We all know that to remove the release agent, the finished job needs to be washed off. Lack of care using the high pressure water jet or uncontrolled application of Acid Wash, will potentially result in damage to the surface of the printed concrete.

At best this damage may be that the surface becomes a little grainy in places and /or the tell tale water jet arcs can be seen on the surface of the new PIC. At worst, the surface is removed and the grey concrete and aggregate shows through. Whatever the damage, it’s VERY difficult to remedy.

It’s easy to forget that this problem is compounded now that the temperatures have dropped. It is important to bear in mind that the concrete (and of particular importance to us, the decorative surface) is going to take longer to become hard. Therefore, Dr Concrete highly recommends that the concrete is left longer before it is washed off and in any event, more care must be taken when washing off the job as it will be easier to damage the surface using the high pressure water jet or the Acid Wash.

Fact: At 5 ̊C concrete takes over twice the amount of time to achieve the same hardness as it would at 25 ̊C.

Please Note: The “Double whammy factor”. This does not take into account an additional consideration, which is that the water/cement ratio will be likely to be far higher in the winter months, which makes the concrete surface more porous and consequently not as strong for a given time and average temperature.

Bottom line: More time needs to be allowed prior to washing off in the colder (and wetter) weather because the concrete will not become hard so quickly and even for a given hardness it will likely be more porous and consequently less strong for a given hardness.

So, guys, as the Doctor always says, prevention is better than cure, so, to avoid the heart ache of a great job being spoilt at the wash off stage, simply leave the concrete longer.

For those of you who would like some facts to back this up, here you go:

The maturity of the concrete (and its relative strength gain) can be calculated. This is a guide, but a fairly accurate one nonetheless:

The hydration process begins at minus 10 ̊C or above.

• Concrete left for 48 hours at 25 ̊C has a ‘degrees C hours’ maturity of:48 x ( 10 + 25 ) = 1680 degrees C hours.
• Concrete left for 48 hours at 5 ̊C has a ‘degrees C hours’ maturity of: 48 x ( 10 + 5 ) = 720 degrees C hours.

Concrete left for 48 hours at 0 ̊C has a ‘degrees C hours’ maturity of: 48 x ( 10 + 0 ) = 480 degrees C hours.

Comparing strength gain for 5 ̊C temperatures compared to 25 ̊C.

Divide 1680 by 720, and you can calculate the strength will be around 2.3 times more after 48 hours at average 25 ̊C than it would be at average 5 ̊C.

Concrete therefore needs to be left 41⁄2 days (112hours) at 5 ̊C in order for the concrete to achieve the same strength as concrete left for 2 days (48 hours) at 25 ̊C.

Calculated as follows: 112 x ( 10 + 5 ) = 1680 degrees C hours.

Comparing strength gain for 0 ̊C temperatures compared to 25 ̊C.

Divide 1680 by 480, and you can calculate the strength will be around 3.5 times more after 48 hours at average 25 ̊C than it would at average 0 ̊C.

Concrete therefore needs to be left 7 days (168 hours) at 0 ̊C in order for the concrete to achieve the same strength as concrete left for 2 days (48 hours) at 25 ̊C.

Calculated as follows: 168 x ( 10 + 0 ) = 1680 degrees C hours.

Simple eh?!

Dr Concrete