Geoscience Reference
In-Depth Information
Concrete
5
The principal applications of petrographic
examination to concrete investigations are:
• Identifying the mix ingredients including coarse and
fine aggregate, cement type, fillers, mineral
additions, and certain additives.
• Determining the mix proportions and air void content.
• Assessing the quality of workmanship.
• Screening for evidence of distress or deterioration.
• Diagnosing the causes, severity, and extent of
defects and deterioration.
Concrete is a mixture of aggregate, water, and a binder
(today usually Portland cement-based). The binder is
chemically activated by the water to form a paste, which
sets the inert aggregate into a hardened stone-like mass.
Today, concrete is the most commonly used construction
material with present global consumption estimated at
11 billion tonnes annually (Metha & Monteiro, 2006). It
owes its popularity to being relatively cheap and readily
available, that it can be poured into a variety of element
shapes and sizes, and that it is water resistant. Concrete
elements are manufactured either '
in situ
' using
prefabricated formwork or are 'precast' in moulds.
The use of concrete-like materials dates back
thousands of years, with early concrete binders including
nonhydraulic lime, hydraulic lime, and lime mixed with
pozzolana. However, the invention of the binder most
commonly used today is credited to John Aspin, who in
1824 patented a cement produced by firing a mix of
ground limestone and clay that would set under water.
Aspin considered his cement to resemble the Jurassic
Portland stone of Dorset (England) and called his product
'Portland cement'. This is the predominant cement variety
in use today.
Plain concrete is inherently strong in compression but
relatively weak in tension. For this reason, steel
reinforcement bars are often cast into the concrete to
improve the tensile strength (reinforced concrete). With
prestressed concrete, by tensioning steel tendons, a
precompression is introduced to counteract tensile
stresses and prevent cracking. Modern concretes also
contain a number of chemical admixtures and/or mineral
additions to impart desirable properties to the concrete,
either in the fresh state or post-hardening. In Europe,
concrete is specified in accordance with EN 206-1 (British
Standards Institution, 2000b) and in America it is
specified to ASTM C94/C94M (ASTM International,
2009).
STRUCTURES
Concrete structures require assessment for a variety of
reasons, which will usually be associated with
investigation of either specification compliance,
maintenance requirements, or structural adequacy. It is
essential to establish the precise aims of inspection and
testing before planning the assessment programme. The
typical elements of an assessment programme include
documentation review, preliminary site visit, access and
safety provision, selection of test methods/numbers/
locations, visual inspection, testing, interpretation, and
documentation of findings. Useful information regarding
assessment and testing procedures is provided by Kay
(1992), Concrete Society Technical Report No. 54
(Concrete Society, 2000) and Bungey
et al
. (2006).
Petrographic examination has an important role in the
assessment of concrete structures and the petrographer
should be involved in planning investigations. The
petrographer should be fully aware of the construction
details, environmental conditions, and history of the
structure under investigation. Ideally, they would have
the opportunity to visit the site and be involved in the
visual inspection and selection of sample locations.
Failing this, all relevant documentation should be made
available including drawings, photographs, inspection
records, and test data.
