I am extending a single skin block built garage to create a double garage, also using blocks. The ground below about 400mm is free draining, well compacted stony soil, on which I built the foundations 600mm wide excavated to 800mm depth with 100mm hardcore and 100mm x 600mm wide concrete.
I was also going to lay the concrete floor using 100mm compacted hardcore and 100mm of ready-mixed concrete but now have some doubt on whether this is acceptable or not. One builder has insisted that the concrete must be 150mm deep, one has said 100mm is okay!
I contacted my local building control (Scotland) - "100mm could be okay, 150mm would be better. I can't find any specific minimum depth"!
Then I asked the ready-mix suppliers - "100mm should be adequate. We can supply stronger grade (C30?) concrete which will help".
The trouble is that nobody seems to know for sure. Can anyone offer any advice?
The trouble is that 100mm hardcore has already been laid down and compacted (I should have explained that) and I would wish to avoid digging it all out again or even scraping 25mm off the top then having to hire the compactor again!
Thanks for the the suggestion of using 125mm. I had already considered doing that and it will just mean a slight height difference between the two sides of the garage with a little ramp where the original block wall will be removed.
Your project sounds interesting.
If all you intend using the floor for is normal cars then 100mm should be fine as long as the sub-base under the floor is well compacted with a vibrating plate compactor.
If you are unsure then you have two options (still keeping 100mm depth). Use re-inforcing mesh in the floor or use a stronger mix of concrete.
If I recall 142 mesh is adequate and not too expensive. This should be supported 25mm above the ground using plastic spacers (supplied by the steel seller). This puts any tension on the slab where the steel is.
A stronger mix is the second option but not as good because concrete strength is usually measured in compression and any tension stresses could still allow it to crack.
In any case, spans of over 5metres should have stress cuts placed in them to allow for movement. This could also be achieved by placing a 5mm dividing strip between two spans of 5m.
A suggested mix for your job if using mesh would be a 25N/sqm (C25).
There are two main factors that contribute to the strength of concrete. Cement water ratio and compaction. There is a third and that is curing.
Cement water ratio is, as it says, the amount of cement in a mix compared to water used. Therefore, the drier a given mix the stronger it will be. The tendency in the building trade when laying floors is to use a slightly more "liquid" mix. This is usually done on site when the truck driver is asked to add water to the mix. This, of course, completely voids the guarantee on the concrete which has been supplied by the ready mix company.
It is harder to spread but gives a much better floor if you specify a 50mm slump. Most truck drivers can tell their concrete slump just by looking at it going round in the drum.
Compaction is achieved either by tamping the floor with a wood pole (very arduous), by using a vibrating poker or by using a vibrating screeder (both hired pieces of kit).
To maximise your floor strength you should do both of the above.
As concrete achieves 60 - 70 % of its strength in the first three days it is also advisable to assist the curing process by keeping the floor wet after it has set for three days. You could wet it then cover it with a sheet of thin plastic to prevent evaporation.
When you order the concrete be aware that many ready mix firms now use PFA (pulverised fuel ash) in their mix. This costs them less than cement and allows them to keep the mix competitive. The problem is that it takes longer for the concrete to set. So if you were planning to cast in the morning and power float the floor for a good finish in the afternoon or evening, you could find yourself up all night long just waiting.
You could ask them for a non PFA mix and avoid the delay.
Very valid point. As you can read in Wikipedia (Google "en.wikipedia.org/wiki/Fiber_reinforced_concrete" a fibre mix would also work but would probably not increase the flexural strength to much extent.
Also, I suspect, it would cost a great deal more as it would be a specialised mix.
I forgot to say, lest anyone picks me up on it, the percentage of PFA used in concrete as a substitute for cement varies. In the 1980's it was common practise to restrict use to 15% of total cementitious content but I know of instances where 50% has been used. I would be very interested to hear of any other facts regarding its use in recent years.
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