CONCRETE:
What is concrete?
Concrete is a mixture of cement, water, sand and gravel (stones, crushed rock). The mixture eventually hardens into a stone-like material. Concrete is an essential material. With a worldwide estimated consumption of between 21 and 31 billion tonnes of concrete in 2006, concrete is the second most consumed substance on Earth after water. A world without concrete is almost inconceivable.
Components of a Basic Concrete Mix
There are three basic ingredients in the concrete mix:
What is concrete?
Concrete is a mixture of cement, water, sand and gravel (stones, crushed rock). The mixture eventually hardens into a stone-like material. Concrete is an essential material. With a worldwide estimated consumption of between 21 and 31 billion tonnes of concrete in 2006, concrete is the second most consumed substance on Earth after water. A world without concrete is almost inconceivable.
What is a precast concrete ?
A precast concrete product is a
factory-made piece manufactured
with concrete and which, later,
together with other pieces, will
become part of a larger structure.
Precast concrete elements are
prepared, cast and hardened at specially equipped plants with a permanent location. Once a precast concrete product is produced and all the undertaken quality controls satisfactory, the unit is stored until delivery. It is then transported for use at another site.
The main advantages of such a process are:
- the intrinsic quality of an industrial product, manufactured in a controlled environment and with accurate methods;
- advanced quality control, which goes far beyond the checking of the fresh concrete, can be introduced. Dimensional accuracy, properties of the hardened concrete and position of reinforcement can all be checked before inclusion of an element in the final work;
- factory-made products are independent of weather conditions and can be proceeded separately from construction work on site.
Components of a Basic Concrete Mix
There are three basic ingredients in the concrete mix:
- Portland Cement
- Water
- Aggregates (rock and sand)
Portland Cement - The cement and water form a paste that coats the aggregate and sand in the mix. The paste hardens and binds the aggregates and sand together. Portland cement is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout. It is a fine powder produced by grinding Portland cement clinker (more than 90%), a limited amount of calcium sulfate (which controls the set time) and up to 5% minor constituents as allowed by various standards such as theEuropean Standard EN197-1:
Portland cement clinker is a hydraulic material which shall consist of at least two-thirds by mass of calcium silicates (3 CaO·SiO2 and 2 CaO·SiO2), the remainder consisting of aluminium- and iron-containing clinker phases and other compounds. The ratio of CaO to SiO2 shall not be less than 2.0. The magnesium oxide content (MgO) shall not exceed 5.0% by mass.
Water- Water is needed to chemically react with the cement (hydration) and too provide workability with the concrete. The amount of water in the mix in pounds compared with the amount of cement is called the water/cement ratio. The lower the w/c ratio, the stronger the concrete. ( higher strength, less permeability)
Aggregates- is a broad category of coarse particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates. Aggregates are the most mined material in the world. Aggregates are a component of composite materials such as concrete and asphalt concrete; the aggregate serves as reinforcement to add strength to the overall composite material. Due to the relatively high hydraulic conductivity value as compared to most soils, aggregates are widely used in drainage applications such as foundation and French drains, septic drain fields, retaining wall drains, and road side edge drains.
Properties of Hardened Concrete
The properties of hardened concrete that are of interests are:
- permeability
- frost resistance
- resistance to abrasion
- strength
Among the above properties, strength is the most important single property, since the first (and most important) consideration in structural design is that the structural members nust be capable of carrying the imposed loads.
Permeability
Concrete which is made with low water/cement ratio and well-compacted has good resistance to the absorption of water, i.e. low permeability. A concrete that is low in permeability has high durability.
Frost resistance
Concrete can be damaged by expansion of ice crystals (in cold weather) form in capillary pores or cracks. Air-entrainment admixtures form discontinuous minute bubbles which improve resistance to frost.
Resistance to abrasion
Depends upon the hardness of the aggregate particles and the ability of the mortar matrix to retain them (i.e. minimal bleeding).
Strength of hardened concrete
As mentioned earlier, strength is the most important single property. Strength of concrete can be compressive or tensile.
Cracking (failure) pattern in normal strength concrete (see Fig 20.5)
Necessary to understand the cracking pattern
The Crack in the Concrete.
Factors influencing strength of concrete
- Water/cement ratio
- Effect of age
- Temperature during hydration
- Humidity during curing
- Aggregate properties
Test Of Concrete
This video show how the compression machine compress the concrete cylinder.
Concrete is reinforced to give it extra tensile strength; without reinforcement, many concrete buildings would not have been possible. Reinforced concrete can encompass many types of structures and components, including slabs, walls, beams,columns, foundations, frames and more. Reinforced concrete can be classified as precast or cast in-situ concrete. Designing and implementing the most efficient floor system is key to creating optimal building structures. Small changes in the design of a floor system can have significant impact on material costs, construction schedule, ultimate strength, operating costs, occupancy levels and end use of a building.
Reinforced Concrete
Reinforced concrete is concrete in which reinforcement bars ("rebars"), reinforcement grids, plates or fibers have been incorporated to strengthen the concrete in tension. It was invented by French gardener Joseph Monier in 1849 and patented in 1867. The term Ferro Concrete refers only to concrete that is reinforced with iron or steel. Other materials used to reinforce concrete can be organic and inorganic fibres as well as composites in different forms. Prior to the invention of reinforcement, concrete was strong in compression, but weak in tension. Adding reinforcement crucially increases the strength in tension. The failure strain of concrete in tension is so low that the reinforcement has to hold the cracked sections together.
For a strong, ductile and durable construction the reinforcement needs to have the following properties:
§ High strength
§ High tensile strain
§ Good bond to the concrete
§ Thermal compatibility
§ Durability in the concrete environment
The Photo Show That The Failure Of Reinforced Concrete.
Blinding concrete
Blinding concrete is required just to form a level base for the waterproofing layer and for placing rebar before casting the raft, so it is not meant to be water-tight by itself. It also can called lean concrete. Therefore there should be no problem in casting this in bays with construction joints. However, there must be a system of temporary dewatering which prevents groundwater from exerting any upwards pressure, otherwise the blinding will heave and you will not be able to lay your waterproofing, etc.
The Worker is pouring the lean concrete on the ground.
Building Made By Concrete
Concrete or a bonding device, was used ever since the Romans
invented it in the 300s BC. Concrete was and is still used
everywhere in arches, buildings and roads. In Rome, for example,
the Romans created the pantheon made of concrete.
It's CITIC Plaza in Guangzhou, its structural height is 391 m
(1,283 ft) 80 stories including two tall antenna-like spires on the
top. Completed in 1997, it is the tallest concrete building in the world.
Thing is, high buildings are generally built from a 'plan libre' -
the façade is rarely load bearing. Concrete is a rather
expensive (and sort of unlogical) material to make a
non-load-bearing façade with. But it has been in fashion
for a while to work with concrete panels.
Water Tower Place in Chicago. The world's tallest concrete tower when it was built in 1973.
This one is some communist concrete supertall! The
Ryugyong hotel is 330m tall and has 105 floors.
Located in Pyongyang, capital of North Korea.
Construction started in 1987 bu stopped in 1992.
1 comments:
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