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Typical problems – quality norms

What are its most typical problems – weight, cracks, holes. Quality norms and limits.

Weight

The main problem that sets restrictions on the form is weight. Similarly to other stones, the density of concrete is 2-2.2 kg/l and, for example, a square meter of worktop with a thickness of 4 cm weighs approximately 82-88 kg (40 liters) or, for example, a worktop with a length of 2500 mm and width of 600 mm, which are relatively standard measurements, weighs around 120 kg (without cut-outs for the stove or sink).

This causes many different problems. Firstly, with such weights one has to consider how to transport and install the worktop – whether the kitchen is on the first floor or the worktop has to be carried up a narrow spiral staircase to the top floor. When moving on stairs, it has to be kept in mind that worktops are long and transporting them is difficult even with many people. This is because the weight is distributed unequally when moving on stairs and the worktop must not be allowed to fall (even in wooden packaging), as concrete cannot sustain a sharp blow or deflection and will break.

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Usually, there are openings in the worktop for the stove or sink, which makes the worktop a little lighter. However, this makes installing it a job that requires diligence. When being put in place, the worktop has to be supported fully and also at the places of the cut-outs, which are thinner. This must be done to avoid cracks appearing in the narrow sections of the worktop.

When already installed, concrete is a surprisingly durable material if used under ordinary conditions.

Cracks – main concern and partially justified

Cracks should not be feared too much. Many types of cracks may appear in concrete, the most typical are the so-called hairline cracks. Cracks caused by deflection may also appear in the narrow sections of the worktop, such as the cut-outs for the stove or sink. Cracks that pass through the concrete may be dangerous if the worktop is not reinforced. However, at the moment we reinforce all our worktops with metal reinforcements to avoid the worktop breaking due to cracks.

Generally, cracks can be classified as either active or stable. An active crack is characterised by changes in either its length or width, a stable crack remains the same. A crack is classified on the basis of three factors, which are 1) direction, 2) width, 3) depth. Cracks can occur along or across the surface, vertically, diagonally or randomly. On the floors, cracks are considered to be wide if their width exceeds 2 mm.

The following classification applies to cracks in concrete floors:

  • Pattern cracking
  • Hairline cracking
  • D-cracking
  • Plastic cracking
  • Settlement cracking:
  • Structural cracking
  • Tension cracking
  • Rust cracking
  • Thermally-induced cracking

When it comes to concrete worktops and sinks, things are a bit simpler. In 99% of the cases, cracks occur either during transport or installation. With worktops, cracks can occur only in the narrower sections of the cut-outs of the stove and sink, no cracks appear in the general surface of the worktop. When it comes to sinks, cracks are even rarer, but the so-called hairline cracks may occur in the thin walls of the sink (here, a difference has to be made between cracks that ruin the worktop and cracks which do not pose a threat to using the worktop).

As stated above, we reinforce worktops sufficiently and they will not break even when dropped on the ground (only large cracks appear and these make the product unfit for use).
However, cracks with a diameter of <0.2 mm (the thickness of ordinary A4 copy paper) are considered natural or safe – i.e. these cracks cannot be felt when touching the surface with a finger.

NB: To avoid cracks completely, it is possible to install smaller sinks in the worktops, in case of which, e.g. upon a 600 mm wide worktop, both edges of the cut-out for the sink are 150 mm thick (i.e. the maximum width of the sink is 300 mm).
The thickness of the worktop can also be increased, which unfortunately is not always effective, as it significantly increases the weight of the worktop!

At the moment, we have achieved the optimal approach to weight and all our worktops are reinforced. We usually offer the thickness of 40 mm, as with this thickness, all kitchen appliances (e.g. faucets) can still be easily installed and do not require additional bolts or other special solutions.

Porosity – pinhole

Concrete is a relatively porous material. To avoid porosity, many substances reducing surface tension have been added to the mixture and all worktops undergo a vibrating table, so that there are no open pores on the surface of the worktop.

Pinhole –air bubbles in mass:
A pinhole is a small bubble of air that has remained in the mixture mass (with a size of 0.5 mm) and may occur more often in case of the so-called pepper polishing, as in such a case, a millimeter or two are polished off the surface and possible openings/holes may be revealed. Due to their small size, they do not weaken the worktop even if they are below the surface. (About the product itself)

Sometimes the so-called pinholes or empty air bubbles are even desired in vertical surfaces (e.g. exterior walls of a sink) if they bring out the aesthetic qualities of the material.

Liquid absorption

One aspect of the porosity of concrete is also liquid absorption, wherefore different surface coatings and impregnation agents have to be used. Surface coatings have been discussed in more detail in article Covering concrete surfaces and selection of surface coatings.

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Customers who have used concrete worktops have later said that many of the fears they had with regard to the concrete becoming dirty, have later turned out to be quite unnecessary.

Concrete floors (both coated and uncoated) provide a good example here. Historically, such floors have been made for technological buildings, warehouses or other rooms where the floors are used intensely and the chances of them becoming dirty are high. If we look at these concrete floors, then regardless of the intense use, they can still be cleaned. Even washing them on a daily basis does not damage their surface for many years.

Here, parallels can also be drawn with the durability of concrete worktops, but as higher demands are set on worktops than on concrete floors, it is necessary to cover worktops with liquid repellent coatings.

The work on repelling liquids begins already when making the mixture for the worktop– additives that significantly reduce liquid absorption are added to the mixture to avoid liquids passing through the surface (for example, marble is a relatively porous material and we all know how stains seep through a marble surface when the plates have been attached with the wrong glue).
Such treatment results in a surface in which liquid spreads only to the thin surface layer without going deep into the material – wherefore it can also be cleaned with stone cleaners (similar to limestone cleaning supplies)

Quality norms – standards developed in the USA

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Thickness of the plate
Nominal thickness cannot vary more than 1/7 cm per running meter of a worktop.
The nominal thickness at joints cannot differ more than 1/10 cm on the joining line.

Worktop flatness
Worktop plates should be level and smooth, deviation of not more than 1/20 cm per square meter.

Straight line of the edge
Straight edges have to be even, without variations in width of more than 1/7 cm per running meter.

Side profiles
Side profiles have to match the profile of the worktop and have to be in as similar a tone as possible with the continued worktop at the joints.
Vertical edges should be square-shaped at 90% and copy the structure of the surface. Angles and rises have to be formed with a sharp cut.

Practice of joining lines
A joining line has to be even in full length, not widening over 1/10 cm throughout the whole joint.
The width of the joint should be as narrow as possible, but not more than 1/4 cm.
The surface of the panels next to the joint should be even and level with a precision of 1/30 cm throughout the whole joining line.

Surface quality – smoothness/porosity
In a horizontal surface, in the kitchen and in the bathroom, there should not be more than 2 pinholes per square foot (i.e. > 20 pcs = per m2). In kitchens, the aim is zero, which is ideal for cleaning the worktop.

Filled caps/holes on the surface cannot be larger than 1/20 cm.

The bottom side of the worktop should be smooth, pieces of mixture or flakes from the mold etc. should not come off it.

Cracks – on the surface
Hairline cracks, which may occur on the end-product, are a natural characteristic of concrete. Hairline cracks do not affect the structural integrity of concrete. Hairline cracks have been defined as cracks with a diameter of less than 0.2 mm, i.e. the thickness of common copy paper.

Stain-proofness – absorption
Finished concrete surfaces should tolerate stain protection in accordance with the purpose and the relevant staining liquids.
The finishing of the concrete surface should protect it against these 12 household products:

  • vinegar
  • lemon juice
  • mustard
  • coffee
  • red wine
  • vegetable oil
  • water
  • ammonia
  • bleach
  • isopropyl alcohol (70%)
  • acetone
  • dishwashing liquid

Stain-proofness is divided into four classes depending on how long the coating can withstand physical damage in case of continued contact and when a permanent change in color occurs.

  •  Class 1: 24 hours resistance
  • Class 2 : 8 hours resistance
  • Class 3: 1 hour resistance
  • Class 4: treated surface behaves similarly to an untreated surface

Surface coating
Treated surface has to be free of stripes, waves, air bubbles or other defects that may occur when coating the surface.
The surface coating cannot peel off or bubble on the treated surfaces.
The gloss should be even and similar in appearance also between different plates.
Treated surfaces must endure a temperature of at least 150 °C for at least 5 minutes.
The finished surfaces cannot change color and the coating cannot break in sunlight (UV radiation).