ROAD PAVEMENT INVESTIGATION BY CORING

CONTENTS

The history of a road pavement
Reading the information
The coring rig
Coring to check crack propagation
Testing for rutting resistance
Testing for Void Content using Cores
Testing for Compaction using Coring
Coring programmes
Reinstatement of Core Holes

THE HISTORY OF A ROAD PAVEMENT

The history of a road pavement can be obtained from a core ( or a trial hole ) if you know how to "read" the information that is presented to you.

READING THE INFORMATION

The core to the left clearly shows the various treatments the road pavement has received over the years culminating in a surface course (wearing course) of hot rolled asphalt and precoated chippings.

It also shows that the more recent overlays are in good condition and that the road pavement is in a strong condition.

The strength of the road pavement being judged, of course, in relation to the amount and weight of traffic passing over it.

The above image clearly shows that a road pavement engineer is never short of records / information on a road pavement providing he is able to read the "information", in this case the evidence that is presented in the core itself.

This process can be taken a step further with appropriate trial holes and the taking of samples of the various layers for further testing, e.g. laboratory analysis of the distinct materials in the construction including binder reclamation and testing.

For information on undertaking trial holes, press --------------------------> HERE

THE CORING RIG

Do you need a big one or a little one, as they are available in different sizes, at different costs, to perform different functions.

The larger rigs are used for taking cores with diameters from 100mm. to 200mm., and they have the power to go to considerable depth, e.g. 500mm..

The small coring machines can be completely self contained and are able to be carried in the boot of the average saloon car.

The smaller machines are mainly used for checking the thickness of reinstatement layers in patching work and the reinstatements of utility works, and you can get a very quick assessment of the reinstatement materials, e.g. cores of a bituminous mixture that when put on the window sill of the office fall apart with the ambient heat, suggesting the reinstatement material has a "cutback" binder when it should be a penetration grade.

When using a large coring rig it is possible to obtain cores of sufficient size to be able to carry out a number of laboratory tests on them, including material analysis providing the nominal aggregate size is not too large.
Percentage refusal density testing, bulk density and rutting tests are amongst other procedures that can be carried out on larger cores.

CORING TO CHECK CRACK PROPAGATION

Here coring has been performed to determine the depth and severity of cracking in a road pavement, and it has been found that the crack travels down through the lean concrete base (roadbase). This type of find in a core would need to be followed up by a trial hole to find out just what had instigated the cracking in the first place.

Note
It has been my experience that cracking that has started in the lean concrete and propagated upwards has not actually been failure of the lean concrete but has been due to settlement from poor earthworks that the lean concrete was eventually unable to bridge.

TESTING FOR RUTTING RESISTANCE

This core is 200mm. in diameter and is just of the wearing course, in this case 14mm. / 30% Hot Rolled Asphalt Wearing (Surface) Course.
The core has been tested in the rutting machine to determine the rutting resistance of the wearing course.
You will note this was a particularly stable design with a minimum of rutting resulting from the test.
This prompts me to say that it was only poorly designed, or poorly manufactured, hot rolled asphalt surface course that actually rutted/wheel tracked.

TESTING FOR VOID CONTENT USING CORES

Cores can easily be taken from the surface course layer and the void content determined, and this can be for a number of reasons, such as :- To determine that there are minimum voids to ensure that the material is impervious.  To determine that there are at least a minimum amount of voids, because a bituminous material without any voids is likely to be of low stability and subject to rutting. To determine that with materials that are designed to be porous, i.e. to allow the flow of water through the matrix, the required minimum amount of voids are present so that drainage will occur.

TESTING FOR COMPACTION USING CORING

This procedure is normally performed using the test known as PERCENTAGE REFUSAL DENSITY and it is a measure of the relative state of compaction of a site sample of laid Dense Bitumen Macadam to that of a similar sample compacted to refusal ( i.e. it will not compact any further even though it may still contain voids ) under laboratory conditions. 
Note - The bituminous mixture described by the term dense bituminous macadam in the UK will be quite similar to bituminous mixtures described as Asphaltic Concrete in other parts of the world, and from 01-01-2008 the term Asphalt Concrete will apply to bitumen macadams in the UK).

(Visit the page  BITUMINOUS MIXTURES / PROCESS DIAGRAMS  to get up to date with modern bituminous mixture terminology.)

Compaction testing is not a test normally carried out on Hot Rolled Asphalt mixtures.

The test which determines the PRD is intended as quality control procedure to ensure a minimum state of compaction of laid bituminous mixtures.
The test is most performed on base (roadbase) and binder course (basecourse) layers. 
Clause 927, "Road-Pavements : Bituminous Bound Materials" of the Specification for Highway Works gives the details on this test. 
Details of the test were/are also in BS 598: Part 104. 
Briefly the principle of the test is that a core is taken of the material in the road, and the density of this core determined. 
This same core is then placed in a mould at a standard temperature and compacted until , it refuses to compact any further. 
The density of the core is measured again. 
We now express the density of the core as it was in the road as a percentage of the density after the core has been compacted to refusal, so the term PERCENTAGE REFUSAL DENSITY. 
The figure of 93% is often regarded as a reasonable minimum percentage of the density of the sample compacted to refusal density, a figure lower than this may require further on site investigation.

I find that Clause 927 is "not used" in the recently introduced Interim Advice Note 101/07, which is the revised Specification for Highway Works "900 Series".
At this time I am not sure if this procedure has been totally excluded as a procedure for assessing the on site compaction of bituminous materials, or if the procedure is contained elsewhere in one of the many new standards introduced on the 1st. of January 2008.
I did not find any equivalent test procedure in a quick perusal of the major documents, but it is possible I have not found it.
However on researching this matter a little further I find that this procedure had been removed from the 2004 edition of the "900 Series".
So It is likely that the Clause 927 did contain the Percentage Refusal Density procedure in 2000/2001 when this page was first written, but ceased to be referred to from 2004.
Rightly or wrongly I think I know more about specifying bituminous mixtures and all that it entails than most. 
So what this highlights, especially with the introduction of the many and various documents from the 1st. of January 2008, is that I would hazard a guess that there are very few, if any, who are totally familiar with all that is happening.
I find that dangerous for the industry, and all who use and pay for the upkeep of the highway network.
From  a technical point of view I wonder how you verify the results that you are obtaining from your Nuclear Density Meter, but perhaps they are no longer used as a compaction testing instrument either.
Who is keeping up with all this change ?  Hopefully your Materials Engineer, or Road Pavement Engineer !
The good news is, if you are looking after a local highway network, that you would probably not have been using Percentage Refusal Density testing anyway. 
Unless you were responsible for the design, and supervision of construction of a local bypass, but that is increasingly unlikely these days.

CORING PROGRAMMES

Coring programmes are usually associated with forms of road pavement survey, e.g. deflectograph, where it is necessary to determine the thickness, type of road making material and its condition to be able to relate found deflection with the condition of the highway.

It is unfortunate that with many of the highways in the U.K. the road pavements have slowly evolved over the years and are not of uniform construction, (see the first picture), in this situation you would need a financially unacceptable close spacing of cores to establish a reasonably clear picture of the road pavement condition.

This problem should not exist with well documented and uniformly constructed motorways, bypasses and other new roads where coring surveys should be able to establish the condition of the road pavement with a high degree of confidence.

REINSTATEMENT OF CORE HOLES

It should be unnecessary to remind you that the core holes must be adequately reinstated and be impervious to the ingress of water. 
I say it should be unnecessary to bring this to your attention, but unfortunately it is necessary.
The hole should be filled with a suitable  bituminous material that can be rendered impervious by compaction, and if the hole is deep add the material in small quantities and compact each layer. Compaction should be with a vibrating "Kango" hammer and a round foot, but a heavy hand tamping bar with a suitable round foot will be the more widely used option.
The small holes can be filled with mastic asphalt of even 50pen. bitumen.
Poorly reinstated core holes in an otherwise sound road should not be the cause of future potholes, and the threat of this possibility should not deter you from taking cores from a road pavement, if you have cause to do so.


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