STONES -QUALITY DETERMINANTS

STONES -QUALITY DETERMINANTS

Post 267 ⇒   by Gautam Shah  →

.

Quality of a stone determines the Quality of stonework. Other factors include quarrying method, seasoning, dressing technique, and masonry or cladding style. The important properties of building stones are: durability, hardness, toughness, porosity and strength under various types of stresses. The structural properties are, mainly governed by: the class of stone (sedimentary, calcareous, igneous, etc.), mode of occurrence (surface, shallow depth, very deep mines), type and condition of the rocks (horizontal or inclined mass). The structural properties do get substantially altered by the subsequent handling of stones, such as: exposure, action of environmental factors (moisture, salt, contaminants, temperature) mechanical pressure, vibration, etc.

Sand Stone block open exposure

Most of the stone materials are extracted from a depth. Such materials have remained buried under heavy over loads, devoid of oxygen and other gases, and free of moisture, or with entrapped moisture or flooded in subsoil water, and without light or radiation exposure, possibly since their age of formation. On extraction the stone material is brought to a completely different environment. Mineral lattices begin to alter due to lessening of pressure and dehydration. Entrapped gases escape and the space begins to get filled in with moisture and atmospheric gases. In sedimentary deposits the excess moisture begins to escape, allowing minerals react, causing expansion or contraction stresses in the mass. Rain water which is slightly an acidic one causes calcium carbonate to change into bicarbonate. In industrial areas sulphur and carbon dioxide enhance the acid action on the freshly quarried stone. Chlorides on sea front and in industrial areas can get converted into weak hydrochloric acid and dissolve the carbonate rocks. Nitric acid produced from oxide or Nitrogen also corrodes stone faces. All corrosive mediums depend on supply of water or moisture, so care of fresh-cut stone is essentially a moisture management exercise.

Bio-weathering

Freshly quarried and exposed stones are affected by: 1 Temperature variations cause differential expansion and contraction of the mass causing cracking or splitting. 2 Wetting and drying cycles of rain or dew, and frost may disintegrate the stone. Other moisture associated, problems are of living organisms that flourish in wet grounds. Stones stored here are affected by algae, lichens destroy lime stones, and worms destroy all stones except granite. Vegetation growth is a problem in tropics and heavy rainfall areas. 3 Polluted atmosphere around cutting centres (industrial areas, dumping sites and sea shores) affects the surface quality. 4 Stones need to be stored in same position as they were in natural state. Layered stones such as sedimentary rocks must never be stored in vertical position before sufficient seasoning.

Roughened surface of Granite due to water and Sun

Seasoning is both drying and wetting cycles on one hand, and airing of stone, on the other hand. There is an increase in strength due to the re-deposition of percolated minerals, surface carbonation, transmission and deposition of minerals on the surface of a stone, by both the evaporating moisture, and addition of water. Dehydration, during seasoning, is more or less an irreversible process, Seasoning applies mainly to soft limestone; hard limestones seem to be less affected by it.

Stones have natural moisture, known as quarry sap. This quarry sap renders the stone blocks comparatively soft and makes them easily workable. Immediately after quarrying, a stone is soft and it is easy to work with it so sized and primarily dressed, but polishing requires a fully seasoned and hardened stone.

Tandur (sedimentary) stone stored upright after weathering

Weathering denotes both desirable and undesirable changes. It is concurrent process of seasoning. It enhances certain qualities, like colour, texture, strength, etc. and reveals micro fissures deep pores on the surface. Weathering is very effective for Lime-based stones, which absorb atmospheric Carbon-dioxide to produce hardened mass.

Salt weathering

Porosity has no direct relation to the weathering resistance of stone materials, such as limestone. It is the shape, size, depth, and nature of pores, that plays an important role in weathering. The action of carbonic acid, increases with greater micro-porosity, capillaries prolong the dissolving action because the water is retained longer in small cavities. Sandstones containing colloidal minerals as a cementing medium has the most pronounced expansion due to porosity.

Slate stone in Polluted Environment

Efflorescence (wall white, stack white or wall cancer), is the appearance of crystallized salts on the stone’s surface. ‘The crystallization of the salts, and their re-crystallization from a lower to a higher hydrate within the range of mineral stability, may develop stresses of high magnitude with quite an appreciable qualitative change.

.

Advertisements

STONE WORK – Quality Parameters

Post 228 – by Gautam Shah

.

640px-Columns_of_the_Temple_of_Apollo_at_Delphi,_Greece

Quality of stonework depends on not only the technique of dressing, cladding and fixing, but also on the stone material itself. A building stone material to be useful requires specific type of extraction, handling skills, seasoning and curing processes.

Delicate_Arch_LaSalle

Most of the stone materials are extracted from a depth. Such materials, since their age of formation, have remained buried under heavy over loads. In many circumstances these have been devoid of oxygen, other gases, moisture, and light or radiation exposures. They may have stayed with entrapped moisture and gases.

 

OLYMPUS DIGITAL CAMERA

On extraction the stone material is brought to a totally different environment, and certain inevitable changes set in. The lessening of pressure allows the entrapped moisture and gases to escape, such as in sedimentary deposits. Some materials absorb fresh moisture and atmospheric gases to fill in the voids. Due to reduced pressure, and exposure the mineral structures begin to alter, causing expansion or contraction stresses in the mass.

Carcassonne City Stone Wall Medieval Castle

The environmental changes continue long after mining, sizing and dressing of stones. Rain water, is slightly acidic, and causes a calcium carbonate to change into bicarbonate. In industrial areas atmospheric sulphur and carbon dioxide enhance the acid action on stone. Chlorides on sea front and in industrial areas can get converted into weak hydrochloric acid and dissolve the carbonate rocks. Nitric acid produced from oxide or Nitrogen also corrodes stone faces. All corrosive mediums depend on supply of water or moisture, so care of fresh-cut stone is essentially a moisture management exercise.

stone-591895_640

Seasoning: Seasoning involves both the drying and wetting on one hand, and airing of stone, on the other hand. There is an increase in strength due to the re-deposition of percolated minerals, surface carbonation, transmission and deposition of minerals on the surface of a stone, by both the evaporating moisture, and addition of water. Dehydration, during seasoning, is more or less an irreversible process and subsequent artificial saturation of stone with water, often lowers its compressive strength. Seasoning is more relevant to soft limestone whereas hard limestone seems to be less affected by it. Immediately after quarrying, a stone is soft, and it is easy to work with it, but polishing to glossy finish requires a fully seasoned and hardened stone.

Weathering_freeze_thaw_action_iceland

Weathering: The term weathering denotes both desirable and undesirable changes. It may even enhance certain qualities, like colour, texture, strength, etc. Following are the main sources for decay or the unwanted effects, on a stone:

Salt_weathering_in_gozo

  • Penetrating water: Effect of acid in air and rain water, mainly sulphur dioxide derived from the combustion of the sulfur constituents of fuel.
  • Surface action of water, gases: Bleaching of colour, water soluble salts’ crystalline deposits, erosion.
  • Effects of temperature variations: Frost, Cracking,
  • Effects of foreign deposits or organisms: Bird droppings, roots of climbers and other vegetative growth weaken the stone and accelerate the decay. Fungus and mildew are other destructive agents.

Desert_varnish

Applied Surface Finishes on Stone: Water repellent or finishes that completely seal a stone surface should be used very judiciously. A sealed surface is likely to allow crystallization of salts beneath the surface instead of its natural leaching out. Water repellent finishes once applied are difficult to remove, and in many cases not re-coatable. Better design techniques, correct type of stone use and use of a biocide treatment give more satisfying result for problems such as algae, fungi, lichens etc. The decay of stone can be minimized by laying the stone with natural grain in horizontal orientation.640px-Olandalbymedbridge

Porosity, has no direct relation to the weathering resistance of stone materials, such as limestone. It is the shape, size and nature of pores, especially the degree of micro porosity that plays an important role in weathering. The action of carbonic acid, increases with greater micro-porosity, and capillaries prolong the dissolving action. Small cavities on the surface and rough finished textures trap dust, bacteria and retain water for longer period. Sand-stones containing colloidal minerals as a cementing medium has the most pronounced expansion due to weathering.

Red_Stones_-_Kerala-_Chenkallu2

Efflorescence (also called wall white, stack white or wall or stone cancer), is the appearance of crystallized salts on the stone’s surface, and the cryptoflorescence, denotes crystallization of salts within the pores. The crystallization of the salts and their re-crystallization from a lower to a higher hydrate within the range of mineral stability may develop stresses of high magnitude with quite an appreciable qualitative change.

stones-2065410_640

Strength of a stone is affected by three types of stresses:

Compressive stresses, decrease the volume of the material, causing breaks with a shattering effect. This is more common in non-homogeneous stones, and stones with inclined grains or stratification.

Shear stresses, move one part of a stone with respect to another, under certain conditions, inducing a permanent change of shape. These are best avoided by appropriate angle of extraction and cut, by careful orientation during coursing a masonry.

Tensile stresses, produce cracks and fissures and torsion (or twisting). Generally, fine-grained rocks are stronger than coarse grained. Rocks with interlocking crystals are stronger than rocks with poor interlocking. Stratified rocks have poor strength along the plane or strata. Stratified rocks as a rule have lower strength than igneous and non-stratified homogeneous rocks.

Flintstone_knife

Fatehpur_Sikri_-_columnWorms_Dom_st_peter_tympanum_006

.