Civil Engineering Reference
In-Depth Information
∑ Polymer-modified concrete (PMC) is prepared by mixing a small amount
of polymer (up to 25% wt) in the fresh concrete mixture. The resulting
composite has an excellent adhesion to steel reinforcement and to old
concrete and good durability.
∑ in polymer-impregnated concrete (PiC), the capillary pores of hydrated
PCC are filled with a low-viscosity monomer that subsequently
polymerizes. As a consequence, the permeability is lowered, while the
compressive and tensile strengths are enhanced.
∑ Finally, in polymer concrete (PC) the polymers are used in total
replacement of the hydraulic cement that, consequently, contains no
Portland cement. Although the cost of the monomer can be quite high,
PC can be useful for maintenance and repairs of highway and aircraft
runways and compares well on a strength-to-cost basis.
Polymers are also used to repair or strengthen damaged concrete, as adhesives,
or to protect it, as coatings.
16.2 Water-reducing admixtures for Portland
cement concrete
Corrosion of reinforcing steel, frost and physico-chemical effects in
aggressive environments are the major causes of concrete deterioration.
There is a general agreement that the permeability of concrete, rather than
normal variations in the composition of Portland cement, is the key to all
durability problems. Among the recent advancements, most noteworthy is
the development of superplasticized concrete mixtures which give very high
fluidity at relatively low water contents and, consequently, lower porosity.
Hardened concretes possessing low porosity are generally characterized by
high durability (Mehta, 1991).
The addition of some water-soluble organic polymers dramatically improves
workability of fresh concrete batches, giving at the same time strengths as
high as those of non-admixtured concretes with the same water/cement (W/C)
ratio, and offers many advantages over casting of concrete.
Water-reducing admixtures are classified broadly into two categories:
normal water reducers (Wr), also called 'plasticizers', and high-range
water reducers (HrWr), called 'superplasticizers'. While the normal Wrs
(generally based on lignosulfonates, obtained as a waste liquor during the
process of paper-making pulp from wood) can reduce the water demand by
5-10%, the HrWrs (based on polymers obtained from chemical synthesis)
can cause a reduction of 15-40%. In general terms, a decrease of workability
from 15 cm slump to 7-8 cm is achievable by producing a concrete with
superplasticizers (Papayianni et al., 2005).
These chemical admixtures, always water-soluble, can be used in several
ways to influence the properties of a concrete mix:
