Case Study: Digging up the Cause of Premature Pavement Deformation

Pavement Management Services were tasked with uncovering the cause behind premature pavement deformation along a section of a street. Pavement cores were unable to determine the cause or the pavement layer causing the premature deformation, so a test pit was undertaken. Previously, a 50mm mill and fill had been adopted at this site. This case study highlights the key observations, issues, causes and the recommended actions following the investigation.

Key Observations

1. New AC layer pulled off with relative ease indicating a poor bond/tack coat.

2. The pavement profile showing the area of noted deformation.

3. The new asphalt layer exhibited crumbling and breakage precisely at the area of the soft asphalt, while on the left side, a distinct sharp edge was evident. Additionally, a crown is observed on the right-hand side of the deformation.

Overall, the pavement comprised of a diverse range of asphalt mixes, along with of geofabric and a seal at lower depths.

What was the issue?

Our findings raised concerns about the quality of the asphalt overlay and the bonding between the new and existing surfaces. The following key issues were identified from the test pit:

1. Density/compaction issues of the new asphalt: The new asphalt showed signs of density problems, with some areas exhibiting crumbling and looseness at the interface and along the edge of the saw cut. This indicated inadequate compaction during the paving process.

2. Soft underlying existing asphalt: The existing asphalt beneath the new overlay was found to be soft, potentially affecting the overall stability and durability of the pavement. The cause of this softness is uncertain but it was postulated that the tack coat might be responsible.

3. Tack coat inadequacy: The tack coat used to bond the new asphalt to the existing surface was found to be inadequate, as it separated easily. This poor bonding could contribute to potential delamination or failure of the pavement over time.

4. Uncertain cause of softness: While the tack coat was considered a possible cause of the softness in the underlying asphalt, the test results were inconclusive. It was also suggested that the softness might have resulted from a hard oxidised surface over the soft asphalt, but milling should have exposed this issue before the overlay.

The Causes

The main cause of the pavement deformation is most likely due to the breakdown of the top of the existing asphalt, affecting the compaction of the new asphalt. The exact cause is of this is inconclusive, but potential reasons include:

• The possibility of the existing asphalt being soft, and milling may have removed a hard oxidised layer, exposing the softer material.

• Compaction over soft materials affecting the density and compaction of successive layers.

• Questionable tack coat may be a contributing factor, indicated by the separating of the overlay from the existing asphalt.

• Tack coat may not have been of standard quality or applied too soon, not allowing enough time to break.

• Use of cutter oils in the tack coat, if applicable, could have caused softening of the asphalt.

• Pavement cracking could worsen the softening as the tack coat disperses through the cracks, and the oils don't have time to disperse or break down if it's not an emulsion.

The Recommended Action

Remedial actions were taken to address these issues and ensure the longevity and performance of the pavement. Due to the presence of soft asphalt and density issues, we recommended the removal of the existing asphalt.

Pavement Management Services also provided two new pavement designs for two pavement types which will exist based on whether there is underlying concrete or not.

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