View Single Post
  #1   Report Post  
tghattaq tghattaq is offline
Junior Member
 
Location: Malta
Posts: 10
Default Cast Concrete Mouldings

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?