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Flexural Strength and Toughness from Round Determinate Panel Tests

RDPT is an essential tool for assessment of post-cracking performance in fibre reinforced shotcrete within the underground construction and mining industries (Parmentier et al. 2008). The specimens can show a very low within-batch coefficient of variation in performance than any other test method for fibre reinforced concrete, mainly because of the large crack length experienced during testing (Bernard 2002). Figure 5.8 and Table 5.3 show the RDPT results of PP fibre reinforced concrete panels. As can be seen from Fig. 5.8, all of the curves attained a peak load at around 30 kN, followed by a sudden drop to 5-10 kN. The curves then kept flat or slightly increased until deflection at 10 mm, before a stable downward trend to about 1 kN.

Energy absorption and load curves from Round Determinate Panel Tests

Fig. 5.8 Energy absorption and load curves from Round Determinate Panel Tests

Table 5.3 Energy absorption (Joules) of the round determinate panel tests

Deflection

5 mm

10 mm

20 mm

30 mm

40 mm

Virgin PP fibre (Line)

44

74

126

166

198

Recycled PP fibre (Line)

42

69

108

133

149

50:50 virgin-recycled PP fibre (Line)

Median

56

99

160

201

225

Standard

deviation

10.0

20.9

36.8

47.7

54.2

Recycled PP fibre (Diamond)

Median

48

80

141

184

213

Standard

deviation

3.7

6.5

12.3

17.1

20.5

5:95 HDPE-recycled PP fibre (Diamond)

Median

47

83

145

187

217

Standard

deviation

1.7

3.7

8.1

10.9

13.5

From Table 5.3 it can be seen that the diamond-indent PP fibres, including recycled PP fibre (Diamond) and 5:95 HDPE-recycled PP fibre (Diamond), showed high post-cracking reinforcement, due to their high Young’s modulus and good bonding with the concrete. The 50:50 virgin-recycled PP fibre (Line) also showed high energy absorption due to its high Young’s modulus and tensile strength. These three kinds of fibres all produced better reinforcement than that of the virgin PP fibre (Line). The recycled PP fibre (Line) produced the lowest reinforcement due to its low tensile strength, Young’s modulus, and poor bonding.

Figure 5.9 shows fracture surfaces of the PP fibre reinforced concrete panels. As can be seen, for line-indent PP fibres (Fig. 5.9a), including the virgin PP fibre (Line) and 50:50 virgin-recycled PP fibre (Line), nearly all the fibres were pulled out instead of breaking, owing to their high tensile strength and poor bonding with the concrete. For the diamond-indent PP fibres, namely, recycled PP fibre (Diamond), and 5:95 HDPE-recycled PP fibre (Diamond), more fibres were broken than pulled out (Fig. 5.9b). This shows that diamond indents are more effective than line indent to improve bonding of fibres with concrete. The broken fibres fully exploited their tensile capacity, thus producing better reinforcement than that of the virgin PP fibre (Line).

Therefore, the reinforcement of the fibres in concrete is determined by tensile strength, Young’s modulus and surface indents of fibres. The diamond-indent PP fibres, including recycled PP fibre (Diamond), and 5:95 HDPE-recycled PP fibre (Diamond), had a good balance of tensile strength, Young’s modulus and surface indents. In the standard mix design of 40 MPa concrete for footpath applications, the diamond-indent PP fibres produced more brilliant post-cracking reinforcement than the commonly used virgin PP fibre (Line), proving their industrial feasibility.

Fracture faces of the PP fibres reinforced concrete round determinate panels

Fig. 5.9 Fracture faces of the PP fibres reinforced concrete round determinate panels: a line-indent PP fibre, and b diamond-indent PP fibre

 
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