Chemical additives are added to the shotcrete mix to modify its properties. The most important one is the hardening accelerator, which modifies the early resistance of shotcrete required in underground work. The use of additives does not compensate for a bad mixture.
Additives are directly included in the mixture, with the exception of the accelerator. Conveyed by an additive pump, it is added immediately before spraying in the spraying nozzle of the equipment.
The correct accelerator dosage is key to the security, quality and economics of the shotcrete application. The concrete spraying equipment automatically supplies the dosage, synchronizing it with the concrete flow through the PLC system (Programmable Logic Control).
How does the synchronization of the additive dosage work?
- The shotcrete recipe is entered into the equipment’s control panel: binder content (cement and fine material), accelerator percentage, and accelerator density
- The additive is automatically metered proportionally to the concrete pump’s output
The accelerator dosage is calculated based on the binder content. For example, if the shotcrete recipe establishes a dosage of 4% and the amount of binder content per m3 of shotcrete mix is 400 kg, 16 kg of accelerator per m3 is applied. Knowing the additive density is essential for correctly calculating the dosage in liters.
Types of additive pumps
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Peristaltic additive pump
Peristaltic additive pumps work by siphoning the additive from the tank. A rotor pressurizes the hose to circulate the additive, so that the compressed side closes forcing the additive to move toward the spraying nozzle.
These pumps are robust and reliable, as well as economical and easy to maintain. A disadvantage is the elevated pulsations of this kind of pump; however, there are pulsation shock absorbers which eliminate them almost entirely.
Operation of a peristaltic additive pump
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Screw additive pump
Screw pumps have a smaller siphoning capacity than peristaltic pumps. Once the accelerator arrives at the pump body, the rotations of the screw make the accelerator flow to the spraying nozzle. With this system, pulsations are less; however, these pumps require more complex maintenance and have higher costs.
Operation of a screw additive pump
Additive flow measurement
The PLC measures the additive dosage through the liters theoretically pumped per hour by counting the revolutions of the rotor or the pump’s screw. For example, if the rotor needs to turn once to pump 0.2 liters of additive, it needs to turn 100 times to pump 20 liters.
Due to wear of the peristaltic or screw pump, the theoretical flow rate may differ from the actual flow rate. To correct this, a flow meter is used, which instantly measures the actual output. Processing this data, the PLC adjusts the pumping mechanism’s speed to ensure the required dosage.
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