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LEARN CIVIL ENGINEERING ON LINE

HIGH PERFORMANCE CONCRETE

High performance concrete is defined as concrete which meets special performance and uniformity requirements that cannot always be achieved routinely by using only conventional materials and normal mixing, placing, and curing practices. The requirements may involve enhancements of characteristics such as placement and compaction without segregation, long-term mechanical properties, early-age strength, toughness, volume stability, or service life in severe environments.
Examples of high performance concretes are:
1. High Workability Concrete
The workability of fresh concrete should be suitable for each specific application to ensure that the operations of handling, placing and compaction can be undertaken efficiently.

EN206 : BS8500 the European and UK standards for concretegives guidance on workabilitys for different uses.

The handling and placing properties of concrete mixes can be improved considerably by the use of cement replacement materials such as pulverised fuel ash or ground granulated blast-furnace slag. Further more, the use of admixtures such as water reducers and superplasticisers have beneficial effects on workability without compromising other concrete properties.

On site productivity can be greatly increased by utilising highly workable concretes. They are especially suitable in the following applications:

• inaccessible locations
• large flat areas
• underwater applications
• pumping concrete over long distances

Pumping
Pumping is one of the most popular techniques worldwide to transport fresh concrete. When placing large quantities of fresh concrete, piston pumps are generally used. Concrete is pushed alternately by two pistons acting in cylinders.


2.0 Self Compacting Concrete (SCC)
SCC has been described as "the most revolutionary development in concrete construction for several decades". Originally developed to offset a growing shortage of skilled labour, it has proved beneficial economically because of a number of factors, including:

• faster construction
• reduction in site manpower
• better surface finishes
• easier placing
• improved durability
• greater freedom in design
• thinner concrete sections
• reduced noise levels, absence of vibration
• safer working environment
Originally developed in Japan, SCC technology was made possible by the much earlier development of superplasticisers for concrete. SCC has now been taken up with enthusiasm across Europe, for both site and precast concrete work

3.0 Foamed Concrete
Foamed concrete is a highly workable, low-density material which can incorporate up to 50% entrained air. It is generally self-leveling, self-compacting and may be pumped. Foamed concrete is ideal for filling redundant voids such as disused fuel tanks, sewer systems, pipelines, and culverts - particularly where access is difficult. It is a recognised medium for the reinstatement of temporary road trenches. Good thermal insulation properties make foamed concrete also suitable for sub-screeds and filling under-floor voids.

4.0 High Strength Concrete
The definition of high strength concretes is continually developing. In the 1950s 34N was considered high strength, and in the 1960s compressive strengths of up to 52N were being used commercially. More recently, compressive strengths approaching 138N have been used in cast-in-place buildings.
High-strength concrete columns can hold more weight and therefore be made slimmer than regular strength concrete columns, which allows for more useable space, especially in the lower floors of buildings.
5.0 Lightweight Concrete
Lightweight concretes can either be Lightweight Aggregate concrete, Foamed concrete or Autoclaved Aerated concrete (AAC). Such lightweight concrete blocks are often used in house construction.
Lightweight Aggregate Concrete
Lightweight aggregate concrete can be produced using a variety of lightweight aggregates. Lightweight aggregates originate from either:

Natural materials like volcanic pumice
The thermal treatment of natural raw materials like clay, slate or shale i.e. Leca
Manufacture from industrial by-products such as fly ash, i.e. Lytag
Processing of industrial by-products like FBA or slag

The required properties of the lightweight concrete will have a bearing on the best type of lightweight aggregate to use. If little structural requirement, but high thermal insulation properties are needed, then a light, weak aggregate can be used. This will result in relatively low strength concrete.

Lightweight aggregate concretes can however be used for structural applications, with strengths equivalent to normal weight concrete.

The benefits of using lightweight aggregate concrete include:

• Reduction in dead loads making savings in foundations and reinforcement.
• Improved thermal properties.
• Improved fire resistance.
• Savings in transporting and handling precast units on site.
• Reduction in formwork and propping
6.0 Foamed Concrete
Foamed concrete is a highly workable, low-density material which can incorporate up to 50% entrained air. It is generally self-leveling, self-compacting and may be pumped. Foamed concrete is ideal for filling redundant voids such as disused fuel tanks, sewer systems, pipelines, and culverts - particularly where access is difficult. It is a recognised medium for the reinstatement of temporary road trenches. Good thermal insulation properties make foamed concrete also suitable for sub-screeds and filling under-floor voids.
7.0 No-fines Concrete
No-fines concrete is obtained by eliminating the fine material sand, from the normal concrete mix. The single sized coarse aggregates are surrounded and held together by a thin layer of cement paste giving strength of concrete.
The advantages of this type of concrete are lower density, lower cost due to lower cement content, lower thermal conductivity, relatively low drying shrinkage, no segregation and capillary movement of water, better insulating characteristics than conventional concrete because of the presence of large voids.
8.0 Sprayed Concrete
There are many applications where traditional formwork is not a suitable method of supporting concrete and so sprayed concretes can be used.
These applications include:
• Shell roofs and domes
• Retaining walls
• Piled wall facings
• Silo structures
• Diaphragm walls
• Blast proof structures
• Bank vaults
• Underground Construction
• Tunnel linings
• Free form structures such as those seen in theme parks
• Water Retaining Structures:
• Sea and river walls
• Reservoirs and dams
• Aqueducts
• Swimming pools

Other typical applications include where existing structures require maintenance and repair.
9.0 Water Resistant Concrete
Water resistant concretes are impermeable to water and other fluids either above or below ground. They are high density concretes that incorporate fine particle cement replacements.
10.0 Autoclaved Aerated Concrete (AAC)

AAC was first commercially produced in 1923 in Sweden. Since then, AAC construction systems such as masonry units, reinforced floor/roof and wall panels and lintels have been used on all continents and every climatic condition. AAC can also be sawn by hand, sculpted and penetrated by nails, screws and fixings
11.0 Roller Compacted Concrete
Roller compacted concrete, a durable paving material that carries heavy loads. It is now developing as a fast, economical construction method for dams, off-highway pavement projects, heavy-duty parking and storage areas, and as a base for conventional pavement. It is a stiff, no-slump concrete mixture with the consistency of damp gravel comprised of local aggregates or crushed recycled concrete, portland cement, and water. The mixture is placed and roller compacted with the same commonly available equipment used for asphalt pavement construction. The process requires no forms, finishing, surface texturing, or joint sawing and sealing.