Civil Engineering Reference
In-Depth Information
et al., 2008). The carbonation rate is controlled by the ingress of co
2
into
the concrete pore system by diffusion which in turn is influenced by the
relative humidity of concrete. The diffusion of co
2
is actually 10
4
higher in
air than in water (Younsi et al., 2011). For a low relative humidity (rh less
than 50%) the diffusion of co
2
into concrete is high but there is not enough
water in the pores to generate carbonation. For a high hr the diffusion of
co
2
is very low also, reducing the carbonation rate (Papadakis et al., 1991,
1992). That is why the majority of the research on concrete carbonation use
rh values between 50% and 70%.
Previous investigations have shown that concrete carbonation is influenced
by several parameters. For instance in a concrete with a water binder (W/b)
ratio = 0.6 a carbonation depth of 15 mm can be achieved after 15 years,
but if the concrete has a lower W/b = 0.45, the same carbonation depth will
take 100 years to reach (Wiering, 1984). Wasserman et al. (2009) found
that for a given W/b the carbonation was independent of binder content
(160-200 kg/m
3
), and this was explained in terms of two competing processes,
of reduction in penetration and reduction in co
2
binding at lower cement
contents. other authors tried to correlate this mechanism with different
concrete properties. Tam et al. (2008) cited an extensive survey by brown
(1991) which found that carbonation depths correlated well with concrete
quality. She argues that the factors that increase concrete permeability can
increase the carbonation rate. atis (2003) reported the existence of a strong
correlation (
R
2
= 0.9) between carbonation depth and compressive strength
for fly-ash (FA) concrete. Muntean and Böhm (2009) stated that carbonation
is strongly dependent on the degree of porosity, which is 'the path for carbon
dioxide and water to transport in concrete'. however, this is in contradiction
with the findings of Schutter and Audenaert (2004) who found no correlation
between carbonation rate and porosity.
roziere et al. (2009) studied the possible correlations between porosity,
chloride diffusivity, gas permeability and carbonation rate, confirming there
is no correlation between them. Those authors mention that porosity, chloride
diffusivity and gas permeability deal with properties of the porous net of
concrete, but they do not take into account chemical reactivity of binder and
carbonatable content. The same authors confirmed the findings of Assie et
al. (2007) about a strong correlation between carbonation rate and chloride
diffusivity, for concrete mixes with the same initial cao content. These
findings highlight the need of review efforts that try to address the gaps and
the contradictions already detected in this field, so they may help to focus
future investigations.
