Alkali Resistance of the 100% Recycled and Virgin PP Fibres

Alkali resistance test was conducted to study possible degradation of the 100% recycled and virgin PP fibres in alkaline environment. As shown in Fig. 4.1, the PP fibres were immersed into Lawrence solution (0.48 g/l Ca(OH)2 + 3.45 g/l KOH + 0.88 g/l NaOH, pH = 12.9), which is considered to simulate pore water composition of a fully hydrated cement paste (Silva et al. 2005). The PP fibres were also immersed in three other alkaline solutions with pH value ranging from 12.3 to 13.5 to study degradation of the fibres in different pH ranges and types. Three © 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_4

Test setup for alkali resistance test

Fig. 4.1 Test setup for alkali resistance test

alkaline solutions used were Ca(OH)2 saturated solution (pH = 12.3), 0.28 mol/l NaOH solution (pH = 13.45), and 0.068 mol/l KOH solution (pH = 12.83).

The PP fibres were immersed into these solutions for 28 days at ambient temperature. The alkaline environment is strong mainly in the fresh concrete when mixing the fibres. When concrete is cured and hardens, the movement of OH- becomes minimal due to absence of water. Based on most of the standards, such as AS 1012.9:2014 (AS 2014c), BS EN 14651-2005+A1-2007 (EN 2005) and ASTM C1550-12 (ASTM 2012), the hardened concrete is tested on the 28 days of curing. Therefore, the 28 days is chosen in this research to simulate the period of fresh and curing concrete. The tensile strength and Young’s modulus of the fibre were measured before and after immersion, according to ASTM D3822-07 (ASTM 2007c). Thirty specimens were tested for each type of fibre and solution.

As can be seen from Fig. 4.2, all the curves representing fibre immersion in the alkaline solutions nearly overlap with the curve of fibre without immersion, indicating that the 100% recycled PP fibre has very good alkali resistance in the various alkaline environments. However, there still were some minor changes on the mechanical properties after immersion. As shown in Table 4.1, NaOH solution slightly embrittled the recycled PP, thus decreasing the tensile strength and

Typical stress-strain curves of the 100% recycled PP fibre before and after immersing in the alkaline solutions for 28 days

Fig. 4.2 Typical stress-strain curves of the 100% recycled PP fibre before and after immersing in the alkaline solutions for 28 days

Table 4.1 Mechanical properties of the 100% recycled PP fibre before and after immersing in the alkaline solutions for 28 days

Tensile strength (MPa)

Young’s Modulus (MPa)

Elongation at break (%)

Average

Standard

deviation

Average

Standard

deviation

Average

Standard

deviation

Without

immersion

284.1

33.7

4582

268.9

6.2

2.3

Lawrence

solution

284.7

22.7

4592

153.4

6.2

0.9

Ca(OH)2

solution

273.4

34.7

4482

380.2

6.1

1.7

KOH solution

261.9

17.4

4516

114.5

5.8

1.0

NaOH solution

273.1

14.2

4965

214.2

5.5

1.1

increasing Young’s modulus of the fibre, due to the its high pH value. The KOH solution slightly decreased the tensile strength of the fibre. It is noteworthy that there was nearly no change after immersing the fibre in the Lawrence solution which simulated a fully hydrated cement paste. Overall, the recycled PP fibre showed good alkali resistance in all the alkaline environments tested.

As a comparison, the degradation behaviour of the virgin PP fibre was also studied. As shown in Fig. 4.3 and Table 4.2, the virgin PP fibre has a minimum

Typical stress-strain curves of the virgin PP fibre before and after immersing in the alkaline solutions for 28 days

Fig. 4.3 Typical stress-strain curves of the virgin PP fibre before and after immersing in the alkaline solutions for 28 days

Table 4.2 Mechanical properties of the virgin PP fibre before and after immersing in the alkaline solutions for 28 days

Tensile strength (MPa)

Young’s Modulus (MPa)

Elongation at break (%)

Average

Standard

deviation

Average

Standard

deviation

Average

Standard

deviation

Without

immersion

356.4

14.6

3129

589

12.6

0.5

Lawrence

solution

372.3

12.5

3377

267

12.1

1.0

Ca(OH)2

solution

358.6

27.3

3502

355

11.2

1.2

KOH solution

377.2

35.4

3318

313

12.5

1.4

NaOH

solution

356.4

22.2

3295

612

11.9

0.6

degradation in the four kinds of alkaline environments, which is consistent with the study of Brown et al. (2002) and EPC (2012).

It is worth to note that the mechanical properties of 100% recycled and virgin PP fibres used in this chapter and following chapters are much lower than those of 100% recycled and virgin PP fibres produced in the Chap. 3. The fibres produced in the Chap. 3 have smooth surface. Chapter 3 studied the molecular orientation, crystal structure and crystallinity of PP fibres, which are not affected by the surface pattern of PP fibres. Moreover, the WAXS test can be conducted only on the smooth surface. When using fibres to reinforce concrete, surface indentation on the fibres is needed to improve bonding with concrete. However, the surface indentation highly decreased mechanical properties of the PP fibres. In this Chapter and following Chapters, the fibres used for reinforcing concrete are all indented, thus having lower mechnical properties.

 
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