SANDSTONE:  CHARACTERISTICS, USES AND PROBLEMS


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


INTRODUCTION

Characteristics of Sandstone:

-    A sedimentary rock consisting of sheets of stone deposited one
     atop another under water.  

-    Very porous and water will penetrate it easily.

-    Brown, red, purple, and pink sandstones are commonly called
     brownstone.

-    Available in a variety of surface textures and earth-toned
     colors.

-    Weathers best when its end-grain faces the weather (naturally
     bedded).  (Face-bedded) stone is subject to greater
     deterioration.  Water damages a face-bedded stone by spalling
     or flaking off entire sheets of sandstone.  Also, freeze/thaw
     cycles allow water to get into the stone and then freeze and
     expand causing some of the top layer to split off.

     NOTE:  In many 19th century applications, the grain was placed
     parallel to the weather side (face-bedded) for aesthetic
     reasons.  This was especially common around doorways.


TYPICAL USES

Typical historical uses for sandstone included:

-    Urban row houses, commercial buildings and churches built from
     the 1840s through the early 20th century (this was usually
     brownstone).

-    Lighter-colored sandstones were used more frequently by the
     end of the 19th century.

Typical current uses for sandstone include:

-    New sandstone is not widely available for use today.

-    The popularity of sandstone today is focused primarily on its
     decay and on how it can be repaired and preserved.


PROBLEMS AND DETERIORATION

Problems may be classified into two broad categories:  1) Natural
or inherent problems based on the characteristics of the material
and the conditions of the exposure, and 2) Vandalism and human-
induced problems.     

Although there is some overlap between the two categories, the
inherent material deterioration problems generally occur gradually
over long periods of time, at predictable rates and require
appropriate routine or preventive  maintenance to control. 
Conversely, many human induced problems, (especially vandalism),
are random in occurrence; can produce catastrophic results; are
difficult to prevent, and require emergency action to mitigate. 
Some human induced problems, however, are predictable and occur
routinely.  


NATURAL AND INHERENT PROBLEMS

1.   Moisture-related problems:  May be evident in sandstone as
     spalling, erosion, cracking, flaking and deteriorated mortar
     joints.

2.   Weathering:  Disintegration of the stone's surface usually
     caused by erosion, chemical action, and moisture freezing in
     the stone.

3.   Exfoliation:  Separation and loss of large areas of stone
     along the bedding planes usually caused by the stone having
     been face-bedded.

4.   Blind Exfoliation:  Separation of stone along bedding planes,
     but where layers are still loosely attached behind the
     surface.  It is often caused by having laid the stone with the
     bedding planes running parallel with surface of the wall
     (face-bedding).  Blind exfoliated stone will sound hollow when
     lightly tapped with a rubber mallet.

5.   Blistering:  Swelling and rupturing of a thin uniform skin
     caused by air-borne chemicals reacting with the stone's
     surface, forming a hard, brittle skin.  The blisters will
     often pop when touched.

6.   Cracking:  Narrow fractures in the stone from 1/16 to 1/2 inch
     wide.

7.   Detachment:  A clean break in the stone often resulting from
     a sharp impact, or from stresses concentrated in a small area
     of stone due to structural settlement.


HUMAN-INDUCED PROBLEMS:

1.   Stone laid with its layers parallel to the wall plane (face-
      bedded) rather than perpendicular to the wall plane
     (naturally-bedded):  Face-bedded stone is more prone to
     deterioration by weathering as entire sheets of stone tend to
     flake off.

2.   Painting over a deteriorated stone surface:  This only leads
     to more serious moisture-related problems.

3.   Applying a cement patch over deteriorated surface areas:  If
     an inappropriate patching mix is used, it may be necessary to
     paint the entire stone surface to lessen the visual disparity
     between the two materials.

                          END OF SECTION