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Historic Preservation - Technical Procedures

Spectitle:

Cast Stone: Characteristics, Uses And Problems

Procedure code:

0472001S

Source:

20Th Century Building Materials (Ed. Tom Jester, Nps)

Division:

Masonry

Section:

Cast Stone

Last Modified:

11/12/2013

Details:

Cast Stone: Characteristics, Uses And Problems



CAST STONE: CHARACTERISTICS, USES AND PROBLEMS


This standard includes general information on the characteristics
and common uses of cast stone and identifies typical problems
associated with this material along with common causes of its
deterioration.


INTRODUCTION

Characteristics of cast stone:

-    Made from Portland cement, sand, crushed stone, fine and coarse aggregates and
    water in varying proportions and formulas.

-    Manufactured in custom molds - either by dry-tamping or wet
    casting.

-    Dry-tamping is cast in two layers - an inner core and a
    facing; due to cost, only the facing material usually contains
    the coloring aggregates and pigments; numerous casts from the
    same mold can be made in the same day.

-    Wet-casting is one integral mix containing enough water for it
    to flow easily into the mold; this method produces a cast with
    integral coloring; typically only one piece can be cast in a
    mold in one day due to the high water content.

-    Typical aggregates used included granites, marbles and blast-
    furnace slag.

-    Can be manufactured in just about any shape or size.

-    The strongest cast stone consisted of varying sizes of
    aggregates; this allowed large and small pieces to fit closely
    together, while cement filled in the voids.  

-    Historically, paint was often applied to the surface for the
    purposes of waterproofing.

-    The aggregate primarily determines the cast stone color.

-    Veining was created by placing dye-soaked strings or thin
    strips of wood into the mold and then removing them before
    casting; the dye could then soak into the concrete mixture;
    veining could also be achieved by applying color or dye to the
    surface using a fine brush

-    Typical finishes include 1) surfaced cast stone, 2) cut cast
    stone, and 3) plain cast stone.

-    Surfaced cast stone includes hand-rubbed, brushed and acid-
    washed finishes.

-    Cut cast stone includes machine-rubbed, planar-rubbed, bush-
    hammered and machine- and hand-tooled finishes.

    References: Cast Stone Institute (CSI), www.caststone.org.;
    American Society for Testing and Materials, www.astm.org, 
    see ASTM C1364, Standard Specifications for Architectural Cast Stone; 
    for general information see: Wikipedia, "Cast Stone".


TYPICAL USES

Typical historical and current uses for cast stone include:

-    Commonly used in the late 19th and early 20th centuries.

-    Used in the form of a veneer, a block or as ornament.

-    Used to simulate evenly veined and colored stones.

-    Used to simulate natural stone by the late 1920s.

-    Commonly used in the construction of houses, banks, churches,
    schools, libraries, and commercial buildings.

-    Used for specific features such as window sill, steps,
    beltcourses, chimney caps, spandrel panels, sculpture and
    other ornament.

-    Cast stone is installed like natural stone, laid in place with
    mortar, or fastened with metal anchors.

-    Due to the high cost of manufacturing cast stone compared with
    lighter weight precast concrete, cast stone companies were
    almost non-existent by the early 1950s; many were absorbed
    into existing precast companies.

-    The compressive strength of new cast stone is 6,500 pounds per
    square inch with an absorption rate not more than 6 percent; in
    the late 1920s, the standard compressive strength of cast
    stone was 5,000 pounds per square inch with an allowable
    absorption rate of 7 percent.


NATURAL OR INHERENT PROBLEMS

-    Facing Delamination: Common with dry-tamp cast stone; can
    result from flaws in manufacturing, or from differences in
    water absorption ratios combined with freeze/thaw cycles.

-    Carbonation: Loss of alkalinity.

-    Aggregate/Alkali Reaction.

-    Freeze/Thaw: May result in surface scaling.

-    Erosion: Visible as weathering of the aggregate and cement
    binder; surfaces look sandy, rough, with exposed aggregate and
    pockmarks; horizontal surfaces are especially vulnerable.

-    Some types of cast stone (those containing calcareous
    sediments such as limestone) are sensitive to acidic
    environments.

-    Cracking and Spalling: Typically caused by corrosion of metal
    reinforcement materials; visible as rust stains.


VANDALISM OR HUMAN-INDUCED PROBLEMS

-    Crazing: Hairline cracks common especially with dry-tamp cast
    stone; a problem often caused by volume differences between
    the facing and backup material, or improper proportioning of
    the facing mix; visible by fine hairline cracks.

-    When aggregates of uniform size are used, the cast stone tends
    to be more porous and less durable.

                         END OF SECTION
 


cast stone, cast stone problems, cast stone uses