Dry screed - Not Great!

[Monsoon]

Today I had a chance to put my research on screeding into practice.

The screed thickness varies from 75mm - 25mm due to the unevenness of the old concrete slab.

Taking advice from an old post I went with
2 part granite chippings
2 part sharp sand
1 part cement

My 1st mixer load was quite dry. The way I would describe it, taking a handful of the mix you could form a concrete snowball, if that makes sense.

Now the problem I found was getting an acceptable finish with this mix. As I tampered it down I expected the surface to smooth as the 'fat' moved to the surface.

This didn't happen and I was left with a grainy gritty finish.

I decided to change the mix and add more water.
1 part granite chippings
3 part sharp sand
1 part cement

With more of the finer aggregate and more water I got my 'fat' and was able to trowel a smooth finish.

Have I compromised the integrity of the screed by making these changes?

Also, using a shovel as my measure of the sand,chippings and cement, I found a slight problem.

The chippings tended to form a small pile on the shovel, where as the sand a large pile. I suspect by weight they would have been roughly the same as the chippings are far denser, but by volume I was getting a more sand than chippings.

 

[^woody^]

Granno is normally mixed something like 6:1:2 (grano, sand , cement)

Did you finish it with a steel or wooden float - steel for grano, wood for screed - wood tends to leave a grainier finish

Also, the mix has to be just right, as you dont want to have to tamp and bring fat to the surface, but rather just compact, level and then trowel smooth - without working it too much.

You should mix by volume - either use a bucket, or just make sure you pick up a bit less sand to match the grano!

 

[Monsoon]

I'll go with a bucket to get the measuring right. I've still got approx 20m2.

The 2,2,1 mix was suggested by Noseall here
//www.diynot.com/forums/viewtopic.php?p=526597#526597

I used a steel bull nose float - so ok there.

Maybe my 1st mix was just too dry.

Will the wetter mix be weaker or does it just mean it will take longer to dry out?

 

[^woody^]

Will the wetter mix be weaker or does it just mean it will take longer to dry out?

Theoretically, there is a point of optimum water content/maximum strength.

But there are so many factors which affect strength involved when you mix, that its not relevent.

The main issue with a 'too wet' mix is that water comes to the top, ponds, makes achieving a flat surface difficult, and when it evaporates, leaves depressions. Also air bubbles will pop up and leave holes.

The biggest problem, is that a traditional 'belle' mixer does not mix screed very well

 

[Monsoon]

The biggest problem, is that a traditional 'belle' mixer does not mix screed very well

Cheers - My get out of jail card ... damn those Belle mixers