SUMMARY
The development in modern times of fibers as reinforcement in the concrete composition can be considered that it started around the ’60. The polymeric fibers have been commercialized around the ’70, those of glass around the ’80, and those of carbon around the 1990.
Fibers are generally used in two forms – as short dispersed fibers in matrix or continuous under the form of tape of reinforcement.
Even if the fibers production increased vary rapid at the world level (based on the industrial source, their production is of 300000 to/year), for the moment only a small part of this are used to realize concrete.
The studies carried out until now distinguished the favorable effect of the fibers addition in the cement matrix composition, transforming this way a fragile material (matrix, concrete) in a ductile material influencing also in a favorable way the cracking process of concrete.
Until today, the most frequent utilization of concrete with fibers are the nonstructural or semi structural elements. However, lately at the world level it was observed the extension of the use of concrete with fibers to realize structural elements, but also appearance of ultra high strength concrete with fibers.
The construction industrial development in an accelerated rhythm, requires a correlation between the construction material production industry with the prefabrication industry. In consequence the implication of modern construction materials is imperious necessary, following:
- reduced costs
- a high volume of concrete production (6 milliards of tones of concrete and ½ milliards of tones of fibers are used annually in constructions)
- a high durability – the concrete structures are designed with a life time of tens of years (reinforced concrete bridges – 75 years)
- service safety ( realizing safe constructions under the seismic action)
A proper selection of fibers and matrix, is the key to realize durable elements and structures. On the other hand the replacement of the traditional reinforcement with fibers leads to an important manual labor saving.
At the same time, the use to realize concrete of certain materials with a negative impact on the environment (materials that are consuming energy and pollution materials- cement) – recalls the replacement of a certain part of the cement dosage with bonding material recycled materials as: silica fume, fly ashes etc.)
From the ratio cost/ benefit point of view, the performance concrete realized with fibers, recommend this material to be used to realize reinforced concrete. Beside this quality it can be underline:
- the important reduction of the unit deformation to shrinkage.
- the impact strength
- the increase of the ductility – high level of the energy absorption
- maximum durability (impermeability, freeze-thaw strength, to corrosion etc)
In what regards the ultra high strength concrete UHSC – realized using very fine powders having a granularity between 800-1000 μm, the researches are still relatively reduced. In the absence of the regular agregates present in normal and high strength concrete, UHSC is characterized by a high content of silica fume and a reduced water/cement ratio (Cheyrezy et al. 1995). Due to the reduced water/cement ratio, this concrete needs a new generation of super plasticizers in an increased dosage, usually of 20 l/m3, to confer it workability.
Generally the concretes (UHSC) exceed the compressive strength to 150 MPa, the tensile strength from bending of 20-50 MPa and an elasticity modulus of 45-65 GPa. By including steel fibers, the concretes became ductile overtaking the high performance concrete ductility.
It presents, also, a very reduced porosity, becoming this way a very durable material. Due to the finesse of the constituents and to the workability, UHSC presents a high grade of finishing (Dauriac, 1997).
Those the have developed the concept of UHSC are those from the Bouyges enterprise from France.
The research begun in 1994 (Richerd and Cheyrezy) studying 2 concrete classes, 200 MPa and 800 MPa with different fabrication process.
It is very clear that, by its constituents, UHSC is different from the conventional concrete. The superior mechanical properties are due to:
- the homogeneity
- the hardness
- the microstructure
- the ductility.
The properties that indicates it as a distinguished construction material, necessary to be extended to realize concrete structures.
When we have regular concrete and high strength concrete the maximum aggregates size are 30mm … 20mm.
When we have UHSC, the sand used has the maximum size of 600-800 μm. By this reduction of cca. 50 times of the aggregates dimensions, it is produced an important reduction of the micro cracking.
The choused cement has to have a reduced content of C3A and to be in a close relation with the selection of the super plasticizer.
The silica fume has the meaning of replenishing the empty spaces between cement particles and it increases the rheological characteristics of the mixture, that gives the result of perfectly spherical particles.
An other method to confine the matrix in steel tubes, a way to improve not only the ductility but also the compressive strength (the effect of the confinement).
The possible advantages of UHSC with steel fibers are:
The high tensile and compressive strength that are parameters of this material that indicates:
- the possibility to realize slender elements and by this way the reduction of the dead load.
- The possibility to create larger spans at the high civil structures, that result a decrease of the transversal sections of the columns or decrease of the infrastructures sections at the bridges.
- The high strength leads also to the optimization of the material use (the transversal sections of the concrete beams having the same bearing).
- The high tensile strength- can lead to the elimination of regular reinforcement provided to undertake the share force. By this it will be reduced the man work and materials (reinforcement) costs.
The high durability:
- by the high reduction of the concrete (matrix) porosity, UHSC becomes virtually impermeable to liquids and gas (ascertainment made by Roux in 1996).
This considerations make UHSC to be an ideal material that can be used in aggressive environment with chemical aggressively, marine, freeze-thaw and to be used in the nuclear industry (to deposit the nuclear residue).
- it presents a high resistance to abrasion, a fact that it gives it a large applicability to bridge planks or to the industrial plates.
From an analysis made by Roux in 1996 it results the following characteristics:
Comparison between UHSC and regular(conventional) concrete:
THE INDICATOR OF DURABILITY COMPARISON
Porosity 4÷6 times smaller
Air permeability 20÷50 times smaller
Water abortion 7÷10 times smaller
Corrosion degree 10÷25 times smaller
Carbonation degree It was not detected to the UHSC
The properties of UHSC are in some regards close to those of metal.
Therefore, for the same bearing, the transversal section of an element realized with UHSC is close to the section realized from metal, but in what regards the cost: 1 cm of steel is approximately 10 times expensive that 1 cm of UHSC.
Between the applications of HSC (already made) it can be remind the pedestrian bridges and for bicycles realized in Sherbrook (fc=200 MPa).
The pedestrian bridge from Seul (over the Han river) realized from 6 sections of post stressed beams, the Maeta bridge from Japan of 50m length. Bridges realized before in Australia (New South Wales) made from precast prestressed elements. Other applications in the civil structures field the can be remind are:
- beams to a nuclear central in France (Catternon Nuclear Power Plant)
- retaining wall in the sea region of Portugal.
Suggestions for further applications:
- high pressure tubes to transport liquids and gas
- the insulation and protection of the nuclear residue
- blast resistance structures
- seismic applications
- high strength liners for tunnels and mining shafts
- industrial floors
UHSC is a material considered to be “high-tech”, used to realize a new technology regarding the composition regarding the concrete composition, its production, the fizico-mechanical properties, and also the methodology of design of the structures realized with this concrete.
The temporary recommendations were for the moment published in France and Germany. Some structural applications are signaled in Canada, Europe and Asia, underlining the benefits of the new technology in what regards the final structure cost, the sustainability and the very good behavior in service. There are necessary, as it had been showed at the “Seventh International Symposium on Utilization of High Strength/High Performance Concrete- Washington-ACI- SP 228- 2005- research regarding this kind of elements under different loads and the ascertainment regarding the fizico-mechanical characteristics and of durability.
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