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How High-Strength Concrete differ from Normal Concrete?
How High-Strength Concrete differ from Normal Concrete? According to the compressive strength value, concrete may be classified as either normal-strength concrete or high-strength concrete (HSC).
Normal-strength concrete usually obtains a compressive strength that ranges between 20 and 40 MPa. On the other side, HSC will possess a compressive strength beyond 40 MPa and can reach up to 140 MPa
The limit that separates the two types of concrete has changed with time. For example, a century ago, a strength above 30 MPa distinguished HSC from normal concrete. Nowadays, researchers have actually designed concretes that can reach 500 MPa.
The current norm in the construction industry is the use of normal-strength concrete. High-strength concrete are employed in situations where there is a need to reduce the dead load of a structure, thin elements to maintain the aesthetic features of buildings, etc.
Features of Normal-Strength and HSC
Regardless of the type of concrete, concrete in the fresh state has to be plastic in nature to allow easy molding, placing, finishing etc.
Uniform distribution of aggregates (sand and gravel) needs to be maintained in the mixture.
Care should be taken to prevent bleeding and segregation.
How Workability differs between Normal concrete and HSC
Workability usually refers to the ease with which concrete activities are performed including casting, compacting, and finishing the surface.
Normal strength concrete has suitable workability features as long as proper mixture design was made by the engineer and all components are in their respective percentages in the batch. Well-graded aggregated assists in such case.
On contrary, HSC can be sticky and harsh in multiple cases. This leads to increased effort in dealing with the concrete activities. Superplasticizers may help with such a situation by it not always the case. The reason why workability is low for HSC is the highly existing content of cement.
How Bleeding differs between Normal concrete and High Strength Concrete
A little time after finishing concrete, solid particles in concrete (aggregate, for example) start to settle in the freshly mixed concrete. This cause a thin layer of water to form on top of the surface of concrete, which is known as bleeding. Little bleeding is alright, whereas excessive bleeding has a negative effect on both the strength and durability of concrete.
The previous description works for normal concrete.
HSC does not bleed due to the significantly lower water content and high cement content in such concrete.
How Permeability differs between Normal concrete and High Strength Concrete
To start, permeability is a key factor in controlling the durability of concrete. Nearly, all damage mechanisms occurring to concrete has something to do with its permeability; whether it is corrosion, sulfate attack, AAR etc. In summary, pores in concrete represent the pathways for harmful substances to enter concrete.
In rapid chloride penetrability test or hydraulic permeability test, HSC shows much lower permeability value compared to normal concrete. For example, charges from RCPT can be 44 for HSC compared to 3000 for normal concrete.
How Carbonation differs between Normal concrete and HSC
Carbonation of concrete starts from the surface exposed to air, proceeding inwards. The end result of carbonation is the conversion calcium hydroxide (also in some cases C-S-H) into calcium carbonates.
Carbonation proceeding at a significantly slower rate for HSC compared to normal concrete due to its lower porosity.
How Cracking differs between Normal concrete and HSC
For the normal type of concrete, micro-cracking starts to develop when concrete is load to 40-50% of its compressive strength. Once loading value reaches 80% or more of the strength value, cracks propagate and connect to each other.
The surface of fracture occurring in normal concrete is rough while that happens in HSC is smooth