Moruzzi

Granite, Marble, Quartz, Limestone, Slate 

CONSIDERATIONS WHEN SELECTING STONE

·      Stain Resistance

·      Strength

·      Resistance to abrasion

·      Durability

Stain Resistance

There are three main factors that determine stain resistance;

·     Water Absorption
·     Composition
·     Appearance

The weighting of each of these factors varies depending on the stone type and application. Water absorption is directly related to staining resistance.  A higher water absorption capacity allows for a higher ‘stain holding’ capacity.  The table below indicates the typical absorption capacity for a range of dimension stone types compared to the relevant ASTM specification.  

 

 

Choosing a stone with a low water absorption is recommended for reducing the risk of staining of the installed material.  The use of polished limestone and marble in entertaining areas or any location where acidic spills may occur can lead to conspicuous, etched spots, which can only be removed by mechanical polishing.  It is important to note that impregnating sealers will not prevent this etching from occurring.  These sealers are designed to restrict the absorption of stains and therefore will not prevent surface etching and subsequent loss of polish.  Topical or sacrificial wax coatings should be used to protect acid-sensitive materials with a fine finish if they must be used in high-risk areas. 

 

Strength

The typical range in compressive strength of different stone types is compared below with the ASTM specification.   

 

 

Flexural strength is a property that is highly relevant to the adequate performance of dimension stone. Modern processing technologies can now produce stone in larger and thinner slabs, which can be used to span greater distances with relatively lightweight units. Products such as large flagstones, wall panels, and large countertops must have an adequate flexural strength to prevent failure. This property is usually determined by a 4-point loading test where the force is applied to the central half of the material.  It can also be evaluated by determining the modulus of rupture (MoR), where the specimen is loaded directly at the mid-point of the span.   As some stones experience a reduction in strength of more than 50% after soaking it is important to determine and consider both the dried and soaked strength.  If this reduction is not taken into account it could lead to failure of the stone in service.  The typical range of flexural strength (or modulus of rupture) for different stone types are given below.  It is essential that these results are considered a minimum whether the material is dried or soaked.

 

 

 

Resistance to abrasion 

Abrasion resistance is important to maintain the intended surface finish of flooring materials. A low resistance to abrasion can result in a significant change in the slip characteristics of the surface by grinding caused by grit and other surface contaminants underfoot.

 

Materials with a textured surface, such as ‘sand-blasted’, are often selected because of their increased slip resistance. In this case, the abrasion resistance of the stone needs to be assessed to determine the effect the anticipated traffic may have on the slip rating. The abrasion resistance of dimension stone has historically been evaluated using the Taber Abraser method (ASTM C1353). This method determines an Abrasion Index (Ha) – the higher the number the greater the resistance to abrasion. Typical values for a range of dimension stone types are given below along with the standard specifications.

 

Stone Type	Typical Abrasion Index (Ha)	ASTM Spec.	Standard Requirement
Granite	50 – 150	C615	25
Marble	15 – 50	C503	10
Sandstone	4 – 24	C616	2 (S) 8 (QS)
Limestone	<1 – 20	C568	10
Slate	4 - 20	C629	8

 

 
Below is a set of recommendations that have been developed  

Abrasion Index	Appropriate Uses
< 8	Suitable only for residential or light external traffic situations.
8-12	Suitable only for light commercial or residential flooring.
12-25	Suitable for general commercial and residential applications.
>25	Suitable for all uses including high traffic prestige commercial flooring.

   

When assessing the suitability of the abrasion resistance of a stone, it is important to consider: 

·       Surface Finish – possibility of loss of polish resulting in track marks, changes to slip resistance 

·       High Traffic Areas – bottlenecks in high traffic areas leading to increased wear 

·       Tracking of Particulates - grit brought in from outside sources may abrade polished surfaces 

·       Adjacent Materials – a significant change in abrasion index of adjoining materials may cause lippage with subsequent cleaning difficulties and tripping              hazards 

 

Durability

Dimension stone is considered a durable, some may say ‘eternal’ material; this is true as long as the stone is matched with its environment.  Decay is normally due to one or all of the following mechanisms:

·  Salt Attack

·  Freeze – Thaw Cycling

·  Dimensional Instability

Durability is a complex criterion determined by inherent strength, water absorption, and pore space.  A higher strength can mitigate the forces applied on the stone by the crystallization of salts or the expansion of ice crystals.  A lower water absorption generally correlates to a greater durability, as it restricts the passage of deleterious solutions, but a larger pore size can assist with durability by reducing the pressure applied by salt or ice crystallization on the walls of the pores.