Page 233 - New Trends in Eco efficient and Recycled Concrete

Recycled mollusc shells 201

ones are those of Sugiyama (Sugiyama, 2004), which substitutes scallop gravel for
all the aggregates (sand 5 15 mm), reaching extremely low-resistance values of
only 0.30 MPa. This should be taken into account with caution, because their stud-
ies are aimed at the manufacture of low-resistance concrete, and as they use a dis-
continuous grain size curve without sand.
The other studies that use sands from mollusc shells are in concordance, in the
case of Yang et al. (2005), as expected; the values remain very similar at both 7
and 28 days, 6 5 MPa, due to low substitution percentages (the differences between
the different series are only in the content of additive and excess of chloride con-
tent). This shows that chloride does not influence percentages lower than 20% in
compressive strength at 7 and 28 days. In the same way, with the work of Yang
et al. (2010), the variations are barely noticeable, replacing oyster sand in percen-
tages of up to 20% in both ages. Nguyen’s work with the substitution of scallop
sand shows that for a 60% substitution for both series, 28-day resistance drops are
approximately 20% and 30%. The same author obtained 28-day compressive
strength drops, with 40% substitutions for coarse aggregate, equal to 36%, 8% and
6%, the most important being the one which does not include sand (,4 mm). This
author also reports that the shape of the crepidula in larger sizes could produce
excessive holes and honeycombing in the mass of the concrete, thus, affecting its
mechanical properties. When the size distribution of the crepidula is reduced to
sand, these hollows are reduced and the decrease in resistance is lower.

8.4.4 Tensile strength
The analysis of the results of the studies regarding the variation of the tensile
strength at 7 days are shown in Fig. 8.7. The general trend is similar and concordant
with the variation of the compression resistance. Again it is shown how the cockle
filler (Othman et al., 2013) is an inert material and has no binding power, with
which the falls of tensile strength are the most notorious (40%). The variations of
the Yang et al. (2005) studies with oyster sand and the Nguyen studies (Nguyen
et al, 2017) with scallop gravel produce differences which are barely perceptible,
maintaining resistance values of 6 0.3 and 21 MPa, respectively. The use of crepi-
dula shells produces more noticeable drops for the same mix composition when par-
ticles are larger than 4 mm; in this case, the drops are higher than 30% with respect
to its reference.

8.4.5 Concrete microstructure

Fig. 8.8 shows a section of a mussel concrete-tested specimen from the work of
Martı ´nez-Garcı ´a et al. (2017) with mussel shells. There can be seen most of the
effects of mussel shells aggregates in concrete microstructure. Firstly, a specific
trend in the orientation of the shell aggregates inside the concrete matrix has been
detected, as they tend to place themselves perpendicular to the vibrating direction.
Therefore, most of the coarse mussel shell aggregate had preferential horizontal