Part I RisksRisks
9.2 What are floors made of?
9.2.1 The substrate
The material that supports flooring, called the substrate or the floor base, has a great impact on the quality of the flooring material. It is either an existing one,
9
Improving the design of floors
B. Carpentier, Agence FrancËaise de SeÂcurite Sanitaire des Aliments, France
which has to be properly prepared, or a new one, which has to be properly constructed and prepared before applying the flooring in order to allow a good adherence of the latter. It must be dry (a concrete slab must be let to dry for a minimum of 28 days but this time may be far greater if climatic conditions are not optimal) and able to prevent humidity reaching the impervious flooring. It must be capable of withstanding all structural, thermal and mechanical stresses and loads that will occur during service and it must be sloped sufficiently in order for liquids to flow to the drains. This is recommended for resin-based floors as well as for ceramic tiles, even though, traditionally, ceramic tiles are applied on a flat substrate, the slope being given by the screed. Particular attention must be given to joints that are an integral part of the floor system.
Nevertheless, it is not the purpose here to detail all the construction rules regarding the substrate. For further information see Timperley (2002).
9.2.2 Flooring
Two families of flooring materials are recommended for food processing areas:
ceramic tiles and resin-based floors. Polyvinyl chloride (PVC) sheets are considered unsuitable because they are too easily worn. They can become cracked after the fall of a knife or other sharp object.
Ceramic tiles
Ceramic tiles are made of clay that after shaping is subjected to high temperature. They are manufactured products of constant quality and have been produced for centuries. Vitrified unglazed ceramic tiles are recom-mended for food processing areas. They are highly resistant to the main constraints that can be encountered in food processing premises, especially to heat shocks. The vitrified tiles can either be pressed (in which case they are usually square or hexagonal) or extruded (they are always rectangular).
Dimension tolerances of the pressed tiles are better than those of extruded ones, allowing thinner joints.
Resin-based flooring materials
The first resin-based flooring, the acrylic cementitious systems, appeared in food industry premises during the 1960s and around two decades later synthetic resin flooring was also proposed, with the prospect of achieving a high standard of hygiene because those floorings are seamless. However, a high degree of technical skill is necessary to obtain in situ a good final product (only to be applied by a trained operative). As this has not always been respected, there have been many problems with such floors.
Resin-based floors are obtained by application of a mortar made of a mix of one or more organic or inorganic binders, aggregates, fillers and additives, and/
or admixture, and can be classified according to the nature of the binder(s) used.
There are two families of binders: synthetic resins and the hydraulic binders.
The first ones are organic polymers comprising one or more components that Improving the design of floors 169
react with a hardener at ambient temperature, whereas the hydraulic binders, as cement or lime, need water to harden.
Hydraulic binder
The hydraulic binder used in the construction of flooring material is cement. The main drawback of the use of hydraulic binders is the high porosity due to water evaporation during hardening. The addition of a synthetic polymer reduces porosity, increases the mechanical resistance and reduces cracking risk. Cement can be used in polymer-modified cementitious screed that is defined as a `screed where the binder is a hydraulic cement and which is modified by the addition of polymer dispersion or re-dispersible powder polymer with a minimum content of dry polymer of 1% by mass of the total composition, excluding aggregate particles larger than 5 mm' (Anon, 2001a). Examples are the acrylic-modified cementitious systems that are the main systems used in the meat industry in France. The main and great advantage of such floorings is that they can be applied onto a damp substrate.
Cement is also used in association with resins, such as epoxy resin and polyurethane. In those cases, resin content is around 5% by mass of the total composition. In such floors, cement is more a filler than a binder, which is why they are considered to belong to the synthetic resins family.
Synthetic resins
Epoxy resins are the more frequently used synthetic binders, followed by polyurethane and methacrylate resins. Polyester resins are seldom used, to the author's knowledge, in the food industry. Characteristics of the different resins change according to the formulation used. The formulation may be changed to adapt to such non-optimal installation conditions as temperature, relative humidity or time available prior to being put into service. Specific formulations proposed may have consequences on the resistance of the final product. It is therefore difficult to give precise rules on curing. The final floor system must be allowed to cure according to the manufacturer's instructions. These generally require 1±3 days at 15±20 ëC before trafficking and 3±7 days before washing, before contact with chemicals or before any ponding tests and high traffic loads (Anon, 2001b).
The only resin that clearly escapes this general rule is polymethylmethacrylate (PMMA), also called methacrylate or methylmethacrylate. It is characterised by a very short time prior to putting in service: 2 hours. It can be applied at low temperature (ÿ10 ëC). However, this resin possesses a strong odour at installation that can irreversibly alter food products present nearby.
The climate above the uncured resin should be maintained at least 3 ëC above the dew point. The substrate humidity must also be correct. It must, for instance, be smaller than 3% for an epoxy resin, or 7% for polyurethane±cement flooring.
Aggregates
Aggregates are granular materials that do not contribute to the hardening reaction of the mortar. Roles of aggregate depend on their size and abrasion 170 Handbook of hygiene control in the food industry
resistance. Small aggregates that may be called fillers have many roles.
Among those are reducing shrinkage and increasing the mechanical resistance.
Such aggregates are often made of sand with high silica content (SiO2 or quartz, 7 on the Mohs scale). Hard aggregates are used to increase resistance to abrasion. Those may be aluminium oxide (Al2O3 or corundum, 9 on the Mohs scale), silicon carbide (SiC, commercial name carborundum, 9.5 on the Mohs scale). It may happen that hard aggregates lead to an accelerated wear of shoes and of brushes used for cleaning. Large aggregates are used to increase slip resistance.
Primer
A primer (one or two coats) is most generally used to aid the adhesion of the final flooring and to seal and consolidate the surface of a porous substrate. It consists of a liquid product, which is often a solvent-based epoxy, applied to a substrate.
Coats
Anti-slip resin-based floorings can be obtained by one-coat or multicoat systems. Multicoat systems that are the more frequently proposed are thin flooring (2±5 mm) made of a self-levelling mortar on which large aggregates are sprinkled. One or two coats and then one or two finishing coats can be applied.
These finishing coats are very thin, which is why they have a poor durability (1 or 2 years). Such finishing coats can be interesting when they fill the bubble gas holes but their role should not be to maintain large aggregates necessary to slip resistance.
One-coat systems, also called monolithic systems, are made of mortar in which all the aggregates are mixed prior to application. The maximum diameter of the aggregates must be smaller than the third of the flooring thickness (Pollet, 2000). They are thicker (4±12 mm) and the large aggregates necessary to obtain slip resistance are often better maintained.
Gas removal
Gas bubble holes are highly undesirable for hygienic considerations (see below).
Surface-active agents can be added to avoid or decrease their formation during polymerisation of resin-based floors. There is also prickle roller, called heÂrisson in French (hedgehog), that is used when the mortar is in the fresh state to release entrapped gas bubbles. In some flooring, such as polyurethane±cement, it is very difficult to remove gas bubbles and to prevent their formation. One important measure is not to apply such flooring when room temperature is increasing.
9.2.3 Jointing Ceramic tile jointing
A jointing material should completely fill the gap between to ceramic tiles right up to the top edge of the tile as shown on Fig. 9.1. This is often not done Improving the design of floors 171
although it is technically possible. The gap must be as small as possible and applicators must wait (for instance around 1 to 2 hours at 18±20 ëC for an epoxy grouting), so that the grout has begun to cure, before the first cleaning of the floor. The jointing material has to absorb dimensional variations of tiles. That is why the better tolerance dimensions of pressed tiles, allow a joint around 5±
7 mm wide instead of 6±10 mm for extruded tiles. The smallest joints may be obtained when ceramic tiles are laid in a synthetic resin bed and then subjected to vibration. However, it is not advisable to have joints smaller than 5 mm because it will be impossible to fill them down to the bottom, as tiles for industrial purpose are thick.
A high diversity of grouting products is available (epoxy, vinyl ester, etc.) and descriptions of all of them are not possible here. The choice of system is governed by the chemical stresses expected on the floor surface. Cement grouts are not suitable for food processing areas because they are highly porous, acid sensitive and have a poor durability when subjected to mechanical stresses. In addition, a simple epoxy grouting will not resist the acidic conditions of some food factories such as dairy factories and an anti-acid grouting must be chosen.
Other joints
Among all the joints necessary in the construction of floors are: construction or day joints, expansion joints, movement joints, and isolation joints. All the joints in the subfloor or floor base should be carried through overlay material and filled with a suitable sealant. Joint fillers have to be flexible and are therefore not as capable of withstanding heavy loads or aggressive chemical as the adjacent floor finish. They must be changed when worn.
Fig. 9.1 Unhygienic and hygienic jointing.
172 Handbook of hygiene control in the food industry