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.