Technical Manual for Cast Stone

1.0 Introduction

CAST STONE HAS BEEN A FAMILIAR AND MUCH-USED ALTERNATIVE TO QUARRIED NATURAL STONE SINCE GEORGIAN TIMES.  IT EVOKES A SENSE OF TIMELESSNESS WHICH FITS IN WITH ANY FORM OF MASSIVE CONSTRUCTION.

Cast stone is comparable to quarried natural stone as a building material in both appearance and performance, yet it is readily available and provides a cost effective alternative on projects from simple domestic housing to complicated schemes such as commercial buildings and cathedrals.  It can be formed to almost any shape and size the designer wishes and can equal, or even surpass, the technical capabilities of quarried natural stone in terms of strength, moisture penetration, colouring and textural consistency.  Stratification is never a problem and it is free from imperfections.  Suitable for period and contemporary styles, it is ideal for both new build and refurbishment work particularly in areas of sensitive planning constraints or where stone is a predominant building material.

Cast stone is a special form of stimulated stone, defined as any product manufactured with aggregate and cementitious binder intended to resemble and be used in a similar way to natural stone.  Cast stone is either homogenous throughout or consists of a facing and backing mix.

Sustainability and Corporate Social Responsibility

In terms of sustainability, cast stone performs well in both production and within its life cycles.  The material is highly durable, non-toxic, re-usable and requires virtually no maintenance or repair over its long lifespan.  It can also contribute to thermal mass.

The scale of its demands on raw materials will never compete with conventional building materials.  The base materials are essentially the by-products of industrial processes or are readily available.

Between 2006 and 2008 members’ collective waste was reduced by 10% and the accident frequency ratio fell by 31%.  Energy data covering electricity, gas and oil are also monitored to enable continuous improvement measures for the future.

In terms of whole life energy consumption, cast stone costs very little to look after once it becomes part of a building.  Its embodied energy becomes insignificant over time.  Furthermore the carbonisation of cast stone captures carbon dioxide from the air.  Concerns about carbon miles dog many natural building materials such as stone and timber, which re sourced from all over the world.  In contrast, cast stone is made and distributed in the UK, often in localised markets.  This provides a minimal carbon footprint and maintains UK employment.

2.0 Manufacture

CAST STONE IS FINDING GROWING FAVOUR WITH BOTH DESIGNERS AND USERS AS A HIGH-QUALITY FACING MATERIAL WHICH LOOKS GOOD AND PERFORMS WELL AT A REALISTIC COST.  THIS SPECIAL FORM OF CONCRETE CALLS FOR PARTICULAR SKILL AND CARE AND APPLICATION OF TECHNOLOGY IN ITS PRODUCTION.

Major advances have been made in mix design, batching, manufacture and curing.  The manufacturer’s aim is to produce a material, which resembles quarried natural stone products as closely as possible.  The colour and texture of most stones can now be matched using crushed rock fines and/or carefully selected and graded natural sands, usually mixed with white cement.  The range of shades can be extended by the use of grey cement and pigments.

Correct curing of all types of cast stone is essential both for the resistance to damage during transport and construction as well as durability and appearance over time.  BS 1217:2008 states “Cast stone should not be transported or installed before it is 14 days old unless accelerated curing processes allow a reduction of this time” and this period may need to be extended for structural units.

2.1 Semi Dry Production

The semi-dry method, as the name implies, involves the use of a low water content or “earth-moist” mix.  Semi-dry products can either be manufactured using a through (facing) mix or employing a backing and facing mix.  Facing mix is used for the complete element if the shape is complex, or if the units are typically less than 75mm thick.  The facing material determines the long term performance of the cast stone and requires careful design.  It must include carefully selected aggregates, graded such that thorough compaction is attainable thus yielding a strong (35-50 MPa) dense product.  It must also incorporate an integral waterproofer, usually a metallic stearate to ensure low absorption and long term performance.

Compaction of the semi-dry material is usually achieved using either pneumatic or electric sand rammers.  The effciency of this process is very important in determining the final quality.  Standard ashlar units can be manufactured using hand-operated compression machinery or, for very large contracts, fully automated plant.

Units are generally cast face down to ensure maximum compaction of the finished surface.  In two-part casting the facing mix is compacted to a minimum thickness of 20mm.  The layer is scratched to form a mechanical key for the backing concrete, which is immediately placed and compacted in successive layers.  Inter-diffusion is an alternative mechanism for ensuring satisfactory bond between successive layers of materials and is dependent upon successful diffusion of two loosely filled layers under force from the compacting hammer.  Both methods ensure that the product is effectively homogeneous.

After compaction the products are often demoulded immediately.  Subject to complexity it is possible to produce up to 80 units per day from one single mould reducing mould commissioning costs and lead times.

In general, dry cast units need no surface treatment after de-moulding.  If a more pronounced or “grainier” texture than the stone like, as-struck finish is called for, this can be achieved after de-moulding.  if the designer or user requires to replicate a natural stone “tooled” finish i.e. striated, rock-faced etc.  this can be achieved by reproducing the pattern in rubber or epoxy resin and then manufacturing the cast stone elements from the pattern.

It is essential that the initial cure is carried out in a controlled environment.  This environment must be protected from direct sunlight and drying winds.  An effective curing regime determines the development of strength and long term durability.  It will also reduce surface shrinkage and therefore crazing and improve resistance to weathering and abrasion.

BS 1217:2008 states “Cast stone should not be transported or installed before it is 14 days old unless accelerated curing processes allow a reduction of this time” and this period may need to be extended for structural units.

The semi-dry process is most suited to traditional sized products such as cills, heads, string courses, cornice, copings etc.  Structural items can be produced so long as the reinforcement requirement does not inhibit compaction.  The ability to reinforce dry cast stone offers a distinct advantage over natural stone.

2.2 Wet Cast Production

This process uses a higher water content mix than semi dry production resulting in a dense product with a typical cube strength of 35-50 MPa.  this is generally a through mix of the finished face material which can be produced in a wide range of shades.

The mix generally will have a slump of up to 100mm and compaction is achieved using conventional wet-casting techniques i.e. table-vibrator, poker or in some cases vacuum casting.

The wet-cast process generally only yields one cast per mould per day.  This is an aspect of wet-casting that both user and specifier should be aware.

Wet casting requires significant work before a satisfactory stone life finish is achieved.  This is due to the formation of a layer of cement laitance typically 0.5-1.0mm thick against the mould face.  This laitance requires emoval if a consistent stone like finish is to be achieved.  it is normally removed by chemical etching although other mechanical methods are used such as hand-rubbing, grinding and polishing.  Its removal reveals the constituents of the mix and therefore quality of aggregate and control of batching is essential if uniformity is to be achieved throughout a particular project.

The product requires a controlled initial cure protected from direct sunlight and drying winds.  As with all cast stone the product should normally be cured for a minimum of 14 days prior to dispatch.

2.3 Fibre Reinforced Cast Stone Production

This process incorporates fibre reinforcement into a higher water content mix that can be either poured or sprayed into moulds, from which the product is demoulded the following day.  A dense product with a close face texture is achieved which can be varied by the use of secondary surface treatments.  Often the units are produced in thin sections which reduces the weight and associated manual handling issues, and offers the opportunity – through careful design and use of fixings – for retrofitting to the structure.

2.4 At a Glance Guide to Manufacturing Method Properties

Extract from research report produced by the Concrete Technology Dept at Dundee University

3.0 Uses & Applications

CAST STONE IS AN EXTREMELY VERSATILE MATERIAL AND OFFERS THE DESIGNER AN OUTSTANDING FLEXIBILITY WHEN DESIGNING FEATURES AND STRUCTURAL ELEMENTS ON PROJECTS IN BOTH TRADITIONAL AND MODERN STYLES.

3.1 Comparison of Properties of Semi Dry, Wet Cast and Fibre Reinforced Cast Stone

The following sections carry information on aspects which may significantly affect the performance of cast stone components:

4.0 Design & Detailing

It would be true to say that cast stone has a place on any project under consideration either as a main construction element or as detailing to improve the appearance and appeal of the property.

Good building practice is essential when considering the design and use of all building materials including cast stone.  General guidance can be found in the relevant British Standards such as BSI Standards Publication PD 6697:2010 Recommendations for the design of masonry structures to BS EN 1996-1-1 and BS EN 1996-2.  See Appendix 4 for associated publicastions.

We are able to offer detailed information on the cost effective application of cast stone for each individual project.  Benefits can include:

• Time savings at design and construction stages
• Early completion of production drawings to minimise disruption to the building programme
• Reduction of manufacturing costs
• Reduction of construction costs
• Minimisation of wastage

4.1 Structural Considerations

Cast stone is often used for structural elements within a buildling.  The inherent strength and density of both semi dry and wet cast material often exceed requirements by a comfortable margin.  However the structural characteristics of cast stone produced by different manufacturers will vary and the supplier should be contacted at an early stage of the project.

As a general guide, cast stone components can be in compression or tension.  Products intended for use in compression e.g. quoins, string courses, ashlar and columns are generally not classed as structural units.  Items such as lintels, which are subjected to both compressive and tensional forces should be specified with care.  Lintels can be supplied as either non loadbearing components (i.e. decorative), or load bearing (i.e. structural).  With decorative heads it is important that they are used inconjunction with a steel support or lintel as they are not able to carry additional loads other than their own self weight.  With structural heads, the loadbearing capacity will be specified by the manufacturer.  In cases of uncertainty, the manfacturer’s advice should be obtained.

Steel support lintels are normally used below the decorative head to support the head itself and the entire brickwork load above.  There are however, cases where it is desirable to fit the support lintel over the decorative head so that the head is self supporting while the lintel carries only the load of the brickwork from above.

4.2 Building Movement

The control of movement should be assessed at the design stage since all building materials are subject to dimensional change during and after construction, due to moisture movement, cyclic thermal movements, and chemical action (i.e. carbonation).  Deflection under load, ground movement or differential settlement may also have to be accommodated.  This in general can be overcome by the use of movement joints, bed joint reinforcement and the correct specification of the mortar.

To reduce the problems with movement, ideally the designer should only specify mortars of strength classes M4 and M2 [designation (iii) and (iv)] (see NA to BS EN 1996-1-1:2005, Table NA.2).  Strong mortars, wherever possible, should be avoided as these can introduce too much restraint into the masonry panel which could induce cracking.

For further information please refer to NA to BS EN 1996-1-1: 2005 and Section 6.5 Mortars.

Any settlement that occurs can put increased loads on some elements of the building, in the case of cast stone this would apply for example to one piece cills and built-in thresholds.  It is good building practice to bed only under the stooled ends of these units and to point up the remainder of the open bed joint at a later date with a mortar of the same strength.  Alternatively, where practical, temporarily bed the cills in a lime: sand mix until the walls are completed, loaded and any initial settlement has taken place.

Further reference should be made to BSI Standards Publication PD 6697:2010 and BS 5642:Part 1:1978.

4.2.1 Differential Movement

Due regard should be given to the possible effects of differential movement between various types of building materials.  Cast stone products in general experience long term shrinkage, conversely, clay bricks suffer from irreversible moisture expansion and so these materials should be seprated because their movements are different in both magnitude and direction.  To reduce the potential for movement it is also important that the units are correctly bedded on a full bed of mortar, except one piece cills and thresholds.  DPC’s should always be sandwiched in the joint so that they are bedded on both sides.  Under no circumstances should cast stone units be laid dry on top of DPC’s.

Where two differing materials are used together in the same construction, then consideration should be given to the use of a slip plane.  Slip planes should be designed to allow parts of the construction to slide, one in relation to the other, to reduce tensile and shear stresses in the adjacent elements.  Often these details double up as DPC cavity trays.  The slip plane may need to contain two layers of smooth incompressible sheet mterial or an applied coating to form a separating membrane.  In principle all details should be checked to ensure that any potential movement is accommodated without adversely affecting the stability and/or performance of the elements and the structure as a whole.

Please refer to BSI Standards Publication PD 6697:2010.

4.2.2 Movement Joints

The risk of cracking in buildings can be minimised by incorporating movement joints according to British Standard recommendations.  With cast stone masonry, movement joints should be no more than 10mm wide and positioned at a maximum of 6m centres.  In external walls with openings, the movement joints may have to be more frequent or bed joint reinforcement included to restrain the masonry.  Around openings, bed joint reinforcement should be positioned in the two courses directly above the opening and in the first and third course below the opening.  The reinforcement should project at least 450mm either side of the opening.  Whilst additional tensile and flexural strength may be obtained by the use of bed joint reinforcement, the size of the bed joint may preclude it’s use or effectiveness.  In terms of walling materials such as ashlar, the risk of cracking increases where the length of panel exceeds twice the height.  Low horizontal walls such as spandrel panels are particularly vulnerable to damage.

Extra consideration should be given to the South and West facing elevations as these suffer more thermal gain.  Movement joints should be positioned wherever there are changes in thickness or directions of walls.  Lightly restrained details such as parapets or boundary walls will need extra thought as these are more prone to movement.

The designer should also consider that brickwork can have movement joints positioned at up to 15m centres.  Slip planes will therefore be necessary to separate the two materials.  Always ensure that movement joints and slip planes do not impair the stability of the wall or its other functions.  Use dowels or straps to provide lateral stability.

Movement joints are usually filled with an easily compressible joint filler of either polyethylene or polyurethane foam however, in narrow joints for cast stone they are usually formed butt jointed.  Optimum performance in butt joints is obtained when the depth to width ratio of the sealant is in the range of 2:1 or 1:1 for two part polysulphide sealants.

Further advice on the design and construction of movement joints can be found in BSI Standards Publication PD 6697:2010, BS 6093: 2006 and BS 6213:2000.

4.3 Prevention of Moisture Penetration

In cavity wall construction, the location of DPC’s should be based on the assumption that rain water will penetrate the outer leaf and, more often that not, run down the itnernal face of the external leaf.  Damp proofing measures are essential in controlling the ingress of rain through locations where the cavity is bridged and where there is a potential for moisture to track across these areas to the internal leaf.  Careful attention to design and detailing, combined with good site practice, is essential in the elimination of damp penetration.

The materials for the damp proof courses and cavity trays are varied.  However they should all comply with the requirements of relevant British Standards:

DPC’s should extend through the full thickness of the wall or leaf, at least 150mm above the external ground.  It is good practice to overlap all DPC materials, by at least 100mm to prevent the upward transference of moisture.  Where downward moisture movements occur, the DPC’s should be lapped and sealed.  Pitch polymers are generally favoured for masonry construction because they do not exude under load, have good bond characteristics and are not easily perforated.  Using the appropriate adhesives, effective joints can be formed giving increased flexibility of use.

The risk of moisture penetration may be reduced by consideration of the following features.

4.3.1 Heads & Lintels

Cavity trays are vital in providing a watertight barrier which will channel and discharge water to the outside face of the masonry.  Cavity trays with stop ends should be incorporated over all openings in external cavity walls and extend a minimum of 150mm on either side of the opening.  The part of the tray that bridges the cavity should be adequately supported and this is particularly important at locations where the tray is to be jointed.  The cavity tray should be bedded on both sides in fresh mortar.  The tray should fall a minimum of 150mm acorss the cavity.  Weepholes, positioned directly above the tray in the external leaf should be located at a maximum of 1m centres.  There should be a minimum of 2 weepholes peropening and each weephole should be at least 75mm high.  In locations where full cavity insulation is aniticipated, the spacing of the weepholes should be reduced.  The use of lable moulding will require additional trays.  The cavity tray should over lap the vertical dpc AT THE jamb.

4.3.2 Jambs

To prevent the transmission of moisture, a continuous vertical damp proof course should be included behind jamb sections and this should extend at least 25mm into the cavity beyond the cavity closer.  The DPC can be fixed to the window frame.  Vertical DPC’s at openings should be located so as to overlap the horizontal DPC at cill level, whilst being overlapped by the horizontal DPC at the head.

4.3.3 Cills

A horizontal damp proof course with stop ends should be provided in all jointed cills or sub-cills, for the full length and width of the cill bed.  If the damp proof course is in contact with the inner leaf or cavity infill, the DPC should be turned up at the back and ends for the full depth of the cill.  A cavity barrier/thermal barrier will be required to prevent cold bridging.  Preformed cavity closers are available which provide both the damp proofing and thermal bridging requirements in one component.

Refer to BSI publication PD 6697 for full details.

4.3.4 Cappings and Copings

BSI Standards Publication PD 6697: 2010 states that “Chimney terminals, freestanding walls, parapet walls and retaining walls exposed to the weather, should preferably be provided with a coping.”  Copings should have a drip on the underside, positioned a minimum of 40 mm from the face of the wall.  Where for aesthetic or other reasons a capping is used, consideration should be given to the durability of the construction beneath which may be exposed to wetting.  A coping with drips positioned as indicated may still not shed rainwater clear of the wall surfaces beneath.  A continuous DPC, supported over the cavity, will be required below a capping or coping detail to prevent the downward transfer of moisture into the wall.  This DPC should be bedded on both sides so that it is sandwiched in the joint and projecting 5 mm beyond each face of the wall below.

In parapet walls, a stepped cavity tray will be required which should fall a minimum of 150mm across the cavity.  Weepholes, 75mm high are required at a maximum of 1m centres.

Further reference should be made to BSI Standards Publication PD 6697: 2010 and BS 5642: Part2: 1983

4.4 Position of Water Bars

All extenal walls should be designed to meet the requirements of the relevant Building Regulations, in particular Approved Document Part L, and the general thermal insulation requirements of the building.  The implication of cold bridging and risk of condensation, for example at exposed internal mullions, and window head and cill positions, should be considered and cold bridge paths should be avoided.

4.6 Prevention of Staining

Where the sectional profile allows it, all projecting components should be detailed with a drip groove in an attempt to shed water clear of the face of the structure and to reduce staining.  However, as indicated in Section 4.3.4 the inclusion of a drip may still not shed rainwater clear of the wall surfaces beneath.

Lead or copper flashing over cast stone can cause staining.  As an alternative, GRP or metal pre-formed flashings should be used.  All flashing or weathering details should be bedded at least 25mm into the works and be provided with sealed joints and adequate overlaps.  Attention to detail can prevent water from the flashing washing over the surface of the cast stone.

4.7 The Design of the Element

A bespoke service can be offered to customers, cost savings can often be achieved by discussing customised components at the design stage.  The following comments are of a general nature only.

• Avoid slender projections from the unit.  These increase the likelihood of damage on demoulding and do not enhance either the appearance or authenticity of the final units.
• Avoid negative rakes as these present difficulties in mould design.
• Avoid ‘U’ sections.  These are very difficult to mould and less robust than creating the same effect using the jointed components.
• Avoid mitred joints.  The use of quoins is traditional for wall construction and jointed returns are an alternative, much more robust solution.

Consideration should also be given to slenderness ratios when designing cast stone units to minimise the risk of cracking.

Please refer to Section 8.4.5 for more information.

4.8 Lifting

Cast stone has an advantage over quarried natural stone in that units can contain cast-in lifting attachments.  These come as M16 OR MORE DELICATE m12 cast-in threaded sockets or as proprietary lifting clutch systems, and assist the Soecifier in meeting their CDM Regulations responsibilities.

For delicate placement of large stones, a rope block-and-tackle system, suspended from a runway beam attached to the top of the scaffold, or even suspended from a crane hook, gives controllable, gentle adjustment.  Chain blocks should not be used as they may mark the stone.

Materials used for lifting inserts depend upon the eventual position in the building, but in the great majority of applications where they are covered by subsequent construction and encased in an alkaline environment (i.e. mortar bed), BZP units are perfectly suitable and more cost-effective than stainless steel.

Safety of lifting operations has to be of paramount concern and relevant sections of HASAWA and Manual Handling Regulations should be observed, and Risk Assessments conducted before work commences.  The following points should be considered:

• Where screw-in wire bond lifting loops are used, it is essential to ensure the threads are screwed fully home, and that a vertical lift is used – lifting capacity reduces very rapidly with angled lifts.
• When a threaded lifting eye has to be used at a right angle (e.g. a socket insert in the back of a panel) then articulated loops are available.
• With two point lifts, use a spreader beam to avoid angled slings.
• Snatch loading by cranes cannot be calculated for and must be avoided as it will damage both stones and lifters.
• Lifting stones directly with slings is unstable and can be unsafe.
• Webbing slings can damage unprotected arrisses.
• Wire rope or chain slings are completely unacceptable.

5.0 Supply & Site Practice

Cast stone supplied by Procter Cast Stone is manufactured to the highest standards and considerable care is taken to ensure that the units are handled and stored correctly prior to delivery.  This section provides guidance on the transportation of cast stone units and their use on site.

Health and Safety information e.g. Control of Substances Hazardous to Health [C.O.S.H.H.] data should be obtained directly from the manufacturer.  Users should be also aware of Health and Safety requirements for handling of units. Manufacturers will generally have discussed handling requirements at the design stage to ensure that consideration is given to points such as the slenderness ratio of the product and it’s characteristics i.e. it’s profile and the ease of handling on site.&bbsp; This may include the incorporation of reinforcement and cast-in lifting sockets for the units.

Cast stone should be treated with care. The units must be handled and stored with care to prevent chipping, cracking or staining, especially those with finely detailed profiles.  Long slender units should be handled in the plane they are designed to be installed, unless otherwise advised by the manufacturer.  A typical example of a product which may not be handled in the same plane as it is installed would be a 65mm thick cill.  Unless supplied in short lengths, this unit can suffer deflection cracks.

5.1 Transporting the Units

During transit the following points should be considered:

• A suitable vehicle e.g. rigid or articulated trailer preferably with air suspension should be used for transport.
• Units should be covered during transit to protect them from saturation and staining.
• Bearers should provide adequate support to prevent incidents of point loading of the units.
• Pallets should be designed to withstand the load which they are to carry
• Palletised deliveries of cast stone should be unloaded by the mechanism for which they were designed, either by grab or forklift, using suitable forks.  Under no circumstances should scaffold poles, timbers etc. be used to carry or support the pallets.
• When using grabs, these should grip the pallet, not the product.
• Slings should not be used unles previously discussed with the manufacturer.
• Where slings are used to lift individual units, the arrises of the unit should be protected and the sling positioned to provide an even support.

5.2 Site Storage

Once the product is on site the following guidelines should be followed:

• Palletised products should be stored on flat, level, dry ground at a safe distance from other trades, roadways, etc. to prevent damage to the aesthetics and structure of the product.  Runners should be used to support pallets on soft ground.
• Never stack pallets of products on top of one another.
• Individual units should not be stacked face to face without appropriate interface material (manufacturers packaging and protective materials should be used for this).
• Individual units should be suitably supported by timber or plastic bearers.
• Products should remain packaged until immediately prior to use.
• When unpacking the product, strapping/ packaging should be carefully cut, not ‘burst open’.  Ensure that care is taken when cutting the packaging so that the face of the stone is not damaged by the knife blade.
• It is important that opened packs of cast stone are covered with polythene sheeting to prevent the ingress of water, dirt or dust.

5.3 Site Handling

The safe handling of cast stone components is essential in order to ensure that they remain undamaged.  Where handling information is not clear, contact the supplier for further recommendations.

• A manual handling assessment should be carried out before the units or pallets are moved.
• Where units are supplied with lifting sockets or eyes these must be used.  Avoid side loading to sockets by using a lifting beam where necessary.
• Always use suitable plant for moving the product around site and ensure wherever possible that units are delivered to the work area before any obstructions are put in the way.
• Units should be adequately supported to ensure ease of handling.  Care must be exercised not to drop the product.
• Re-use interior packing to protect faces, arrises etc.  during site handling.
• Care should be taken not to slide the units across each other.

Don’t store pallets on sloping or uneven ground.  Make sure storage area is flat, level and dry

Never stack pallets and large units on top of each other

When unpacking products, do not burst open the wrapping either by hand or with site tools.  It should be cut open with a knife, taking care not to damage the faces.

When moving pallets of cast stone units, always re-use interior packing to prevent damage to faces, arises and profiles.

6.0 Installation

This section provides guidance on protecting cast stone during construction, laying, bedding, joiing, cutting, the recommended grade of mortars for cast stone products, and casting-in any required fixings.

Cast stone units should only be installed by suitably qualified personnel.  During construction, the units should be protected at the end of each day and it is advisable to protect finished work.

The fixing of cast stone should be considered at the design stage so that any required fixings can be cast-in during production.

Cast stone units should be designed to minimise on site cutting.  They are typically designed to be fixed with joint sizes of between 5-10mm and should be laid and adjusted to final position while the mortar is still plastic.  It is vital to specify the correct mortar designation, which is often different to that used for the surrounding brickwork.  Mortars containing lime give a stronger bond than those containing air-entrainment.

6.1 Protecting During Construction

Protect the units at the end of each day with polythene sheeting to prevent contamination and use edge protectors to prevent impact damage to the arrises.  Allow an air space to let air circulate within the polythene sheeting, to prevent condensation forming.

6.2 Laying

Cast stone should only be installed by masons or suitably experienced personnel.  Below are some general precautions that should be observed.

• During construction it is advisable to protect finished work using appropriate gauge polythene sheeting.  This prevents mortar drops, mastic, paint and other construction materials staining or adhering to the cast stone.
• Mortar stains can be removed by using a dilute hydrochloric acid (typically 7-10%) solution.  The masonry should be wetted down with water to reduce the initial suction by the cast stone.  Under no circumstances should the masonry be saturated.  The stain should be agitated with a nylon brush to break up the surface of the motar stain.  The acid should then be washed from the surface of masonry.&bbsp; Care should be taken to ensure that the acid washings are collected and conveyed to a safe place for disposal.   Protective equipment will be required by the operatives and this will include goggles, rubber gloves and protective overalls.
• Brace constructions to prevent damage to freshly assembled materials.  It is also advisable to limit the height and number of courses constructed in any one day, depending upon the width of the wall, mortar strength, exposure, unit density and weight.  Typically, individual lifts should be imited to 1.2m in any one day unless restrained.

6.3 Bedding and Jointing

Typically, cast stone products are designed to be fixed with joint sizes of between 5-10mm between the units.  All units should be laid and adjusted to final position while the mortar is still plastic.  Mortar exuding from joints should be cut away without smearing the face of the unit.  Use load shedding (plastic) spacers to support heavy stones and to prevent the mortar being extruded until it has cured sufficiently.  Locating holes for dowel joints should be completely filled with either mortar or resin.

• Do not leave pockets that could collect water.  Protect all unfinished masonry with polythene.
• During hot dry weather, the faces to be jointed should be lightly sprayed with clean water to reduce initial suction and to prevent the cast stone from removing too much moisture from the mortar.  If this does occur, there may be insufficient water left in the mortar to fully hydrate the mix and this will result in a dry, powdery joint which may be substantially weaker than anticipated in terms of bond strength.  However, it should be noted that the use of water reducing admixtures or other water resistant additives, introduced into the cast stone during manufacture may reduce the effectiveness of spraying the joint with water.  Ideally, the correct designation of mortar should be specified in the first instance, to suit the environmental conditions.

6.4 Cutting

Cast stone units should be designed to minimise on-site cutting.  If it is unavoidable on site, units should be cut with a diamond tipped masonry blade which should ideally be water fed.  Once cut, all units should be washed down to remove any excess dust.  Due regard should be given to protecting the operative in accordance with the current health & Safety requirements.

6.5 Mortars

It is vital that the correct mortar designation is specified when using cast stone products.  The designer should note however that this is often of a different designation to that used for the surrounding brickwork.  Failure to allow for this, particularly where the mortar is stronger, can result in cracks appearing in long units as a result of concentrating the effects of differential movement.  The cracks are usually of little structural significance but are unsightly.  The reason is that too much restraint is offered by strong mortars and that this can cause distress to the cast stone by preventing the shrinkage process taking place.  The cracks will occur both in the cast stone and in the mortar below onto which the units are bedded.  This can cause debonding of the unit from the mortar and hence instability.  If strong mortar is used for the jointing/pointing process then damage may occur to the arrises as the strong mortar shrinks away, perhaps pulling some of the arris with it.  Strong mortars shrink considerably and also have a higher bond strength.

• Sand and cement mortars are not recommended because they lack the required workability to allow the masons to lay the units at an economic rate.
• Mortars should be able to resist frost and develop durability fairly quickly.  However, as has been stated earlier, the strongest mortars are not always the best.  Where a mortar of a stronger strength is rquired for durability reason, reference should be made to Table NA.2 of NA to BS EN 1996-1-1: 2005.

The recommended grade of mortars are:

6.6 Fixings

Consideration should have been given to the fixing of cast stone at the design stage so that any required fixings can be cast-in during production.  This will facilitate easy installation and reduce unnecessary costs and delays.

Here are some typical examples of fixing solutions.

Illustrations used courtesy of Ancon Building Products

7.0 Aftercare & Surface Effects (open in a new window)

Back to Cast Stone Technical Properties Content

Appendix 1 – Flatness of Plane

The equipment required to perform this test shall be:

• Straight edge of equal or greater length to the maximum dimension over which the flatness is to be measured or 1.5 m whichever is the lesser.
• Feeler gauges (two sets).

Method

Concave Surfaces: Place the straight edge on the surface of the item to be tested.  Insert a feeler gauge blade between the straight edge and the surface of the cast stone.  Measure the deviation from the straight and compare with the value given in 8.4.4.

Appendix 2 – Capillary Absorption Test

Measure the visual face of each product by suitable means and record the area of each in square millimetres to the mearest 100mm².  Record A₁, A₂, A₃.

At not less than 14 days old, dry the three products in a well ventilated oven or ovens at 70+5^。Celcius for at least 72h and allow to cool to room temperature 15-25^。Celcius in an air-tight enclosure.  Weigh and record the mass of each unit to the nearest 0.1% of the product weight and record as W₁, W₂, W₃.

Place each product, supported on suitable spacer devices, into a try and fill with cold water so that the visual face under test is under a maintained 5+1mm head of water.  After 10+0.5min, remove each product; remove excess water with a damp rag and within 30s of the removal, reweigh and record the masses as above, Record as X₁, X₂, X₃.

Calculate the capillary absorption C₁, in mg/mm², to the nearest 0.1 mg/mm²:

C_{1 =} 1000₍X₁-W₁₎

A₁

Similarly reepeat the calculation for the other samples C₂ and C₃ substituting the relevant values of X and W.

Finally calculate the mean value C (in mg/mm²) as:

C = C₁+C₂+C₃

3

Record the mean value to one decimal place.

Appendix 3 – The Specification

•   Cast stone units:

– Method of manufacture:      Wet cast*
&bbsp;                              Semi-dry cast*
Fibre reinforced cast stone*
(* include the method of manufacture required as appropriate)
– Product references:             To approved details
– Absorption:                         To BS 1217: 2008, CAT method
– Compressive strength:         To BS 1217: 2008
Cube strength testing to be carried out on a minimum of a weekly basis
Cube curel in accordance with BS EN 12390-2
Cube crushed in accordance with BS EN 12390-3
– Average cube strength:        Minimum 35 MPa (35 N/mm²)
Single cube strength:           Minimum 28 MPa (28 N/mm²)
– Finish:                                To approved sample
– Colour:                               To approved sample

• Mortar:                      To BS 5628-3:2005
• Joints:                       Flush pointed

– Width:                                6 mm **
10 mm **
(** include the joint width required as appropriate)

• Other Requirements:   None
  

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Appendix 4 – British Standard Reference

British Standards give guidance and information on various aspects of the design and construction of masonry structures.  It is strongly recommended that all concerned are acquainted with the relevant aspects of the Standards prior to commencing on a project.  See this page for a table of Cast Stone British Standards.

Appendix 5 – Acceptability Guidance

Chips, Scuffs, Blemishes, Hairline Cracks, Crazing
Shall not be obvious under direct daylight illumination from a distance of 6m.
CrackingMinor hairline cracking can seal itself through the action of autogenous healing and as such is unlikely to be a major problem.

However, a live crack, one that is continuing to move due to external forces, is unlikely to heal quickly as the walls of the crack will be moving and the crack width changing.

The cause of the cracking may be a more important factor than the crack itself.  For instance, cracked cills are usually caused by incorrect installation, eg fully bedding the cills instead of just bedding under stools, the cills not being able to move independently to the brickwork around and being pulled apart by the brickwork movement at the opening.  In this brickwork around and being pulled apart by the brickwork movement at the opening.  In this instance the incorrect installation should be dealt with rather than the result ie the cracked cills, and a replacement cill should be correctly installed.

Colour Variation

Cast stone items are manufactured with natural products and colour variations are inevitable and should not be a cause for rejection.

Cast stone is trying to replicate the appearance and character of quarried stone with it’s inherent variations in both colour and texture.

Initial colour variations can often be down to the age of the products.  There could be 6-8 weeks differences in the age of products when initially installed, leading to colour differences due to inherent moisture content, weathering etc.  These will all equalise in time when the pieces are installed and subject to the same water content, weathering and sunlight conditions.

Colour differences can also arise due to the water content of individual castings.  This may simply be due to the position on the building which allows one piece to get wetter than another during normal weather conditions, or may be due to problems with the waterproofer in the castings.

Colour Variations (due to Efflorescence)

Lime bloom, or efflorescence is a temporary, naturally occuring pheomenon that occurs to varying extents on all items containing camentitious binders.  It will, with time, disappear as a result of normal weathering.  The length of time will depend on many factors such as rainfall, atmospheric pollution etc.  The bloom can also be removed by the judicious use of a mid brick cleaner.

Surface Texture Variations

All surfaces intended to be exposed to view shall exhibit a texture approximately equal to the approved sample when viewed under direct daylight illumination from a distance of 3m.

Surface Texture Variations (Wet Cast)

All surfaces intended to be exposed to view shall have no air voids greater than 0.8 mm and the density of such voids shall be less than three occurences per 25 mm square.
Repairs
Repairs to cast stone shall be acceptable if the repairs conform to the other reqiurements above.Repairs can range from minor filling of blemishes through to a reconstruction of the piece with a full surface coating.  Significant differences in colour may exist between the property repaired areas and the original castings when the time elapsed between the date of manufacture and the date of the repair is great.  The repaired areas should be left alone and should blend in over time due to the action of curing, natural weathering and sunlight, as mentioned under colour Variations.

Dimensional Requirements

Acid Etching Process of applying a solution of hydrochloric or muriatic acid to the exposed surface of cast stone in order to remove the laitance from the aggregates, thus achieving a fine grained finish which simulates natural cut stone.

Anchor Metal device used for securing cast stone to a rigid structure.

Arris The sharp edge at the junction of two adjacent surfaces of a cast stone unit.

Ashlar Masonry constructed of stones to a rectangular shape and laid in courses, as opposed to rubble work which is uncoursed masonry of random shaped stones.

Backing Mix Concrete, normally sand, gravel, and grey cement: used for the unexposed portion of cast stone.

Bed Joint The joint which the stone sits on.  It is normally filled with mortar.

Capping Cast stone unit intended to protect the top a wall, balustrade or parapet as well as adding aesthetic value to the wall, but not necessarily designed to shed rainwater clear of the surfaces beneath.

Cast Stone Any material manufactured with aggregate and cementitious binder that is intended to resemble the appearance of, and be used in a similar way, to quarried stone.  Cast stone is either homogenous throughout or consists of a facing mix and backing mix.

Colouring A process of (or material used for) tinting the hue of cast stone.  It is normally achieved through the use of aggregates or inorganic iron oxide pigments.

Coping Cast stone unit intended to protect the top of a wall, balustrade or parapet as well as adding aesthetic value to the wall, and designed to shed rainwater clear of the surfaces beneath.

Crazing A series of hairline cracks, normally less than one millimetre in depth, in the outer surface of a concrete product.  Crazing does not normally affect the life of a concrete product.

Curing The process of hydrating the Portland Cement in cast stone to a specified age or compressive strength.

Dowel Round (usually non-corrosive) metal pin used in anchoring and aligning cast stone.

Dressings Brickwork or stonework flanking a wall opening or adjacent to a corner, treated distinctly from the remainder of the wall face.

Drip Continuous groove cut or cast into the bottom of the projecting edge of cast stone in order to disrupt the path of the water to the wall below.

Efflorescence Also known as lime bloom, may appear as a white deposit covering part or all of the surface of products containing cement.  It is a temporary, naturally occurring phenomenon that disappears as a result of normal weathering.

Exposed Face Any face which is not bedded or ortherwise protected in the works (e.g. with mortar or bitumen).  Visual Faces are Exposed Faces but not necessarily vice versa.

Facing Mix Materials used for both homogenous cast stone and, when a backing mix is used, the visual face of cast stone.

Fibre Reinforced Cast Stone A wet cast manufacturing process incorporating alkali-resistant extruded fibre reinforcement.  Allows thin and lightweight sections to be produced.

Fines Aggregates passing 6mm sieve.

Finish Final exposed surface of cast stone.  It is independent of colour, but it will control the colour intensity.  Acid etching is a cast stone finish.

Full bed A horizontal joint completely filled with mortar.

Grout Mortar of pouring consistency.

Head A unit spanning an opening but not necessarily intended to carry the weight of the construction above.

Homogenous A single continuous mix throughout the section of the unit.

Insert A metal device cast into a unit normally used for anchoring or handling.

Jamb The vertical side of a door or window frame or opening.

Joint Gap between masonry units filled with mortar or backer rod and sealant.

Jointing Scheme The jointing pattern shown on contract documents.

Lift Socket A metal device embedded into the cast stone for the purpose of lifting and/or anchoring.

Lintel A unit spanning an opening and intended to carry the weight of the construction above.

Monolithic Of, say, a column when it is made of a single block of stone.

Mortar A blend of cement, lime, sand and water which is applied at a pliable onsistency to bond masonry units.

Mould A form in which cast stone is shaped.  it can be constructed from wood, rubber, fibreglass and other materials.

Precast A concrete product not poured in place.

Quoins Mansory blocks placed to give emphasis to the corner of a building.

Rebar A deformed steel bar used for reinforcing cast stone.

Reiforcing Rebar, basalt fibre composite reinforcing bars, or alkali-resistant extruded fibre placed into a cast stone unit during the manufacturing process to augment the unit during handling or to enable it to carry a structural load (ie lintel).

Return The side or face of a surface or moulding at right angles to the main face.

Reveal The return of a wall surface into a door or window opening, normally at right angles to the main wall face.

Rustification Masonry of stone, brick or stucco with the joints between the blocks recessed with V-joints or other profiles imparting additional emphasis and visual strength to the wall.

Semi Dry Cast Stone A manufacturing process giving components a slightly open textured face, similar to sawn quarried stone.

Shop Drawing The drawing which the cast stone manufacturer submits for approval, showing the shape of pieces, exposed faces, jointing, anchoring, reinforcing and unit cross section.

Soffit The underside of a projecting element such as a cornice, or any flat underside.

Splay A large-scale chamfer, such as a door or window reveal, wider at the wall surface than at the frame.

Slip Cill A cast stone window sill that fits within the masonry opening.

Tolerance Allowable deviation from specified dimensions.

Tooled Finish A finish obtained by texturing the cast stone eg bush hammering or needling.

Trowel Finish A finish normally given to the back or unformed side of cast stone.  This finish may look slightly different than the moulded sides of the piece.

Visual Face Any face or part of a cast stone unit visible after completion of works.  Visual Faces are Exposed Faces but not necessarily vice versa.

Wet Cast Stone A manufacturing process giving a close face texture and allowing large components and those with complex reinforcements to be produced.

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F136

Procter Cast Stone