Crack Tip Opening Displacement (CTOD) and Crack Mouth Opening Displacement (CMOD) tests are normally used to study the effect of fibres on the post-cracking behaviour of concrete (Fraternali et al. 2011). According to ASTM E1290 (ASTM 2008b), CTOD is the displacement of the crack surfaces normal to the original (unloaded) crack plane at the tip of the fatigue pre-crack. However, due to inherent difficulties in the direct determination of CTOD, CMOD test is a preferred test to assess post-cracking performance of fibre reinforced concrete (Zhijun and Farhad 2005). According to BS EN 14651:2005+A1:2007 (BSI 2007), CMOD test measures the opening of the crack at mid-span using a displacement transducer mounted along the longitudinal axis. Both tests can clearly display the ability of fibres to redistribute stresses and bridge the cracks formed.
Fraternali et al. (2011) performed CTOD tests on the PP and recycled PET fibre reinforced concrete specimens. The PP fibre had 1.04 mm2 of cross section, 47 mm
Fig. 2.5 Load-deflection curves of PP fibres reinforced concretes (Hsie et al. 2008) of length, 250 MPa of tensile strength, 1.1 GPa of Young’s modulus, and 29% of ultimate strain, while the recycled PET fibre had 1.54 mm2 of cross section, 52 mm of length, 274 MPa of tensile strength, 1.4 GPa of Young’s modulus, and 19% of ultimate strain. The results can be seen in Fig. 2.6 (Fraternali et al. 2011). The peak load was reached at a corresponding CTOD of less than 0.6 mm for all the specimens. However, compared to the plain concrete, ductility of the specimens after the peak load was significantly improved in the PP and PET fibre reinforced specimens. This clearly exhibits the ability of macro plastic fibres to improve post-cracking performance of concrete.
Round Determinate Panel Test (RDPT) is considered to better represent the relative behaviour of different fibre reinforced concretes. This test has a significantly lower variation in post-cracking performance than that of either CMOD or CTOD test (Bernard 2002). The panel-based performance assessment is desirable because panels fail through a combination of stress actions that reflects the behaviour of a fibre reinforced concrete more closely than other mechanical tests (Cengiz and Turanli 2004). RDPT, based on ASTM C1550 (ASTM 2012), involves bi-axial bending in response to a central point load, and shows a mode of failure related to the in situ behaviour of structures such as concrete slabs-on-grade and sprayed tunnel lining construction (Parmentier et al. 2008).
Cengiz and Turanli (2004) compared the shotcrete panels reinforced by macro PP fibre, steel mesh and steel fibre. The PP fibre had a length of 30 mm, a diameter of 0.9 mm, a tensile strength of 400 MPa, a Young’s modulus of 3.5 GPa, and ultimate strain of 11%. The steel fibre had a length of 30 mm, a diameter of
Fig. 2.6 Load-CTOD curves of recycled PET and PP fibres reinforced concrete (Fraternali et al. 2011)
0.6 mm, a tensile strength of 1.2 GPa, a Young’s modulus of 200 GPa, ultimate strain of 0.6%, and flattened ends with a round shaft. The steel mesh had a diameter of 8 mm and intervals of 150 mm. As can be seen from Fig. 2.7, 0.45% of steel fibre reinforced concrete showed 65 kN of peak load and 664 J of energy absorption until 25 mm deflection, while 0.78% of PP fibre reinforced concrete showed better post-cracking performance with 70 kN of peak load and 716 J of energy absorption. Steel mesh showed much more brilliant post-cracking performance (1308 J in energy absorption) than either of steel or PP fibres. However, until deflection of 2.5 mm, the PP fibre reinforced concrete exhibited comparable load with that reinforced with steel mesh.