testing of concrete and quality control

8/12/2019 Testing of Concrete and Quality Control

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CHAPTER VTESTING OF CONCRETE AND QUALITY CONTROL

6.1 Various Strength of Concrete: Tensile, Compressive, Shear andBond

In concrete design and quality control, strength is the property which is generally specified. This is

because, compared to other properties, testing of strength is relatively easy. Furthermore, many

properties of concrete, such as elastic modulus, water-tightness or impermeability and resistance to

weathering agents including aggressive waters, are directly related to strength and can therefore be

deduced from the strength data.

Compressive strength of concrete is many times greater than other types of strength and a majority of

concrete elements are designed to take advantage of the higher compressive strength of the material.

The uniaxial compression tests are the easiest to perform in the laboratory.

The other types of strength required for concrete are:

- Tensile Strength- Shear Strength- Bond and Bearing Strengths

5.2 Compressive Strength Test

In order to determine the compressive strength of concrete, three types of specimen can be used:

i) Cubesii) Cylindersiii) Prisms

CUBE TESTS

Generally, specimens are cast in steel or cast iron moulds of 150 mm dimensions, which should

confirm to cubical shape. The dimensions and plainness should be within the limits of tolerance.

i) The cube is filled in three layers and well compacted.ii) After compaction, the top surface is made flush with edges of mould and finished by means

of trowel.


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iii) The finished surface is left undisturbed for 24 hours at a temperature of 66F - 70F andrelative humidity not less than 90 percent.

iv) After 24 hours, the mould is stripped and the specimen is stored in water for furthercuring. As far as possible the curing temperature should be maintained at 66F - 70F.

The specimens are usually cured for 28 days. In some cases, testing of specimens is carried out at the

age of 1 day, 3 days, 7 days, 14 days, 21 days, 45 days and 90 days.

CYLINDER TESTS

The standard cylinder mould is 150 mm in diameter and 300 mm height made of cast iron or steel.

Cylinder specimens are prepared as cube specimens. As the finished surface of cylinder may not be

smooth, sulphur caping is done before testing.

The strength of cylinder is equal to 4/5 thof the strength of a cube, but experiments have shown that

there is no simple relation between the strengths of the specimens of the two shapes.

LHermite suggested the following relation:

Clinder strength.76.2log ( f284)

Where, fcu= strength of the cube in pounds per square inch.

Cylinders are believed to give a greater uniformity of results for nominally similar specimens as their

failure is less affected by the properties of the coarse aggregate used in the mix and the stress

distribution on horizontal planes in a cylinder is more uniform than on a specimen of square cross-

section.

EFFECT OF HEIGHT/DIAMETER RATIO ON CYLINDER STRENGTH:

Standard clinders are of height h equal to twice the diameterd, but sometimes specimens of otherproportions are encountered.

Height to diameter ratio (h/d)Strength Correction Factor

ASTM C 42 77 BS 1881 : 19702.00 1.00 1.00

1.75 0.98 0.98

1.50 0.96 0.96

1.25 0.93 0.94

1.00 0.87 0.92


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PRISM TESTS

Dimensions = 75 mm x 75 mm x 300 mm

GAIN OF STRENGTH WITH AGE:

The concrete develops strength with continued hydration. The rate of gain of strength is faster to start

with and the rate gets reduced with age. It is customary to assume the 28-day strength as the full

strength of concrete.

Age (in days) Compressive Strength

1 16 %

3 40 %

7 65 %

14 90 %

28 99 to 100 %

5.3 Tensile Strength Test

Concrete being a brittle material is very weak in tension and is not expected to resist tensile forces, in

general, from external loading conditions. But, due to restrained conditions of shrinkage and

temperature effects, tensile stresses are developed in concrete.

DIRECT TENSION TEST

This type of test is carried out under axial loading. The strength obtained under such type of testing is

small compared to other types of tensile tests. Generally, this test is not performed.

Direct tensile strength, f .35 f

Where, fck= characteristic strength of concrete

specimen.

INDIRECT TENSION TEST

- Splitting Test- Flexural Test i. Centre Point Loading ii. Third Point Loading


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SPLITTING TEST/BRAZILIAN METHOD

A cylinder of the type used for compression test is placed with its axis horizontal between the plates of

testing machine and the load is increased until the failure by splitting along the vertical diameter takes

place. The load is applied through narrow strips of packing material such as ply-wood. Usually these

strips are 3 mm thick and 12.5 mm to 25 mm wide.

The horizontal tensile strength is obtained by the formula:

T 2DL

Where, P = compressive load on the cylinder

D = diameter of the cylinder

L = length of the cylinder

T = splitting tensile strength of the specimen

This test gives strength very close to the true tensile strength of concrete than the modulus of rupture.

MODULUS OF RUPTURE (FLEXURAL TEST)

The tensile strength produced due to bending is known as modulus of rupture. Central point loading

and third point loading are two loading schemes in which modulus of rupture is determined. Third

point loading is the more appropriate method and gives lesser value compared to central point

loading.

CENTRAL POINT LOADING:


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e have,

Hence, . L4

112 bd

d2 3L

2bd

Where, b= breadth and d=height of section

THIRD POINT LOADING:

. L6

112 bd

d2 L

bd


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5.4 Variabilit of Concrete Strength and Acceptance Criteria

Since, strength is a variable quantity, when designing a concrete mix, we must aim at a mean

strength higher than the minimum required from the structural standpoint so that we canexpect every part of the structure to be made of concrete of adequate strength.

Concrete strength follows normal distribution curve.

i. Every sample should have a testing strength not less than the characteristic strength ofconcrete. Or,

ii. The strength of one or more samples though less than the characteristic value, is in each casenot less than the greater of the following:

a) the characteristic strength minus 1.35 times the standard deviation; that is, fck1.35s

b).8 times the characteristic strength; that is, .8 f

ck.

Whichever is greater in the above two conditions and the average strength of all the samples

should fulfill the following condition:

. f [1.65 1.65 ]

here, tested strength from supplied concrete,

avg.= average or mean strength of concrete supplied,


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fck= characteristic strength,

n = number of samples (minimum 30)

s = standard deviation

5.5 Non Destructing Testing of Concrete

1. Destructive Tests

2. Non Destructive Tests

a. Schmidts Rebound Hammer Test

b. Ultrasonic Pulse Velocity Test

c. Profometer

d. Vibroscanner

REBOUND TEST

The rebound hammer test measures the elastic rebound of concrete and is primarily used for

estimation of concrete strength and for comparative investigations.

It consists of a spring control hammer that slides on a plunger within tubular housing. When the

plunger is pressed against the surface of the concrete, the mass rebounds from the plunger,

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.


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The results of this test are affected by:

i. Smoothness of surface under testii. Size, shape and rigidity of the specimen

iii. Age of specimeniv. Surface and internal moisture condition of the concretev. Type of coarse aggregate

vi. Type of cementvii. Type of mould

viii. Carbonation of concrete surfaceULTRASONIC PULSE WAVE VELOCITY TEST

It consists of measuring the time of travel of an ultrasonic pulse, passing through the concrete to

be tested. The pulse generator circuit consists of electronic circuit for generating pulses and a

transducer for transforming these electronic pulses into mechanical energy having vibration

frequencies in the range of 15 to 50 KHz. The time of travel between initial onset and the

reception of the pulse is measured electronically. The path length between transducer divided

by the time of travel gives the average velocity of wave propagation.

PROFOMETER/COVERMETER TEST

Profometer/Covermeter test is conducted to determine the presence of reinforcing bars, laps,

transverse steel, metal tie, wires or aggregates with magnetic properties. It determines the size,

depth and position of reinforcement buried in concrete.

Purpose of application:

Quality control, to ensure correct location and cover to reinforcing bars after concreteplacement.

Investigation of concrete members and locating reinforcement as a preliminary to someform of testing, such as, core extraction.

Location of buried ferromagnetic objects other than reinforcement.

testing of concrete and quality control

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