Dr. L. R. Manjunatha, Honorary Chairman, Indian Concrete Institute, Bangalore
Concrete is a durable and versatile construction material; it is not only strong and economical, but it also takes the shape of the form in which it is placed, and is aesthetically satisfying. However, experience has shown that concrete is vulnerable to deterioration unless precautionary measures are taken during the design and production stage.
The term ‘high performance’ is somewhat pretentious because the essential features of this concrete is that its ingredients and proportions are specifically chosen so as to have particularly appropriate properties such as high strength and low permeability, for the expected use in the structure.
High-strength concrete and high-performance concrete are not synonymous because strength and performance of concrete are different properties of concrete. High-strength concrete is defined based on its compressive strength at a given age whereas high-performance concrete is defined based on performance criteria, namely, high durability, high strength, and high workability.
There are no unified definitions for High Performance Concretes (HPC) and different institutions and experts define High Performance Concrete differently. The American Concrete Institute defines HPC as: “Concrete which meets special performance and uniformity requirements that cannot always be achieved routinely by using only conventional materials and normal mixing, placing and curing practices”. The requirements may involve enhancement of characteristics such as placement and compaction without segregation, long-term mechanical properties, and early age strength or service life in severe environments.
In simpler words, HPC is a concrete that has at least one outstanding property viz. Compressive Strength, High Workability, Enhanced Resistances to Chemical or Mechanical Stresses, Lower Permeability, Durability etc. as compared to normal concrete. For example, Self-Compacting Concrete is a specific part of High Performance Concrete, which distinguishes itself with self-consolidation properties coupled with high flowability.
Earlier, to fulfil the structural needs, cement content in the concrete mixture was increased in order to produce high strength concrete. But it was done with least concern towards reducing the carbon footprint of the concrete as well as durability parameters. In this regard, as per IS – 456: 2000, maximum cement content has to be restricted to 450 kg/m3. With such limitation on use of cement, Supplementary Cementitious Materials (SCMs) such as Fly Ash, ground granulated blast furnace slag (GGBS), Silica fume (SF), Rice husk ash (RHA), Ultrafine slag and ultrafine flyash, etc. have been used to reduce the amount of total cement content and thereby produce sustainable concrete, without compromising on the performance part of concrete. Research has shown that addition of mineral admixtures to substitute Ordinary Portland Cement (OPC) in concrete results in more hydrated products and reduces the porosity as compared to conventional concrete.
Composition of High-Performance Concrete
HPC is comprised of the same materials as that of conventional cement concrete. The use of some mineral and chemical admixtures enhances the strength, durability, and workability qualities to a very high extent. The composition of HPC usually consists of the following materials:
- Cement: Chemical and physical properties of cement can help in selecting the desired cement to produce high-performance concrete. For instance, cement with lower C3A is the most desired type of cement to produce high-performance concrete because the C3A creates incompatibility of cement with a superplasticizer. Nonetheless, a certain quantity of C3A is important for cement from a strength point of view.
- Water: Water is a crucial component in HPC, and it should be compatible with cement and mineral/chemical admixtures. The water used for mixing and curing should be clean and free from injurious quantities of alkalis, acid, oils, salt, sugar, and any organic materials
- Fine Aggregate: Coarse fine aggregate is desired compared to finer sand to produce high-performance concrete, since finer sand increases the water demand of concrete.
- Coarse Aggregate: The selection of coarse aggregate is crucial since it may control the strength of high-performance concrete. It is advisable to avoid flaky and angular aggregates.
- Superplasticizer: It is an essential component of HPC and is added to the concrete mix to reduce water to cement ratio.
- Cementitious Materials: Ground Granulated Blast Furnace Slag is suitable for use in high-strength concrete at dosage rates of about 50% and above, based on the applications and performance requirements. However, for very high strengths of more than 100 Mpa, it is necessary to use the slag in conjunction with ultrafine materials like silica fume, Ultrafine slag, Ultrafine Flyash, Metakaolin etc.
Features of High-Performance Concrete
- Compressive strength > 60 MPa
- Quite brittle but introduction of fibers can improve ductility
- Water binder ratio (0.25-0.35), therefore very little free water
- Densified cement paste
- Low bleeding and plastic shrinkage
- Less capillary porosity is achieved through use of low water to cementitious materials that produce dense micro-structure; so migration of aggressive elements would be difficult; hence, durability is improved greatly
- Stronger transition zone at the interface between cement paste and aggregate
- Low free lime content
- Low heat of hydration due to use of supplementary cementitious materials