18-09-2014, 10:49 AM
Concrete as a Material
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Concrete as a Material
Concrete is a material that literally forms the basis of our modern society. Scarcely any aspect of our daily lives does not depend directly or indirectly on concrete. We may live, work, study, or play in concrete structures to which we drive over concrete roads and bridges.
Our goods may be transported by trucks traveling on concrete super high ways. by trains that run on rails supported on concrete crossties., by ships that moor at concrete piers in -harbors protected by concrete breakwaters, or by airplanes landing and taking off on concrete runways.
Water drinking and for raising crops is stored behind massive concrete dams and is distributed by systems of concrete waterways., conduits., and pipes..
Concrete plays a key role in oil production, being used to construct massive off shore platforms for drilling .
We take concrete for granted in our everyday activities and tend to be impressed by the more dramatic impacts of technology.
it can be truly said that many of the achievements of our modern civilization have depended on concrete, just as many of the enduring achievements of the earlier civilization of Rome were ;made possible by the use of the forerunner of modern concrete.
The word concrete comes from the Latin verb "concretus," which means to grow together.
The Nature Of Concrete
Concrete is a composite material composed of coarse granular material
(the aggregate or filler) embedded in a hard matrix of material (the cement or binder) that fills the space between the aggregate particles and glues them together. Aggregates can be obtained from many different kinds of materials, although we mostly make use of the materials of nature common rocks.
They are essentially inert, filler materials that, for convenience. are separated into fine and coarse factions. Similarly, the cement can be formulated from many diverse chemicals. "Cement"' is a generic term that can apply to all binders. Therefore, descriptors must he used to qualify this term when referring to specific materials. A civi1 engineer may have cause to use
Advantages Of Concrete
concrete properties can vary significantly from the figures given, depending on the choice of materials and proportions of a particular application.
For example, subsequent chapters will show how concrete can be designed to have compressive strengths <10 MPa (1500 Ib/in.2) or > 100 MPa (15,000lb/in.2) with concomitant changes in modulus of elasticity.
The ability of concrete to be cast to any desired shape and configuration is an important characteristic that can offset other shortcomings.
Concrete can be cast into soaring arches and columns, complex hyperbolic shells, or into massive, monolithic section; used in dams, piers, and abutments. On-site.
construction means that local materials can be used to a large extent, thereby keeping costs down. Cement costs only about 7-10 cents/kg (3--4 cents/lb) (2001) and aggregates less than 2 cents/kg (<1 cents/b).
Furthermore. fabricating concrete on site, its properties may be tailored for the specific application.
On the other hand, on-site production is a mixed blessing because the quality of concrete must be carefully controlled. Environmental conditions fluctuate, so that it is difficult to assure uniform process41g of concrete throughout a job
Constituent materials are less carefully characterized than they might be and can have undesirably high variations in properties. The use of an unskilled or semiskilled work force means that in the absence of proper supervision on the job site, undesirable practices they be adopted and tolerated.
Casting of concrete can also be adapted to factory-controlled production.
precast building elements for standardized low. cost building systems arc more common in European countries, but have also been developed in the United States.
Limitation Of Concrete
Even in compression, concrete has a relatively low strength-to-weight ratio, and a high load capacity requires comparatively large masses of concrete, although, since concrete is low in cost, this is economically possible.
The volume instability of concrete must also be allowed for in design and construction.
It shows volume stability that is more characteristic of timber and quite unlike that of steel, which is a volume-stable material under normal conditions of service
Concrete undergoes considerable irreversible shrinkage due to moisture loss at ambient temperatures and also creeps significantly under an applied load even under conditions of normal service.
Awareness of these problems with concrete enables us to compensate for them, by using suitable designs and by controlling them, in part, through a suitable choice of materials and construction practices.
A great deal of research effort has been devoted to ameliorating these problems and now ready-mixed concrete with compressive strengths of 100 MPa (15,000 Ib/in.2) can be routinely produced in some areas.
Over the last 30 years, new types of concrete have been developed, such as fiber. Reinforced concrete, shrinkage-compensated concrete,
Cement based materials with flexural strengths exceeding 150 MPa (22,000 Ib/in.2) or with tensile
strains greater than 1% have been produced