High Strength Concrete

High Strength Concrete

High compressive strength

Concretes with high compressive strengths (> 60 MPa) are classified in the high performance concretes group and are used in many different structures. They are often used in the construction of high load bearing columns and for many products in precast plants.

Conventional high strength concrete mixes

In conventional high strength concrete production, the mix and the constituents require particular care, as does the placing.

·     High strength aggregates with a suitable particle surface (angular) and reduced particle size (< 32 mm)

·     A highly impermeable and therefore high strength cement matrix due to a substantial reduction in the water content

·     Special binders with high strength development and good adhesion to the aggregates (Silicafume)

·     Use of a soft concrete consistence using concrete admixtures to ensure maximum de-aeration

Sample mix:

CEM I 52.5                                      450 kg/m

Silicafume                                       45 kg/m

Aggregates                                     Crushed siliceous limestone0–16 mm

Eq. w/c ratio                                  0.28

Strength after 7 days 95 MPa

Strength after 28 days                110 MPa

Strength after 90 days                115 MPa

Innovative high strength concrete mixes

Many different alternative mixes for high strength concrete (and mortars) are being developed alongside conventional concrete mixes. The search for high strength constituents and a minimum water content is common to them all. Special aggregate particles and gradings with superplasticizers are used to achieve this. Strength development is also boosted by new drying and hardening techniques (such as compression hardening). Concretes produced in this way, which are more usually mortars, can reach strengths of 150 MPa to 200 MPa plus.

Note in particular that:

·     High strength concrete is always highly impermeable

·     Therefore the curing of high strength concrete is even more important than usual (inadequate supply of moisture from inside the concrete)

·     High strength concrete is also brittle because of its strength and increased stiffness (impact on shear properties)

·     By reducing the water content to below 0.38 some cement grains act as aggregate grains because not all of the cement can be hydrated

·     Apart from Portland cement, high strength concrete uses large quantities of latent hydraulic and pozzolanic materials which have excellent long term strength development properties