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The properties of hardened concrete

Hardened concrete has a number of properties, including:

1) Mechanical strength, in particular compressive strength. 
The strength of normal concrete varies between 25 and 40 MPa. Above 50 MPa, the term High Performance Concrete is used (50 MPa corresponds to a force of 50 tonnes acting on a square with sides of ten centimetres).

2) Durability. 

Durability refers to the resistance to the forces of environment such as weathering, chemical attack and fire. Freezing and thawing result in adverse stresses which develop due to the presence of water in the pores of the concrete. Generally, dense and strong concretes have better durability in extreme weather conditions. The cube-crushing strength alone is not a reliable guide to the quality and durability of concrete. It should have an adequate cement content and a low water-cement ratio. Concrete is extremely resistant to the physico-chemical attack emanating from the environment (frost, rain  atmospheric pollution, etc...) It is particularly well-suited for structures exposed to demanding and extreme conditions.

3) Porosity and density. 
These properties are responsible for the first two. The denser (or the less porous) the concrete the better its performance and the greater its durability. 
The density of concrete is increased by optimizing the dimensions and packing of the aggregate and reducing the water content.

4) Fire resistance.

5) Thermal and acoustic insulation properties.

6) Impact resistance.

7) Impermeability

This is the resistance of the concrete to the flow of water through the pores. Excess water during concreting leaves a large number of continuous pores leading to permeability. Since permeability reduces the durability of concrete, it should be kept very low by using low water-cement ratio, dense and well-graded aggregates, proper compaction and continuous curing at low temperature conditions. The cement content used should be sufficient to provide adequate workability with a low water cement ratio and with the available compaction method.