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?

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:
 
 

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


**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.

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Useful Diagrams


        
 
 

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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 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 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:
 


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:


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