IMPORTANCES OF TESTING
Testing Of Concrete Done On Fresh Concrete As well As Hardened Concrete.
Basically, Concrete Test Performed To Check Out Properties Of Concrete Like Durability, Workability, Strength Etc
Testing plays an important role in controlling and confirming the quality of concrete.
Systematic testing of raw materials, fresh concrete and hardened concrete an inseparable part of any quality.
It helps to utilize the full potential of the material used and greater assurance of the performance of concrete.
The test methods should be simple, direct and convenient to apply.
This test also has a detecting effect on those responsible for construction work.
The test results of the hardened concrete help to reveal the quality of the concrete and enable any adjustment made in the production of further concrete.
Tests made by casting cubes or cylinder from the representative concrete.
Following are the test includes for making the quality of the concrete better:-
- Slum Test
- Compaction Factor Test
- Vee-Bee Test
- Flow Table Test
- Compression Test (Cube Test)
- Split Cylinder Test
- Rebound Hammer Test
- Pulse Velocity Test
- Combined Methods
- Radioactive Methods
- Nuclear Methods
- Magnetic Methods
- Electrical Methods
- Physical Method
- Chemical Method
SLUM TEST:-
It Consist Of Mould That Is In The Form Of Frustum Having To Diameter Of 10 cm, Bottom Diameter Of 20 cm And Height Of 30 cm.
Concrete To Be Tested Is Filled In Mould In 4 Layers.
Where Each Layer Compacted 25 Times With The Help Of Rod.
After The Mould Completely Filled It Lifted Immediately Vertically Upward Direction.
Which Causes The Concrete To Subside And Subsidence Of The Concrete Referred to As Slump.
Which May Also Be Define As The Difference Level Of The Height Of Mould And Highest Point Of Subside Concrete.
The Slump Of Concrete Represents Is Workability As Higher Is The Value Of Slump Higher Its Workability.
The Test Is Not Suitable For The Concrete That Either Possessing Very High Workability Or Very Low Workability.
COMPACTION FACTOR TEST
The Degree Of Compaction Is Represents In Terms Of Compaction Factor.
That Represents Density Ratio That Is The Ratio Of The Density Achieved During The Test To The Density Of The Fully Compacted Concrete.
The Result Obtained From Compaction Factor Test Is More Uniform Than Those Obtained From Slump Test.
VEE-BEE TEST
It Consists Of A Mould In The Form Of The Frustum That Is Placed Inside The Cylinder Which Is Further Placed Over The Vibrating Table.
Concrete To Be Tested Is Filled In The Mould Which Is Raised Immediately And Cylinder Is Subjected To Vibration.
The Time Required By The Concrete To Assume The Cylindrical Shape Is Noted That Is Referred to As Vee-Bee Degrees And Its Is Further Used To Represents The Workability Of The Concrete.
FLOW TABLE TEST
This Test Is Performed For The Concrete Which Posses Very High Workability.
It Consist Of A Circular Table 76 cm In Diameter And A Mould That Is In The Form Of Frustum Having The Top Diameter Of 17 cm And Bottom Diameter Of 25 cm And Height Of 12 cm.
Concrete To Be Tested Is Filled In The Mould In Two Layers That Is Placed Over The Flow Table.
When The Mould Is Completely Filled, It Is Removed Immediately And The Table Is Raised And Dropped By 12.5 mm, 15 Times In 15 Seconds.
The Spread Of Concrete On The Table Is Noted At Least 6 Direction That Is Further Used To Represent Its Workability.
COMPRESSION TEST (CUBE TEST)
A Compression Test Is The Most Common Test Conducted On Hardened Concrete Because It Is Easy Test To Perform.
The Compression Test Is Carried Out On Specimens In Cubical Or Cylindrical In Shape.
In India The Standard Size Of The Cubic Specimen Is Of Size 15*15*15 cm Is Used.
After 28 Days Of Casting The Cube Is Taken Out From The Mould And Then Placed In The Compression Testing Machine (CTM).
Following This, The Machine Is Turned On And The Reading Is Noted Until The Crack Appears On The Cube.
SPLIT CYLINDER TEST
This Is Also Sometimes Referred To As The “Brazilian Test”.
This Test Was Developed In Brazil In 1943.
At About The Same Time This Was Also Independently Developed In Japan.
This Test Is Carried Out By Placing A Cylinder Specimen Horizontally Between The Loading Surfaces Of A Compression Testing Machine And The Load Is Applied Until The Failure Of The Cylinder Occurs On The Vertical Side As Shown In The Figure
This Test Is Made Of 15 cm In Diameter And 30 cm Long Which Is Placed In A Cylindrical Mould During Casting.
The Cylindrical Mould Is Made Of Metal Which Shall Not Be Less Than 3mm Thick.
Each Mould Is Capable Of Being Opened Longitudinally To Facilitate Removal Of The Specimen.
Due To Compression Load, The Cylinder Undergoes Lateral Expansion Owing To The Poisson’s Ratio Effect.
Comparison Between Cube And Cylinder Strength
It Is Difficult To Say Whether The Cube Test Gives More Realistic Strength Properties Of Concrete Or Cylinder Gives A Better Strength Of Concrete.
However, It Can Be Said That The Cylinder Is Less Affected By The End Restraints Caused By Platens And Hence It Seems To Give a More Uniform Result Than A Cube.
Therefore The Use Of A Cylinder Is Becoming More Popular Particularly In Research Laboratories.
Cylinders Are Cast And Tested In The Same Position Whereas Cubes Are Cast In One Direction And Tested From The Other Direction.
The Points In Favor Of The Cube Specimen Are That The Shape Of The Cube Resembles The Shape Of The Structural Member.
The Cube Does Not Require Capping Whereas Cylinder Requires Capping.
The Capping Material Used In The Cylinder May Influence To Some Extent The Strength Of The Cylinder.
REBOUND HAMMER TEST
The Rebound Hammer Test Measures The Elastic Rebound Of Concrete And Is Primarily Used For Estimation Of Concrete Strength And For Comparative Investigations.
Schmidt’s Rebound Hammer Developed In 1948 Is One Of The Commonly Adopted Equipment’s For Measuring The Surface Hardness.
It Consists Of A Spring Control Hammer That Slides On A Plunger Within A Tubular Housing.
When The Plunger Is Pressed Against The Surface Of The Concrete, The Mass Rebound From The Plunger.
It Retracts Against The Force Of The Spring.
The Hammer Impacts Against The Concrete And Spring Control Mass Rebounds, Taking The Rider With It Along The Guide Scale.
By Pushing A Button, The Rider Can Be Held In Position To Allow The Reading To Be Taken.
The Distance Travelled By The Mass Is Called The Rebound Number.
It Is Indicated By The Rider Moving Along A Graduated Scale.
PULSE VELOCITY METHOD
This Can Be Sub-Divided Into Two Parts
(a) Mechanical Sonic Pulse Velocity Method
which involves measurement of the time of travel of longitudinal or compressional waves generated by a single impact hammer blows or repeated blows.
(b) Ultrasonic Pulse Velocity Method
which involves measurement of the time of travel of electronically generated metallurgical pulses through the concrete.
Out of these two, the ultrasonic pulse velocity method ha gained considerable popularity all over the world.
When mechanical impulses are applied to a solid mass, three different kinds of waves are generated.
These are generally known as longitudinal waves, shear waves or surface waves.
Ultrasonic pulse velocity method consists of measuring the time of travel of an ultrasonic pulse, passing through the concrete to be tested.
The pulse generator circuit consists of an electronic circuit for generating pulses and a transducer for transforming these electrical pulses into mechanical energy with vibration of 15 to 50 kHz.
The pulses can be generated either by hammer blows or by the use of an electroacoustic transducer.
Electroacoustic transducers are preferred as they provide better control over the type and frequency of pulses generated.
The instrument used is called “Sonoscope”
Recently, battery-operated fully portable digitized units have become available in the U.K.
One such unit is called “PUNDIT” (Portable Ultrasonic Non-destructive Digital-Indicating Tester)
It only weighs 3 kg.
COMBINED METHODS
One of the most important objectives of non-destructive methods of testing of concrete is to estimate the compressive strength of concrete in the structure.
Use of anyone methods may not give reliable results regarding the strength of the structure.
The most popular combination was found to be the ultrasonic pulse velocity method in conjunction with hardness Measurement techniques and the Rebound Hammer method.
RADIOACTIVE METHODS
The use of X-rays and gamma-rays as a non-destructive method for testing properties of concrete is relatively new.
X-ray and gamma-rays both components of the high energy region on the electromagnetic spectrum penetrate concrete But undergo an attenuation process
The intensity of the incident gamma-rays and the emerging gamma-rays after passing through the specimens are Measured.
Gamma-rays transmission method has been used to measure the thickness of concrete slabs of known density.
Gamma radiation source of known intensity is made to pass and penetrate through the concrete and the intensity at the Other face is measured.
From this, the thickness of concrete is calculated.
NUCLEAR METHODS
Use of nuclear methods for non-destructive measurement of some properties of concrete is of recent origin.
Two principal techniques have been reported, namely neutron scattering methods for determining the moisture content Of concrete and neutron activation analysis for the determination of cement content.
These methods are not suitable for finding out the strengths of concrete.
MAGNETIC METHODS
Battery operated magnetic devices that can measure the depth of reinforcement cover in concrete and detect the position Of Scanner to locate the reinforcement and to determining spacing, diameter, and concrete cover.
The apparatus is known as cover-meter or profilometer.
This can be used for measuring the cover given in the reinforced sections and the diameter of the bars.
ELECTRICAL METHODS
Recently some electrical methods have been employed for determining the moisture content of hardened concrete
Tracing of moisture permeation through concrete and determining the thickness of concrete pavements.
Electrical resistivity methods have been used to find out the thickness of concrete pavements.
The method is based on the principle that the materials offer resistance to the passage of an electric current.
Concrete pavement has a resistivity characteristic that is different from that of the underlying subgrade layers.
A change in the slope of the resistivity versus depth curve is used to estimate the depth of concrete pavement.
CHEMICAL METHOD
The method of finding out cement content based on silicates in Portland cement is much more readily decomposed and made soluble in dilute hydrochloric acid than the solubility of silica contained in aggregates.
Similarly, the lime contained in cement is much more soluble than lime content in aggregates, with the exception in limestone aggregates.
The procedure is to crush a representative sample of the concrete and dehydrate it at a temperature of 550c for 3 hours.
A small portion of the sample taken and treated with 1:3 hydrochloric acid.
This will liberate the silica contained in the cement.
All these tests determined by chemical methods.
PHYSICAL METHOD
Polivka has successfully used a “Point-count” method on a sawn and varnished surface of a dried concrete specimen to determine its cement content, total aggregate count, and fine/coarse aggregate ratio.
The test determines the cement concrete within 10 per cent but the original water content or voids ratio cannot be estimated
Since no distinction made test between air and water voids.
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Author & Editor Of This Blog.
Founder Of “KPSTRUCTURES”
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