Issue 43

S. Jing et alii, Frattura ed Integrità Strutturale, 43 (2018) 33-42; DOI: 10.3221/IGF-ESIS.43.02 40 Figure 7 displays the form of the specimen at the peak load and after the failure. No burst fracture is observed in the test. At the peak load, the multiple tiny vertical cracks on the specimen gradually developed into major connected cracks; the crack development is accompanied by the sound of fibers being strained or broken. Whereas the fibers hold the matrix together and the material has the strain-hardening property, the stress state of the ECC in uniaxial loading is similar to that of the confined concrete under conventional triaxial loading. In this test, the specimen exhibited the squeezing flow failure similar to that of confined concrete [18]. When macroscopic cracks begin forming (Fig.8, a), fibers between the crack sections are interlaced and evenly distributed. Fibers at the crack parts bear the majority of the tensile force, which prevents further development of cracks. The cracks at this stage are mainly the outcome of the bridging role of fibers. In other words, on the crack surface, the concrete matrix delivers the stress to fibers which rely the surface to convey the stress to the concrete matrix around that doesn’t have cracks. When the cracking strength of the matrix around reaches the limit, new cracks will appear, which is reflected in increasing cracks on specimens. Strain hardening is realized through formation of multiple cracks. The process will last until the surface fibers are collectively pulled out or broken. Specimens show remarkable extensibility [19]. Fig. 8(b) displays the failure mode of specimens in the direct tensile test. It can be noted that a number of cracks show up on the specimen surface, which indicates that desirable adhesive performance exists between PVA fibers and the matrix. It prevents further expansion of tiny cracks. With such strain hardening behavior, the strength and toughness of materials are significantly enhanced [20]. Figure 7 : Failure mode comparison of ECC cube specimens in different phases of loading. (a) (b) Figure 8 : (a) Micro-crack bridging and ductility enhancement effect of PVA fibers; (b) Typical multiple cracks on ECC thin plate specimen under tensile load.