Hollow Core Slab Production

[Albertina Drybread]

Thehollowcore production process begins with the drawing and design of a floor layout within a building. The usage of the building and loads applied will determine the depth, prestressed wire patterns and span of the slabs.

The design is accomplished using 3D CAD software. This can then be translated into a production plan for the hollowcore factory. Any additional details, such as the insertion of lifting pins, cut-outs and narrow units are included.

Production is accomplished by a team of factory operatives, and is often spread over multiple work shifts. Production planning is an important part of maximising efficiency and minimising waste of raw materials.

Hollowcore slabs are prestressed concrete elements. This means that they are reinforced by high strength, low relaxation steel wire or strand. The tendons are tensioned to a predetermined load prior to casting the slabs.

Due to the high forces involved, prestressing is a potentially dangerous operation, requiring strict health and safety procedures. It also requires specialist hydraulic equipment, either for single strand or multi-stressing.

Once the concrete has been batched to the requirements of the casting machine, it is delivered via a distribution system. Casting machines typically fall into 3 categories: extruders, slipformers and flowformers.

These can include: cleaning, oiling and wiring the beds, marking and detailing slabs, cutting slabs to required lengths, and equipment for lifting and stacking the finished product. Factories can range from labour intensive through to highly automated production.

The floor layout of hollowcore slabs in a building is designed using 3D CAD software. At this stage, design loads are used to calculate the depth and wire patterns of the slabs, depending on the spans required.

This is then translated into a production plan for the factory to maximise efficiency and minimise waste. Production plans include information such as cutting slabs to length, and detailing requirements such as insertion of liftings pins, notches and exposed cores.

Have you pondered how to increase the productivity of your precast plant cost-efficiently? How to save cement and assure capacity of your hollow-core slab production? If yes, read how a modern shear compaction extruder was developed and how it nowadays positively impacts hollow-core production.

Back in the 80s, we wanted to change the situation, so we came up with the first ever shear compaction extruder in 1985. Ever since, machine manufacturers have preferred the shear compaction method due to the many benefits it brings to the factory.

How does a shear compaction extruder work? The concrete is compacted by the means of a controlled three-dimensional motion. The compaction frequency is approximately 10 Hz. It is considerably less compared to vibrating extruders. Efficient feeding screws also play as essential role. They push the Extruder forward on the casting bed while compacting concrete.

Also, you may have spotted that problems sometimes arise because of a change in resistance due to friction from the strands, or a change in the concrete amount in the hopper. These problems can be avoided with automatic compaction. It resists or assists the extruding movement, whichever is needed at a given moment.

Comparing to the old Elematic EL900, and other extruders on the market, the Extruder E9 2010 has a more rigid frame. Thus the frame does not give in and the power goes to the concrete. The rigidity is achieved by using cast parts instead of parts made out of steel plate.

In addition, if you want to produce slabs with different cross sections, several nozzle units are available and we can advise you on the most suitable one. Also, if your customers need slabs of different heights, you can meet the need with Extruder E9 2010. It is designed for casting slabs of up to 500 mm high.

Efficient compaction means less strand slippage and more geometrically perfect slabs. You will also significantly save cement and decrease the amount of production errors. Your customers will thank you for fast deliveries of high quality slabs and thus you will win more projects in the future.

Spans can be anywhere up to around 20m, and applications range from individual houses to residential apartments, office buildings, hotels, schools, hospitals, supermarkets, industrial units and car parks.

Prestressed hollow core requires the use of high-quality raw materials, such as high strength concrete and low relaxation steel wire/strand. The produced elements have high load resistances thanks to a low water/cement ratio of concrete from 0.32 to 0.38. Moisture, temperature and plasticity control are considered of paramount importance for the mixing plant in order to get excellent hollow core slab finishing: concrete quality consistency must be assured for every batch of concrete.

MCT Italy has decades of experience in the development of technologies meant to ensure the best quality concrete as well as high performances for the entire production process, through the automation of its plants and machines specifically designed for this type of business.

MCT batching plants are designed to fulfill the strict demands in terms of concrete consistency through their humidity and plasticity controls. The long experience in this field allowed MCT engineering departments to develop and test a range of solutions such as moisture detection on aggregates and concrete, materials dosing control, water trimming and all related automation controls by customized software platforms.

Concrete distribution systems by flying buckets, one of the fields where MCT plays a leading role with its technology in the precast business, are one of the most important tools to ensure the quickest, cleanest and safest delivery of concrete to the production machines.

The Concrete Distributor type CG specifically designed for extruders and slipformers feeding for hollow core and wall panel production is a customizable machine with buckets installed on dedicated cranes. A large number of options feature the Distributor type CG, which is designed upon the actual factory and type of production machine, to become the smartest component of the entire production through special tools such as concrete consumption and flow rate control and lifting devices for the production machine.

Thanks to MCT in-house software engineering departments, the control platform for these plants has been developed to have the entire system working in fully automatic mode, in order to avoid the requirement of human operators or ground means to feed the production machines: through integrated sensors, concrete is automatically ordered to the batching system. The CG Concrete Distributor positions itself at the flying bucket discharge in order to receive concrete and it follows the extruder or slipformer during its casting operation, automatically managing the concrete feeding to the production machine at the proper rate as required.

Hollow core is extremely durable, retaining its structural capacity for a lifespan of 100 years or more. Production of elements in a controlled factory environment reduces waste, noise and emissions. Hollow core slabs contribute to the thermal mass of a building, and innovations enable reductions in the energy used for air conditioning.

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The Modifier E9 is an automatic machine for digging openings and recesses, for example for HVAC ducts and plumbing, into fresh hollow core slabs. It also drills waterholes and plots cutting lines, openings and text. The Modifier can recycle cut-off concrete back into slab production when paired with Extruder E9 or P7.

The Elematic Modifier E9-1200m makes digging openings into fresh concrete slabs on a hollow core production line accurate and flexible. Furthermore, the machine reduces the need for manual work in the modifying phase of the hollow core slab production process, which improves production efficiency, eliminates mistakes and improves safety on the shop floor. Furthermore, thanks to the smart concrete recycling system, the Modifier E9 also significantly reduces concrete consumption in precast plants.*

The Modifier E9 is designed for automatic operation but can also be used in semi-automatic or manual mode. Production plans can be transferred to the machine either via a wireless connection, or if needed, via USB. Wi-Fi communication hardware is included as standard. The Modifier E9 features a laser positioning system to identify the slab position.

NOTE! In order to use the automatic features , the machine requires a FloorMES machine control module (see below). When the Modifier is equipped with a FloorMES machine control module it is compatible with BIM design systems.

The Modifier E9 enables efficient concrete recycling. This is possible because it does not use any water in the digging process, which means that any concrete removed from openings can be recycled back into the slab production process. To ensure slab quality, the recycled concrete is mixed gradually into the fresh concrete according to a time limit; recycled concrete will not be fed back into the process if it is older than a preset factory-specific time value. The Modifier E9 and Extruder E9 are a perfect match, and together they increase the efficiency and sustainability of hollow core slab production. The Modifier E9 further reduces concrete consumption by using an advanced digging mechanism that ensures the minimum amount of concrete is left on the bed.

Like all Elematic machinery, safety features are built in. The Modifier E9 is fitted with safety buffers on both ends, including light sensors, which protect the area between the buffers. In addition, there are also ultrasonic sensors, which increase safety and also make it possible to pause operation when it comes close to an obstacle, like an extruder, for example. Operation can be resumed when the obstacle moves away.

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