Bubbles when precasting concrete step

[mrbrunes]

Hi

I'm casting some concrete steps with a C30 mix in a timber former, but have found after two attempts that air bubbles are persisting on the step surface i.e the bottom of the former. This is despite lots of tamping down and using a wetter mix on the second attempt. I was thinking about doing the step in separate layers and using an even wetter mix for the first layer but was concerned that using a too wet mix might affect the durability of the step surface.

Any ideas how to precast steps without bubbles?

 

[Tigercubrider]

one suggestion would be to tap the former a lot to encourage bubbles to rise?
although only with plaster, I know people have used sanders to vibrate the moulds and get air out.
Maybe one of those oscillating cutters with a modified blade (bit of rod added?) would act as a poker?

 

[^woody^]

Use less water, not more

Use a good release agent on the shutter

Use different fines, something like granno or more sand than stone

Use an anti-foaming agent in the mix

Use a poker or a hammer drill on the shuttering

Strike the shutter as soon as the concrete has set enough and brush on/in some cement slurry while the concrete is still wet

 

[tell80]

1:2:3 mix (cement, sand and stone) using chippings with small stones will work.
Any re-bar?
Veg oil or cooking oil will work for oiling the forms.
Bucket the mix in from close range - no dropping from a mixer etc.
wet mix bad - have the mix just so it flows and tamp in layers with a 2 x 4 offcut.
lightly tapping either side with two hammers will bring air up - so will steel trowelling after the mix flashes.

assume you are using individual steps - full single pour flight of conc steps are well different.

 

[^woody^]