Facades get soiled due to weather conditions and environmental impacts. Dirt deposits on the facade surface alter the original appearance requiring regular cleaning. Soiling can take many forms depending on the building material, the way it has been treated and the building’s location.
Dust and soot or airborne particulates: If a facade is porous, dirt particles do not get washed off by rain but get trapped in recesses.
Oily and greasy substances: They adhere even to smooth surfaces. Being sticky, they also attract other dirt deposits.
Aggressive gases: These combine with water to form acids that attack the facade material and lead to chemical weathering.
Usually, several types of soils are found simultaneously on facades making it difficult to give them the appropriate cleaning as applicable to treating different soils.
Generally, three methods are now used for cleaning facades:
• Wet cleaning
• Moist cleaning
• Dry cleaning
Wet cleaning methods include mechanical cleaning, particle blasting with air, and dry ice blasting. Moist cleaning is particle blasting with the addition of air and water.
Wet Cleaning Methods
Wet cleaning methods include pressure washing with cold or hot water, with or without the addition of cleaning agents. They also include particle blasting with water and ultra-high pressure washing.
Dirt clings to facades mainly as a result of electrostatic attraction and also by mechanical inter linkage due to the surface structure. Chemical binding forces can further encourage this.
After identifying the mechanisms that bind dirt to the surface, the next step is to apply suitable methods to make forces that are stronger than the dirt particles’ bond with the surface act on those particles. Only this can deliver a reliable cleaning outcome.
In the case of pressure washing, mechanical, thermal and chemical factors play a role. The crucial factor for dislodging a dirt particle is contact pressure per surface unit, or the pressure at which the water jet meets the surface to be cleaned.
Contact pressure depends on the spraying distance, spraying angle, nozzle pressure and flow rate. The nozzle pressure is determined by the flow rate and the nozzle profile. As the distance from the surface increases, contact pressure drops rapidly. Depending on the nature of the surface and the type of soiling, operatives should work at distances of between 10 and 30cm.
The influence of the spraying angle on contact pressure is also crucially important. A narrow spraying angle (of 0°, 15°, 25°) achieves a high contact pressure on a small area. Wide spraying angles (40° or more) result in correspondingly large contact areas and lower contact pressures. To remove stubborn dirt, a spraying angle of 0° to 25° is recommended, for moderate soiling 25° to 40°, and for lightly soiled surfaces 40° or more.
Volume of water
The volume of water is of critical importance for contact pressure. Large quantity of water produces a high contact pressure because the jet of water remains more concentrated (less atomisation). This effect is reinforced by the special contours of power nozzles, which increase contact pressure by up to 40%. The volume of water also determines the extent to which the dirt dislodged is washed away. These are the two main factors that determine the time frame for the work assignment. A water volume of around 1,000 litres per hour is therefore recommended for facade cleaning.
Water is also the medium for temperature and cleaning agent. Raising the temperature from 12°C to around 80°C with the help of a hot-water pressure washer can achieve the following:
– The same cleaning result at lower contact pressure
– A better cleaning result at the same contact pressure (more gentle!)
– Better removal of oily and greasy dirt
– Shortening of cleaning time by up to 40%
– Faster drying of the warmed facade
Cleaning agents in two-step method
Occasionally, the mechanical and temperature factors described above do not deliver a satisfactory cleaning result. In such cases the only solution is to add cleaning agents that improve the water’s cleaning properties by increasing its wetting capability and emulsification and by direct chemical reaction with the dirt components.
As a rule, the two-step method is applied in such cases.
The first step is to spray cleaning agent (in the concentration prescribed for the particular degree of soiling) on the surface to be cleaned. The cleaning chemical is left for an appropriate time to take effect but must not be allowed to dry. It is important to apply the cleaning agent section by section, working from bottom to top. Direct sunlight must be avoided. The second step is to use a pressure washer to rinse off the loosened dirt with clear water, working from top to bottom. In this case, the mechanical factor and the cleaning agent work in parallel.
The advantage of the two-step method is that the cleaning agent has time to take effect, which has a positive impact on dirt removal. The second step using clear water reliably removes all the loosened dirt residues, along with cleaning agents.
Gerd Heidrich, Training Instructor
at Alfred Kärcher-Vertriebs GmbH
Niels W. Buhrke, freelance journalist