Ultra-High Performance Concrete (UHPC)
What is it called internationally?
These are fiber-reinforced concretes with very high fiber content, to the point that internationally they are no longer only referred to as UHPC, but more precisely as UHPFRC, meaning Ultra-High Performance Fiber Reinforced Concrete; in French BFUP: Béton Fibré à Ultra-Haute Performance.
What type of fibers are used? What advantages do they provide?
The fibers, very fine (around 0.2–0.3 mm thick), are typically steel fibers coated with copper or brass. They are used in proportions as high as 150 kg/m³. Their role is not only to achieve strength, but especially to provide ductility, preventing brittle failure.
What is the consistency of UHPC?
They are generally “wet-cast” concretes rather than “dry-cast,” although both are possible. Wet-cast mixes allow better workability and performance in achieving the required strength and ductility.
Is there any standardization?
As of June 2018, there is no Spanish standard or instruction yet, though development work is underway. French and Swiss standards already exist.
What are the compressive characteristics? Water/cement ratio?
UHPC requires a minimum characteristic compressive strength of 150 MPa (around 1,500 kg/cm²). It is also tested in direct tension and flexural tension. Failure must occur beyond the proportional limit, ensuring ductile behavior.
In some architectural cases, 130 MPa may be accepted for aesthetic requirements.
Water/cement ratios are very low, typically between 0.18 and 0.22, achieved through high-range water-reducing admixtures and very fine particle packing. Cement contents are high (around 800 kg/m³), plus approximately 200 kg/m³ of silica fume.
What durability does UHPC have?
It is extremely compact, with very low absorption and permeability, resulting in significantly higher durability than conventional concrete.
It can be reinforced and prestressed, enabling very lightweight structural elements. In Switzerland it is used for bridge deck rehabilitation, and in France for architectural precast elements.
Some experts even state: “UHPC is not concrete”, considering it a new material.
Javier Regás — Engineering Consultant at Architectonic Concrete S.L. since 1999.
Surface finishes of UHPC concrete
Simple finish “as demoulded”
A piece can be cast with a perfectly smooth appearance given by the mold itself. The surface may remain clean and untreated, possibly including specific textures or reliefs integrated into the mold design.
This allows virtually unlimited design possibilities, from smooth or textured surfaces to engraved drawings, photographs, or patterns.
STRIP finish (acid-washed / exposed aggregate)
The strip finish is slightly rough to the touch. It exposes the natural color and texture of the aggregates on a smooth but sand-like surface. It produces bright, mineral-like finishes with high definition and durability.
The process removes a thin surface layer using retarders and high-pressure washing after demoulding.
Technical summary of UHPC
Compressive strength: Typically above 45 MPa for early demoulding, reaching up to 150 MPa or more in UHPC.
Flexural strength: Generally above 5 MPa, critical for crack resistance and durability.
Water absorption & freeze resistance: Key factor for durability and aesthetics, especially in cold climates.
Abrasion resistance: Strongly influenced by fine aggregate grading.
Impact resistance: According to UNE EN 127748-1/2006, steel ball drop height resistance > 200 cm.
Differences and common factors between UHPC and GRC concrete
https://www.youtube.com/watch?v=MFRIaH2YKXQ
Bibliography: Architectonic Concrete SL
