Compressive Strength of Concrete

Compressive strength tests were performed on the PP fibre reinforced concrete specimens according to AS 1012.9:2014 (AS 2014c). Fibre concrete cylinders of 100 mm diameter by 200 mm height were tested at an age of 28 days. The diameter, height and mass of the cylinders were recorded before testing. The concrete specimen was placed in the test rig, ensuring the rubber cap had a snug fit, and the safety guard was closed. The axial load was incrementally applied at a rate of 20 ± 2 MPa compressive stress per minute until specimen failure in the universal testing machine with a maximum load capacity of 2000 kN. The compressive strength results for concrete reinforced with each fibre type were based on an average value of four specimens.

The compressive strength (fC) is calculated by dividing the ultimate load at failure (F, kN) by the cross sectional area of the specimen (A, mm2), and then converted into Mega Pascals (MPa) as seen in Eq. 4.1.

Residual Flexural Tensile Strength with CMOD

In this study post-cracking behaviour of the recycled PP fibre reinforced concrete was quantified and compared with that of virgin PP fibres through the Crack Mouth Opening Displacement (CMOD), according to BS EN 14651-2005+A1-2007 (EN 2005). The CMOD test demonstrates the relationship between residual flexural strength and cracking behaviour, reflecting how the fibres control the cracks. The CMOD test can clearly assess the ability of the fibres to redistribute the stresses and bridge the cracks formed (Buratti et al. 2011).

As shown in Fig. 2.15, the flexural beams were of size of 150 mm x 150 mm x 600 mm. A notch of 2 mm wide and 25 mm depth were cut at mid-span of each beam. The notched beams were loaded using a 500 kN hydraulic testing machine on a three-point loading setup. The CMOD was measured using two clip gauges installed at the centre of the notch and averaged CMOD values were adopted. The clip gauges attached to knife edges glued to the bottom of the beam were connected to a data acquisition system and a Linear Variable Displacement Transformer (LVDT), as shown in Fig. 4.4. The tests were displacement controlled to achieve a constant rate of CMOD at 0.05 mm/min. The

Schematic diagram of the CMOD test

Fig. 4.4 Schematic diagram of the CMOD test

tests were carried out at K&H Geotechnical Services Pty Ltd., Australia. Three samples for each fibre and concrete type were tested. One plain concrete beam was tested as a control specimen.

The residual flexural tensile strength fR,j is given by the Eq. 4.2:

where, fR,j, is the residual flexural tensile strength corresponding with CMOD = CMODj (j = 1, 2, 3, 4), in Newton per square millimetre; Fj, is the load corresponding with CMOD = CMODj (j = 1, 2, 3, 4), in Newton; l, is the span length, in millimetres; b, is the width of the specimen, in millimetres; hsp, is the distance between the tip of the notch and the top of the specimen, in millimetres.

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