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Historic Preservation - Technical Procedures

Spectitle:

Concrete Block: Characteristics, Uses And Problems

Procedure code:

0422001S

Source:

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

Division:

Masonry

Section:

Concrete Unit Masonry

Last Modified:

12/10/2012

Details:

Concrete Block: Characteristics, Uses And Problems


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CONCRETE BLOCK: CHARACTERISTICS, USES AND PROBLEMS


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

 

Reference: National Park Service Preservation Brief #15: Preservation of Historic Concrete

 

http://www.nps.gov/history/hps/tps/briefs/brief15.pdf


INTRODUCTION

Characteristics of concrete block:

-    Made from a mixture of Portland cement, blended cement, various types of aggregates, and water.

-    Also referred to as concrete masonry units (CMU).

-    Advantages: Inexpensive, lightweight, durable, easy to
    install, fireproof, low maintenance, and could be ornamented.

-    Face plates were used to create a variety of surface finishes,
    including cobblestone, brick, ashlar and rockface (the most
    common type); more decorative finishes included designs of
    scrolls, wreaths and roping.

-    Typical size manufactured is nominally for a stretcher block 8 by 8 by 16 inches; this was the
    standard size manufactured by 1930 (actual dimensions 8 by 7 3/4 by 15  3/4 inches).

-    They may be solid or hollow with two or three cores for such stretcher blocks; various other types of standard shapes are also often available          and one should consult the local market to determine availability.

-    Block ends may be flat or flanged.

-    Compressive strength and fire resistance of the each block is
    dependent upon the block's configuration.

-    Lightweight aggregates were introduced around 1917 and cinder
    blocks were patented.

-    Advantages of using cinder blocks included its strength,
    ability to receive nails and ease of installation.

-    Lightweight aggregates were either natural materials, by-
    products or manufactured.

-    Natural aggregate materials included pumice.

-    By-products aggregate materials included cinders and slag;
    Pottsco or Celocrete is one example of slag product used
    around 1930 in the manufacture of blocks; Waylite is another
    example introduced in the late 1930s.

-    Manufactured aggregate materials included expanded shale, clay
    and slate; Haydite is one example of an expanded shale product
    used in the early 1920s in the manufacture of blocks.


TYPICAL USES

Typical historical uses for concrete block include:

-    Foundation walls - typically rockfaced.

-    Basement walls.

-    Partition walls - usually plainfaced.

-    Exterior walls - usually plainfaced and then often covered with
    stucco.

-    Most concrete block was used as a back-up material or for
    cavity wall construction.

-    Coatings are often are applied to concrete block in order to
    prevent water penetration; some of these include Portland
    cement paints, latex paints, oil- and rubber-based coatings,
    epoxy coatings, alkyd paints, urethanes and silicones; a
    single type may be selected for a specific function including
    its water resistance; other factors to consider might also
    include  its resistance to ultraviolet rays, its
    breathability, its resistance to alkalis, and its coloration
    or visual appearance when applied to the block.


NATURAL OR INHERENT PROBLEMS

-    Cracking: Often due to shrinkage of the concrete or movement
    of the wall.

-    Efflorescence: Occurs when accumulations of salt are carried
    to the surface by water migrating through the masonry.

-    Staining: Staining may appear in many forms, including dirt
    build-up, metallic staining or painted graffiti.

-    Rising Damp: When ground water enters the wall from the base
    and migrates upward.


VANDALISM OR HUMAN-INDUCED PROBLEMS

-    Spalling:   May be caused by the composition of the concrete
    mixture, prolonged exposure to water which has infiltrated the
    wall, or mechanical failure.

                         END OF SECTION
 


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