Hot Rolled Asphalt (HRA)
surface course mixtures
All the HRA mixtures are specified
in BS EN13108-4.
All laying thicknesses, tolerances etc are
specified in BS594987.
There are two types
of HRA surface course,
chipped and unchipped**
**( also known as “55/10”, “55/14”, “high stone content
HRA” and “medium temperature HRA”).
Un-chipped and chipped HRA’s
Chipped HRA is a mat of sand/filler/binder,
bulked out with 14mm crushed rock aggregate,
onto which high psv crushed rock chipppings,
pre-coated with 50 pen bitumen, are spread and
rolled into the mat.
We speak of 30/14 HRA and 35/14 HRA.
These contain 30% and 35% of 14mm aggregate
The bitumen is grade 40/60pen.
HRA surface course, binder and aggregates,
(chippings not shown)
Old photo of crew laying chipped HRA
A typical chipped HRA site, texture depth 1.2 mm
Another typical chipped HRA site, texture depth 1.5mm
A typical 55/10 HRA site, TD about 1mm
The 55/10 and 55/14 have 55% of 10mm stone
and 14mm stone respectively, but all high psv.
Both are usually made with 100/150pen** grade
bitumen.
Both never deform by wheel-tracking and they
are also first class patching materials
(**If you use grade 40/60pen, it is extremely
difficult to hand lay)
30/14 surface course, no chippings shown, it is about 98% impermeable. NB NO aggregate interlock
Note the aggregate interlock. This is Close graded AC surface course, cheaper than HRA, but not as durable. Not
impermeable. OK for some sites.
BSEN 13108-4 specifies 2 fine aggregate types,
Type F and Type C
Type F uses either natural fines OR
a blend of natural fines and crushed rock fines
Type C uses only crushed rock fines.
Typical natural sand as
seen through a microscope
Crushed rock fines to the same magnification
In my experience, Type C HRA, using only
crushed rock fines, is totally resistant to
deformation but extremely difficult to lay and
compact.
Some years ago, on the M5, an HRA
surface course made with crushed rock fines
failed within 5 years because of its poor
compaction
Most of HRA surface course is sand. HRA made with wet
sand can cause serious loss of level tolerances when paving
( It should be stored under cover to keep it dry)
COLD 20mm chippings of high psv
aggregate, pre-coated with bitumen, are
scattered onto, then rolled-into the HOT
mat using a high degree of skill to avoid
them being pushed too deeply into the mat.
Note that these are the only high psv aggregates used in
chipped HRA , a very sustainable use of a scarce
resource
Some images of work before paving and chipping
machines in the 1920’s ( from Tarmac’s archives)
Hand laying HRA, in the 1940’s (from Tarmac’s archives)
Paving machines were invented in the USA
in 1930 and were brought to the UK in
about 1942 for building US Army Airforce
bomber bases for in East Anglia.
They were first used on civilian sites after
WW2
Hand-chipping before the chipping machine and
the Health and Safety Act !!
One idea was to modify a paver to apply pre-coated
chippings to HRA
But the chipping machine, designed by TRRL,
eliminated hand chipping
Prior to 1978, all HRA’s were “recipe” mixes,
ie so much sand,
so much filler,
so much bitumen,
plus 30% by mass of 14mm single size
crushed rock
However, following very severe deformation
of surface courses in the UK 1976 and 1977,
HA then moved to “design” mixes, involving
the Marshall test method
Producers used the Marshall test to achieve
target “ Marshall stabilities” (stiffness’s)
and limits of “flow” (deformation of the test
sample)
Also, to optimise the binder content for use in
achieving these stabilities with the sand the mix
producer planned to use.
The Marshall test machine
Mix design, with acknowledgements to the late
Colin Loveday
How changes of binder and filler affect deformation
The first uses of “design” mixes
eliminated many previously widely used
sands as “unsuitable”
(ie mixes made with them deformed too easily)
Subsequently, the HRA’s made using the
Marshall design method, eliminated wheel
tracking problems for many years, until the
early 1990’s
Digressing slightly.....
In 1984, some county council Agent
Authorities used “designed” HRA’s made
with modified bitumens on jobs laid in winter,
to get better workability in cold weather.
The trick was to forget 50pen bitumen, instead
use 70pen, which deforms more easily ...BUT..
To stop the 70pen mix deforming in
hot summers, the binder was blended with either
SBS or EVA before being added to the mixer.
This increased the cost of the HRA
BUT ONLY MARGINALLY.
One site was the M6, Junctions 10A to 11.
This included Hilton Park Service Area,
so works were not allowed to start until
October 31.
The contract period was 5 months so work
continued all winter.
HRA surface course work in February 1985
Trying to melt the snow with an infra-red heater!
The modified 70 pen HRA
lasted 21 years on one of the most heavily
trafficked lengths of motorway in the UK with
more than 100,000 vpd and 30% hgv’s
There was absolutely NO wheel-tracking
deformation
End of digression..............
In the early 1990’s some HRA’s started to
deform on SOME, not all, parts of the most
heavily trafficked sections of some
motorways and trunk roads.
The first uses of “design” mixes
eliminated many previously widely used
sands as “unsuitable”
(ie mixes made with them deformed too easily)
Subsequently, the HRA’s made using the
Marshall design method, eliminated wheel
tracking problems for many years, until the
early 1990’s
HRA on the M6, as seen in 1993, deformed by very
heavy traffic
A typical 1980’s UK HGV artic and trailer
Stress in pavement, one end of an axle with two tyres, 5.5 tonne load, from the “old” type of UK
artic tractor and trailer
The French had similar deformation problems
with their asphalt concretes, (dense macadams
with optimised bitumen contents)
They had invented the three axle artic trailer,
the super-single tyre and have a 13 tonne axle load,
a triple whammy
Triple-axle artic and trailer, with “super single” high pressure tyres, increasingly seen in the UK in the 1990’s
and ever since !!
Super single tyre, same load as on the twin tryes
French test specimens, failed!
The French solution:- Use very stiff binder courses, 15pen EME2 + very thin mixtures with modified bitumens These were too thin to deform more than 5mm (eg Safepave, UL-M) These were introduced into UK by small surfacing companies in 1991/2
HA’s solution, Clause 943,
“PERFORMANCE SPECIFIED” HRA,
made with modified bitumens
introduced in 1994.
HA’s Clause 943 solution ,
10 years after the Agent Authorities had first
successfully used modified binders,
included testing cores removed from jobs
and testing them in the UK wheel-tracking
machine.
The UK wheel-tracking machine, now called the
small device
In 1994, if the cores deformed by more than 7mm at
60 C. or at a rate greater than 5mm/hr **
then the contractor had to plane out the HRA and
start again.
Contractors very sensibly priced this risk
and the rate /sq.m for HRA doubled overnight.
**NB The test method has been slightly modified on becoming a
BSEN test, but the BS 598(pt110) method and values are still the
current. The values are listed in Appendix C.3 of PD6691
Plot of a wheel track test, sample fails,
deformation greater than 7mm.
So, HA had solved the problem but could
not afford the solution!
Instead, they looked to see how the French
and Germans dealt with such problems.
This resulted in the introduction of the many
BBA/HAPAS materials we have seen since
(and record breaking numbers of potholes.)
So now we had ( and still have)......
“recipe” chipped HRA
“designed” chipped HRA
and “performance specified” chipped HRA
But in BSEN 13108-4 for HRA, “design” mixes
are specified to also meet wheel-tracking test
criteria.
Virtually all “recipe” mixes have disappeared
Map of UK showing ALL flexible roads NOT surfaced with
HRA in 1993 ( no other map shown)
A myth has grown up about the deformation of
chipped HRA.
Next is an example of a deformed HRA at a bus
stop.
It also could deform at traffic signals
BUT NOT ALL HRA’s DEFORMED AT
BUS STOPS AND TRAFFIC SIGNALS
Aberdeen, HRA deformed at a bus stop,
elsewhere along the road it is 100% OK
A length of unchipped, 55/10 HRA ,
laid at bus stops
in place of the chipped HRA would solve
this deformation problem.
From the contractor’s viewpoint,
even normal, unmodified, chipped HRA is a
high risk material, since if chippings are
lost in the first 2 or 3 weeks,
this is usually entirely his fault.
Chipping loss due to end of load being too cool.
Chipping loss is exacerbated if the crew
were working in weather which was
very cold or windy
or
they were working in heavy rain
Chipped in heavy rain to meet political deadline
( which was not in the contract!!)
Another cause of chipping loss.....
If the chippings were frosty,
because they had been stockpiled on site in
winter but not covered with sheets to keep
them free of frost.
Problem due chipping machine not working
properly and dropping the chippings in rows.
Another cause of chipping loss.....
If they were dusty,
or had been stored too long in sunlight
which tended to harden the bitumen on the
chippings
so it was no longer a good adhesive when
they were rolled into the mat.
Oxidised chippings
HRA was marginally too cool when it was laid
and chipped
The contractor usually needed at least 11 men
to lay chipped HRA
whereas with SMA’s etc he only needed 7 men,
the same as for 55/10 and 55/14.
The SMA’s and thin AC mixes were both cheaper
and quieter
so many LA’s started to specify them instead of
chipped HRA, and the rest, as they say, is history.
A crew laying chipped HRA, 8 here plus loading
shovel driver, a very slim crew! Normally 11
Before starting paving try to get 60 tonnes in
front of the paver and make sure all the prep
work is completed,
ie ironwork adjusted to new levels
vertical faces of kerbs and ironwork all painted
with hot bitumen
any regulating course or planing completed
surface to be overlaid swept clean and properly
tack-coated
Don’t let the truck bump into the paver!
Truck protects asphalt in paver
Chippings being spread onto the just laid
HRA mat
To be successful,
the cold chippings, which are rolled into the mat,
need to absorb sufficient heat from the asphalt
mat for their bitumen coating to melt.
They then glue themselves into the hot mat.
( If they suddenly “Glisten” as the bitumen
coating melts, you’ll have no problems)
The cool “end of load”, the cause of so many of our
problems, even today with sma’s etc.
The same mat showing new hot mix
How Tarmac used to reduce cool “ends of loads” and
subsequent “chipping loss” problems
JOINTS
New BS 594987 clause 6.8
“..Joints in surface courses shall be cut…”
Note neatly cut edge to mat no 1, the cut should be the thickness of mat from edge.
Applying bitumen to mat edge
Attention to joint (2)
Fortunately, chipping loss from HRA
can be
remedied by re-heating and re-rolling,
as long as it is done within a few days
of the chip loss.
Re-heating with infra-red heater
Re-heated mat
Adding new chippings, mat then rolled with Bomag 100
or similar
Good quality, well designed, chipped HRA’s,
laid and compacted by competent crews,
should last for up to 30 years on typical
city centre sites and more than 20 years on
motorways.
This is because its void content is about 3%
to 4% so it is virtually impermeable to both
air and water.
HRA 22 years old, never bonded, but strong and watertight.
Even 30 years old and failing, the lowest part of the
matrix still protects the underlying road pavement
35 year old HRA in central Milton Keynes, 2012.
18 years old and never a pothole!!
If you want an attractive, very durable
asphalt, look no further !
With such a long life, the chippings can
become polished whilst the HRA still has
years of life.
However, the surface can easily and cheaply
be “re-textured” using one of several
systems,
See HD 37/99
And finally, no asphalt type is easier to patch
successfully if some ****** digs it up!
Thank you for listening.