Are you selecting either natural or engineered stone for use in a commercial lobby floor? Make sure you’ve considered these important criteria.
Article: Jim Mann
Commercial lobbies are not just for passing through; they set the tone and ambience of a building. They prepare a visitor to expect a Soviet style mausoleum a Disney palace or something in between. From a functional perspective, the materials used must be able to withstand the daily impact of possibly thousands of tenants and visitors entering, exiting, eating, drinking and socialising not to forget the impact of couriers, cleaners, maintenance staff and their equipment. The floor finish is subject to the impact of thousands of feet as well as the grime and surface contaminants they bring.
As part of the challenge in designing these lobbies, architects are presented with an ever-widening range of natural and engineered products for use as floor finishes. Selection may primarily be based on aesthetics but determining fitness for purpose of the product is vitally important to maintain the desired appearance. Your selection process (whether it be a natural or engineered stone) should include a review of the following performance criteria:
– surface finish (slip and wear resistance)
– strength and load-carrying capacity
– stain resistance
Slip resistance would have to be the hottest topic discussed during the selection of floor finishes and in many projects is the determining factor for the type and finish of the material to be used. Despite the high level of importance given to slip resistance, the guidelines for selection of a ‘slip resistant finish’ could be currently considered arcane and ambiguous.
With the introduction of a new standard in Australia the slip resistance of new pedestrian surfaces is now determined according to Australian standard AS 4586:2013 Slip resistance classification of new pedestrian surface materials. The standardbrings with it some significant changes including a new classification system for the wet pendulum method. While the previous version of the test method provided a five step classification system ranging from V (representing a very low contribution to risk of slipping when wet) to Z (representing a very high contribution to risk of slipping when wet), the new method provides a six step classification system ranging from P0 to P5 to represent the contribution to risk of slipping.
The new classification system splits the old ‘Z’ classification into two classes (P0 and P1) which provides more sensitivity in the classification of less slip-resistant surfaces. Another major change in the test is the method of preparing the rubber slider used to perform the test. The slider preparation method now includes a ‘polishing stage’ which produces a smoother finish on the test surface of the slider and a subsequent lower slip resistance value evident on some smoother finishes. The new ‘polishing stage’ could be considered analogous to the effect experienced by a shoe sole as a person walks across a polished granite floor where it becomes smoother and potentially more slippery as you continue to walk on the polished surface.
Currently within Australia, the adequacy of the slip resistance of a pedestrian surface is evaluated according to the recommendations of Standards Australia handbook HB197:1999. Although a new classification system is now in effect, an updated version of the guide is still to be published. To provide some assistance in evaluation of test results in the absence of an updated guide it is necessary to use the 1999 version by translating the ‘P rating’ back into the old ‘V to Z’ classification system.
Table 3 of HB197:1999 provides a list of locations where commercial lobbies could be considered as “entry foyers hotels, office, public buildings”. The table provides different classifications for dry and wet locations within the foyers. Although the handbook does not provide a definition for dry or wet foyers, a wet foyer could be defined as the zone that may be expected to become wet due to tracking of water through external entry doors or other similar locations. The 1999 version of the handbook recommends a minimum X classification for wet foyer zones which may be considered to be similar to (but not the same as) the new P3 classification. Conversely a dry foyer could be considered as any zone within the lobby that would be expected to be dry during its normal use. The handbook recommends a minimum Z classification for dry foyer zones which is now encompassed by both P0 and P1 classifications. In this instance, it should be confirmed that a floor requiring a P0 or P1 classification has adequate slip resistance in a dry condition.
There is usually no clear delineation of the wet zone within a foyer and therefore the extent of the zone should be determined following a risk assessment. The use of water trapping devices such as mat wells, revolving doors and airlocks may significantly reduce the risk of tracking of water although some risk may still be present from dripping umbrellas and apparel.
The updated version of HB197 may include a recommendation for ‘transitional areas’ within lobbies (possibly P2) which would assist with evaluating the ambiguous transition from wet to dry floors.
Smoother surface finishes that meet the recommended slip resistance classification for wet foyers are limited, with some honed finishes and micro-etched polished surfaces satisfying the slip resistance recommendations. A perceived higher risk of slipping within commercial lobbies has led to the introduction of more heavily textured finishes such as exfoliated (flamed), brushed and sandblasted. These coarser finishes bring with them different challenges especially related to cleaning and maintenance.
While a surface finish may achieve the required P3 classification at the time of installation the maintenance of this classification throughout its life cannot be determined from the current test method. The use of more highly slip resistant finishes such as exfoliated granite (which usually achieves a P4 or P5 classification) allow for a reduction in slip resistance over time due to wear due primarily to foot traffic.
The aim of maintaining adequate slip resistance throughout the service life has led to the development of a test that measures the slip resistance of the surface after accelerated wear. Slip resistance is usually measured at intervals of 100, 500, 1000 and 5000 cycles of wear. Each installation has unique conditions including environmental conditions and foot traffic volume, preventing a reliable conversion of the number of test cycles to years of service.
Evaluation of the change in slip resistance values at each stage gives a useful insight into the stability of the slip resistance of the surface over time as well as providing guidance on the likely ‘worst-case’ scenario. As an example, testing carried out by Stone Initiatives to method SI-SAW-13 found that the slip resistance of an exfoliated granite dropped from a P5 classification to P4 after 500 cycles and the surface maintained this rating to 5000 cycles. In comparison, the slip resistance of a sample of different exfoliated granite had an initial slip resistance classification of P5 which reduced to P3 after only 100 cycles and was further reduced to a P2 classification at the end of 5000 cycles.
A change in slip resistance may be considered a significant consequence of wear with some finishes but change in appearance also needs to be taken into account. When used in high traffic and bottle-neck areas such as doorways and turnstiles, the combination of dark-coloured materials with a highly polished finish can lead to the development of tracking marks as the level of gloss is reduced with wear. Uniformly dark-coloured marble and high-density limestone is particularly sensitive to this phenomenon due to their lower resistance to abrasion.
Strength / load-carrying capacity
Nothing detracts more from the appearance of a lobby than the sight of broken tiles. Ensuring the load carrying capacity of the tile is adequate is an important first step towards reducing the risk of tile failure.
In a commercial lobby the tiles are subjected to regular live loads from pedestrians, trolleys and cleaning equipment and the occasional exposure to scissor lifts during maintenance. These loads usually present a very small portion of the load carrying capacity of a tile under compression although these loads are significant in flexure. In this situation the load carrying capacity of the tile under flexural load should be considered as a critical design criteria.
There is scant guidance within standards for determining if the load carrying capacity of a tile proposed for a particular location is adequate. British Standard BS 5285-5:2009 provides some specific information on the design and installation of natural stone wall and floor tiling. Section 5.3 of this standard advises that at the planning stage all stone types should be considered individually for their merits in use as flooring. After providing this prudent advice the standard goes on to state that in a commercial setting, where floors exposed to occasional over run of light maintenance vehicles, slabs not exceeding 600 mm x 600 mm with a minimum thickness of 20 mm should be used. Given the variable strength of natural stone this specific advice does appear to contradict the earlier advice to evaluate each stone on its merits.
In evaluating a specific tile at the design stage, it is necessary to evaluate both its strength and load carrying capacity and as a first step it is important to understand the difference between these two properties.
Strength is the alibility of a material to withstand a force (load) per unit of cross sectional area. Strength of a material (e.g. compressive or flexural) is usually expressed in megapascals (MPa) which is defined as force of 1 Newton (0.1 kg) applied over an area of 1 mm2. By determining the strength of a material we can determine the approximate load carrying capacity of any object. As an example, if an object has a compressive strength of 50 MPa and a cross-sectional area of 100 mm2 then it can withstand a load of approximately 5000 Newtons (i.e. 50 MPa x 100 mm2) equivalent to approximately 500 kg.
Evaluation of the flexural load carrying capacity of a floor tile is usually determined under three-point load where the tile is supported across a span and loaded to failure at its midpoint. During the design phase of a project it is often not possible to test the tiles with the proposed project dimensions. In this instance the approximate load carrying capacity can be determined indirectly by determining the modulus of rupture (3-point bending strength) of the stone.
Table 1 shows the theoretical breaking load of a range of tiles with varying dimensions all made from a material that has a modulus of rupture of 10 MPa (typical for a granite tile).
|Tile Plan Size (mm)||Length to Width Ratio||Theoretical Breaking Load (kg)|
|15 mm thick tile||30 mm thick tile|
|800 x 400||2 : 1||82||328|
|600 x 400||1.5 : 1||111||444|
|400 x 400||1 : 1||175||1050|
Table 1: Examples of the effect of variable thickness and size on the breaking load of the tile.
The table shows that an 800 mm x 400 mm x 15 mm thick tile that is only supported at each end can withstand a mid-point load of approximately 82 kg. By increasing the thickness of the tile to 30 mm the breaking load increases dramatically to 328 kg. This example illustrates the basic relationship where doubling of the thickness provides a fourfold increase in the load carrying capacity of the tile regardless of the tile format.
The table also shows that the geometry of the tile has a significant effect on its load carrying capacity. A reduction in length to width ratio (e.g. from 2:1 to 1:1) significantly increases the load carrying capacity of the tile. As an example, the table shows that a 30mm thick tile with a length to width ratio of 1:1 can carry 3.2 times the load of a tile of the same thickness that has a length to width ratio of 2:1. As a rule, the lower the length to width ratio the greater ability to withstand flexural loads.
In the absence of a specified minimum strength and breaking load requirement for natural stone tiles, test data could be compared with the breaking strength and breaking load requirement for terrazzo tiles which have a history of use within commercial lobbies and similar locations.
Standard BS EN 13748.1 covers terrazzo tiles for internal use and specifies a minimum mean bending strength (modulus of rupture) of not less than 5.00 MPa with no individual result less than 4.00 MPa. The standard also sets a minimum breaking load requirement where no individual result shall be less than 3.0 kN (306 kg) for tiles with a surface area greater than 1100 cm2. A review of Table 1 shows that in this case an 800 mm x 400 mm x 30 mm tile complies marginally with this requirement.
Although the load carrying capacity of an installed tile should be significantly greater than an unsupported tile, the unsupported breaking load is useful in evaluating the worse-case scenario, for example where the tiles are significantly drummy. The use of the unsupported breaking load as the requirement within a specification also provides a safety factor for the load capacity of the floor finish.
To optimise the performance of the floor finish it is important to identify all live loads including maintenance and fit-out vehicles (e.g. scissor lifts) as well as dead loads (e.g. soft drink machines or ATM’s) so that the maximum floor point-load can be determined.
Inadequate load carrying capacity is not the only cause of the cracking of tiles. Poor joint design, excessive lippage, inadequate adhesive coverage or substrate strength can all contribute to the failure of tiles in service. The use of hard-wheeled vehicles such as pallet trucks on the floor finish should be reviewed as they exacerbate weaknesses in the installation due to the concentrated point-loads applied to the tiles through the wheels.
Floor finishes within commercial lobbies are continually exposed to grime tracked into the building. The general soiling deposits are often combined with oily residues which can permeate the intergranular structure of the stone or become trapped on the surface of coarser finishes. Commercial lobbies often include cafes which further expose the floor finish to beverage spills such as the ubiquitous café latte. Coffee is a staining agent that is slightly acidic and also contains very fine particulates therefore it can etch acid-sensitive materials as well as leave grimy stains. Food spills also present a staining risk especially foodstuffs containing oil, such as meat pies and salad dressings.
The apparent stain resistance of a floor finish is determined by a complex relationship between colour, composition and the absorption characteristics of the stone. Oil stains are more conspicuous on materials with a mid-tone especially mid-greys and browns. The deposition of an oily film on a surface changes the optical characteristics of the material producing a darker colour. This effect occurs on all materials, whether they are stone, ceramic or painted finishes. The ability to remove the oil stain is affected by the depth of penetration which is determined by its absorption characteristics.
The general appearance of the stone can also affect its apparent stain resistance. A highly figured and variegated granite has more potential to hide a stain than a plain light coloured stone with similar properties (like trying to hide a gravy stain on a white shirt).
Composition plays a major role in determining the degree of stain resistance of a stone. Some stains are caused by a chemical interaction between the stone and the staining agent. For example, most limestone and marble types contain calcite and are therefore sensitive to acidic solutions. When a substance such as wine is spilt on these stones, staining occurs not only through the absorption into the pores, but also by the etching of the surface. Stone composed predominantly of chemically inert minerals such as quartz and feldspar (as found in granite) are more resistant to etching.
Determining the stone’s water absorption capacity and mineralogy can assist in evaluating stain resistance although these tests do not take into account the colour or surface finish of the tile. A practical method of determining stain resistance is through application of staining agents directly onto the surface of the tile. Stone Initiatives test method SI-STAIN-12 involves application of a range of common staining agents onto the surface of the tile for periods of one hour and twenty four hours. The short dwell term represents the scenario where cleaning of the stain occurs soon after the spill while the longer dwell time represents a floor finish where spills are only attended to once a day. After the dwell time, attempts are made to remove the staining agents using cleaning practices similar to those that may be used on site. Once the tiles have been cleaned and dried the tiles are graded according to the visibility of the staining agents. This simple but effective test method allows the inherent stain resistance of the tile to be evaluated as well as the efficacy of impregnating sealers and surface treatments when applied to the tile.
There is no doubt that the judicious use of impregnating and topical sealers can assist in the maintenance of stone floor finishes and their use is now considered de rigueur. In specifying a sealer for a floor finish it is important to treat each installation as unique and ensure the sealer has the properties that match the requirements for your project. The selection of a sealer is a case of ‘horses for courses’ and not a case of ‘one size fits all’.
The varying chemistry of sealers produces varying strength and weaknesses. Some sealers provide excellent resistance to water-based stains but have poor resistance to oil stains and vice versa. Sealers can also affect the optical properties of the stone depending on its colour, porosity and surface finish. Correct sealer selection requires review of the project requirements and environment of the final installation. What type of staining agents are likely to be present? Are they mainly water-based, oil-based or both? Do I want to maintain the natural appearance or a colour enhanced wet-look? Does the sealer need to be water-based or low VOC?
Slip resistance requirements also need to be taken into account when selecting a sealer. Although impregnating sealers do not usually have a significant effect on slip resistance, their improper application can affect the slip resistance of smoother finishes such as honed or micro-etched. By definition topical sealers produce a coating on the surface of the tile which usually produces a smooth finish with a low coefficient of friction that is only suitable for dry areas. Topical sealers produce a wearing face on the tile that has a much lower resistance to wear than the stone tile itself and therefore issues such as possible change in gloss especially in high traffic areas may need to be addressed.
Stability is a broad term covering issues such as change in appearance, structural integrity and sensitivity to warping. The stability of the appearance of the tiles is of course an important requirement to maintain the design intent of the lobby. The appearance of stone tiles may be affected by the absorption of moisture around the joint lines producing a ‘picture frame’ effect. The moisture may originate from the substrate or the abundant use of water during cleaning or other external sources but in both cases the moisture is preferentially absorbed by the grout (due to a higher absorption capacity) and it is then transferred laterally into the tile at the perimeter.
Tiles with a light grey tonality are particularly sensitive to colour change and in this situation the increased moisture content produces a darker frame around the tile. The risk of picture-framing developing can be reduced by restricting the absorption of moisture from the screed through six-sided sealing of the tiles prior to installation. The use of adhesives designed for moisture-sensitive stone and epoxy grouts which are less permeable can also reduce the risk of picture-framing.
The appearance of light coloured granites and marble are occasionally affected by the formation of iron stains. This colour change is due to the presence of soluble iron within the tile that has originated from altered iron-bearing minerals. As the moisture within the tile gradually evaporates the dissolved iron oxidises and produces a yellow-brown blush on the surface of the tile. It is difficult to predict the formation of these stains as the soluble iron may have been introduced into the stone from altered minerals within a different region of the quarry. Rust stains on granite tiles can be removed using proprietary rust removers if they are treated prior to sealing. The removal of rust from the surface of marble is much more difficult and requires specialty treatments.
High-density limestone is currently a popular choice in commercial lobbies as it is available in a palette of neutral tones to suit current trends. Limestone often contains stylolites which are concentrations of clay minerals that appear as veins throughout the stone. The regular wetting and drying of installed tiles during the cleaning process can cause the expansion and contraction of the clay minerals. Decay is usually limited to loss of the clay filling and undercutting of the veins if the stylolites are generally oriented perpendicular to the surface of the tile. Stylolites that are oriented sub-parallel to the surface can cause spalling and loss of the tile surface.
Tiles containing unstable stylolites must be protected from moisture and should only be considered suitable for use in permanently dry locations or when protected by a water repellent topical coating.
The bid to gain a competitive advantage in the highly competitive global market has led to the production of larger format tiles that are also thinner and therefore cheaper to manufacture. This reduction in thickness combined with a larger length to width ratio has significantly increased the sensitivity to warping of some tiles. This sensitivity is an interrelation between the total porosity and pore size distribution of the material in conjunction with the dimensions of the tile. Pore size is the predominant physical property related to warping as the finer the pore structure within a material the stronger the capillary action. In practice this stronger ‘sucking power’ increases the ability of the tile to draw moisture out of the substrate which can lead to the development of a saturated zone on the bottom face of the tile while the upper face stays relatively dry. In an everyday sense, this effect is similar to leaving a dry plank of wood on the lawn overnight and coming back and finding it has warped due to the absorption of moisture on the lower face. If the tile is allowed to move freely while it dries the warping may recede. If the tile is fixed in place by adhesive this warping is permanent.
The degree of warping of a tile is determined by its thickness and dimensions. By reducing the thickness of the tile its stiffness is reduced, which in turn reduces its ability to resist any tendency of the tile to warp. The apparent sensitivity of a tile to warping is further exacerbated by an increase in length as this allows the deflection to accumulate over a larger dimension. Large rectangular tiles are more likely to warp at the ends while large square tiles are more likely to curl at the corners. With regards to natural stone, the warping issue is mainly limited to finely-pored basalt which can be manufactured as thin, large format tiles. Some types of engineered stone materials have also been found to show dimensional instability. In summary, when assessing the overall stability of a tile it is important to take into account its colour, composition, porosity and dimensions.
The selection of a floor finish that is fit for purpose is not a decision that should be made lightly. The four selection criteria we have discussed are Surface finish, Strength, Stain resistance and Stability. A thorough review of these points during your evaluation will assist you in achieving a fifth S – Success!