Producing fibre concrete, especially when shotcrete is concerned, might sometimes cause headache for concrete producers. Usually, fibre concrete is required for special projects only and if so, the concrete plant needs to be equipped accordingly and must work reliably. In the following some important points are mentioned, which might help to produce high quality fibre concrete for shotcrete purposes.
Raw materials and mix-designs for fibre concrete
The most important thing when producing fibre concrete to be used as shotcrete is to only use materials of high quality and constant composition.
The maximum aggregate size for shotcrete is 8-10 mm, otherwise problems when spraying would be programmed, as bigger sized grains might get stuck in the nozzle.
The amount of cement required is quite high, sometimes even 500 kg/m3. Different binders are used to replace some of the cement, e. g. micro silica or limestone powder. In fibre concrete for shotcrete purposes the w/b-ratio usually is below 0.45, which means that it is almost impossible to get acceptable concrete workability without the help of chemicals. With the right amounts of suitable admixtures though, fibre concrete can be mixed, which is optimal for pumping and spraying.
Fibres with concrete normally are used as reinforcement, to avoid cracks in the hardened concrete or the increase the resistance against fire. The most commonly used types are steel, plastic or glas fibres.
When spraying, in the spraying machine an accelerator is added to the concrete. The accelerator speeds up the solidification and hardening process and with this improves the ability of the concrete to stick to walls and especially ceilings.
Fibre concrete requirements for the concrete plant
Producing high quality fibre concrete makes some demands on the concrete plant.
First of all, in the manufacturing process, accurately working dosing devices and weighing systems are absolutely essential, with the standard dosing deviation from batch to batch being as low as possible. If there is a high variation in the amount of aggregates, there also is a high variation in the amount of binders required and the demand of water in a mix.
The second relevant point in the manufacturing process is measuring the aggregate moisture. Measurement of the aggregates’ moistures and correction of the additional water to be added to the mix, must be done automatically. Especially for mixes with a low W/B-ratio, even a small mistake in the calculation of the amount of water caused by a wrong value of total moisture in the aggregates will affect the concrete quality. Inaccurate dosing and ineffective moisture measurement usually will be detected by laboratory tests only, negative influences on the workability and solidification however, already when spaying shotcrete or pouring.
Other relevant features for the concrete plant are a sophisticated control system and a high-performance mixer. The control system must be able to care for accurate doing of all ingredients, for precise measuring of the aggregate moisture and for automatic control of the W/B-ratio as well as of the concrete consistency. Mixers perfectly suitable to mix fibre concrete, are, for example, planetary countercurrent mixers or twin-shaft mixers.
How to dose the fibres and what are the pros and cons of different dosing methods?
1. Dosing fibres directly into a concrete truck mixer’s drum
After the concrete has been discharged into the truck mixer drum, the truck drives to the fibre dosing station, where the fibres are added to the concrete, either manually or via a dosing belt conveyor.
Pros+ The mixing of the concrete with the plant’s mixer requires less time.
+ There are no fibres left on conveying equipment, therefore no fibres in non-fibre concrete mixes.
Cons- Adding of fibres and mixing takes a long time.
- Despite the longer mixing time, the concrete may still be very inhomogeneous, with fibres not equally spread. There might occur fibre clumps, which will easily get stuck in hoses, pipes or nozzles.
- This dosing method is not suitable for plastic fibres.
2. Dosing fibres straight into the plant’s mixer
When using this dosing method, a proper dosing device to the mixer is required.
+ There are no fibres left on the conveyor.
Cons- Space for the dosing device and the conveyor required in the mixer area.
- There might occur fibre clumps, which will easily get stuck in hoses, pipes or nozzles.
- This dosing method requires a prolonged mixing time, which reduces the capacity of the concrete mixing plant.
3. Dosing fibres at the end of the aggregate weighing belt or lifting conveyor
In most concrete plants, weighing of the aggregates is done by a weighing belt conveyor. If so, it might be possible to install the fibre dosing equipment beside the belt conveyor, allowing to dose the fibres on top of the aggregates on the belt.
+ There are no fibres left on the conveyor.
Cons- Space for the dosing device and the conveyor required in the aggregate bin unit area.
- There might still occur fibre clumps, which will easily get stuck in hoses, pipes or nozzles.
- This dosing method requires a prolonged dosing and mixing time, which reduces the capacity of the concrete mixing plant.
4. Dosing fibres on top of the aggregates flowing on the “flying” weighing belt conveyor
This method applies with Tecwill’s “flying” aggregate dosing and weighing system only and an automatic fibre dosing system installed beside the aggregate bin unit.
While the aggregates are “flowing” on the weighing belt conveyor, the fibres get dosed continuously on top of the thin aggregate bed. This leads to a premixing of fibres and aggregates already before they reach the aggregate holding hopper. With this method fibre clumps are avoided and the plant’s mixing capacity does not get affected.
Pros+ There are no fibres left on the conveyor.
+ Fibre clumps do not occur. Fibres will get mixed in homogenously.
+ Short mixing time.
- Extra space for the dosing device and the conveyor required in the aggregate bin unit area.
Producing fibre concrete, especially when shotcrete is concerned, requires materials with good quality and a goal-orientated mix design. Accurately working dosing devices, a low standard deviation from batch to batch, precise moisture measurement of aggregates and a compulsory mixer are absolutely essential. A not suitable fibre dosing method may affect the quality of the concrete and lead to problems especially when spraying shotcrete.
Tecwill concrete plants and fibre concrete
Automatic fibre dosing in connection with Tecwill’s “flying” aggregate dosing and weighing system is the best possible way to constantly produce homogenous and high quality fibre concrete, without negative influence on the production capacity of the concrete mixing plant.
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