THE CONCRETE SURGERY
Concrete and freeze thaw cycles and de-icing salts
Dr. Concrete, PICS UK.
29th AUGUST 2018
This cold climate and freeze thaw issue has been around for as long as I’ve been involved in the industry (25 years) and as each year passes, I realise that when the job is undertaken to the correct standard, (as indeed the majority are) the concrete is not damaged by these conditions, whether that is in Iceland, Czech Republic or the UK.
I see many jobs on my travels, in the UK and other countries, installed by countless companies. The installations can be 5, 10, 15, 20, 25 years old and have not been damaged by freeze thaw cycles or the use of de-icing salts.
I sometimes hear people advising that de-icing salts should not be put onto PIC. Some end users will follow this advice. However, even those end users that do follow this advice and do not put de-icing salts onto their PIC driveways, in reality actually do put de-icing salts on their driveway – EVERY DAY.
EVERYBODY does, by virtue of the fact that they park their car on the driveway, in exactly the same place, every day, and salty water, collected from the roads, drips from the car, off onto the driveway, in the same place every day, with no damage whatsoever.
If de-icing salts were to damage PIC, then it would be entirely reasonable to expect to see an area looking great, everywhere, except where the car is parked.
Sure, ANY concrete product does not like freeze / thaw cycles nor does it like de-icing salt. HOWEVER a good quality concrete product WILL stand the test of time. A poor quality concrete product WILL NOT. Standard, but suitable, concrete is used for airport runways, roads, sea defences, bridges, car parks, buildings, footpaths, as well as domestic driveways and patios. Clearly, if the correct concrete is used and the correct installation procedure is followed, then concrete will last for many years with no signs of damage.
It is also very important to understand that for any given type of concrete, applying colour surface hardener will produce a surface that is even more resistant to freeze / thaw cycles or de-icing salts than the concrete itself.
Secondly, (but less dramatically) a sealed surface will also add to the resistance of the surface of the concrete to freeze thaw cycles or de-icing salts.
In the UK, the construction industry specifies a concrete that is to be exposed to freeze thaw cycles and de-icing salts, should be a minimum of 30Nmm2 and air entrained. If the concrete is not air entrained it should be stronger. (British standards say 50Nmm2, which, depending on many factors, equates to a cement content of around about 480kgs/m3)
In the decorative concrete industry we do not order a guaranteed minimum strength. Instead, we order a minimum cement content of 320 kgs cement per m3. This is the MINIMUM. Not 300, not 280, not 260!!! Some installers will use a cement content lower than 320 for two reasons. Firstly, the concrete sets slower, which is useful in the summer and secondly a lower cement content equates to cheaper concrete. However, a concrete with less cement will produce a weaker concrete and is therefore more likely to fail.
So, bottom line is if the correct concrete is used, i.e. min 320 + AEA, the concrete will be fine, as laid out by the construction industry in the UK.
That is for plain, grey concrete.
PIC is concrete which is coloured and hardened with CSH. It is also sealed.
The CSH used to colour and harden the concrete, would, under laboratory conditions, reach 70Nmm2. When applied to the surface of concrete it produces a surface even more resistant to freeze thaw cycles or de-icing salts. Therefore, a colour hardened concrete will be even more durable than a normal minimum standard concrete as required by the construction industry in the UK.
Furthermore, the colour hardened concrete surface is then sealed with a penetrating acrylic sealer which further increases the surface resistance to freeze thaw and de-icing salts.
A thru coloured / integral coloured concrete or a plain uncoloured concrete with no CSH is more likely to show early signs of surface deterioration because of a potentially poor water / cement ratio at the surface when the concrete was laid.
A correctly installed job will not fail.
So, why do some jobs fail, or show early signs of deterioration?
- Cement content of concrete too low.
- CEM3 was used.
- Concrete not air entrained.
- Concrete was laid too wet.
- Poor quality CSH used.
- Insufficient CSH used.
- Water (even a small and carefully applied spray) added to the concrete surface (in the
summer) to make the surface easier to float or trowel.
- Rainfall on the concrete while installing.
- A plain concrete or integrally coloured concrete was used with no CSH.
Often, it may be a little bit of each which contributes to the failure. Imagine a beautiful summers day. A contractor is installing a large driveway. He has ordered a low cement content because the concrete will set slower and is cheaper to buy. He has also ordered a blended, CEM3 mix, because it sets slower and is cheaper to buy. He purchases an inferior CSH, because it’s cheaper to buy. He then tries to get the CSH to cover as many m2 as possible to save money on materials and also save time applying the CSH and / or floating and / or trowelling the CSH. The sun is shining (and the bluebirds are singing) and there’s a gentle dry breeze over the concrete surface, so a ‘light spray’ of water is applied to the surface to make the floating and trowelling easier and / or reduce the chance of surface opening fissures.
Hey presto! ……It’s not one thing…..A little bit of many things….produces a poor quality job.
Red Shute Hill Industrial Estate