SITE EFFECT ON FRACTURE ENERGY OF CONCRETE AND STABILITY ISSUES IN 3-POINT BENDING FRACTURE-TOUGHNESS TESTING

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) b eams. Most of the beams with target concrete cylinder compressive stre ngths 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 reco rded. The LPD was measured by two direct current displacement transduc ers (DCDTs), supported by an aluminum frame attached to both sides of the beam at its mid-height. Three beam sizes (S1, S2, S3), a constant width b = 127 mm, span-to-depth ratio S/d = 4, and notch length-to-dep th 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(F)(R) values are consistently higher than the c alculated G(f)(SEM) values [Bazant's size effect model (SEM)] and the equivalent G(Ic) values based on LEFM and the Jenq and Shah two-parame ter model (TPM). A macro- and microscale effect is observed in the fra cture energy values. The RILEM C-F(R) values for the smaller d(max) (6 mm) increased by only 35 to 59 percent, which is four times the incre ase in beam size (S1 to S3), while the larger d(max) (25 mm) only incr eased by 24 to 36 percent, which is two times the increase in beam siz e (S2 to S3), The G(F)(R) values are influenced more drastically by d( max). Increasing d(max) from 6 to 25 mm, G(F)(R) increased independent ly by about 100 percent of the beam size. The G(F)(R) values are influ enced rather mildly by the concrete compressive strength. For d(max) = 25 mm and normal concrete compressive strength, rougher crack surface s are observed and higher G(F)(R) values are obtained, compared to tho se for d(max) = 6 mm. Symptoms of a possible snap-back instability wer e detected in the load-LPD diagram of the beams with the shot rer notc h a(o) = 0.1 d. Depending on the relative stiffness of the beam and te sting frame, snap-back may occur in the load-stroke diagram, Extraneou s influence of the load actuator stroke signal on the measured LPD may result in an ''apparent'' instability in the load-LPD diagram that ma y be inadvertently presumed to be a true snap-back instability.