The discoloration is understood to mean a change in the color of the concrete surface, which z. B. can be caused by the concrete composition, the formwork system, the execution, and/or the environmental conditions.
Concrete can have color deviations due to raw materials and processing. Concretes with the same type of cement but coming from different cement factories may have different shades of gray.
Concretes with a high water/cement ratio are lighter than concretes with a low water/cement ratio. Concretes with the same water/cement ratio is lighter if they are soft than if they are stiff.
Discoloration caused by design may represent a defect but does not result in any structural damage. As a rule, they only have an impact on the aesthetic appearance of the concrete surface and are therefore particularly important in the case of fair-faced concrete.
Low temperatures during the pouring of concrete also promote the occurrence of discolorations. With additional measures such as increasing the temperature of the fresh concrete, heating and/or coating the parts, concreting of fair-faced concrete parts is possible to a certain extent even at low temperatures.
Discoloration (patina) caused by aging and weathering is due to the material and design. Discoloration, on the other hand, is considered staining if it has been caused by external influences on a surface that originally had a perfect color (e.g. adhesive tape, wooden slats, plastic sheeting, etc.). Stains should be evaluated differently from discoloration.
Why do black spots appear on the concrete?
Despite professional planning and flawless execution, some dark and light spots occur again and again when manufacturing fair-faced concrete surfaces in winter conditions.
In very cold seasons, the high concentration of moisture in the environment is of decisive importance for the visual appearance of the concrete surface.
Upon drying, a moisture gradient develops from the inside to the outside of the concrete.
This causes moisture movement in the capillary pore system, which transports the dissolved calcium hydroxide in solution from the pores to the evaporation horizon.
Depending on the water content, the porosity of the concrete, and the ambient conditions, the evaporation rate at the surface of the concrete is greater than that of the pore solution flowing inward, so that the evaporation horizon moves from the surface of the concrete to the interior of the concrete.
Calcium hydroxide precipitates as carbonate when it dries at the level of the evaporation horizon.
If the surface of the concrete remains wet, which means that the evaporation horizon is on the surface of the concrete, slight calcareous efflorescence forms. If the evaporation horizon moves into the concrete, the calcium hydroxide crystallizes as carbonate in the pore space.
At low temperatures and high relative humidity, the period of time until the evaporation horizon moves into the interior of the concrete is long, so that a lot of calcium hydroxide can collect just below the concrete surface.
This compacts the cement stone close to the surface and the surface structure becomes even and more closed. Such surfaces have a lower degree of reflection, which makes them appear even darker.
The light-dark discoloration due to calcium hydroxide accumulation can be influenced by postponing the concreting at times with favorable climatic conditions or by carrying out winter construction measures in such a way that increased carbonate precipitation on the concrete surface is prevented.
What causes concrete to turn yellow?
Yellow-brown discoloration can occur in concrete that has been manufactured with phenolic resin-coated formwork panels whose phenolic resin coating has not cured sufficiently or is insufficiently weather and alkali resistant after curing.
Exposure of the plastic-coated formwork to ultraviolet light and weathering, as well as the type of on-site storage, determine the development and intensity of concrete to yellow discoloration.
The increase in component temperature that occurs during hydration can accelerate the chemical breakdown of the phenolic resin film.
If a gap occurs between the surface of the concrete and the formwork due to shrinkage or loosening of the formwork anchors, condensation water may form due to the penetrating cooler outside air.
The condensation water absorbs the phenolic compounds and runs off between the formwork and concrete, leaving yellow-brown rivulets on the concrete surface of the wall.
Before using plastic modified formwork, especially for fair-faced concrete applications, the alkali resistance of the formwork skin can be tested with the bull’s eye test (brief exposure of the phenolic resin film surface with a potassium hydroxide solution).
When producing smooth concrete surfaces, it is essential to ensure that the formwork coating does not discolor. It is recommended that absorbent wood formwork be artificially aged by spreading a cement slurry on it before use.
Avoid exposing the formwork to ultraviolet light and weathering, as well as improper storage on site.
What causes concrete to turn red, brown, or orange?
Red, brown, or orange discoloration can occur if wall and roof bonding reinforcement is exposed to the weather. Rust that forms on the surface of the steel can mix in rainwater.
If this rusty water runs down the surface of the concrete, reddish-brown discoloration or streak marks occur.
It is very difficult to remove rust stains from the concrete surface. Rust water often penetrates so deeply into the concrete that surface cleaning is not sufficient.
There are cleaning products that can be used to remove this dirt, but surfaces cleaned in this way become much lighter. Therefore, if these cleaners are used, it is advisable to treat the entire concrete surface.
Protruding reinforcing steel should be covered with sheeting and protected from water penetration. “Enclosing” the component also provides the necessary protection.
Another possibility is to coat the reinforcing steel with cement paste and thus create corrosion protection.
Why do black streaks appear in the concrete?
Black streaks may appear on the surface of self-compacting concretes and concretes of fluid consistency. Black streaks are not visible in rigid to soft consistency concretes. The causes may be:
- black and organic inclusions in certain limestones in concrete
- unburned coal dust in fly ash or silica dust
Organic components can be contained in the aggregates of cement, in the aggregates, or in the concrete admixtures, depending on the raw material. In fresh concrete, the coloring components are transported to the surface due to their low density.
For concretes with a fluid consistency, it is recommended to use types of cement, aggregates and admixtures with a low content of black organic components. Loss on ignition can be the first benchmark for evaluation.
What causes concrete to turn blue, green?
The blue discoloration is due to the low sulfide content in the slag sand. Sulfides are converted to calcium hydrosulfides and calcium polysulfides when they react with water.
In the absence of air and in an alkaline environment, as is the case with formed concrete surfaces, these polysulfides can react with dissolved metal ions from the blast furnace slag and cement (e.g. iron, manganese) to form metallic sulfides of very intense green or blue color.
On the drying surfaces, these green or blue metal sulfides are oxidized by oxygen in the air to form colorless metal compounds (sulfates, sulfites).
The rate of this oxidation, and the resulting discoloration of concrete made with blast furnace cement, depends on several factors:
- Cold, damp weather delays the drying of the surface and thus the oxidative discoloration.
- High water/cement ratio porous concretes discolor quickly so that a light-colored surface appears immediately after stripping. In contrast, the process is slower in the case of dense concrete.
- Horizontal elements (ceilings) that remain in the formwork longer than vertical elements (walls) discolor more slowly.
Depending on the environmental conditions and the structure of the concrete, the surface discoloration process can last from a few days to a few months.
However, in the central area of dense concrete, the green or blue discoloration remains for decades. This has a special effect on subsequently processed concrete surfaces.
It is assumed that the subsequent processing closes the pores close to the surface and thus the oxidation process slows down.
For example, earth surfaces show a persistent discoloration over a very long period of time, which is still visible after several months. Re-wetting can also cause the discoloration to become visible again. This phenomenon has not been fully elucidated.
Green or blue discoloration usually only occurs on particularly dense concretes and disappears on its own within a few days or months.
However, if the desired light shade is desired after a short time, helping the surface to dry can have an accelerating effect.