Concrete Technology
Introduction
The concrete is obtained by mixing cement, aggregates and water in required proportions, with or without a suitable admixture. The subject which deals with the fundamental principles of concrete is known as Concrete Technology. The practical utility of Concrete Technology for civil engineers is to enable them to know how to stock properly the materials required for concrete and to perform different tests for concrete.
Properties of Concrete
Classification of Concrete
Functions of Ingredients of Cement
Composition of Cement Clinker
Setting and Hardening of Cement
Types of Cements
Testing of Portland Cement
Aggregate
Specific Gravity and Bulk Density of Aggregate
Properties of Aggregate
Bulking of Sand
Deleterious Materials in Aggregates
Fineness Modulus
Water
Admixtures
Water Cement Ratio
Workability
It is defined as that property of freshly mixed concrete or mortar which determines the case and homogeneity with which, it can be mixed, placed, compacted and finished. The concrete mix is said to be workable, if it has mixability, stability, flowability or movability, compatability and finishability. The optimum workability of fresh concrete varies from situation to situation.
Sometimes, the terms consistency and plasticity are used to denote the workability of concrete mix. Following are the factors which affect the workability :
(a) Water Content : The workability of concrete largely depends upon its water content. It increases with increase in water content on account of greater lubrication.
(b) Size of aggregates : The concrete having large size aggregates is more workable than that containing small size aggregates because the smaller size aggregates require more quantity of water for lubrication.
(c) Shape of aggregates : The round shape aggregates increase the workability whereas the angular, flaky and elongated aggregates reduce the workability considerably.
(d)Surface texture of aggregates : The smooth surface aggregates increase the workability because less quantity of water is required for lubrication. On the other hand, rough surface aggregates reduce the workability because more quantity of waler is required for lubrication.
(e) Grading of aggregates : The grading of aggregates affect the workability and should be continuous lean concrete mix.
(f) Air entraining agents : The air bubbles produced due to adding air entraining agents in the concrete mix, act as rollers and thus increase the workability.
(g) Temperature : The workability of concrete mix reduces at high temperature.
Measurement of Workability
Gradation of Concrete
Methods of Proportioning Concrete
Storing of Ingredients of Concrete
The ingredients of concrete should be stored in a warehouse whose walls are of water proof masonry construction and the roof is leak proof. The importance of storing the various ingredients of concrete is to maintain the uniformity of grading and moisture. The capacity of a warehouse depends upon the floor area occupied by one cement bag and the height to which the cement bags are piled. In designing a warehouse, it is assumed that each bag contains 50 kg of cement and the floor area occupied by one bag of cement is 0.3 m². The height of each cement bag containing 35 litres of cement is 0.18 m. In order to prevent the cement bags from any possible contact with moisture, the cement bags should be placed closed together in the piles and the space between the exterior walls and piles should be 300 mm. The width and height of the pile should not exceed 3 m and 2.7 m respectively. The cement bags should be rolled on the floor when it is taken out for use. The ‘first-in, first-out’ rule should be applied when cement bags are to be removed from the warehouse. It may be noted that the strength of cement decreases with the passage of time.
Production of Concrete
Stripping of Forms
The removal of forms after the concrete has set is termed as stripping of forms. The period upto which the forms must be left in place before they are stripped is called stripping time. Under normal circumstances, the vertical sides of columns may be stripped after 1 to 2 days and the beam soffits may be removed after 2 days.