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Three main points to check before laying asphalt

In this post we will discuss the three main points to be checked prior to asphalting works to make sure that the underlying materials are up to standard.


Plate bearing tests shall be carried out on the compacted unbound layer as per testing frequency required by the contracting authority. Make sure that:
is greater than the required bearing capacity Ev2 stipulated in the road structure catalog (usually should be greater than 180n/mm2).
the ratio between Ev2 and Ev1 is less than 2.2.


Use the String Method to check the levels at 3 points along the carriageway.

Take surveyed points every 10m,
A) On the gutter of both sides of the carriage way,
B) On the underlying material on the left, middle and right side of the carriage way,

Compare the surveyed points on the gutter with the:
A) proposed levels in the construction drawings.
B) existing surveyed levels on the underlying material and make sure that there is enough space for all the asphalt layers. I am attaching a sample spreadsheet to help you with this task.

If there is not enough space for all the asphalt layers then the existing unbound material has to be scarified and re-laid to the appropriate levels accordingly.

If the existing underlying material is found to be too low, do not add layers less than 150mm thick above it to correct levels. Layers thinner than 150mm will form ‘biscuit’ layers which are to be avoided. The problem can be resolved by first scarifying the existing unbound material to a depth where the new layer and the scarified layer are 150mm thick.


Removal of 'biscuit Layers'

The minimum thickness of Type 1 layers shall be 150mm. Any layer smaller than that will create thin ‘biscuit layers’ which have to be removed prior to asphalt laying.

Check that:

  • All unbound material next to manholes is properly compacted,
  • All loose material is removed
  • There is no debris such as wood, plastics in the unbound material. Remove any such materials.
  • There are no ‘biscuit’ layers. Remove all such layers.
  • The unbound surface is swept and free from dust.
  • The side of the gutter is coated with Tar.
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