Roads need to be hard-wearing.
Our roads have to stand up to constant stresses and strains. Concrete pavers or slabs have to be tested for resistance to the abrasive action of traffic, amongst other things. As there was no existing European standard for such a test, seven partners from six Member States came together to develop and validate a test method. Following much research and two rounds of extensive testing, a precast concrete paving product abrasion resistance test has been adopted in five draft European standards, and more than 20 test devices have been built.
Every year, in Europe, 300 million m2 of concrete paving units are laid. These pavers or slabs need to be able to survive the impacts of traffic. The concrete must not crack when the weather changes and it must be strong enough to carry heavy loads. The paving also has to withstand the abrasion that can really wear roads down. Replacing concrete is not cheap either: if just 5% of the paving laid each year needs be replaced due to poor quality products, the cost of replacement is estimated to be 350 million ECU.
Concrete producers need to be able to show their customers that their products make the grade both in their own country and across borders. If standards are insufficient, there is potential for national authorities to create trade barriers.
The search for an abrasion test
A European Directive for building and construction materials aims to create a single market for these products. It spurred the European Committee on Standardisation (CEN) to begin work to standardise tests for precast concrete paving products. At that time, CEN's special Technical Committee realised that a standard abrasion test would be beneficial to distinguish those products capable of withstanding intense traffic.
As a result, two concrete users, a concrete producer, three research institutes and a national testing laboratory began to discuss their experiences of abrasion testing. They decided to develop and validate an appropriate test method.
They started by looking at existing test methods. They found two candidates. The first is the 7 cm wide wheel test defined in a French standard. The second is the German standard test, also known as the Böhme test. The partners decided that the French test would be more appropriate. This test lends itself well to "type testing" and factory production control of newly made product samples. It can also be used to test existing concrete directly or in situ.
What does the test involve?
The test is quite simple in theory. A concrete sample is held vertically against a "wide wheel". This sample is pulled towards the wheel with a constant force and a constant flow of abrasive material is poured over the point at which the wheel meets the sample. At the end of the test, the tester removes the concrete sample and measures the groove cut into the block by the wheel. If the concrete resists abrasion, this groove is small and if it does not the groove is much larger.
Of course, there is much potential for varying results if the test is not performed in a standard way; the devil is in the detail. The partners had to check the performance of every part of the test apparatus. Both the apparatus and the abrasive must match precise specifications if the results are to be useful. The conditions for prior calibration are also vital. The operator must measure the abrasive flow rate under precisely defined conditions, measure the groove on reference specimens, check alignment of specimen support and check the dimensions of the abrasion wheel. The project team researched and tested all these criteria. They found that a marble sample would be the most useful as a reference.
Another aspect of the test is the sample preparation. When a product is deliberately designed to have an irregular surface, the abrasive can get stuck between the gaps and alter the test results. In these cases, the sample's surface has to be ground down so that it becomes more even.
Validating the test
In fact, all the details of how to perform the test, measure the groove and analyse the scatter of results had to be well-detailed. Once this had been done initially, a round robin of testing began. A number of laboratories used the test to see if they found similar results on two different samples: a British-made smooth finish slab and a French-made textured slab. The first intercomparison showed little variation between the laboratories but the description of the testing equipment, calibration and procedure still needed some improvements.
After five months of work, the partners were ready once again to subject the test to the scrutiny of a round robin test. This time, the results were very positive indeed and they showed that the groove measurements can be measured within 0.5 mm. What's more, the test can also be correlated with the German Böhme test. Overall, international agreement among laboratories was achieved and many technicians received training for the test method.
The results of this research project were transferred to the European Standardisation Bodies and, with just a few small adjustments, the CEN Technical Committee received recommendations for three draft European standards. These now form three draft standards covering concrete paving blocks, flags and kerb units. The test is also included in another two draft standards for natural stones and brick paving units.
According to the project's coordinator, Michel Vallès of CERIB, "All the clients buying pavings units in Europe will benefit from the project's results." This refers mainly to the construction sector where civil engineering contractors will be able to perform quality assurance tests more reliably. During the project, the partners have also made links to the national regulatory bodies for manufactured concrete testing so the tests will be used widely.
In economic terms, the test will ensure that suitable paving products are accurately identified for road building. This will mean that more road paving manufacturers will be forced to make more durable and longer lasting concrete, according to Mr Vallès. The market for the testing devices will also grow to some degree and manufacturers will benefit from the experiences of building about 20 of them.
The project participants all gained experience in concrete testing. Each developed a particular area of work and then tested the resulting machine. The team will work together again if the occasion arises. Results like these pave the way for future concrete durability research.