Civil Engineering Reference
In-Depth Information
along the concrete low direction and lubrication properties produced by low
molecular weight polymers (Uchikawa et al., 1995).
Fresh concrete is well known to lose its workability with time. This
phenomenon is called 'slump loss'. The slump loss reduces the beneficial
effect of high workability at the time of placement. When a concrete mix
must be transported for a long time, particularly in hot weather, it should
keep as far as possible its initial slump level to avoid the practice of re-dosing
the concrete with water above that required in the mix design. Factors that
determine slump loss include initial slump value, type and amount of cement,
type, amount and time of addition of the superplasticizer, temperature, mixing
criteria and the presence of other admixtures in the mix.
The chemical composition of the cement, in particular the C
3
A content,
is critical to ensure good compatibility between cement and superplasticizer
(Sugiyama et al., 2003; Nkinamubanzi and Aitcin 2004; Zimmermann et al.,
2009). According to Bundyra-oracz and Kurdowski (2011) the amount of
ettringite arising from the C
3
A is responsible for the slump loss. Bensted
(2002) reported, on the other hand, that all the aluminatic phases present in
cement, i.e. C
3
A + C
4
AF, are responsible for slump loss. The slump loss
increased significantly also with the increase in cement fineness (Aydin et
al., 2009).
Several methods have been adopted to slow the rate of slump loss. The
first is to add the superplasticizer at the point of discharge; however, there
are some practical problems associated with this approach. other methods
include the addition of a higher dosage of superplasticizer or the use of some
type of retarding admixture in the formulation (Uchikawa et al., 1992; Aiad
et al., 2002). However, there are some limits, mainly economic, even in this
approach. Slump retention characteristics are also improved by blending
SnF with lignosulfonates (Heikal and Aiad, 2008). Another common
problem with SMF and SnF admixtures is the excessive set retardation. The
low molecular weight fractions, commonly present in the polymers, cause
excessive retardation by covering reactive sites on the cement surface and
inhibiting reactions.
Pei et al. (2000) synthesized a sodium sulfanilate-phenol-formaldehyde
(SSPF) condensate. The latter has the advantage over the SnF and SMF
admixtures of maintaining around 80% of initial slump values for about
120 min, so it is more suitable to prepare pumping concrete (Fig. 16.1).
The smallest slump-loss rate of SSPF was attributed to the higher number
of functional groups in its molecule, which results in a strong adsorption
to cement particles and an increase in the density of electric charges on the
surfaces of particles.
Compressive strengths of concrete at 7 and 28 days (cement = 300 kg/
cm
3
, slump 8 ± 1 cm) with SSPF were compared with those of concretes
with either SnF or SMF, as shown in Fig. 16.2. The compressive strengths
