Whatever the causes of deterioration, careful analysis, supplemented
by testing, is vital to the success of any historic concrete repair
project. Undertaken by experienced engineers or architects, the
basic steps in a program of testing and analysis are document
review, field survey, testing, and analysis.
Document Review. While plans and specifications for older concrete
buildings are rarely extant, they can be an invaluable aid, and
every attempt should be made to find them. They may provide information
on the intended composition of the concrete mix, or on the type
and location of reinforcing bars. Old photographs, records of
previous repairs, documents for buildings of the same basic construction
or age, and news reports may also document original construction
or changes over time.
 It is important to match the visual qualities, such as color and texture, when repairs or replacement sections are undertaken. Photo: NPS files. |
Field Survey. A thorough visual examination can assist in locating
and recording the type, extent, and severity of stress, deterioration,
and damage.
Testing. Two types of testing, onsite and laboratory, can supplement
the field condition survey as necessary. Onsite, nondestructive
testing may include use of a calibrated metal detector or sonic
tests to locate the position, depth, and direction of reinforcing
bars. Voids can frequently be detected by "sounding"
with a metal hammer. Chains about 30 inches long attached to a
2-foot-long crossbar, dragged over the slabs while listening for
hollow reverberations, can locate areas of slabs that have delaminated.
In order to find areas of walls that allow moisture to penetrate
to the building interior, areas may be tested from the outside
by spraying water at the walls and then inspecting the interior
for water. If leaks are not readily apparent, sophisticated equipment
is available to measure the water permeability of concrete walls.
If more detailed examinations are required, nondestructive instruments
are available that can assist in determining the presence of voids
or internal cracks, the location and size of rebars, and the strength
of the concrete. Laboratory testing can be invaluable in determining
the composition and characteristics of historic concrete and in
formulating a compatible design mix for repair materials. These tests, however, are expensive. A well-equipped
concrete laboratory can analyze concrete samples for strength,
alkalinity, carbonation, porosity, alkali-aggregate reaction, presence
of chlorides, and past compostion.
Analysis. Analysis is probably the most important step in the
process of evaluation. As survey and test results are revised
in conjunction with available documentation, the analysis should
focus on determining the nature and causes of the concrete problems,
on assessing both the short-term and long-term effects of the deterioration,
and on formulating proper remedial measures.
"Preservation of Historic Concrete" an Historic Preservation Brief
