There are basically 2 single application methods for shotcrete placing.
1. Dry process
The dry mixture of shotcrete is conveyed inside a hose by air. Just before the end of the nozzle, water and accelerator will be added in the same time than air in order to push the materials up to a velocity of 70m/s. Water addition is under control of the nozzle-man.
The dry-mix method summary
Low investmentSimple handlingNearly unlimited distancesDamp surfacesConveyance of gravel, sands
High dust emissionHigh rebound (trials with dust-binder, pre-wetted raw material are reducing these disadvantagesWater/Cement ratio depends on the operatorApproximately 1.7 m3 dry mix for 1 m3 concrete at the wall
2. Wet process
Wet process consists on normal mixed concreted at the batching plant, of which the maximal size is 16mm. The accelerator is added just before the end of the nozzle. Wet process consists of 2 ways to convey shotcrete: thin stream and thick (dense) stream methods.
The Dense Stream Method (to pump)
Low air consumptionDistances up tp 100-150 metersCan be used for pumping/ backfillingConstant low W/C ratioSteel finer sprayingHigh output up to 30 m3/hLow wear costs
Approx 1.1 me wet mix for 1 m3 concrete on the wall
Complicated start and cleaning procedure, if not special retarders are used in the last batchPumpable concrete is neededSlump (cone) limited 10-15cmRigid control of aggregates in the mixRigid time control for the transportation of the wet mix or retarders have to be added
The Thin Stream Method (Air Suspended)
Simple HandlingNo need of pumpable concreteStops and starts without cleaning and pre-lubricationLow additives consumptionDry spraying without change of equipmentConveyance of gravel, sands
High flexibility with the slump (cone)
Low W/C and constant W/C ratio
Steel fibre spraying
Approx 1.1 m3 wet mix for 1 m3 at the wall
Higher conveying air consumptionLimited distancesRigid time control for transportation of the wet mix or retarders have to be addedProblems with fine aggregatesHigher wear costsDistance up tp 40-6- meters in wet methods
Nowadays, around 70% of the shotcrete are wet process application. World-wide tendency is wet process, except Austria (>95% dry process) and Germany (>60% dry). Total shotcrete production is greater than 20 million cubic meters per year all over the world.