Civil Engineering Reference
In-Depth Information
the cement particles. in conclusion, a shorter mean backbone chain length
gave more fluidity at the same dosage, along with a slightly longer setting
time; while it did not affect the loss of workability with time. The concrete
produced with this kind of superplasticizer, even displaying a slow initial
development of mechanical resistance due to the delay of the cement
hydration as the effect of the high number of adsorbed molecules on the
cement surface (Hamada et al., 2009), is able to achieve the typical value of
strength in a few (two to three) days. This concrete mix can be transported for
long time, even in hot weather, keeping its initial slump level, avoiding the
practice of re-dosing the concrete with water (Collepardi, 2005). However,
its applications in preparing flowing concrete and high-performance concrete
are still limited due to economic reasons. in other words, SnF is still the
main superplasticizer used because of its relatively low cost.
16.2.3 Influence on durability
The influence of a superplasticizer on the pore structure of hardened
cement paste mainly depends on its chemical nature. When polycarboxylate
superplasticizer types are added to concrete, the amount of pores greater than
0.1 mm diameter decreases, while the amount of those < 0.1 mm increases
(Sakai et al., 2006). it is commonly known that capillary pores of diameters >
0.1 mm adversely influence paste tightness and permeability (Deja, 2002).
The general effect of SnF and polycarboxylate-type superplasticizers
is to retard the initial cracking time of mortars and decrease their cracking
sensitivity (Ma et al., 2007). The maximum crack width of mortars with
polycarboxylate-type is reduced with respect to reference ones.
16.2.4 Applications
A brief overview of the application fields of superplasticizers is given. The
most important innovative products that can be manufactured by the use of
superplasticizers, due to the remarkable reduction of W/C ratio, are: high-
performance concrete (HPC); ultra-high-performance concrete (UHPC);
self-compacting concrete (SCC); and reactive powder concrete (rPC).
The term high-performance concrete refers to cement mixtures with a
water-binder (W/B) ratio as low as 0.30-0.35, so that a 28-day compressive
strength as high as 70-100 MPa and even 1-day compressive strength as high
as 45-55 MPa can be obtained (Ran et al., 2009b). UHPCs exhibit exceptional
tensile and compressive strengths and high durability. They possess a very
low W/C ratio (< 0.25). over the last years UHPCs have become a vanguard
product in industrial and structural applications thanks to outstanding properties,
such as compressive strength of 150-200 MPa, tensile strength of 8-15 MPa,
with significant remaining post-cracking bearing capacity, and remarkable
