Ultra high performance fibre concrete

Ultra High Performance Fiber Concretes (UHPC) are cementitious matrix materials, reinforced with fibres and offering compressive strengths of between 150 and 250 MPa.

Advances in the field of admixtures, formulation methods and the use of ultrafine have led to a revolution of concretes. The range has expanded: from standard concretes with compressive strengths of 30 mpa to high performance concretes (bhp: more than 50 mpa) and very high performance (bthp: more than 80 mpa). A technological breakthrough occurred in the early 1990s with the development of concrete whose resistance exceeds 150 mpa.

Their formulation uses superplasticising adjuvants and specific granular compositions as well as fibres (metal or organic fibres). For structural bfup the presence of fibres and tensile performance lead to pseudo-ductile behaviour to overcome all or part of passive frames.

These concretes revolutionize the techniques and methods of construction and allow the design of new structures.

These concretes offer exceptional performances:

• A very good workability,
• Compactness
• Low permeability
• Characteristic compressive strengths at 28 days (for structural bfups) between 150 and 250 mpa,
• Early high mechanical resistance.
• Exceptional durability (which allows them to be used in very aggressive environments).
• Ductility (deformability under load without brittle fracture).
• High tenacity (resistance to micro-cracking).
• Very low desiccation shrinkage and creep.
• Surface hardness.
• High resistance to abrasion and shock.
• A particularly aesthetic facing and very fine facing textures.

The first companies that used UHPFC were in France, and were EDF, Eiffage, Structalis, and Bouygues.

The evolution of UHPCs compared to high performance concretes (BHP) is characteris ed by:

• Their great resistance in compression and in traction.
• Their specific composition and their high dosage in cement (700 to 1000 kg / m3) and in adjuvants.
• Their specific granular skeleton (4 to 5 grain scales) and the optimisation of their granular stacking.
• The use of aggregates of small dimensions.
• A much lower water content.
• The presence of fibres (at a high rate).
• The UHPC may be associated with prestressing by pre-tensioning or post-tensioning, more rarely with passive reinforcement.
• The various formulations of the UHPFs make it possible to give them additional properties adapted to the specific requirements of the projects.

Obtaining high resistances and low permeabilities to aggressive agents requires a very significant reduction in porosity and more specifically in the network of connected pores, by acting on two parameters.

Extremely low water content, thanks to the optimised use of superplasticisers which deflocculate the cement grains (e / c ratio of 0.15 to 0.25).

Maximum compactness, obtained using components corresponding to several granular classes (typically four to five which include cement, ultrafine, fillers and sand). The size and quantity of larger grains are considerably reduced (maximum diameter ranging from 1 to 7 mm, aggregates being specially selected). The optimisation of the granular stack makes it possible to reduce the volume of the voids.

The bfup thus have a very low capillary porosity. Ultrafine used in UHPCs are generally silica fumes that fill the intergranular spaces, optimising the compactness of the material, and react thanks to their pozzolanic power with the lime resulting from the hydration of the cement. They improve the rheology of fresh concrete, actively participate in the resistance of the whole and close the network of pores to the diffusion of ions and gases. Other ultrafine can also be used such as calcareous or siliceous microfillers and natural or artificial pozzolans.

The use of adjuvants, such as water-reducing plasticisers and superplasticisers, makes it possible to formulate bfups with a very low equivalent eeff / binder ratio.

Fibres, a key component of UHPCs, give the material its ductility. These fibres have a length adapted to the size of the larger grain and a weak section. They generally have a diameter of 0.1 to 0.3 mm and a length of 10 to 20 mm.

Metal fibres are used for structural applications requiring significant mechanical strength.

[edit] Related articles on Designing Buildings Wiki

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• Blockwork.
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• Cement.
• Cement mortar.
• Concrete.
• Concrete superplasticisers.
• Earthquake resistant building materials.
• Efflorescence.
• Grout.
• Hot-mixed mortars: the new lime revival.
• Lime mortar.
• Mortar analysis for specifiers.
• Plasticiser.