Design
i) The grade designation that is giving the characteristic strength requirement of concrete.
ii) The type of cement that influences the rate of development of compressive strength of concrete.
iii) Maximum Nominal size of aggregates to be used in concrete may be as large as possible within the limits prescribed by IS:456-2000.
iv) The cement content that is to be limited from shrinkage, cracking and creep.
v) The workability of concrete for satisfactory placing and compaction is related to the size and shape of section, quantity and spacing of reinforcement and technique used for transportation, placing and compaction.
WATER CEMENT RATIO
The ratio of water and cement by weight, in a concrete mix. As stated by Duff Abram with given concrete materials and conditions of cement determines the strength of the concrete so long as the mix is of workable plasticity.Abrum Equation: S=A/B^(w/c)
Where A and B are constants
S=Compressive Strength in MPa
w/c=water to cement ratio
The strength of concrete is inversely proportional to water-cement ratio i.e., lower the water-cement ratio, greater is the strength of concrete and vice versa. The relationship between water cement ratio by weight and the developed compressive strength of concrete at 23 days may be shown graphically.
when water cement ratio becomes less than 0.28, the curve bends downward, which indicates that the concrete is not workable. if such a concrete used for construction then a honeycombed structure will be obtained.
Properties Of Fresh Concrete
Workability of Concrete:- The property of concrete which determines the amount of concrete which determines the amount of useful internal work, necessary to produce full compaction i.e., workability is the amount of energy to overcome friction while compacting. Also defined as the relative ease with which concrete can be mixed, transported, moulded and compacted.
Workability Tests:
i)Slump Test:- Slump cone of bottom dia 20cm, top dia 10cm and height 30cm. Three layers of concrete. Each layer tamped for 25 times by a standard tamping rod of 16mm diameter and 60cm, length. The subsidence of concrete under gravity in 'mm' is called slump.
Recommended slumps for various concrete works (in 'mm')
| Type of member | Minimum | Maximum |
|---|---|---|
| Pavement | 25 | 50 |
| Mass Concrete structure | 25 | 50 |
| Unreinforced footing | 25 | 75 |
| Reinforced slabs beams foundations footings and walls |
50 | 100 |
| pumped concrete, slip form work | 75 | 100 |
| trench fill, insitu piles, tremie conc | 100 | 150 |
Advantage of slump cone:- Suitable for field application, suitable for concrete of high and medium workability.
Compaction Factor Test:
compaction factor=(weight of partially compacted concrete)/(weight of fully compacted concrete)
Suitability:- For determination of workability in a laboratory. It gives fairly good results for concrete with low workability.
Compaction Factor Values For Different Workability:
| 0.7 to 0.8 | Very low workability |
| 0.8 to 0.85 | low |
| 0.85 to 0.95 | Medium |
| >0.95 | High |
Vee-bee Consistometer:-
The time required for complete re-moulding of concrete in seconds after placed in the mould with a slump cone. Expressed in "seconds".
Suitability:- For dry concrete having very low workability.
| >20sec | Very low workability |
| 6-12sec | low |
| 3-6sec | Medium |
| 0-3sec | High |
Flow table test:- Based on % flow workability can be estimated.
Kelley ball test:- Based on the depth of penetration workability can be assessed.
Segregation and Bleeding:
Segregation:- segregation is defined as separating out of the constituent parts of a concrete mix so that the mix is no longer in a homogenous and stable.
Bleeding:- Bleeding is the separation of grout i.e., (cement + water) from the mix.
Laitance:- Due to over vibration, coarse particles settle and scum rises to the top. The formation of scum is called laitance. which causes "dusting off" of the surface i.e., after setting of concrete in powder form.
MIXING CONCRETE:
i) Hand Mixing ii) Machine Mixing
Types of Mixers:
i) Tilting drum type Mixers
ii) Non-tilting type
iii) Pan Mixers
Mixing time in Mixers: Not less than 1 minute Generally 1.5 to 2 minutes (revolutions per minute)
i)Hand Mixing;- Aggregate should be spread first on a clean surface. Cement is then spread over aggregate and dry materials are mixed by turning over from one end of try to another and cutting with a shovel until the mix appears uniform. Turning 3 times is usually required.
Usually, 10% more cement should be added to cater for the inferior hand mixing.
TRANSPORTATION OF CONCRETE:
Plants for transportation used are wheelbarrows, chutes, dump buckets, trucks endless belt conveyors, pneumatic pressure equipment and displacement pumps. concrete should be transported early to avoid segregation and to have uniform consistency of concrete.METHODS OF COMPACTING CONCRETE:
i) Tamping: Process of ramming of concrete with flat faced heavy tools. suitable for slabs and elements of small depth. Not suitable for all grades including lightweight concrete.
ii) Vibrating:- Widely used method better compacting. Period of vibration depends upon workability of a mix.
Types; Immersion vibrators, shutter vibrations or form vibrators, surface vibrators (pan vibrators, vibrating screeds), vibrating tables.
iii) vibro pressing: used for the production of concrete kerbs and concrete paving flagstones.
iv) Rolling With rollers
v) centrifugation:- for concrete pipes. poles
vi)High pressure and shock: Precast concrete products.
METHODS OF CURING:
The development of strength of concrete is due to "Hydration of cement" that takes place only in water-filled capillaries. If strength development has to continue, water shall be present in pores without evaporation till the development of strength is complete.The environment provided for proper hydration of cement paste in cement concrete is called curing of concrete.
METHODS:
i) Methods which reduces partly the water loss in concrete: Ponding water over concrete surface, covering concrete with a wet straw of damp earth, or jute or hessian covering, sprinkling water.
ii) Methods preventing loss of water: Waterproof paper, Membrane curing using sealing compounds.
iii) Accelerated Curing:
a) Using infrared radiation
b) Electrical curing
iv) Steam curing for precast products
PROPERTIES OF HARDENED CONCRETE
a) Compressive Strength: As per BIS 15cm cubes cured for 28 days, crushing strength determined. Grades are classified based on 28-day strength.
Eg: M15, M20, M25
Cylinders of 15cm dia and 30cm height are also used. Cube Strength = 1.25 X cylinder strength.
b) Modulus of Rupture: Flexure test using a prism of section 15cm X 15cm and 70cm long. it gives Flexural Tensile strength of concrete.
i) Tensile Strength: Indirectly determined by split-cylinder test.
ii)Bond Stresses
iii) Modulus of Elasticity: stress-strain curve drawn in compression test on standard cylinder. Generally 'Schmidt test hammer' or UPV for determination of compressive strength.
NON DESTRUCTIVE TESTING OF CONCRETE
- Testing methods of concrete members in a structure without disturbing its state.
- Gaining importance nowadays.
METHODS OF NON DESTRUCTIVE TESTING:
a) Surface Hardness Test:- This method is based on hardness of surface
Eg:- William Testing Pistol, Schmidt Rebound Hammer(widely used)
This is a quick testing method based on surface hardness.
This method is not giving the strength of the concrete in its core area
b) Pulse Velocity Method(UPV):
Ultrasonic pulses are used to test the strength of concrete.
In this method, the concrete strength is assessed by sending ultrasonic pulse waves through the concrete.
PUNDIT(Pulse Ultrasonic Digital Indicating Tester) is a widely used apparatus based on pulse velocities.
BIS CLASSIFICATION OF CONCRETE AS PER ULTRASONIC PULSE VELOCITIES
| Pulse Velocity | General condition of concrete |
|---|---|
| >4.5 | Excellent |
| 3.5-4.5 | Good |
| 3-3.5 | Medium |
| >3 | Doubtful |
Code of practice:
IS 13311: Method of non-destructive testing of concrete:
Part 1: 1992 Ultrasonic pulse velocity.
Part 2: 1992 Rebound Hammer.
c) Resonant Frequency Method:- The quality of the concrete is determined by obtaining the natural frequency of the concrete specimen.
d) Pulse-Echo Method:- Ultrasonic stress waves are introduced into the concrete with a piezo-electric element. The waves are analyzed by computer with the required software and the quality of concrete will be assessed. This Method gives satisfactory results for deep beams(Depth up to 1m to 10m)
ADMIXTURES: To modify the properties of concrete as per special requirements.
Classified as:
i) Accelerators
ii)Retarders
iii)Water Reducing admixtures
iv)Air
i)Accelerators:- To shorten the time of setting or increases the rate of hardening or strength development
Eg: Calcium Chloride
uses i) Repair works
ii) Precast Production
iii) Cold Weather
ii)Retarders:- To delay the setting of cement
Eg: sugar, carbohydrate derivatives, soluble zinc salts gypsum and lignosulphonates etc.
Used in hot weather.
iii) Water Reducing Admixtures:
- To increase workability without increasing the water content.
- Useful for very heavily reinforced sections or where repaid placing of concrete is desired.
Eg: Soda, calcium soaps, vegetable oils, fats, waxes, coal tar, ammonium state, oleate.
iv) Air Entraining Agents:- Air to be incorporated in the form of minute bubbles in concrete during mixing to increase the workability and resistance to freezing and thawing.
Eg: Vinsol Resin, Aluminum Powder, Hydrogen peroxide
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