Concrete admixtures are used to improve and/or change concrete properties which cannot, or cannot correctly be controlled by the cement, aggregate and water. Admixtures are also added to sprayed concrete during the spraying process to regulate the start of setting. Concrete admixtures and additives make concrete a complex multi-material system.

Sprayed concrete admixtures are added as a percentage of the cement or binder weight. They are added in an approximate range of 0.5 % to 7.0 %. This gives quantities of 2 kg/m3 to 32 kg/m3, that is in the range of thousandth parts of the total concrete volume. All the admixtures used are fed into the concrete during its production at the mixing plant after the initial water metering. Main exception is the shotcrete accelerator, which is adding immediately before spraying.

Target specifications for the use of sprayed concrete and mortar additives / admixtures

Sprayed Concrete
Target Specifications
 
Control Parameters   Concrete Admixtures for
Target Achievement
 
Compressive strength
Flexural strength
Durability
Set concrete characteristics Water reducer
Additives
Fiber reinforcement
Curing agents
Pumpability
Sprayability
Spraying configuration
Workability Mix stabilizers
Additives
Water reducer
Strength development Setting and hardening Shotcrete accelerators
Water reducer
Working time Open time Setting retarders
Slump keeper