Civil Engineering Reference
In-Depth Information
conservation of damaged or obsolete structures (Van gemert, 2011). in
addition to the increase in adhesion strength, PMCs are also characterized by
an increase in tensile strength, resistance to dynamic loads, impact toughness
and durability, the latter due to the reduction in permeability. on the other
hand, the elastic modulus of concrete is reduced, even though no changes in
the compressive strength are observed. A reduced elastic modulus might be
particularly helpful when PMC is applied as a bridge deck overlay or repair
surface, since it implies a reduction of the stress due to different shrinkage
and thermal coefficients. PMC can also improve the resistance to corrosion,
chemical attack and severe environments (acid rain and freeze-thaw cycles)
(Su et al., 1996; Kardon, 1997; Silva et al., 2001; Barluenga and olivares,
2004; Morlat et al., 2007; Bode and Dimmig-Osburg, 2010).
The properties of the fresh and hardened PMC are affected by several
factors that tend to interact each other: the nature of materials used as latex,
cement and aggregates; the mix proportions (W/C, polymer/cement ratio,
etc.); air content; type and amount of surfactants and anti-foaming; and
curing methods (de S. Almeida and Sichieri, 2006). The influence of any
parameter depends on the type of polymeric backbone and, for the same
backbone chain, on its molecular weight (nair and Thachil, 2011).
16.3.2 Types of polymeric latex
A great variety of latexes, of a single or combinations of polymers, is
available for use in PMC, such as: polyvinyl acetate, copolymers of vinyl
acetate-ethylene, styrene-butadiene, styrene-acrylic and acrylic and
styrene-butadiene rubber (SBr) emulsions (Beeldens et al., 2005; Wong et
al., 2003). As already underlined, the chemical nature of the polymer has a
profound effect on the properties of the relative PMC.
The molecular structure of SBR comprises both the flexible butadiene
chains and the rigid styrene chains; their combination offers to the SBr-
modified mortar and concrete many desirable characteristics, such as good
mechanical properties, water tightness and abrasion resistance, especially
when an appropriate polymer/cement ratio is used (Yang et al., 2009).
SBR-modified lightweight aggregate concrete is an ideal material for precast
components due to its low weight, high strength and high performance under
severe service conditions (Rossignolo and Agnesini, 2004). SBR-cement
composites give better strength than acrylonitrile butadiene rubber (nBr)
(nair and Thachil, 2011). However, the chemical resistance of nBr-cement
composites is slightly better than SBr-ones. Styrene-acrylic ester copolymer
(SAe) latex decreases the elastic modulus and increases the toughness of
cement mortar to a larger extent.
The addition of methylcellulose to cement concrete increases tensile strength
and tensile ductility, and compressive modulus; on the other hand, tensile
