Post-cracking Performance of Concrete Reinforced by Various Newly Developed Recycled PP Fibres

In the previous chapter (Chap. 4), the reinforcement of 100% recycled PP fibre with line indentation was studied to ascertain the feasibility of using 100% recycled PP fibre in concrete. In this chapter, various newly developed recycled PP fibres described in Chap. 3 were used to reinforce concrete, and their performance in concrete was determined.

Target compressive strength of concrete used in the concrete footpaths, based on industrial practice, normally ranges from 25 to 40 MPa, and the standard fibre dosage is 4 kg/m3. In the previous chapter (Chap. 4), 4 kg/m3 fibre reinforced 25 MPa concrete for the application of concrete footpaths has been assessed. In order to evaluate fibre’s reinforcement in other concrete-footpath applications, 4 kg/m3 newly developed recycled PP fibres were used to reinforce standard mix design of 40 MPa concrete in this chapter.

The effects of tensile strength and Young’s modulus of these recycled PP fibres on their reinforcing effects in the concrete were studied. In order to enhance the fibre-concrete bonding, different types of surface indentations (line and diamond) were made on fibre surface, as shown in Fig. 5.1, and their ability to bond with concrete were also observed in the RDPT and CMOD tests. All the fibres have 0.9-1 mm2 of cross section, 47 mm of length and 0.1-0.2 mm of indentation depth. Table 5.1 lists these newly developed recycled PP fibres. They were virgin PP fibre (Line), recycled PP fibre (Line), 50:50 virgin-recycled PP fibre (Line), recycled PP fibre (Diamond), and 5:95 HDPE-recycled PP fibre (Diamond). Their details can be seen in Table 5.1.

As can be seen in Fig. 5.2, all the fibres show a brittle mode of failure, with a short elastic period of steep slope and a regime of sharply rising stress until fracturing occurred at strains between 5% and 14%. Thirty specimens for each type of fibre were tested. Table 5.2 presents the averages of the tensile strength, Young’s modulus and elongation at break of these fibres and their standard deviation. The virgin PP fibre (Line) showed high tensile strength (359.2 MPa) and elongation at © Springer Nature Singapore Pte Ltd. 2017

S. Yin, Development of Recycled Polypropylene Plastic Fibres

to Reinforce Concrete, Springer Theses, DOI 10.1007/978-981-10-3719-1_5

a Line-indent PP fibre, and b diamond-indent PP fibre

Fig. 5.1 a Line-indent PP fibre, and b diamond-indent PP fibre

Table 5.1 Details of PP fibres

Specimen

Indent

type

Details

Virgin PP fibre (Line)

Line

100% virgin PP

Recycled PP fibre (Line)

Line

100% recycled PP

50:50 virgin-recycled PP fibre (Line)

Line

Mixture of 50% of virgin PP and 50% recycled PP

Recycled PP fibre (Diamond)

Diamond

100% recycled PP

5:95 HDPE-recycled PP fibre (Diamond)

Diamond

Mixture of 5% of HDPE and 95% recycled PP

break (14%), but lowest Young’s modulus (only 3801 MPa). The recycled PP fibre (Line) showed much lower tensile strength (284.7 MPa), but much higher Young’s modulus (6272 MPa) than those of the virgin PP fibre (Line). When 50% of the virgin PP was mixed with 50% of the recycled PP, the Young’s modulus of the 50:50 virgin-recycled PP fibre (Line) increased to 7513 MPa, and the tensile strength was even higher than the virgin PP fibre at 392.9 MPa. When the surface indent of recycled PP fibre was changed to diamond indents, Young’s modulus in the recycled PP fibre (Diamond) was considerably improved to 8727 MPa, with no

Typical stress-strain curves of PP fibres

Fig. 5.2 Typical stress-strain curves of PP fibres

Table 5.2 Mechanical properties of PP fibres

PP fibre

Tensile stress (MPa)

Young’s modulus (MPa)

Elongation at break

(%)

Average

Standard

deviation

Average

Standard

deviation

Average

Standard

deviation

Virgin PP fibre (Line)

359.2

30.6

3801

564

14.0

2.8

Recycled PP fibre (Line)

284.7

21.0

6272

1661

7.4

2.2

50:50 virgin-recycled PP fibre (Line)

392.9

18.2

7513

1460

12.5

1.4

Recycled PP fibre (Diamond)

279.4

26.5

8727

1337

5.1

1.1

5:95 HDPE-recycled PP fibre (Diamond)

287.0

23.2

7294

1649

6.1

1.3

obvious change in the tensile strength, compared to the recycled PP fibre (Line). This indicates that these two patterns of indents do not affect the tensile strength, but considerably affect Young’s modulus of the fibres. HDPE brought ductility to the fibre, hence, 5:95 HDPE-recycled PP fibre (Diamond) showed lower Young’s modulus and higher elongation at break than those of the recycled PP fibre (Diamond).

Compressive strength of the PP fibre reinforced concrete cylinders

Fig. 5.3 Compressive strength of the PP fibre reinforced concrete cylinders

 
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