Plaster

This is the material applied on internal walls and ceilings to provide a smooth, easily decorated surface, external plastering is normally, called rendering. Plaster is a mixture that hardens after application; it is based on a binder (gypsum, cement or lime) and water with or without the addition of aggregates. Depending on the surface being plastered plastering schemes may require the application of either one coat, or undercoats to build up a surface followed by a finishing coat. The main plasters in use are:

1. Gypsum plaster is for internal use. Different grades of gypsum plaster are used according to the surface and coat.

2 Cement-sand plaster is used for external-rendering, internal undercoats and water-resisting finishing coats.

3. Lime-sand plaster is used for both under and finishing coats, although lime can be added to other plasters to improve their workability.

The object of plastering is to provide a finish to internal surfaces. The two principal binders are: Portland cement and gypsum plaster. Hydraulic limes are alternatives to Portland cement. In each case sand or lime may be added. Also, there are some special thin wall plasters in use, which consist of finely ground mineral substances with an organic binder such as polyvinyl acetate (PVA). The number of plaster coats applied to a surface may be one, two or three. Undercoats are from about 5 mm thick and final coats about 3 mm thick.

DURABILITY OF CONCRETE

Concrete must be hard, durable, dense, non-porous, fire-resisting and economical.

Concrete is to be durable when made of good materials, well mixed, and properly cured. Failures can be found in concrete work, but the trouble is usually caused by poor material, faulty foundations, lack of knowledge of the properties of concrete or poor workmanship. For example, some cements will give better results in sea water than others. This fact had to be established by experience and experiments.

It is more difficult to secure durable reinforced concrete than mass concrete. This is due to the reinforcing steel and the additional water required making the concrete flow around the steel bars. When moisture reaches the steel, it will rust and the expansion caused by the rust will crack the concrete, resulting in an unsightly structure and necessary repairs. In all structures exposed to the weather reinforcing steel must be carefully placed and well secured so that it cannot be displaced with concreting. No metal should project to the surfaces. Small wires will soon cause rust spots on the surface of the concrete if they are exposed.

Concrete, to be durable, must be made of good materials, uniform in quality, mixed with a minimum amount of water, and properly placed and protected with curing. Concrete exposed to sea water and the rise and fall of water levels, especially in cold climates where ice forms on the structures, requires special attention in the selection of the cement, aggregates, mixing, placing and curing.

LIME

Lime is a calcium oxide. It is used in great quantities for mortar and plaster. Lime (quicklime) is a white solid that reacts violently with water to form calcium hydroxide. It is made by heating limestone in a special kind of furnace called a "kiln". Lime must be stored in a dry place, otherwise it will absorb moisture.

Limes may be divided into three distinct classes:

1. Rich limes that contain not more than 6 percent of impurities, slake rapidly, and are entirely dependent on external agents for setting power. These are widely used for interior plasterer's works.

2. Poor limes that contain from 15 percent to 30 percent of useless impurities and possess the general properties of rich limes, only to a lesser degree.

3. Hydraulic limes that contain certain proportions of impurities, which when calcinated, combine with the lime and endow it with the valuable property-of setting under water or without external agents.

Lime is a basic building material extensively used all over the world, but it was not until the later years of the 19¹ century that a greater appreciation of the fuel-burning problems involved became apparent. Until this time the requirement for lime agricultural and it was produced by farmers or by small builders who used it for making mortar and plaster.

BRICK

A brick is best described as a "building unit". It may be made of clay by molding and baking in kilns, of concrete, of mortar or of a composition of sawdust and other materials. In shape it is rectangular solid and its weight is from 6, 5 to 9 lb.

There exists variety of bricks for different purposes: ordinary, hollow or porous, lightweight, multicolor bricks for decorative purposes, etc. Bricks are usually laid with help of mortar.

The shape and convenient size of brick enables a man to grip it with easy confidence and, because of this, brick building has been popular for many hundreds of years. The hand of the average man is large enough to take a brick and he is able to handle more then 500 bricks in an eight-hour working day.

It is necessary, therefore, for the "would be" bricklayer to practice handling a brick until he can control it with complete mastery until he is able to place it into any desires position.

The brick may be securely handled by placing the hand over the surface of the upper part of a brick and by placing the thumb centrally down the face of the brick with the first joins of the fingers^[1] on the opposite face. It is better to protect the thumb and the fingers with leather pads, which also prevent the skin from rough bricks.

Sometimes natural stones such as marble, granite, basalt, limestone and sandstone are used for the construction of dams and foundations. Marble, granite and sandstone are widely used for decorative purposes-as well, especially with public building.