Surface Metrology is defined by Wikipedia as “the measurement of small scale features on surfaces, and is a branch of metrology. ... Surface metrology is the study of surface geometry, also called surface texture or surface roughness. The approach is to measure and analyze the surface texture in order to be able to understand how the texture is influenced by its history, (e.g., manufacture, wear, fracture) and how it influences its behavior (e.g., adhesion, gloss friction).”
Wikipedia also says Surface Roughness “plays an important role in determining how a real object will interact with its environment. Rough surfaces usually wear more quickly and have higher friction coefficients than smooth surfaces (see Tribology).”
There are portable instruments called surface profilers used in other industries such as aerospace, ship building, mechanical engineering, automotive and healthcare to measure surface profiles that have been processed with abrasives. These instruments have the ability to record data and print reports. In the concrete polishing industry, these reports could be submitted to a GC, architect or owner as proof of testing.
There are more than 25 different measurements used in various industries. Focusing on Ra for now would best fit the concrete polishing industry. Ra is the Arithmetic Mean Roughness of a surface. It measures from a mean line that lies between the highest and lowest point on a sample length. Ra measures the average length between a scratch pattern’s peaks and valleys and the deviation from the mean line. The measurement is expressed either in micro inches or meters. ISO standards express Ra as CLA, or Center Line Average.
I have completed some initial tests based on the information I have presented above. This concept will need to be put through additional testing on polished concrete to see exactly how it can be applied to the concrete polishing industry. My raw data is presented in Chart 1.
The floor was an old warehouse space once used as a machine shop. The concrete was ground to sand aggregate and no densifiers were used. The planetary machine I used for grinding had a main head and satellite heads that all rotated in the same direction. The equipment weighed 600 pounds and had 18 3-inch round abrasives underneath it. For the surface profile testing I used a Mahr Federal profile meter. Also, keep in mind my testing was focused on a mechanical process, not chemical.
I ground the entire 20-foot by 25-foot test area using a 70-grit metal bond abrasive with two passes at a slow rate of speed making sure the floor was fully refined and creating an equal base scratch pattern. An approximately 3-foot-wide by 20-foot-long area was left at the 70 grit metal grind. I then divided the remaining test area into two sections equaling two 10-foot by 25-foot areas. One side received one pass of subsequent abrasive grits and the other side received two passes of subsequent abrasive grits. I then proceeded across a second section of the test area with a 120-grit metal bond abrasive. Leaving a section refined to the 120-grit metal bond abrasive, I proceeded in the sequence with a 3-foot-wide section with a 50-grit resin bond abrasive. I repeated this for 100-, 200-, 400- and 1500-grit resin bond abrasives. See Chart 1 for results.
The following are first impressions of the readings:
- Random readings within the same grit produce a wide range of readings.
- The areas that received two passes have a lower reading than one pass except at 50 resin which could be an anomaly.
- As the process is performed, generally speaking, the higher the grit the lower the reading showing a decrease in surface roughness.
- The areas that received one pass compared to two passes showed a greater inconsistency of results.
What could cause the wide range of readings?
- Micro fissures
- Degree of concrete consolidation around aggregates
It would have to be determined how to best use this raw data in the concrete polishing industry. Here is a proposed method, and of course, numbers would have to be refined after further testing. At this point it is assumed fine aggregate, sand aggregate and exposed aggregate standards would be individually produced. To keep this example simple it shows resin bond abrasives only. Metal and hybrid bond abrasive readings would also be incorporated into the overall formula: