ISO 1920-7:2004 download.Testing of concrete Part 7 : Non-destructive tests on hardened concrete.
1 Scope
This part of ISO 1920 specifies non-destructive test methods for use on hardened concrete. The methods included are
a) determination of rebound number,
b) determination of ultrasonic pulse velocity, and
c) determination of pull-out force.
NOTE These test methods are not intended to be an alternative for the determination of compressive strength of concrete, but wth suitable correlations they can provide an estimate of in-situ strength.
2 Terms and definitions
For the purpose of this document, the following terms and definitions apply.
NOTE Additional terms are defined in other parts of ISO 1920.
2.1
rebound number
(rebound number test) reading on a rebound hammer, which is related to the proportion of the energy returned to the hammer after striking the surface of the concrete
2.2
test area
(rebound number test) region of concrete that is being assessed and which, for practical purposes, is assumed to be of uniform quality
2.3
median
(rebound number test) middle value of a set of numbers when arranged in size order
NOTE If the set has an even number of items, the median is taken as the mean of the middle two.
3.3.2 Preparation
Heavily textured or soft surfaces and surfaces with loose mortar shall be ground smooth using the abrasive stone (3.2,3).
Smooth-formed or trowelled surfaces may be tested without grinding.
Remove any water present on the surface of the concrete.
3.4 Procedure
3,4.1 Preliminaries
Use the rebound hammer (3.2.1) in accordance with the manufacturer’s instructions for its operation. Activate it at least three times before taking any readings, to ensure that it is working correctly.
Before a sequence of tests on a concrete surface, take and record readings using the steel reference anvil (3.2.2) and ensure that they are within the range recommended by the manufacturer. If they are not, then clean andlor adjust the hammer.
The hammer should normally be operated at a temperature within the range of 10 DC to 35 DC.
3.4.2 Determination
Hold the hammer firmly in a position that allows the plunger to impact perpendicularly to the surface being tested. Gradually increase the pressure on the plunger until the hammer impacts.
After impact, record the rebound number.
NOTE There are hammers with automatic writing equipment and, In these cases. the rebound number is recorded automatically.
Use a minimum of nine readings to obtain a reliable estimate of the rebound number for a test area.
Record the position and orientation of the hammer for each set of readings.
No two impact points shall be closer together than 25 mm and none shall be within 50 mm from an edge. NOTE It is preferable to draw a regular grid of lines 25 mm to 50 mm apart and take the intersections of the lines as the test points.
3.6 Test report
An example of a test report is given in Annex C.
In addition to the details required by Clause 6, the report shall include the following:
a) identification of the rebound hammer;
b) reference anvil readings, before and after tests;
c) test result (mean value) and hammer orientation for each test area;
d) individual rebound hammer readings (when specified);
e) test result adjusted for hammer orientation (if appropriate).
4 Determination of ultrasonic pulse velocity
4.1 Principle
A pulse of longitudinal vibrations is produced by an electro-acoustical transducer held in contact with one surface of the concrete under test. After traversing a known path length in the concrete, the pulse of vibrations is converted into an electrical signal by a second transducer and electronic timing circuits enable the transit time of the pulse to be measured.
4.2 Apparatus
The apparatus comprises the following.
4.2.1 Electrical pulse generator
The pulse velocity of the apparatus should be calibrated against a standard calibration bar, generally supplied by the manufacturer of the apparatus.
4.2.2 Pair of transducers
4.3 Performance requirements of apparatus
The apparatus shall conform to the following performance requirements:
a) it shall be capable of measuring transit times in the calibration bar to an accuracy of ± 0,1 ts;
b) the electronic excitation pulse applied to the transmitting transducer shall have a rise time of not greater than one-quarter of its natural period; this Is to ensure a sharp pulse onset:
c) the pulse repetition frequency shall be low enough to ensure that the onset of the received signal is free from interference by reverberations:
d) the apparatus shall be used within the operating conditions stated by the manufacturer:
e) the apparatus shall be in calibration at the time of the test.
4.4 Procedure
4.4.1 Factors influencing pulse velocity measurements
In order to provide a measurement of pulse velocity that is repeatable, it is necessary to take into account the
various factors that influence the measurements. These are set out in Annex E.
4.4.2 Transducer arrangement
Place the two transducers on opposite faces (direct transmission), or on adjacent faces (semi-direct transmission), or on the same face (indirect or surface transmission) (see Figure 1). Although the direction in which the maximum energy is propagated is at right angles to the face of the transmitting transducer, it is possible to detect pulses that have travelled through the concrete in some other direction.
It may be necessary to place the transducers on opposite faces but not directly opposite each other. Such arrangements shall be regarded as a semi-direct transmission (see Figure 1 b)I.
The indirect transmission arrangement is the least sensitive and should be used when only one face of the concrete is accessible, or when the quality of the surface concrete relative to the overall quality is of interest.
See Annex D for the method of determining the ultrasonic pulse velocity by indirect transmission.
The semi-direct transmission arrangement has a sensitivity intermediate between the other two arrangements and should only be used when the direct arrangement cannot be used.
5.4.1 Installation of inserts
Securely fix the cast-in inserts to the formwork or locating device at the required test positions.
NOTE A small separately removable panel may be incorporated in the formwork when the test is being used to determine the formwork stripping time. It is important to ensure that the shafts are disconnected from the focrnwork before its removal.
Drill holes for other types of inserts, under-reamed, and assemble the inserts according to the manufacturer’s instructions.
5.4.2 Loading
Do not apply the test to frozen concrete.
First remove the tapered shaft of a cast-in insert and then connect the loading system to the insert in accordance with the manufacturer’s instructions.
Apply the load and increase at a steady rate of 0,5 kN/s ± 0.2 kNIs, without shock, until either the concrete is fractured (if the strength is to be estimated) or to the specified proof load (if the purpose of the test is to verify that the concrete has achieved a required minimum strength).
Record the maximum indicated force.
5.5 Expression of results
The maximum indicated force shall be expressed to the nearest 0,05 kN or to three significant figures.
If there is a requirement to determine the pull-out strength, then the procedures given in Annex H should be
followed.
5.6 Test report
An example of a test report is given in Annex I.
In addition to the details required by Clause 6. the report shall include the following.