BITUMINOUS (THERMOPLASTIC) MATERIAL TEMPERATURES

CONTENTS

WARNING
BITUMINOUS MATERIALS SPECIFIED IN BRITISH STANDARDS
A QUICK ACCESS TO A GUIDE TO THE MORE COMMONLY NEEDED TEMPERATURES
STORAGE OF BITUMEN AND BITUMINOUS MATERIALS
PROPRIETARY BITUMINOUS MATERIALS
RECLAIMING BINDER
TAKING OF TEMPERATURES
PRACTICAL TEMPERATURE TAKING
LASER THERMOMETERS
COMPACTION
WIND CHILL FACTOR WHEN LAYING ASPHALT AND BITUMINOUS MACADAM
WIND CHILL TEMPERATURES - GUIDANCE FOR ASPHALT / BITUMINOUS MACADAM LAYING

BITUMINOUS MATERIALS SPECIFIED IN BRITISH STANDARDS

Hot Rolled Asphalt Temperatures:-

The relevant temperatures for this material (roadbase, basecourse and wearing course) are to be found in :-

BS 594:Parts 1 and 2

BS 594 HOT ROLLED ASPHALT FOR ROADS AND OTHER PAVED AREAS
Part 1:- Specification for constituent materials and asphalt mixtures.
Part 2:- Specification for the transport, laying and compaction of rolled asphalt.

Coated Macadam Temperatures:-

The relevant temperatures for this material (roadbase, basecourse and wearing course) are to be found in :-

BS 4987:Parts 1 and 2

BS 4987 COATED MACADAM FOR ROADS AND OTHER PAVED AREAS
Part 1:- Specification for constituent materials and for mixtures.
Part 2:- Specification for transport, laying and compaction.

I strongly recommend all staff involved in the laying of materials to have access to the above documents.

A QUICK ACCESS GUIDE TO THE MORE COMMONLY NEEDED TEMPERATURES

MATERIAL                                    DELIVERY TEMP     ROLLING TEMP
                                                MAX   MIN         MIN
H.R.A. WEARING COURSE (50 PEN) .................190   140          85
H.R.A. WEARING COURSE (70 PEN) .................185   140          80
H.R.A. BASECOURSE/ROADBASE (50 PEN) ............170   120          85
DENSE/CLOSE GRADED MACADAMS (100 PEN BIT.) .....160   120          95
DENSE/CLOSE GRADED MACADAMS (200 PEN BIT.) .....150   110          85
DENSE/CLOSE GRADED MACADAMS (300 PEN BIT.) .....130   100          80
OPEN GRADED MACADAMS (200 PEN) .................125    95          75

( The above temperatures may well have changed slightly, if for no other reason than available binder grades have changed, please refer to the current relevant British Standard. )

STORAGE OF BITUMEN AND BITUMINOUS MATERIALS

Bitumen Coated Macadam :-
In Appendix A of BS 4987 Part 1, there is in table 42 a list of recommended storage temperatures for certain grades of binder.

Hot Rolled Asphalt :-
There is a similar table i.e. Table 1, Section 2, in BS. 594 Part 1.

Please refer to these British Standards.

These temperatures can be regarded as applicable to mixed material held in storage-bins as it does state "prolonged subsequent storage above the temperatures given in this table should be avoided".

Individual proprietary modified binders may have even more stringent conditions relating to storage and mixing temperature, the appropriate temperatures and storage times will be provided by the binder supplier / manufacturer.

The engineer needs to be aware of the temperature parameters of a modified binder when purchasing proprietary bituminous mixtures incorporating these binders, because without this information he cannot ensure any special conditions of mixing and storage, and laying are complied with by the producer / supplier of the mixed bituminous material and the laying contractor.

It is pointless specifying a modified bitumen, at increased cost, to achieve enhanced engineering characteristics from the binder if the enhanced properties are destroyed through overheating at the time of mixing or subsequently being held at a high temperature for too long a period.

PROPRIETARY BITUMINOUS MATERIALS

With these materials you will need to obtain the specification for the product, this should  include the various temperature criteria.

RECLAIMING BINDER

If you have reason to believe the binder of laid material has been hardened or damaged to such a degree it is unacceptable, it is a relatively easy and inexpensive test to remove a material cut-out, extract the binder and perform penetration and softening point tests on the reclaimed binder to compare with the requirements of the specification. 


TAKING OF TEMPERATURES
 The temperatures quoted above from British Standards are delivery temperatures, (or more accurately "maximum temperature at any stage"), and as such should be taken within 1/2 hour of delivery on site.
Although material not tested within that time will still be unacceptable if it is too hot when tested at a later time.
But material tested for temperature after 30 minutes that is too cold could be deemed to have been hot enough on delivery if it had been tested at that time.

The taking of temperatures as early as possible and informing the contractor/supplier of any problem is good practice.

The detail of how to take the temperature of hot bituminous material, the type of thermometer to be used and the calibration of the thermometer is all to be found in:-

B.S.598 : SAMPLING AND EXAMINATION OF BITUMINOUS MIXTURES FOR ROADS AND OTHER PAVED AREAS.
Part 109:-Methods for the assessment of the compaction performance of a roller and recommended procedures for the measurement of the temperature of bituminous mixtures.

BUT basically the British Standard recommends the use of a 300mm. long probe and an electronic thermometer accurate to +/- 2 degrees with the probe inserted at least 250mm. into the material, 500mm. from any edge of the lorry body, moving the probe frequently before recording the final temperature.
It is also good practice to have access to a "back-up" thermometer in case of disputes.

It is a fact that with modern lorry sheeting methods and health and safety regulations it is not that easy to be able to gain access to the bituminous material in the back of a lorry, and so temperatures are often taken at the time of discharging the material into the paver hopper.

Most times this will not be a problem as the temperature will be in specification, but when it is too hot there is a considerable amount of pressure put on the operative taking the temperature to allow the continued laying of that load, especially as the laying contractor will want confirmation that the thermometer is accurate.

Once you are in the situation that you know overheated material is being brought to site it is necessary to provide some safe means of testing the temperature of other material on site while it is still in the lorry body, not a convenient process but one that is required.

LASER THERMOMETERS
"Laser" thermometers have been around for a few years now but until recently (in my opinion) have been too expensive to use as a general on site piece of testing equipment.
However as with most things related to modern electronics and mass production the prices of this type of equipment have fallen dramatically in the last year, and they are now very affordable and very useful at what they do, i.e. they take take surface temperatures very easily and quickly, and from a safe position.  
The pictures to the left show an example of what is available. I have missed out my usual "scale", but it is about 10 centimetres long (4 inches in real money) and is quite sturdy. It incorporates a red laser to "aim" at what you want to know the temperature of, but I believe the actual temperature measurement is based on the use of the infra red wavelength.
This particular model comes with a little "holster" which you can place on your belt, this may seem amusing but carrying around your temperature probe and read-out box can be awkward on site, and site staff are far more likely to use devices that are easy to use.
This thermometer costs about £80:00, is quite robust and has a reasonable range of about two metres with good accuracy.
There are other models available that are accurate up to about one metre that are small enough to be easily carried in your pocket, and can be purchased for about £40:00.
I must stress  these devices measure surface temperature, so in my opinion it will always be necessary to have a sturdy long probe thermometer available for taking the temperatures of  loads of "hot" material before tipping, as once tipped it is not easy to get it back in the lorry if it is two hot in the bulk of the load.
It is also necessary to use a "conventional" thermometer to comply with the British Standard when taking temperatures of bituminous mixtures, (see the previous section).  
But I believe these, now affordable, laser thermometers are two be recommended for use by all staff who work on site where hot thermoplastic materials are used, and this means such work as thermoplastic road marking, hot high friction surfacing as well as the more usual laying of hot bituminous materials.
When an operation is easy to perform site staff are far more likely to be prepared to carry out the function far more readily and far more often, I know I am.
A quick temperature check of "bucketfuls" of road marking thermoplastic or hot applied high friction surfacings as they are being transferred to the laying apparatus could go a long way to improving the durability of these products.
Over heating bituminous and thermoplastic materials, although it is costing the producer money, which is why I find it difficult to understand why it happens, is one of the most common causes of poor performance of a range of hot applied road surfacing materials. 

COMPACTION

I have been concentrating on the damage that can be done to a bituminous mixture by overheating, but of course the strength of the road pavement can be seriously impaired by laying too cold and thus preventing full compaction.

Bituminous material MUST be fully compacted with minimum air voids to achieve maximum pavement layer strength and hence be best able to distribute load over the road foundation.

So, it is also extremely important to ensure rolling of bituminous material takes place before it falls below the minimum rolling temperature in order that complete compaction can take place.

With regards to "rolling" temperatures there is some interesting terminology.
There is "Minimum rolling temperature", this means compaction of the material shall be substantially completed while the mixed material is greater than this temperature,
and,
there is "Minimum temperature immediately prior to rolling".

In my opinion we can take the "Minimum temperature immediately prior to rolling", as the minimum delivery temperature, these are the figures I quote in the table under that heading.

You may not receive full temperature information from suppliers of proprietary "Thin Surfacings", if this is so it is my opinion we should defer to the temperature requirements stated in British Standards for the particular binder grade.

WIND CHILL FACTOR WHEN LAYING ASPHALT AND BITUMINOUS MACADAM

It is extremely important to take into account the WIND CHILL FACTOR when laying hot bituminous materials, especially when laid in thin layers. 

This problem relates especially to  H.R.A. WEARING COURSE when applying PRECOATS, and in particular with a "dry" H.R.A. DESIGN WEARING COURSE. 
The minimum air temperatures laid down in specifications allowing the laying of bituminous materials do not always take account of wind speed. 
The rate at which a "hot material" looses heat is greatly increased with increase in wind speed.

Graphs are available in the "700" series in the March 1998 DOT Specification for Highway Works.

WIND CHILL is a VERY VERY real problem, and if you do not take account of it in your site management the quality of work will suffer. 

ALL MATERIAL TO BE LAID TO BE ON SITE BEFORE YOU START LAYING,  HAVE THE CHIPPER CLOSE BEHIND THE PAVER, AND THE ROLLER CLOSE BEHIND THE CHIPPER 

A hot material laid at an actual air temperature of plus 7 degrees centigrade and a wind speed of 25 m.p.h. will loose heat at a rate equal to a still air temperature of minus 5 degrees. 

TRRL Research Report 4 : Cooling of bituminous layers and time available for compaction 

This is a report on the cooling of bituminous layers, the factors affecting cooling, and the time available for compaction. 

In simple terms the thicker the layer the longer it will take the material to cool and the longer time you will have for compaction. 

This is especially important with WEARING COURSE layers in winter, i.e. they are thin and cool quickly, and even with thicker layers you can get a "skin" of chilled surface material.
This can be a particular problem with precoated chippings on hot rolled asphalt wearing courses, if the surface of the laid mat chills quickly there will not be enough heat to melt the precoated chipping bitumen coating which bonds it to the bituminous material.

You can even have situations where chippings are pushed into the mat, because underlying material is still hot and plastic, but precoats have not formed a bond with the wearing course.   

You MUST remember however that thick layers of hot material in hot summer  weather can also be a problem because of the length of time it will take to cool.
You may also lose PRECOATS in the HRA WEARING COURSE if you do not control rolling carefully. 

WIND CHILL TEMPERATURES - GUIDANCE FOR ASPHALT / BITUMINOUS MACADAM LAYING

This table is an INDICATION of the wind chill effect of certain wind speeds at certain temperatures.   
There are NO absolute wind speed criteria written in British Standard specifications,

But,  BS 594:1992:PART 2:CLAUSE 6, will give guidance,
        also
        BS 4987:PART 2:CLAUSE 5.2.2 

Below is a guide to indicate what a difference the cooling effect of the wind does make.

WIND SPEED  |                ACTUAL TEMPERATURE, (DEGREES C)
.....................13.............7..............2............-4
            |   Below, Wind-chill temperature, 
            |   i.e. equivalent still air temperature
..5mph......|........12.............7..............0............-7
.15mph......|.........8.............0.............-9...........-17
.25mph......|.........3............-5............-13...........-22
.35mph......|.........1............-7............-16...........-25

There is  very good information on  WINDCHILL in the DTP DESIGN MANUAL HD 27/94 : Pavement construction methods

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