INTRODUCTION UHPFRC is much denser. In order

                                                          INTRODUCTIONWhat is Ultra-High Performance-Fiber Concrete?Fordecades, many scientific researchers around the world have tried to investigateand develop concretes that possess higher compressive strength, flexuralstrength, tensile strength, ductility, durability, and performance than that ofconventional concretes. No precise definition of ultra-high performance-fiberreinforced concrete found in the research reviewed but there seems to be acommon conclusion that, it is a concrete which have compressive strength morethan 150Mpa and tensile strength exceeding 7-8Mpa, water to cement ratio lessthan 0.25 which typically is between 0.16 and 0.

20, UHPFC should also have higherbinder content which served and enhanced the absence of porosity, addition offibers ensures ductility as well. Thedifference between UHPFRC and conventional concrete mix design lies inparticular in the amount of binder, the size of the aggregate and the presenceof fibers. Compared to a conventional concrete, the matrix of the UHPFRC ismuch denser. In order to produce this type of concrete, it is important toachieve the maximum possible packing density of all granular constituents. Useof a quite large amount of super-plasticizers in order to obtain an acceptableworkability is also a characteristic of the UHPFRC. SometimesUHPFRCs are subjected to a thermal treatment during curing. The heat treatmentinitiates the formation of more hydrates, which give the raise to the improvedcharacteristics. The typical compressive strength of UHPC is in the range of150-220MPa, but higher strengths can be obtained.

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Still, high compressivestrength is not always the most important feature of an UHPFRC; the tensile andflexural strength are often of higher importance. Without fibers, UHPC canexhibit a direct tensile strength in the range of 7-10MPa. The tensile strengthmay be doubled when fibers are added to the mix. The increase depends on theamount, type and orientation of the fibers.

The flexural strength of UHPFRC isusually much higher than the direct tensile strength. Generally, UHPFRC showsimprovedCharacteristicsin permeability, heat resistance and impact strength. UHPFRC development foundits origin in the studies of Odler, Brunauer and Yudenfreund in the beginningof the 1970s. Yudenfreund et al., (1972) 1, they investigated high strengthpastes with low w/c-ratios (w/c = 0.2 to 0.

3) whose main characteristic was thelow porosity leading to high compressive strengths (up to 200MPa) and to lowdimensional changes. Strength enhancement by hot pressing techniques was firstapplied by Roy and resulted in very high strength cement pastes withcompressive strengths up to 680 MPa. Roy et al., (1972) 3. With thedevelopment of super plasticizers and pozzolanic admixtures such as silicafume, two kinds of materials emerged in the 1980s: Birchall developed polymermodified cementinuos materials called Macro-Defect-Free (MDF) cements.

UHPFRCdevelopment found its origin in the studies of Odler, Brunauer and Yudenfreundin the beginning of the 1970s. Yudenfreund et al., (1972) 2, theyinvestigated high strength pastes with low w/c-ratios (w/c = 0.2 to 0.3) whosemain characteristic was the low porosity leading to high compressive strengths(up to 200MPa) and to low dimensional changes.

Strength enhancement by hotpressing techniques was first applied by Roy and resulted in very high strengthcement pastes with compressive strengths up to 680 MPa. Roy et al., (1972) 3.With the development of super plasticizers and pozzolanic admixtures such assilica fume, two kinds of materials emerged in the 1980s: Birchall developedpolymer modified cementinuos materials called Macro-Defect-Free (MDF) cements.  The pores are filled by polymerizationleading to a compact matrix. However, these concretes are susceptible to waterand have high creep. Kendall et al.

, (1983), Alford et al.,(1985) 4,5. Bachedeveloped the DSP (Densified Small Particles) which use the interaction ofsuper plasticizers and silica fume to decrease the porosity of the material andto increase the strengths. That way, he prepared the ground for modern UHPFRCdevelopment.

Bache., (1987) 6. However, theses high strength cement pastesand mortars are very brittle. Consequently, the addition of fibers is necessaryto enhance ductility (increase of SF). Three tendencies are distinguished byRossi., (2002) 7: DSP with an addition of 5 to 10% short steel fibers (lf = 6mm), commercialized under the name. the so-called Reactive Powder Concrete(RPC) with 2.

5% of short slender steel fibers (lf = 13 mm), developed byBouygues, Lafarge and Rhodia and commercialized under the name Ductal . and theMulti-Scale Cement Composite (MSCC) using a mixture of short and long steelfibers. MSCC are developed at the LCPC in France and are known under the nameCEMTEC. 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 bygraded natural aggregate (max size 8 mm). Studies revealed that (i) there is nochange 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 isreduction in the cement factor, and hence decrease in compressive strength.(iii) Flexural strength was lower when graded coarse aggregate replaced all thequartz sand. Steam curing done at 90°C and 160 °C provided lower dryingshrinkage and creep strain. Feylessoufi.

A. et al., (1997) 9 stated thatxonolite is one of the most important crystalline hydrates in RPC. The heatingmode 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 andkinetically controlled thermal curing (CRTA) technique. Results showed controlrate of heat treatment at a definite water vapour pressure is required in orderto get precise control of hydrate crystallization. Garas, Kahn, and Kurtis.,(2009)10 conducted trials to study the stress/strength ratio, thermaltreatment and fiber reinforcement consequences on the tensile creep behavior,tensile strength and free shrinkage using different UHPC mixes.

They concludedthat usage of fibers and the application of thermal treatment decreased 14-daydrying shrinkage by more than 57% and by 82%. Increasing the stress-to-strengthratio from 40% to 60% increased the tensile creep coefficient by 44% and the specificcreep by 11%, at 14 days of loading. Incorporating short steel fibers at 2% byvolume decreased the tensile creep coefficient by 10% and the specific creep by40%, 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 thespecific creep by 77% at7 days, as compared to ordinarily cured companionmixes.


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