Building Stones

A good building stone should have the following characteristics.

1. Strength: high crushing strength (not less than 3.5 N/mm² for ordinary work; granite is 100 N/mm² or more).
2. Durability: resists weathering, frost and chemical attack.
3. Hardness and toughness: good resistance to abrasion and impact, with hardness coefficient above 17 for roads.
4. Porosity and water absorption: low, not more than 5% of weight after 24 hours, otherwise frost and salts damage it.
5. Specific gravity: at least 2.6, giving a dense, heavy stone.
6. Fire resistance: should not crack or lose strength on heating. Granite is poor, sandstone is good.
7. Appearance: uniform colour, fine texture, free from patches, and able to take a good polish.
8. Workability: easy to dress and carve without much cost, though it should still be hard after dressing.
9. Seasoning: quarry sap should have evaporated, so the stone is stronger after exposure.
10. Structure and soundness: free from cracks, cavities, veins, clay, soft patches, iron pyrites and flaws; a dull sound on striking shows defects.
11. Weathering resistance and cost: should be economical and available near the site.
12. Resistance to acids: should not contain soluble salts like carbonates and sulphates in polluted environments.

Natural bed

In a stratified (sedimentary or metamorphic) rock, the layers formed during deposition lie in parallel planes. The plane along which the layers were deposited is the natural bed (bedding plane). The stone obtained from the quarry has these layers, and the bed is easily seen as the plane of weakness.

   ______________   <- layers (bedding planes)
  |______________|
  |______________|
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Importance

1. The stone is strongest against loads that act perpendicular to the natural bed, and weakest along it, because layers can split or slide.
2. In masonry, stones are laid with their natural bed perpendicular to the direction of the load. In arches the bed is placed radial, i.e. normal to the thrust. In cornices and projecting parts the bed is kept vertical.
3. If the bed is placed vertical in a wall, water enters between layers and frost makes the layers flake off (scaling).
4. Identification of bed is therefore necessary during dressing and placing to obtain strength and durability.

Unstratified igneous rocks like granite have no natural bed.

Dressing of stone is the process of giving the required shape, size and surface finish to the quarried stone blocks by cutting and chiselling with hand or machine tools.

Why dressing is carried out

1. To obtain the required shape and size so that stones fit properly.
2. To get even beds and joints, thin mortar joints and uniform load distribution.
3. To give a good appearance by smooth, plain or ornamental finishing.
4. To remove irregularities, cracks, loose and weak parts.
5. To make stones easy to lift, handle and transport.
6. To make stones suitable for special work like arches, cornices and carvings.

Surface finishes (common types)

• Rough tooled, chisel drafted, furrowed, punched, boasted, sunk and polished finish.

It is done using chisels, hammers and mallets, or by sawing, planing and polishing machines in factories.

Deterioration of stone is the loss of its strength, shape and appearance with time. Its main causes are:

1. Rain and moisture: water enters pores, dissolves minerals and causes frost damage and weakening of the stone.
2. Atmospheric gases: CO₂, SO₂ and acid fumes in polluted air form carbonic and sulphuric acids that attack limestone and sandstone (acid rain).
3. Temperature variation: daily and seasonal expansion and contraction cause cracks, particularly in stones made up of different minerals.
4. Frost action: water freezing in the pores expands by about 9% and splits the stone.
5. Wind and abrasion: wind-borne particles wear the surface.
6. Crystallisation of salts: soluble salts in the stone or in the ground water crystallise on the surface and spall the stone.
7. Chemical action: sulphates and chlorides from soil, sea water or mortar react with the stone.
8. Vegetation and organisms: roots, moss, lichens and fungi hold moisture and produce acids that corrode the stone.
9. Poor selection and workmanship: wrong bedding, porous stone, poor dressing or weak mortar.
10. Fire and earthquakes may also crack stone.

Preservation protects a stone from weathering and chemical attack so that it lasts longer. The main methods are:

1. Proper selection and seasoning: choose a dense, low-porosity stone and allow quarry sap to dry before use.
2. Correct placing: lay stratified stones with their natural bed perpendicular to load, and give projecting parts a drip course.
3. Good drainage and design: avoid water pockets; provide slopes, copings and damp-proof courses.
4. Smooth, fine dressing: a smooth surface sheds water and dust.
5. Coating with paints or varnish: oil paint, linseed oil, or a coat of wax or silicone to seal the pores.
6. Surface treatment (solutions): application of Baryta water (barium hydroxide) or a solution of alum and soap, or silicone water repellents.
7. Chemical treatment: the solution of fluosilicate or silicate and the "Kessler" process produces insoluble compounds in the pores.
8. Pointing with good mortar and cleaning to remove algae and salts.
9. Regular maintenance and cleaning with water or brushes.

Stone is a natural building material taken directly from rock. It is preferred when strength, durability and good appearance are needed with low maintenance.

Benefits of natural stone

• Strong and durable: high compressive strength and long life (granite, basalt).
• Hard and resistant to weather, fire (to a degree), insects and rot.
• Low maintenance, since it needs no paint.
• Pleasing appearance: natural colour and texture, can be polished and carved.
• Locally available in hilly Nepal, so cheap and easy to obtain.
• Heavy, giving stability to retaining structures.
• Thermal comfort: stone walls keep buildings cool in summer and warm in winter.

Where it is preferred

• Foundations and masonry walls of buildings in hilly areas.
• Retaining walls, check dams, gabion walls, bridge piers and abutments, and river training works.
• Road aggregates, ballast for railways and concrete aggregate.
• Flooring, steps, facing and cladding (marble, granite, slate).
• Roof covering (slate), kerbs, paving, monuments and decorative work.

Deterioration of stone is the gradual decay of stone in which it loses strength, colour and surface because of weathering and chemical action over time.

Causes

• Rain, moisture and frost action in the pores.
• Acid gases (SO₂, CO₂) in the air forming acids that dissolve limestone and sandstone.
• Temperature changes causing expansion and contraction.
• Crystallisation of salts in the pores and growth of lichen or roots.
• Wind abrasion and wrong laying of the natural bed.

Preservation

1. Select dense, low-porosity stone and season it.
2. Dress the surface smooth and provide proper drainage and projections.
3. Place stratified stone with the bed perpendicular to the load.
4. Coat the surface with oil paint, linseed oil or wax, or silicone water repellents; use Baryta water or alum-soap solution to fill the pores.
5. Maintain the stone by regular cleaning and good pointing.

Dressing of stones is the process of cutting and shaping the quarried stone to the required size, shape and surface finish.

How it is done

1. Sawing: large blocks are cut with hand or machine saws.
2. Hammer dressing: the rough surface is removed with a hammer (quarry-faced or rock-faced finish).
3. Chisel and punch: stone is chiselled to a plane surface (chisel drafted, punched, furrowed or tooled finish).
4. Boasting or bush hammering: the surface is made even by a boaster or bush hammer.
5. Rubbing, grinding and polishing: machines with abrasive carborundum and polishing powder give a smooth glossy surface (marble, granite).
6. Sunk and moulded finish: carved by hand or machine for cornices and ornamental work.

Dressing is done at the quarry or the site using hand tools or machines.

Cataclastic metamorphism

Cataclastic (dynamic) metamorphism is the change of a rock into a new form by intense mechanical stress and crushing, such as during faulting and mountain-building (folding), without much heat. The minerals are broken, ground and recrystallised (example: mylonite, fault breccia).

Plutonic metamorphism

Plutonic metamorphism is the change of rocks at great depths in the crust under very high temperature and high pressure, caused by deep burial and by hot magma. The rock texture and minerals are recrystallised (example: gneiss from granite).