MECHANICAL PROPERTIES
OF STEEL FIBER REINFORCED CONCRETE (SFRC)
Toughness
Toughness is the total energy absorbed prior
the complete separation of the specimen. It can be calculated as the area under the load-deflection curve
plotted for beam specimen used in a flexure test. Although, it was well
established that the steel fibers significantly improve concrete toughness and
it is widely agreed that toughness can be used as a measure of the energy
absorption of the material, there is a doubt about the way that SFRC toughness
should be measured and used. Two methods to interpret and calculate the
toughness of SFRC are widely used. The method in which the energy absorbed up
to a certain specified deflection is normalized by the energy up to a point of
fast cracking. The Japanese Institute of Concrete standards interprets the
toughness in absolute terms, as the energy required to deflect the beam
specimen to a mid-point deflection. The ASTM method evaluates the flexural
performance of toughness parameters derived from SFRC in terms of areas under
the load-deflection curve obtained by testing a simply supported beam under
third-point loading. It provides for determination of a number of ratios called toughness
indices that identify the pattern of material behavior up to the selected
deflection. These indices are determined by dividing the area under the
load-deflection curve up to a specified deflection by the area up to the
deflection at first crack.
Abrasion and Skid Resistance
Knowledge
of abrasion and wear resistance of concrete is essential especially for
pavement due to the continuous nature of its loads. Difficulties might be
encountered concerning of the wear and abrasion resistance, as the damaging
action varies depending on the cause of wear, and no single test procedure is
satisfactory in evaluating the resistance of concrete to the various conditions
of wear.
Skid Test
Tests on hydraulic
structures, which have the same effect of wear on slabs under traffic loads,
revealed that the abrasion resistance of SFRC is not improved over that of the
plain concrete. Significant increases of abrasion resistance were found by
other researchers, with about 15% higher resistance reported under drying, wet and
frozen surface conditions. Wear tests were carried
out using a pair of hardened steel wheels running in a circular path under load
on flat specimen slabs. It was found that for specific number of cycles,
the SFRC exhibits average groove depths less than that of plain concrete, which
in turn proves that the SFRC has a better wear resistance
relative to an identical plain concrete. The skid resistance of SFRC was found
to be same as that of the plain concrete at early stages prior the
deterioration of the surface. In later stages, where abrasion and erosion of
the surface had to take place, steel fiber reinforced concrete has an up to 15
% higher skid resistance relative to plain concrete. It can be concluded that the SFRC has better performance
regarding its erosion, abrasion and skid resistance, but how much better is
dependent on the case of application.
Thermal Properties and Fire Resistance
Fire resistance of
concrete
There are three thermal properties that may be significant
in the performance of concrete, coefficient of thermal expansion, specific heat
and conductivity. Thermal expansion is seen to be the most relevant to the
ground slabs applications especially for concrete subjected to thawing and
freezing action. Specific heat and conductivity are normally relative to
applications whereby thermal insulations are provided, or other applications
such as rocket launch facilities or mass structures.
The effect of steel fibers on coefficient of expansion
factor was studied using beam specimens that have various steel fibers content
(ranges between 0 and 2 % by volume). Specimens were subjected to temperatures
ranges between 38 and 66 degrees Celsius. Tests results indicate that the
coefficient of thermal expansion factor was not significantly affected by fiber
content. Tests on relatively dry SFRC specimens at ages of about 220 to 250
days and 27-degree Celsius temperature rise, revealed that addition of steel
fibers marginally influence the thermal expansion coefficient. Just to give an
indication, for SFRC containing 75 kg/m3 of enlarged-end steel fibers,
the typical expansion coefficient is found to be 8.2 x 10-6 per degree Celsius.
It can be seen from the above discussion
that the expansion of SFRC is the same (if not less) than plain concrete for
identical mixes. The author's opinion is that, the only hazard is the expansion
coefficient of the steel fibers, in other words, large differences between
thermal coefficients of steel fibers and paste might cause the interface layers
between them to damage and damage in many surfaces in different dimensions
might weaken the entire matrix.
Electrical Conductivity
Electrical Conductivity
Steel
fibers contents of up to 1 % by volume (80 kg/m3) has no significant effect on
electrical conductivity, hence, wire guided vehicles may be operated without
difficulties on SFRC floors, which can be taken as an advantage if compared
with steel bars or mesh floors. It can also be beneficial where traffic devices
are needed e.g. vehicle detection loops for traffic counting and
classification.
Durability
Porosity and permeability are
primary factors affecting the durability of the concrete due to its effect on
alkali-acid reaction, leaching characteristics, resistance to chloride or
sulphate attack, reinforcement corrosion, and freezing and thawing
characteristics. Initially SFRC mixes had high porosities and permeability due
to the higher W/C used to increase the workability. Recently, reductions
in W/C ratio are possible, which result in relatively low porosities and
permeability’s. Tests indicated that the SFRC has permeability values typical
of those for the plain concrete, therefore, apart from corrosion of steel
fibers, the SFRC has the same durability (if not better) than the identical
plain concrete.
Attention
has to be given to the question of the corrosion of the steel fibers when added
to concrete. Theoretically, one of the main problems associated with the use of
steel fibers is their durability in concrete structures. In severe exposure
condition, corrosion of steel fibers is more aggravated than that of steel
bars, in other words, a significant decrease to the steel fibers diameter,
contribute significantly to lessen the load capacity of the structure at
service. In contrast, unlike steel bars, only limited expansion force develops
due to the corrosion of steel fibers [14], which means less paste disruption
and eventually minimal breakdown and weathering rates in comparison to
conventional concrete reinforced by steel bars.
Concrete Durability
There is evidence that in practice, in good
quality concrete, fibers corrosion does not penetrate into the concrete.
Laboratory studies have shown that, stainless steel fibers can perform well
even in a very aggressive type of exposure conditions while the carbon steel fibers
invite the corrosion and cracks development. SFRC specimens exposed to a marine
environment for about 10 years, show that the corrosion of fibers is limited to
the surface of the un-cracked specimens and no noticeable reduction in flexural
strength was found, whilst, for cracked specimens, corrosion does occur through
the depth of the crack and reduction on flexural strengths were encountered. Under normal finishing processes very few
fibers will be left exposed at the surface of slabs and any such fibers exposed
to the surface is assumed to corrode and blow away under trafficking. It was
found that the corrosion depth is usually confined to the first 5 mm, therefore, designs should consider
cover depths of about 10 mm apart from recommending the knocking down of steel
fibers while finishing the concrete surface.
