Historically, the most common term used for describing the appearance of polished concrete is gloss: how shiny it is. Gloss has traditionally been measured using a glossmeter, a handheld instrument that projects a beam of light at a fixed intensity and angle onto a surface and measures the amount of reflected light at an equal but opposite angle.
Many international technical standards are available for these measurements, such as ASTM D523, Standard Test Method for Specular Gloss. They explain how to use different types of glossmeters on different materials, including paint, paper, metals, and plastic. Many industries use glossmeters for quality control to ensure consistent manufacturing processes.
Various geometries are available for gloss measurement. Measurement angle refers to the angle between the incident light and the perpendicular. Three measurement angles – 20°, 60°, and 85° – are specified to cover the majority of applications. The angle is selected based on the anticipated gloss range, as shown in the following table.
Gloss Level | Value at 60° | Action |
Low | <10 gloss units (GU) | Re-measure using 85° |
Medium | 11 GU to 69 GU | Use 60° value |
High | >70 GU | Re-measure using 20° |
The measurement unit for gloss is the gloss unit (GU) and is scaled according to the angle of measurement:
Gloss Level | Angle | Scale |
Low | 85° | 0 (matte finish) to 199 GU where 199 is a perfect mirror |
Medium | 60° | 0 (matte finish) to 1,000 GU where 1000 is a perfect mirror |
High | 20° | 0 (matte finish) to 2,000 GU where 2,000 is a perfect mirror |
For example, if the measurement made at 60° is greater than 70 GU, the measurement angle should be changed to 20° to optimize measurement accuracy.
Gloss measurement of polished concrete has historically been made with a 60° glossmeter, which is not best-suited for measuring highly polished surfaces. However, the greatest limitation of relying on a glossmeter to quantify the overall finish of a concrete floor is that it doesn’t assign a measurement to effects that are visible with the eye. Gloss is only a part of what is noticeable to the observer. Two floors may have the same reading, even when using the correct angle, but look very different.
To fully quantify quality, we need to consider more than just gloss.
A Prime Example
The gloss measured for the floors shown in Image 1 and Image 2 would be very close at a 60° gloss, although Image 2 doesn’t meet expectations if Image 1 is the desired finish. They look different because of reflectance haze, an optical effect caused by microscopic texture or residue on a surface, which results in a milky finish. Haze is caused by microscopic surface damage from burnish and polishing marks, fine scratches, lack of refinement, poor cleanliness/residue, and resin transfer. The texture produces halos and patterns around reflections of high-intensity light sources and reflected images appear “shallow.
Haze is a common problem associated with coatings and polished materials and can be measured with a handheld hazemeter/glossmeter
Image 3 shows the same floor with another undesirable effect, known as orange peel because the surface resembles the bumpy skin of an orange. Orange peel dramatically reduces appearance quality without affecting gloss readings. Orange peel is measured using an instrument that measures distinctness of image (DOI). DOI is a measure of how clearly or sharply a reflected image will appear in a reflective surface and is used to quantify surface waviness and orange peel, and is an indication of the clarity of reflected images. A high DOI measurement tells us light is almost perfectly reflected: The surface is smooth and flat.
Floors that have similar gloss readings will have a better perceived appearance with high DOI and low haze. They will also be more durable than floors that only have a high gloss level.
Gloss, haze, and DOI can be evaluated with a clarity meter, which profiles how a surface reflects light. Rhopoint Instruments’ Concrete Clarity Meter also measures reflected image quality (RIQ), a DOI measurement used for very highly polished surfaces. Take the meter for a test drive on actual surfaces during World of Concrete’s Polishing Luncheon on Tues., Jan. 2, 2019, or at the Concrete Polishing Council’s booth (C4511).
