Sprayed Concrete Admixtures and the Process
Sprayed concrete or Shotcrete is pumped to the point of application projected in high velocity used for rabid construction.The applications are frequently vertical or overhead and this requires rapid stiffening if slumping or loss by concrete detaching from the substrate under its own weight is to be avoided. In tunneling applications, sprayed concrete is often used to provide early structural support and this requires early strength development as well as very rapid stiffening. While spraying the concrete maintained the distance between at least 1 to 2 meters from the nozzle.
Admixtures can be used in fresh concrete to give stability and hydration control prior to spraying. Then by addition of an accelerating admixture at the spray nozzle, the rheology and setting of the concrete are controlled to ensure a satisfactory build-up on the substrate with a minimum of unbonded material causing rebound.
There are two spraying processes:
- The dry process where the mix water and an accelerator are added to a dry mortar mix at the spray nozzle.
- The wet process where the mortar or concrete is pre-mixed with a stabilizer/retarder prior to pumping to the nozzle where a liquid accelerator is added.
The wet process has become the method of choice in recent times as it minimizes dust emissions and gives more controlled and consistent concrete.
Accelerators are of two types:
- Alkaline, which gives the quickest set, often less than 1 minute, and very early strength development but presents a safety hazard to the applicators and tends to give low later age strengths.
- Alkali-free, which can take tens of minutes to set but is safer for the applicator and has little effect on later age strength.
Sprayed concrete admixtures are often affected by the cement chemistry and their use requires technical and practical expertise. For these reasons, their use should be left to specialist applicators.
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