For Protecting Against Frost Damage
When the thermometer shows low or even sub zero temperatures and again it’s time to think about protecting freshly laid concrete to prevent frost damage whilst it is setting.
Increasing the cement content and lowering the slump will not only mean that you get to go home earlier, but will also help to reduce the chance of frost damage occurring in the time between laying the concrete and it gaining sufficient strength to prevent frost damage. The reasons for this are:
- The concrete will set (and gain strength) faster.
- As the cement hydrates, the chemical reaction gives off heat.
- Lower slump concrete has less water to freeze.
However sometimes this isn’t enough.
The construction industry states that concrete should not be laid unless temperatures are +5°C and rising and that the temperature of the concrete should not fall below 5°C until the concrete reaches a strength of at least 5 N/mm². Obviously the lower the temperatures are, the longer it will take the concrete to reach this minimum strength.
Therefore, once the concrete has been imprinted, it is recommended that the exposed surface is covered with an insulating material if you think temperatures may drop overnight and cause the surface of the concrete to freeze.
The three common ways that contractors will protect the concrete.
- Frost blankets.
- Thin polythene and hessian / Polystyrene sheets or similar.
- Extra release agent power.
Each of these methods helps to prevent the heat in the concrete from escaping, because they trap air and insulate the concrete. (Exactly like wearing an insulated and windproof coat). The heat generated from the hydration process is then not lost and the concrete stays ‘warm’.
Not many PIC guys use frost blankets, although they are made for the job. Generally frost blankets come on a roll. e.g. 75m x 1.5m x 8mm thick. There are a few different types, but all are variations on a theme and all are reusable.
Probably the most common way to protect concrete is to lay a sheet of thin polythene over the job (to stop the hessian marking it) and then top off with a couple of layers of hessian. Again, what this is doing is entrapping a layer of air and consequently insulating the concrete.
Lastly, I know some guys will simply apply another layer of release agent over the surface after the pattern is imprinted. Since the RA is a light fluffy powder, it entraps air and helps to insulate the concrete. There is a tendency to adopt this method if it is awkward to cover the job because of access restrictions and / or if it is thought the temperature isn’t going to drop ‘too much’. Although Dr Concrete can appreciate how this would help, he wouldn’t recommend this as the only method for protecting against cold weather.
Obviously the lower the temperatures, the more robust any frost protection needs to be.
Wherever possible Dr Concrete highly recommends covering the area properly as frost damage to concrete works out to be expensive!
So, how do you know if you’ve got frost damage?
Don’t worry, you WILL know!
When water freezes, it gets bigger. If the water in the concrete freezes before the concrete has attained sufficient strength, the water will expand and push the concrete matrix apart and make it significantly more porous and this will cause the affected area to look much darker. Since the concrete has been ‘expanded’ before it has hardened, it will be, and will remain being, very weak and the surface will easily be removed even to the extent that the surface could be scraped away with a persons finger nails. The water jet will rip it off.
Fixing frost damaged areas is very difficult and expensive and often repairs are not successful because they are noticeable and depending on how well the repair is undertaken the long term durability is questionable. Best to avoid frost damage in the first place by ensuring the concrete is well protected. Sure, it can be a hassle to do this, but it doesn’t cost very much and it’s significantly less hassle than trying to repair damaged patches or digging up and reinstating the whole area!
Covering up does work.
Dr Concrete can remember doing a driveway twenty odd years ago when we used to print with the metal moulds through polythene. (and it was NOT 5°C and rising!). The job was covered with a couple of layers of polythene which were held off the surface of the concrete with an assortment of screwed up polythene balls and sausages. All tucked up and secure.
One corner of the polythene had ‘blown away’ overnight and exposed the front left hand corner of the driveway (I think what actually did happen was that the customer pulled the polythene back to have a peek, but didn’t put it back again properly) and the 2m² of PIC which was exposed was wrecked. Ended up digging up half the driveway to the contraction joint (it was a small driveway). The rest of the driveway, which had been covered was absolutely perfect, remained in place and still looks good today, over thirty years later.
Bottom Line:
Make sure your concrete is securely tucked up and cosy at night and you won’t have any unpleasant problems with frost damage.
Notes:
The colder the temperatures the more robustly the concrete needs to be covered. Fortunately we don’t get very cold weather in the UK and if it is that cold, concrete plants generally don’t operate.
Dr Concrete doesn’t get to hear of many people having frost damage problems. The rain remains, as always, the biggest enemy.
When it gets round to Spring and the days start warming up, don’t get lulled into a false sense of security. It can be a lovely warm day and get jolly cold at night. In fact, it’s the time of the year that people get caught out because they don’t think it’s going to be necessary to cover up.
The extra Release Agent trick may work for very small minus temperatures, as it’s just enough to stop too much heat being lost from the concrete, BUT when it’s significant minus numbers, I do question how effective it is and I have known of people getting frost damage when they have only used this method of protection.
I am not aware of an area of concrete suffering frost damage when the area has been adequately protected prior to leaving site.
Dr Concrete