ASTM C1399 (ASTM 2011a) is a very useful method of assessing the toughness of fibre reinforced concrete, especially for the concrete reinforced by relatively low fibre volumes (Banthia and Dubey 1999). This test is an open-loop test, so the testing machine does not need a displacement control. It was found by Banthia and Dubey (1999, 2000) that the load versus deflection curves obtained in this way were very similar to those obtained using a closed-loop testing machine with proper displacement control.
As shown in Fig. 2.11, a four-point loading is carried on a beam of 350 x 100 x 100 mm, which is the same size with the beam in ASTM C1018. Different with the ASTM C1018, a steel plate is used under the beam before concrete cracks. After the beam is cracked, the steel plate is removed and the cracked beam is reloaded to obtain data to plot a reloading load-deflection curve. An open-loop testing system is used with the rate of displacement of loading head at 0.65 mm/min.
ASTM C1018 uses relative toughness value descriptions, which normalise the energy absorbed up to a specified deflection by the energy absorbed up to the first crack, while the ASTM C1399 describes absolute toughness values, which involve the average energy absorption [i.e. average residual strength (ARS)]. The ARS is calculated using the loads determined at reloading curve (Fig. 2.12) deflections of 0.50, 0.75, 1.00, and 1.25 mm as follows:
Fig. 2.11 Schematic of ASTM C1399 (ASTM 2011a)
Fig. 2.12 Load-deflection curves (ASTM 2011a)
ARS average residual strength, MPa,
PA + PB + PC + PD sum of recorded loads at deflections of 0.50, 0.75, 1.00, and 1.25 mm, N,
L span length, mm,
b average width of beam, mm, and
d average depth of beam, mm.
This standard, however, has some disadvantages (Banthia and Mindess 2004). Firstly, since the test procedure is divided into two parts, the effect of the fibres on the behaviour just after first cracking is lost. Another concern is that in an uncontrolled open-loop test, during initial loading, the deflection is very hard to control and the net deflection requirements are seldom met. This is of particular concern for very high strength matrices. Doubts are often raised as to the capacity of the pre-cracking procedure (with steel plate) to effectively replace proper re-loading test setup. Moreover, the length of the pre-crack obtained is unknown, and is variable for different fibre reinforced concrete systems. This makes comparison between different fibre reinforced concrete beams difficult.