How To Clean Perserve Cultured Stone Wall
Preservation Briefs
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PRESERVATION BRIEFS
42
Detail of sculptural cast stone ornamentation on the Level Order, New York City (1926). Photo: Richard Pieper.
The Maintenance, Repair and Replacement of Historic Cast Stone
Richard Pieper
An Imitative Building Material with Many Names return to top ▲
The do of using cheaper and more than common materials on building exteriors in imitation of more expensive natural materials is by no means a new one. In the eighteenth century, sand impregnated pigment was practical to wood to look like quarried stone. Stucco scored to simulate stone ashlar could fool the eye as well. In the 19th century, cast iron was likewise often detailed to appear like stone. Another such imitative building material was "cast rock" or, more precisely, precast physical edifice units.
The prominent Delaware and Hudson Edifice, Albany, New York, (1916) made extensive utilise of cast rock equally trim combined with a random ashar facing of natural granite. Photo: Richard Pieper.
Cast rock was simply one name given to various concrete mixtures that employed molded shapes, decorative aggregates, and masonry pigments to simulate natural stone. The bones mixtures included water, sand, coarse aggregate, and cementing agents. Natural cements, portland cements, oxychloride cements, and sodium silicate based cements were all used every bit binding agents. The differences in the resulting products reflected the different stone aggregates, binding agents, methods of manufacture and curing, and systems of surface finishing that were used to produce them. Versatile in representing both intricately carved ornament and plain blocks of wall ashlar, cast stone could be tooled with a diversity of finishes.
During a century and a one-half of use in the United States, bandage stone has been given various names. While the term "bogus rock" was unremarkably used in the 19th century, "concrete rock," "cast rock," and "cut cast stone" replaced it in the early 20th century. In improver, Coignet Stone, Frear Rock, and Ransome Rock were all names of proprietary systems for pre-cast concrete building units, which experienced periods of popularity in unlike areas of the The states in the 19th century. These systems may be assorted with "Artistic Concrete," decorative molded concrete construction, both precast and cast-in-identify, which made little endeavor to simulate natural stone.
Sculptural decoration was oft produced in cast stone. Repetitive item, such as these banding course panels on the Level Gild in New York Metropolis (1926), were produced much more than economically than they could be in natural rock. Photo: Richard Pieper.
Having gained popularity in the United States in the 1860s, cast rock had become widely accustomed as an economical substitute for natural stone by the early decades of the 20th century. Now, it is considered an important historic material in its own right with unique deterioration problems that require traditional, likewise every bit innovative solutions. This Preservation Brief discusses in item the maintenance and repair of historic cast stone-precast concrete edifice units that simulate natural stone. Information technology also covers the weather that warrant replacement of historic cast rock with advisable contemporary concrete products and provides guidance on their replication. Many of the issues and techniques discussed here are relevant to the repair and replacement of other precast concrete products, likewise.
History of Use and Manufacture return to top ▲
Early Patented Systems
While some use of cast stone may exist dated to the Middle Ages, more recent efforts to replicate stone with cementitious materials began in England and France at the stop of the 18th century. Coade Stone, i of the all-time known of the early on English manufactures, was used for architectural ornament and trim, and saw express use for interior decoration in the The states as early as 1800. Pregnant advances in the artificial stone industry in the United States were tied to the production of natural cement or hydraulic lime, which began about 1820.
A large number of patented American, English language, and French systems were marketed immediately afterward the Civil War. One of the primeval American patents for cast stone was awarded to George A. Frear of Chicago in 1868. Frear Rock was a mixture of natural cement and sand, to which a solution of shellac was added to provide initial curing strength. Frear's arrangement was widely licensed around the country, and the resultant variation in materials and manufacturing methods apparently resulted in some pregnant failures.
Constructed in 1868 of Beton Coignet, the Scissure Ridge Bridge in New York Urban center's Prospect Park is one of the earliest extant cast stone structures in the United States. Photograph: Richard Pieper.
Some other production which utilized natural cement as its cementing agent was Beton Coignet (literally, "Coignet concrete," too known as "Coignet Stone"). Francois Coignet was a pioneer of concrete construction in France. He received United States patents in 1869 and 1870 for his system of pre-cast physical structure, which consisted of portland cement, hydraulic lime, and sand. In the United States the formula was modified to a mix of sand with Rosendale Cement ( a loftier quality natural cement manufactured in Rosendale, Ulster County, New York). In 1870 Coignet's U.S. patent rights were sold to an American, John C. Goodrich, Jr., who formed the New York and Long Island Coignet Rock Company. This company fabricated the cast stone for i of the earliest extant cast stone structures in the United States, the Scissure Ridge Bridge in Prospect Park, Brooklyn, New York.
Some proprietary systems substituted other cements for the portland cement or hydraulic lime. The British patent process of Frederick Ransome utilized a mixture of sand and sodium silicate, combined with calcium chloride, to course blocks of calcium silicate. The sodium chloride past-product was intended to be removed with h2o washes during the curing process. The Sorel cement process, developed in 1853 and later practical to the industry of grindstones, tiles, and cast stone for buildings, combined zinc oxide with zinc chloride, or magnesium oxide and magnesium chloride, to form a hydrated oxychloride cement mixture that leap together sand or crushed stone. The Spousal relationship Rock Visitor in Boston manufactured cast stone using the Sorel process. Ultimately, withal, alternate cementing systems were abandoned in favor of portland cement, which proved to be more than undecayed and less expensive.
Late 19th and 20th Century Development
The apply of cast stone grew speedily with the extraordinary evolution of the portland cement and concrete industries at the terminate of the 19th century. In the early on decades of the 20th century, cast stone became widely accepted as an economical substitute for natural stone. It was sometimes used as the only outside facing material for a edifice, but was more than oftentimes used as trim on a rock-faced natural stone or brick wall.
Cast stone was commonly used for molded trim in conjunction with brick or natural stone. This brick building in Rochester, New York, uses bandage rock for the entry surroundings, and natural stone for unornamented windows sills, thresholds, and water tabular array units. Photo: Richard Pieper.
In most early on 20th century installations, cast stone was used for exterior window and door surrounds or lintels, copings, parapets and balustrades, banding courses, cornices and friezes, and sculptural ornament. On occasion, decorative interiors were also finished with cast stone, although elaborate interior cornices and ornaments were more frequently made of plaster.
Manufacture
Manufacturers of bandage stone used graded mixes of crushed marble, limestone, granite, and smelting slag to produce a variety of rock effects. A lite cement matrix with an aggregate of crushed marble could replicate limestone, while a mix of marble and minor amounts of smelting slag would give the effect of white granite. Some manufacturers added masonry pigments and varied colors on the faces of the stone to give a somewhat stylized upshot of variegated sandstone. Each manufacturer prepared a multifariousness of stock mixes too. Not surprisingly, aggregates varied in different localities. In New York State, for instance, crushed Gouverneur and Tuckahoe marbles were popular facing aggregates; in other areas crushed feldspar or granite and even silica sand were usually used.
The ii basic cast rock production systems were "dry tamp" and "wet cast." The dry tamp process employed a stiff, low slump concrete mix that was pressed and compacted into the molds. The decorative aggregate mix was ofttimes distributed only on the exterior facing of the cast units (typically 3/4" to one Ω" thick,) while the cores of the units were common concrete. Because of the stiff mix, dry tamp units required a relatively short period of fourth dimension in the molds, which could then be used several times a 24-hour interval. After removal from the molds, the dry out tamp units were often cured in steam rooms to clinch proper hydration of the cement. The moisture cast procedure, on the other hand, used a much more plastic concrete mix that could be poured and vibrated into the molds. This system used significantly more water in the mix, assuring proper hydration of the cement mix without elaborate curing, but requiring that the units be left in the molds for at least a day. Because of this method of fabrication, wet cast products necessarily distributed their decorative aggregate mix through the entire unit, rather than only an outer facing.
Concrete was cast in molds of wood, plaster, sand and, early in the 20th century, even hide glue or gelatin, depending upon the product method, the intricacy of the piece to be cast, and the number of units to be manufactured. Metallic molds were sometimes used for stock ornamental items, less frequently for custom architectural work. When the units were adequately difficult, finish surfaces were worked to expose the decorative stone aggregate. When removed from the mold, wet cast units exhibit a surface motion-picture show of cement paste, which must be removed to expose the amass. Partially cured units could be sprayed with water, rubbed with natural bristle brushes, etched with acrid, or sandblasted to remove the cement layer. The surface of dry out tamp products required less finishing.
Historically, a point stop was put on with a unmarried pointed chisel. Photograph: From Onondaga Litholite Company Catalog. Courtesy, Collection of Michael F. Lynch.
High quality bandage stone was frequently "cutting" or tooled with pneumatic chisels and hammers similar to those used to cut natural stone. In some cases, rows of minor masonry blades were used to create shallow parallel grooves similar to lineal chisel marks. The results were often strikingly similar in appearance to natural stone. Motorcar and hand tooling was expensive, however, and simple molded cutting bandage stone was sometimes only slightly less plush than similar piece of work in limestone. Significant savings could exist accomplished over the cost of natural stone when repetitive units of ornate carved trim were required.
For this column, the appearance of a "pink granite" was false past using a pinkish matrix with white and black amass. Erosion of a tinted matrix results in a significant lightening of the cast rock surface. Photo: Richard Pieper.
Finally, cast stone is sometimes today used to supercede natural stone when the original historic rock is no longer bachelor, or the greater strength of reinforced concrete is desired. Reinforced cast rock columns, for instance, are often used to supervene upon natural stone columns in seismic retrofits of historic structures. Fine-grained stones, such as sandstones, may be very successfully replicated with cast rock. Coarse-grained granites and marbles with pronounced patterns or banding are, for obvious reasons, not so successfully matched with cast stone. The replacement of natural stone with cast stone requires careful attention to selection of fine aggregates and the pigmentation of the cementing matrix. Fibroid aggregate, which is generally used in cast stone to control shrinkage and assure acceptable compressive strength, tin can nowadays an aesthetic problem if it is visible at the surface of cast stone elements which simulate sandstone. Careful control of aggregate sizes in the mix formulation can reduce this trouble.
Mechanisms and Modes of Deterioration return to top ▲
The all-time historic cast rock can rival natural stone in longevity. Many quality bandage rock installations from the first decades of the twentieth century are notwithstanding in fantabulous condition, and require petty repair. Like any other building material, however, bandage rock is subject field to deterioration, which may occur in several ways:
- Separation of the facing and core layers
- Deterioration of the aggregate
- Deterioration or erosion of the cementing matrix
- Deterioration of the iron or steel reinforcement
- Deterioration of cramps and anchors used in its installation.
Separation of the Facing and Core Layers
Separation of the facing and core layers of dry tamp units is not uncommon, and often reflects fabrication defects such as poor compaction, lengthy fabrication time, or improper curing. Where separation of facing and cadre layers is suspected, cast stone units may be "sounded" to establish the extent of delamination.
Deterioration of the Amass
Bandage stone failure caused past deterioration of the aggregate is uncommon. Granites, marbles, and silica sand are generally durable, although limestone and marble aggregate are subject to the same dissolution problems that bear upon quarried units of these stones. In rare instances, a reaction between the alkalis in the cement matrix and the stone amass may also cause deterioration.
Deterioration or Erosion of the Cementing Matrix
Scaling of bandage stone units signals problems with the cementing mix and method of industry. Serious deterioration of cast stone, such as this, warrants replacement. Photos: Richard Pieper.
While it is relatively uncommon in twentieth century bandage rock, serious deterioration of the cementing matrix tin cause all-encompassing harm to cast stone units. A properly prepared cementing mix will be durable in almost exterior applications, and any flaking of exterior surfaces signals problems in the cementing mix and in the method of manufacture. The utilise of poor quality or improperly stored cement, impure water, or set accelerators tin can cause cement problems to occur years after a structure is completed. Improper mixing and compaction tin can as well result in a porous concrete that is susceptible to frost impairment and scaling. Severe cement matrix problems may be incommunicable to repair properly and often necessitate replacement of the deteriorating cast stone units.
More common and less serious than flaking or scaling caused past deterioration of the cementing matrix is the erosion of the surface of the matrix. This usually occurs on surfaces of projecting features exposed to water runoff, such as sills, h2o tables, and window hoods. In these areas, the matrix may erode, leaving small grains of aggregate projecting from the surface. The resultant crude surface is non at all the intended original appearance. In some celebrated cast rock installations, the sparse layer of cement and fine sand at the surface of the bandage stone units was not originally tooled from the molded surface, just was finished with patterns of masonry pigments in a stylized imitation of highly figured sandstones or limestones. Erosion of the pigmented surface layer on this type of bandage rock results in an even more than dramatic change in advent.
Deterioration of the Iron or Steel Reinforcement
Rusting of reinforcement about the surface of the stone may result in spalling. Sections of reinforcement, such as this, may be cut out and the spall repaired with a matching blended mortar. Photograph: Richard Pieper.
During their original industry, unusually long and thin bandage stone units, such as window sills or balcony railings, and units requiring structural chapters, such equally lintels, were generally reinforced with mild steel reinforcing bars. Big pieces sometimes had cable loops or hooks bandage into them to facilitate handling and attachment. On occasion, this reinforcement and wire may be besides close (less than two") to the surface of the piece and rusting volition cause spalling of the surface. This often happens to sills, copings, and water tables where repeated heavy wetting leads to loss of alkalinity in the concrete, allowing the reinforcement to rust. If damage from the deteriorating reinforcement is extensive, equally for case, the splitting of a baluster from the rusting of a central reinforcing rod, the cast stone unit may require replacement.
Deterioration of Cramps and Anchors
Even when reinforcement has not been added to individual cast stone units, balmy steel cramps may have been used to anchor a cast stone veneer to backup masonry. Where spalls take occurred primarily at the tops of ashlar or frieze units, this is generally the crusade.
Maintenance of Cast Stone Installations return to top ▲
Cleaning
Cast stone installations with marble or limestone aggregates may sometimes be cleaned with the same alkaline pre-launder/acrid afterwash chemical cleaning systems used to clean limestone and other calcareous natural stones. If no marble or limestone aggregates are nowadays, acidic cleaners, such as those used for natural granites and sandstones, may be used.
Sandblasting and wet grit blasting can seriously erode the surface and should non be used to make clean cast stone surfaces. Photograph: Richard Pieper.
In either case, dark particulate staining in protected areas may exist persistent, nevertheless, and crave experimentation with other cleaning methods. Some micro-abrasive cleaning techniques used under very controlled circumstances past skilled cleaning personnel tin can be appropriate for removing tenacious soiling. Ordinary sand blasting or wet grit blasting tin can seriously impairment the surface of the cast stone and should non be used.
Repointing
Early cast stone installations may have been constructed with natural cement mortars, just in late nineteenth century and twentieth century installations, cast rock units were generally bedded and pointed with mortars composed of portland cement, lime, and sand. When repointing or replacement of the historic mortar is required, a Type N mortar (virtually ane part cement, and one part lime to six parts of sand) is generally appropriate. When repointing any historic masonry, it is of import to match both the character and color of the sand and colour of the cement matrix in the historic mortar. Cement matrix colour can often be adapted by using combinations of white, "low-cal," and grayness portland cement in the mortar.
Joints in historic cast rock installations can be quite thin and the dense mortar thus hard to remove. Unnecessary repointing can cause significant damage to celebrated cast stone. Cracked and open joints will near often be plant on exposed features such as balustrades and copings and, of form, require repointing. When a difficult and tenacious mortar was used in the original installation or a after repointing, the removal of the mortar tin can easily chip the edges of the bandage stone units.
While the careless employ of "grinders" to remove mortar has damaged endless historic masonry buildings, a skilled mason may sometimes utilise a hand held grinder fitted with a thin diamond bract to score the center of a joint, and so remove the rest of the mortar with a paw chisel. If this method is not done carefully, however, wandering of the bract tin can widen or alter joints and cause pregnant damage to the bandage rock. Care must be taken to prevent damage from over cutting of vertical joints by stopping blades well brusque of adjacent units. The use of small pneumatic chisels, such as those used to tool rock, can too work well for mortar removal, only even this method can cause chipping to the edges of cast stone units if it is not done carefully.
Methods of Repair return to top ▲
Much celebrated bandage stone is unnecessarily replaced when it could easily be repaired in situ, or left untreated. This is especially true of areas that exhibit isolated spalls from rusting reinforcement confined or anchorage, or installations where erosion of the matrix has left a crude surface of exposed amass.
The weathering of cast stone, while dissimilar from that of natural stone, produces a patina of age, and does non warrant large-scale replacement, unless severe cement matrix problems or rusting reinforcement bars have acquired extensive scaling or spalling. Astringent rusting of reinforcement bars on minor decorative features, such every bit balusters, may bespeak carbonation (loss of alkalinity) of the matrix. Where carbonation of the matrix has occurred, untreated reinforcement volition proceed to rust. Replacement may be an acceptable approach for exposed and severely deteriorated features, such as paw railings, roof balustrades, or wall copings, where disassembly is unlikely to damage adjacent construction. Conversely, pocket-size areas of harm should generally be repaired with mortar "composites," or left alone.
A delaminated layer of ornamental cast rock on the Orpheum Theater, San Francisco, California (1925), was successfully re-attached using epoxy. The multiple delivery ports for the epoxy are removed afterward treatment and the holes patched. Photograph: David P. Wessell.
Re-securing Separated Surface Facing
Where the decorative facing of dry out tamped cast stone has separated from core layers, injected grouts may be used to re-secure the facing. Re-attachment of a separated facing layer may be time consuming, and should exist undertaken by a conservator, rather than a stonemason. This technique may be the best, nearly economic, approach for repair of figurative sculpture or unique elements that are not repeated elsewhere on a edifice. Theoretically, cementitious grouts are most appropriate for re-attaching separated facings, just hairline fissures may require the use of resin adhesives. Depression-viscosity epoxies have been used for this purpose, and may exist practical through minor injection ports. Cracks that would permit agglutinative to leak must be repaired prior to injection, of grade. Holes made for adhesive injection will crave patching after re-zipper is consummate.
Repairing Reinforcement Spalls and Mechanical Damage
Drilled holes, mechanically damaged corners, and occasional spalls from rusting reinforcement bars and anchorage are repairable weather condition that do not warrant the replacement of cast stone. Pocket-sized "composite " repairs to damaged masonry units tin can exist made with mortar formulated to visually match the original material, and may be successfully undertaken by a competent and sensitive mason. If deterioration appears widespread, however, or if big surface areas are spalling or cracking and replacement appears necessary, the owner may wish to consult a preservation architect or consultant to decide the cause of deterioration and to specify necessary repairs or replacement, as appropriate.
The damaged area is cutting out to create a shallow void of even depth, half inch or more than. Photo: Richard Pieper.
The composite mortar is applied to the void with a small spatula or trowel. Photo: Richard Pieper.
This completed blended repair could have been improved by brushing to remove the matrix residue at the edges of the repair earlier the surface cured. Photograph: Richard Pieper.
The methods of composite repair used for stone masonry are also mostly applicative for the repair of celebrated cast stone. For repairs to damaged cast rock to be successful, nevertheless, both the cement matrix colour and the amass size and coloration must match that of the historic unit. Crushed stone and slag (such as "Blackness Beauty" annoying grit), which are similar to many mutual traditional aggregates, are widely available, although some boosted burdensome and/or sieving may be necessary to obtain aggregate of an appropriate size. Recall that half or more of a weathered surface is exposed amass, and so careful aggregate selection and size grading is extremely important for patching. Even differences in aggregate angularity (rounded pebbles vs. crushed stone) will be noticeable in the final repair. If more than 1 aggregate was used in the cast stone, the ratio of the selected aggregates in the mix is, of course, equally important. Variation in coloring of the cement matrix may be accomplished through the apply of either white, "light, " or gray portland cement. If additional tinting is required, only inorganic alkali-resistant masonry pigments should be used. Considering nearly historic cast rock was manufactured primarily from portland cement and aggregate (with a less than fifteen% lime/cement ratio), it is not necessary to add together large amounts of hydrated lime to cast stone composite repair mixtures. Small amounts of lime may exist added for plasticity of the working mix.
To repair a spall caused past deterioration of a ferrous reinforcement bar or anchorage, information technology is necessary to remove all cracked concrete next to the spall; grind and brush the reinforcement to remove all rust and scale; and paint the metallic with a rust-inhibiting primer prior to applying the cast rock composite. If the reinforcement bar is much besides close to the surface of the rock, information technology may be advisable to cutting out the deteriorating section of reinforcement later on consultation with a structural engineer. If deteriorating cramps are removed, it may be necessary to install new stainless steel anchorage.
Where spalls take a feather edge, it volition be necessary to cutting back the repair area to a uniform depth (1/2" or more). Equally with natural rock composite repairs, a bonding agent may aid adhesion of the repair material to the original concrete. For unusually large or deep patches, mechanical anchoring of the repair with modest nylon or stainless steel rods may be required. If the adjacent bandage rock is tooled or weathered, it volition be necessary to scribe or brush the repaired surface area to give it a matching surface texture. Adding enough coarse aggregate to lucifer adjacent original material will sometimes interfere with adhesion of the composite, and it may exist necessary to press additional aggregate into the practical patch prior to finishing. If this is not skillfully done, however, the surface of the patch may take on a mosaic appearance. For this reason, it is advisable to undertake test composite repairs in an unobtrusive location first.
Surface Refinishing
While re-tooling of deteriorated natural stone may sometimes be advisable, restoring the original appearance of cast stone where surface erosion has occurred is difficult or impossible.
Unlike natural stone, cast stone mostly may non be tooled in place to reduce lippage of uneven surfaces at joints. Tooling often exposes fibroid amass from below the surface. Photo: Richard Pieper.
Tooling or grinding of the surface of the bandage stone may expose coarse amass beneath the surface and will non, in whatsoever case, restore original patterned pigmentation that has weathered away. Silicate paints or masonry stains may exist applied in patterns to replicate the original appearance, but may non exist durable or completely successful aesthetically. Where matrix has eroded, it is appropriate to accept the weathered appearance of the cast stone, unless all-encompassing replacement is mandated by other factors.
Because cast rock depends on exposed aggregate to reach its artful upshot, the use of an applied cementitious surface coating dramatically alters the visual effect of the material and is inappropriate as a bandage stone repair technique. A cementitious surface blanket tin too trap moisture in the cast units and cause surface spalling.
Replacement of Celebrated Bandage Stone Installations return to top ▲
Private cast stone units, which are subject area to repeated wetting (such every bit copings, railings and balusters) and showroom severe failure due to spalling or reinforcement deterioration, may require replacement with new cast stone and tin replicate deteriorated units in existing buildings.
After removal from the mold, the new cast stone units are scrubbed to remove surface paste and expose underlying aggregate. End cast pieces, such every bit this sculpture, may accept numberous air bubbles at the suface.Photograph: Richard Pieper.
Fortunately, a number of companies custom industry precast concrete units. The variables involved in manufacture are considerable, and it is wise to use a firm with experience in ornamental and custom work rather than a precast concrete firm which manufactures stock structural items, concrete pipe, or the like. Several merchandise organizations, including the Cast Rock Institute, the National Precast Concrete Clan, and the Architectural Precast Association, take developed recommendations and/or guide specifications for the manufacture of bandage rock and precast physical. These specifications set standards for characteristics such equally compressive forcefulness and water absorptivity, and talk over additives such as air entraining agents and water reducing agents, which influence the longevity of new cast stone. Trade references and guide specifications should be consulted earlier contracting for replacement of historic cast rock.
The holes on the surface of the casting (see in a higher place, left) are beingness filled with a mortar similar to the concrete mix used to cast the element. Photo: Richard Pieper.
Fabrication defects in new cast rock. While the cement matrix coloration and aggregate considerations previously mentioned require the nigh conscientious attending, project staff should also await for defects which are common to cast stone fabrication:
Air bubbles. Small pits on the surface of the stone may class if the unit of measurement is non given adequate vibration to release trapped air during pouring. Bubbles can also be a problem when stop casting long items such as columns or railings, where information technology is difficult to vibrate bubbles away from the finish surface of the unit.
Surface cracking or checking. Overly wet mixes and insufficient moisture during curing can result in surface keen of large castings, such every bit columns. Such cracking dramatically reduces the durability of new bandage stone. Small reinforced elements, such as balusters, besides oftentimes crack at sparse "necks" in the castings.
Aggregate segregation. Cast stone formulations generally include a range of coarse aggregates (crushed stone) and fine aggregates (sand). When units are vibrated to assure compaction of the mix and liberate trapped air bubbles, fibroid aggregates may begin to settle and separate from the paste of cement and sand. Aggregate segregation results in a visible concentration of coarse aggregate at 1 end of the casting. Segregation is more problematic when stop casting long pieces such as columns.
Surface cracking may reduce the durability of cast rock units. Cracking is often problematic on reinforced elements with thin "necks," such equally balusters, unless curing is advisedly controlled. Photograph: Richard Pieper.
Surface rippling or irregularity. Production molds for fabrication are oftentimes made of rubber mold facings encased in larger "mother molds" of plaster and forest. Vibration tin loosen the rubber facing from the outer mold and outcome in rippling or irregularities on the surface of the finished casting. Even when rippling is not noticeable, irregularity caused by mold movement tin can make information technology difficult to line up surfaces of side by side units when assembling cast stone installations.
Mold lines. Freestanding elements, such as columns, must be cast in two-part molds, which are separated to release the completed cast piece. If the mold parts do non join tightly, some leakage of cement paste will occur at the mold articulation, resulting in a projecting line on the surface of the casting. This is generally tooled off before the casting completely cures. A mold line will be visible on the completed piece if the projecting fabric is not completely removed, or if the tooling at the mold line does not match the adjacent surface of the casting. Tooling at mold lines may too expose contrasting coarse aggregate beneath the surface of the casting.
Other Considerations for Replacement of Cast Stone
Several other considerations are worth noting when it is necessary to supercede historic cast stone elements with matching new cast stone.
Reinforcement. The alkalinity of new concrete generally provides adequate protection to steel reinforcement. In exposed areas where deterioration due to rusting of reinforcement has previously been a trouble, however, the use of stainless steel reinforcement is recommended.
Production molds fabricated of durable rubbers backed with wood and plaster supports are used to fabricate new cast rock. Photograph: Richard Pieper.
Surface finishing. Postal service-fabrication surface tooling of new cast rock is not currently common. Sandblasting is typically used to remove the surface picture of cement and expose the amass. For replacement units replicating historic bandage stone pieces in highly visible locations, it is sometimes possible to make a mold of a audio or repaired existing piece to incorporate the original tooling in the casting procedure. If the historic unit is as well deteriorated to use every bit a design, a plaster model may be made to replicate the damaged piece. This is tooled to replicate the desired surface treatment or appearance, and a production mold is so made from the plaster model.
Moist curing. Surface crystallization of soluble salts (efflorescence) during curing may lighten the surface of some precast units, especially those simulating darker stone. Some manufacturers use a serial of wet/dry out curing cycles or washing with acerb acid to remove soluble salts that might otherwise discolor finished surfaces. For virtually wet cast products, simple moist curing nether a plastic cover is sufficient.
Appropriateness of Glass Fiber Reinforced Physical equally a Replacement Material return to top ▲
Lite-Weight Alternative
GFRC is sometimes used to replicate deteriorated elements of cast or fine grained natural rock. Because the GFRC element is a rigid, but relatively sparse shell, information technology must be supported and attached with an interior framework of steel.
The attachment hardware inside this GFRC cartouche (left) will not exist visible when the unit is installed (armature visible at right). Photos: Towne House Restorations, Inc
Glass fiber reinforced concrete (GFRC) is more and more ofttimes encountered in edifice restoration and is used to replicate deteriorated stone and cast stone, and even architectural terracotta. This is a relatively new material that uses short chopped strands of glass fiber to reinforce a matrix of sand and cement. GFRC has become a popular low cost culling to traditional precast concrete or stone masonry for some applications. Fabricators utilize a spray gun to spray the mortar-like mix into a mold of the shape desired. The resulting concrete unit, typically only æ" thick, is quite rigid, but requires a metal frame or armature to secure it to the building substrate. The metallic frame is joined to the GFRC unit with modest "bonding pads" of GFRC.
GFRC has a dramatic advantage over traditional precast concrete where the weight of the installation is a business concern, such as with cornices or window hoods. Many cast stone mixes tin successfully be replicated with GFRC. Where it is used to simulate natural stone, GFRC, similar bandage rock, is most advisable for simulation of fine-grained sandstones or limestones.
Non for Use in Load Begetting Applications
Because the GFRC arrangement is in outcome a "skin," GFRC cannot exist used for load begetting applications without provision of additional support. This makes it unsuitable for some tasks such as replacement of individual ashlar units. It is also not appropriate for small-scale freestanding elements such as balusters, or for most columns, unless they are engaged to surrounding masonry or can exist vertically seamed, which may significantly alter the historic appearance. GFRC units must also allow for expansion and contraction, and are generally separated by sealant joints, not by mortar. A sealant joint may be unacceptable for some historic applications; however, substitution of GFRC for cast rock may exist advisable when an entire assembly, such every bit a cornice, roof dormer, or window hood, requires replacement. Great care must exist taken when detailing a GFRC replacement for existing bandage stone.
Deterioration of GFRC
Because information technology is a relatively new material, the long term immovability of GFRC is still untested. When GFRC was starting time introduced, some installations experienced deterioration caused past alkaline sensitivity of the drinking glass fiber reinforcement. Alkali resistant glass is now used for GFRC manufacture. Even when the GFRC skin is well manufactured, however, the steel armature and bonding pad system used to mount the textile is vulnerable to damage from leakage at sealant joints or pocket-size cracks in wash surfaces. The use of galvanized or stainless steel armatures, and stainless steel fasteners and bonding pad anchors is appropriate.
Summary and References return to elevation ▲
Cast stone-a mixture of water, sand, coarse aggregate, and cementing agents—has proven over time to be an attractive and durable building material, when properly manufactured. It gained popularity in the 1860s and, past the early decades of the 20th century, became widely accepted as an economical substitute for natural stone. Unfortunately, much historic bandage rock is unnecessarily replaced when it could easily exist repaired and preserved in situ, or left untreated. Advisable repair of damaged units tin can extend the life of any bandage stone installation. Because of the necessity of matching both matrix color and aggregate size and ratio, conservation projects which involve repair or replication of cast stone should allow adequate lead time for the assembly of materials and the preparation of examination samples. Understanding which atmospheric condition require repair, which warrant replacement, and which should exist accepted as normal weathering is key to selecting the about appropriate approach to the protection and care of historic cast stone.
Helpful Organizations
- Cast Stone Institute
- 10 West Kimball Street
- Winder, GA 30680-2535
- National Precast Concrete Association
- 10333 North Meridian Street, Suite 272
- Indianapolis, IN 46290
- Architectural Precast Association
- P.O. Box 08669
- Fort Myers, FL 33908-0669
Acknowledgements
Richard Pieper, Manager of Preservation for January Hird Pokorny Assembly, Inc., New York, has considerable experience in varied aspects of architectural conservation, including documentation of historic architectural engineering and analysis and conservation of building materials. The author wishes to thank Alan Barr of Towne House Restorations, Inc., Ron Moore of Western Edifice Restoration, architect Theo Prudon, and conservator David Wessell of the Architectural Resources Group for their assistance in the preparation and review of this brief. Chuck Fisher and Anne E. Grimmer, Technical Preservation Services, NPS, offered valuable comments during its development. MJM Studios provided admission for photography of bandage stone fabrication. Michael F. Lynch, Vice President, SPNEA, and Michael Devonshire, Main, Jan Hird Pokorny Associates, generously lent images from their personal collections for the Cursory. Kay D. Weeks, Heritage Preservation Services, National Park Service (NPS), served equally project director and full general editor.
This publication has been prepared pursuant to the National Historic Preservation Deed of 1966, as amended, which directs the Secretary of the Interior to develop and make bachelor data apropos historic properties. Technical Preservation Services (TPS), National Park Service prepares standards, guidelines, and other educational materials on responsible historic preservation treatments for a broad public.
Photographs included in this publication may non exist used to illustrate other publications without permission of the owners.
September 2001
Reading List return to top ▲
Childe, H.L. Manufacture and Uses of Concrete Products and Cast Stone, London: Concrete Publications Limited, 1930.
Jester, Thomas C., ed., Twentieth Century Building Materials, New York, NY: McGraw-Hill, 1995.
Precast/Prestressed Concrete Establish, Architectural Precast Concrete, 2nd ed., Chicago, Illinois: Precast/Prestressed Physical Constitute, 1989.
Whipple, Harvey, Concrete Stone Manufacture, Detroit: Physical-Cement Age Publishing Company, 1918.
Source: https://www.nps.gov/tps/how-to-preserve/briefs/42-cast-stone.htm
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