HISTORICAL PURSUIT OF CONCRETE
The term UHPC stands for Ultra High Performance Fiber Reinforced Concrete. This term encapsulates various features and technologies that are disguised beyond the true meaning of each word and require further explanation based on concrete technology background. In this article, we are trying to provide a broad perspective of the continuous process that resulted in the birth of the UHPC and a definition of what this term actually means. The way and reasons why traditional concrete is transforming into UHPC is the first step we need to take to start understanding UHPC.
Although the history of concrete dates back to the Roman Empire, the history of modern concrete used today can be thought to have started in 1824, when Joseph Aspdin patented a material called Portland cement. It consisted of a mixture of a certain proportion of limestone, clay and other minerals that was calcined and ground into fine particles. It didn’t take long for this cement being mixed with gravel, brick dust, sand, crushed stone, or other aggregates to produce what we now call concrete.
The main problems with the use of concrete in engineers practice were related to its low tensile strength and the material’s lack of ductility. In the mid-19th century, researchers realized that these problems could be partially resolved by adding a metallic reinforcement to the concrete mix. Presumably, what is now known as ‘reinforced concrete’ began with the French gardener Joseph Monier, who added iron reinforcement made up of a portion of steel rods to large flower pots in 1849. The first iron reinforced concrete structure application which was probably made in 1853, is still standing.
In this application, steel was used to keep the walls standing in one piece and to add volume rather than to add strength to concrete. The main advantages of using reinforced concrete with steel bars later were demonstrated mainly by the development of various patented construction methods for beams, pipes, piles and floor slabs by Monier and Hennebique. The foundation of today’s construction technology was laid with these developments. Combining steel with concrete in the 19th century enabled the development of concrete, the perfect building material of the 20th century.
Scientists and engineers focused on improving concrete technology and design throughout the 20th century. During this time, the properties of the concrete matrix also improved: However, some problems of concrete and reinforced concrete remained. These are weight, low tensile / compressive strength, high weight / strength ratio, poor volumetric stability, poor toughness and insufficient toughness.
Scientists and engineers have worked hard to correct these deficiencies in concrete, and they have achieved this, albeit partially.
Increased compressive strength
Adding fibers to the mix
By optimizing the particle density in concrete.
The race to increase concrete compressive strength began in the mid-20th century, as it also increased other properties such as tensile strength, shear strength or bond strength. In the 1980s, the way to produce high strength concrete became known for its low water-binder ratios, special selection and classification of aggregate particles and water reducing additives. It was then that researchers around the world originated the terms HSC and HPC, which mean High Strength Concrete and High Performance Concrete respectively. The term High Strength Concrete refers to a concrete with a characteristic compressive strength between 50 and 120 MPa; The term High Performance Concrete included HSC with improved durability properties.
However, the higher the compressive strength, the more fragile the matrix becomes. This is an undesirable effect in concrete, which can be dissolved by adding fibers to the mixture. In the early 1970s, the term Fiber-Reinforced Concrete (FRC) was first used to refer to a concrete with any type of fiber reinforcement, regardless of fiber shape, aspect ratio, quantity, or material used. Since then, research on fiber reinforced concrete has never stopped, dealing with different fiber types (shape, aspect ratio, material type) and quantities, and showing that the use of fiber improves tensile strength, ductility, hardness, crack control, toughness.
The combination of the three technologies described above evolved into Reactive Concrete Powder (RPC), a dense mixture of particles made of very fine powders (no cement, sand, quartz dust and silica fume, coarse aggregate or gravel) in 1995. Extraordinary results were obtained with extremely low water-binder ratios by adding steel fiber and subsequently super plasticizers to this new mixture.
From the 21st century, the term RPC was replaced by UHPFRC (Ultra High Performance Fiber Reinforced Concrete) gradually, retaining historical terms used in concrete technology evolution around the world. It is more common (and simpler) to use the term UHPC to refer to Ultra High Performance Fiber Reinforced Concrete, as it is assumed that Ultra High Strength Concrete requires a minimum amount of fiber that provides a certain degree of ductility. Therefore, the term
UHPC currently means:
Ultra High Strength Concrete with compressive strength over 120 MPa.
Ultra High Performance Concrete with optimized particle packaging using a special selection of fine and ultra fine particles, low porosity and high
Using minimum fiber content to ensure a minimum degree of matrix ductility.
UHPC is a natural evolution of concrete according to the increasing demands of my societies. The efforts of scientists and engineers will continue to improve and keep concrete perfect as a building material in the coming years.