Due to the drawbacks of ASTM C1018, this standard has been replaced by ASTM C1609 (ASTM 2011b) since 2005. The ASTM C1609 has similar procedures with ASTM C1018 for obtaining the load versus deflection curve, but the resulting curve is analysed in a totally different way. Therefore, the faults in the ASTM C1018 were excluded.
Based on the ASTM C1609, toughness tests are carried out on concrete beams with size of 350 x 100 x 100 mm and 500 x 150 x 150 mm. A closed-loop, servo-controlled testing system and roller supports are used in this test. Flexural
Fig. 2.13 Test apparatus of ASTM C1609 (ASTM 2011b)
load is applied under constant rate of displacement (not exceeding 0.05 mm/min) at one-third of test specimen spans. As shown in Fig. 2.13, a frame (referred to as a ‘yoke’) is mounted to the beam specimens, which allows direct measurement of the net central deflection of the beams. The use of the ‘yoke’ eliminates extraneous deflections arising from support settlements and results in load-deflection curves which are considerably different from those obtained by using the cross-head displacement of the testing machine.
Different to ASTM C1018, this standard calculates residual strength (fD00 and f1D50) at net deflection of L/600 and L/150, respectively. Absolute toughness (T1D50) and equivalent flexural strength ratio (TTD, 150) are also presented. As shown in Fig. 2.14, this test uses first-peak instead of first-crack, which is more accurate and objective. The equivalent flexural strength ratio, which is based on the first-peak strength, is more accurate. However, some difficulties still arise when this standard is applied to ultra-high performance fibre reinforced concrete containing very high volume fraction of fibres and exhibiting deflection-hardening behaviour (Wille et al. 2014). The peak load in ASTM C1609 is defined as the first point on the load-deflection curve where the slope is zero. Clearly, deflection-softening fibre reinforced concrete exhibits such a response. However, a deflection-hardening fibre reinforced concrete may have a stable deflection-hardening response without a sudden load drop after peak load, so there is no point with a zero slope (Yehia 2009).