Page 92 - New Trends in Eco efficient and Recycled Concrete

66 New Trends in Eco-efficient and Recycled Concrete

3.4 Mechanical properties of recycled plastic concrete

3.4.1 Compressive properties
The compressive behaviour of recycled plastic concrete is influenced by different
parameters such as the water-to-cement ratio, RPA%, RPF% and type, shape and
geometry of the recycled plastics. Fig. 3.4A and B shows the variation of the
28-day compressive strength of concrete containing plastic coarse and fine aggre-
gates with RPA%, respectively. As can be seen in the figures, the compressive
strength of PA concrete decreases with an increase in RPA%. This can be attributed
to the higher elastic modulus of NAs compared to that of PAs, low bonding strength
between the cement paste and surface of PA and higher air content and porosity of
PA concrete (Ismail and Al-Hashmi, 2008). As can be seen in Fig. 3.4A and B, the
incorporation of PAs with lower elastic modulus (i.e., EVA) leads to a more signifi-
cant decrease in the compressive strength of concrete compared to that of PAs with
higher elastic modulus (i.e., PET).
Fig. 3.5 shows the variation of the 28-day compressive strength of PF concrete
with RPF%. A few of the existing studies showed that the compressive strength of
concrete increased by the addition of PF into the mixture (Toutanji, 1999; Yao
et al., 2003; Song et al., 2005; Han et al., 2005; Suji et al., 2007; Hsie et al., 2008;
Khadakbhavi et al., 2010; de Oliveira and Castro-Gomes, 2011; Fraternali et al.,
2011; Kakooei et al., 2012), which can be explained by the control of the initiation
and propagation of micro-cracks to macro-cracks by PFs in the concrete. As can be
seen in Fig. 3.5, the use of PFs with a higher tensile strength (i.e., PP fibre) leads to
a more significant increase in the compressive strength of concrete compared to
that with a lower tensile strength (i.e., PET fibre). However, as can be seen in
Fig. 3.5, some of the existing studies showed that an increase in RPF% led to a
decrease in the compressive strength of concrete (Meddah and Bencheikh, 2009;
Kim et al., 2010; Pelisser et al., 2012; Fraternali et al., 2014), which can be because
of the poor dispersion of fibres in concrete when a high fibre content is used, result-
ing in a poor workability and incomplete consolidation. It can also be seen in
Fig. 3.5 that, in a few studies (Kayali et al., 1999; 2003; Wang et al., 2000;
Bagherzadeh et al., 2011; Karahan and Atis, 2011; Ramadevi and Manju, 2012;
Nibudey et al., 2013), the compressive strength of PF concrete increased up to an
optimum fibre content (i.e., 0.5% for PP fibre and 1% for PET fibre) and then
decreased with further increase in the fibre content. Khadakbhavi et al. (2010)
investigated the effect of PF geometry on the compressive strength of concrete and
showed that concrete containing 0.6% HDPE fibres with aspect ratios (as the ratio
of the fibre length to diameter) of 20, 40, 60 and 80 respectively exhibited 5%, 8%,
14% and 3% higher compressive strength, whereas the same concrete containing
HDPE fibres with an aspect ratio of 100 exhibited 6% lower compressive strength
compared to that of the conventional concrete.
Density, elastic modulus of main constituents (i.e., fine and coarse aggregates)
and properties of the transition zone are the most influential parameters affecting
the elastic modulus of concrete (Mehta and Monteiro, 2014). Fig. 3.6 shows the