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The Idiots' Guide to Highways
Maintenance |
SPREAD RATES OF BITUMINOUS MIXTURES, DENSITY, VOIDS
CONTENTS
RATES OF SPREAD OF
MATERIALS FROM FIRST PRINCIPLES
DENSITY OF
CONSTRUCTION MATERIALS
RATES OF SPREAD AS
AN AID TO BUDGETING
BITUMINOUS MATERIAL
DENSITY
PERCENTAGE REFUSAL
DENSITY
RATES OF SPREAD OF
MATERIALS FROM FIRST PRINCIPLES
How many
square metres to the tonne is it possible to obtain from a
particular material at a given thickness ?
There are available in various publications tables of spread
rates for particular materials,
E.g.
BS 4987 : Part 2 : Table 5, Approximate rates of spread of coated
macadam.
However these
tables, although very useful, give a range of rates of spread for
a particular thickness of material.
But it maybe worth relating a quite simple process of determining
quite accurately the rate of coverage of any material, providing
you know the density of the material used and the precise
thickness at which it is intended to be laid.
The trick in this process is to imagine all the square metres of
material of the appropriate thickness laid on top of one another
as part of a cube metre rather than laid flat on the ground.
Example
1
In a cube metre of any material you have in
fact 1000 square metres of 1mm. thickness laid on top of one
another.
If the rate of spread of material you require to determine is
being laid 100mm. thick, then in a cube metre you will have,
1000 divided by 100, square metres,
i.e. 10 square metres laid on top of one another.
You now need to know the density of the material in question in
tonnes per cubic metre.
Let us assume the material being laid is dense bitumen macadam
basecourse If you now divide the amount of square metres of
material you can obtain from a cubic metre by the density, in
this case 2.3, the rate of spread is 4.35 square metres per
tonne.
(You do not get many square metres to the tonne when you are
laying it 100mm. thick.)
This principle works for all materials providing you know their
density, and since nearly all of us have a pocket calculator to
hand it is quite simple to work out.
Example
2
Let us assume you wish to know how many
square metres you can obtain from a tonne of 35%/14mm. HRA
wearing course laid 45mm. thick.
a) Divide 1000 by 45 = 22.2 square metres (Remember
this is from 1 cubic metre of the material.)
b) Divide 22.2 by 2.35 (Approx. density of 35%/14mm. HRA)
= 9.45 square metres per tonne
e) i.e. if you are going to lay 35%/14mm. HRA 45mm. thick it is reasonable to
expect that one tonne of asphalt will provide 9.5 square metres of laid
surfacing, with a little bit of a plus or minus factor depending upon the mix
design and the density of the aggregate in the mix, this variability for
practical purposes will not be great.
Note : If you are laying on existing (variable) road
surfaces in traffic sensitive areas it does not pay to try and be too accurate,
there is nothing worse than "dying in the hole", i.e. having
insufficient material available at the end of a day, with no more supply available,
for the last few metres of the job.
This is embarrassing, causes further disruption to the motoring public, and
costs you money due to disruption of the works programme.
If the production plant fail to supply the quantity ordered that is a different
situation.
DENSITY OF
CONSTRUCTION MATERIALS
The density figures included here are good "working"
figures and so may be a useful guide in working out rates of
spread, quantities of material required, etc., but because of
differences in aggregate gradings and the actual density of the
source of the aggregate there may be some difference, but they
should not be far off.
Density
of often used materials
Dense
bitumen macadams
2.3 / 2.4 tonnes per cubic metre
Hot rolled asphalts
2.3 / 2.4
Open graded bituminous macadams 1.9 / 2.1
Slurry macadams
2.2 / 2.3
Well graded granular sub-base 2.0 / 2.2
Concrete
2.3 / 2.5
Pulverised fuel ash
1.2 / 1.4
Single size aggregate
1.7 / 1.9
Other
useful density figures
Bitumen
1.0 tonnes per cubic metre
Aggregate (most "granite" types) 2.7
Note : The variation in
aggregate density, and the effect it can have on binder content and air void
content, as well as over-all bulk density, is now being taken into account in a
number of the new "bituminous mixture" specifications,
e.g. BS EN 13108-5:2006 :
Bituminous Mixtures - Material Specifications - Part 5:Stone Mastic Asphalt
:
Clause 5.2.3 Binder Content
and you should be making yourselves aware of the situation, if the
suppliers/production plants have not already brought it to your attention.
The difference in aggregate density can make a difference to sample results that
are near to the specification limits on binder content and air void content.
(You might like to be aware that the more dense the material the less coverage per tonne.)
However my argument is that materials should be designed to the mid point of the
specification limits to allow for variation in production, I thought that is
what the tolerance around a mid-point target was for.
If this practice is followed it is unlikely that differences in aggregate
density will take a mixture outside the specification limits.
I do not like the practice of designing bituminous mixtures to be just within
the specification limits, I can understand the commercial pressures but lament
the loss of engineering quality, and you only need a small hiccup at the plant
for material to fall outside the specification envelope.
If you are not purchasing to a production specification, but to a performance
specification it might be a little time before the situation will bother
you.
All bituminous mixtures,
including proprietary mixtures, have
to be produced to a "recipe" at the production plant, that is the only
way to ensure conformity, therefore it does not seem unreasonable to me that a person/body
who is purchasing this product should be aware of the "recipe", and be
able to sample and test the supplied product if they so wish.
Many respected Engineers responsible for highways maintenance are questioning
whether,
Sector Scheme 14 : For the
quality assurance of the production of asphalt mixes
is actually achieving the
desired outcome of continuous quality in bituminous mixtures employed in
highways construction and maintenance..
The CSS (County Surveyors Society) have published the "Advice
Note for the Specification of Thin Surfacings"
to assist Engineers in the "specifying" of proprietary bituminous
mixtures.
It really is necessary
to read and understand the implications of (new) specifications and other
documents that govern the production and supply of materials used in highways
maintenance.
I include above the density of bitumen while we are talking on the subject because binder
percentage in a bituminous mixture is measured and stated by
mass, but in fact for every one percent increase in binder
content by mass you are obtaining a much larger increase (approximately 2.5
times) in
bitumen volume in the mix to coat the aggregate and improve
durability, all other design aspects of the mixture being
correct.
The figures I suggest may not be appropriate to the actual
materials you are using and so may be disputed by the contractor,
or engineer if you are a contractor, "massaging" the
true figures even slightly can make a big difference on a job
where large quantities are involved.
Do not worry if the difference cannot be resolved amicably,
testing materials for density on site or in the laboratory is one
of the easiest and least expensive tests to undertake, so do not
be afraid to have a number of density tests performed if a large
amount of money is at stake, the cost will easily be repaid.
If you are performing Percentage Refusal Density tests you will
have exact densities available for macadam base (roadbase) and binder course (basecourse).
RATES OF SPREAD AS
AN AID TO BUDGETING
Many proprietary materials are sold only at a cost per square
metre, if knowing the thickness at which the material is laid it
is possible, by the calculation already described, to work out
the cost of a tonne of material, this cost can be quite
surprising compared to a generic product, and it is an exercise
well worth performing.
Example 3 :- You are considering a single application of a
"Thin Surfacing" laid 20mm. thick and you wish to
find the cost per tonne of the material laid.
a) Divide 1000 by 20 = 50 square metres from a cubic
metre
b) Divide 50 by 2.1 (Open graded density) = 24 square
metres from a tonne of material
If the cost per square metre of the material is £7:50, which it
could well be, it means the cost per tonne of the laid material
is £180:00, i.e. 24 x £7:50
Having arrived at this information you can then consider other
materials that you buy on a laid tonnage basis, i.e. materials
supplied to a British Standard, as a matter of routine to see
which is the most cost effective material when purchased in tonnes.
BITUMINOUS MATERIAL
DENSITY
In view of recent introductions of more open proprietary
bituminous materials it is necessary to understand the different
density terms used :-
Laid
Density / Bulk Density
This is the density of the material in a
laid and fully compacted condition, it includes the voids present
in the layer due to the open nature of the grading of the
bituminous material.
The laid / bulk density will be quite a bit less dense than Mix
Density according to the degree of "openness" of the
grading.
It is important to use the Laid / Bulk Density when calculating
rates of spread.
Mix
Density
This is the density of the actual mixed
material excluding the large voids found in the mixed material
once it has been laid.
TRL Project Report 78 : Tests for voids and compaction in rolled
asphalt surfacing,
is an
excellent reference to understand the different terminology
regarding voids and density with regard to bituminous mixes.
PERCENTAGE REFUSAL
DENSITY
Percentage refusal density tests are performed on dense bitumen
macadam roadbases and basecourses to ensure these materials have
been fully compacted during laying.
Percentage refusal density results are an excellent source of
laid / bulk density figures.
But also be aware that when you see a figure of 93% of maximum
refusal density as an acceptable compacted density figure this
does not mean an air void content of 7% in the compacted
material.
The figure 93% of maximum refusal density means exactly what it
says, but the maximum compacted density could well have 5% voids
in it if the grading of the material is rather open, so the laid
material will have an even greater void content and still be
acceptable with regards to compaction.
This
page is not meant as a "knocking the contractor / supplier" exercise,
it is meant as a know what you are doing exercise.
By knowing what you are doing you are able to assess whether each contractor /
supplier is pricing for work on an equal footing.
There are many good
contractors and suppliers out there with good people working for them, who will
provide you with materials and workmanship you require.
In my opinion if you claim to be a highways engineer or an engineering
consultancy it is not the contractor's or supplier's duty to select the correct
bituminous mixture for your work, it is your responsibility, I assume that is
what you are being paid for, amongst other things.
Not fully understanding materials, especially bituminous mixtures, may cause you
to choose an option that may well cost you less at the time, but will be a less
cost effective option over the following years.
It really does "pay" to know what you are doing, if you do not, find a
man/woman who does, somebody like an experienced Materials Engineer, perhaps,
this way you may actually be able to select a bituminous mixture that costs less
and will meet the requirements of the site in question.
You do not need a Ferrari to do the shopping, in fact a Ford Focus would be far
more suitable for the task.
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