Concrete as a construction material are grouped as normal concrete or high strength concrete based on its compressive strength. The compressive strength of normal concrete has a value ranging between 20 and 40 MPa. The high strength concrete will have strength above 40MPa.
Examples of high strength concrete that have compressive strength between 40 and 140MPa is discussed in this article.
With time and changes in the history, the distinguishing factors between the normal and the high strength concrete have also changed. Say 100 years before, the concrete with compressive strength of 28 MPa was considered as a high strength concrete. But now, the concrete can attain strength that is greater than 800 MPa. These are also called as reactive powder concrete.
In terms of application, the normal strength concrete is the most used type compared with high strength concrete. The main objective of using high strength concrete is to reduce the weight, creep or the permeability issues, to improve the durability of the structure, to consider special architectural considerations that demands for elements that carry smaller loads.
Fig.1.High Strength Concrete Used in Buildings From 1980s to 1990s
Properties of Normal and High Strength Concrete
Whatever be the type of concrete say normal or high strength, the mixed fresh concrete must be plastic or semi-fluid in nature so that it can be molded by hand or by using any mechanical means.
All the particles of sand and the coarse aggregates in a fresh concrete mixture are encased together and they stay at suspension.
It is very necessary that the mixture must not undergo bleeding or segregation during the handling or transportation. Uniform distribution of aggregates in the concrete helps in controlling the segregation.
Workability Factors of Normal and High Strength Concrete
As we know, the workability factor reflects the ease in which the concrete is placed, compact and finished in its fresh state.
The Normal strength concrete possesses good workability given that all the concrete ingredients are in proper and accurate proportions. These aggregates must be of proper gradation.
High strength concrete mix is often sticky and is found very difficult to be handled and placed. This is the condition even if the plasticizers are used. This condition is mainly due to the high cement content in it.
Bleeding Factors Normal and High Strength Concrete
The settlement of solid particles of the cement and the aggregate in the fresh concrete mix results in the development of a layer of water on the top of the concrete surface (fresh concrete state), this is called as bleeding. Smaller bleeding has no issues. But large-scale bleeding affects the durability and the strength of the concrete.
When compared to normal strength concrete, the high strength concrete does not bleed. This is because the high strength concrete has smaller water content and a high amount of cementitious materials. The air entrained concrete also has less chances to bleed.
Permeability of Normal Strength and High Strength Concrete
All the durability concerns like corrosion resistance, resistance to chemical attacks, creep have a direct relationship with the permeability of the concrete. Only if a foreign substance enters inside the concrete, damage occurs. The permeability of concrete depends on the permeability property related with the paste and the aggregates present in the concrete.
Decrease in permeability helps in
- Improving sulfate and chemical attack resistance
- Resistance to corrosion
- Resistance to chloride penetration
The table-1 below shows the test results of various permeability tests conducted on different concrete mix. The table differentiates the normal strength concrete and the high strength concrete in terms of water cement ratio.
Lower the water cement ratio with adequate curing period helps in having a concrete of lower permeability. For a normal strength concrete the permeability is found to be in the range of 1 x 10-10 cm/sec.
Table.1: Various Permeability Tests on Concrete as per American Concrete Institute, 1988
Addition of supplementary cementitious materials into the concrete mix like silica fume, fly ash and GGBFS helps in reducing the permeability of the concrete.
The high strength concrete has a lower value of permeability compared to normal strength concrete. This is because the high – strength concrete is designed with lower water cement ratio. They commonly use silica fume in their mix. The high strength concrete has a permeability coefficient that ranges from 1 x 10-11 to 1 x 10-13 cm/sec.
The high strength concrete thus has lower permeability and higher resistance to chloride attack and this makes it suitable for the construction of bridges, parking decks and those structures that are more exposed to seawater or de-icers.
Carbonation of Normal Strength Concrete and High Strength Concrete
The carbonation occurs on the surface of concrete. The phenomenon of carbonation is related to the permeability of the concrete. The carbon dioxide in the air reacts with the compounds present in hardened cement paste. The reaction gives away carbonates, which are called as calcium carbonates.
This effect of carbonation as mentioned in permeability factor is less in high strength concrete compared with the normal strength concrete.
In addition to the mix criteria of both the type of concrete, the engineers specify essential amount of protective concrete cover for the reinforcement steel. This is to reduce easy reach to the reinforcement.
Other Difference Between Normal Strength Concrete and High Strength Concrete
In normal strength concrete, at 40% of compressive strength value, the micro cracks are formed. These interconnects and propagates when reaches 80 to 90 % of the strength.
The fracture surface formed in normal strength concrete is very rough. This zone is formed along the transition zone between the paste matrix and the aggregates. The fracture surface in case of High strength concrete is smooth.