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3b. - Testing of Concrete 1 Quick revision 1.1 Test of fresh concrete (refer to CS1) a. Slump test – CS 1 Section 2

b. Compacting factor – CS 1 section 3

c. Vebe time – CS 1 section 4

d. Density of compacted fresh concrete – CS 1 section 5

1.2 Compressive Strength of Concrete 1.2.1 Compressive Strength of concrete cubes

Standard: HK – CS 1 Section 12

British – BS 1881 Part 116

normally 150 mm cubes are used

100 mm cube can be used for maximum aggregate Size < 20 mm

Compressive strength of a cube, fcu

fcu = A

F (N/mm2 or MPa)

where F is the maximum load attained (in N)

A is the cross-section area of the cube (in mm2)

1.2.2 Compressive strength of concrete cylinder

Standard: USA – ASTM C39

Size of specimen – 150 mm diameter 300 mm height, or

100 mm diameter and 200 mm height

Cylinder strength is usually lower than cube strength.

It is used in the USA and some other countries but not in Hong Kong.

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2 Core Strength Standard: HK – CS 1 Section 15

British – BS 1881 Part 120

The main purpose taking core samples in a structure is to determine the actual

concrete quality in the structure.

Cores shall be drilled perpendicular to the surface using a diamond core drilling

bit.

Drilling through reinforcement shall be avoided wherever possible.

The test specimen shall preferably be 150 mm diameter and in no case shall it be

less than 75 mm diameter.

The ratio of diameter to the maximum aggregate size shall be not less than 3.

For compressive strength testing the length/diameter ratio shall be between 1.0

to 1.2.

When it is necessary to reduce the length of the core, the core shall be swan

perpendicular to its longitudinal axis. Inclusion of reinforcement in the

specimen shall be avoided wherever possible.

Grinding is the preferred method of end preparation by capping is acceptable.

Capping material may be a mixture of sulphur and fine siliceous sand or a

mixture of sulphur and pulverized-fuel ash.

Coring Machine and Diamond Bit (Source: ELE)

Core Cutter (Source: ELE)

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The load shall be applied steadily at a rate of 0.2 to 0.4 MPa/s.

2.1 Calculation of compressive strength of core, fcore

fcore = A

F (MPa)

where f is the maximum load attained

A is the cross-section area of the core

2.2 Estimated in-situ cube strength, fce

For cores free of reinforcement, the estimated in-situ cube strength, fce shall be

calculated from the equation:

fce =

α

1 1.5

D

fcore (MPa)

Where

D is 2.5 for cores drilled horizontally or 2.3 for cores drilled vertically

α is the length/diameter ratio

For cores with reinforcement, different equations are used

(refer to CS1 section 15).

Grinding Machine (Source: ELE)

Capping Apparatus (Source: ELE)

Capped Cylinders (Source: Standard Australia)

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3 Tensile Splitting Strength Standard: HK – CS 1 Section 13

British – BS 1881 Part 117

“Tensile splitting strength test” is an indirect method for the determination of the

tensile strength of concrete.

It involves the making of a standard 150 300 mm test cylinder, which is then

placed, with its axis horizontal axis, in a testing machine and applying a

compressive force to it.

3.1 Calculation of Tensile Splitting Strength, fct

fct = d L

F 2

(MPa)

Where

F is the maximum load (in N)

L is the average measured length (in mm)

D is the average measured diameter (in mm)

(Source: ELE)

Tensile Splitting Strength Test (Source: CS 1)

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4. Flexural Strength Standard: HK - CS 1 Section 14

British – BS 1881 Part 118

Usually splitting cylinder method is used to determine the tensile strength of

concrete.

In occasions where the strength of concrete in flexure, or bending, is of prime

importance such as pavement or similar project, flexural strength is specified.

The test involves loading a standard 150 150 750 mm beam in a flexural

loading device until broken. (BS also allows the use of 100 100 500 mm

beams.)

Failures outside the middle one-third of the distance between the supporting

rollers shall render the test invalid.

Calculation of Flexural Strength, fcf

fcf = 221 d d

F L

(MPa)

Where

F is the maximum load (in N)

d1 and d2 are the width and depth of the specimen respectively (in mm)

L is the distance between the supporting rollers (in mm)

Flexural Loading Frame

(Source: ELE)

Flexural Strength Test (Source: CS 1)

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5. Static Modulus of elasticity in compression Standard: HK - CS 1 Section 17

British – BS 1881 Part 121

Static modulus of elasticity in compression is the ratio

between compressive stress and strain, expressed in

terms of the secant modulus.

Ec = strain

stress

Just before commencing the static modulus of elasticity

test, the compressive strength of the concrete shall be

determined from two standard test cubes of the same

batch.

The equivalent cylinder strength (fc) shall be determined by multiplying the

average cube strength by 0.8.

The specimen shall then be loaded to 1/3 fc and then unloaded for three times at

the rate of 0.6 MPa/s.

Then the measurement of elasticity shall be made for stress between 0.5 - 1/3 fc

MPa and with the corresponding strain being measured.

Calculation of Static Modulus of elasticity, Ec a - b

Ec = a - b

Where

a is the upper loading stress (a = 1/3 fc )

b is the basic stress (0.5 MPa)

a is the mean strain under the upper loading stress

b is the mean strain under the basic stress

Determination of Modulus

of Elasticity of Concrete (Source: ELE)

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6. Surface hardness Standard: BS1881 : Part 202

The surface hardness test is based on the principle that the rebound of an elastic

mass depends upon the hardness of the surface which it strikes.

The equipment for the test is known as the Schmidt rebound hammer and is a

non-destructive method of testing concrete.

The results are expressed in terms of the rebound number.

The test is particularly suited to comparative surveys and checking the

uniformity of concrete.

It may also be correlated with other properties of the concrete. In this case a

specific calibration for the type of concrete under investigation should be

established.

6.1 Method of obtaining a correlation between strength and rebound number

To prepare a correlation between rebound number and strength it is necessary to

test a number of specimens which encompass the likely range of strength

expected.

The reliability of the correlation is increased by increasing the number of

specimen.

Then construct a correlation curve from the mean rebound number and strength

for each test specimen.

The most convenient method of producing a correlation between strength and

Schmidt rebound hammer

(Source: ELE)

Hardness test using cube specimen

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rebound number is by tests in which both measurements are made on concrete

cubes.

The cubes should be held in a compression testing machine under a stress of 7 -

10 N/mm2.

Take nine readings using the rebound hammer on each of the two side faces

accessible in the compression machine.

The points of impact on the specimen should not be nearer an edge than 20 mm

and should not less than 20 mm from each other.

Alternatively, hardness tests are made on the concrete in-situ at proposed core

position and cores subsequently cut and tested for strength.

Twelve readings are usually sufficient to obtain a reliable estimate of the surface

hardness at one location.

6.2 Advantages

The tests are quick and easy to carry out,

of low cost, and

non-destructive.

6.3 Limitations

The results are influenced by many factors:

- surface texture,

- moisture condition of the surface,

- cement type,

- mix characteristics, and

- type and rigidity of the structure.

There is no unique relation between hardness and strength of concrete and the

use of universal calibrations can give seriously erroneous results.

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7. Measurement of velocity of ultrasonic pulses in concrete Standard: BS1881 : Part 203

The apparatus consists of an electrical pulse generator, a pair of transducers and

an electronic timing device to measure the time taken by the pulse to traverse the

length.

Ultrasonic pulse velocity equipment measures the transit time of a pulse

between transducers placed on the surface of a body of concrete.

The pulse velocity can then be calculated using the measured path length

through the concrete. V = T

L

Calibration of Ultrasonic Tester Using Standard Calibration Bar

Measurement of Ultrasonic Pulse Velocity

Method of Propagating and Receiving Ultrasonic Pulses

(Source: BS1881:203)

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7.1 Principal application 7.1.1 Determination of concrete uniformity

Heterogeneities in concrete within or between

members cause variation in pulse velocity.

The test provides a quick means for checking the

uniformity of concrete.

However, existing of steel reinforcement may

influence the test result.

7.1.2 Detection of defects

When an ultrasonic pulse traveling through concrete meets a concrete-air

interface, there is negligible transmission of energy across this interface.

Therefore the test may be use to detect large voids or cavities in concrete,

to estimate the depth of a surface crack, and

To estimate the thickness of a layer of inferior quality concrete.

(Concrete may be suspected of having a surface layer of poor quality. This

may occur during manufacture or arise as a result of damage by fire, frost,

sulpahte attack, etc.)

Influence of steel reinforcement

on pulse velocity (Source: BS1881:203)

Determination of crack depth (Source: BS1881:203)

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7.1.3 Correlation of pulse velocity and strength

It is possible to establish a correlation between ultrasonic pulse velocity and

strength of a particular type of concrete.

This correlation has to be established experimentally by testing a sufficient

number of specimens to cover the range of strengths expected and to provide

statistical reliability.

7.1.4 Determination of the dynamic elastic modulus and dynamic Poisson’s

ratio.

The pulse velocity depends upon the dynamic Young’s modulus, dynamic

Poisson’s ration and density of the medium.

It is possible to estimate these two parameters from empirical data.

Pulse velocity determination by indirect (surface) transmission

(Source: BS1881:203)

Empirical relationship between static and dynamic modules of elasticity and pulse velocity

(Source: BS1881:203)

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7.2 Advantages

The tests are quick and easy to carry out,

of low cost, and

non-destructive.

7.3 Limitations

Although non-destructive, the use of some couplants may cause staining of the

surface.

Measured values may be affected by:

- Surface texture,

- Moisture condition of the surface,

- temperature,

- specimen size,

- reinforcement, and

- stress.

Reference: Construction Standard – Testing Concrete CS1:1990 Volume 1 & 2, Hong Kong Government British Standard, British Standard Institute:

BS 1881 : Part 116 BS 1881 : Part 117 BS 1881 : Part 118 BS 1881 : Part 120 BS 1881 : Part 121 BS 1881 : Part 201 BS 1881 : Part 202 BS 1881 : Part 203