INTRODUCTION concrete which have compressive strength more than


                                                          INTRODUCTION

What is Ultra-High Performance-Fiber Concrete?

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For
decades, many scientific researchers around the world have tried to investigate
and develop concretes that possess higher compressive strength, flexural
strength, tensile strength, ductility, durability, and performance than that of
conventional concretes. No precise definition of ultra-high performance-fiber
reinforced concrete found in the research reviewed but there seems to be a
common conclusion that, it is a concrete which have compressive strength more
than 150Mpa and tensile strength exceeding 7-8Mpa, water to cement ratio less
than 0.25 which typically is between 0.16 and 0.20, UHPFC should also have higher
binder content which served and enhanced the absence of porosity, addition of
fibers ensures ductility as well.

The
difference between UHPFRC and conventional concrete mix design lies in
particular in the amount of binder, the size of the aggregate and the presence
of fibers. Compared to a conventional concrete, the matrix of the UHPFRC is
much denser. In order to produce this type of concrete, it is important to
achieve the maximum possible packing density of all granular constituents. Use
of a quite large amount of super-plasticizers in order to obtain an acceptable
workability is also a characteristic of the UHPFRC.

Sometimes
UHPFRCs are subjected to a thermal treatment during curing. The heat treatment
initiates the formation of more hydrates, which give the raise to the improved
characteristics. The typical compressive strength of UHPC is in the range of
150-220MPa, but higher strengths can be obtained. Still, high compressive
strength is not always the most important feature of an UHPFRC; the tensile and
flexural strength are often of higher importance. Without fibers, UHPC can
exhibit a direct tensile strength in the range of 7-10MPa. The tensile strength
may be doubled when fibers are added to the mix. The increase depends on the
amount, type and orientation of the fibers. The flexural strength of UHPFRC is
usually much higher than the direct tensile strength. Generally, UHPFRC shows
improved

Characteristics
in permeability, heat resistance and impact strength. UHPFRC development found
its origin in the studies of Odler, Brunauer and Yudenfreund in the beginning
of the 1970s. Yudenfreund et al., (1972) 1, they investigated high strength
pastes with low w/c-ratios (w/c = 0.2 to 0.3) whose main characteristic was the
low porosity leading to high compressive strengths (up to 200MPa) and to low
dimensional changes. Strength enhancement by hot pressing techniques was first
applied by Roy and resulted in very high strength cement pastes with
compressive strengths up to 680 MPa. Roy et al., (1972) 3. With the
development of super plasticizers and pozzolanic admixtures such as silica
fume, two kinds of materials emerged in the 1980s: Birchall developed polymer
modified cementinuos materials called Macro-Defect-Free (MDF) cements. UHPFRC
development found its origin in the studies of Odler, Brunauer and Yudenfreund
in the beginning of the 1970s. Yudenfreund et al., (1972) 2, they
investigated high strength pastes with low w/c-ratios (w/c = 0.2 to 0.3) whose
main characteristic was the low porosity leading to high compressive strengths
(up to 200MPa) and to low dimensional changes. Strength enhancement by hot
pressing techniques was first applied by Roy and resulted in very high strength
cement pastes with compressive strengths up to 680 MPa. Roy et al., (1972) 3.
With the development of super plasticizers and pozzolanic admixtures such as
silica fume, two kinds of materials emerged in the 1980s: Birchall developed
polymer modified cementinuos materials called Macro-Defect-Free (MDF) cements.  The pores are filled by polymerization
leading to a compact matrix. However, these concretes are susceptible to water
and have high creep. Kendall et al., (1983), Alford et al.,(1985) 4,5. Bache
developed the DSP (Densified Small Particles) which use the interaction of
super plasticizers and silica fume to decrease the porosity of the material and
to increase the strengths. That way, he prepared the ground for modern UHPFRC
development. Bache., (1987) 6. However, theses high strength cement pastes
and mortars are very brittle. Consequently, the addition of fibers is necessary
to enhance ductility (increase of SF). Three tendencies are distinguished by
Rossi., (2002) 7: DSP with an addition of 5 to 10% short steel fibers (lf = 6
mm), commercialized under the name. the so-called Reactive Powder Concrete
(RPC) with 2.5% of short slender steel fibers (lf = 13 mm), developed by
Bouygues, Lafarge and Rhodia and commercialized under the name Ductal . and the
Multi-Scale Cement Composite (MSCC) using a mixture of short and long steel
fibers. MSCC are developed at the LCPC in France and are known under the name
CEMTEC. Collepardi et al., (1996) 8 investigated in three sets (i)
replacement of ground fine quartz sand (0.15-0.40 mm), (ii) a part of (cement +
silica fume) of the cementinuos binder and (iii) the whole of fine sand by
graded natural aggregate (max size 8 mm). Studies revealed that (i) there is no
change in the compressive strength of the RPC at the same water-cement ratio.
(ii) An increase in the water-cement ratio is observed, due to which there is
reduction in the cement factor, and hence decrease in compressive strength.
(iii) Flexural strength was lower when graded coarse aggregate replaced all the
quartz sand. Steam curing done at 90°C and 160 °C provided lower drying
shrinkage and creep strain. Feylessoufi.A. et al., (1997) 9 stated that
xonolite is one of the most important crystalline hydrates in RPC. The heating
mode studied were putting the specimen directly in the oven preset at 300°C,
conventional thermo gravimetric analysis(TGA) in vacuum at a rate of 100 °C and
kinetically controlled thermal curing (CRTA) technique. Results showed control
rate of heat treatment at a definite water vapour pressure is required in order
to get precise control of hydrate crystallization. Garas, Kahn, and Kurtis.,
(2009)10 conducted trials to study the stress/strength ratio, thermal
treatment and fiber reinforcement consequences on the tensile creep behavior,
tensile strength and free shrinkage using different UHPC mixes. They concluded
that usage of fibers and the application of thermal treatment decreased 14-day
drying shrinkage by more than 57% and by 82%. Increasing the stress-to-strength
ratio from 40% to 60% increased the tensile creep coefficient by 44% and the specific
creep by 11%, at 14 days of loading. Incorporating short steel fibers at 2% by
volume decreased the tensile creep coefficient by 10% and the specific creep by
40%, at 14 days. Also, subjecting UHPC to a 48-h thermal treatment at 900C,
after initial curing, decreased its tensile creep coefficient by 73% and the
specific creep by 77% at7 days, as compared to ordinarily cured companion
mixes.

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