Surface cracking sounds like a moisture problem to me.

Possibilities:
The forms are dry (absorb water) or allow water to escape rather
than hydrate and cure the concrete. Are you using some type of
form release? You might try wetting the form work to aid in
concrete placement and to prevent form material absorbing
moisture.

You mention applying fiber reinforcement. It would be normal here
to blend the fiber throughout the concrete mixture.

You mention keeping the mixture quite dry and plasticizing with a
waterproofing agent. I assume this is to avoid shrinkage cracking
and to maximize ultimate strength. You have no aggregate to speak
of and are using dust rather than sand. Many powder type products
demand mixing time and a resting time to absorb water and a
retempering.
Are you bulking by volume or by weight? It still sounds like loss
of moisture, insufficient moisture, or absorption. I would not
depend on a waterproofing additive to provide curing. I assume
your mixture is a hydraulic concrete that will set under water.
Can you provide wet cure? - flood, submerge, steam, tented moist
cure.



(top posted for your convenience)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Keep the whole world singing . . . .
DanG (remove the sevens)




"tghattaq" wrote in message
...

For decades I was fascinated by the concept of using decorative
concrete
mouldings cast in situ. At last given now the time and the
possibility
of having made up in my own workshop the complex shuttering that
is
required to cast classic mouldings onto concrete slabs I have
partially
achieved success.
To give an example - as this generalised description is a little
nebulous - where you cast a concrete floor out into a short
cantilevered balcony, you can apply to the flying edge an
attractive
moulded shape by the use of a complex shuttering. This is
primarily a
work of joinery and there is a lot of planning into how the
shuttering
progressively dismounts.

I have been very successful in creating the shapes and achieving
a
clean release of the shuttering. In fact I am more than pleased
with
results.
BUT one problem is an occasional hairline cracking in the cast
surface.

Can anyone with a better knowledge of concrete technology help
me?

I am using a very fine aggregate of about 1/4" hard limestone
and a
mixture of ground hard limestone and marble dust obtained from
the
poishing of marble. Silica sand is not available to us. I use a
mixture
of light coloured Russian grey cement and the common white
cement. To
this mixture I add a waterproofing preparation which acts as a
plasticiser and greatly inhibits drying so that it cures to
become
extremely hard and dense. The mix is made in a standard cement
mixer
and kept very dry. When I need reinfocement I use glass fibre
strands
laid in the concrete. It is well vibrated with a very effective
small
wand.
These mouldings have been between 150mm and 350mm in height and
a
projection of 50mm up to 150mm from the surface to which they
are
bonded.

The external finish is most satisfactory - it has an appearance
similar
to the grey hardstone used in central Italy since the
Renaiassance for
stairs and architectural mouldings. However hairline cracks do
appear
in the surface on some of the mouldings. I can repair with a
slurry of
cement and marble dust which sands out quite well and maintains
the
shaping very crisply, but the delightful sheen that direct
contact with
shutter confers is lost and the surface becomes somewhat dull
and
lifeless.

Can anyone advise me how to avoid this occasional surface
cracking on
the profile?


--
tghattaq