Prestressed concrete

Reinforced concrete

High-performance concrete

High-strength concrete

High-strength concrete has a compressive strength generally greater than 6,000 pounds per square inch (40 MPa). High-strength concrete is made by lowering the water-cement (w/c) ratio to 0.35 or lower. Often silica fume is added to prevent the formation of free calcium hydroxide crystals in the cement matrix, which might reduce the strength at the cement-aggregate bond.

Low w/c ratios and the use of silica fume make concrete mixes significantly less workable, which is particularly likely to be a problem in high-strength concrete applications where dense rebar cages are likely to be used. To compensate for the reduced workability, superplasticizers are commonly added to high-strength mixtures. Aggregate must be selected carefully for high-strength mixes, as weaker aggregates may not be strong enough to resist the loads imposed on the concrete and cause failure to start in the aggregate rather than in the matrix or at a void, as normally occurs in regular concrete.

In some applications of high-strength concrete the design criterion is the elastic modulus rather than the ultimate compressive strength.

High-performance concrete (HPC) is a relatively new term used to describe concrete that conforms to a set of standards above those of the most common applications, but not limited to strength. While all high-strength concrete is also high-performance, not all high-performance concrete is high-strength. Some examples of such standards currently used in relation to HPC are:

· Ease of placement

· Compaction without segregation

· Early age strength

· Long-term mechanical properties

· Permeability

· Density

· Heat of hydration

· Toughness

· Volume stability

· Long life in severe environments

Reinforced concrete contains steel reinforcing that is designed and placed in structural members at specific positions to cater for all the stress conditions that the member is required to accommodate.

Reinforced concrete structures are normally very heavy and they have to be designed to carry their own weight as well as the superimposed design loads. The high compressive forces found in concrete columns present few problems, but the tensile stresses found in slabs and beams present design challenges to engineers. Prestressed concrete provides a way to overcome the combined tensile stresses, due to own weight and design loads in beams and slabs, by introducing a compressive stress in the structural element prior to the superimposed design loads coming into play. The net effect in a properly designed prestressed structural element is a stress condition that satisfies the stress limits in the concrete for both compression and tension.

The prestressing is achieved by using steel tendons or bars that are subjected to a tensile force prior to casting the concrete, in pre-tensioned concrete, or only later once the concrete has cured, in post-tensioned concrete.

Дата добавления: 2014-12-23