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High-performance concrete can be produced through the use of low w/c ratio, pozzolanic materials, packing aggregates densely, and improve transition zone strength.
Superplasticizers reduce water content by dispersing cement particles and maintain the required workability. Concerning pozzolanic materials, it improves concrete strength and reduces its permeability
Low permeability decreases the ingression of aggressive substances and hence improve concrete durability. The dual requirements of high strength and low permeability linked to each other through the need to reduce the volume of relatively larger capillary pores.
The reduction of capillary pores is achievable by a low water-cement ratio as well as dense packing of fine particles.
Mechanism of High-performance Concrete
The function of superplasticizers is to reach to cement grains and impart a negative charge on them. As a result, the cement particles get deflocculated and disperse. The dispersion of cement particles improve concrete workability.
Additionally, make concrete more flowable for the same water to cement ratio, or reduces water content for the same workability. The latter leads to increase strength and decline permeability. The superplasticizer is a long chain molecule organic compound.
2. Pozzolanic Materials
The role of pozzolanic materials such as silica is many folds due to large surface area and relatively high content of glassy silica, which is very reactive.
The influence of silica fume on the strength of concrete is paramount. A small amount of silica, for instance, 20Kg, can replace about 60Kg of cement to produce the same compressive strength.
Silica fume would increase the early strength of concrete and decrease the heat of hydration. This property makes the use of silica fume considerably advantageous in mass concrete.
Nonetheless, silica fume rises water demand in a concrete mixture that is why superplasticizer should be added to the mix.
3. Dense Packing of Concrete Constituents
Dense packing is another basis for the achievement of high-performance concrete. The superplasticizer disperses cement particles. And fine particles of pozzolanic materials like silica fume fill the space between particles of dispersed cement and normally packed fine and coarse aggregate.
The overall result is a denser microstructure. The concrete exhibit less porosity with no evidence of capillary pores, and only very narrow less than 0.5mm gel pores is visible under high magnification in the electron microscope.
C-S-H gel particles in concrete containing silica fumes appear not as individual particles, but rather as a massive, dense structure. Fig. 2 Shows the packing of cement that contains superplasticizer and silica fume.
4. Improvement of Transition Zone
Enhancement in the strength of the transition zone in concrete is another important mechanism in producing high-performance concrete. In the presence of silica fume, dense C-S-H occupies all the space around aggregate, and a direct bond with aggregate is established.
The result of the strengthened transition zone is a reduction in micro-cracking at the interface between cement paste and aggregate.
In normal strength concrete with only cement, the transition zone around aggregate is 20mm to 100mm wide and richer in calcium hydroxide and ettringite, as against the C-S-H phase in the bulk matrix. The porosity is also higher. Thus, the transition zone forms a weak link.