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Dry Goods Sharing | How to Make high quality Shotcrete?

Release Time:

2021-09-15

The production of fiber concrete is often a headache for concrete producers-especially when it comes to shotcrete production. Usually, fiber concrete is only used for special projects. However, the production of it requires the factory to be equipped with the corresponding equipment. We present important points for producing high quality fiber concrete for shotcrete.

Design of raw materials and mixtures for fiber concrete

Fibers are commonly used in the reinforcement of concrete. They help to avoid cracks in the hardened concrete or to improve the fire resistance of the concrete. The most commonly used fibers are steel, plastic, fiberglass and polypropylene.

When producing fiber concrete for shotcrete, it is necessary to use materials of high quality and constant composition.

The maximum aggregate size produced by shotcrete is 8-10mm, and larger particles may get stuck in the nozzle during the spraying process.

The amount of cement required to produce fiber concrete is quite high, sometimes up to 500kg/m³. In order to replace part of the cement, different binders are usually used, such as microsilica or limestone powder.

The water-cement ratio (w/c) is typically below 0.45. This means that it is almost impossible to obtain acceptable concrete workability without the addition of chemicals. Only with the appropriate amount of admixture, fiber concrete can be the best choice for pumping and spraying.

In addition, the concrete in the spraying machine also needs to add the accelerator. Accelerators can accelerate the setting and hardening process. This improves the ability of the concrete to adhere to walls and ceilings.

What do concrete plants need to consider?

The production of high-quality fiber concrete has certain requirements for concrete plants.

First of all, accurate dosing devices and weighing systems are absolutely essential. The deviation of each batch of ingredients should be as small as possible. If the aggregate is very different, the amount of binder and water required in the mixture will also be very different.

Secondly, the aggregate moisture must be measured automatically. This includes calculating the amount of water subtracted from the ingredients. Especially for mixtures with low water-cement ratio, even small calculation errors caused by incorrect aggregate moisture value will affect the quality of concrete.

Inaccurate batching or moisture measurements can have a negative impact on workability and solidification when spraying concrete. However, these problems can only be discovered through laboratory testing.

Other important factors include the control system and the mixer. The control system must be able to accurately meter all ingredients, accurately measure aggregate moisture, and automatically control the water-binder ratio and concrete consistency.

The most suitable mixers for mixing fiber concrete are planetary countercurrent mixers or twin-shaft mixers.

It is also very important to maintain a stable fiber concrete or shotcrete temperature. Any temperature change will affect the concrete hardening process. In addition, these changes are easy to see, for example, when spraying concrete for tunnel lining. Therefore, automatic concrete temperature regulation is essential for aggregate and water heating systems and plant batch control systems. The concrete temperature must be measured and recorded within the batch control system.

Green energy technology CSM three-dimensional shear countercurrent type mineral material mixing mixer

The advantages and disadvantages of different fiber addition methods:

1. Add the fiber into the drum of the concrete mixer truck

After the concrete is poured into the drum of the mixer truck, the mixer truck drives to the fiber batching station. There, fibers are added to the concrete: they can be added manually or through a batching belt conveyor.

Advantages: the use of concrete concrete mixing station mixer concrete mixing speed is very fast.

Since no fibers remain on the conveying equipment, there is no risk of fibers entering the non-fiber concrete mix.

Disadvantages: It takes a long time to add fiber and mix.

Despite the long mixing time, the concrete may still be uneven and the fibers may not be distributed evenly. As a result, clumps of fibers may occur, which may get stuck in the hose, pipe or nozzle. This metering method is not suitable for loose plastic fibers.

2. Add the fiber to the concrete mixer

Advantages: There is no risk of fibers entering the non-fiber concrete mix.

Disadvantages: It takes a long time to add fiber and mix.

① A suitable dosing device is necessary and sometimes difficult to adapt;

② The mixer area needs space for placing the batching device;

The need for longer mixing time, thereby reducing the capacity of the concrete mixing station;

Fiber clumps may still occur and may get stuck in hoses, pipes or nozzles.

3. Add fiber at the end of the aggregate weighing belt or lifting conveyor

In most concrete plants, aggregate weighing is carried out by a weighing belt conveyor. In this case, the fiber metering device can be installed next to the belt conveyor. This allows the fiber to be metered to the top of the aggregate.

Advantages: It is easier to add fibers on the belt conveyor than in the concrete mixer.

Disadvantages:

① Space for batching device and conveyor is required in the unit area of aggregate warehouse;

The batching and mixing time is longer, reducing the capacity of the concrete mixing station;

③ Fiber clumps may still appear and may get stuck in hoses, pipes or nozzles.

4. Add fiber on top of aggregate flowing on weighing belt conveyor

This method of aggregate batching and weighing system to organic linkage is possible to achieve. In this case, the automatic fiber batching system is installed next to the aggregate warehouse unit.

The fibers are continuously fed to the top of the thin aggregate bed as the aggregate flows on the weighing belt conveyor. This allows the fibers and aggregates to be pre-mixed before reaching the aggregate storage hopper.

Advantages:

① The fibers are evenly mixed; there will be no fiber lumps;

The mixing time is short, does not affect the capacity of concrete mixing station;

Applicable to all fiber types;

④ Easy to connect to aggregate box, even as a retrofit.

Disadvantages:

Some fibers may remain on the conveyor belt and may be mixed into concrete batches that are not intended for use. The production of fiber concrete, especially fiber concrete for shotcrete, requires many factors to be considered in the concrete mixing plant. It only needs high-quality materials and proper mixing design. In addition, accurate batching equipment, low batch-to-batch deviations, accurate aggregate moisture measurement and efficient mixers are all essential.

The fiber compounding method also plays a role. After all, it can significantly affect the quality of the concrete and the capacity of the plant.