Understanding Cast Stone
A Guide to Cast Stone Products for Architects and Specifiers
Cast stone products enable architects to incorporate high-quality details features such as cills, mullions, copings, quoins, columns, arches and pilasters in a rang of colours that closely match those of natural stone. Many manufacturers offer both standard ranges and the ability to custom-design units. In terms of appearance and performance, cast stone is comparable with natural stone. However, the cost of cast stone can be significantly less than that of natural stone, especially where repetitive work is concerned.
Cast stone achieves the following...
AestheticsAlmost any three-dimentional geometric form can be cast, allowing architects to design elegant curves as well as straight-edged features. Additional details can also be incorporated into the surface, such as brick-effects or channels, and the edges of units can be square, bevelled or radiused. Cast stone units can b used to complement brickwork, natural stone, flint or rendering.
Standard colours are available to match those of natural stone, such as York stone, Bathstone, Red Sandstone or Portland stone, while non-standard colours can also be produced to meet a client's specific requirements. Over time, cast stone weathers the same way as natural stone.
Surface FinishTo provide sufficient inherent strength during manufacture and handling, there is a limit to the slenderness ratio for a cast stone product. The slenderness rtio (S) is defined as the ratio of the Length (L) to the diameter (d), where the diameter refers to an inscribed or superscribed circle on the section of the product, determined by the support plane (ie. S=L/d). Unless otherwise agreed between the specifier and manufacturer, S should be less than or equal to 15.
Where large openings are required, due consideration should be given to incorporating bed joint reinforcement above and below. It is recommended that the maximum distance between vertical movement joints should be 6m. To reduce the risk of cracking in stooled cills only, the ends should be bedded in mortar; the joint below should be left open and, upon completion of the brickwork, the joint should be pointed.
Mechanical and Structural StrengthThree cube samples shall be tested to ensure that the material meets the rquirements specified in BS EN 12390-3:2002 (Testing hardened concrete. Compressive strength of test specimens).
Where possible, all units are cast in a two-stage process. A facing mix provides the stone appearance on the exposed faces and a backing mix adds strangth and forms a good bond with the facing mix and reinforcement, where used. Products intended for use in compression, such as quoins, string courses and walling units, can be readily used as structural units.
Where a steel lintel is used, the lintel would normally b positioned beneath the head to support the entire lintel and brickwork load. However, if the lintel is to be installed over a self-supporting cast stone head such that the lintel supports the brickwork load only, the manufacturer of the cast stone should be consulted first.
Differential MovementIt is essential to consider the possible effects of differential movement between various types of building materials. As a matter of principle, the details should be checked to ensure that differential movement can take place freely (see BS 5628-3:2005, Code of practice for the use of masonry. Materials and components, design and workmanship).
Damp ProofingTo prevent the ingress of rainwater and damp, attention should be paid to the design and detailing of damp proof membranes and/or cravity trays. Good site practice is also essential.
For cills, a horizontal damp proof membrane should be provided for the full length and width of the cill bed. This membrane should be turned upward to cover the vertical inner face of the cill and extend as far as the cavity tray or window board (see BS 5628-3).
Beneath copings to freestanding walls, parapet walls and similar structures, it is good building practice to provide a continuous damp proof membrane sandwiched in mortar and projecting beyond each face of the wall by at least 5mm.
Design for Long-Term Maintenance of Good AppearanceTo avoid staining, projections should incorporate a drip groove. Cills and heads should also be provided with drip grooves in the soffit.
Care should also be taken to avoid the possibility of water running onto cast stone from lead or copper flashing. Alternatively, non-metallic flashing may be used, but the manufacturer of the cast stone should be consulted prior to specifying the flashing.
Good Building PracticeIt is essential to use good building practice when designing all building materials - includng cast stone. General guidance can be sought from manufacturers, and relevant standards should be consulted, such as BS 5628-3.
Design-for-ManufactureCast stone is normally produced in a mould that is open on one side; the product is turned out onto this side for finishing. It is therefore advisable to design cast stone products such that there is one major flat unexposed surface, otherwise the cost will rise to take account of the addtional manufacturing difficultly.
Because of the way cast stone is produced in moulds, certain design details can be easier or more difficult to produce in custom-designed products - and the difference is reflected in the price of the products. The manufacturer should therefore b consulted at the design stage in order to avoid unnecessary costs being incurred.
Larger components are often produced with integral threaded sockets to enable lifting hooks to be inserted for moving and positioning the units on site. The manufacturer will normally locate the threaded sockets to suit the functional requirements and the production process, while taking care to avoid faces that will be visible once construction is complete. It is therefore important that unit rawings clearly show which of the unit faces will and will not be visible.
Handling and StorageAs with all other building materials, cast stone units must be handled and stored with care to avoid damage such as staining, chipping and cracking - particularly items with fine detail or sharp arrises, Slender units should be handled in th eplain in which they are to be installed, unless the manufacturer provides alternative instructions.
Palletised deliveries of cast stone products should be unloaded using a grab or forklift with suitable forks. Slings, scaffold poles andsimilar arrangements should not be used.
Palletised loads are best left in their protective packaging until required, but individual items should be visually inspected upon delivery. Any damange should be noted on the delivery note and reported to the manufacturer by telephone, fax or email.
On-Site StorageCast stone products should preferably be stored well away from areas of heavy traffic. Palletised products should b stored on flat, level, dry ground, and pallets should never be stacked. Individual items should also be stored on flat, level, dry ground, supported by suitable bearers placed one-quarter to one-third of the way in from each end to provide adequate support wile preventing point loading.
On-Site HandlingAs with natural stone, careful handling of cast stone is essential in order to avoid damage. If additional handling instructions are required, these can be provided by the manufacturer.
Exterior strapping an dpackaging should be cut away with a sharp knife, taking care to avoid scoring the surface of the cast stone. Once the packaging has been opened, the cast stone should be loosely covered with polythene to prevent contamination while maintaining an adequate airflow to avoid the formation of condensation.
Interior packing should b reused to protect faces and arrises during site handling. Additional packaging materials can be provided upon request.
Suitable plant should always be used for moving cast stone products around the site. Wherever possible, units should be delivered to the work area before any obstructions are put in the way. Cast stone should not be moved around a site in mechanical plant, as unpackaged units can be easily damaged.
If any machinery is to be used to handle the cast stone units, contact the manufacturer to discuss the best types of grabs to use. threaded sockets are often incorporated within cast stone units to facilitate movement using threaded eyes and appropriate lifting equipment.
Shock loading can cause damage to cast stone, so should be avoided at all times - such as when pallets are being moved. Units should be adequately supported to ease handling and eliminate the need to 'drop' the product if being handled manually.
Individual units should not be slid across any supporting surface or across each other.
InstallationCast stone products should only be installed by suitably experienced opearatives who should be made aware of the guidelines below.
When units are removed from storage they should be protected until they have been installed.
All units should be laid and adjusted to their final position while the mortar is still plastic. Mortar exuded from joints should be cut away, taking care not to smear the face of the cast stone. During cold weather, fresh mortar should be protected from frost damage.
Cast stone products are generally designed to be fixed with 6mm joints between units. Locating holes for dowel joints should be completely filled with an appropriate material such as a proprietary resin mortar.
Constructions should be braced to avoid freshly assembled materials being damaged.
At the end of each day it is recommended that the top of all of that day's work should be protected to prevent contamination and the ingress of water. Nevertheless, airflow around the cast stone units should not be restricted, and on no account should anything be stuck directly to the unit faces.
It is recommended that finished work b protected with appropriate gauge polythene sheeting to avoid mortar drips, mastic, paint and other construction materials from staining or adhering to the cast stone. Mortar, in particular, is difficult to remove.
Unprotected items such as scaffold poles and planks should not be supported directly on cast stone.
FixingsDue consideration should be given to the type and size of fixings at the design stage. It is recommended that the cast stone manufacturer be consulted on the applicability of various alterntive fixings.
MortarsIt is essential to use the correct grade of mortar when installing cast stone products, as incorrect mortars can result in cracks appearing in long units due to differential movement. While these cracks are seldom of any structural significance, they detract from the visual appearance.
Note that the mortar is frequently different from that used in surrounding brickwork; plain send and cement mortars are not recommended. However, mortars containing lime are strongly recommended, as they are, to a certain extent, self-healing.
Exposed joints may bnefit from the water-repellence imparted by proprietary water-proofers added to mortars. The following table gives recommended mortars for different exposure conditions:
EfflorescenceEfflorescence is a temporary, naturally-occuring phenomenon that manifests as a white deposit on all or part of the surface. It occurs to varying extents on all items containing cementitious binders, and mortar is particularly susceptible to efflorescence. While it can affect other materials, such as brick or cast stone, it is not detrimental to the materials' structural integrity.
Generally it is recommended that the phenomenon be allowed to disappear naturally but, should chemical treatment be deemed necessary, the manufacturer of the cast stone should be consulted prior to any chemicals bing appplied to its products.
The risk of efflorescence occuring is reduced by protection on site before and during installation.
Maintenance and CleaningHigh-quality cast stone is remarkably durable, being capable of lasting for many dcades given reasonable care and maintenance. For best results, it is particularly important to avoid contamination during transit, storage, installation and completion of surounding construction works.
Under most conditions, cast stone will weather in a similar way to natural stone and will require no maintenance for many years. However, it may be desirable to clean cast stone, in which cast the main methods that can be used are: water washing, mechanical cleaning, air abrasives, the proprietary JOS method, chemical cleaning, alkaline cleaning, acid cleaning, and soaps and poultices. Most techniques have both advantages and disadvantages, and it is recommended that the manufacturer be consulted prior to the selection of any cleaning treatment.
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