What is Ultra High Performance Concrete?
Ultra high performance concrete (UHSC) is an advanced concrete mix that has a minimum specified compressive strength of 120MPa, obtained by eliminating coarse aggregate and increasing packing density of powders.
Classification of Concrete based on its strength Grade
Every design standard uses its own ranges to classify concrete grades. In India, grades of concrete are indicated by the letter M which means mix and is followed by a number that is the compressive strength of concrete at 28 days in N/mm2. As per IS 456:2000, concrete grades are designated under three categories as follows
- Ordinary Concrete – M10 to M20
- Standard Concrete – M25 to M60
- High Strength Concrete – M65 to M100
Few studies consider concrete with compressive strength of 120MPa and above as Ultra High Strength Concrete, while few consider concrete with compressive strength 100 – 150 MPa as Very High Strength concrete and >150MPa as Ultra High Strength Concrete.
Advantages of Ultra High Performance Concrete
When compared with normal concrete, UHSC has
- Higher compressive and tensile strength due to reduction in the size of micro cracks
- Higher modulus of elasticity will be the result of higher resistance to deformation
- Reduced creep due to less deformation
- Greater ductility can be achieved by the addition of steel fibres
- Higher density and lower porosity are obtained by optimizing the packing density of cementitious materials
- Lower water to cementitious materials ratio due to the presence of superplasticizers
- Reduced member sizes due to the usage of high strength concrete, thus reducing self-weight of structure on the foundation
- Lower usage of steels in case steel fibres are used in the production of UHSC
Limitations of UHPC
- High quality control is required in order to obtain required properties
- Low water to cementitious materials ratio requires special curing methods
- Higher cost of production compared with normal concrete production makes the usage of UHSC limited
- Absence of widely accepted design provisions
The methods followed in the production of UHSC are:
- Compaction by pressure
- Helical binding
- Polymerization in concrete
- Reactive powder concrete
Compaction by Pressure
Strength as high as 680 MPa can be obtained by subjecting the cement paste to 357 MPa pressure under a temperature of 250οC. This technique is known as “Hot Pressing”. The water/cement ratio will be 0.093. The micro structure shows that the materials are very compact, consisting of intergrowth of dense hydrated cement gel surrounding residual unhydrated cement grain cores. The lowest porosity of the materials measured is about 1.8%.
It is an indirect method of achieving ultra high strength. Here, high tensile steel wires are bound externally over concrete cylinder which results in high strength.
Polymerization of monomers that are impregnated into the pores of hardened concrete results in the development of very high strength. Polymerization can be either by irradiation or by thermal catalytic process. This method holds much promise.
Reactive Powder Concrete
The conventional sand and aggregate are replaced by ground quartz less than 300 micron, silica fume, synthesized precipitated silica, steel fibres about 1 cm in length and 180 micron in diameter. By using this method, compressive strength of 200 to 800 MPa can be achieved along with required ductility.
Over the past few decades, UHSC is gaining interest in a wide range of applications covering tall structures, high rise buildings, long span bridges, walkways, precast industries, military structures and nuclear waste containment structures.
- Pedestrian/bikeway bridge, Sherbrooke
Spanning 197 ft. (60 m), this precast prestressed pedestrian/bikeway bridge in Sherbrooke, Quebec with no conventional steel reinforcement is the first Reactive powder concrete structure erected in 1997.
- Taxiways in an extension of Haneda airport in Tokyo
The taxiways in new landing strips in an extension of Haneda airport in Tokyo were built with precast pre-tensioned slabs made of UHSC.
- Museum of Mediterranean Civilizations (MUCEM) in Marseille
The first museum dedicated to Meditterranean civilizations is one of the finest examples of UHSC. The columns which support the façade were designed as a forest of tree-shaped elements, precast and prestressed across their full height.
- The Louis Vuitton foundation building in Paris
This French art museum and cultural center is made up of an assembly of white concrete blocks or “icebergs” that are enveloped by 12 huge glass facades. The iceberg’s façade comprises of 19,000 white custom-cut ductal panels covering 9200 sq.m.
Mechanical Properties of Ultra High Performance Concrete
|Compressive Strength||150 – 800 MPa|
|Tensile Strength||6 – 13 MPa|
|Flexural Strength||40 MPa|
|Modulus of Elasticity||50 – 60 GPa|
|Fracture Energy||20,000 – 40,000 J/m2|
Design Provisions for Ultra-High Performance Concrete
UHSC is a mixture of very fine powders (cement, sand, quartz powder and silica fume), steel fibres and superplasticizer.
Recommended principles to develop UHSC include
- Removal of coarse aggregate to enhance homogeneity of the concrete
- Use of silica fume for pozzolanic reaction
- Addition of steel fibres to achieve ductility
- Optimization of granular mixture for the enhancement of compacted density
- Optimal usage of superplasticizer to reduce w/c ratio and improve workability
- Application of pre-setting pressure for better compaction
- Post-setting heat treatment to enhance microstructure
A very dense matrix has to be achieved by optimizing the granular packing of the dry fine powders in order to obtain ultra high strength and durability.
Cement: Cement with the lowest C3A content (maximum 5%) and not too high C3S (maximum 50%) and have a low specific surface area of about 350 m2/kg will be the best choice to make UHSC.
Superplasticizer: It must provide a good rheology long enough to pour the concrete and should entrap as little air as possible. It should not retard the hardening of concrete excessively. Sodium salt of a polynaphthalene sulphonate was used in Sherbrooke pedestrian bridge. Polyacrylate superplasticizers are also commonly used.