Floor & Paving Stone Geotechnics
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.
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 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
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):
-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.