Introduction
Floors and paving are presently the major application of stone materials being marketed constituting over 35%. In the last few decades igneous rocks, like the granite varieties and quartzitic stones have been considered, for quality and cost saving, maintenance and "durability", the most expedient materials for the purpose. The extensive colour ranges monopolized by the non-igneous rocks have now been supplemented by globalization of imports from new sources.
The traditionally easier workable sedimentary carbonates, often of qualities superior to the igneous and other quartz-based varieties, have by non-judicious use caused legislative problems. Casual selection and grading especially of stone materials emanating from unproven global sources have resulted in much litigation. These phenomena are gradually being corrected by the intensive attention to the geotechnics and expressed in the ongoing stone standardization by CEN. Approved certification confirm statutory reliability but awareness of the suppliers responsibility is advised when drawing up the specifications and supply contracts.
These rather recent developments in flooring and paving require a new look at a rather staid sector of the stone trade, trailing behind advances in the other sectors.
Flooring vs. Cladding
Although both stone applications except those with loadbearing functions are considered coverings, the stone selection is more difficult for flooring than stone for walls. Vertical forces acting on cladding are mainly static whereas the forces to be reckoned with on horizontal or near-horizontal surface like floors are dynamic and require at times different testing procedures.
Abrasion, usually negligible in vertical surfaces, is a factor to consider in stone selection for flooring and paving, and has to be balanced with slip protection.
Flooring
Characteristics required for stone used in flooring include resistance to stress (concentrated & distributed loads) abrasion, shock, chemical agents. More than in any stone application, flooring is depended on the sublayer it rests on. The elasticity of the flooring must be compatible with the structural elements e.g. a concrete sub-base; it is additionally influenced by deflection on the structural elements due to applied loads, thermal insulation, sound proofing and waterproofing requirements especially where heating is installed beneath the flooring. Tiles used for flooring can be pre-polished or to be polished in situ after laying cut-to-size or in standardized sizes cut to a pre-set architectural pattern of a specific design including motifs, inlays, inserts etc. The designer has to decide at an early stage on the spacing of the jointing, especially in exposed areas. He has to decide whether to use calibrated tiles implying a product submitted to specific mechanical finishing to obtain more precise dimensions; they are suitable to be fixed by thin mortar bed or adhesives
Stylolites are perhaps the major inherent properties causing default in calcareous stones including some major marble varieties. The origin, characteristics, classifications have recently been described in Litos 72 (May 2004).
Paving
Paving implies exterior use, at times in areas partially enclosed by walling. In contrast, the term flooring is generally used to denote its use in covered areas enclosed by walls. The surface treatment for paving is similar to that of flooring. However a large percentage of paving tiles or slabs are used with a natural riven finish i.e.slabby stone split along cleavage planes.
Traditionally natural faced slabs were used for paving, formalized by the Romans, with the sides trimmed to obtain various geometrical patterns. In modern practice this has changed by sawing with diamond discs enabling rectangular patterns, easier for maintenance, and predominant in architectural use. Another innovation has been the use of open joints to be lead, along sloping subsurface, to concealed drain holes, typically for plazas, promenades, terraces and flat roofs.
Technical requirements are similar for pavers and flooring stone i.e. low water absorption, high compressive strength and resistance to weathering. Flexural strength is particularly important for pavers that take the eccentric loads particularly of heavy traffic. Surface design has to consider the nature and intensity of the expected traffic, human and mechanical, ranging from baby carriages to vehicular invalid wheelchairs to delivery transportation.
Important progress has been achieved with the new EU standards for external paving and pertaining to slabs, kerbs and setts, for the first time harmonized in Europe and giving a clear definition of terms.
In EN 1341 three slab types are defined:
Slab: any unit of natural stone used as a paving material, in which the working width exceeds 150 mm and generally exceeds two times the thickness.
Riven slab: slab with split face.
Textured slab: slab with a modified appearance resulting from one or several surface treatments (for example mechanical or thermal).
In EN 1342:
Sett: small natural stone paving block with work dimensions between 50mm and 300 mm and no plan dimension generally exceeding twice the thickness. The minimum nominal thickness is 50 mm
Textured sett: sett with a modified appearance resulting from one or several mechanical or thermal surface treatments
In EN 1343:
Kerb: unit greater than 300mm in length, commonly used as edging to a road or footpath
Concave kerb: kerb, curved in plan with a concave face
Convex kerb: kerb, curved in plan with a convex face
Textured kerb: kerb with a modified appearance resulting from one or several mechanical or thermal surface treatments
Surface finish
Besides technical properties, the surface appearance of the tiles and the slabs is a decisive factor in selection.
Surface finishes (EN 12670) are to extend uniformly to the edges of the modular tiles and may typically involve the use of patching, fillers or other similar products for natural holes, faults or cracks as part of the normal processing. The type of treatment and the nature of additional materials are to be pointed out by the supplier.
To ensure the finish required the following descriptions be to be specified. The F stands for Federation of European Producers of Abrasive Products and the number indicates the grain size of the abrasives to be used.
Surfaces obtained by grind can be:
-rough ground, e.g. by means of size F 60;
-medium ground surfaces, size F 120;
-fine ground surfaces, size F 220;
-matt or honed finished surfaces by a polishing/-grinding disk, grain size F 400. The polishing disk or felt is used for-highly polished surfaces.
Surfaces obtained by means of hammer type tools are, for example:
-bush hammered surfaces* (see EN 12670);
-trimmed surfaces: finish obtained by using pointed chisel and mallet or a grooving machine;
-striated surfaces: finish obtained by using a claw chisel (percussion tool for roughening a surface, with the cutting end covered by several teeth of various size) or a ruling machine.
Other surface finishing operations include (see EN 12670):
-flamed finish;
-sand blasted finish;
-water jet streamed finish: a matt textured surface finish, accomplished by exposing the surface to a steady jet of water under pressure;
-machine tooled finish);
-riven cut finish: rugged surface produced by splitting stone with a guillotine or chisel.
The selection of the stone requires the following geotechnical considerations, ranked below in order of resistance to abrasion vs. slip control.
Quartzites, often called flagstones, together with quartz sandstones have good abrasion resistance if carefully chosen; the quartz sandstones need a closer attention, as they are dependent on the cementing matrix. Besides uniform and pleasing colours, an important merit is their easy splitting characteristics saving considerably quarrying costs.
Granites, containing several minerals of different hardness may assume differential wear in use and depend on the grain size and mutual contact.
Careful consideration enables the selection of a surface texture likely to reach equilibrium during wear between the harder and softer mineral components. A useful guide is quartz and mica contents as noted in some of the UN published stone monograph mentioned under references at the end. For instance in Brazilian granites (p.63), or in the Nordic stones a minimum quartz content of 25% and mica up to 10% are reasonable averages, depending on the hardness of the feldspars and other minerals present. The marbles and limestone may at times have more abrasion resistance than the granite and quartzose varieties, but have the disadvantage, especially in exterior paving, to abrade smoothly with wear and to loose gradually slip resistance.
Slates depend on their hardness that varies greatly within the variety, and require more complex testing than the aforementioned varieties.
Although travertine is used mainly for interiors, their application in streetscapes is increasing because of lower cost and architectural preferences. The coarseness of the texture is by nature anti-slip. One of the earliest uses in recent times busy is a sidewalk in 45th Avenue in New York. The travertine stands up very well not to speak of the well-known examples of floorings dating to antiquity.
Finally when considering the rock types for hot climates the exterior use of black or very dark varieties are to be avoided, unless safeguard against expansion is taken and barefooted traffic is not expected. The opposite applies to cold climates.
For playing safe stone is to be judged by its performance in existing applications, especially for exterior use. Varieties used for flooring or paving internationally include Carrara marble, Trani Chiaro, Botticino, Tennessee Limestone, Scottish flagstones, Norwegian Quartzite and Indian Kota stone to name a few. At one time, much flooring and especially paving came from slabby deposits (ref).
Arguably, Kota stone is one of the most intensively worked slabby deposits in the world, with layers of uniform thicknesses and easy cleavability. The quarry faces stretch along many kilometres and thousands of people are employed daily in the workings. Kota stone has superior characteristics both for quarrying and application. Professional geotechnical promotion could turn the product into a world class generic trademark comparable to flagstones and quartzites in other parts of the world.
Cleavable schists and flagstones are intensively quarried in Northern Norway and after proving their quality locally for centuries have now been a hot export item for some time. A flagstone is generally a sedimentary rock easily separated into flat slabs due to stratification in the deposit and make the textured flags a special product.
Grinding
For flat surface finish, grinding is used universally, from roughening to the finest honing. Roughening of the surface from a technical, safety and economical point of view has to be sustainable for a longer period allowing lower cost of conservation procedures. To generate brake on slip mostly a F120 powder size is used. Shape, hardness of the grinding material, resistance of stone etc are not taken under consideration. The higher the F number the more flattening of the surface is obtained and the weaker the slip resistance.
High polish slip proofing
A high polish surface finish is still favourite in architectural applications notwithstanding the slip factors involved. There is presently no agreement for a common regulation or standard method of measuring different aspects of anti-slippery surface, especially polished natural stone floors. In many cases insurance (in case of accidents) and regulations dealing with the floor use control norms. There is no guaranteed protection from slip in public applications. Local slip protection regulations have reduced the number of accidents in Europe, but uncertainty exists. Therefore, a niche is emerging for services specializing in anti-slippery finish especially on natural stone floors.
Interior high polished stone floors should be characterized by anti-slippery surface, easy maintenance and durability. Easy maintenance exists but in many cases does not give protection from slip. On the other hand, floors with a rough surface and protection from slip usually are tough to clean. Where high polish is not required safe level of slip protection are achievable by proper roughness.
In exterior applications the ideal natural stone floor surface should have an anti-slippery finish and be characterized by durability, easy maintenance and low pollution adhesion. Roughness of the surface determines the extent of maintenance costs, and can be achieved by grinding, chemical processing, laser processing, plasma processing and with traditional dressing, hammering, thermal treatment etc. However, the traditional methods are presently being challenged by new slip proofing developments to reduce soiling proneness.
Slip Prevention by Laser
Cooperation of industrial interests with academe in Germany during the late 90's succeeded in devising an anti slip treatment without loosing polish or damaging rock structure. Free off acid based chemical treatment, laser structurization is especially suited for polished and precisely ground (F 320) natural stones. With the use of a laser stream, microholes are created on the surface of the stone. Microholes have dimension of about 200 micrometers and depth varying between 30-100 micrometers. Myriad's of such microholes give a significant increase of slip brake level, with only 20% of surface material being evaporated by the heat.
Certification of such a floor is easy to obtain because the laser treatment is being done before installing the floor on slab sizes up to 92 cm and with length of 240 cm.
The application of the diode laser is characterized by steadiness, claims reproducibility of result and the colour impact of a polished natural stone surface with the laser is almost unchanged. Maximum loss of glare from the polished surface is put at 20% but usually it stays below 10% mark. On a polished or well-honed surface, a dulling effect will not take place.
The surface can only undergo roughening for example due to gathering of sand in the microholes or other mechanical reasons, which are not possible to remove because microholes have very smooth edges and dirt particles hardly ever stay in such microhole. Abrasion takes place only on the remaining part of polished surface (about 80%) so at the same time it is considerably slower compared to other rough surfaces without the microholes.
Efflorescence
Excess moisture is always existent when a floor is constructed. Also the stone material may contain excess moisture and is transmitted by the laid floor via the porous surface of the tiles and slabs and joints. This process continues for some time until the floor is 'dry' i.e. until equilibrium is reached between the 'free' moisture in the floor and the air moisture content.
The floor sub-base may contain soluble salts and lime, or in some cases insoluble salts, which solve or are drawn to the floor surface as the case may be, to form a dullish or powdery looking deposit. This is called efflorescence.
Moisture movement through the floor is usually largest during the first month of the laying. With time, possibly years the amount of moisture diminishes gradually but does not stop completely for various reasons. E.g., the relative moisture content of the air is not constant; the moisture drawn into the floor from the sub-base; by rain; or water used during maintenance.
The deposition may also occur just below the stone surface in pores, veins or stylolites. During the consolidation the volume of the salts (ferrous etc) may increase and in the worst case may cause some blistering. These problems can be avoided by proper stone selection and by reducing the possibility of moisture movement. In exterior use, peeling may occur caused by increase of volume in or below the tile by frost.
Discoloration may occur in the long run, typically along the tile joints and is known as 'framing'. As a rule, a moist floor assumes a soiled look faster than a dry floor space. A light coloured stone becomes soiled faster than darker varieties. Risk may be avoided by the thermal shock resistance test (prEN14066).
Whereas in stone with large pores the salts are likely to wash out, in a fine-pored stone crystallization known as cryptoflorescence can cause mechanical damage to the stone structure.
Legislative
Considering the importance of responsibility on quality and avoidance of claims, attention is to be to that:
- The stone supplier/ manufacturer is expected to draw attention to the natural variations of the stone materials; deviations from the declared values may occur.and they shall be declared by the manufacturer.
- Whenever stone processing is likely to change the characteristics of the initial material (e.g. a consequence of the type of processing or because the use of patching, fillers or other similar products for natural holes, faults, cracks and similar) to be taken into account.
The following characteristics shall be declared where requested by this standard or with reference to the intended use conditions:
- The denomination shall always be declared (meaning traditional name, petrological family, typical colour and place of origin) and the petrographic name.
- The colour, veining, texture, etc.of the stone shall be identified visually, typically by a reference sample of the same stone suitable for providing a general description of visual appearance.
Reference Sample
To safeguard the supplier and the client, the reference sample, provided by the supplier shall refer to the above requirements be an adequate number of pieces of natural stone of sufficient size to indicate the general appearance of the finished work. The dimensions of individual pieces shall be at least 0,01 square metres (typical values are between 0,01 and 0,25 square metres in face area but may be more), and shall indicate the range of appearance regarding the colouring, the vein pattern, the physical structure and the surface finish. In particular the reference sample shall show specific characteristics of the stone, such as holes for travertine, wormholes for marble, glass seams, crystalline veins and rusty spots.
The reference sample does not imply strict uniformity between the sample itself and the actual supply; natural variations may always occur.
If the processing of the stone involves the use of patching, fillers or other similar products for natural holes, faults or cracks, then the reference sample shall similarly display the impact of the same on the finished surface.
All the characteristics as shown by the reference sample shall be considered typical of the stone and not as flaws, therefore they shall not become a reason for rejection, unless their concentration becomes excessive and the typical character of the stone is lost.
The name and address of the producer or the supplier, as well as the denomination of the stone, shall be indicated on the reference sample.
Contractual specifications may be used to establish reference values, for example stated in a design or supplier data sheet.
Additional reading:
Manual de Rocas Ornamentales (1995), Entorno Grafico, S.L., 28034, Madrid
SHADMON, A. (1976): "Posibilidades de explotación de la piedra de cantera", United Nations, N.Y., Sales no. S76, II A4.
SHADMON, A. (1996):"Stone-An Introduction", 2nd edition, IT Publications, London, U.K.
FRATTARI, A., STENICO, F. (2001): "Porphyry-Architecture and Technique", Faenza Editrice, 48018, Faenza RA
