Does anyone know why, precast concrete, like used on prefab garages,
pipes etc, is waterproof, and has a fine surface texture, whereas the
concrete you mix yourself, is porous and nothing like the precast?
Is there a special sand and cement used or is it made under
pressure? I am repairing concrete garage, and between the sections
it was originally filled with a very fine texture grey waterproof
"concrete", does anyone know what type of mixture this is?

"John" wrote in message
oups.com...
Does anyone know why, precast concrete, like used on prefab garages,
pipes etc, is waterproof, and has a fine surface texture, whereas
the
concrete you mix yourself, is porous and nothing like the precast?
Is there a special sand and cement used or is it made under
pressure? I am repairing concrete garage, and between the
sections
it was originally filled with a very fine texture grey waterproof
"concrete", does anyone know what type of mixture this is?


It will have a high cement and fines content, and will have been
vibrated into the moulds.

AWEM

On 19 Jun, 09:50, "Andrew Mawson"
wrote:
"John" wrote in message
oups.com...

Does anyone know why, precast concrete, like used on prefab garages,
pipes etc, is waterproof, and has a fine surface texture, whereas
the
concrete you mix yourself, is porous and nothing like the precast?
Is there a special sand and cement used or is it made under
pressure? I am repairing concrete garage, and between the
sections
it was originally filled with a very fine texture grey waterproof
"concrete", does anyone know what type of mixture this is?

It will have a high cement and fines content, and will have been
vibrated into the moulds.

AWEM

It also is usually fibre cement, and compressed.

Getting a smooth surface diy is achievable using polythene sheet, and
if even smoother is desired, wiping fine mortar into any pinholes left
on the surface after wards.

But to get the same strength per size you need fibres. Alkali
resisting glass fibre is the modern replacement for asbestos fibre.


NT

roups.com...

Does anyone know why, precast concrete, like used on prefab garages,
pipes etc, is waterproof, and has a fine surface texture, whereas
the
concrete you mix yourself, is porous and nothing like the precast?

more on mixes:
http://wiki.diyfaq.org.uk/index.php?title=Mortar_Mixes


NT

Many thanks for all your responses. John

John wrote:
Does anyone know why, precast concrete, like used on prefab garages,
pipes etc, is waterproof, and has a fine surface texture, whereas the
concrete you mix yourself, is porous and nothing like the precast?
Is there a special sand and cement used or is it made under
pressure? I am repairing concrete garage, and between the sections
it was originally filled with a very fine texture grey waterproof
"concrete", does anyone know what type of mixture this is?

Just add more cement. The porosity is because the gaps between the sand
grains are not full of cement.

I think up to 1:2 cement/sand is possible.

You can also add e.g. PVA which tends to fill the gaps between the
cement grains.. ;-)

Use builders or playpit sand rather than sharp sand as well..its finer
grained.