Materials

Question 1

Why was Hot rolled asphalt used?

Answer 1

• Client decision
• Historical performance
• Typically meets or exceeds 20 year design life
• SMA performed poorly in the early 2000’s
• Political pressure to avoid SMAs
• SMAs being tested now on select sites

Question 2

What table and document is used to determine surface course material for LCC?

What informs the material selection? Describe the parameters for Northgate

Answer 2

Table 2 from LCC’s Highway Works: standards, materials and testing documents.

The material choice of LCC standard mix was defined by the site category (F), single carriageway with major junctions, and the traffic, 250-750 CV/lane/day

Question 3

How would you select a surfacing and binder material and PSV to DMRB/MCHW?

Answer 3

Surface course to CD 236 and relevant MCHW clauses

PSV to CD 236 using site category and traffic figures

Binder material to CD 226

Question 4

How do you select a binder material for LCC?

Answer 4

AC20 40/60 pen binder always used unless sites meet specific requirements.

Question 5

Describe the composition of HRA

(grading and 5 ingredients)

Answer 5

A gap-graded mix, therefore containing a high proportion of fine material and fewer intermediate size aggregates.

Ingredients:
- Bitumen binder (40/60)
- Sand (fine aggregate)
- Filler (limestone dust)
- Coarse aggregate (14/20mm chips)
- Pre-coated chippings (PSV)

Question 6

What does bitumen binder contribute in material composition?

What is it measured in and typical values (describe in mm)

Answer 6

Bitumen binder binds the aggregate together and provides flexibility.

It is measured by penetration and is typically 40/60 pen (4-6mm)

Question 7

What does the sand (fine aggregate) contribute in material composition?

Answer 7

Fills voids and contributes to the dense structure, supports gap-graded design

Question 8

What does the filler (limestone dust) contribute in material composition?

Answer 8

Improves cohesion and reduces air voids

Question 9

What does the coarse aggregate contribute in material composition?

What are the typical sizing’s?

Answer 9

Provides structural strength

Typically 14/20mm sizing

Question 10

What do pre-coated chippings contribute to in material composition?

Answer 10

Improves skid resistance

Question 11

What is PSV?

Define it

Answer 11

Polished stone value, it is an aggregates resistance to polishing when subjected to simulated traffic wear

Question 12

What is AAV?

Define it

Answer 12

Aggregate abrasion value, a measure of the resistance of aggregate to surface wear caused by friction

Question 13

What does a 30/14 Surf HRA mix mean?

Answer 13

A HRA surface course where 30% by mass of the total mix is made up of coarse aggregate of 14mm nominal maximum size.

Question 14

How is binder course selected in LCC

Answer 14

The design charts indicate the use of dense asphalt concrete binder course (DBMBC) 40/60 pen.

Question 15

Can other materials be used?

Answer 15

For bespoke designs or major schemes we would refer to national standards and select materials through the DMRB and MCHW that comply with LCC’s approved materials and suppliers lists.

Question 16

Describe the individual aspects of the following mix:

30/14 F Surf 40/60 rec HRA

Answer 16

30% by mass of the mix is
14mm nominal aggregate size used in surface course
F: filler-rich mix
Surf: surface course application
40/60: bitumen penetration grade
Rec: recipe mix (standard proportions)

Question 17

Why is penetration grade shown as 40/60?

Answer 17

It means the range of penetration falls within 4-6mm

Question 18

Describe the composition of AC20 binder course

(grading and ingredients)

Answer 18

Continuously graded mix with a higher proportion of coarse aggregate contributing to load distribution.
- 20mm coarse aggregate
- fine aggregate (sand)
- filler (limestone dust)
- bitumen

Question 19

Why does binder and base course contribute the majority of pavement strength? (4)6

Answer 19

• Larger coarse aggregate size
• Continuously graded as opposed to gap graded, which increases load distribution
• Lower binder content and higher aggregate contents
• Compacted to achieve high density and low air voids

Question 20

What is the function of the surface course layer? (4)

Answer 20

Provides skid resistance
Waterproofing/shedding properties
Even running surface
Resist deformation under traffic loading

Question 21

What is the function of the binder course layer? (4)

Answer 21

Provides some strength
Load spreading layer between surface and base
Regulating course to lay surface evenly
Waterproofing properties

Question 22

What is the function of the base course layer? (3)

Answer 22

Provides most of the pavement strength
Protects lower layers from stress
Provides a platform for binder and surface layers

Question 23

What is the function of the sub-base layer? (4)

Answer 23

Platform for the flexible pavement

Deals with stresses imposed by construction traffic

Provides frost resistance and protection to sub-grade

Contributes to overall pavement strength

Question 24

What is the function of the sub-grade layer? (3)

Answer 24

Protect from water ingress
Provide platform to construct sub-base
Ensures the sub-base can function effectively

Question 25

How does the binder course help to distribute the load? Why does a thicker pavement provide a greater design life?

Answer 25

Vertical loading from tyres is applied downwards and travels in a triangle formation through the pavement, becoming more widely distributed at lower layers. Thicker layers allow the load to spread over a wider area before reaching the weakest layer (the sub-grade), reducing the stress and potential for foundation failures

Question 26

How are minimum layer thicknesses defined? Take Ac20 binder course for example (20mm aggregate size)

Answer 26

General rule of thumb is 2.5-3.0x the nominal maximum aggregate size I.e. 50-60mm minimum thickness.

Question 27

Why is the minimum layer thickness defined by the maximum aggregate sizing?

Answer 27

- Ensures aggregate orientation - Ensures adequate density in compaction Compaction poor and voids present in too thin layers leading to permeability and premature failure

Question 28

How are maximum layer thicknesses defined? Take Ac20 binder course for example (20mm aggregate size)

Answer 28

General rule of thumb is 5.0-6.0x the nominal maximum aggregate size I.e. 100-120mm maximum thickness.

Question 29

Why is the maximum layer thickness defined by the maximum aggregate sizing?

Answer 29

If a layer is too thick, rollers cannot achieve uniform compaction and air voids will be too high. Excessive thickness can lead to segregation of aggregates and uneven stiffness Thick layers cool unevenly, causing variations in density

Question 30

Why do we design to a 20 year design life

Answer 30

The difference in pavement thickness, and therefore money, between a 10 and 20 year design life is not significant compared to doubling the design life (220mm to 250mm for a Type 2). In urban situations the pavement can fail prematurely due to utility intervention and other forms of failure.

Question 31

What depth of black provides the thermal properties to resist frost below this depth?

Answer 31

275mm of black

Question 32

What is the minimum construction thickness for a type-1 sub-base to provide frost protection?

Answer 32

450mm

Question 33

Why do you need to provide frost susceptibility?

Answer 33

to protect the sub-grade, which could be clay or peat, from expanding and shrinking through freeze thaw weathering FROST HEAVE

Question 34

What is msa?

Answer 34

msa= million standard axles It is a measure of the cumulative traffic loading a pavement will experience over its design life

Question 35

What is a standard axle?

Answer 35

An axle load of 80kN (8.16T)

Question 36

Does WMA have any impact on performance?

Answer 36

Does not degrade binder performance, but can lead to improved workability in cold weather

Question 37

Does RAP have any impact on performance?

Answer 37

20% RAP content can increase overall stiffness, but otherwise is very comparable

Question 38

What is the LA coefficient?

Answer 38

Los Angeles Coefficient, or LA abrasion value, measures the resistance of an aggregate to abrasion and impact

Question 39

How is the LA coefficient measured?

Answer 39

Los Angeles abrasion test where aggregates are placed in a rotating drum with steel balls.

Question 40

Why is no foundation required at 30% or higher subgrade CBR?

Answer 40

Foundation designs compensate for weak ground. At 30% CBR, the subgrade is as strong as, or stronger than a typical foundation.