Page 186 - New Trends in Eco efficient and Recycled Concrete

158 New Trends in Eco-efficient and Recycled Concrete

GC as a sand replacement in concrete blocks had a negative effect on the compres-
sive strength, which was related to the decrease in bond strength between the glass
particles and the cement paste. Also, a major concern for using recycled glass in
the concrete blocks is the potential risk of expansion due to ASR. In this regard,
Lam et al. (2007) proved that the addition of supplementary cementitious materials,
like pulverised fuel ash or metakaolin, were able to suppress ASR in the concrete
glass blocks.
In general, there are two main casting methods for the production of waste glass
concrete, one is using a conventional wet-mixed method (Park et al., 2004; Topc¸u
and Canbaz, 2004; Wang and Huang, 2010) and the other is by using a dry-mixed
method (Ganjian et al., 2015; Zhao et al., 2011; Zhan and Poon, 2015). In the dry-
mixed method, only a small amount of water is added to the concrete mixture to
provide sufficient cohesiveness, but no workability. Then, the mixture is cast into a
steel mould and compacted by a large compressive force immediately followed by
demoulding. The potential detrimental effect of using glass material due to ASR in
cementitious materials still a real concern. Lee et al. (2011) found that concrete
blocks produced by the dry-mixed casting method contained relatively higher
porosity, which enables the blocks to accommodate the volume increase due the
formation of ASR gel without causing detrimental expansion and cracking. The
ASR expansion of the dry-mixed concrete blocks containing 100% glass aggregates
was found to be 44% lower compared to concrete blocks produced by using the tra-
ditional wet-mixed method. Furthermore, the dry-mixed casting method has practi-
cal advantages as it is a more cost-efficient manufacturing operation with a shorter
production cycle. Given these benefits, the use of GC as aggregates to produce dry-
mixed concrete paving blocks has been successfully commercialised in Hong Kong
as “Eco-blocks” (Ling et al., 2013). Sites paved with Eco-blocks in Hong Kong are
illustrated in Fig. 6.4. Currently, up to 25% of the total aggregates is replaced by
waste GC for the production of Eco-blocks. It is, therefore, worth exploring the fea-
sibility of increasing the content of GC in paving blocks. Maximising the applica-
tion of waste glass in such paving blocks may help expand the potential outlet for
recycling waste glass containers in Hong Kong.
Fig. 6.5A shows the influence of different GC contents on the compressive
strength, density and water absorption of paving blocks. It is observed that the
values of compressive strength were relatively stable and were above 60 MPa as
the replacement of aggregates by GC was increased from 0% to 55%. Meanwhile,
it is worthwhile to point out that, regardless of the GC content, the compressive
strength can meet the requirements of the Hong Kong General Specifications (i.e.,
30 and 45 MPa for footpaths and carriageways, respectively). This provides a strong
support for increasing the use of GC for the production of concrete paving blocks.
To address the ASR problem with increased an amount of GC in the concrete
blocks, 10% of cement was replaced by milled GP (,50 μm) in concrete paving
blocks. From the mechanical test results, the replacement of 10% cement by GP did
not lead to a reduction in compressive strength as the strength of paving blocks
incorporating GP satisfy the minimum strength limit.