Posted on Aug 22, 2007 in Construction Technology & Machines
Gautrain’s high-tech Pre-cast Yard is in the final stages of construction on the future Bus Depot site between Midrand and Malboro. The Pre-cast Yard will manufacture all the pre-cast components that make up the viaducts and bridges on Gautrain’s 80-kilometre route.
A twin batch plant with an average generating capacity of 120 cubic metres of concrete per hour will supply the various areas in the Pre-cast Yard as well as the related construction sites in the vicinity with ready mixed concrete. The pre-cast yard will be operational for the next two years. This Pre-cast Yard is one of the biggest in Africa.
The various sections to be cast in the pre-cast yard can be split up into M-beams,
Viaduct segments, Parapet sections for bridge decks and general pre-cast work. The biggest being the M-beams and the viaduct segments.
Pre-casting at Midrand Pre-cast Yard
M-Beams are mainly used in the construction of bridges. These concrete components rest on piers and abutments that support the bridge decks. M-Beams measure 1m wide and can vary in height (+/- 1,5m) depending the span.
These will be pre-cast in formwork (moulds). When ready to be installed, they are transported by means of low bed trucks to the various construction sites. Cranes will place the M-Beams in their required positions according to the bridge designs. The benefit of transporting ready-made M Beams to the construction sites is that scaffolding is not required on each site to construct the bridge decks and is time saving.
To allow for expansion and contraction of rail bridges, special bearing pads will be inserted between M-Beams and piers. M-Beams comprises mostly of pre-cast elements produced at the Pre Cast Yard. M-Beams will be pre-stressed. During pre-stressing, steel cables, running through the concrete sections, are put under tension and anchored at the ends. With this method of working the height of the beams can be reduced.
Viaducts are distinctly designed, multi-spanned bridges for rail over roads or rivers or valleys. A viaduct is thus a long bridge spanning in the order of 50m between piers. Viaduct Segments are cast in sections of about 2,5m in length can weigh up to 55 tons and assembled by means of a launching girder between piers. There are various viaducts along the Gautrain route with the longest spanning 650m.
The trapezoidal segments will be cast and later be attached to one another in line with the geometric shape and grading of the viaduct. They are 10,1m wide to accommodate two rail lines for trains travelling in both directions.
The viaduct segments are ”glued” together as part of the process to form the shape of the viaduct and to span the distance between the piers. Inside the viaduct sections are ducts for the placing of cables to do post-tensioning to keep the sections intact and to be able to carry the required/ design stresses.
Various heavy-duty cranes, that can handle up to 60 tons, are used to lift the pre-cast sections from their formwork in the Pre-cast Yard and placed in stacking areas. When the construction sites are ready to receive the required sections, it will be transported via road to the sites where cranes will put them into place.
A weigh bridge is also installed at the Pre-cast Yard to ensure that loaded vehicles leaving the site do not exceed their maximum road freight limit.
The first viaduct segment was cast at the Pre-cast Yard on 6 July 2007.
Intelligent twin batch plant
To suit the needs of a modern rapid rail link, concrete produced at the Pre Cast Yard will vary in strength up to 50MPa. The concrete mix of cement, aggregates (sand and stone) and water will also have admixtures so that pre-cast elements are not only strong enough but have improved properties to enable easier placement into the formwork and also rapid development of strength, particularly within the first three days.
Each batch plant is fitted with bins containing the required sand and stone. Concrete mix requirements are typed into a computer and an intelligent conveyor belt fitted with a weigh in motion scale that will automatically ensure that the correct amount of each material is released into the mixer from the bins.
Full time quality assurance will take place to ensure that all concrete batches do comply with design requirements.
After being removed from the formwork, the pre-cast elements such as M-Beams and Viaduct segments will be stacked on site until needed.
The production, storage and transportation of these pre-cast elements will certainly produce a challenge for engineers who need to correctly manage the supply and demand for these elements.
When the Pre-cast Yard has completed its function, it will be demolished to make way for the storage area of Gautrain’s fleet of air-conditioned buses. These buses form part of the 36 feeder and distribution services, serving all stations within a radius of 15km.