Prestressed concrete sleepers for the American continent

GIC Ingeneria y Construcción has been one of the leading building contractors in North America for over 30 years. Precast concrete elements such as hollow decks, wall elements, columns and beams for residential and industrial construction as well as concrete girders for bridge and road construction can now be found in over 700 building projects.

For further growth the GIC group invested in a new precast plant for prestressed concrete sleepers in 2013. “A sustainable plant concept, flexible processes for an increase in capacity later on and high safety at work were important to GIC in the design of the mould circulation plant. That’s what we focused on”, says Steffen Schmitt, Senior Sales Manager at Vollert Precast Solutions.

Thanks to its compact plant structure, the pallet circulating plant has a footprint of 1,200 sq. metres. In several stages of development, a maximum annual production capacity of 350,000 prestressed concrete sleepers is to be achieved. In 2-shift operation that is equivalent to a daily output of over 1,300 concrete sleepers. 280 sleeper moulds are in constant circulation for this.

Production of the prestressed concrete sleepers began in March 2014. The quadruple sleeper moulds are oiled and cleaned first before the installation of the anchors for the fastening of the rails later on. Subsequently, the prestressing wires are installed in the mould prior to concreting. With the aid of a button heading machine, four individual wires are connected with two anchor plates to make an assembly before the anchor plates are fastened to the tensioning and anchor bolts in the sleeper mould. Subsequently, the wires are prestressed by means of the tensioning bolts whilst constantly monitoring and recording the tightening torque of each prestressing wire. In order to ensure high safety at work, pressure-sensitive mats are installed in this work area. As soon as a worker steps on the pressure-sensitive mats the prestressing procedure is interrupted and the transport of material is stopped.

The preassembled mould is transported onwards by roller conveyors to the soundinsulated concreting chamber. The semiautomated concrete distributor fills the moulds precisely via electrically driven discharge screws. A high frequency vibrating station ensures homogeneous concrete compaction. Floor space for a further compaction station has already been earmarked in this work area for an extension in capacity later on. The concreted mould is subsequently transported via a corner transfer conveyor and chain conveyors to the hardening chamber. There, an overhead crane takes up the quadruple mould by means of a suspended spreader bar and stacks up to eight moulds on a prepared trolley car. The hardening chamber is thermally insulated and equipped with a heating device. The trolley cars move through the hardening chamber on rails according to predefined cycle times. To this end a chain pusher pushes the entire train of cars one car length further at a time. In the exit area a further chain pusher pulls one run-off carriage at a time out of the hardening chamber following a hardening time of about 20 hours. A total of three transport for the transport and for the turning of the mould”, explains Steffen Schmitt. First of all the moulds are taken off the run-off carriage and stacked on a chain conveyor. The prestress release process is subsequently initiated via a semi-automated detensioning station. To do this the prestress is introduced into the concrete sleeper.

At the end of May 2021, the central metro station ‘November 8’ was opened within Metro Line 3. In the process, the section between the Avtovaghzal, Memar Ajami and 8th November metro stations was newly built on the basis of the Vollert Precast Solutions-Rheda system. This is special because ‘November 8’ is considered an important junction between metro lines 3 and 4 and plays a central role in connecting downtown Baku to the metro network. This was preceded by a number of important planning phases. In the process, a whole series of questions were solved: For example, how must the substructure for the slab track in the tunnel sections lined with tubbings be constructed? What type of reinforcement is to be used? How strong must the concrete be and what type of concrete is best suited? What is the best solution for waste water? What kind of turnouts can be planned and how are they fixed to the concrete? And what needs to be considered at the transition to the sections with wooden sleepers?

In the actual, first construction phase, the old wooden sleepers were removed in the affected section and the existing concrete foundation was demolished. At the same time, extensive pipelines were laid to bring the ready-mix concrete to the construction site and important wastewater issues were resolved. In the second construction phase, a concrete layer was first laid for a smooth underside of the track before the steel plate was fixed, further superstructure elements were laid, the conductor rails were installed and the concrete sleeper blocks were connected to the supporting plate via the reinforcement installed. The sleepers were transported to the construction site just-in-time in individual daily sections during the entire construction phase.

“For the concrete sleepers themselves, we didn’t want to rely on imports from abroad,” explains Zaur Huseynov, CEO of Baku Metropolitan. “Just-in-time means we have to supply the construction sites quickly and reliably. During the preliminary planning, it also became clear that the plant location should be as central as possible within reach of the planned modernization and new construction sections in order to keep transport times short and costs low.” At the beginning of 2017, the first sleeper block was produced at the new Baku Metropolitan concrete plant. For the plant technology, the company relied on a partially automated concept and the proven machine technology from Vollert Precast Solutions. For the special environmental conditions in Baku with maximum temperature differences in summer and winter, a proprietary concrete recipe and a test plan for continuous quality assurance for fresh and hardened concrete testing were created. The novel sleeper design was laid out by Vollert Precast Solutions engineers with regard to the maximum axle load of 15 tons and the maximum top speed of the metro trains of 50 km/h, and the design parameters for the structural analysis were also adjusted. All this ensures optimum stiffness and uniform distribution of the vibration force. With 20 formwork molds, up to 30,000 sleepers are currently produced annually, which corresponds to a daily production of approx. 1,300.

Subsequently, the turning spreader bar turns the mould by 180 degrees, before the shake-out process is carried out by means of an electric lifting mechanism. After shaking out, the mould is turned back by 180° and fed back to the mould circulation. The manufactured prestressed concrete sleepers are delivered ready for laying. For this the entire reinforcement is installed in the sleeper and prestressed according to the standards. The rail fastening is also ready installed. The demoulded sleepers are transferred to the rail support assembly for this.

“Vollert Precast Solutions was the right know-how partner for GIC for both the engineering and the plant technology. We have found the correct balance between the degree of automation and high plant productivity. In further stages of development we can easily adapt our new precast plant to the output quantities that we are aiming for”, says Mauricio Gutiérrez, Sales Director at GIC. “At the same time, particular attention has been paid to safety at work in every work procedure.”

In further stages of development we can easily adapt our new precast plant to the output quantities that we are aiming for”, says Mauricio Gutiérrez, Sales Director at GIC. “At the same time, particular attention has been paid to safety at work in every work procedure.”

With this investment GIC has taken a further important step into the future. It is now aiming not only at the Mexican sales market, but also at international markets and in particular the US market.