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The Idiots' Guide to Highways
BITS AND PIECES ABOUT ROAD PAVEMENTS AND ROAD SURFACE FAILURE
|HIGHWAY FAILURE||FAILURE OF THE ROAD PAVEMENT - DEFLECTOGRAPH|
|INVESTIGATING ROAD PAVEMENT FAILURE - DTP ADVICE NOTE HA 30/85||PAVEMENT STRUCTURAL FAILURE|
|DETERMINING CAUSE OF ROAD PAVEMENT FAILURE - TRIAL HOLE - SITE INVESTIGATION||TRRL SUPPLEMENTARY
REPORT 550 : ROAD FAILURE WARNING LEVELS
- STRUCTURAL MAINTENANCE
|TRL PROJECT REPORT 30 - VEHICLE WEIGHT LIMITS - ROAD PAVEMENT WEAR / FAILURE||FAILURE OF A ROAD PAVEMENT DESIGN - DRAINAGE - CBR|
FAILURE - WHEEL TRACK RUTTING - WHEEL TRACKING
- BASECOURSE DENSIFICATION
|ROAD PAVEMENT FAILURE - WATER DAMAGE - POOR DRAINAGE|
|PREMATURE FAILURE OF A ROAD||ROAD SURFACE FAILURE|
|ROAD SURFACE FAILURE - FRETTING - FRETTING OUT OF ROAD SURFACE||SURFACE COURSE (WEARING COURSE) FAILURE|
|RESTORING ROAD SURFACE SKID RESISTANCE FAILURE BY RETEXTURING||HOT ROLLED ASPHALT SURFACE COURSE (WEARING COURSE) FAILURE - PRECOAT LOSS|
|FAILURE OF PRECOAT
ADHESION - PRECOAT LOSS - HOT SAND TEST
- HRA SURFACE COURSE (WEARING COURSE)
|HOT ROLLED ASPHALT SURFACE COURSE
(WEARING COURSE FAILURE)
- WET SAND - RIDE QUALITY FAILURE - PRECOAT LOSS
|IRONWORK FAILURE, AND HOW TO AVOID IT||FOOTWAY FAILURE - TRL PROJECT REPORT PR/H/88/94 - FOOTWAY DESIGN|
|WHY DID IT FAIL ? - WORKMANSHIP|
|WHY DID IT FAIL ? - BITUMINOUS MATERIALS|
|WHY DID IT FAIL ? - SURFACE DRESSING FAILURE|
There is not just one type of road failure, and there is not just one reason for each type of failure, and it is in these situations where it is very difficult to replace an experienced engineer/technician with a guide such as this, but an attempt is made to give guidance and supply some information.
The information is in small paragraphs because it is drawn together on this page from other sources, and at the moment I do not have the time to present it in a different manner.
So, work through the items provided, or use the "Find" function of your web browser to search for specific items.
FAILURE OF THE ROAD
PAVEMENT - DEFLECTOGRAPH
DEFLECTOGRAPH usually refers to the machine i.e. the vehicle with the specific deflectograph apparatus built into it.
The DEFLECTOGRAPH MACHINE is used to carry out a DEFLECTOGRAPH SURVEY.
The Deflectograph Survey measures the amount a road pavement will deflect when subjected to a standard load, the load being supplied by the rear wheels ofthe vehicle being correctly weighted.
Pivoted measuring arms pass between the pairs of rear wheels and measure the deflection of the road, in one hundredths of a millimetre, when the wheel load is applied.
Basically the more a road deflects the weaker it is BUT other factors have to be applied to the deflection results before assessing the road strength, e.g. the temperature at which the survey took place and the nature of the construction of the road pavement.
The process has been developed by the TRRL ( now the TRL ) and is fully covered in TRRL REPORTS 833, 834 and 835.
The surveys are carried out to determine weak areas of highway prior to actual failure and determine road strengthening measures to prevent failure.
INVESTIGATING ROAD PAVEMENT FAILURE - DTP ADVICE NOTE HA 30/85
The title of this Advice Note is :-
DTP Advice Note HA 30/85 : Structural examination of bituminous pavements.
Although this may still be quoted it has been SUPERSEDED by DTP DESIGN MANUAL HD 29/94, and DTP DESIGN MANUAL HD 30/94.
This Advice Note gives recommendations for the structural examination of bituminous pavements to identify the causes of deterioration and failure.
PAVEMENT STRUCTURAL FAILURE
This is when the PAVEMENT has failed, maybe not completely, but in a major way.
The PAVEMENT is no longer able to absorb and transmit the wheel loading through the fabric of the road without causing fairly rapid further deterioration of the road pavement.
The layers making up the PAVEMENT have failed for various reasons, E.g. through age, inadequate design, or an altering in requirements of the strength of the PAVEMENT by increased traffic flow/weight, impeded drainage decreasing subgrade strength, etc..
When you have STRUCTURAL FAILURE the solution is always a form of RECONSTRUCTION.
I say a form of RECONSTRUCTION because it is not always necessary or even advisable to remove material that is still sound and will provide a good base to new work.
If total RECONSTRUCTION is needed so be it, but you really need the experts in here to advise,
I mean real experts with a few years of real experience not just somebody with a fancy machine, however much data they can provide.
SEEK GOOD ADVICE !
OF ROAD PAVEMENT FAILURE - TRIAL HOLE - SITE INVESTIGATION
If you do not know what is in the existing road pavement, dig a hole and have a look, providing you know what you are looking at, and if you do not, get a man who does!
Even something as simple as the degree of difficulty in digging the hole will give an indication of the strength of the materials that make up its construction.
There is very little substitute for experience in trial hole assessment but some things are common sense:-
1) How thick is the construction, how thick are the various layers.
2 ) What type of material is it? bituminous, concrete, granular, what condition is it in.
3) What is the height of the water table.
4) What is the type of subgrade, what is the CBR of the subgrade.
5) Is the road pavement material excavated in sound condition or stripping and loose.
ALL THESE QUESTIONS AND MORE NEED ANSWERING TO ARRIVE AT THE NEW ROAD DESIGN.
Trial holes are not expensive and can save you a lot of money, an intimate REAL knowledge of the road you propose to design/strengthen is NEVER wasted.
TRRL SUPPLEMENTARY REPORT 550 : ROAD FAILURE WARNING LEVELS - STRUCTURAL MAINTENANCE
The full title is :-
TRRL Supplementary Report 550 : Proposed warning levels for the structural maintenance of flexible roads
The report details various warning levels recommended for use in highway maintenance rating systems.
TRL PROJECT REPORT 30 - VEHICLE WEIGHT LIMITS - ROAD PAVEMENT WEAR / FAILURE
This report was produced as a result of a questionaire survey of goods vehicle operators conducted by TRL.
The survey data was processed in such a manner as to give estimates of numbers of goods vehicles, amounts of goods vehicle travel, perhaps most importantly an indication of increased road wear attributable to the increased weight limits.
FAILURE OF A ROAD PAVEMENT DESIGN - DRAINAGE - CBR
DRAINAGE is VERY! VERY! VERY! IMPORTANT.
You MUST keep the water-table low to prevent the moisture content of the SUBGRADE increasing, and hence decreasing the CBR VALUE on which the ROAD PAVEMENT was designed.
If this is not done by the use of FRENCH DRAINS, or even open ditches the road will weaken and fail.
The water-table of naturally occurring ground will rise and fall from winter to summer, bear this in mind when designing the road, and design for the highest water table conditions.
Also the ROAD PAVEMENT itself must be constructed so that it will drain in the event of a failure of the integrity of the surfacing layers.
The internal drainage function of a ROAD PAVEMENT is usually performed by the GSB layer, this itself MUST be drained.
Water below the ROAD PAVEMENT must be kept low and not be allowed to rise up into the construction layers, (and water CAN flow upwards, by capillary action).
Water that enters the ROAD PAVEMENT from the surface MUST have a drainage path out.
ROAD PAVEMENT FAILURE - WHEEL TRACK RUTTING - WHEEL TRACKING - BASECOURSE DENSIFICATION
WHEEL TRACK RUTTING is where, during hot weather, the surface course (wearing course) bituminous mixture in the vehicle wheel tracks softens and is displaced / pushed to each side of the actual track of the vehicle wheel when meeting resistance from the stable binder course (basecourse) material beneath the surface course (wearing course), with rutting you actually have a depression in the wheel track and a hump each side.
It is possible to have only WHEEL TRACKING, with no associated rutting, i.e. the twin humps.
This is where compaction in the wearing course and underlying binder course (basecourse) and even base (roadbase) has been inadequate.
So, during hot weather and the weight of traffic the various bituminous layers will compact causing a depression in the wearing course, often referred to as DENSIFICATION.
The most appropriate way to study the nature of the deformation is to actually "saw" transverse cross sections of the failed pavement for visual study.
ROAD PAVEMENT FAILURE - WATER DAMAGE - POOR DRAINAGE
Once water has entered a road pavement, WATER DAMAGE is initially caused by hydraulic pressure,
i.e. vehicles passing over the road pavement impart considerable sudden pressure on the water, this pressure forces the water further into the road fabric and breaks it up, this process can be very rapid once it begins.
Water that has entered the road pavement and is subject to the process of freezing and thawing during the winter also brings about the swift failure of the road pavement.
Eventually the water will descend to the subgrade layer below the road pavement and weaken this layer thus lowering the CBR of the subgrade which the road pavement design was based upon, and deep seated failure of the road will begin.
PREMATURE FAILURE OF A ROAD
The term PREMATURE FAILURE means what it says.
That is, the material, what ever it is, hot rolled asphalt (HRA) surface course (wearing course), surface dressing or structural concrete etc. has failed before its predicted design life has expired.
This should not happen, something was/is wrong.
It is necessary to determine whether the design, specification, supplied materials or workmanship was inadequate.
Except in obvious cases of failure this can be difficult to determine, especially as financial considerations often outweighs revealing the true problem, and reputations can be at stake.
ROAD SURFACE FAILURE
This term means what it says, only the surface of the road has failed, usually the bituminous SURFACE COURSE (WEARING COURSE).
The ROAD PAVEMENT is still sound, the SURFACE COURSE (WEARING COURSE) is still structurally entire, but the surface no longer performs the function it was designed to do, or the surface is beginning to oxidise and fret.
That can mean the SKID RESISTANCE is no longer adequate, or the surface is not water tight.
If a surface impervious to water is what you require, (you do not from all surface courses), a suitable SURFACE DRESSING will seal the surface and provide good skid resistance as long as the traffic loading on the road is not very severe.
Where a SURFACE COURSE (WEARING COURSE) is designed to be porous, e.g. FRICTION COURSE / POROUS ASPHALT and you have actually designed for the passage of water through the wearing course, you may wish to consider the change in nature a surface dressing will bring to the road surface before taking that course of highway maintenance.
The life of a SURFACE DRESSING on a VERY heavily trafficked road can be relatively short.
Unfortunately ROAD SURFACE FAILURE is often due to poor SURFACE DRESSING, i.e. FATTED surfaces, or lost chippings.
These can be rectified with remedial surface dressings performed with more care, but very often fatted surfaces need to be replaced with thin bitumen macadam wearing course, e.g 10mm CGM.
ROAD SURFACE FAILURE - FRETTING - FRETTING OUT OF ROAD SURFACE
This term, or the term FRETTING OUT are the terms used to describe a surface material that is beginning to lose its surface gradually, usually due to age,
BUT it can happen when the BINDER has been overheated in mixing thus prematurely ageing the binder.
This is not general failure or structural failure, but as I have said surface failure.
As long as overheating of the BINDER is not the cause of the failure, further life may be obtained from the surfacing material by the application of a suitable SURFACE DRESSING.
This can delay for many years the need to OVERLAY or reconstruct the road.
The SURFACE DRESSING will seal and hold the the FRETTING surfacing material.
SURFACE COURSE (WEARING COURSE)
It is necessary to decide whether the wearing course material has failed altogether, if so the material will need replacing.
If it is in a relatively sound condition an OVERLAY could be used if surrounding surface "levels" permit the addition of another road pavement layer, this process will increase the strength to the road pavement.
On the other hand perhaps the situation is that the RUNNING SURFACE is no longer satisfactory but the material itself is still good.
The ANTI SKID properties of the surface may be restored with a suitable SURFACE DRESSING, or even RETEXTURING.
Problems such as WHEEL TRACK RUTTING also have to be considered, both the material and RUNNING SURFACE may be adequate but there may be ruts of such depth as to be dangerous to vehicles passing through them on overtaking movements.
This will mean replacement or OVERLAY with WEARING COURSE and on occasions even the BASECOURSE as well.
RESTORING ROAD SURFACE SKID RESISTANCE FAILURE BY RETEXTURING
These are physical processes that will restore a texture to sound surfaces that have become smooth through wear.
One type of retexturing machine retextures by impacting the aggregate in the road surface with numerous small hardened steel hammers to roughen the aggregate surface
Another machine impacts the road surface with free moving discs mounted on a revolving drum.
Although the texture can be improved with these processes it must be remembered that the Polished Stone Value (PSV) of the aggregate will remain the same.
An improvement in SKID RESISTANCE VALUE of the road surface is NOT to be confused with PSV value of the aggregate, the PSV of an aggregate cannot be improved, it is what it is !
The improvement in the skid resistance of the road surface is caused by the creation/breaking of the exposed aggregate surfaces and once again providing "sharp" angular edges to the exposed aggregate, similar to the state of newly crushed aggregate.
The duration of the improvement in skid resistance will depend upon the actual quality / characteristics of the the aggregate being retextured, but with good aggregate an improvement can be sustained for several years.
An improvement is also achieved by removing the bitumen/fines matrix between larger aggregate particles and thus increasing the macrotexture of the road surface.
There are various contractors in the country able to provide machinery capable of this type of RETEXTURING process, both large lorry mounted and small hand operated units.
HOT ROLLED ASPHALT SURFACE COURSE (WEARING COURSE) FAILURE - PRECOAT LOSS
PRECOATS are lost in two ways.
1) They are lost into the hot rolled asphalt (HRA) mat at the time of application and rolling,
2) They are lost from the mat due to poor bonding and embedment.
However there are a number of reasons for both these types of chipping loss!
Reasons for PRECOATS lost into the mat:-
 Material too hot, if temperature is within specification delay rolling.
 HRA mat too thick, regulate prior to wearing course to achieve correct nominal thickness.
 Unstable material, WET SAND (usually after heavy rain), or bad mix proportions.
Reasons for PRECOATS lost FROM the mat:-
 Material too cold, PRECOAT binder coating is not melted to create bonding, check ALL temperatures
 HRA mat too thin, check thickness.
 Over chipping, chips on chips, check rate of spread.
 Delay in first pass of roller.
 PRECOAT bitumen coating too thin OR burnt/carbonised so no bonding of chips to asphalt.
FAILURE OF PRECOAT ADHESION - PRECOAT LOSS - HOT SAND TEST - HRA SURFACE COURSE (WEARING COURSE)
The HOT SAND TEST is a simple little test for assessing the quality of the bitumen coating on the chipping, i.e. the "precoating".
It is fully specified in BS 598:Part 108.
Basically it consists of shaking a certain quantity of PRECOATS in a given quantity of hot sand, (the temperature is specified).
It is necessary for the PRECOATS to retain a certain minimum quantity of sand for them to meet the specification.
The test is to ensure the binder on the precoated chippings has not been carbonised by overheating in the coating process.
It is necessary to have active binder coating the chippings to ensure good retention of the PRECOATS by the HRA WEARING COURSE.
During laying, the HRA at the correct temperature will melt the bitumen binder covering the PRECOAT and on cooling will bond the chipping into and onto the HRA wearing course.
HOT ROLLED ASPHALT SURFACE COURSE (WEARING COURSE FAILURE) - WET SAND - RIDE QUALITY FAILURE - PRECOAT LOSS
This is a phenomena which DOES occur whatever suppliers claim.
It is a rare happening, but when it does happen it is usually after a period of heavy or continuous rain that has saturated the sand stockpiles.
The time for the sand spent in the drier will not have been increased to remove the extra water,
OR production pressure does not allow for the increasing of drying time.
The water is not of course water but steam trapped around the sand particles, this produces instability in the asphalt.
It will show itself in the lorry body by being absolutely flat with usually free (displaced) bitumen on the surface.
If stood on, (on top of the sheet), in extreme cases you will be able to make the asphalt move like a liquid.
Because of its instable nature it is difficult to control the "bed" of the Blaw Knox making it difficult to lay the material to levels, also PRECOATS applied to the surface may disappear into it.
IRONWORK FAILURE, AND HOW TO AVOID IT
The term IRONWORK used in relation to highways maintenance and construction means gulleys (drains), manholes, stop-tap covers, inspection chamber covers, etc. etc..
All these products will be manufactured to the appropriate British Standard with regards to approved designs, dimensions and strength.
To achieve maximum effectiveness and strength IRONWORK must be lifted / installed / bedded in the correct way, poor workmanship in installation is the biggest cause of IRONWORK failure.
IRONWORK that has been lifted poorly will "move" and settle, making the road unsafe, and cause premature failure of the pavement construction / surfacing / overlay, and even failure of the manhole cover, gully. etc..
Obtain a copy of the IRONWORK LIFTING specification for full details,
but basically the chamber shall be soundly constructed and pointed (if brickwork), some chambers / gulley pots are pre-made in concrete.
Or, preformed in plastic which is then completely surrounded with fresh concrete for strength.
When/if the plastic "former" is not completely surrounded with suitable concrete this is often the cause of structural failure of the gully, sometimes within the first few years of construction.
The IRONWORK shall be bedded on an approved mortar, quite often a rapid setting mortar,
e.g. a strength of 10n./mm.squared at 2 hours.
IRONWORK can be lifted before or after the wearing course has been laid, but it is preferred to lift before final surfacing if possible as this allows more thorough inspection of the IRONWORK LIFTING, and it avoids extra joints in the surfacing and allows better compaction of material.
FOOTWAY FAILURE - TRL PROJECT REPORT PR/H/88/94 - FOOTWAY DESIGN
This report reviews the present methods for the design of footways and the causes of deterioration.
It also considers new approaches to the design of footway construction
WHY DID IT FAIL ? -
Before you start blaming materials as the cause of failure because it may be "convenient" to both Contractor and Engineer I suggest you look closely at the standard of workmanship employed in laying / using the various construction materials.
(But do bear in mind that some materials, especially hot bituminous materials, do need the right weather conditions to allow successful laying, however good the possible workmanship may be.)
First determine whether the quality of workmanship available is suitable for the nature of the work to be done.
Contrary to some opinion held by some management who believe road maintenance and construction can be performed by any type of workman, and achieve quality of work.
This is just NOT so, you need quality men and good quality plant for quality work and enough of them / it for the job in hand.
And, If you send a good kerb-layer, (with no black-top experience) to lay H.R.A. you have got problems!
Similarly, an experienced "black-top" operative is not likely to be to good at laying kerbs, etc..
WHY DID IT FAIL ? - BITUMINOUS MATERIALS
The following are a few possibilities:-
1] Did you order the correct material, and did you receive the material you specified, did you check the delivery ticket.
2] Was the material too hot/cold, did you check the temperature.
2a] Did the material look "dead", lacking any "shine", was it difficult to lay.
These factors indicate that the mixture may have been overheated in production, and/or stored at too high a temperature thus destroying the binder qualities of the bitumen.
It has then been allowed to cool down before delivery, but the damage to the binder has already occurred, the failure can often be seen early in the form of potholing in areas of stress.
3] Did the material visually appear to be of the correct composition, did it look dry / rich, uncoated, coarse / fine, segregated, make site notes, take sample for testing.
4] Was material stable in lorry body, i.e. still in noticeable heaps, not flat and "fatty"
5] Was there segregation within the material, a problem with larger aggregate, e.g. 32MM. BASE (ROADBASE), when the larger aggregate can roll to the outside edges of the lorry body as the material is load, and the lower the binder content of the mixture the more this is likely to happen.
So, take particular caution when using low binder content 32mm. "design" base (roadbase) mixtures.
6] With material that is flat and fatty was there condensation on underside of covering sheet, these conditions tend to indicate moisture in still present in material due to incomplete drying, and this will mean you have bituminous material with poor stability.
This can be a problem in hot rolled asphalt after periods of heavy rain when sand stockpiles at quarries have become saturated.
The above points assume you have correctly specified the material and the laying procedure has been correct and you still have a failure.
N.B. If you have doubts about a material, record these doubts even if there is no failure at that time, it may still happen!
WHY DID IT FAIL ? - SURFACE DRESSING FAILURE
SURFACE DRESSING fails for many reasons and it MUST be said most of the causes of failure can be controlled.
For those of you with little experience of SURFACE DRESSING I would recommend you first read and digest Road Note 39, and TRRL Report 627.
If you do not have immediate access to them read all items in this database with SURFACE DRESSING included in their title.
BUT I will list a few factors for you to consider, BUT in some cases (and ONLY some) the failure can be obvious:-
 Road hardness assessed wrongly.
 Wrong size chipping used.
 Wrong rate of spread of BINDER chosen.
 Different rate of spread of BINDER sprayed to that chosen, e.g.
a) Speedometer of sprayer inaccurate.
b) Incorrect "rate of spread speed chart" for a particular sprayer.
c) Tank pressure incorrect.
d )BINDER VISCOSITY not that specified.
 Wrong BINDER VISCOSITY specified for ambient temperature.
 Badly formulated BINDER e.g. unsuitable BASE BINDER.
 Poor traffic control.
 Site not suitable for surface dressing, e.g. areas of "screwing" traffic.
 Ambient temperature too cold.
 Dirty/dusty chippings.
For further information on surface dressing failure access the pages "linked" below.
|"Fatting up" failure||"Chipping loss" failure|
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