The car manufacturer Porsche assigned STRABAG in Bratislava to find a solution for a problem that was not at all trivial: For a planned factory access road, the previously contracted earthworks company had constructed the so-called subgrade much too high. The subgrade is the upper connecting surface of the subsoil on which the actual road is built. This subsoil must be level and true to profile and may only deviate minimally – no more than a few centimetres – from the target height.
In this case, however, the corresponding gravel fill was much too high. There was not enough space left for the planned pavement surface – so, what could be done? Zsolt Boros, Division Manager TPA Society for Quality Assurance and Innovation s.r.o. – Central Division TPA 04, Subdivision Czech Republic / Slovakia, Slovakia Business Unit – was one of the contacts for this project at the time. He recalls: ‘What we were looking for in order to solve the problem was a synergy between road design and construction technology, using all the possibilities of the available construction machinery. Together with the BMTI, we decided that we are able to solve the problem.’
The aim of all considerations was to leave the gravel layer that had already been applied. At the TPA laboratory in Bratislava, tests with different materials for road construction were immediately prepared. The subsoil of the site was carefully investigated and a new assessment approach for an alternative road structure design was tinkered with. This was developed on the basis of standard procedures applicable in Slovakia for the design of flexible and semi-rigid pavements. These calculations also included the results of engineering geological investigations, a geotechnical report and the results of the bearing capacity tests that the team had carried out on the ballast layer at hand.
Paving stones instead of cement
The solution developed on the basis of these preliminary investigations is a road construction with a surface layer of concrete paving stones. Originally, a cement concrete layer was planned here. The pavement consists of partially automated conventional L-shaped Uni-Coloc concrete paving stones and a special unbound binder course, which was placed and pre-compacted with a paver. It requires a much smaller thickness than cement concrete and is also much more cost-effective to produce. The necessary load transfer is provided by the alternative hydraulically bound robust base course. In Germany, this type of road construction for heavily loaded areas is less widespread, whereas it is very common in England, Holland and Asia.
An unconventional idea was also found for the base course underneath, for which special evenness was required: A hydraulically bound base course (HGT) built using the cold recycling method with an earth-moving machine and levelled with a laser-leveled precision grader, which was installed directly under the pavement (including unavoidable bedding). The material of the gravel layer was crushed on site using an earthmover and bound with a custom-mixed hydraulic binder. This made it possible to make positive use of the given boundary conditions: The lower soil layer was not affected, and the original gravel material was adapted to with a hydraulically bound mix that ensured sufficient strength and evenness, even at the existing height level. Moreover, additional material did not have to be transported away, but was processed directly.
This boldly unusual, alternative way of road construction achieves comparable results in tests with conventional constructions in terms of evenness and strength and yet meets all the evaluation criteria of the relevant regulations. Zsolt Boros also praises the good cooperation between TPA and BMTI: ‘Our success is based in equal parts on the innovative road and technology design as well as the high precision in the work of the machines in use.’ Boros sees future applications for such base layers in large industrial areas and also in motorway construction.