waterproof concrete questions

[John]

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?

[Andrew Mawson]

sections

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

AWEM

[meow2222]

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

[The Natural Philosopher]

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.

[meow2222]

[John]

Many thanks for all your responses. John