Introduction of Concreting in Construction
Concrete is most frequently used man-made construction material in the world and is second only to water as the most used substance on Earth. It’s obtained by mixing cementing materials, water, and aggregates, and at times admixtures, (shown in as per below figure) in required proportions.
The mix, when put in forms and allowed to heal, invisibly to a rock-like mass called concrete. The hardening is due to a chemical reaction between cement and water, and it lasts for quite a while, and thus, the concrete grows stronger with age.
The hardened concrete also is considered as an artificial stone where the voids of larger particles (coarse aggregate) are stuffed with the smaller particles (fine aggregate) and the voids of fine aggregates arc full of cement.
Basic of Concrete
At a concrete mix, the cementing material and water form a paste called cement—water paste which along with filling the voids of fine aggregate, coats the surface of both coarse and fine aggregates and binds them together as it heals secures, thereby cementing the particles of the aggregates together in a compact mass.
The durability, strength, and other characteristics of concrete depend upon the attributes of its ingredients, on the proportions of a mix, the method of compaction and other controls during placing, compaction, and curing.
The popularity of the concrete is a result of the fact that by the typical ingredients, it’s possible to tailor the properties of concrete to meet the demands of any particular situation. The images in as below figure illustrate the mouldability of concrete in architectural forms.
Architral of Concrete
The improvements in concrete technology have paved the way to make the best use of locally available materials by judicious mix proportioning and appropriate workmanship, in order to produce concrete satisfying performance demands.
The secret to producing a solid, lasting, and durable concrete, i.e., high-performance concrete lies at the careful control of its fundamental and process elements. These are the following:
Portland cement, the most widely used cementing ingredient in present-day concrete, comprises Phases that consist of compounds of calcium, silicon, iron, aluminum, and oxygen.
All these are primarily naturally occurring, inert granular materials like sand, gravel, or crushed stone.
But, technology is broadening to include using recycled materials and artificial products.
The water content, as well as the minerals and chemicals dissolved in it, are critical to attaining quality concrete.
These are the Ingredients in concrete besides Portland cement, water, and aggregates which are added into the mixture
immediately prior to or during mixing to reduce the water requirement, accelerate/retard setting or improve specific durability characteristics.
Supplementary Cementing Materials
Supplementary cementing materials, also called mineral additives, Contribute to the properties of hardened concrete through hydraulic or pozzolanic action.
Average examples are natural pozzolans, fly ash, ground granulated blast-furnace slag, and silica fume.
After the concrete is placed, these elements have to be cured at a satisfactory moisture content and temperature has to be carefully kept for a sufficiently long time to allow adequate maturation of the strength of the concrete.
Classification of Concreting in Construction
As stated before, the main components of concrete are cement, fine aggregate (sand) and coarse aggregate (gravel or crushed rock).
It’s usual to define a particular concrete from the proportions (by weight) of those constituents and their features, e.g., a 1:2:4 concrete describes a particular concrete made by mixing cement, sand and broken stone at a 1:2:4 ratio (using a specified form of water-cement ratio, cement, maximum size of aggregate, etc.).
This classification specifying the proportions of components and their traits is termed as prescriptive specifications and is founded on the expectation that adherence to these prescriptive specifications will result in satisfactory performance Rewrite Sentence.
Alternately, the specifications specifying the requirements of the desirable properties of concrete like strength, workability, etc., are specified, and these are termed as performance-oriented specifications. Based on these considerations, concrete can be classified either as nominal mix concrete or designed mix concrete.
Sometimes Sometimes concrete is categorized into controlled concrete and ordinary concrete, depending upon the levels of control exercised at the works along with the way of proportioning concrete mixes.
Accordingly, a concrete with ingredient proportions fixed by designing the concrete mixes with preliminary tests are called controlled concrete, whereas ordinary concrete is one where nominal mixes are adopted.
In IS:456-2000, there is nothing like uncontrolled concrete: only the degree of control varies from very good to poor or no control.
In addition, to mix proportioning, the quality control includes a selection of appropriate concrete materials after proper tests, proper workmanship in batching, mixing, transportation, placing, compaction and curing, coupled with necessary checks and tests for quality acceptance.
Properties of Concreting in Construction
Concrete making is not just a matter of mixing ingredients to produce a plastic mass, but good concrete has to satisfy performance requirements in the plastic or green state and also the hardened state.
In the plastic state, the concrete should be workable and free from segregation and bleeding.
Segregation is the separation of coarse aggregate, and bleeding is the separation of cement paste from the main mass.
The segregation and bleeding result in poor quality concrete. In its hardened state, concrete should be strong, durable, and impermeable, and it should have minimum dimensional changes.
One of the various attributes of concrete, its compressive strength is regarded as the most important and is accepted as an index of its general quality.
A number of different properties of concrete appear to be generally linked to its compressive strength.
Grades of Concreting in Construction
Concrete is usually graded in accordance with its compressive strength. The different grades of concrete as stipulated in IS:456-2000 and IS:1343-1980 are given in as per below table.
|Group||Ordinary Concrete||Standard Concrete||High Strength Concrete|
|Specified Charactistic strength at 28 Days, Mpa||10||15||20||25||30||35||40||45||50||55||60||65||70||75||80|
In the designation of concrete mix, the letter M refers to the mix and the number to the specified characteristic strength of 150 mm work cubes at 28 days, expressed in MPa (N/mm2).
The concrete of grades M5 and M7.5 is suitable for lean concrete bases, simple foundations, foundations for masonry walls, and other simple or temporary reinforced concrete constructions.
These need not be designed. The concrete of grades lower than M 15 is not suitable for reinforced concrete works and grades of concrete lower than M30 are not to be used in the prestressed concrete works.
Advantage & Disadvantage of Concreting in Construction
Advantage of Concrete
Here, the pros of concrete are as follows.
- Concrete is inexpensive in the long term compared to other engineering materials. Except for cement, it can be made from locally available coarse and fine aggregates.
- Concrete owns a higher compressive strength, and the climatic and climatic effects are nominal. When properly prepared, its strength is potency to that of a tough All-natural stone.
- The green or newly mixed concrete can be easily handled and molded or formed into virtually any shape or size according to specifications. The formwork can be reused a number of times for similar jobs resulting in economy.
- It’s strong in compression and has boundless structural applications together with steel reinforcement. Steel and concrete have approximately equal coefficients of thermal expansion. Steel and concrete have approximately equal coefficients of thermal expansion.
- Concrete May even be sprayed on and Stuffed into fine cracks for repairs from the uniting Procedure.
- Concrete may be pumped, and therefore it may be laid in difficult positions too.
- It is durable and fire-resistant and requires very little maintenance.
Disadvantage of Concrete.
Here, the cons of concrete are as follows.
- Concrete has reduced tensile strength and therefore cracks easily. Consequently, concrete is to be reinforced with steel bars or meshes or fibers.
- Fresh concrete shrinks on hardened and drying concrete expand on wetting. Provision for contraction joints must be made to prevent the development of fractures due to drying shrinkage and moisture movement.
- Concrete expands and contracts with all the changes in temperature. Consequently, expansion joints need to be offered to avoid the creation of cracks due to thermal movement.
- Concrete under sustained loading undergoes creep, resulting in the reduction of prestress in the prestressed concrete construction.
- Concrete isn’t completely impervious to moisture and contains soluble salts that might lead to efflorescence.
- Concrete is responsible for floating by alkali and sulfate attack.
- The lack of ductility inherent in concrete as a material is disadvantageous with respect to earthquake-resistant design.
Quality Control in Concrete
Quality in general terms is a totality of features and characteristics of a product or service that bear on its ability into satisfying the stated or implied needs.
The stated or implied needs are those derived by balanced excellence and equity within the sustainable regime and in the given socio-techno-economic scenario.
The quality management has evolved over the period through:
Acceptance and Rejection of thorough inspection and assessment by a user.
Confidence building through third party judgment.
Ensuring this quality of the final product by attending to quality at all intermediary stages such as in Certification Marking Schemes.
Concrete, generally manufactured at the site, is likely to have variability of performance from batch to batch and also within the batch.
The magnitude of the variation Depends upon several factors, such as the variation in the quality of constituent materials, variation in mix proportions due to batching process, variations in the quality of hatching and mixing equipment available, the quality of overall workmanship and supervision at the site, and variation because of sampling and testing of concrete specimens.
The preceding variations are inevitable throughout production to varying levels. By Way of Example, cement from different batches or sources may exhibit different strengths.
The grading and shape of aggregates, even in precisely the exact same origin, varies widely. Considerable variations occur partly because of the quality of the plant accessible and partially on account of the efficacy of the operation.
Some of the variations in test results are due to variations in Sampling, making, curing, and testing the specimen when completed with regard to relevant specifications.
The quality control of concrete is thus to reduce this variation and to produce concrete of uniform quality consistent with specified minimum performance requirements which can be achieved by good workmanship and maintenance of the plant at peak efficiency.
The concrete industry strives at making ‘quality,’ a way of life and a way of management through Quality Systems Approach covering all aspects of ISO 9000 series.