Issue 30

A. Fernández-Canteli et al., Frattura ed Integrità Strutturale, 30 (2014) 383-393; DOI: 10.3221/IGF-ESIS.30.46 384 amount of material (especially for 3PB and 4PB) is needed. Searching for an alternative and simpler way of testing, the idea of using a compact tension (CT) specimen has emerged. Figure 1. Load- Figure 1 : Load-displacement curve of concrete specimens In metallic materials, testing CT specimens represents a usual procedure to determine fracture parameters. Thus, ASTM Standard E-399 [11, 12] specifies the geometry and general configuration of this kind of tests in metallic materials. The idea of its application to testing concrete on fracture is not new: previous researchers, like Wagoner et al. [13], attempt to perform this kind of tests in concrete according to the above mentioned standard though they ran into troubles obtaining premature failures in 50% of the tests at the pulling load holes thus evidencing the disability of the new procedure. With the purpose of avoiding such a high percentage of invalid tests, a different CT test methodology, denoted modified compact tension test (MCT), is developed by the authors getting practically nil quote of premature failures. Basically, the new procedure consists in applying the pulling force by means of reinforced bars built in the specimen, which are clamped in the machine during testing [7, 14]. In this way, the need of providing holes in the specimen, as in the metallic materials, dismissed. M ODIFIED COMPACT TENSION TEST : CONFIGURATION he modified compact tension test (MCT) solution was initially developed for slide shaped specimens cut off from cylindrical specimens used in standard compression tests, which can be obtained as a drill core from real constructions for evaluating age and conditions of the material. The holes are drilled into the specimens in perpendicular direction to the milled notch into which the pulling bars are allocated and glued. Nevertheless, in the meantime more advanced, practical and simple solutions, the denoted “ad-hoc” specimens, have been developed that facilitate its preparation in the laboratory. In the two above mentioned MCT solutions, i.e. “drill-out” and “ad-hoc” specimens, the geometry is apparently the same consisting in notched cylinders of a suitable thickness b furnished with the pulling bars, see Fig. 2, but they result from two different manufacturing procedures. The characteristic dimensions of the two specimens tested in this work are summarized in Tab. 1, where :  cs : specimen diameter [mm],  sb : pulling bar diameter [mm], W : specimen width, i.e., distance from the load axis to the back side of the specimen [mm] a: notch length measured from load axis [mm], b : specimen thickness [mm], e : notch width [mm], A lig : ligament area [mm 2 ], and α : relative notch length [-]. With these values, the ligament area, defined as the area of specimen that has to be fractured, is calculated, as the product of the length of the ligament times the thickness of the specimen. On its turn, the relative notch length α , defined as the dimensionless parameter in Eq. (2) like in the original compact tension tests [15], is considered a reference parameter to be studied in the modified compact tension tests of concrete. a W   (2) T Displacement (mm) Load (N)