Plain concrete is known as concrete without steel reinforcement. It is an intimate mixture of cement, fine aggregates, coarse aggregates and water. It can be easily molded to desired shape and size before it hardens. Plain concrete is strong in compression but weak in tension.

Major ingredients of plain concrete are;

• Binding materials such as cement, lime, polymer.
• Fine aggregate material such as sand.
• Coarse aggregate materials like crushed stone, gravel, and granite.
• Clean water.

Common cement, sand and coarse aggregate ratios are;

1:1:2, 1:2:4, 1:3:6, 1:4:8

Constituents of plain concrete.

Cement:

Cement is the binding material. It binds and hardens when water is added. Generally a concrete mix with more cement gives more strength.

Coarse aggregate:

Coarse aggregates consists of crushed stones of igneous origin. They should be clean, hard, angular and sharp. They give mass to the concrete and prevent shrinkage of cement.

Fine aggregates:

River sand is the common fine aggregate for plain concrete. Its gives mass to concrete and fills voids created by coarse aggregates.it adds density to concrete. A denser concrete is higher in strength.

Water:

Water used for making concrete should be clean. It hydrates the cement to form a plastic mass which hardens gradually. Water makes it possible to mix the concrete with ease and place it in final position. The more water the better, however, excess water reduces the strength of concrete. A water – cement ratio of 0.5-0.6 is used to achieve desired concrete strength when hand mixed.

Preparing and placing of concrete.

Batching:

Batching by volume or weight are the 2 common methods of measurement.

Mixing: 

Thorough mixing to produce good quality concrete. Cement, sand and coarse aggregates is mixed in dry condition, then in wet condition after adding water. Hand and machine mixing are 2 common methods of mixing of concrete mixing

Transport and placing:

Fresh concrete is transported on site using head pans, wheel barrow on small scale work while concreting chutes, bell conveyors or pumps are used in large scale construction.

Compaction:

Compaction of concrete is necessary to remove entrapped air during placement of concrete. Compaction can be carried out by hand or with help of  concrete vibrators

Cement mortar is an intimate mixture of cement , sand and water. Cement is the binding material, and sand is the fine aggregrate. The binding material (cement) develops when water is added to the mixture. Sand suitable for cement mortar include, river sand, gutter or pit sand.

Sand used for cement mortar must have the following properties;

• should be chemically inert.
• It should be free from organic or vegetable matter.
• Should not contain salt.
• Should contain sharp, angular, coarse grain that is well graded and hard.

Preparing cement mortar.

Cement and sand in the required ratio is first thoroughly mixed in dry condition then water gradually added and mixed until required consistency is achieved.

Cement to sand ratio for various works;

• Masonry Works :   1:6 to 1: 8
• Plastering:    1:3 to 1:6

Mortar Curing.

Curing is the process of ensuring sufficient moisture during hardening of mortar. Curing is ensured by spraying of water. It starts 6 – 24 hours after mortar has been applied

Characteristics of Cement Mortar;

• Cement sand ratio mix richer than 1:3 is prone to shrinkage.
• Mortar with well-proportioned mix gives an impervious surface.
• Mortar with leaner mix may become porous.
• Strength of mortar depends on the cement sand ratio.

Compressive Strength of various cement sand ratios;

• 1:3 –   10N/mm2
• 1:4 –  7.5N/mm2
• 1:5 –   5.0 N/mm2
• 1:6 –   3.0 N/mm2
• 1:8 –    0.7 N/mm2

Uses of cement mortar.

• To bind masonry units like blocks and bricks.
• Plaster walls and slab.
• Give neat finishes to walls and concrete works.
• Fill joints and cracks in walls.
• As filler material in stone masonry.