F. K. Kong MA, MSc, PhD, CEng, FICE, FIStructE, R. H. Evans CBE, DSc, D ès Sc, DTech, PhD, CEng, FICE, FIMechE, FIStructE (auth.)-Reinforced and Prestressed Concrete-Springer US (1987)

44 Properties of structural concrete
carried out in Cambridge in which two of the principal stresses are of equal
magnitude, and the third principal stress is smaller (i.e. ft = / 2 2: /J 2: 0). It
was observed that a splitting failure (which Johnson and Lowe referred to
as a cleavage failure) could occur in the plane of the principal stresses f 1
and /z. Their research, which is continuing, has since been indirectly
supported by results obtained independently elsewhere [28). In the mean
time it is worth noting that the simplified failure criterion represented by
the dotted square in Fig. 2.5-10, though usually accepted as conservative
for biaxial compression, may be unsafe under certain special conditions.
For example, Johnson and Lowe's work [27, 28) has indicated that in a
state of biaxial compression, it is possible that cleavage failure will occur
when / 1( = /z) is only about 50% off~.
2.5(g)
Non destructive testing of concrete
We have earlier referred to standard tests carried out on control specimens
to determine the cube strength, the cylinder strength, the modulus of
rupture, and so on. These may all be classified as destructive tests in the
sense that the test specimens are destroyed in the course of the tests. There
is another class of tests known as non-destructive tests, [29-31), which are
particularly useful for assessing the quality of the concrete in the finished
structure itself.
The ultrasonic pulse velocity method [29-31) is described in BS 4408:
Part 5. It consists basically of measuring the velocity of ultrasonic pulses
passing through the concrete from a transmitting transducer to a receiving
transducer. The pulses are short trains of mechanical vibrations with
frequencies within the range of about 10-200 kHz. They travel through
concrete at velocities ranging from about 3 to 5 km/s. In general, the
higher the velocity, the greater the strength of the concrete. Ultrasonic
pulse velocity in concrete depends mainly on its elastic modulus and, since
this is closely related to mechanical strength, the pulse velocity can be
correlated to that strength. This correlation, however, is not unique
but depends on the mix proportions and type of aggregate used, and also
to a smaller extent on the moisture content of the concrete, its curing
temperature and its age. The apparatus for this test generates ultrasonic
pulses at regular intervals of time (usually from about 10 to 50 per second)
and measures the time of flight between the transducers (that is the transit
time) to an accuracy of better than ± 1%. The distance between the
transducers (that is the path length) must be measured to the same
accuracy thus allowing the pulse velocity to be calculated to an accuracy of
better than ±2%. Originally the apparatus incorporated a cathode-ray
oscilloscope, but recently a portable form of the equipment has been
developed (and marketed under the trade name Pundit) which indicates
the transit time directly in digital form.
As mentioned above, estimation of strength from pulse velocity
measurements requires a correlation curve. It has been found that such a
correlation is of the form
equivalent cube strength = Kekv (2.5-6)