The castable mortar must not come direct contact with the insulation lining or else the water in the mortar gets absorbed by the insulation lining resulting reduced castable strength due to low water.
Most of the castable cracking occurs due to the above said reason only. 

So cover the insulation with polythene cover/sheet and then pour the castable mortar.This is one of the best method followed which avoid direct contact between castable mortar and insulation lining.
                                                                                                                                      
Now while placing the castable use vibarator to avoid blow holes or air holes/voids.This usage of vibrators provides excellent compaction and prevents air cracks.

What kind of vibrator is useful for castables?


Type:Needle type vibrator/poker type vibrator
Drive: Electrically driven
Type: High frequency
Poker diameter: 10 mm to 50 mm (as per requirement)
Capacity: 11000 to 16000 vibrations per minute

Even pneumatically driven vibrators are also available in the market which can used.

How to purchase an efficient vibrator?


There are major companies which manufacture vibrators.You can select any company based on:

·         Your own experience 
·         Performance of the vibrators in other companies
·         Warranty and maintenance conditions provided by the supplier
·         Life of the vibrators in other companies
It is safe to purchase two vibrator machines,keeping one for operational and one as stand by.
It was strongly recommended to purchase different diameters of pokers/needles to use at different places. 

 How to use the vibrators in castable mortar?



As per experience, it was suggested to insert the vibrator needle to maximum inside the castable mortar while installing the castables to get maximum compaction.
Never use vibrator at a single place.Move the vibrator poker slowly entirely along the castable mortar area.

How long we must use the vibrator inside the castable mortar?


 It is clearly based on experience or else use the vibrator:

·         Till completion of castable work
·         Upto 3 to 5 minutes after completion of castable work or Until the castable mortar attains its viscosity                                                                                                                                                                                                                                                               
(images of different size pokers, courtesy: www.healthdeep.com)            

What to be done if vibration to be done at places where vibrator cannot be used?


At narrow passage areas,use slightly more water mixed mortars as they pass very easily.You can use plain metal rods and poke inside to and fro inside narraow passages which also gives good compaction.
You can use very gentle hammering at required places.

How to find whether the area got good compaction?


After removal of the forms,if the area was without unusual grooves,ups and downs on surfaces,gaps,voids,irregularities etc., then it means that the good compaction of castable mortar.

Also if the surface was shining with almost flat appearance,then it is clear that the castable mortar attained good compaction.

Note: At any cost, for finishing appearance never try for more smoothening the surface using trowel.