Ultrasonic Pulse Velocity Tests
The standard method for the assessment of concrete strength for concrete employed in structures is carried out on specially prepared test cubes in the laboratory. It is therefore difficult to assess the effect of compaction on the strength of the in-situ concrete since the concrete may not be as well compacted as that used for test purposes. Cores can of course be cut from a structure and tested, but this is expensive and damages the member.
For these reasons, attempts have been made to measure a property related to strength in a non-destructive manner.
The measurement of the velocity of a longitudinal pulse of ultrasound has achieved considerable success in this application. While strength and ultrasonic pulse velocity are not uniquely related, under specified conditions a direct relationship does exist: a change in the concrete density gives mix, the ratio of the actual and fully compacted densities of the concrete and the resulting strengths are closely related. The lowering of density caused by an increase in water cement ratio or by poor compaction causes both a lowering in strength of the concrete and a reduction in the velocity of a pulse of ultrasound transmitted through it. Furthermore, the presence of cracks across the path of the sound causes diffraction of the pulse and thus a reduction in its velocity.
The presence of cracks can in some circumstances be detected and it is also possible to determine the crack length. Cracks travelling longitudinally in the path of the beam do not cause significant attenuation as the pulse can pass either side of the crack.
Equipment and test method
The equipment most frequently used for this is the "Pundit" apparatus manufactured by CNS Electronics, which employs a digital read-out of the time for the pulse to travel a measured distance. The pulse velocity may then be simply determined. In use, a transmitting transducer is placed on one side of the concrete to be measured and a receiving transducer placed directly opposite on the other side. A grease such as "Castrol LM" is used as a coupling medium between transducers and concrete to ensure adequate contact. The path length must then be accurately measured and the velocity determined.
If access to only one side of the member is available it may be possible to make an indirect measurement, although this is difficult, in particular because the path length cannot easily be determined. The technique is covered in ASTM C597-1980 and also in BS 4408.
Cracks depths can be measured using ultrasonics and the method is described in the CNS Electronics 'Pundit Manual'.
Estimating the Depth of Surface Cracks
An estimate of the depth of cracks visible at the surface can be obtained measuring the transit times across the crack.
One suitable arrangement is to have the transmitting and receiving transducers on opposite sides of the crack and equidistant from it. The receiving transducer is then moved in increments away from the crack, recording the values obtained.
An equation can then be derived by assuming that the plane of the crack is perpendicular to the concrete surface and that the concrete in the vicinity of the crack is of reasonably uniform quality.
The velocity of sound will vary between different concrete of similar strength but made with different materials. However a guide figure for good quality concrete is in the range 4.0 to 4.5 km/s.
Within samples, or areas, of concrete made with similar materials the pulse velocity will increase with the strength.
The measurement of pulse velocity along a member and a statistical analysis of these results can provide a good indication of the quality control exercised during construction. Values given in the C and CA Advisory Data Sheet ADS/34 issued in October 1977 as typical for construction are:
|1.5% or less||Should be expected from units containing a single load of concrete|
|2.5% or less||Typical of several loads of concrete on a site with good supply and even quality of workmanship|
|3% -5%||Some marginal non uniformity should be suspected, a typical case being the supply of low slump concrete to a member with a high concentration of reinforcement|
Research in Canada has indicated that a coefficient of variation of 3% to 6% might be reasonably expected from an existing building constructed to a good standard of workmanship.
Floor slabs may be expected to produce coefficients about 1% higher than those quoted above.
ASTM C597-1980. BS4408: Part 5:1971.
C and CA Advisory Data Sheet 34:October 1977. Drysdale R. C. "Variation of Concrete Strength in Existing Buildings".
Magazine of Concrete Research, Volume 25:No. 85: pp 201-207: December 1973.