Report on CE 512 Structural Engineering Laboratory

DEPARTMENT OF CIVIL ENGINEERING INDIAN INSTITUTE OF TECHNOLOGY GUWAHATI

Submitted by

Batch II Deeptimayee.B(124104012)

Gowtham Sriram Vetsa(124104009) Jim Noble Thomas(124104008)

Kavita Deka (124104073) Mohd Abdul Raheem Wasay(124104016)

Mohamed Sajeer M(124104017)

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INDEX

NON-DESTRUCTIVE TESTS

1.REBOUND HAMMER TEST

1.0 Aim

1.1 Apparatus required

1.2 Principle

1.3 Operating instructions

1.4 Measuring procedure

1.5 Interpretation of observations and results

1.6 Discussions

2.UTRASONIC PULSE VELOCITY

2.1 Aim

2.2 Apparatus required

2.3 Description of apparatus

2.3.1 Electrical pulse generator

2.3.2 Transducer

2.4 Principle

2.5 Performance of assembly of apparatus

2.6 Procedure

2.7 Testing methods

2.8 Influence of test conditions

2.9 Interpretation of observations and results

2.10 Discussions

Reference

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List of tables

Table no Title Page no

1.0 Age of concrete Vs time factor 4

1.1 Average rebound number readings and quality of concrete 5

1.2 Rebound number readings 6

2.0 Natural frequency of transducers for different path lengths 8

2.1 Velocity Criterion for concrete Quality Grading (IS: 13311 Part I) 14

2.2 Ultrasonic velocity readings 14

List of figures

Figure no Title Page no

1.0 Rebound hammer equipment 1

1.1 Rebound Hammer with 900 to the concrete surface 3

2.0 Electronic timing device and transducers 8

2.1 Different testing methods 12

2.2 Electronic device display unit 13

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Experiment no-1

REBOUND HAMMER TEST

1.0 AIM

To find out the compressive strength of concrete as per IS: 13311 (Part 2) - 1992 by

measuring the surface hardness of concrete using rebound hammer.

1.1 APPARATUS REQUIRED

Rebound hammer by Schmidt principle DIGI-SCHMIDT 2000.

Fig1.0. Rebound Hammer equipment

1.2 PRINCIPLE

The underlying principle of the rebound hammer test is:

The rebound of an elastic mass depends on the hardness of the surface against

which its mass strikes. When the plunger of the rebound hammer is pressed against the

surface of the concrete, the Spring-controlled mass rebounds and the extent of such a rebound

depends upon the surface hardness of the concrete. The surface hardness and therefore the

rebound is taken to be related to the compressive strength of the concrete. The rebound value

is read from a graduated scale and is designated as the rebound number or rebound index.

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The compressive strength can be read directly from the graph provided on the body of the

hammer

The Schmidt rebound hammer is principally a surface hardness tester with little apparent

theoretical relationship between the strength of concrete and the rebound number of the

hammer.

1.3 OPERATING INSTRUCTIONS

The DIGI-SCHMIDT 2 concrete tester is designed for non-destructive testing of

the uniformity of concrete & for estimating the compressive strength.

The test hammer strikes the concrete with a defined energy. Body rebounds

depending on the hardness of the concrete. The rebound number is measured by sensor.

1) Putting into operation:-

a) Connect test hammer to indicator unit.

b) Press ON key and the following are displayed briefly.

i. The number of impact device of the indicator unit.

ii. The installed software version.

iii. The remaining battery life.

iv. If no display appears, the batteries must be replaced.

Basic settings:

1. Impact direction

2. Calculation of mean value.

3. Measurement series number.

4.1 Units

4.2 Curve

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5.1 Shape factor ( s)

5.2 Eliminate outlier

6. Indication range

7. Limits

8. Display of rebound values of bar chart.

1.4 MEASURING PROCEDURE

- Input basic settings.

- Rub test surface with grinding stone

- Release the impact bolt by applying pressure to it.

- Place test hammer perpendicular to the test surface.

- Press housing against the test surface at moderate speed until the impact is triggered.

Each test point should be tested with at least 8 to 10 impacts. Minimum distance between

impact points is 20 mm. after the hammer has been used lock the impact bolt by means of

push button after an impact has been triggered.

Fig 1.1. Rebound Hammer with 900 to the concrete surface

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The indicator has a menu system with user guidance. Please follow the instructions in the

respective display field. The MENU key displays the following

1. Impact direction: conversion curves are always based on a horizontal impact

direction. For testing in a non-horizontal direction, the rebound value is automatically

corrected according to the pre-selected direction.

2. Calculation of mean value: mean value X is automatically calculated after n test

impacts (or) when the END key is pressed.

3. Measurement series number: the number is automatically incremented after the end

of a measurement of series.

4. The Compressive Strength () of the specimen is displayed after n test impacts.

5. Corrections:

i) Shape factor ( s): the PROCEQ standard curves 7 & 28 were determined using

200x200x200mm test specimens. If the strength is defined for samples of other dimensions,

the respective shape factor described in the standard must be used.

ii) Time factor ( t):

The influence of the age of the concrete can be corrected using the time factor. The

following factors are known from GRADE/SCHMIDT

Table 1.0 Age of concrete Vs time factor

Age in days 7 50 100 200 400 800

Time Factor 1.21 1.0 0.94 0.87 0.79 0.7

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6. Indication range: - this can be adapted to the testing range

7. Limits: - the range within which the rebound values are to lie can be selected. These limits

are not stored.

8. Data /Time: - the integral clock stores the measured values together with the date of the

test & time.

9. Performance check: - After thousand measurements the test hammer should be checked on

the testing anvil.

The testing anvil must be placed on a rigid and smooth surface. The contact surfaces of

anvil & impact bolt must be cleaned. The calibration value stated for the test hammer on the

anvil must be reached. If this is not the case, the unit must be overhauled by a service center.

1.5 INTERPRETATION OF OBSERVATIONS AND RESULTS

The rebound reading on the indicator scale has been calibrated by the manufacturer

of the rebound hammer for horizontal impact, that is, on a vertical surface, to indicate the

compressive strength. When used in any other position, appropriate correction as given by the

manufacturer is to be taken into account.

Table 1.1 Average Rebound number and quality of concrete

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Table 1.2. Rebound number readings

Rebound Number

Face 1 Face 2 Face 3 Face 4

49 46 45 48

42 42 52 40

47 45 40 48

Mean Value of Rebound Number = 28.78

Therefore quality of concrete is fair.

1.6 DISCUSSIONS

The rebound hammer method provides a convenient and rapid indication of the

compressive strength of concrete by means of establishing a suitable correlation between the

rebound index and the compressive strength of concrete. In general, the rebound number

increases as the strength increases but it is also affected by a number of parameters such as

the type of cement and aggregate, surface condition and moisture content, age of concrete and

extent of carbonation of concrete.

As such, the estimation of strength of concrete by rebound hammer method cannot be held to

be very accurate and probable accuracy of prediction of concrete strength in a structure is

25 percent.

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Experiment no-2

ULTRASONIC PULSE VELOCITY TEST

2.1 AIM

This test is done to assess the quality of concrete by ultrasonic pulse velocity method

as per IS: 13311 (Part 1) 1992.

The ultrasonic pulse velocity method is used to establish.

a) The homogeneity of the concrete

b) The presence of cracks, voids and other imperfections.

c) Changes in the structure of the concrete which may occur with time.

d) The quality of concrete in relation to standard requirements.

e) The quality of one element of concrete in relation to another.

f) The values of dynamic elastic modulus of concrete

2.2 APPARATUS REQUIRED

The apparatus shall consists of the following,

a) Electrical pulse generator (Pulser)

b) Transducer (Transmitter)

c) Amplifier (Receiver)

d) Electronic timing device (Timer)

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Fig 2.0 Electronic timing device and transducers

2.3 DESCRIPTION OF APPARATUS

2.3.1 TRANSDUCER

Any suitable type of transducer operating within the frequency range of 20Hz to

150Hz may be used. Piezoelectric and magneto-strictive types of transducers may be used,

the latter being more suitable for the lower part of the frequency range.

Table 2.0 Natural frequency of transducers for different path lengths

Path length(mm) Natural frequency of

transducer(KHz)

Minimum transverse

dimensions of members(mm)

Up to 500 150 25

500-700 >60 70

700-1500 >40 150

Above 1500 >20 300

2.3.2 ELECTRONIC TIMING DEVICE

It shall be capable of measuring the time interval elapsing between the one set of a

pulse generated at transmitting transducer and the one set of its arrival at the receiving

transducer. Two forms of the electronic timing apparatus are possible, one of which uses a

cathode ray tube on which the loading edge of the pulse is displayed in relation to the suitable

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time scale, the other uses on interval timer with a direct reading digital display. If both the

forms of timing apparatus are available, the interpretations of results become more reliable.

2.4 PRINCIPLE

The ultrasonic pulse velocity is generated by an electro acoustical transducer, when

the pulse is induced into the concrete from transducer; it undergoes multiple reflections at the

boundaries of the different material phases with in the concrete.

A complex system of stress waves is developed which includes longitudinal

(compressional), shear (transverse) and surface (Rayleigh) waves. The receiving transducer

detects the one set of the longitudinal waves, which is the fastest.

Because the velocity of the pulses is almost independent of the geometry of the

material through which they pass and depends only on its elastic properties, pulse velocity

method is a convenient technique for investigating structural concrete.

The underlying principle of assessing the quality of concrete is that competitively

higher velocities are obtained when the quality of concrete in terms of density, homogeneity

and uniformity is good. In case of poor quality, lower velocities are obtained. If there is

crack, void or flow inside the crack which comes in the way of transmission of the pulses, the

pulse strength is attenuated and it passes around the discontinuity, thereby making the path

length longer. Consequently lower velocities are obtained. The actual pulse velocity obtained

depends primarily upon the materials and mix proportions of concrete. Density and modulus

of elasticity of aggregate also significantly affect the pulse velocity.

2.5 PERFORMANCE OF THE ASSEMBLY OF APPARATUS

The apparatus should be capable of measuring transit times to an accuracy of 1%

over a range of 20 microseconds to 10 milliseconds.

The electronic excitation pulse applied to the transmitting transducer should have a

rise time of not greater than of its natural period. This is to ensure a sharp pulse onset.

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The interval between pulses should be low enough to ensure that the onset of the

received signal in small concrete test specimen is free from interference by reverberations

produced with in the preceding working cycle.

The apparatus should maintain its performance over the range of ambient

temperature, humidity and power supply voltage stated by the supplier.

2.6 PROCEDURE

In this method, the ultrasonic pulse is produced by the transducer which is held in

contact with one surface of concrete member under test. After traversing a known path length

(L) in the concrete, the pulse of vibrations is converted into an electrical signal by the second

transducer held in contact with the other surface of concrete member and an electronic timing

circuit enables the transit time of the pulse to the measured. The pulse velocity is given by

V = L/T.

Once the ultrasonic pulse impinges on the surface of the material, the maximum

energy is propagated at right angles to the face of the transmitting transducer and best results

are therefore obtained when the receiving transducer is placed on the opposite face of the

concrete member.

To ensure that the ultrasonic pulses generated at the transmitting transducer pass in to the

concrete and are then detected by the receiving transducer, it is essentially that there be

adequate acoustical coupling between the concrete and the face of each transducer. Typical

couplings are petroleum jelly, grease, liquid soap and kaolin glycerol paste. If there is very

rough concrete surface, it is required to smoothen and level an area of the surface where that

transducer is to be placed. It is necessary to work on concrete surfaces formed by other

means. E.g.: Trowelling. It is desirable to Measure pulse velocity over a longer path length

than would normally be used. A minimum path length of 150mm recommended for the direct

transmission method involving one unmoulded surface and a minimum of 400mm for the

surface probing method along an unmoulded surface.

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The N.F of transducer should preferably be within the range of 20-150 KHz. Generally,

high frequency transducers are preferable for short path-lengths and low frequency

transducers for long paths. Transducers With the frequency of 50-60 KHz are useful for most

all-round applications.

Since the size of the aggregate influence the pulse velocity measurement, it is

recommended that the minimum path length should be 100mm for concrete in which the

nominal maximum size of aggregate is 20mm or less and 150mm for concrete in which the

nominal maximum size of aggregate is between 20-40mm.

Transducers are held on corresponding points of observation on opposite faces of a

structural element to measure the ultrasonic pulse velocity by direct transmission i.e. crossing

probe. If one of the faces is not accessible, ultrasonic pulse velocity is measured on one face

of the structural member by surface probing.

Surface probing in general give lower pulse velocity than in case of cross probing and

depending on number of parameters, the difference could be of the order of about 1 Km/sec.

2.7 TESTING METHODS

a) Direct Method (Cross Probing)

b) Semi-Direct Method

c) Indirect Method

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Fig 2.1 Different testing methods

2.8 INFLUENCE OF TEST CONDITIONS

i) Influence of surface conditions and moisture content of concrete

ii) Influence of path length, shape and size of concrete members.

iii) Influence of temperature of concrete.

iv) Influence of stress.

v) Effect of reinforcing bars.

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Fig.2.2 Electronic device display unit

2.9 INTERPRETATION OF OBSERVATIONS AND RESULTS

The quality of concrete in terms of uniformity, incidence or absence of internal flaws,

cracks and segregation, etc., indicative of the level of workmanship employed, can thus be

assessed using the guidelines given below, which have been evolved for characterizing the

quality of concrete in structures in terms of the ultrasonic pulse velocity.

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Table 2.1 Velocity Criterion for concrete Quality Grading (IS: 13311 Part I)

Table 2.2 Readings of Ultrasonic velocity

Methods Length, l

(m)

Time, t

(s)

Velocity, v

(m/sec)

Concrete

Quality

Grading

Specimen 1

Direct 0.445 119.8 3720

Excellent Direct 0.445 118.9 3708

Semi-direct 0.227 62.0 3660

2.10 DISCUSSIONS

Ultrasonic pulse velocity measures provide the quality of concrete relation to the

standard requirements and the homogeneity of the concrete. In general, the velocity of pulses

higher when the quality of concrete in terms of density, homogeneity and uniformity is good.

In case of poorer quality lower velocities are obtained. If there is crack, void or flaw in the

concrete lower velocities obtained. The actual pulse velocity obtained depends primarily

upon the materials and mix proportions of concrete. Density and modulus of elasticity of

aggregates also significantly affected the pulse velocity.

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REFERENCE

IS 13311(part 1) :1992 Non destructive testing of concrete Methods of test(part 1-

Ultrasonic pulse velocity)

IS 13311(part 2) :1992 Non destructive testing of concrete Methods of test(part 2-

Rebound hammer)

IS 516 :1959 Methods of tests for strength of concrete

M.S.Shetty, (2008).Concrete Technology. S.Chand and Company Ltd, New Delhi,

India.

http://www.spectro.in/Rebound-Hammer-Test.html

http://www.mhtesting.co.uk/schmidt.html

http://www.spectro.in/Ultrasonic-Pulse-Velocity-Test.html

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