Macro Plastic Fibre Reinforced Concrete

Fresh Concrete Properties Slump

Workability of fresh concrete can be determined through a slump test (AS1012.3.9 1993). Table 2.3 shows slump test results of macro plastic fibre reinforced concrete. The results indicate addition of macro plastic fibres decreases slump, thus decreasing workability of fresh concrete. This is due to the fact that the addition of fibres can form a network structure in the concrete matrix, thus restraining mixture from segregation and flow. Moreover, due to high content and large surface area of the fibres, the fibres can easily absorb cement paste to wrap around, hence increasing viscosity of the concrete mixture (Soroushian et al. 2003). Mazaheripour et al. (2011) made following two suggestions to improve the workability of fibre reinforced concrete: (a) to limit the volumetric content of macro plastic fibres to a range of 0.1-1% and (b) to add more water. However, addition of water will

Table 2.3 Properties of macro plastic fibre reinforced concrete

Macro plastic fibre

Fibre dimension

Fibre

volumetric

content

(%)

Slump

(mm)

Compressive

strength

(MPa)

Splitting

tensile

strength

(MPa)

Macro PP fibre, wavelength shape (Choi and Yuan 2005)

0.9 mm in diameter, 50 mm in length

0

102

35.0

2.2

1

38

35.4

3.2

1.5

6.5

30.7

3.2

PP fibre,

620 MPa tensile strength and 9.5 GPa Young’s modulus (Hasan etal. 2011)

40 x 1.4 x 0.11 mm

0

N/A

38.9

3.6

0.33

N/A

40.5

3.9

0.42

N/A

41.4

4.1

0.51

N/A

41.6

4.1

negatively affect concrete strength; hence plasticiser or water reducing admixtures are often used in fibre reinforced concrete to improve workability without increasing water content (Hasan et al. 2011).

Plastic Shrinkage

Plastic shrinkage cracking is caused by moisture loss after casting (Banthia and Gupta 2006). Generally, if the moisture evaporation rate exceeds 0.5 kg/m2/hr, it causes negative capillary pressure inside the concrete, resulting in internal strain (Uno 1998). Plastic shrinkage can cause cracks during the initial stages, when the concrete has not yet developed adequate strength (Sanjuan and Moragues 1997). Kim et al. (2008) reported that although the macro plastic fibres do not affect the total moisture loss or rate of moisture loss, they still can effectively control the plastic shrinkage cracking by improving integrity of the fresh concrete. They also found that once the fraction of fibre volume exceeds 0.5%, a sufficient number of fibres are involved in controlling plastic shrinkage cracking, so the fibre geometry had no further effect. Najm and Balaguru (2002) studied the effects of fibre aspect ratio on the plastic shrinkage crack areas. They found that longer fibres (aspect ratio with length/width = 167) were extremely efficient and provided a crack-free surface at a fibre dosage of 9 kg/m3, while shorter fibre (aspect ratio with length/width = 67) could eliminate 94% cracking at a dosage of 18 kg/m3.

 
Source
< Prev   CONTENTS   Source   Next >