Coring is one of the most useful tools for investigating structural condition in pavements. As well as providing important information about layer thicknesses, necessary for the correct interpretation of other surveys, cores provide an opportunity to examine the visual condition of the various materials in the pavement.

Why take core samples?

In flexible pavements, cores through cracks provide a means of determining the depth of cracking, which has serious implications for the maintenance of the pavement. Coring also provides material that can be subjected to further testing in the laboratory, where appropriate, and it effectively opens a window in the pavement through which Dynamic Cone Penetrometer tests can be carried out to reveal further information about the condition of the underlying unbound layers.

General Principles

To take cores, the following equipment is required:

• Coring rig (see figure 1). This equipment contains a diesel engine, hydraulic pump, water tank and drill assembly (including drill controls).
• Coring Barrels (See figure 2). These are manufactured from steel with metallic teeth segments containing synthetic diamonds.
• Backfill material and associated compaction tools and accessories.

The coring rig is located where a sample is required to be extracted and set up. The barrel is attached and drilling commences.

For coring to be possible, the material which is being ground away must be flushed out of the hole (see figure 3), so the barrel is free to continue cutting. Commonly, water from an on- board tank is used, but for weaker materials pressurised air can be used instead (see figure 4).

Location

Cores need to be carefully targeted. For example, taken through cracks or at locations identified from deflection surveys as being representative of either good, bad or intermediate condition.

Extraction

Cores shall normally be extracted to the full depth of the bound material present in the pavement and shall generally be 150mm in diameter. If cores are taken from concrete slabs, where the slab thickness is less than 150mm, then 100mm diameter cores will be sufficient although they may not provide enough material for laboratory testing.

After removal of the core, water should be removed from the core hole when coring is by the water flush method, the total depth of the bound material recorded and the sides of the core inspected, as far as possible, for voids and cracks. The high shear forces induced by coring may cause some bound materials, in particular old tarmacadams, to break up in the core. The sides of the core hole should then be inspected to see if materials, which have disintegrated, have deteriorated in the pavement.

Handling of cores

All cores should be logged to record the layer thickness, material type and condition. The following should also be noted:

• Aggregate type (e.g. crushed rock, gravel, slag), nominal size and shape.
• The presence and position of any reinforcement.
• The relative quality/density/void content of the material.
• The bond between layers.
• The presence of detritus where there is a lack of bond between layers.
• The presence and depth of any cracking or loose material.
• Any missing layers (i.e. not recovered by coring).
• Any unusual features.

When measuring layer thicknesses, individual layer thicknesses should be recorded to the nearest 1mm. Thicknesses should be measured at not less than 2 points diametrically opposite each other on the surface of the core and an average determined. The total thickness of each group of combined asphalt layers should also be recorded in addition to the individual layer thicknesses.  

Each core should be photographed in colour, with a scale strip and with the core reference number clearly visible. A core log should include all of the above information as well as the photograph of the core that should ideally be 150mm by 100mm or greater.

TRL Coring Facilities

TRL currently have an ‘X-Calibre Neptune' Coring Trailer. Air compressors can be hired in, if air flush is required.

For more information contact:

Kevin Green
Lead Laboratory/Research Technician
Telephone: +44 (0)1344 770903
Email: kgreen@trl.co.uk