You have me a bit confused. What are you doing?
In real concrete each gallon of water added to a yard of concrete
reduces the ultimate strength of the concrete by 500 PSI and
reduces the slump about 1". Not knowing what you are doing makes
it hard to help you, and I have no idea what strength material you
need or what strength design mix you are starting from.
Sand and cement with no stone is called grout. Grout is usually
used for a thin topping. It costs quite a bit more and is weaker
than concrete which adds well graded stone to the mix to extend
the materials. If you are pouring over 2" thick, use concrete.
A screed is a temporary guide to establish finish grade. It can
be a wet screed based on pins driven to grade, pipe screeds are
well supported steel pipe, lumber can be used, and metal screed
key can be used and left in. Screeds can be set with their tops
at finish grade or their bottoms set at finish grade. If you set
the tops to grade and pull them, they are pulled as soon as they
have been used and you float in fresh material to fill the ditch
they leave immediately so it is homogenous with the rest.
If the rod you are using to pull grade on the screeds is tearing
the top, try sawing it back and forth. The surface left by the
rod is usually not meant to be finish. The finishing comes from
hand or machine floats and hard trowels.
I have no idea if I've helped or not.
Keep the whole world singing . . . .
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