What are in this post?
This post covers the factors affecting ultrasonic pulse velocity test condition. Some methods to avoid these factors while performing the test are also described here. So scroll and read this most important post.
If you don't know about UPV test well, then you must read my previous post. In that post the definition, apparatus list, IS code and test procedure, test result and it's interpretation for ultrasonic pulse velocity test on concrete as per IS 516 Part-5 Sec-1 are covered. Here is the link
Factors Affecting Ultrasonic Pulse Velocity Test and Way to Avoid
Ultrasonic pulse velocity test (or UPV test) is a non-destructive test which is performed on concrete to check their quality, homogeneity and cracks, voids, cavities and defects in the concrete.
There are some factors that affect the UPV test result. Also there are some methods to avoid these factors.
Factors Affecting Ultrasonic Pulse Velocity Test
The important factors that affect/influence the ultrasonic pulse velocity test are:
- Surface Conditions of Concrete
- Moisture Content of Concrete
- Path Length of Concrete Structure
- Shape and Size of Concrete Structure
- Temperature of Concrete
- Stress to Which the Structure is Subjected
- Reinforcing Bars
- Contact Between the Transducer and Concrete
- Cracks and Voids in Concrete
- Density and Modulus of Elasticity of Aggregate
Now read about these factors in detail.
1. Surface Conditions of Concrete
Smoothness of contact surface of concrete structure under test affects
the measurement of ultrasonic pulse velocity.
If there is any flaws or damage is present in the traverse path of ultrasonic pulse then the pulse traverses around the damage and takes more time than direct traverse. So the pulse velocity obtained in such conditions is lower than the direct traverse.
How to Avoid Surface Condition Effect
If the concrete surface is rough and
uneven, necessarily smooth the surface applying coupling medium (grease, petroleum jelly, liquid soap or kaolin glycerol paste) between the concrete surface and the face of each transducer.
2. Moisture Content of Concrete
Generally, ultrasonic pulse velocity through concrete
increases with increased moisture content of
concrete. The affect of moisture content in concrete is more for low strength
concrete than high strength concrete. The pulse
velocity of saturated concrete may be up to 2
% higher than that of similar dry concrete. In general, drying of concrete may result in somewhat lower pulse velocity.
How to Avoid Moisture Content Effect
If possible dry the concrete to obtain somewhat apparent pulse velocity. Performe the test at dry location in the concrete structure.
3. Path Length of Concrete Structure
The path length of concrete also affects the pulse velocity because concrete is inherently heterogeneous.
In field work, path length does not pose any difficulty
as pulse velocity measurements are performed on a thick structural concrete member.
However, in the laboratory where generally small specimens are used, the path
length can affect the pulse velocity readings.
How to Avoid Path Length Effect
The path length must be sufficiently long so as to avoid any error introduced due to its heterogeneity. In direct transmission, the minimum recommended path length for concrete is 100 mm, in which the nominal maximum size of aggregate is 20 mm or less and the minimum recommended path length for concrete is 150 mm, in which the nominal maximum size of aggregate is between 20 mm and 40 mm.
4. Shape and Size of Concrete Structure
The shape and size of the concrete member do not influence the velocity of short pulses of vibration unless the least lateral dimension is less than a certain minimum value. Below this value, the pulse velocity may be reduced appreciably. The extent of this reduction depends mainly on the ratio of the wave length of the pulse vibrations to the least lateral dimension of the specimen but is insignificant, if the ratio is less than unity.
How to Avoid Shape and Size Effect
The below table gives the relationship between the pulse velocity in the concrete, the transducer frequency and the minimum permissible lateral dimension of the specimen.
Table: Minimum Specimen Dimensions
If the minimum lateral dimension is less than the wavelength or if the indirect transmission arrangement is used, the mode of propagation changes and, therefore, the measured velocity will be different. This is particularly important in cases where concrete elements of significantly different sizes are being compared.
5. Temperature of Concrete
The temperature of concrete out of range 5°C and 30°C affects the pulse velocity. At temperatures between 30°C to 60°C, the pulse velocity may decrease by up to 5%. Below freezing temperature, the
free water freezes within the concrete, resulting in an increase in pulse velocity up to 7.5%.
How to Avoid Temperature Affect
Variations of the concrete temperature between 5°C and 30°C do not significantly affect the pulse velocity measurements in concrete. So, While doing the test, keep in mind that the temperature is between 5°C and 30°C.
6. Stress to Which the Structure is Subjected
When concrete is subjected to a stress which is abnormally high for the quality of the concrete, the pulse velocity may be reduced due to the development of micro-cracks. This influence is likely to be the greatest when the pulse path is normal to the predominant direction of the planes of such micro-cracks. This occurs when the pulse path is perpendicular to the direction of a uniaxial compressive stress in a member.
This influence is generally insignificant unless the stress is greater than about 60 percent of the ultimate strength of the concrete.
How to Avoid Stress Affect
The stress should be greater than about 60 percent of the
ultimate strength of the concrete.
7. Reinforcing Bars
The pulse velocity measured in reinforced concrete in the vicinity of reinforcing bars is usually higher than in plain concrete of the same composition. This is because, the pulse velocity in steel is 1.2 to 1.9 times the velocity in plain concrete and, under certain conditions, the first pulse to arrive at the receiving transducer travels partly in concrete and partly in steel.
The apparent increase in pulse velocity depends upon the proximity of the measurements to the reinforcing bar, the diameter and number of the bars and their orientation with respect to the path of propagation.
8. Contact Between Transducer and Concrete
Poor contact between transducer and concrete structure affects the reading. It is essential to use grease or other couplants to improve the contact.
How to Avoid Reading Affect
Use adequate acoustical couplants like grease, petroleum jelly, liquid soap, kaolin glycerol between the concrete structure and the face of each transducer to improve the contact between them.
9. Cracks and Voids in Concrete Structure
When an ultrasonic pulse travelling through concrete meets a concrete-air interface, there is negligible transmission of energy across this interface. Thus, any air-filled crack or void lying immediately between two transducers will obstruct the direct ultrasonic beam when the projected length of the void is greater than the width of the transducers and the wavelength of sound used. When this happens, the first pulse to arrive at the receiving transducer will have been diffracted around the periphery of the defect and the transit time will be longer than in similar concrete with no defect.
Note: This effect is used for locating flaws, voids or other defects greater than about 100 mm in diameter or depth.
Relatively small defects have little or no effect on transmission times, but equally are probably of minor engineering importance. Plotting contours of equal velocity often gives significant information regarding the quality of a concrete unit.
10. Density and Modulus of Elasticity of Aggregate
The ultrasonic pulse velocity of concrete is mainly related to its density and modulus of elasticity. This in turn, depends upon the materials and mix proportions used in making concrete as well as the method of placing, compaction and curing of concrete.
For example, if the concrete is not compacted as thoroughly as possible, or if there is segregation of concrete during placing or there are internal cracks or flaws, the pulse velocity will be lower, although the same materials and mix proportions are used.
How to Avoid the Factors Affecting Ultrasonic Pulse Velocity Test
- Keep these things in mind before testing for better results.
- Remove loose particles like dust, gravel and unnecessary fine aggregates.
- If the surface is roughed then make it smooth and clean with a grinding wheel or stone.
- Apply coupling medium (grease, petroleum jelly, liquid soap or kaolin glycerol paste) between the concrete surface and the face of each transducer.
- Choose a dry location for performing UPV test.
- If possible, dry the concrete structure.
- Take concrete of minimum path length 100 mm, in which the nominal maximum size of aggregate is 20 mm or less or take concrete of minimum recommended path length is 150 mm, in which the nominal maximum size of aggregate is between 20 mm and 40 mm.
- Use specimen of minimum permissible lateral dimension.
- Perform the UPV test in the temperature range of 5°C to 30°C.
After doing these, the effects on the test can be reduced to great extent.
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