Types of Formwork (Shuttering) for Concrete Construction:

Types of Formwork (Shuttering) for Concrete Construction:

Timber Formwork:

Timber for formwork should satisfy the following requirement:

It should be

1. well seasoned
2. light in weight
3. easily workable with nails without splitting
4. free from loose knots

Timber used for shuttering for exposed concrete work should have smooth and even surface on all faces which come in contact with concrete.

Compressive Strength Definition

Compressive Strength Definition

Compressive strength is the ability of material or structure to carry the loads on its surface without any crack or deflection. A material under compression tends to reduce the size, while in tension, size elongates.

Cement and History of cement

Cement

• A cement is a binder, a substance used in construction that sets and hardens and can bind other materials together.

• The most important types of cement are used as a component in the production of mortar in masonry, and of concrete, which is a combination of cement and an aggregate to form a strong building material.

Cement History

• Joseph Aspedin of Yorkshire (U.K.) was the first to introduce Portland cement in 1824 formed by heating a mixture of limestone and finely divided clay in a furnace to a temperature high enough to drive off the carbonic acid gas.
• In 1845, Issac C. Johnson invented the cement by increasing the temperature at which the mixture of limestone and clay were burned to form clinker. This cement was the prototype of the modern Portland cement.
• From then onward, a gradual improvement in the properties and qualities of cement has been made possible by researchers in U.S.A., U.K., France and Germany

Cement Clinker

In the manufacture of Portland cement, clinker occurs as lumps or nodules, usually 3 millimeters (0.12 in) to 25 millimeters (0.98 in) in diameter, produced by sintering (fused together without melting to the point of liquefaction) limestone and alumino silicate materials such as clay during the cement kiln stage.

Today cement finds extensive use in all types of construction works; in structures where high strength is required e.g. bridge piers, light houses, lofty towers, and large structures such as bridges, silos, chimneys. And also in structures exposed to the action of water, e.g. reservoirs, dams, dock yards etc. Cement mortar, concrete, reinforced brick work, artificial stones, plastering, pointing and partition walls are routinely used in buildings.

Lime Production

Lime Production

When limestone (calcium carbonate) is heated, at about 1000 °C it undergoes thermal decomposition. It loses carbon dioxide and turns into quicklime (calcium oxide).

The reaction is carried out in specially constructed lime kilns(a kiln is a high temperature oven). Limestone is added at the top and quicklime is removed from the bottom in a continuous process.

Lime Cycle

Lime Production

FACTORS AFFECTING CONCRETE ADMIXTURES PERFORMANCE

The various factors which affect the performance of concrete admixtures are:

1. Type of super-plasticizer:

The admixture will be more effective if molecular weight of the super-plasticizer is high.

2. Dosage:

The quantity of admixture should be optimum. Excess of admixture may cause segregation or bleeding. It may also cause excessive retardation. The optimum does should be estimated by trials.

3. Compatibility with Cement:

All admixtures may not produce same results with different cements. Therefore before using any admixture, its compatibility with cement has to be established. Properties of cement like fineness, chemical-composition, C3A content etc. affect the performance of admixture. Therefore, trials have to be made before finalizing an optimum does of admixture.

4. Mix Design:

All constituents of mix affect the performance of the super-plasticizer as given below:

• Water: more water in the mix improves the physical interaction and dispersion of admixtures.
• Coarse aggregate: proportioning and grading of coarse aggregates influence the performance of concrete admixture.
• Fine aggregate: proportioning, grading and silt content also influence the performance of concrete admixture.
• Cement: its fineness, C3A content influence the performance of admixture. Higher C3A reduces efficiency of admixture.
• Other admixture: presence of other admixtures also influences the performance of concrete admixtures.

Therefore, proper trials before actual use are very vital for effectiveness of admixture.

Other factors admixture performance:

Certain other factors like temperature and humidity at the time of concreting also affect the performance of the concrete admixtures.

Drum mixtures are considered ideal for mixing admixtures, instead we should use pan or compulsive shaft mixes.

What are the reasons of establishing minimum area of reinforcement and maximum area of reinforcement?

Beams may be designed to be larger than required for strength consideration owing to aesthetics or other reasons. As such, the corresponding steel ratio is very low and the moment capacity of pure concrete section based on the modulus of rupture is higher than its ultimate moment of resistance. As a result, reinforcement yields first and extremely wide cracks will be formed. A minimum area of reinforcement is specified to avoid the formation of wide cracks.

On the other hand, a maximum area of reinforcement is specified to enable the placing and compaction of fresh concrete to take place easily.