1. Lean concrete subbases
Performance requirements under concrete Bases

2. The opinions expressed in this presentation are those of the author and do not necessarily reflect the views of the Roads & Traffic Authority NSW (RTA).
Some aspects do not appear in official RTA publications.
Geoff Ayton

3. - a pavement layer - a material Terminology
lean concrete subbase (LCS) or
lean-mix concrete subbase (LCS)
- a pavement layer
- a material
lean-mix concrete (LMC)

4. pre -1960s concrete pavements ….

5. Subgrade pre -1960s concrete pavements ….
s: non-erodable subbases

6. pre -1960s concrete pavements ….
Warringah Freeway 1960s 
"a broken stone base course of six (6) inches compacted thickness"
July 1964
De Leuw Cather & Co
MDCS ≥ 250 lb/sq in. (1.75 MPa)
"...shall include such added quantity of gritty cementitious binder as is necessary..."

7. Subbases - non-erodable
during the 1960s s ….
Subbase
Subbases - non-erodable
USA: permeable granular, LCS (econocrete) Europe: bound granular, rolled lean-mix, LCS
RTA: since ~1975: LCS or rolled lean-mix

8. LCS today’s overseas practices …. now not so popular overseas 
Asphalt subbase (USA)
Roger Larson FHWA, 1996
LCS
now not so popular overseas 
partly due to variable performance
USA visitors noticeably impressed by local LCS
we treat it as a structural layer, not merely a mudmat (thanks to Ed Haber et al)

9. LCS today’s overseas practices …. now not so popular overseas 
David Dash & Carlos Kraemer (Spain, 1994)
LCS
now not so popular overseas 
partly due to variable performance
Carlos Kraemer (1994): Belgian Motorway failure
(~100 km, within 12 months of opening)
"...yours are serious LCS, this was cement bound soil..."

10. Lean concrete subbase (LCS) (150 typ)
typical RTA pavement structure ….
Base (250 typ)
Lean concrete subbase (LCS) (150 typ)
SMZ (300 typ)

11. Pavement structure Base Subbase Performance requirements
Specification requirements
 compressive strength MPa min
Base
Subbase
Select material zone
(SMZ)
 main structural layer
flexural fatigue
joint performance
durability of surface texture
 flexural strength MPa min
 erosion resistance
volumetric stability
shear capacity under Base joints
water-shedding
 compressive strength 5 MPa min by 42D
 etc
etc

12. Pavement structure Base Subbase LCS Performance requirements
Specification requirements
Base
Subbase
Select material zone
(SMZ)
 erosion resistance
volumetric stability
shear capacity under Base joints
water-shedding
 compressive strength 5 MPa min by 42D
 adequate strength (> 5 MPa) for erosion resistance
 very fine cracks
 low shrinkage

13. Yass Field Trials
LCS: typical characteristics

14. Subbase compressive strength (cores & cylinders)
7 days
Subbase
LCS cf Base concrete
Subbase compressive strength (cores & cylinders)
Yass Field Trials

15. Subbase compressive strength (cores & cylinders)
Yass Field Trials

16. Nil RTA specification R82 Slag Typical : 160kg 90 kg 250 kg
Lab Trial Mix : 6 MPa 28D
15 MPa 28D
Beware : cement < 90 kg/m3 may struggle to achieve 5 MPa in colder climate.

17. RTA specification R82
Shrinkage & AEA content

18. Field controls UCS: 5 MPa from 100 mm cores within 42 days;
(cylinders not required)
Flexural: not specified
Slump: mm for slipformed (± 10)
55-65 mm for fixed-formed (± 15)

19. * specification requirement
Pavement structure
Lean concrete subbase (LCS)
Performance requirements
Specification requirements
Base
Subbase
Select material zone
(SMZ)
 erosion resistance
volumetric stability
shear capacity under Base joints
water-shedding
 compressive strength 5 MPa min by 42D
* specification requirement
not
design requirement
 compressive strength yrs
* unworkable in a contract situation
* hence, minimum binder levels (90 kg PC, 250 kg total)

20. Pavement structure
LCS
Mooney Mooney Bridge

21. Pavement structure  LCS hopefully a water-shedding layer LCS
Transverse joint
 LCS hopefully a water-shedding layer

22. P14 P14 Lean concrete subbase Tweed Heads Bypass (~1986)
Photos by Graham Foley

23. Fall line
Fall line
P14
Photos by Graham Foley

24. Fall line
P14
Photos by Graham Foley

25. * specification requirement
Pavement structure
Lean concrete subbase (LCS)
Performance requirements
Specification requirements
Base
Subbase
Select material zone
(SMZ)
 erosion resistance
volumetric stability
shear capacity under Base joints
water-shedding
 compressive strength 5 MPa min by 42D
* specification requirement
not
design requirement
 compressive strength yrs
ie a mix of methods and end product spec

26. AC Pavement structure Under an AC Base :
Lean concrete subbase (LCS)
Performance requirements
Specification requirements AC
 erosion resistance
volumetric stability
shear capacity under Base joints
water-shedding
Subbase
Assumed modulus 10,000 MPa
a bound/stabilised granular is not an equivalent alternative !
(nor is a rolled lean concrete likely to be equivalent)

27. LCS cracking Base Subbase water-shedding (SMZ)
Select material zone
(SMZ)
 strength: MPa (for erosion resistance)
 erosion resistance
volumetric stability
shear capacity under Base joints
water-shedding
 very fine cracks
 low shrinkage

28.  What’s Good LCS cracking Narrow (fine) cracks Strength
· days
 What’s NOT
Wide cracks
· reduced shear capacity
· increased water ingress
Strength (in Trial Mix)
· < 5MPa
· > 15MPa
Cobblestone or block cracking

29. - R82 criteria
LCS cracking
3-15 m typ

30. - R82 criteria LCS cracking 5.1.1 Typical Subbase Cracking
Lean-mix concrete subbase will typically form full-depth transverse cracks continuous for the full width of the paving run at approximately 3-15 m centres.
3-15 m typ
Transverse
cracks
In subbase placed in a single pass more than 6 m wide, longitudinal full-depth cracks might also typically occur at spacings of approximately 4 m and in continuous lengths exceeding 4 m.

31. LCS cracking
12 m

32. - R82 criteria LCS cracking 5.1.1 Typical Subbase Cracking
Lean-mix concrete subbase will typically form full-depth transverse cracks continuous for the full width of the paving run at approximately 3-15 m centres.
In subbase placed in a single pass more than 6 m wide, longitudinal full-depth cracks might also typically occur at spacings of approximately 4 m and in continuous lengths exceeding 4 m.
3-15 m typ
Transverse
cracks
Cracking of this type and extent will not be deemed to be nonconforming unless subsequent deterioration occurs prior to its being covered with base.

33. - R82 criteria LCS cracking 5.1.2 Plastic Shrinkage Cracks
Plastic shrinkage cracks are discrete cracks of length less than 300 mm and a depth less than 50% of the slab thickness which do not intersect a formed edge.
3-15 m typ
Transverse
cracks
No remedial action is required if a bond breaker is applied, but the Contractor must immediately implement Corrective Action.

34. - R82 criteria LCS cracking
5.1.3 Additional Longitudinal and Transverse Cracks
Subbase cracking other than typical cracking as described in Clauses and is nonconforming if the cumulative length of cracking in any 25m2 area of subbase exceeds two metres.
The Contractor must immediately implement Corrective Action.
3-15 m typ
Transverse
cracks

35. - R82 criteria LCS cracking nonconforming LCS cracking 3-15 m typ
Transverse
cracks
nonconforming LCS cracking

36. - R82 criteria LCS cracking nonconforming LCS cracking 3-15 m typ
Transverse
cracks
nonconforming LCS cracking

37. - R82 criteria LCS cracking nonconforming LCS cracking 3-15 m typ
Transverse
cracks
nonconforming LCS cracking

38. - R82 criteria LCS cracking nonconforming LCS cracking 3-15 m typ
Transverse
cracks
nonconforming LCS cracking

39. - R82 criteria LCS cracking nonconforming LCS cracking 3-15 m typ
Transverse
cracks
nonconforming LCS cracking

40. LCS cracking

41. non-structural layer ?

42. LCS cracking
nonconforming LCS cracking
End