CONCRETE HONEYCOMB-TEST
This method statement describes the procedure of the Ultrasonic Pulse Velocity (UPV) test as a method for assessment of the uniformity of concrete (quality of concrete).
1. ULTRASONIC PULSE VELOCITY MEASUREMENT
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.
The pulse velocity depends upon the dynamic Young’s modulus, dynamic Poisson’s ratio, and density of the medium.
Advantages
The principal advantages of ultrasonic pulse velocity measurement are that it is totally non-destructive, quick to use, and reflects the properties of the interior of a body of concrete. It is particularly valuable in circumstances where a considerable number of readings are required for the assessment of the uniformity of hardened concrete.
Limitations
Although non-destructive, it is essential not to overlook surface staining from the use of some complaints. The pulse velocities for most practical concrete mixes lie within a narrow range and it is, therefore, necessary to measure both the transit time and path length to an accuracy of the order of ±1% if the results are to be of the greatest value.
Principal Applications
The principal applications are as follows
a) Determination of concrete uniformity, which is most reliably achieved by taking measurements on a regular grid on a member, or at comparable locations on similar members.
b) Detection of cracking and honeycombing, which will increase the effective path length resulting in a higher measured transit time. If such defects lie between the transducers, a higher measured transit time will not necessarily occur if the crack or voids are water filled or bridged by reinforcement. The depth of surface cracks may be estimated by placing transducers on either side. Honeycombing or voids may be identified by taking a series of measurements through a member on a regular grid. The minimum size of detectable defects depends upon the transducer size and the distance between them.
c) Assessment of concrete deterioration, using a general comparative survey to locate suspect areas. Techniques are also available to estimate the depth of surface fire damage or chemical attack. The long-term performance of concrete may also be monitored by conducting repetitive tests at a location.
Transducers Arrangement
There are three basic ways in which the transducers may be arranged, as shown in figure 1. These are:
- Opposite Face (Direct Transmission)
- Adjacent Faces (Semi-Direct Transmission )
- Same Faces (Indirect Transmission)
3. TESTING EQUIPMENT
The apparatus consists essentially of an electronic pulse generator, a pair of transducers, an amplifier, and an electronic timing device for measuring the time interval between the onset of a pulse generated at the transmitting transducer and the onset of its arrival at the receiving transducer.
The elements of the ultrasonic instrument Proceq Pundit Lab PL-200 are illustrated in Figure 2.
4. DETERMINATION OF CONCRETE UNIFORMITY
Heterogeneities in the concrete within or between members cause variations in pulse velocity which, in turn, are related to variations in quality. Measurements of pulse velocity provide means of studying the homogeneity and, for this purpose, a system of measuring points that cover uniformly the appropriate volume of concrete in the structure has to be chosen.
The number of individual test points depends upon the size of the structure, the accuracy required, and the variability of the concrete. In a large unit of fairly uniform concrete, testing on a 1m grid is usually adequate but, on small units or variable concrete, a finer grid may be necessary. It should be noted that, in cases where the path length is the same throughout the survey, the measured time may be used to assess the concrete uniformity without the need to convert it to a velocity. This technique is particularly suitable for surveys where all the measurements are made by indirect transmission.
It is possible to express homogeneity in the form of a statistical parameter such as the standard deviation or coefficient of variation of the pulse velocity measurements made over the grid. However, such parameters can only be properly used to compare variations in concrete units of broadly similar dimensions.
Variations in pulse velocity are influenced by the magnitude of the path length because this determines the effective size of the concrete sample which is under examination during each measurement. The importance of variations should be judged in relation to the effect which they can be expected to have on the required performance of the structural member being tested. This generally means that the tolerance allowed for quality distribution within members should be related either to the stress distribution within them under critical working load conditions or to exposure conditions
5. PROCEDURE
Mark the measurement points of the UPV test, and be sure that the marked test point to be having a smooth surface.
Measure the distance between the measurement points accurately to 1% of the length, Maximum distance of direct measurement is 15m, and the distance for direct measurement is 0.25 to 0.50m.
Apply coupling paste to contact surfaces of the transducers (thin coat for fine concrete surface, thicker coat for rough surface).
Position transducers exactly on the measurement points, and start taking by UPV instrument.
Making statistical analysis of results for comparison and determination of substandard areas in the slab and getting the quality of the concrete.
6. QUALITATIVE DESCRIPTION OF CONCRETE
|
APPROXIMATE
PULSE VELOCITY (m/s) |
CONCRETE QUALITY |
|
4500 |
Excellent |
|
3500
– 4500 |
Good |
|
3000
– 3500 |
Questionable |
|
2000
– 3000 |
Poor |
|
<
2000 |
Very
Poor |
7. REFERENCES
BS EN 12504 – 4, 2004 and Guideline from ACI Manual of Concrete Inspection.
8. FIGURE
