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TECHNICAL PAPER COLLECTOR’S EDITION

Also included in this table are the results of the be negligible, and failure loads are consistent with
companion modulus of rupture and compressive the modulus of rupture on the nett section. It may
strength tests, and the nominal failure stress on also be concluded that concrete members may be
the nett section, and for the cracked specimens, notch insensitive for small laboratory specimens,
the computed value of d for which equations (8) but susceptible to crack propagation for structural-
o
and (9) intersect for each series. sized members. This effect may influence the
application to structural members of laboratory

The results are presented in a non-dimensional results for such properties as cracked stiffness,
form in Fig 2 where the ratio of the nominal failure shear and torsional strengths, or the failure of
stress to the modulus of rupture is plotted against unbonded prestressed concrete.
the ratio . From this figure it can be seen that
the dual failure criterion of equations (8) and (9) is
quite consistent with the experimental results. The
figure also demonstrates the marked reduction in
nominal failure stress that occurs with increasing
specimen size.

For the application of linear fracture mechanics
to the strength of notched specimens, must be
greater than unity, and to avoid transitional effect
should preferably be greater than two. Fig 2 shows
the accuracy with which equation (8) applies to
results in this range. From a reference to the values
of d in Table 1, it would suggest that a minimum Fig 2 Failure stress of notched specimens
o
value of d of 6 in (150 mm) and consequently a
minimum depth of beam specimen of 9 in (225 mm) References
should be used for fracture testing of concrete. It 1. Kaplan, M. F. Crack propagation and the
would, therefore, appear that the specimens tested fracture of concrete. Journal of the American
by Kaplan and Welch were too small. C o ncr e t e Insti t ut e , N ov e m be r 1 9 6 1 .
Proc Vol 58, pp. 591-610.
CONCLUSION 2. Welch, G. B. and Haisman, B. Fracture toughness
measurements of concrete. UNICIV report no R42,
The fracture load of a pre-cracked concrete January 1969. University of New South Wales,
member can be based on a limiting stress intensity Australia.
factor in accordance with the theory of linear sharp 3. Welch, G. B. and Haisman, B. The application of
crack fracture mechanics, if the specimen is large fracture mechanics to concrete. UNICIV report no
enough. For concrete flexural tests the minimum 112, November 1968. University of New South
desirable size of prism is indicated by the tests Wales, Australia.
to be 9 in (225 mm). For small prisms, say of 3 in 4. Walsh, P. F. Stress intensity factors by a calibrated
(75 mm) size, the stress concentration effects can finite element method. (To be published).

P. F. Walsh, Research Scientist, Division of Building Research, Commonwealth Scientific and Industrial
Research Organisation, Melbourne, Victoria, Australia

36 The Indian Concrete Journal | November 2018