SPREAD RATES OF BITUMINOUS MIXTURES, DENSITY, VOIDS

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.

RATES OF SPREAD GUIDE FROM BRITISH STANDARDS

If you wish a simpler guide to approximate rates of spread of bituminous mixtures look at,
BS 594987 : 2007 : Asphalt for roads and other paved areas - Specification for transport, laying and compaction and type testing protocols - Annex B - Approximate rates of spread of asphalt
You will note there is a range in the figures provided, especially the thinner a material is laid, this is why I suggest that the spread rate obtained from first principles is the better figure to use. 

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.


Guide Figures for Bulk Density of often used Materials

                             Tonnes per cubic metre

Dense bitumen macadams             2.3 / 2.4    (compacted)
Hot rolled asphalts                2.3 / 2.4        "   
Open graded bituminous macadams    1.9 / 2.1        "
Slurry macadams                    2.2 / 2.3    (after "setting")       
Well graded granular sub-base      2.0 / 2.2    (compacted)
Concrete                           2.3 / 2.5    (compacted, hardened structural concrete)
Pulverised fuel ash                1.2 / 1.4    (drained)
Single size aggregate              1.7 / 1.9

Other useful density figures

                               Tonnes per cubic metre

Bitumen                                   1.0   

Aggregate (most "granite" types)          2.7   ( as solid rock, i.e. density/specific gravity)


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 you obtain when you are laying the bituminous mixture.)
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, but early "tired" roads may be the result.
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, especially if you take the cores while existing traffic management for the work is still in place.

If you are performing Percentage Refusal Density, or any other form of compaction test you will have exact densities available for the macadam base (roadbase) and binder course (basecourse) that you are using.

RATES OF SPREAD AS AN AID TO BUDGETING

Many proprietary materials are sold only at a cost per square metre, by 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, quite surprising.

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.

If you are not acquainted with the tolerances that can be applied to the layers of the road pavements, particularly the bituminous layers, you should be.
If you are paying for your bituminous materials on a square metre basis a good contractor, and I mean this on the basis that he knows the contract specifications, and knows what is permissible, he will bring the binder course slightly high and take advantage of the +/- 5mm. tolerance on the finished surface course thickness to save him 5mm. of the expensive binder rich surface course, this is assuming a 40mm. nominal thickness of surface course.
This is all entirely correct, that is why some highways maintenance organisations pay on tonnage with a supervisor on site collecting delivery tickets. If the contractor lays it a little thicker at least you are getting what you pay for and the result is a stronger more durable road.
For guidance on this matter see, 
BS 594987 : 2007 : Asphalt for roads and other paved areas - Specification for transport, laying and compaction and type testing protocols - Para. 5.2 Surface level tolerances
Large surfacing contracts are won and lost on the difference in costs that the application of tolerance allowances can produce. 
But of course you need a quality paving gang to work to the tolerances, as simple coring will easily tell if the permissible tolerance has been exceeded.

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

NOTE : This test has largely been discontinued as a "standard" test for determining the compaction of bituminous base and binder course, however, at this time, I still believe it exists as a British Standard test and can therefore be included in a contract if you so stipulate.

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.

Current requirements and guidance for compaction and compaction control is to be found in,
BS 594987 : 2007 : Asphalt for roads and other paved areas - 
Specification for transport, laying and compaction and type testing protocols

Personal Note
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, or indeed the thickness it is to be laid, 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, and less mental effort, 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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