The field and laboratory description of concretes
Lime cement-based products in the form of mortars, concretes and
plasters, being synthetic rocks, may be easily described in language borrowed
from geologists and ceramic petrologists. Descriptive methods for ceramic
petrography is particularly useful and is discussed in Whitbread (1986),
Whitbread (1989) and Freestone (1995). For description of sedimentary rocks
in thin section,see Adams et al. (1991). Materials may be examined in situ
on the building they were used to construct, or as a hand specimen. In
addition, thin sections of the material can be made for examination under
the petrological microscope. If possible both hand and microscopic descriptions
should be made, but in cases where destruction of buildings to obtain samples
is unfeasible, then descriptions may be made without removing material.
Description of materials in the field
Description of materials using a petrological microscope
Description of materials in the field
Whether a sample is to be removed for microscopic examination or
not, it is imperative that all material should be described in situ on
the building of origin. These contexts of a material are important because
they will have affected the original manufacturing processes and the selection
of aggregates and/or pozzolanas. Frequently a number of different materials
will be used in the construction of a single building, depending on their
appropriateness. Phases of occupation of buildings may be recognised by
rebuilding or retouching using different compositions of concretes, mortars
and plasters from the originals. When constructing a log of materials used
in a single building the following guidelines and points to recognise should
i. For what reason has the mortar, concrete or cement been applied?
ii. Is there a single application or have a series of layers been built
structural - for the large scale construction of a cast foundation, wall
binding medium - for cementing together bricks or masonry, or securing
facings (i.e. decorative stonework) to walls.
flooring - either as a material to give a smooth floor, or the binding
medium for tiles or mosaics.
waterproofing - for the lining of cisterns, ornamental pools, aqueducts,
baths, etc. In these circumstances the material is generally an hydraulic
cement, particulary in Roman or later constructions.
decorative - for mouldings or as a base for paintwork or frescoes.
miscellaneous - for constructing vessels, making pipes and other non-typical
This is most important for material covering walls and floors,
where a succession of layers have been applied to produce a surface of
particular quality or for particular aesthetic use. In such cases the following
aspects should be noted:
iii. Aggregate description. Wherever possible, the following characters
of the aggregate should be described.
thickness of the various layers. Are they uniform or is there a variation?
If the latter, do they become thicker or thinner in one direction, or are
the layers of random thickness?
were layers applied in quick succession or was each layer left to set before
the next was applied? If a layer is applied to an underlying layer that
was still wet, some mixing of the layers may be seen at the boundary (this
is more evident under the microscope). If a layer was applied to an already
cured layer, there may be pecking of the underlying plaster or mortar
to form a rough key to aid adherence of the next coat.
aggregate variation. Is the aggregate graded in size through the layers?
Are specific aggregates used for aesthetic or waterproofing purposes?
special preparations. Has the outer surface been treated in any way? For
example burnishing (rubbing the surface with a stone to produce a glassy
surface layer was used as a means of waterproofing in pre-Roman times).
average size grade of the aggregate. This is essential for classifying
a material as a lime plaster, mortar or concrete. Lime plasters should
have an aggregate grain size less than 1 mm; mortars have aggregate of
1-5 mm size; concretes have aggregate greater than 5 mm.
occurrence of fine aggregate. If the dominant aggregate is mortar or concrete
grade, is there an additional fine aggregate? This may not be visible in
occurrence of pozzolanic additives. Is the plaster discoloured? A pink
hue may denote the presence of crushed ceramic materials. Addition of volcanic
ashes may produce a variety of colours. Microscopic and other analytical
techniques may be required to recognise these components.
aggregate abundance. What (estimated) percentage of the bulk sample does
the aggregate make up?
aggregate sorting. Is the aggregate all of a similar size range or is there
a wide variety in particle size? A sample may be described as well sorted if all aggregate particles are of a similar size, and conversely if there
is a wide variation in particle size, the sample may be described as poorly
aggregate morphology. Is the aggregate rounded or angular? spherical or
aggregate composition. Are distinct materials identifiable? If so are they
synthetic (usually ceramics), organic (plant fragments or natural fibres
such as hair. Such materials may not be preserved in tact but may leave
recognisable moulds) or geological (lithic fragments). The nature of
lithic fragments may be distinguished if they have characteristic features
and the fieldworker has some geological knowledge. If there is a variety
of material used, estimates of the percentage abundances of each should
**WARNING: Some materials are believed to contain horsehair infected
with anthrax. Seek advice before removing materials from Victorian era
Ideally, this set of criteria should be recorded for each plaster,
mortar or concrete used in a single edifice or single construction phase.
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of materials using a petrological microscope
If a sample is to be removed from a structure and subjected to analysis
under a petrological microscope, a sample of sufficient size needs to be
removed. Ideally, a piece of dimensions c. 5 x 5 x 5 cm is required. This
will of course depend on the thickness of the layer of material being sampled.
The material sampled should also be selected as a representative of the
overall construction if possible. Removal of samples and preparation of
thin sections is destructive, and the original sample cannot be replaced.
The preparation of thin sections of plaster, mortar and concrete
is a skilled job and should only be attempted by those trained in sectioning
rocks. A slab of the material of dimensions c. 1.5 x 3 x 2 cm is cut and
this is glued to a glass microscope slide using araldite˘ or a similar
adhesive. Then sample is then impregnated with resin to consolidate it
before grinding it down to the standard thickness of 30 µm. At this
stage the sample may be either polished or covered with a glass cover-slip
The petrological microscope is an optical microscope equipped
to operate using polarised transmitted light. In this way an analyser
can be inserted to view the sample under crossed polars. This enables
aspects of the mineralogy to be identified which are not visible in transmitted
or plane polarised light. Also essential is a flat stage that can be rotated
through 360°. Magnifications between x5 and x100 are standard. Identification
of cement matrices and the composition of aggregate clasts in thin sections
requires an amount of experience and training and should only be attempted
by those qualified to do so.
Obviously, examination using a microscope will considerably refine
observation made by the naked-eye or with a hand lens. The following aspects
should be noted:
i. Overall textures. The term texture is routinely used in petrology
(rock description) to describe the relationships between the components
of a rock to describe that rock as a whole. For plasters, mortars and concretes,
generally this involves honing of information gleaned from field observations,
namely aggregate sorting, percentage abundance, morphology and grain size.
ii. Matrix cements. Much of the crystalline features of cements are
not visible using an optical microscope. However, the cement matrix, binding
the aggregate together, should be described in terms of:
colour. The colour of the cement in plane polarised light should be recorded,
along with variations in colour density and hue, and the nature of these
variations. For example the material may appear mottled or speckled.
interference colour. This is a feature observed under cross polarised light
(see for example, Gribble and Hall, 1992). A non-hydraulic lime cement
should show interference colours typical of micritic calcite. These are
equivalent to fourth-order pinks and greens on Newton's Scale of colours.
If pozzolanic materials are present, then the cement will become more glass-like
and is therefore isotropic. Under cross-polarised light, this will appear
black. If the cement is partially hydraulic, then a speckled or mottled
texture may be observed.
reaction rims between aggregate clasts and the cement. Some aggregate clasts
may bond very well with the cement and react with it. This is sometimes
observed as a rim around the clasts with may blur the outline of the clast,
show crystal growth at the junction or produce a localised hydraulic set.
Thickness of reaction rims, and the clast-types they are associated with
should be noted.
non-calcined limestone. This material, also called schwachbrand (lightly-burnt
in German) occurs where calcination has not gone to completion and lumps
of the original limestone remain. These may show relict textures of the
parent rock and can be an enormous aid in identifying limestone quarry
cracks, fractures and voids. The presence and morphology of these features
should be noted and described.
iii. Aggregate composition. Lithic (rock fragments) ceramic and
organic fragments should be identified. The field of descriptive micro-petrology
is vast and well beyond the scope of this paper. A geologist should be
employed to identify rock and mineral fragments used as aggregates. Such
information can give clues as to the source of the aggregate used especially
if coupled with geological field observations. Locally derived material
may be distinguished from imported materials used to produce special properties,
i.e. pozzolanic additives or materials to impart colours. Mineral phases
may be identified with references to texts such as Gribble and Hall (1992)
and Deer et al. (1992).
iv. Aggregate morphology. Apart from sorting and roundness, more specific
properties describing aggregate regardless of mineralogy may be applied.
the nature of the boundaries of the grains. Are they sharp or diffuse?
the shape of the grains.
the optical density of the grains (i.e. how well they stand out from the
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