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

Useful Diagrams

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 be noticed;

i. For what reason has the mortar, concrete or cement been applied?

structural - for the large scale construction of a cast foundation, wall or vault.
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 uses.

ii. Is there a single application or have a series of layers been built up?

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:

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).

iii. Aggregate description. Wherever possible, the following characters of the aggregate should be described.

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 hand specimen.
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 sorted.
aggregate morphology. Is the aggregate rounded or angular? spherical or elongate?
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 be made.

**WARNING: Some materials are believed to contain horsehair infected with anthrax. Seek advice before removing materials from Victorian era walls!**

Ideally, this set of criteria should be recorded for each plaster, mortar or concrete used in a single edifice or single construction phase.

Useful Diagrams

Description 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 aralditeô 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 for analysis.

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 sources.
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. These are:

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 matrix).

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