Abrasion Resistance Abrasion (wear) resistance is achieved by controlling a whole series of factors. It is not sufficient to specify just an appropriate concrete strength. This must be complemented by proper construction practices, e.g. placing, compaction, finishing and curing. Where very high abrasion resistance is required, special aggregates or dry shake may be needed, either added to the surface or as a topping. NZS 3101 sets out requirements for the minimum f'c depending on member and type of traffic. These are summarised in Table 1. It must be emphasised that these are minimum strengths and serve as a guide only.

Member and Type of Traffic Minimum characteristic strength, f'c (MPa) Floors in commerical areas subject only to pedestrian and/or light trolley traffic 25 Floors subject only to light pneumatic-tyred traffic (< 3t gross) 25 Floors in warehouses and factories subject to medium or heavy pneumatic-tyred traffic (> 3t gross) non-pneumatic-tyred traffic  steel-wheeled traffic     30 40 ≥ 40 (to be assessed) Table 1. Minimum concrete strength for abrasion resistance

Effect of Various Factors on Abrasion Resistance

The abrasion resistance of concrete is directly related to its strength and the increase in resistance is principally due to an increase in cement content and reduction of water content. The quality of the mortar is important - the hardness of the coarse aggregate only becomes significant under exceptionally abrasive conditions i.e. when the surface matrix has been worn away. The good wear-resistance properties of granolithic concrete arise mainly from its being a very rich concrete and less from the aggregate it contains.

In general, well-graded natural sands free from soft materials should be used, with coarse aggregates which need only be especially selected for conditions of exceptionally heavy wear. Coarse aggregates should be free from soft sandstone or soft limestone.

Apart from the direct relationship between abrasion resistance and concrete compressive strength, other factors also have a major effect on abrasion resistance. Methods of construction such as the finishing process can have an influence.

Curing and the type of surface treatment are other important factors. The relative effect of each of these variables is illustrated in Figures 1 to 5. This data is based on work carried out by the University of Aston and the Cement & Concrete Association of New Zealand.

The accelerated abrasion test method adopted in the research project allowed a reliable determination of surface wear against time. The extent of abrasion was measured by a micrometer at intervals of 5, 10, 15 and 30 minutes of test, and these are plotted in Figures 1 to 5 for various test conditions.

The results show that the finishing technique, especially the use of repeated power trowelling, has the greatest influence on abrasion resistance, followed by curing, then concrete mix proportions. The study also found that:

• a change from Grade 40 to Grade 25 concrete will result in an increase in wear of about 20%;
  
  
• not using the appropriate finishing technique can increase the wear by 3 to 4 times;
  
  
• repeated power trowelling is an effective finishing technique to improve abrasion resistance;
  
  
• the use of surface treatments, such as polyurethane or epoxy, were found to significantly enhance the abrasion resistance;
  
  
• failure to cure the slab compared to covering with polythene sheeting can result in more than doubling the wear; and
  
  
• surface hardeners seemed to provide initial improvement but once the hardener layer was penetrated, the abrasion resistance reverted to that of an untreated concrete.

The repeated power trowelling using a solid disc power float machine consisted of three periods of power trowelling separated to allow the bleed water to reach the surface and evaporate.

Figure 1. Water-cement ration's effect on abrasion resistance - power float finishing and polythene sheet curing. Figure 2. Finishing method's effect on abrasion resistance - w/c ratio of 0.65 and polythene sheet curing.

Figure 3. Curing method's effect on abrasion resistance - w/c ration of 0.65 and repeated power float finishing. Figure 4. Liquid treatment's effect on abrasion resistance - w/c ratio of 0.65, repeated power float finishing, and polythene sheet curing.

Figure 5. Dryshake treatment's effect on abrasion resistance - w/c ratio of 0.65, repeated power float finishing, and 90% efficiency resin membrane curing.