Once the metal bond grinding process has been completed, it is time to polish the surface, for which finer diamonds are required. This is where alternative bonds come in as metal bonds can scratch the surface or provide a rougher finish once the diamond grit exceeds 100-150.
Ceramic bonds are the happy medium between metal bonds and metal resin bonds in terms of aggressiveness and can only be used wet. They provide exceptionally long life on hard and medium concrete, and a beautiful finish.
Ceramic cups are used for edging, in many cases eliminating the need for a metal bond cup by prepping and polishing simultaneously. Ceramic pads on the other hand are designed for polishing under walk-behind grinders, working a little slower but therefore lasting longer than resin pads.
Baked at 400°C, ceramic pads have a very delicate, porcelain-like structure and are only recommended for professional use. Similar to dropping a plastic plate vs. a porcelain plate, ceramic tools can easily shatter when used incorrectly.
METAL RESIN / HYBRID BOND
As the name indicates metal resin bonds consist of a mix of metal powders and resins. Offering double the lifespan to regular resins, hybrids are harder wearing, however, are only available up to 400 grit. If a higher grit is required, resins are the ideal solution. Unlike ceramics, metal resins can be used wet and dry.
As with metal bonds, there are varying levels of hardness for different applications to ensure the bond wears away at the optimal rate. Holding diamonds up to 3000 grit, resin pads quickly remove scratches caused by metal bonds and are best suited for applications on hard materials as the resin wears fast on abrasive floors.
When determining the hardness of the concrete surface you are wanting to grind down, several factors need to be considered. In an ideal world, you can just go by the specifications the concrete manufacturer has supplied. These, however, are not always accurate as several variables can influence the curing process and leave you with different results - weather, concrete mix, placement and finishing techniques.
The initial setting phase that allows the concrete to be tread upon takes 24 to 48 hours. After 7 days concrete is about 50% stronger and after 28 days it is fully cured. In this period the concrete hydrates, forming crystals from a reaction between the cement and water that increase the material’s compressive strength and make it durable. How the concrete is treated during the curing process, especially during the first week, has a high impact on the final result in terms of hardness. The amount of moisture, temperature and weight fresh concrete is exposed to, influences the way it cures. Not to forget, if curing compounds were used, this also has an effect.
Therefore, our recommendation is to always test the concrete to determine how hard it is. This can be done by either using a Mohs scratch test kit, verifying the concrete floor’s resistance to abrasion or with a rebound hammer (also known as Schmidt hammer or sclerometer), supplying approximate values of the concrete’s compressive strength in psi (pounds per square inch). It is important to note that slabs usually show different strengths in different areas, so sampling different areas can help.
RESIN BOND CONTINUED
If too much weight is applied or the machine is operating too fast, the resin can burn. Therefore, the down pressure on each resin should never exceed 45kg. What is important to note here is that the weight of the machine does not equal the down pressure as the weight on each resin depends on various factors such as the number of resin pads, the way the machine is balanced and whether it is a planetary or oscillating grinder.
Burnishing pads are made of real or synthetic hog hair that is then sprayed with a fine-grit diamond mixed with a resin. They are used as the final steps in a polishing process to give an even higher level of shine than what can be achieved by normal polishing with resin diamonds. Often, burnishing pads are used in conjunction with sealers that protect the floor, for example against stains from red wine or vinegar spills in supermarkets. The heat generated by the burnishing pads while polishing the floor activates the sealer, forming a protective layer. Burnishing pads are available in a wide selection of diamond grits to cover all applications from floor maintenance under auto scrubbers to terrazzo and concrete applications under high-speed burnishing machines.
SPONGE RESIN BOND
Sponge resin pads were initially designed to restore or maintain already polished concrete and terrazzo floors that had lost their shine due to heavy foot traffic, providing the benefit of simultaneously cleaning and polishing the floor thanks to their nylon texture. They can, however, also be used for polishing cementitious overlays and concrete, when wanting to achieve a so-called ‘cream polish’ that doesn’t expose the aggregate. With their flexible backers, sponge resin pads contour to the floor, allowing them to reach minor low spots and take out resin swirl marks.
WET VS. DRY GRINDING
Surface prep tools can be used in two different ways: wet and dry. As opposed to cutting and coring tools, the preferred option for grinding tools is the dry method. This is due to the cost of the disposal of dust vs. slurry as well as reducing the overall time to complete a job.
The fine dust created when dry grinding hard materials presents a safety hazard to those in the vicinity. This dust, however, can be contained within the rubber lips underneath a floor grinder and simultaneously removed with the help of a vacuum extractor that is equipped with a HEPA filter. The most common kind of collecting the dust, which is by using sausage-shaped Longo bags that can be sealed with two cable ties at either end, reduces any contact with the dust to a minimum.
When wet grinding, water helps minimise the amount of dust generated. Due to health and safety regulations throughout many countries, the disposal of slurry, however, has become less cost-effective, providing a further reason to resort to dry cutting. Situations that do call for wet cutting include outdoor work without a sufficient power supply. Either way, we always recommend wearing a dust mask to prevent yourself from inhaling dust which can lead to serious lung disease.
- The finer the grit, the slower the speed of the grinder should be.
- If you are producing dust, you are grinding. If not, the machine is drifting over the surface and you should change the tool.
- Always clean up! Keeping the surface clean not only ensures no foreign objects are causing scratches, but it is also easier to see the results you are achieving with the tool.
- Do not polish green concrete that hasn’t cured yet.