During construction, prevention against staining is mandatory and becomes the best solution for a clean finished project. Every effort should be taken to protect the Cast Stone during storage, setting, and after installation. Storage of Cast Stone should be above ground on non-staining planks or pallets. The storage site should be away from heavy construction traffic. Cast Stone stored for an extended period of time should be kept on pallets or non-staining planking and covered with non-staining tarpaulins. After setting, columns, pilasters, entry jambs, windowsills and all stone with projecting profiles should be protected with non-staining materials during the remaining construction.

Regardless of the degree of care exercised during construction, a final washdown will be needed and, normally, whatever is specified to clean the brickwork will adequately clean the Cast Stone. A variety of commercial cleaners are available and most contain detergents combined with mild solutions of phosphoric and/or muriatic acids. Extreme care should be taken when applying acidic cleaners to areas where joints are left open or where sealant is used as jointing material. The sealant manufacturer should be contacted to ascertain compatibility with cleaning materials. Acids left behind the stone on masonry wythe may cause corrosion problems later on.

The most common stains due to construction are dirt and mortar. Dirt can be removed by scrubbing with a mild detergent and water. Mortar stains require brushing a solution of one part hydrochloric acid to six parts water on the stain. Soak the stone with water prior to adding any acid solution to prevent surface burning. Consult the brick supplier prior to applying acids to trim items. Insure that lower stone courses are frequently drenched with water because as acid is rinsed down the wall it can gather strength when reapplied. Take necessary steps to protect windows, door and grade materials.

When unusual stains are encountered, the same procedures, which are recommended to clean concrete, will normally clean Cast Stone. The Portland Cement Association publishes a guide for the removal of stains in concrete. The most important step to stain removal is identifying the stain and its cause. There are degreasers and paint removers readily available. Any treatment should be tested on a small inconspicuous area prior to cleaning the main units.

Dunnage materials used in the packing and transport of Cast Stone can leave stains (or clean spots) after becoming wet. Wood packaging products can transfer resins to the surface, which may be easily removed. However, solid dunnage made from fresh timber can cause dunnage marks, which become difficult to remove. Packing and dunnage materials should always allow the exposed surfaces to breathe, especially when stones are palletized or placed into storage shortly after manufacture. This will avoid color differential due to moisture becoming trapped on the surface of the stone.

Power washing and sandblasting are not recommended procedures for cleaning Cast Stone. Metal fiber brushes are not to be used for scrubbing Cast Stone.

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The on-site personnel should be familiar with the applicable sections of the Cast Stone Institute® Specifications and the Project Specification pertaining to delivery, storage, setting, patching, cleaning, pointing, caulking and sealing. In case of a conflict between the two specifications, the Project Specification should prevail. Where the Project Specification may not include a particular issue, the Industry Standards should be followed. The following checklist has been developed for Cast Stone installations.

  1. Prior to delivery there should be a set of the approved shop drawings and the approved color and texture sample on file. All test reports specified should be submitted as required.
  2. Upon delivery, all Cast Stone should be checked for chips, cracks, stains, or broken pieces. Any damage should be noted on the delivery slips and communicated to the manufacturer or the sales representative.
  3. Color and texture should be inspected in accordance to approved color sample or mock-up panel set up at the job site. In general, the color and texture of the Cast Stone delivered to the job site should be approximately equal to the approved sample when viewed in good typical daylight conditions at a ten foot distance. (See technical literature on Inspection and Acceptance.)
  4. Storage of Cast Stone should be above the ground on non-staining planks or pallets. The storage site should be away from heavy construction traffic. Cast Stone stored for an extended period of time should be kept on pallets or non-staining planking and covered with non-staining tarpaulins. Allow for air circulation.
  5. Prior to setting, insure climatic conditions are within thermal limitations of mortar. Mortar retarders and accelerators should be used according to manufacturer’s directions but not with patching material. Set stone in full mortar joints and fill all dowel holes and anchor slots completely with mortar. Insure uniform joint widths within specifications tolerances.
  6. Ensure that all specified flashing and dampproofing is installed. Flashing pierced by stone anchors must be sealed either by metal thimble, grommet or approved sealant.
  7. Concrete should never be poured against unprotected Cast Stone. Where poured in place, concrete is placed against Cast Stone sills, separate with appropriate material prior to pouring concrete.
  8. Stone anchors must meet specified standards and be non-corrosive. Stone slots to receive anchors should be completely filled with mortar.
  9. Prior to setting insure that the surfaces set in mortar are drenched with water. This will secure a good bond and help to prevent mortar shrinkage.
  10. Weep holes must be installed over windows, at relieving angles and at the V bottom of walls. No mortar drippings shall be allowed in the wythe between back of stone and face of back-up structure.
  11. All head joints at coping and sills, and joints at column covers, soffits, and, in general, all stone sections with projecting profiles, exposed top joints or rigid suspension connections to the supporting structure should be sealant joints. Only the ends of load bearing lug sills shall be set in a full bed of mortar to prevent cracking from future wall settlement. After setting, prime the joints, insert properly sized backup rod and gun in sealant.
  12. All trim items except parapet coping must align with control joints. Do not bridge coping over expansion joints.
  13. Cast Stone should be handled to minimize chipping. Care must be taken not to bump the stone into anything. Handle stones with the wide portion of the cross section in the vertical position to minimize breakage.
  14. After setting, columns, pilasters, entry jambs, window sills and all stone with projecting profiles should be protected during the remaining construction.
  15. During construction, cover open walls when rain is anticipated.
  16. Chipped Cast Stone must be patched by skilled mechanics. A trial patch must be approved before general patching is to commence.
  17. Planter coping, fountain coping, swimming pool coping, treads, risers, stone pieces at grade, and pavers should be treated with a silane or siloxane water repellent after setting. This will minimize the likelihood of dirt and groundwater entering the surface of the stone; a frequent cause of staining, efflorescence and enhancement of crazing. Check that water repellent does not affect color or texture when dry.
  18. Load bearing units should be reinforced as necessary. They may not be designed to be handled in a different orientation than they will be installed in the structure. Lintels and large panels must be kept vertical.

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Selection of the correct grade of mortar is perhaps the most important factor in the performance of a masonry wall. The mortar must have sufficient strength, bedurable, and resist rain penetration as much as possible and yet be flexible enough to accommodate slight movement within the wall.

Mortars used in the setting of Cast Stone should meet the requirements of ASTM C270, Type N mortars. These Portland cement/lime mortars consist of one part cement (ASTM C150), one part lime (ASTM C207) and six parts of clean, washed masonry sand (ASTM C144). They may also contain iron oxide coloring pigments (ASTM C979) up to 5% of the weight of the cement when pointing mortars are not used.

The 1/1/6 mixture provides good bond strength with desired weather resistance and moderate compressive strength relative to the stone when cured. The lime enhances the workability of the mortar while reducing shrinkage. The practice of wetting the head and bed joints of the stone will further protect against joint shrinkage.

Although Type N mortar is the standard used in Cast Stone (as well as many natural cut stone) applications, the proportions may be varied to suit specific applications.

Proper mixing is essential to good consistency. All materials are measured by volume. Sands should be placed in the spiral-blade or paddle type mixer first, followed by pigments (if required), pre-water, lime and cement, final water and 5-7 minutes of mixing time. Mortars unused after 90 minutes should be discarded.

Head joints in most hand set stones may be set with the usual wet consistency mortar used in setting brick and block. Stiffer mortar must be used when setting larger stones and shims are recommended for all pieces over 300 lbs. When setting, fill all dowel holes, anchor slots and similar building stone anchor pockets completely with mortar. Non-shrink grout or anchoring cement may be specified for dowel connections. Avoid placing mortar across a full bed of flashing. The stone needs a bond with the masonry wall below.

Only the ends of the lug windowsills and stair treads should be set in mortar. This prevents the stone from cracking due to loading and future settlement.

Rake all stone-to-stone joints to a depth of 3/4”± for pointing later. Stone-to-brick joints are usually struck and tooled to a slight concave (See Technical Bulletin #44 on Pointing). Sponge all mortar smears from face of stone with water. Hardened, smeared mortar is difficult to remove from the surface of cast stone. Clean with commercial masonry cleaner such as SureKleen #600 or Vanitrol with water and a stiff fiber brush. Consult Cast Stone manufacturer first. Power washers should not be sued to clean Cast Stone. (See Technical Bulletin #39 on Cleaning).

The decision on whether to use mortar/pointed joints or sealant joints is a common one. All head joints at coping and joints at column covers, cornices, platforms, soffits, and in general, all stone sections with projecting profiles, exposed top joints or rigid suspension connections to the supporting structure should be “soft” sealant joints. After setting, prime the ends of the stones, insert properly sized backup rod and gun in sealant (See Technical Bulletin #43 on Sealants).

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Always rake and point mortar joints rather than full-bed setting and finishing in one operation. Mortar joints are best suited for masonry-bound trim items such as belt courses, lintels, window surrounds, date stones, inscription blocks, quoins, keystones and similar applications.

Not all joints between stones or between stone and other material should be filled with mortar. All head joints at coping stones and joints at column covers, cornices, platforms, soffits, window sills and in general, all stone sections with projecting profiles, exposed top joints or rigid suspension connections to the supporting structure should be 'soft' sealant joints (See Technical Bulletin #43 on Sealants).

Pointing is required because mortar shrinks and settles as it cures. Since mortar beds harden from the face in, stresses can be applied to the edge of the stone, which can cause spalling later. Shrinkage also can create cracks at the joints; a condition which causes leaking. Regardless of whether the mortar or sealant is selected as the face joint material, the mortar must be raked out of the joint to a minimum depth of 3/4". If sealant is to be used at the head joints, then mortar should not be used there at all.

Pointing is usually done in 1 or 2 stages to allow maximum sealing of shrinkage cracking in the mortar. It should not be done in areas exposed to hot sunshine and it is suggested that pointing be accomplished after patching of Cast Stone and before final wash-down.

Apply pointing mortar using proper tools to compress the material against the edges of the stone. A concave joint is recommended for the best protection against leakage although other joint types are often available in the stone setting trade.

Pointing mortar should be softer than the stone so that thermal stress will not cause spalling at the edges of the joints. It is usually slightly drier than normal setting mortar consistency to prevent shrinkage and is usually composed of the following:

  • 1 part Portland cement, ASTM C150
  • 1 part hydrated lime, ASTM C207
  • 6 parts masonry sand, ASTM C144

Coloring may be added to achieve almost any hue, however pointing mortar which sharply contrast the color of the stone may cause staining. Excess pointing material must be sponged away from the face of the stone immediately. Colors added must be natural or synthetic mineral oxides, which meet the requirements of ASTM C979 (sun-fast, lime-proof, alkali-resistant) and the dosage must not exceed 10% of the weight of the cement used. Carbon black or ultramarine blue pigments should not be used. In general, pigmentation types and amounts used in the manufacture of Cast Stone can also be used as a starting point when custom blending the pointing mortar to match or complement the color of the Cast Stone.

Always specify a mockup wall when approving final colors and be sure that it has been properly cleaned because cleaning will usually affect the color of pigmented masonry materials.

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The decision on whether to use mortar with pointed joints or sealant joints between stones is a common one. All head joints at coping stones and joints at column covers, cornices, platforms, soffits, window sills and in general, all stone sections with projecting profiles, exposed top joints or rigid suspension connections to the supporting structure should be “ soft” sealant joints.

Mortar joints are best suited for masonry-bound trim items such as belt courses, lintels, window surrounds, date stones, inscription blocks, quoins, keystones and similar applications. Always rake and point mortar joints rather than full-bed setting and finishing in one operation (See Technical Bulletin #44 on Pointing).

Sealant joints allow for movement at the vertical joints. Leave head joints dry when setting. It is a mistake to use mortar in combination with sealant in the same joint. An allowance for compression is required for the system to be effective. After setting, prime the ends of the stones, insert properly sized foam backup rod and gun in sealant.

Since sealant systems are not intended to bear weight, use plastic setting pads or lead shims when setting the stones on a soft bed joint. The sealant is not intended to adhere to the foam backer rod. The sealant should adhere to the parallel surfaces only. The foam rod should be placed to a depth approximately equal to the width of the joint.

Sealants are specified under section 07920. The most common types are one-part “moisture cure” or “air cure.” Two part systems are also available which require the mixing of materials together to allow chemically induced curing.

The inherent properties of silicone products make them excellent sealant materials. Silicones provide superior weathering resistance and perform over a wide range of service temperatures. They are easy to apply, have low shrinkage rates, and can accommodate high movement. While organic materials tend to crack, dry up, and become brittle or even revert with age, silicones remain flexible and durable.

Two component, polyurethane sealants are tough and elastic, allowing for movement of up to 50% of the joint width. They are also durable, flexible and form a watertight bond with most building materials. According to the manufacturers, these formulations offer weathertight seals in caulking joints today for as long as 20 years under normal application conditions and ten years under severe conditions.

Allowance for thermal and other movement should be within 25% of the joint size. For instance, a normal 3/8” joint should be expected to compress to approximately 1/4” and expand to approximately 1/2” during elongation.

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