Introduction
Concrete a composite man-made material, is the most widely used building material in the construction industry. It consists of a rationally chosen mixture of binding material such as lime or cement, well graded fine and coarse aggregates, water and admixtures (to produce concrete with special properties).
In a concrete mix, cement and water form a paste or matrix which in addition to filling the voids of the fine aggregate, coats the surface of fine and coarse aggregates and binds them together. The matrix is usually 22-34% of the total volume. Freshly mixed concrete before set is known as wet or green concrete whereas after setting and hardening it is known as set or hardened concrete.
The moulded concrete mix after sufficient curing becomes hard like stone due to chemical action between the water and binding material. It would be impossible to discuss all the aspects of this material in few pages and the discussion is confined to the general characteristics and quality tests necessary for its use by civil engineers and architects.
Most of the ancient structures and historical buildings had been constructed with lime concrete.
With the advent of cement, the use of lime concrete has been confined to making bases for concrete foundations and roof terracing.
The major factors responsible for wide usage of cement-concrete are mouldability, early hardening, high early compressive strength, development of desired properties with admixtures to be used in adverse situations, suitability for guniting, pumpability and durability.
Most of the chapter is devoted to cement concrete because of its versatility. The simple reason for its extensive use in the construction of almost all civil engineering works is that the properties can be controlled within a wide range by using appropriate ingredients and by special mechanical, physical and chemical processing techniques.
Buildings—from single storey to multistorey, bridges, piers, dams, weirs, retaining walls, liquid retaining structures, reservoirs chimneys, bins, silos, runways, pavements, shells, arches, railway sleepers are but a few examples of cement concrete applications.
Classifications.
1. Based on cementing materials.
Concretes are classified as lime concrete, gypsum concrete and cement concrete.
2. Based on perspective materials.
2. Based on perspective materials.
The cement concrete is specified by proportions of different ingredients, e.g., 1 (cement) : 1.5 (fine aggregate) : 3 (coarse aggregate). It is presumed that by adhering to such perspective specifications satisfactory performance may be achieved.
This type of concrete mix is also known as nominal mix.
Conventional nominal mix proportions have limited significance, since the quantity of fine aggregate is fixed irrespective of the cement content, water-cement ratio and the maximum size of aggregate to be used. The proportions of materials of nominal mix concrete are prevalent in field. However, IS: 456 restricts its use only up to M-20 grade
Conventional nominal mix proportions have limited significance, since the quantity of fine aggregate is fixed irrespective of the cement content, water-cement ratio and the maximum size of aggregate to be used. The proportions of materials of nominal mix concrete are prevalent in field. However, IS: 456 restricts its use only up to M-20 grade
3. Based on performance oriented specifications.
When the concrete properties such as strength, water-cement ratio, compaction factor, slump, etc., are specified the concrete may be classified as designed-mix concrete.
For a design mix concrete the mix is designed to produce the grade of concrete having the required workability and a characteristic strength not less than the appropriate values.
4. Based on grade of cement concreting Depending upon the strength (N/mm2) of concrete cubes (150 mm side) at 28 days
It is further classified as low strength concrete (< 20 N/mm2), medium strength concrete (20–40 N/mm2) and high strength concrete (>40 N/mm2).
5. Based on buk density.
On the basis of density, concrete is classified as super heavy (over 2500 kg/m3), dense (1800-2500 kg/m3), light weight (500–1800 kg/m3) and extra light weight concrete (below 500 kg/m3).
6. Based on place of casting.
When concrete is made and placed in position at the site it is known as in-situ concrete and when used as a material for making prefabricated units in a factory is known as precast concrete.
0 comments:
Post a Comment