Civil Engineering Reference
In-Depth Information
to be less dependent on the cement type (Coppola et al., 2010). Used at low
dosage, polycarboxylate admixtures still can reduce water as much as high
dosage of conventional admixtures based on lignosulfonates.
Tanaka et al. (1996) studied the effect of a partially cross-linked
superplasticizer based on copolymer of acrylic acid and polyethylene glycol
mono-alkyl ether (CLAP) on the slump-loss of concrete mixture. According
to the authors, the cross-linked polymer is hydrolyzed by the alkaline water
phase of the cement paste and, than, converted into a polycarboxylate-based
polymer. The negative carboxylic groups, due to the alkaline hydrolysis,
would be adsorbed on cement surface of cement particles and then would
be responsible for the dispersion of cement particles and the fluidizing
action of the admixture. The low slump-loss effect of this superplasticizer
was attributed to the increasing number of the extension side chains of the
acrylic polymer which would prolong the dispersion of hydrated cement
particles through a steric hindrance effect.
other polycarboxylic acid-based copolymers with block and graft groups
of polyethylene oxide (Peo) chains were synthesized by Li et al. (2005). The
effects of the different PEO chains on the fluidity and adsorption in cement
paste and on performance of the copolymer in concrete were studied. The
properties of the copolymer were found to be affected by the length and density
of Peo graft and block chains; copolymers with some block Peo chains
at a certain length had good performances in the water-reducing capability
and fluid-retaining ability. A PEO-based superplasticizer with much longer
side chains of ethylene oxide (eo) - 130 moles of eo instead of 10-25
moles as in traditional acrylic polymer-based superplasticizer - produces a
lower adsorption speed and reduces the typical retarding effect related to the
early adsorption (Hamada et al., 2003; Qiu et al., 2011). Longer Peo side
chains gave more fluidity at the same dosage, along with a shorter setting
time. Concrete cured at low temperature when added with a polycarboxlate-
based superplasticizer with longer graft chain possess good early strength
development at and better durability (Dan et al., 2001). Polycarboxylate-type
superplasticizers find, therefore, large applications in the precast industry as
an alternative of the steam curing, for quick turnaround of forms and casting
beds. With the use of these new generation superplasticizers it is possible,
in fact, to overcome the negative effects of steam curing on strength loss,
permeability, shrinkage, creep and frost resistance (Khurana and Torresan,
1997).
A modified PE-based superplasticizer, where a great number of carboxylic
groups are replaced by a slump-loss controlling agent, is able to achieve a
still higher slump retention with minimal setting retardation. Subsequently
to the hydrolytic effect related with the oH
-
presence in the aqueous phase
of the cement paste, the slump can still increase by prolonging the mixing
time, owing to the increasing adsorption of the polymer on the surface of
