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Cabins have just been installed on an asphalt car park. The portable cabins weigh 4000 kg with 4 no. 300 mm square spreaders under each foot.

The car park is constructed from: 1" thick rolled asphalt 2" thick bitumen macadam 6" thick granular subbase well compacted ground

The effective pressure on the asphalt is: 4000 kg = 40 kN / (4 × 0.3²) = 111 kN/m²

The question is whether this load is likely to cause permanent deformation of the asphalt leading to problems with ponding of water in the future.

I know this is a complicated question depending on the semi-solid nature of the asphalt and its rheological characteristics. The temperature for the duration of the portable cabins is not expected to exceed 30°C. The cabins are expected to be in place for approximately a month.

Any guidance, rules of thumb or a point in the right direction would be appreciated.

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I wouldn't expect this to be a big problem.

Would you be worried about leaving a car parked in one place on this pavement for a month? Typical car tire pressures are at least 2 bar or 200 kN/m^2, about twice as high as your cabin.

Note, the weight of the car is almost entirely supported by the air pressure in the tires, so the pressure on the pavement equals the tire pressure - and the contact area of a tire would be smaller than your 300mm square spreader, so it is more likely to make a dent.

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It depends on the tar and bitumen creeping properties.

Some asphalt perform well under transient loads but slowly creep under steady load. Hence the concrete is material of choice for runways.

Although the max 30 sounds like a mild temperature, some asphalt which has been exposed previously to high temps tend to let the aggregate sink and tar float.

Bottom line, in the absence of lab tests, one has to try to check for any signs of pavement deterioration most visible around the posts penetration or intersection of the curbside and street asphalt and the potential depression under street lane painting.

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