AGGREGATE PROPERTIES

CONTENTS

Aggregate Abrasion Value, (AAV)
Aggregate Crushing Value, (ACV)
Ten Percent Fines Value, (TFV)
Aggregate Impact Value, (AIV)
Polished Stone Value, (PSV)
Artificial Aggregates (Hardness) 
Magnesium Sulphate Soundness Value, (MSSV)
Aggregate Size
Aggregate Grading
Flakiness
Grading Zone
Well Graded
Poorly Graded
Gap Graded
Moisture Content - Water Absorption - Frost Susceptibility

British Standards - Formerly , and may still be, relevant to aggregate properties 
BS EN Standards - Relevant to aggregate properties 

Informative Links

Above are listed a number of aggregate properties that it is important to have a basic knowledge about in order to be able to specify materials correctly, and to ensure the materials you specify will perform as you want them to.
It is important to note that after initial trafficking, and removal of any surface bituminous coating, vehicles will be traveling on the actual aggregate used in the mixture for the bulk of the life of the road surface. 
Therefore selecting aggregate with the necessary characteristics for a particular site is paramount.
There are plenty of British (European) Standards, DOT Design Manuals, TRL Reports, CSS Reports, and good text books about how to find further and more complete information on the topics included here.
I have compiled a list of new (BS EN) specifications at the foot of this page. I am not claiming it is comprehensive but it will give some help in keeping up with the changes "Europeanisation" of standards relating to aggregate is having.

The current "overall" standard for aggregate properties is,
BS EN 13043 : 2002 : Aggregates for bituminous mixtures and surface treatments for roads, airfields and other trafficked areas,
with cross reference to other new BS EN Standards for more specific requirements.
(Although reference may still be made to the more familiar, and easily understood, "old" British Standards.)

The report, 
"The Sustainable Use of High Specification Aggregates for Skid-Resistant Road Surfacing in England"
has recently been published and is available on a CD-ROM, from :-

Capita Symonds,  
Tel.   01342 327161,
Fax.  01342 315927,
or, email : claire.huxley@capita.co.uk,
asking for the Project Manager, Claire Huxley, a CD-ROM copy will be sent to you without charge.

This 162 page A4 report represents part of the output from research carried out by Capita Symonds Limited (CSL - formerly Symonds Group Ltd., and before that, Travers Morgan Ltd.), for the Office of the Deputy Prime Minister (ODPM) under research contract SAMP/1/039.
I understand that the report will eventually be obtainable from the MIRO (Mineral Industry Research Organisation) website, after some refurbishment of the site.
This is a comprehensive report on the subject of High Specification Aggregates, and aggregate properties, containing a wealth of information on the subject of highway surfacing, aggregate procurement and its efficient use.
This report raises serious issues on the use of scarce resources of premium quality aggregate in England, and in my opinion needs to be obtained and studied.
This document contains a number of tables that give an indication of the properties of aggregates from various sources in the UK, however the properties of aggregate within quarries can change as blasting moves into different seams of stone within the quarry and verification of current aggregate properties is always a good idea.

AGGREGATE ABRASION VALUE, (AAV)

It is important that we know that aggregate will not wear away, abrade, too quickly. This applies in particular to aggregate present in wearing courses and surface treatments.

Therefore we need to specify aggregates that have a maximum abrasion value when they have been tested in accordance with a standard testing regime.

This standard U.K. testing regime is ,

BS 812 : Testing Aggregates : Part 113 - Method of determination of aggregate abrasion value
or it may now be,
BS EN 1097 : 1998 : Tests for mechanical and physical properties of aggregates : Part 1 : Determination of the resistance to wear (micro-Deval) 

With the test in BS 812 : Part 113 a known mass of aggregate chippings are set in resin in a small flat test panel and subject to abrasion on a grinding apparatus that is fed with a known type of sand. The loss in weight of the aggregate after a specified time as a percentage of the original weight is the AAV.

There are other types of abrasion test where a known mass of aggregate is placed in a small rotating steel drum with a number of hardened steel balls.
The drum is rotated for a set time and the material passing a specified fine sieve, at the end of the test, is regarded as abraded aggregate and this loss as a percentage of the original weight is the AAV by this method.

Maximum AAV's for various highway situations are now stated in the,

Dot Design Manual for Roads and Bridges, HD 36/99 - Surfacing Materials for New and Maintenance Construction
The information was formerly included in,
DOT Design Manual for Roads and Bridges, HD 28/04 - Skid Resistance

AAV's required differ according to "weight" of commercial traffic per lane.
A maximum value of 10 is specified for chippings/precoats for lanes carrying over 3250 commercial vehicles per day. 
A maximum value of 14 is sufficient for less than 250 commercial vehicles per lane per day.

N.B. The smaller the number, the less the aggregate abrades.
The exclusion of aggregate with low AAV's is particularly relevant to coated 20mm chippings, i.e. "precoats" applied to hot rolled asphalt wearing course layers.

AGGREGATE CRUSHING VALUE, (ACV)

The aggregate crushing value is a value which indicates the ability of an aggregate to resist crushing. 
The lower the figure the stronger the aggregate, i.e. the greater its ability to resist crushing.

The UK test to determine the aggregate crushing value is ,

BS 812 : Testing Aggregates : Part 110 - Method of determination of the aggregate crushing value
or it may now be,
BS EN 1097-2 - Tests for mechanical and physical properties of aggregates : Part 2 - Methods for the determination of resistance to fragmentation

In brief, in BS 812:Part 110, a sample of 14mm. size chippings of the aggregate to be tested is placed in a steel mould and a steel plunger inserted into the mould on top of the chippings.
The chippings are subject to a force rising to 400kn. over a period of 10 minutes.
Usually by placing in a concrete crushing apparatus.
The fine material, (passing a 2.36mm. sieve), produced, expressed as a percentage of the original mass is the aggregate crushing value, (ACV).

TEN PERCENT FINES VALUE, (TFV)

This test is not is not too dissimilar from the test to determine the aggregate crushing value.

But instead of using a standard force of 400kn., the force at which 10% of fines is produced is noted as the Ten Percent Fines Value.
This usually requires a number of tests and a graph to establish the exact figure.

This test tends to be used for softer aggregate where a force of 400kn. would crush most or all of the aggregate.

The test is set down in,

BS 812 : Testing Aggregates : Part 111 - Methods for determination of ten percent fines value

AGGREGATE IMPACT VALUE, (AIV)

The aggregate impact value is a strength value of an aggregate that is determined by performing the Aggregate Impact Test on a sample of the aggregate in question.

The test is fully described in,

BS 812 : Testing Aggregates : Part 112 - Method for determination of aggregate impact value

Basically the AIV is the percentage of fines produced from the aggregate sample after subjecting it to a standard amount of impact.

The standard amount of impact is produced by a known weight, i.e. a steel cylinder, falling a set height, a prescribed number of times, onto an amount of aggregate of standard size and weight retained in a mould.

Aggregate Impact Values, (AIV's), below 10 are regarded as strong, and AIV's above 35 would normally be regarded as too weak for use in road surfaces.

Aggregate Impact Values and Aggregate Crushing Values are often numerically very similar, and indicate similar aggregate strength properties.

POLISHED STONE VALUE, (PSV)

The PSV of an aggregate is a measure of the resistance of an aggregate to polishing, and the value is established by subjecting the aggregate to a standard polishing process, and then testing the aggregate with the Portable Skid Resistance Tester.

The testing procedure and description of the process is set out in,

BS 812 : Testing Aggregates : Part 114 - Method for determination of polished-stone value

N.B. The PSV is a value applicable to a particular aggregate and NOT the road surface.

The property that an aggregate possesses which gives it a good PSV is often referred to as its' MICRO-TEXTURE.

Aggregate that has a PSV over 60 is regarded as a High Skid Resistant Aggregate, but aggregate with a PSV over 65 is needed for particularly high stressed sites.
The higher the the PSV figure the greater resistance the aggregate has to polishing, and the greater the ability the aggregate has to retain its inherent very fine surface texture, i.e. MICRO-TEXTURE.

ARTIFICIAL AGGREGATES (HARDNESS)

"High Friction (Skid Resistant) Surfacings" for approaches to pedestrian crossings and difficult roundabouts will have an artificial aggregate such as calcined bauxite, this aggregate should have a PSV in excess of 70.
That calcined bauxite is so successful in the applications where it is used, achieving high Skid Resistance Values (SRV), is due as much to its extreme hardness, as its high polished stone value. 
In my opinion it is not wise to use processes where an unknown amount of calcined bauxite has been replaced by a natural aggregate. 
Even though the natural aggregate may have a high PSV, it will not have the hardness of a calcined bauxite and therefore will loose its angularity and eventually "smooth" over, whereas the calcined bauxite will retain its "sharp" edges and high levels of friction/skid resistance.

MAGNESIUM SULPHATE SOUNDNESS VALUE

This is often referred to as AGGREGATE SOUNDNESS.

This is a test to simulate weathering characteristics of an aggregate, or more pecisely its ability to resist weathering.
The particular aggregate being tested is subject to a number of immersions in an aggresive solution of magnesium sulphate to hasten the degrading process the environment has on a road aggregate.
After the aggregate has been subject to the testing regime the remaining weight of the aggregate is expressed as a percentage of the original weight.
It is a general requirement that road aggregate shall have a MSSV above 75.

Aggregate Soundness tends to be related to the water absorption of an aggregate, i.e. an aggregate with a high moisture absortion value tends to have a low Magnesium Sulphate Soundness Value.

The test is fully described in :-

BS 812 : Testing Aggregates : Part 121 - Method of determination of aggregate soundness

AGGREGATE SIZE

The size of an aggregate is not quite what it seems.
The size of a particular aggregate will depend on what sieve sizes determine the grading of an individual material, or in the case of a quarry what screen sizes are used to separate out crushed aggregate.

If you have a material where the normal sieve/screen sizes are, 37.5mm., 28mm., 14mm., 10mm., 6.3mm. etc.,

A 28mm. aggregate will be that aggregate which passes the 28mm. sieve and is retained on the 20mm. sieve.

So, in the case of a 28mm. aggregate the size could be 27.9mm or 20.1mm., and still be regarded as a "28mm. aggregate".

This variance in true size can be a particular problem with surface dressing chippings, which are single size.

It leads to such expressions as a "bold" 10mm. chipping, or a "small" 10mm. chipping, meaning the bulk of the chippings are quite near the 10mm. size or the 6.3mm. size.

Chippings being "bold" or "small" can necessitate a change in binder spread rates to ensure retention of the chipping, or to prevent "fatting up" of binder.

AGGREGATE GRADING

Aggregate grading is the term given to the percentages of the different size fractions, after sieving, that go to make up the whole material.

To obtain the different size fractions for weighing, the sample of aggregate is sieved on the appropriate sieve sizes for the particular material, and the retained aggregate amounts weighed.

This process is known as "grading", or, more scientifically put you are determining the particle size distribution of the material.

The necessary sieve sizes for a particular material will be found in the appropriate specification the material is supplied to.

The test for particle distribution of a "dry stone" aggregate is fully described in,

BS 812 : Testing Aggregates : Part 103 - Method for determination of particle size distribution

The reverse process to performing a grading on a material is a supplier blending appropriate amounts of single size aggregates to create the correct blend of aggregate to satisfy the "mix" specified.

The Client/Engineer will in due course perform a grading on supplied material to ensure it meets the specification.

FLAKINESS

Flakey is the term applied to aggregate or chippings that are flat and thin with respect to their length or width,

Aggregate particles are said to be flakey when their thickness is less than 0.6 of their mean size.

The flakiness index is found by expressing the weight of the flakey aggregate as a percentage of the aggregate tested.

This is done by grading the size fractions, obtained from a normal grading aggregate, in special sieves for testing flakiness.
These sieves have elongated rather than square apertures and will allow aggregate particles to pass that have a dimension less than the normal specified size, i.e. 0.6 of the normal size.
This grading process is normally performed by hand because flakey chippings tend to 'lie' on the sieve surface rather than fall through the apertue.

There are a number of material and aggregate specifications that have a maximum amount of flakey material allowed, e.g. surface dressing chippings.

Flakey aggregate has less strength than cubical aggregate, and does not create the dense matrix that well graded cubicle aggregate is able to do, and it will provide less texture when used in surface dressing.

E.g. Granular sub-base with a high proportion of flakey aggregate tends to segregate and be difficult to compact, although performing a normal aggregate grading test will show it conforms to specification.

Flakey chippings do not create the surface texture that a cubicle or angular chipping is able to produce.

The test for flakiness is described in,

BS 812 Testing Aggregates : Part 105.1 - Flakiness Index

GRADING ZONE

A grading zone is more easily explained when set down on logarithmic graph paper, (see diagram above).

To explain it in words, it is the area contained between a line drawn through the maximum amounts permissible to pass any particular specified sieve, and a line drawn through the minimum amounts permissible to pass the same specified sieves.

The area contained between these two lines is known as the "grading zone".

When plotting an actual grading result, providing the ‘‘plot’’ remains within the zone/envelope the aggregate tested is within specification.

If the line of the ‘‘plot’’ leaves the grading zone the aggregate sample is out of specification.

Recording the results of tested materials on a graphical basis makes it far easier to assess the quality of a material than looking at a string of numbers, you are able to tell at a glance whether a material is well graded or gap graded, a fine material or a coarse material.
But results presented graphically are not easy to store on databases.

WELL GRADED

Well graded means that within a material that is well graded there is a good distribution of all the aggregate sizes from largest to smallest, coarse aggregate to ‘‘dust’’.

With a well graded material all the different size aggregate particles will position themselves within the total matrix in such a way to produce a tightly knit layer of maximum possible density, when compacted correctly.

A well graded material is better able to carry and spread load imposed on it than a poorly graded material.

A well graded material will possess good stability, with good distribution of load / stress spreading out uniformly through the material to the road pavement layer below.

POORLY GRADED

A poorly graded material is one where the size / particle distribution of the supplied material is out of balance with the intended specification /design of the received product.

There may be too high a percentage of fines or coarse within the material, and maximum density by proper compaction will not be achievable.

Segregation, i.e. separation of particular aggregate sizes, usually the larger sizes, is much more likely to occur in a poorly graded material.

Segregation leaves laid areas with too many fines, or areas that are ‘‘open’’ due to patches of coarse material. Both conditions making the particular pavement in question less able to perform its load spreading function.

GAP GRADED

Macadams

The term gap graded refers to a material when one or more of the aggregate sizes in a normal downward distribution of aggregate particle sizes are missing, hence producing a "gap" in the grading where there is little or no aggregate of a particular size to be found.

This can be quite detrimental to the strength of a macadam which relies on mechanical interlock of aggregate particles for its strength.

Although it has to be noted some of the "newer" bituminous materials such as Porous Asphalt/Pervious Macadam are designed to have very little fines in them to be able to create the open interconnecting void structure for drainage to occur.

This is not quite the same as being gap graded by poor production control, but more a grading specifically designed to have a low fines content, it will still be structurally strong with aggregate to aggregate contact spreading
loading through the surface.

In a designed gap graded material the lack of binding fines is compensated for by using a stiffer binder than is usual for a macadam, and the inclusion of hydrated lime to further stiffen the binder and prevent stripping of binder from the aggregate that can happen more readily in an open graded material.

Hot Rolled Asphalts

Gap grading is designed into hot rolled asphalt, with the almost single size coarse material being present in a very stiff fine (usually sand) matrix, like plums in a pudding.

The stiff matrix consisting of a stable sand, a stiff 50 pen. binder, and a filler normally ground limestone to stiffen the binder even further.
The wheel loading in this case will be spread uniformly through the entire material, the stiffness of the matrix, if designed correctly, being capable of resisting deformation.

The smaller stone size HRA wearing courses that have only 30% or 35% aggregate are true gap graded materials, and when this material is cored you will observe distinct separation of the coarse aggregate particles within the mix.

When you have larger percentages of coarse aggregate in the mixes, (50% to 60%), as with the HRA basecourses, although there is still the gap in grading between coarse and fine sizes you will find considerable mechanical interlock of the coarse aggregate.

MOISTURE CONTENT - WATER ABSORPTION - FROST SUSCEPTIBILITY

The amount of water that an aggregate can absorb tends to be an excellent indicator as to the strength of the aggregate, or should I say weakness.
Strong aggregates will have a very low absorption figure, i.e. below 1%.

Above 4% absorption you need to perform further tests on the aggregate to determine its' acceptability, it may be frost susceptible.
A test for water / moisture absorption is described in, 

BS EN 1097-6 : 2000 : Tests for mechanical and physical properties of aggregates : Part 6 : Determination of particle density and water absorption

An aggregate with a high moisture absorption is not likely to be an acceptable road building material, e.g. soft oolitic limestones.

Aggregate that has a honeycombed structure and can ''hold'' water rather than absorb it need not have a high moisture absorption in the actual solid part of the aggregate, e.g. some slags and good quality furnace bottom ash.
These materials themselves will not be subject to frost action providing the road pavements in which they are used are well drained.

INFORMATIVE LINKS

An exceptionally informative website from the British Geological Society, press ----------> HERE

I try to refer to as few commercial sites as possible in compiling my site, but when a site offers particularly useful information about a subject I make an exception, the following sites contribute excellent information on aggregate, its properties and methods of testing.
To gain access to this information, press -----------------------> HERE, and then ---------------------> HERE

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