How long does tarmac take to set?
‘Tarmac’ is one of those terms that has a specific meaning, but is also used commonly to refer to any similar method of surfacing a road, pavement or driveway.
Examples of alternative methods include asphalt and bitmac – this being similar to tarmac but containing bitumen rather than coal tar.
Newly laid tarmac needs time to set before it is exposed to heavy traffic, as when it is first poured the surface is soft and at risk of indentations from wheels and shoe heels.
But how long does tarmac take to set? That depends on a few variables, and there is no one time when newly laid tarmac goes from ‘not set’ to ‘set’ – it all depends on what you want to do on it.
What affects the drying time of tarmac?
Fresh tarmac has a few important properties:
- Penetration grade is a measure of how hard the cured tarmac will be at normal temperature.
- Cutback is the addition of a solvent such as kerosene to slow down the setting process and keep the tarmac workable for longer.
Including a cutback solvent delays the drying time and the solvent must evaporate out of the tarmac before it will continue to fully set.
Penetration grade, meanwhile, can mean that even when it is fully dry, one grade of tarmac may never be as hard as another.
Environmental factors that affect tarmac drying time
There are certain environmental factors that affect tarmac drying time, so these should be noted and the drying time increased if necessary.
One of them is heat – and although hot weather can encourage the evaporation of solvents and liquids within the tarmac, this usually will not speed up the process.
That is because the higher the ambient air temperature, the longer it typically takes the tarmac itself to set, and on very hot days in the future even set-solid tarmac may soften slightly.
Water reaching the surface from the environment may also have an effect – and while rainwater may help to cool the tarmac on a hot day, it’s not wise to artificially cool it too fast in an attempt to speed up the drying process.
How long should tarmac be left to dry?
If you need to walk on newly laid tarmac in order to complete a job, then using flat shoes, careful footsteps and ground mats to distribute pressure more evenly, it is usually OK to do this within the first few hours after laying.
However it can be 2-3 days before the surface solidifies enough for heavier use, and you should avoid scuffing it – for example by turning vehicle tyres while the vehicle is stationary – for at least a few weeks.
Finally if the tarmac stays soft after several days, there may still be solvent in the mix. Given enough time this can evaporate out naturally, but will prolong the tarmac drying time.
Alternatively there are surface treatments including fine sand and sealing compounds that can help a tacky tarmac surface to harden faster, but these should only be used if the process is not taking place naturally.
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