Stress-intensity factor for plain concrete in bending—Prenotched versus precracked beams

A series of experiments conducted to compare the behavior of beams with notches to those with natural cracks has recently been completed. A total of 42 beams with notches formed by casting teflon strips into the concrete were tested to failure. A companion series of 42 beams were statically precracked following the procedure described in Ref. 5. The ranges of crack depth to beam depth varied from 0.3 to 0.7 (nominally). Two strengths of concrete were used and the 3-in.×4-in.×15-in. (span) beams were tested on three-point or four-point bending. Comparisons are made on the basis of computed stress intensity using a finite-element program developed by the writers. Results of this study show the following: (1) in all cases the naturally cracked beams yielded higher failure loads and stress-intensity values than notched beams with the same crack length; (2) the averageK ₁ values for precracked beams were approximately 38 percent, 77 percent and 96 percent greater than for notched beams for crack-depth ratios of 0.3, 0.5 and 0.7 respectively.