Travertine is often associated with the Colosseum in Rome and its history of use over thousands of years. Now, this unique stone type has made a comeback. But what is it?
Article: Jim Mann
How does Travertine differ from Limestone?
Although both are composed of calcium carbonate, travertine is different from limestone due to its mode of formation and structure. For limestone, the predominant source of calcite is from marine organisms that either die or secrete material that settles to the ocean floor. The limestone subsequently formed may be reinforced at a later date by secondary calcite provided from supersaturated waters.
Travertine owes its origins to limestone deposits that have been dissolved by warm carbon dioxide laden (slightly acidic) water. When this carbonate-saturated water resurfaces at springs, the change in pressure and temperature results in the release of the carbon dioxide causing precipitation and recrystallisation of the calcium carbonate. In most cases the precipitation settles on aquatic plants eventually encasing the vegetation within the newly formed stone producing the typical pores or spongy appearance.
The elongated cavities found in most travertine also change its physical structure and characteristics compared to limestone. Although most stone types have some form of ‘grain’ or rift which can vary the stones’ appearance to some degree, the porous structure of travertine can change the stone markedly when viewed in different orientations.
Travertine is typically light cream to tan in colour although some localised deposits have been found to have a light grey-blue colouring. The stone can be produced in a range of finishes including polished, honed, sawn, tumbled and grit blasted.
As discussed earlier, travertine is a highly anisotropic material which gives the stone a distinctly different appearance depending on which way it is cut. Cutting travertine perpendicular to the bedding accentuates the grain and is identified as vein-cut. The vein-cut finish exposes the natural elongated cavities within the stone which accentuate the texture of the stone but may also trap dirt and general grime. Filling the open cavities on one face of the slab with a cement or resin based filler makes cleaning easier by producing a consistent surface finish.
Travertine that is cut parallel to the bedding is called cross-cut. This orientation displays loose ‘flowery’ concentric blotches giving rise to the alternative name of fleuri-cut.
Travertine is easy on the eye and soft to the touch making it a popular material for use inside and out. It is predominantly used as floor tiles, veneer cladding and paving. The natural cavities usually exclude the stone from use as bench top material although when filled with a durable epoxy or polyester could be used in areas that are predominantly dry.
Travertine is rarely used for monuments although its textural characteristics make it an interesting statuary and landscaping material. The stone is quarried in large block form. Finished unit thickness varies typically from 10 – 30mm with internal tiles typically 10mm, large format tiles and paving 20 – 30mm and external cladding 30mm thick.
Both cross-cut and vein-cut are popular products and selection is usually based on the desired aesthetic. As there can be a significant difference in the strength between cross-cut and vein-cut travertine it is important to consider the static and live loads that may be imposed on the stone.
By definition, vein-cut exposes the vein and presents the material in its weakest condition although this can be offset to some degree by the use of square units or by ensuring the vein is oriented down the length of rectangular units.
The cavities in cross-cut travertine are oriented parallel to the slab which exposes fewer cavities than vein-cut. The random distribution of cavities means that occasionally cavities will lie just below the surface and can be exposed by being punched through by shoe heels or hard wheels on trolleys. In a commercial environment it should be expected that the occasional punch-through will occur and these can be repaired by use of a matching filler.
Like marble and limestone, travertine is etched by acidic substances such as wine, soft drinks and some liquid soaps. The application of an impregnating sealer will not prevent etching as they do not protect the surface of the stone. If the stone is likely to be exposed to acids the use of a honed or matt surface will make etching less conspicuous.
Testing and Specification of Travertine
The specification of any dimension stone should be based on location, design and engineering considerations specific to the intended use. That said, standard specification ASTM C1527-09 provides a guide to the selection of travertine dimension stone suitable for general building and structural purposes. The physical requirements for this specification are given in Table 1.
TRAVERTINE INTERNAL USE
TRAVERTINE EXTERNAL USE
|Bulk Specific Gravity – min (kg.m-3)||2305||2305|
|Water Absorption – max (% by weight)||2.5||2.5|
|Flexural Strength – min (MPa)||4.8||6.9|
|Modulus of Rupture – min (MPa)||4.8||6.0|
|Compressive Strength – min (MPa)C||34.5||52|
|Abrasion Resistance – min (Ha)||10||10|
The physical specification requirement is primarily intended for evaluation of raw (unfilled) travertine although it provides valid criteria for evaluation of the many filled travertine products on the market. It is important to note that the flexural strength requirement in the specification is based on the testing of vein-cut travertine with the loaded applied across the grain which is the weakest orientation.
The ASTM specification is a useful guide for performance of travertine as flooring as the higher the density and lower the water absorption, the less likely the stone is to suffer from punch-throughs. Water absorption and flexural strength are the key performance indicators for this stone and should be evaluated closely throughout the project supply phase to ensure adequate performance in service.