Shown here are scratches made with different pick numbers. The #4 scratch is barely visible so the reading would be between #4 and #5.
Metal-bond diamond polishing pads (hard-bond tools) are used for polishing soft concrete, while softer diamond matrixes (soft-bond tools) are used for polishing hard concrete. Before the advent of the scratch test, contractors wouldn’t know the hardness of the floor surface so they generally started with a hard-bond tool and worked up to softer matrixes.Tools that achieve the best productivity and yield good life were used to start the polishing process. But this search could be costly in terms of labor and diamond tooling. Having a good estimate of concrete hardness beforehand greatly increases productivity.
Conducting a scratch test
Testing slabs with scratch kits helps contractors choose the best tool to start a job. The concrete surface is scratched with as many as eight numbered “picks.” With each pick, watch for the first appearance of a scratch on the concrete. The number on the pick that first makes the scratch represents the surface’s approximate hardness. Prescott, Ariz.-based Mineral Lab makes these numbered picks, which allows you to start by selecting the appropriate polishing tool based on the hardness of the surface. The science isn’t exact and each tool manufacturer has slightly different ways to match its tools to hardness. But from the pick number, you can quickly zero in on the best tool choice, and it’s more accurate than a seat-of-the-pants perception of hardness. Hardness varies around the country. What’s considered hard concrete in Florida is different than hard concrete in Montana.
Selecting a diamond tool
When you know the surface hardness based on the scratch kit, you can select the correct polishing pads. Diamond tool manufacturers know that concrete hardness and abrasive-wear resistance relate to the productivity of the polishing process, especially for the beginning metal-bond steps. For example, manufacturers offer three to six blends of matrix metals and diamonds for a 40-grit pad. A selection based on perception adds uneccessary costs. Today, contractors can use the scratch test to reduce the number of steps and achieve more accuracy.
Formulating Diamonds Pads
Matrix metals used to hold diamonds in place are made from expensive, exotic varieties such as titanium, cobalt, tungsten carbide, and carborundum, says Clif Rawlings, product manager and training coordinator for Knoxville, Tenn.-based HTC. Titanium is the hardest, while lower grades of steel are the softest. Each manufacturer has its own blend of alloys.
“The purpose of the matrix is to keep diamonds in position, wearing away as the diamonds wear and exposing new diamonds at the right time,” says Rawlings. It’s a balancing act for manufacturers: Selecting the right hardness for the metal bond matrix and pairing it with the proper diamond mixture for the concrete hardness. Tools must cut efficiently yet last to give contractors the best cost per square foot.
The quality of the diamond is also important; fracturing and breaking as they grind so good cutting surfaces are always present. Manufacturers often blend quality levels to provide the right cutting surface for different concrete hardnessess. HTC makes four different segment series per metal bond type.