Abrasion Resistance of Warehouse Floors

The first observation on abrasion resistance, or more correctly a lack of it, is that it is very rarely a problem. In 15 years of experience, Tony Hulett of Face Consultants says he has never come across a case of poor abrasion resistance in a newly constructed warehouse floor. This may have been a problem in the past but modern construction techniques and good curing regimes have eliminated the problem. It is well understood that heavy power finishing creates a densified surface layer with closely integrated particle packing and low water:cement ratio. This has often been referred to as the creation of a ‘case hardened’ surface layer that is in the order of 1–2mm thick. In practice, it is also observed that when the surface layer has been worn away in aggressive environments or has been removed by grinding, the underlying concrete is usually still highly resistant to wear.   Abrasion Mechanisms Abrasion resistance is the ability of the surface to resist loss of surface from rubbing or scraping/plucking actions that dislodge surface particles. In typical warehouse or distribution centres, conditions are relatively benign. Neoprene truck tyres do not cause scraping or plucking actions and in many cases, it can be seen that the tyres have a polishing effect rather than a wearing effect. Hard-wheeled pallet trucks can be aggressive on joints but there is little evidence of them causing direct wear on the floor surface. Most loss of surface appears to be caused by scraping actions, typically from nails protruding from pallets that are pushed or dragged around. Dirty dusty floors can suffer wear when the dust is ground into the surface by rubber tired forklift trucks, a scenario that is more commonly in industrial buildings. In extreme cases such as waste transfer stations, the action of front loader buckets is very aggressive.   Abrasion Resistance Testing The most commonly used method for testing in-situ floor surfaces is described in BS EN 13892-4(1). This Standard prescribes a machine, known as the BCA (British Cement Association) test, which creates a wearing process. The machine, shown in Figure 1, simulates a wearing mechanism by the use of three hardened-steel wheels mounted on a revolving plate. The plate revolves at a set speed for a set time under a prescribed load. The resultant annulus of wear is measured at eight points and the average depth of wear is reported to the nearest 0.01 mm. Historically, floors have been classified in accordance with Tables 3 and 4 of BS 8204 -2(2). It is now appreciated that those classifications are of little value as the difference between the applications related to each of the classifications is totally subjective and therefore unhelpful. It has also been shown that in practice, the difference between for an example an AR1 and AR2 floor has not been reflected in real long-term wear rates of the floor. For TR34 4th Edition(3), it was concluded that the test method was not useful in characterising long-term wear rates under typical warehouse use. This is to say that there are no data to show that, for example, a floor with a test classification of AR1 will have a longer service life than a floor that is compliant one with AR2. However, the test was accepted as being a useful indicator of a minimum acceptable floor surface for this type of application. For that reason a maximum test limit of abrasion of 0.2mm is required using BS EN 13892-4. TR34 notes that resin-based curing compounds create a layer or ‘skin’ on the surface of the floor that can be impenetrable to the test machine. Caution should therefore be exercised when interpreting results. In parts of Europe, BS EN 13892-3(4) is commonly cited in specifications for floors. This is a laboratory test known as the B?hme test and is used for testing screed materials. It is usually cited in respect of dry-shake toppings used on floors. It is difficult to see the relevance of this test method as the manufactured test specimen used in the laboratory cannot be representative of the actual floor. ASTM C779(5) prescribes three methods using different machines each of which apply a different abrasion mechanisms. It can be assumed that the three different methods are intended to simulate different types of abrasive action. However, no guidance is given in this standard to suggest the appropriate method for simulating any particular use of a floor. It is understood that floors are rarely tested by any of these methods.   Factors Affecting Abrasion Resistance:-Concrete Mix Design Historically it was suggested that there was a relationship between increased cement content and greater resistance to abrasion resistance, suggesting that higher cement contents produced more durable floors. This relationship has been discounted since the late 1990s but it is still held to be the case in some of the English-speaking countries that were influenced by UK guidance in the 1980s and 1990s. A literature review in 2002(6) demonstrated that, in line with flooring industry perceptions of the time, abrasion resistance does not increase with increased cement content above an upper limit of about 350kg/m3. Having revisited that literature review, it is probable that a lower limit of 325kg/m3 can be safely applied provided that water:cement ratios are controlled. The significance of avoiding higher than necessary cement contents is that higher cement contents are a major contributory factor to greater shrinkage. For this important reason the updating of BS 8204, where minimum cement contents of 400kg/m3 are given, is long overdue.   Curing Effective curing to ensure maximum hydration of the concrete surface is of paramount importance. Curing can be through the use of sprayed-on curing compounds, which create a waxy coating, polyethylene sheeting or water or combinations of these methods. Curing compounds are often described as ‘hardeners’. It is doubtful if they have that effect in the true sense in that they are unlikely to modify the surface matrix of the concrete. These compounds are sprayed on when the surface pore structure of the concrete is saturated and it is therefore difficult to see how the compound can penetrate the surface. Curing compounds are, however, effective in slowing water loss from the surface and therefore will create ‘harder’ surfaces in the same way as any other effective curing regime. It is well known that the compounds are worn off or degrade in use over a period of around a year, although this period is dependent on use and exposure to light. Surface hardeners, such as sodium silicate, which can be used for improving weaker surfaces in hardened concrete, cannot be used as a curing medium on fresh concrete.   Dry-shake Toppings Dry-shake toppings are blends of cement and fine aggregates, sometimes together with wetting agents and or pigments. In principle, a dry-shake topping has the same basic components as the fine aggregate and cement in concrete, ie, the mortar paste element. It follows that for the dry-shake topping to enhance abrasion resistance over and above the base concrete in a floor, the topping must have property enhancing characteristics. In most cases, toppings are supplied as fibre suppressants and are associated with steel-fibre-reinforced floors. The prime purpose of the topping is to boost the volume of mortar paste at the surface so as to give greater coverage potential over the close-to-surface fibres. It should not be assumed that toppings will increase abrasion resistance, although it is certainly the case that some toppings when tested exhibit better results than others. It is generally held that toppings that contain metallic particles will enhance abrasion resistance and it is, therefore, perhaps not surprising to find that toppings are marketed with product names that infer metallic attributes. Before committing to unnecessary cost, specifiers should seek clarification and test data based on real floors in realistic conditions and bear in mind that in the UK, most floors do not have any toppings. In extremely aggressive environments such as waste transfer stations, it is doubtful if any form of surface treatment can extend service life.   Surface hardeners A recent trend outside of the UK has been to use lithium or sodium silicate as surface hardeners. Traditionally, sodium silicate (waterglass) was used on concrete floor surfaces to deal with dusting problems caused by inadequate curing or other deficiencies. The silicate compound reacts with excess portlandite formed during cement hydration to form stable compounds. While there is little doubt that these silicates can play a beneficial role in improving poor quality surfaces on completed floors, these products are being applied as soon as 12 hours after concrete placement, as part of the finishing process. In other cases, they are being applied at around seven days after finishing following the removal of a wet curing medium such as polyethylene sheeting. It is difficult to see how this can be effective, as at such early stages in the development of the maturity of the concrete, the pore structure is saturated and therefore impenetrable to surface applied compounds in solution. For most effective use of soluble sodium silicate compounds the concrete is required to have a surface porosity and therefore a period of drying before application. It is also worth reminding ourselves that well-finished and well-cured concrete surface have been shown to be highly durable without the use of such surface hardeners. However, it is the case that surface hardeners can play a useful role in improving mature floor surfaces in some cases.   Concluding remarks Poor resistance to abrasion is highly unusual in modern floor construction, where good-quality concrete and good finishing and curing techniques are used. High cement contents do not enhance abrasion resistance and should be avoided because of their effect on shrinkage. Dry-shake toppings do not necessarily enhance abrasion resistance and although some exhibit greater potential from test results, they are not generally needed in typical warehouse applications. The use of surface hardeners on new immature concrete is questionable although they can be useful in improving deficient mature concrete.