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High-performance concrete (HPC) is produced by careful selection and proportioning of its constituents namely cement, sand, gravel, cementitious materials such as fly ash; silica fume; and slag, and chemical admixtures for instance high range water reducing admixtures. The strength and durability of the high-performance concrete exceed that of ordinary concrete.
Therefore, the composition of high-performance concrete is almost the same as that of conventional cement concrete. However, it has many features such as high strength, smooth fracture surface, low permeability, discontinuous pore, etc. which are different from those of ordinary concrete.
This is due to low water to cementitious material ratio, and the presence of cementitious materials and chemical admixtures. Curing of HPC is considerably important and critical curing period runs from placement or finishing up to 2 to 3 days later.
Composition of High-Performance Concrete
The composition of high-performance concrete usually consists of the following materials:
Chemical and physical properties of cement can help in selecting desired cement to produce high-performance concrete. For instance, cement with low C3A is the most desired type of cement to produce high-performance concrete because the C3A creates incompatibility of cement with a superplasticizer.
Additionally, the rheology of cement containing low C3A can be controlled easily. Nonetheless, a certain quantity of C3A is important for cement from a strength point of view.
Water is a crucial component in high-performance concrete which should be compatible with cement and mineral/chemical admixtures.
3. 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.
4. Coarse Aggregate
The selection of coarse aggregate is crucial since it may control the strength of high-performance concrete.
It is an essential component of high-performance concrete that is added into the concrete mix to reduce water to cement ratio.
6. Cementitious Materials
The cementitious component of high or any combination of cementitious material such as slag, fly ash, silica fume.
6.1 Silica Fume
Silica fume is a waste by-product of the production of silicon and silicon alloys. Silica fume is available in different forms, of which the most commonly used now is in a densified form. In developed countries, it is already available readily blended with cement.
It is possible to make high strength concrete without silica fume, at compressive strength of up to 98 MPa. Beyond that strength level, however, silica fume becomes essential. With silica fume, it is easier to make HPC for strengths between 63-98 MPa.
6.2 Fly Ash
Fly ash has been used extensively in concrete for many years. Fly ash is, unfortunately, much more variable than silica fumes in both their physical and chemical characteristics. Most fly ashes will result in strengths of not more than 70 MPa.
Therefore, for higher strengths, silica fume must be used in conjunction with fly ash. For high strength concrete, fly ash is used at dosage rates of about 15 % of cement content.
6.3 Ground Granulated Blast Furnace Slag (GGBFS)
Slags are suitable for use in high-strength concrete at dosage rates between 15-30 %. However, for very high strengths, more than 98Mpa, it is necessary to use the slag in conjunction with silica fumes.
Sometimes, quartz flour and fiber are the components as well for HPC having ultra-strength and ultra ductility, respectively.
Features of High-Performance Concrete
- Compressive strength > 80 MPa ,even up to 800 MPa
- High-performance concrete is quite brittle but the introduction of fibers and can improve ductility.
- High durability
- Water binder ratio (0.25-0.35), therefore very little free water
- Reduced flocculation of cement grains
- Wide range of grain sizes
- Densified cement paste
- Low bleeding and plastic shrinkage
- Less capillary porosity is achieved through the use of low water to cementitious materials that produce dense microstructure, hence migration of aggressive elements would be difficult. Hence, durability improved greatly.
- Discontinuous pores
- Stronger transition zone at the interface between cement paste and aggregate
- Low free lime content
- Endogenous shrinkage
- Powerful confinement of aggregates
- Little micro-cracking
- Smooth fracture surface
- Low heat of hydration