Read the guidelines for mixing cement

By Don MacAlister Time of article published Jun 18, 2015

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Clanwilliam - Two questions this week I believe are applicable to many people.


Questions and answers

My old friend Tony B has another teaser for me:

Below our outside deck is a basement area and we decided to lay a cement surface over the basement’s concrete slab.

I now notice when sweeping this cement floor with a firm outdoor broom that the surface is breaking away in small pieces in some places. These pieces are soft and brittle.

Although I advised the contractor that I wished to have a hard cement finish on which to lay future tiles, the surface topping is definitely more brittle than the usual brick wall plaster finish.

Watching the contractor do the cement topping work, unlike the usual (well watered) plaster mixing process, the topping mix was done by way of a damp method. This was then stamped down with a stamper before finishing with a hand float. Monitoring the contractor’s mix calculation it appeared to be four barrows of sand to one bag of cement used.

Because it is a basement area used for storage of garden tools, etc, my wife has now decided not to lay tiles but stay with the raw cement look.

My questions are: why is the topping so soft/brittle; is a soft surface topping an industry standard; what can be done to repair or strengthen the current topping; and what should be done better if a future repeat topping is planned?

Answer: To start with, I would suggest that when next in the local hardware store, all readers pick up a copy of the PPC Cement brochure called the The Sure Way to Estimate Quantities. It is a handy guide to mix proportions, strengths and what mixes to use for which job.

The first problem is that the mix was definitely wrong. The brochure I mentioned says: “Floor screeds: two pockets of cement to three wheelbarrows of sand.” It further states that sand-cement screeds are for light duty only – they are not suitable for use in workshops or factories. A cement screed is usually covered with something, such as tiles or carpets etc. For a harder finish, fine grit or fine stone chips can be added to the mix to form a grano finish.

The brochure says screed mixes should be mixed to a plastic consistency. “The use of very dry screed mixes is a common cause of failure.” I haven’t seen a dry screed mix in years; at one stage they were laid under heavy flooring tiles, but the tiles were placed on top and tamped in as the screed was laid.

I have consulted with both a renowned engineer and a young man who is making great strides in repairs to floors and creating an epoxy finish (Technikote) about your floor, and the news is not good.

First, you must check if what is left of the floor is solid. This can be done with a simple tap check – tap the floor with a broom handle and listen for a hollow sound. Follow this by removing or digging through a section of the floor and see if the screed is sandy all the way down to the base screed. If either of the above shows a negative result, your cheapest solution is going to be to hack up the screed and start again, as any repairs are going to be temporary.

However, if your problem relates to just the surface of the floor, there are liquid chemicals available that can be applied to strengthen the remaining screed, but this can be expensive and may not be a permanent solution. Once the screed has been stabilised, it can then be over-coated with an epoxy finish which will both fill in the indentations and provide a hard and durable finish.

I would suggest that you obtain quotations to replace the floor in its entirety, or to use a chemical and epoxy repair.

Zainab asks:

I am a trustee at a residential complex in Wynberg in Cape Town. All the geysers are above the baths, including the ones on the top floor. A top floor flat was sold recently and the new owner wants to move the geyser into the roof. Is this acceptable? If so, will the owner then become responsible for any damage as the roof is common property? Would this affect our insurance in the event of a geyser burst?

Answer: This is an interesting question as many variables come into play, and hopefully some of you may respond with different ideas.

First, what is common property? Is it just the roof and the supporting structure, or is it the roof void as well? As far as I am aware the roof void cannot just be an open space – the void above each flat must be divided into sections by means of fire walls going up from the dividing walls of the flat up to the underside of the roof.

In other words if flat one has a fire, the flames cannot spread into flat one via the roof, so if the block is constructed properly any burst from the geyser would not spread to other flats.

I cannot see any problem with the owner installing his geyser in the roof space, as long as the geyser is installed from his flat. If the roof had to be opened to allow the geyser to be fitted he would become responsible for any initial or subsequent damages caused by a leaking roof.

* Keep your questions or comments coming to [email protected], or SMS only to 082 446 3859.

Weekend Argus

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