Lean concrete subbases Performance requirements under concrete Bases

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

- 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)

pre -1960s concrete pavements ….

Subgrade pre -1960s concrete pavements …. 1960 - 70s: non-erodable subbases

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..."

Subbases - non-erodable during the 1960s - 1970s …. Subbase Subbases - non-erodable USA: permeable granular, LCS (econocrete) Europe: bound granular, rolled lean-mix, LCS RTA: since ~1975: LCS or rolled lean-mix

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)

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..."

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

Pavement structure Base Subbase Performance requirements Specification requirements  compressive strength 35 MPa min Base Subbase Select material zone (SMZ)  main structural layer flexural fatigue joint performance durability of surface texture  flexural strength 4.5 MPa min  erosion resistance volumetric stability shear capacity under Base joints water-shedding  compressive strength 5 MPa min by 42D  etc etc

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

Yass Field Trials - 1994 LCS: typical characteristics

Subbase compressive strength (cores & cylinders) 7 days Subbase LCS cf Base concrete Subbase compressive strength (cores & cylinders) Yass Field Trials - 1994

Subbase compressive strength (cores & cylinders) Yass Field Trials - 1994

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

RTA specification R82 Shrinkage & AEA content

Field controls UCS: 5 MPa from 100 mm cores within 42 days; (cylinders not required) Flexural: not specified Slump: 20-40 mm for slipformed (± 10) 55-65 mm for fixed-formed (± 15)

* 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 15-20 MPa @ 1 -2 yrs * unworkable in a contract situation * hence, minimum binder levels (90 kg PC, 250 kg total)

Pavement structure LCS Mooney Mooney Bridge

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

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

Fall line Fall line P14 Photos by Graham Foley

Fall line P14 Photos by Graham Foley

* 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 15-20 MPa @ 1 -2 yrs ie a mix of methods and end product spec

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)

LCS cracking Base Subbase water-shedding (SMZ) Select material zone (SMZ)  strength: 15-20 MPa (for erosion resistance)  erosion resistance volumetric stability shear capacity under Base joints water-shedding  very fine cracks  low shrinkage

 What’s Good LCS cracking Narrow (fine) cracks Strength · 7 - 12 MPa @ 42 days  What’s NOT Wide cracks · reduced shear capacity · increased water ingress Strength (in Trial Mix) · < 5MPa · > 15MPa Cobblestone or block cracking

- R82 criteria LCS cracking 3-15 m typ

- 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.

LCS cracking 12 m

- 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.

- 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.

- R82 criteria LCS cracking 5.1.3 Additional Longitudinal and Transverse Cracks Subbase cracking other than typical cracking as described in Clauses 5.1.1 and 5.1.2 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

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

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

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

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

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

LCS cracking

non-structural layer ?

LCS cracking nonconforming LCS cracking End