Types of Stone Masonry: Ashlar Masonry, Rubble Masonry
When stones are used as building units, the masonry is called stone masonry. This article is about stone masonry, types of stone used in stone masonry, specifications of stone masonry, etc.
Stone is a natural material derived from rocks. It has no definite shape but is a mixture of two or more minerals. Masonry is the construction of building units bonded together with mortar.
Types of stone masonry
a) Rubble masonry
In rubble masonry, the blocks of stones that are used are either undressed or comparatively roughly dressed. The masonry has wide joints since stones of irregular size are used.
i) Random rubble
• Un-coursed
It is the roughest and the cheapest form of stonewalling. Since stones are not of uniform shape and size, they are arranged with great care so as to distribute pressure over the maximum area and at the same time avoid long vertical joints.
• Built to course
It is similar to un-coursed except that the work is roughly leveled up to form courses 30 to 45 cm thick.
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ii) Square rubble
• Un-coursed
Stones having straight beds and sides are arranged in an irregular pattern to give a good appearance and avoid the formation of long, continuous joints.
• Built to course
Stones having straight beds and sides are leveled up to form courses of varying depth.
• Regular course
Stones having straight beds and sides are leveled up to form courses of varying depth but the height of stones in each course is the same.
iii) Miscellaneous type rubble
. Polygonal rubble masonry
In this type, stones are hammer-finished on the face to an irregular polygonal shape.
If stones are roughly shaped to form rough fitting, such a work is called rough picked work. If stones are carefully shaped to form close fitting such a work is called close-picked work.
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• Flint rubble masonry
In this type, the main material is “flints or cobbles” and is used that have varied in width and thickness from 7.5 cm to 15 cm and length from 15 cm to 30cm. The stones are extremely bard but brittle.
The strength of such walls can be increased using a lacing course of thin long stones or bricks at 1 to 2m vertical intervals.
iv) Dry rubble masonry
In this masonry, as the name suggests the mortar is not used in the joints, therefore it is the cheapest but more skilled manpower is required in construction. The use of dry rubble masonry is for non-load-bearing walls like compound walls.
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b) Ashlar masonry
In ashlar masonry, square or rectangular blocks that are used are dressed and have extremely fine bed and end joints.
i) Ashlar fine tooled
It is the finest stone masonry. The beds, joints, and faces of the stones are chisel-dressed to remove all unevenness and obtain perfectly horizontal and vertical joints.
ii) Ashlar rough tooled
The beds and sides are finely chisel-dressed but the exposed face is dressed by rough tooling.
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iii) Ashlar rock faced.
The exposed face of the stones is not dressed but is kept as such to give a rough facing.
iv) Ashlar chamfered
It is a special type of ashlar rock-faced in which the strip provided around the perimeter of the exposed face is chamfered at an angle of 45° to a depth of 25 mm.
v) Ashlar block in course
It is immediately between ashlar and rubble masonry. The faces of each stone are hammer-dressed but the vertical joints are not as straight and fine as in ashlar masonry.
Vi) Ashlar facing
It is provided along with brick or concrete blocks to give a better appearance. The outer faces of the stone are rough-tooled and chamfered.
Types of stone used in stone masonry
a) Igneous stones
These stones are obtained from igneous rocks, formed from the cooling and solidification of magma generated within the Earth at high temperatures during volcanic activity.
They are generally used for purposes that require strength and durability, such as kerbstones, countertops, floorings, and breakwaters. The igneous stone ranges from very soft rocks such as pumice and scoria to somewhat harder rocks such as tuff and hard rocks such as granite and basalt.
b) Sedimentary stones
They are obtained from sedimentary rocks that are formed by the accumulation and cementation of mineral grains (sediments) transported by wind, water, or ice to a basin or by precipitation at a site. There are mainly two types of sedimentary stones used in masonry work, limestone and sandstone. One of the sedimentary rocks is Limestone which is composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate.
Limestone is the main construction material and used in various fields such as a building material, as aggregate for the base of roads, as white pigment or filler in products such as toothpaste or paints, and as a chemical feedstock.
On the other hand, sandstone is an elastic sedimentary rock composed mainly of sand-sized minerals (mostly quartz and/or feldspar) or rock grains. It is relatively soft, making it easy to change the required shape.
Typically used as flooring or paving material, it has also been used for artistic purposes to create ornamental fountains and statues.
c) Metamorphic stones
They are obtained from metamorphic rocks, which are pre-existing rocks that have been altered by great heat and/or pressure or by chemical conditions. They have traditionally been used for carving statues, and as facing in many Byzantine and Renaissance Italian buildings. Slate and marble are metamorphic stones commonly used in the building industry.
Slates are typically used for flooring, roofing, and countertops whereas marbles are suitable for wall-cladding, roofing, flooring, and all other interior and exterior applications.
Requirements of stone
a) Appearance and general structure:
Stones used for face work should have homogeneous color, should be resistant to weathering agencies, and must be able to receive polish.
b) Weight:
Stones should be dense and less porous; good building stones should have a specific gravity between 2.4 and 2.8. When used for domes, roof coverings, etc., lighter varieties of the stones are preferred.
c) Hardness and toughness:
Stones used in floors and pavements should be hard enough to resist abrasive forces caused due to wear and friction. They should also be tough enough to withstand stresses developed due to vibrations of machinery and moving loads over them.
d) Durability:
Stones should possess natural durability to withstand the destructive effects of various agents continuously operating on them. The durability of a stone depends upon the relation between its chemical composition and the atmospheric surroundings. The durability of the stone also depends on the texture of the stone. Crystalline homogeneous and close-grained varieties of stones with dense structures should be selected for good works. The surface of a freshly broken stone should be uniform in texture, color, and hardness
e) Fire-resistance The minerals composing the stone should be such that the shape of the stone is preserved when there is fire. Limestone resists fire up to a temperature of 800°C. Sandstone with silicates as binding material can resist fire very well.
f) Workability
The stones should be such that they can be easily carved, molded, cut, and dressed. It is an important consideration from an economic point of view. However, this property of stone is opposed to its strength, durability, and hardness. Hence, it is to be properly correlated with respect to the situation in which the stone is to be used.
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Specification of stone masonry
Following are the specifications of stone masonry with Ashlar Masonry is as below:
a) The mortar used in work shall have a strength of not less than 5 N/mm2 or 7.5 N/mm2 at 28 days specified. Mortar shall be fluid, mixed thoroughly, and then poured into the joints.
b) All stratified stone possessing bedding planes shall be laid with the natural bed as nearly as possible at right angles to the direction of load. The natural bed shall be radial for use in arch rings.
c) Except for dry rubble walling, all joints shall be sufficiently thick to prevent stone-to-stone contact and the gap shall be completely filled with mortar.
d) Stones shall be clean and properly wetted before they are used so as to avoid sucking water from mortar.
e) The bed which is to receive the stone shall be cleaned, wetted, and covered with a layer of fresh mortar.
f) All stones shall be laid full in mortar both in bed and vertical joints and settled carefully in place with a wooden mallet
immediately after placement and solidly embedded in a mortar before it has set.
g) Clean and wet chips and spalls shall be wedged into the mortar joints and bed whenever necessary to avoid thick joints or beds of mortar.
h) Dressing of the stones shall be done before they are laid and as per the stipulated requirement.
i) Continuous vertical joints shall be avoided.
j) The verticality of the wall shall be frequently checked with a plumb-bob.
k) Through stones shall be used within a 1.5 m distance.
l) TI1e height of masonry shall be used uniformly.
m) Under the beams, trusses, sills, etc., large flat stones shall be used.
n) Before continuing work, the masonry built on the previous day shall be well-cleaned and freed from loose particles.
o) Curing shall be done properly for 2 to 3 weeks.
p) Masonry shall not be laid when the temperature is less than 3 C. Newly laid masonry shall be protected from the harmful effect of weather.
Use of different stones
a) Granite: It is used in facing work walls, steps, sills, bridge piers, columns, and road metal.
b) Gneiss: It is used in street paving and rough stone masonry walls.
c) Marble: It is used in flooring, steps, and ornamental work as it can be carved easily and at the same time, it can take a nice polish.
d) Slate: It is mostly used in roofing work and in sills and damp-proof courses.
e) Quartzite: It is used in retaining walls supply , road metal, concrete aggregate, pitching rubble masonry, and facing buildings.
f) Sandstone: It is used in facing work, steps, walls, road metal, and ornamental carving.
g) Limestone: It is used in floors, steps, walls, road metal, manufacture of lime in a blast furnace, etc.
h) Basalt and trap: It is used in road metal, for rubble masonry, foundation works, etc.
i) Laterite: It is used as building stone and in road metal and rough stone masonry work.
j) Murum: It is a metamorphic rock, which is brown or red in color. It is used in road metal and garden walls.
k) Chalk: It is pure White limestone which is soft and easy to form a powder. It is used in the manufacture of Portland cement and the penetration of glazier’s putty.
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