Civil Engineering Reference
In-Depth Information
fracture energy of 20-30 kJ/m
2
. The superior performance of these products
have been achieved by tailoring their microstructures, that is by maximizing
the packing density with very fine minerals, quartz powder and silica fume,
and by enhancing the matrix toughness with an optimal fiber reinforcement
(Sorelli et al., 2008; Plank et al., 2009; Schröfl et al., 2010).
SCC is considered to be a concrete that, once poured, will compact under
its self-weight with no requirement for vibration. Moreover, it remains
cohesive enough to be handled without segregation or bleeding (Selvamony
et al., 2010). These characteristics allow SCC to completely fill formwork
even when congested with reinforcement. A more complete description of
SCC was given in Chapter 9.
By replacing a part of Portland cement with fly ash, slag or pozzolans
in the concrete, it is possible to obtain the required features of the concrete
mixture and, at the same time, contribute to the environment protection by
reducing the consumption of raw materials, lowering the energy consumption
and reducing the emissions of greenhouse gases (Malhotra, 2006; Damtoft
et al., 2008; Mehta and Walters, 2008).
Uzal et al. (2007) indicated that concretes with high-volume natural
pozzolan, in the presence of high range water reducers, are suitable for
structural concrete applications with 12-14 MPa and 29-38 MPa compressive
strengths at 3 and 28 days, respectively. SCCs made with a high volume
of mineral admixtures achieve good workability, high long-term strength,
good de-icing salt surface scaling resistance, low sulfate expansion and
very low chloride ion penetrability (Swamy, 1997; Uysal and Summer,
2011).
16.2.5 Underwater concreting
Placing concrete under water can be particularly challenging because of the
potential for washout of the cement and fines from the fresh mixture, with
relative reduction in strength and integrity of the in-place concrete. Although
several techniques have been used successfully to place concrete under water,
other situations, requiring enhanced cohesiveness of the concrete mixture,
necessitate the use of an anti-washout or viscosity-modifying admixtures
(AWA). As a long-chain polymer 'bridge', AWA enables mixture to form a
stable network structure with a flexible space, improves the cohesion of fresh
concrete, minimizes the dispersion and segregation of fresh concrete and, in
presence of superplasticizers, makes it possible to form high-quality, uniform,
self-compacting concrete. Hu et al. (2011) compared the compatibilities
between AWA and different superplasticizer, finding that compatibility of
AWA with PC is better than that with SnF.
