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, fine and coarse aggregates and
     water.

-    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.

     Refer to Cast Stone Institute (CSI) Standard Specification
     04720-97 for additional information.  See also CSI Technical
     Bulletins for specific technical guidance.  Cast Stone
     Institute, 10 West Kimball Street, Winder, GA 30680-2535,
     770/868-5909.


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 6,000 pounds per
     square inch with an absorption rate less than 5 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