Introduction
Concrete is effectively an artificial stone or rock. Its primary properties
are that is is workable before hardening, strong in compression and stays strong for extremely long
timescales. Concrete is a strong hard building material composed of sand and gravel and
cement and water. It is used for making buildings, roads, bridges, vessels
pipes etc etc etc. As the concrete formulations develop concrete is increasing its
range of applications such that it is making inroads into those presently monopolised by
metals. To enable concrete to withstand tensile loads it is often reinforced with steel rebars or
with natural or artificial fibres...
Concrete is a mixture of a binding agent (generally cement) to bond the other
materials together :, fine aggregate (sand), coarse aggregate ( gravel/stones ), and water.. A typical
composition is about 7-15% cement, 14-21% water and the rest agregate.
The water/cement ratio (w/c) of the mixture has
a control over the final properties of the concrete.
The water/cement ratio is the relative weight of the water to the
cement in the mixture.
The water/cement ratio is a factor selected by
the civil engineer. Selection of a w/c ratio gives the engineer
control over two desirable properties: strength and workability.
A mixture with a high w/c will be more workable than a mixture with a low w/c:
it will flow easier. But the less workable the mixture,
the stronger the concrete will be. The civil engineer must decide what
ratio will give the best result for the given situation.
The water/cement ratio needs to be about 0.25 to complete the hydration reaction.
Typical values of w/c are between 0.35 and 0.40 because they give a
good amount of workability without sacrificing a lot of strength.
Types of Concrete. (selection of types available).
| Type of Concrete | Description |
| Cement concrete |
This is the most common type of concrete and is made mostly from portand
cement, sand, aggregate and water. It is used reinforced and un-reinforced for structures,
roads, foundations. The compositions of cement, sand and aggregate vary from 1: 1: 2
(a richest practical mixture) to 1: 3 :6 (a lean mixture used for concrete filling) |
| Plain mass concrete |
Concrete not strengthened by reinforcement. Used for foundations and mass structures
such as damn, and gravity retaining walls.. also called non-reinforced concrete |
| Lightweight cellular aerated concrete |
Concrete with a density of 320 kg/m3 upwards is suitable for wide varieties of duties including thermal insulation applications. it has lower strength
than ordinary dense concrete . This product is made using one of the following methods
1)Use of lightweight agregate e.dg foamed blast furnace slag, clinker , pumic etc.
2) Use of agregates from which smaller particles have been removed (no -fines concrete)
3) Aeration, foaming , /cellular. Involving introduction of entrapped bubles in mixture by mechanicl or chemical processes. |
| Lean concrete |
A plain concrete with a large ratio aggregate to cement than structural concrete. It is used for filling and not structural duties |
| Structural concrete |
Lightweight concrete of such a quality that it is suitable for load-bearing members of structures.
If it is a compact concrete made with stone aggregate , it is of comparatively high density ( about 2.4 ) and great strength. If it is based
on lightweight aggregate then high strengths are available but the design generally requires special considerations |
| Reinforced concrete |
Lightweight concrete of such a quality that it is suitable for load-bearing members of structures.
If it is a compact concrete made with stone aggregate , it is of comparatively high density ( about 2.4 ) and great strength. If it is based
on lightweight aggregate then high strengths are available but the design generally requires special considerations |
| Prestressed concrete |
Structural concrete which is subjected to compression in those parts which in service are subjecte to tensile forces so
that generally,the concrete is nowhere is a state of tension under the working load. |
| Cast in Place /Cast in Situ Concrete |
This is deposited in its permanent position to harden. This is the most common method of construction and when to concrete
is not deposited on the ground as for roads and similar it is generally placed in temporary
moulds or is contained within formwork or shuttering. |
| Precast Concrete |
This is concrete which placed in separate moulds, under controlled factory
conditions, to harden and when required transferred to site for final erection.
This procedure allow high quality concrete castings to be made at low relative costs.
This method is used for the production of paving slabs, bricks, road
channels , kerbs lintels, fence posts, bridge beams etc etc. Precast units can
include re-inforcement and Engineered steel inserts. |
| Vacuum Concrete |
This is concrete which includes high water content to allow sufficient workability
to enable it to be placed into complicated moulds or around extensive reinforcement.
The concrete is then subject to a vacuum which removes significant quantities
of water resulting in a stronger concrete on hardening. Pumped concrete needs
to include higher water content to improve the flow characteristics. If a high
strength concrete is required then special additives are use in place of the additional water.
Concrete pumping stations may be static or mobile
|
| Pumped Concrete |
Concrete which is conveyed from the mixer to the point of deposit through pipes.
The concrete is discharged from the mixer into a hopper which feeds it inot a pump which forces it through the pipe.
The pipe is 100 or 150mm dia and the method can be used to pump over distances of
650m horizontally or 50m vertically., or some combination of these lengths..
lengths
|
| Spun Concrete |
This process is used for the production of vessels and pipes. The process involves
feeding relatively dry concrete into a rotating cylinderical mould. The concrete is flung against
the wall be centrifugal action to form a dense hard impermeable wall.
|
| Ready mixed Concrete |
Concrete which is made at a mixing plant and delivered to the site in special transport vehicles.
The transport includes a rotating drum in which the concrete is continuously mixed until it
is discharge on site. The mix specification is agreed between the supplier
and the user prior to delivery and generally a high quality product results .
|
| Water Resistant Concrete |
Water Resistant concrete can either be water proofed or watertight.
- Waterproofed concrete is formed with a water resistant layer or surface whilst the mass of concrete being ordinary concrete. The water tight layer
can be formed using a spray of lacquer, or applying a coat of asphalt or bitumen or using a wash of soda (water glass)
- A watertight concrete can be produced by ensuring and dense product using tight quality control
of the production process. The concrete so formed can be sufficiently water-tight to enable use
for tanks retaining water
|
| High density Concrete |
High density concrete for use as nuclear shield walls and ballast blocks and sea walls can be produced
by using different materials for the aggregate. Candidate materials include
barytes, haematite, iron shot, steel shot and lead shot.
|
| Fibre reinforced Concrete |
High strength high performance concrete can be produced by including short fibres in the mix.
A number of reinforcement materials are available including glass, nylon, polypropylene, carbon
and steel. Concrete so formed as increased strength, impact resistance and greater strength.
This is an area of concrete development which is under continuous development.
|
| Fire resistant concrete |
The fire resistance of ordinary cement concrete depends on the agregate. For fire resistant concrete limestone and blast furnace slag is
preferred to flint , granite, and other natural stone agregated. Pumice, foamed blastfurnace slag, crushed brick various burnt clay products, and burned clinker
provide very good fire resitant properties but these agregates are not commonly used for reinforced concrete. |
| Spun concrete |
This is obtained by feeding dry concrete into a rapidly rotating cylinderical mould. It is forced by centrifugal force against the sides
and produces a dense hard impermeable wall ideal for concrete pipe. |
Concrete Strength / Designation
Relevant Standards..
BS EN 206-1:2000 Concrete. Specification, performance, production and conformity
BS 8500:2002, Concrete. Complementary British Standard to BS EN 206-1. Method of specifying and guidance for the specifier
Note: BS 5328: Parts 1-4: 1997- ..Has been superseded by the above standards..
According to the above standards the compressive strength class of a concrete is expressed
as
- Prefix: �C� for normal-weight and heavyweight concrete or �LC� for lightweight concrete
- The minimum characteristic 150 � 300mm cylinder strength required
- A Backslash (/)
- The equivalent minimum characteristic 150mm (and, in the UK, 100mm) cube strength required.
|
The compressive strength classes are in N/mm (MPa)
Typical concrete classes include
- Normal Concretes
C8/10, C12/15, C16/20, C20/25, C25/30, C28/35,
C30/37, C32/40, C40/50, C50/60, C55/67, C60/75, C70/85, C90/105, C100/115
- Lightweight Concretes
LC8/9, LC12/13, LC16/18, LC20/22, LC25/28, LC30/33,
LC35/38, LC40/44, LC50/55, LC55/60, LC60/66, C70/77, LC80/88
|
Density of Concrete
Concrete is produced in a range of densities as listed below
- Plain Concrete, with natural stone aggregate ...2300 kg/m3
- Plain Concrete, with natural Broken brick aggregate...2000 kg/m3
- Reinforced Concrete, with dense aggregate ...2400 kg/m3
- Reinforced Concrete, with dense aggregate ...2400 kg/m3
- Lightweight cellular (aerated) Concrete ...641 kg/m3
- Lightweight aggregate structural grade Concrete ...1760 kg/m3
- Lightweight aggregate (structural grade) Concrete ...1760 kg/m3
- Steelshot aggregate Concrete ...5285 kg/m3
|
Mechanical/Physical properties structural concrete
Young's Modulus : 15 - 40 kN/mm2
Poissens ratio :0,2 (BS 8110 part 1)
Thermal conductivity (5% moisture ):1,6 - 2,2 W/m/K
Specific Heat : 1,1 KJ/Kg/K
Coefficient of theraml expansion 7x10-6 - 14x10-6
The above values are typical of structural concrete..other grades on concrete have very different values
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