Normal concrete versus high strength concrete: properties and differences

Concrete is categorised as normal or high strength based on compressive strength. The compressive strength of normal concrete is between 20 and 40 MPa, while high strength concrete exceeds 40 MPa, ranging up to 140 MPa. Definitions have evolved over time. Concrete once considered high strength now appears ordinary, as technology today produces materials exceeding 800 MPa.

Workability

Normal strength concrete shows better workability when ingredients are properly proportioned. High strength mixes tend to be sticky and difficult to handle despite the use of plasticisers, due to their elevated cement content.

Bleeding

Bleeding occurs when solid particles settle, leaving water on the surface. High strength concrete experiences minimal bleeding because it has a smaller water content and a high amount of cementitious materials.

Permeability

Permeability directly affects durability, corrosion resistance and chemical attack resistance. Normal strength concrete typically shows a permeability of 1 x 10⁻¹⁰ cm/sec, while high strength concrete ranges from 1 x 10⁻¹¹ to 1 x 10⁻¹³ cm/sec. Additives such as silica fume, fly ash and GGBFS reduce permeability.

Carbonation

Carbonation occurs when atmospheric carbon dioxide reacts with compounds in hardened cement paste. High strength concrete shows reduced carbonation effects compared with normal strength concrete.

Fracture characteristics

Normal strength concrete develops rough fracture surfaces, with visible micro cracks forming at around 40 percent of compressive strength. High strength concrete shows smooth fracture surfaces, with crack propagation occurring at higher stress levels.