Ceramic & Stone Close-up: Clean 'em up and lay 'em out -- Then what? A look at Dimensional Stones
As any one who has ever strapped on a tool belt can tell you, there are pluses and minuses to every building material. Dimensional stones are no exception. While space constraints limit the discussion here, let's look at some of the fundamentals involved in working with this important material
Dimensional stones fall into two groups: siliceous or calcareous. The group we see most is calcareous stone, made up of calcium carbonate and sensitive to acid cleaning. This group includes marble, travertine and limestone. Marble is divided into four groups-"A," "B," "C" or "D"-according to soundness and suitability for prospective use. The classification "C" or "D" means the material is comparatively fragile and may need additional fabrication.
Before cleaning the stone you may want to place a few drops of muriatic acid on the stone's face using an eye dropper. If effervescence starts, acid cleaners should not be used. A neutral pH compound will be required. And remember: Stone coated with sealers or polish must have the "coating" removed, before applying muriatic acid.
The siliceous stone group includes; granite, quartz, and serpentine. The body of the stone is basically silicone or quartz and feldspar. Additional classifications include metamorphic (marble, serpentine, and slate), igneous (granite) and sedimentary (limestone, sandstone, travertine, and quartzite).
Let's focus on concrete since most dimensional stone is applied to a Portland cement substrate. Concrete shrinks as it dries, so naturally its mass is greatest when it is first poured. Still, there is more water in the mix then is needed for curing (hardening). This is "free water" and it must go somewhere. It either evaporates into ground soil or into a sand layer on top of the vapor retarder but its removal can take months or even years. Consequently shrinking occurs.
An engineer specializing in concrete construction defects told me that half of the concrete shrinkage occurs in the first 60 days after casting. The rest of the shrinkage occurs over years. As a rule of thumb concrete slabs should not have additional shrinkage cracks after three years.
Helpful Hints: Due to concrete's propensity to shrink and crack and the effects of thermal or structural movement, it is wise to use a crack isolation product. Remember though, these crack isolation products only protect against horizontal movement. The plane of the concrete slab must remain level. Also, cracks over 1/8" wide must be repaired. Joints in the concrete slab should be 24" to 36" in each direction (interior). Slab deflection should meet the L/720 requirement.
Water based adhesives can be a problem since the water content may cause staining. In addition, the water can cause curling of certain stone products susceptible to water's effect, notably certain greens, blacks, and reds. Epoxy bonding agents should be used with these "sensitive" stones. Aggregate marble tile also falls into curling categories and need proper setting materials.
It may be noted that gray mortars can have a darkening effect on stone, especially light colored stones. This is why I prefer using white mortars at all times. I also prefer a latex modified thinset for stronger bonds.
Helpful Hint: Make certain to "wash" the back of the stone to remove dust that can inhibit a better bond.
Structual engineers say the safest way to forestall a tile cracking problem is by employing a crack suppression systems. These are many and varied. If you decide on a system, study your options and remember these systems are not complicated. Pick the one suited to your specific job and get the warranty you want. Some of the available systems include sheet membranes. The category offers five options: vinyl sheets, polyethylene sheets butyl /bitumene and cork.
Then there are liquid membranes. These may come in acrylic versions, or asphalt modified versions, and of course there are liquid/fabric products. National standards are in the process of gaining approved.
Helpful Hint: Do not create your own membrane. If you do, you are heading for trouble. Use only those products that have a successful history. Take your time and chose wisely.
No matter how carefully you work, you may still encounter one of the these situations:
Hollow Sounding Tone: Frequently a hollow sound is interpreted to mean that the stone was not bonded properly. Keep in mind that hollow sounds may be an acoustical problem or maybe just an echo. But also remember:
• Air may be entrapped in the bed or slab causing one part of the floor to sound different than the other.
• Waterproofing or crack isolation membrane installed between the slab and the setting bed always affects the sound
• Irregular substrates causing one part to sound different than the other.
• The back surface of the stone may be irregular.
• There may be voids within the stone
Lippage: This is a concern. An accepted elevation difference between the stones should not exceed 1/32". Again the substrate could be the villain. Or perhaps your stone is not consistently thick. Maybe the stone is not properly seated in the bonding material.
Staining or Discolorings: This may simply be a by-product of the water used in the organic adhesive or exterior water percolating through the slab (this may also cause spalling). Other suspects include gray thinset, topical spills and asphalt saturated membranes.