At the end of last year, staff at the Fraunhofer Institute for Building Physics IBP’s Holzkirchen site received an e-mail with a curious request. Chewing gum would now be available in the cafeteria, it said, and asked the recipients if they wouldn’t mind chewing a piece of gum for five minutes and then throwing it into special containers provided. But what was it all about? Behind the odd proposition, sent by qualified mineralogist Sebastian Dittrich from the Concrete Technology and Functional Construction Materials working group, part of the Building Chemistry, Building Biology, Hygiene department, was a project relating to the cleanability of paving stones. Removing tough dirt from sidewalks poses big challenges for local and municipal authorities every year.
Countless events of all different sizes take place every year in Germany’s public squares and pedestrian zones. These events leave a lot of garbage as well as permanent traces behind – and paving stones take the brunt of it. For example, the crowds attending street events and other passersby drop cigarette butts and chewing gum on the ground, while stalls and kiosks often leave behind oil, drink, ketchup and mustard stains. The cleaning costs involved in keeping cities looking tidy and well-maintained are high, and when not covered by the event organizer, it is taxpayers who foot the bill. In the city of Duisburg alone, A staff of 212 persons is responsible for cleaning 107,000 square meters of what are known locally as “bazaar streets” – in other words, shopping streets – as well as other roads, sidewalks and bike paths. Depending on the local street-cleaning statutes pedestrian zones must be cleaned several times a day. How long cleaning takes and how much it costs varies according to the type and quality of the paving stones or other surfaces in place. Certain physical properties make some paving stones easier to clean than others. "If this was taken into account when planning public spaces, it would be possible to make a rough advance estimate of the costs for removing persistent dirt and even to reduce some of the costs," says Dittrich.
On the initiative of municipal cleaning services, nine major German and Austrian cities have come together for a project to tackle this problem. Three paving stone manufacturers, a company that makes cleaning equipment, and a producer of cement-based joint mortar and epoxy paving joint mortar have also come on board. The consortium’s objective is to study the influence of various physical parameters on the cleanability of paved inner-city areas under real weather conditions.
This is where Fraunhofer IBP comes in. As well as possessing valuable know-how, Fraunhofer IBP researchers also have the world’s largest outdoor testing site along with suitably equipped laboratories at their disposal to provide the project with scientific support. As a first step, 30 different types of paving stones were laid across a total of 420 square meters on the testing site. "The paved areas consist of concrete blocks with different surface characteristics. We used both coated and uncoated blocks. As the jointing medium, we are mainly using sand, which is the usual option for paving stones, but we are also analyzing solid jointing materials based on cement and synthetic resin," says Dittrich. At regular intervals over the course of two years, the blocks will be exposed to various specific types of dirt and stains – including ketchup, cola, pigeon droppings, chewing gum and oil – and then cleaned using an ordinary commercially available street sweeper. The types of dirt used will be those that cleaners in urban centers typically find so difficult to remove. For the most part, a dry brush is used for the cleaning. In accordance with a defined cycle, the researchers carry out wet cleaning with a so-called scrubbing deck about once a month.
2D cameras and 3D scanners are used to assess cleanliness before and after cleaning. They help the Fraunhofer scientists evaluate the efficiency of cleaning methods in relation to the dirt type and the stone surface. The tools also aid the scientists as they use the surface topographies of the stones to determine whether their surfaces change over time as a result of the dirtying and cleaning cycle; for instance, becoming smoother or more slippery. Surfaces in public areas have to meet a defined numerical skid resistance value, which can be measured using a British pendulum skid resistance tester (SRT). "The SRT measures the microroughness – essentially the grip – of surfaces and is currently approved in Germany for control measurements in road construction," explains Dittrich. These and other mechanical tests to characterize the blocks, such as determining compressive strength, abrasion resistance, and wetting ability (i.e. how well liquids can spread out), are to be carried out at the beginning and the end of the project, in order to detect any influence from dirt and cleaning on the durability of the blocks.
In the long term, the findings from the experiments should increase the efficiency of street cleaning, bringing down the very high cleaning costs in various areas of city centers faced by cities and municipalities. In addition, the results should also serve as a guide for city planners when choosing the most suitable combination of paving stone and jointing to use in cities. Ideally, a catalog will have been compiled by the end of the project, which will specify in euros per square meter the estimated cleaning costs for any kind of paved area and dirt type. To help make this a reality, the selfless personnel at Fraunhofer IBP are happy to keep chewing gum for a while yet.