Functional requirements - Stones used in building.

Tuesday, February 8, 2011

Strength and stability.

The strength of sound building stone lies in its very considerable compressive strength. The ultimate or failing stress of stone used for walling is about 300 to 100 N/mm3 for granite, 195 to 27 N/mm3 for sandstone and 42 to 16 N/mm3 for limestone. The considerable compressive strength of building stone was employed in the past in the construction of massive stone walls for fortifications and in other large structures. The current use of stone as a facing material makes little use of the inherent compressive strength of the material.

The stability of a stone wall is affected by the same limitations that apply to walls of brick or block. The construction of foundations and the limits of slenderness ratio, the need for buttressing walls, piers and chimneys along the length of walls and the requirements for lateral support from floors and roofs up the height of walls apply to stone walls as they do for brick and block walls.

Resistance to weather an ground moisture.
To prevent moisture rising from the ground through foundation walls it is necessary to form a continuous horizontal dpc some 150 mm above ground level. One way of achieving this is to construct foundation walls of dense stone, such as granite, that does not readily absorb moisture. More usually one of the damp-proof materials described for use with brickwork is used. A sheet lead dpc is commonly used as it is less likely to be squeezed out and forms a comparatively thin and therefore less unsightly joint than a bitumen felt dpc.
The resistance to the penetration of wind driven rain was not generally a consideration in the construction of solid masonry walls. The very considerable thickness of masonry walls of traditional large buildings was such that little, if any, rain penetrated to the inside face.

With the use of stone, largely as a facing material for appearance sake, it is necessary to construct walls faced with stone as cavity walling with a brick or block inner leaf separated by a cavity from the stone faced outer leaf, as illustrated in Fig. 107.

The outer leaf illustrated in Fig. 107 is built with natural stone blocks bonded to a brick backing, with full width stones in every other course and the stones finished on face in ashlar masonry. This is an expensive form of construction because of the considerable labour costs in preparing the ashlared stones. As alternatives the outer leaf of small buildings may be constructed with stone blocks by themselves for the full thickness of the outer leaf, or with larger buildings the outer leaf may be constructed of brick to which a facing of stone slabs is fixed.

The leaves of the cavity are tied with galvanised steel or stainless steel wall ties in the same way that brick and block walls are constructed and the cavity is continued around openings, or dpcs are formed to resist rain penetration at head, jambs and cills of openings.

Fig. 107 Cavity wall faced with ashlared stone and brick backing.

Durability and freedom from.
Sound natural stone is highly durable as a walling material and will maintenance have a useful life of very many years in buildings which are adequately maintained.

Granite is resistant to all usual weathering agents, including highly polluted atmospheres, and will maintain a high natural polished surface for a hundred years or more. The lustrous polish will be enhanced by periodic washing.
Hard sandstones are very durable and inert to weathering agents

but tend to dirt staining in time, due to the coarse grained texture of the material which retains dirt particles. The surface of sandstone may be cleaned from time to time to remove dirt stains by abrasive blasting with grit or chemical processes and thorough washing.
Sound limestone, sensibly selected and carefully laid, is durable for the anticipated life of the majority of buildings. In time the surface weathers by a gradual change of colour over many years, which is commonly held to be an advantage from the point of view of appearance. Limestones are soluble in rainwater that contains carbon dioxide so that the surface of a limestone wall is to an extent self- cleansing when freely washed by rain, while protected parts of the wall will collect and retain dirt. This effect gives the familiar black and white appearance of limestone masonry. The surface of limestone walls may be cleaned by washing with a water spray or by steam and brushing to remove dirt encrustations and the surface brought back to something near its original appearance.
In common with the other natural walling material, brick, a natural stone wall of sound stone sensibly laid will have a useful life of very many years and should require little maintenance other than occasional cleaning.

Fire resitance.
Natural stone is incombustible and will not support or encourage the spread of flame. The requirements of Part 13 of Schedule 1 to the Building Regulations for structural stability and integrity and for concealed spaces apply to walls of stone as they do for walls of block or brick masonry.

Resistance to the passage of heat.
The natural stones used for walling are poor insulators against the transfer of heat and will contribute little to thermal resistance in a wall. It is necessary to use some material with a low U value as cavity insulation in walls faced with stone in the same way that insulation is used in cavity walls of brick or blockwork.

Resistance to the passage of sound.
Because natural building stone is dense it has good resistance to the transmission of airborne sound and will provide a ready path for impact sound.

Ashlar walling.
Ashlar walling is constructed of blocks of stone that have been very accurately cut and finished true square to specified dimensions so that the blocks can be laid, bedded and bonded with comparatively thin mortar joints, as illustrated in Fig. 107. The very considerable labour involved in cutting and finishing individual stones is such that this type of walling is very expensive. Ashlar walling has been used for the larger, more permanent buildings in towns, and on estates where the formal character of the building is pronounced by the finish to the walling. Ashlar walling is now used principally as a facing material.

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