Issue 31

A. Abrishambaf et alii, Frattura ed Integrità Strutturale, 31 (2015) 38-53; DOI: 10.3221/IGF-ESIS.31.04 49 2 SPLT F ld    (13) where F is the applied line load, d is the diameter of the cylinder (150 mm) and l is the thickness of the net area in the notched plane (50 mm). 0.0 0.5 1.0 1.5 2.0 2.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 EXP UTT Envelope NUM SPLT EXP UTT Avg EXP SPLT Stress [MPa] w [mm] 0.0 0.5 1.0 1.5 2.0 2.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 EXP UTT Envelope NUM SPLT EXP UTT Avg EXP SPLT Stress [MPa] w [mm] (a) (b) Figure 14 : Comparison of the uniaxial stress – crack width relationship, σ – w , for: (a) θ= 0 ° and (b) θ = 90 ° . The σ – w relationship obtained by the inverse analysis procedure rendered a relatively good approximation of the uniaxial tensile response, principally, for the series θ = 90 ° . Regarding the θ = 0 ° series, NUM SPL T and EXP SPLT methods showed a very close numerical tensile strength, which were higher than EXP UTT Avg. However, as expected, splitting tensile test tends to slightly overestimate the tensile strength compared to the uniaxial tensile test. At the early cracking stages (w < 0.6 mm) NUM SPL T and EXP SPLT methods rendered σ – w responses nearby upper bound limit of the EXP UTT Envelope, this overestimation could be correspondent to the effects of the compressive stress along the loading plane. For higher crack opening widths, since in EXP SPLT method, stress was determined from Eq. 13 that assumes a linear elastic stress distribution even after cracking of the matrix, this approach was unable to predict post-cracking tensile response with enough accuracy. On the other hand, NUM SPL T started to get closer to the response obtained from the uniaxial tensile test. Regarding the θ = 90 ° series (see Fig. 14(b)), the inverse analysis procedure of the splitting tensile tests also overestimated the tensile strength when compared to the tensile strength obtained from the uniaxial tests, although it was within the experimental envelope. Based on the EXP UTT results, a sharp stress reduction happened once the crack initiated due to the brittle nature of the matrix and lower number of effective fibres at the fracture plane. The sudden stress decay occurred until the beginning of the hook mobilization, which happened at a crack width of around 0.3 mm. The result of the inverse analysis method reproduced the EXP UTT response with a good accuracy since unlike the θ = 0° series that showed higher residual stresses, the θ = 90° series had lower post-cracking residual stresses, therefore the load bearing capacity of the specimen has decreased and the compressive stresses were not so preponderant in the overall response. C ORRELATION BETWEEN THE FRACTURE AND FIBRE DISTRIBUTION PARAMETERS ab. 5 shows the fracture parameters obtained experimentally (uniaxial tensile test) and numerically (inverse analysis of splitting test) for the two series ( θ = 0 ° and 90 ° ). In this table, σ peak , σ 0.3 , σ 1 and σ 2 represent the stress at peak, 0.3, 1 and 2 mm, respectively; G F1 and G F2 are the dissipated energy up to a 1 and 2 mm crack opening width. It was noticeable that the influence of the notch orientation towards the concrete’s flow on the post-peak behaviour of the material was quite high. The series with a notch inclination of θ = 0º revealed higher residual stresses and hence larger dissipated energy than the specimens with θ = 90º. The observed variation in the post-cracking parameters could be ascribed to a preferential fibre orientation at the crack surface. On the other hand, in the casting process of the panels from the centre, since the wall effects are negligible, the flow velocity is uniform and diffuses outwards radially from the casting point, see Fig. 15. Therefore, the fibres have a tendency to reorient perpendicular to the concrete flow direction. Consequently, in the θ = 0° series because of the high number of effective fibres with favourable orientation, the composite showed a semi-hardening response while in the θ = 90° series, since fibres were rotated due to the concrete flow velocity, the number of the effective fibres was reduced and lower residual forces were achieved. T