Title:
Size Effect on Fracture Energy of Concrete and Stability Issues in Three-Point Bending Fracture Toughness Testing
Author(s):
Philip C. Perdikaris and Alberto Romeo
Publication:
Materials Journal
Volume:
92
Issue:
5
Appears on pages(s):
483-496
Keywords:
aggregates; compressive strength; concretes; cracking (fracturing); failure; loads (forces); microstructure; Materials Research
DOI:
10.14359/900
Date:
9/1/1995
Abstract:
The effect of the beam size, aggregate size, and compressive strength on the specific fracture energy of plain concrete is studied, based on three- point bending (TPB) static tests on single-edge notched (SEN) beams. Most of the beams with target concrete cylinder compressive strengths of 28 and 55 MPa and two maximum aggregate sizes of 6 and 25 mm were tested under crack mouth opening displacement (CMOD) control. The CMOD, applied load, load-point deflection (LPD), and stroke were recorded. The LPD was measured by two direct current displacement transducers (DCDTs), supported by an aluminum frame attached to both sides of the beam at its midheight. Three beam sizes (S1, S2, S3), a constant width b = 127 mm, span-to-depth ratio S/d = 4, and notch length-to-depth ratio a o /d = 0.3, were considered. Two beams with a shorter notch length of a o = 0.1 d were tested to compare their postpeak response. The measured RILEM G R F values are consistently higher than the calculated G SEM f values [Bazant's size effect model (SEM)] and the equivalent G Ic values based on LEFM and the Jenq and Shah two-parameter model (TPM). A macro- and microscale effect is observed in the fracture energy values. The RILEM GG R F values for the smaller d max (6 mm) increased by only 35 to 59 percent, which is four times the increase in beam size (S1 to S3), while the larger d max (25 mm) only increased by 24 to 36 percent, which is two times the increase in beam size (S2 to S3). The GG R F values are influenced more drastically by d max . Increasing d1 max from 6 to 25 mm, G R F increased independently by about 100 percent of the beam size. The G R F values are influenced rather mildly by the concrete compressive strength. For d max = 25 mm and normal concrete compressive strength, rougher crack surfaces are observed and higher G R F values are obtained, compared to those for d max = 6 mm. Symptoms of a possible snap-back instability were detected in the load- LPD diagram of the beams with the shorter notch a o = 0.1 d. Depending on the relative stiffness of the beam and testing frame, snap-back may occur in the load-stroke diagram. Extraneous influence of the load actuator stroke signal on the measured LPD may result in an apparent instability in the load-LPD diagram that may be inadvertently presumed to be a true snap-back instability.