Abstract:
Polymer cement concretes have high tensile strength, good ductile
behavior and high impact resistance capability due to the formation of a three dimensional
polymer network through the hardened cementitious matrices. Because of the void-filling
effect of this network and its bridging across cracks, the porosity decreases
and pore radius are refined. Furthermore, the transition zone may be improved
due to the adhesion of a polymer. A styrene butadiene rubber emulsion is incorporated
to improve the ductile behavior and flexural strength
of steel fibre reinforced cement concretes (SFC). Silica fume and
fly ash are also used to enhance the densification of cementitious matrix. The mechanical
properties, microstructure, porosity and pore size distribution of polymer modified
steel fibre reinforced concrete are studied.
Steel fibre reinforced concretes are structural materials that are
gaining importance quite rapidly due to the increasing demand of superior structural
properties. These composites exhibit attractive tensile and compressive strengths,
low drying shrinkage, high toughness, high energy absorption and durability. This
is due to the tendency of propagating micro-cracks in cementitious matrices to
be arrested or deflected by fibres, which is guaranteed by the local bond between
fibres and matrix. Studies show that fibre-matrix interfacial bond is provided by
a combination of adhesion, friction and mechanical interlocking (Li, 2007). Thus
fibre reinforced concrete has superior resistance to cracks and crack propagation.
GENERAL
Plain, unreinforced concrete is a brittle material, with a low tensile
strength and a low strain capacity. Steel fibre reinforcement is widely used as
the main and unique reinforcing for industrial concrete floor slabs, shotcrete and
prefabricated concrete products. It is also considered for structural purposes
in the reinforcement of slabs on piles, tunnel segments, concrete cellars, foundation
slabs and shear reinforcement in prestressed elements. In tension, SFC fails only
after the steel fibre breaks or is pulled out of the cement matrix. The role of
randomly distributed discontinuous fibres is to bridge across the cracks that develops
and
provide some post- cracking ductility. The real contribution of
the fibres is to increase the toughness of the concrete under any type of loading.
When the fibre reinforcement is in the form of short discrete fibres, they act effectively
as rigid inclusions in the concrete matrix.
Properties Of SFC
Compressive strength
Fibres do little to enhance the static compressive strength of concrete,
with
increases in strength ranging from essentially nil to perhaps 25%.
Even in members which contain conventional reinforcement in addition to the steel
fibres, the fibres have little effect on compressive strength. However, the fibres
do substantially increase the post-cracking ductility, or energy absorption of the
material.
Tensile strength
Fibres aligned in the direction of the tensile stress may bring about
very large increases in direct tensile strength, as high as 133% for 5% of
smooth, straight steel fibres. However, for more or less randomly distributed fibres,
the increase in strength is much smaller, ranging from as little as no increase
in some instances to perhaps 60%, with many investigations indicating intermediate
values, as shown in Fig. 2.1. Splitting-tension test of SFRC show similar result.
Thus, adding fibres merely to increase the direct tensile strength is probably not
worthwhile. However, as in compression, steel fibres do lead to major increases
in the post- cracking behaviour or toughness of the composites.
Flexural strength
Steel fibres are generally found to have aggregate much greater effect
on the flexural strength of SFC than on either the compressive or tensile strength,
with increases of more than 100% having been reported. The increase in flexural
strength is particularly sensitive, not only to the fibre volume, but also to
the aspect ratio of the fibres, with higher aspect ratio leading to larger strength
increases. Fig. 2.2 describes the fibre effect in terms of the combined parameter
Wl/d, where l/d is the aspect ratio and W is the weight percent of fibres. It
should be noted that for Wl/d > 600, the mix characteristics tended to be
quite unsatisfactory. Deformed fibres show the same types of increases at lower
volumes, because of their improved bond characteristics.
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