A1_Subgrades & Subbases for Concrete Pavement_Rodden

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Subgrades and Subbases January 2008

9th Pennsylvania Concrete

Conference 1

Subgrades and Subbasesfo r Concre t e Pavements

Presented to Ninth AnnualPennsylvania Concrete

ConferenceJanuary 29, 2008

Robert RoddenDirector of Technical Services

Subgrades and Subbases

Roadbed (subgrade and subbase) design is key

to long-term performance and smoothness of concrete pavements.

TerminologyDesign PrinciplesSubgradesSubbases

Termino logy

A concrete pavement structure typically consistsof a concrete surface andsubbase(s) placedupon a prepared subgrade. A “base” is part of anasphalt pavementstructure, while asubbase is an optional elementof a concrete pavement structure.

Termino logy

Why the difference in terminology?Pressures imposed on a base (under asphalt) aredramaticallydifferentthan those imposed on asubbase (under concrete) due to differences inmoduli (stiffness).Material requirements for a subbase may be relaxedwhen compared to a base.

Design Pr inciples

Roadbeds for a concrete pavement structure should:Be free from abrupt changes in character of the materials(should be uniform and constructed of a material that willprovide requisite stability over the life of the pavement)Resist erosionBe engineered to control subgrade soil expansion and frostheave.

Above all other design concerns,uniformity is of utmost importance.

Design Pr inciples

Because of the rigid natureof concrete pavements,loads are distributed over relatively large areas,greatly reducing stresseson the subgrade/subbase;thus, concrete pavementsdo not necessarily requireexceptionally strongfoundation support.

UNIFORMITY >> STRENGTH





Conference 2

Design Pr inciples

Proof of the point:Concrete pavement design thicknessis relatively insensitive to support stiffness (modulus of subgrade reaction), so it is improper engineering to makea subgrade/subbase stronger or thicker in an attempt todecrease concrete pavement thickness…

Design Pr inciples

Overzealous engineering of a roadbed could have

a negative effect once all loads are considered.

Design Pr inciples

Subgrade/subbase design is engineering:The pavement design engineer should consider allsubgrade/subbase types (stabilized or unstabilized)and available materials (recycled or virgin) for eachpavement design; there is no standard recommendedsubgrade/subbase combination for any concretepavement.

Subgrade/subbase selection/engineering isthe designer’s option!

Subgrades

Options for any subgrade:UnstabilizedChemically Stabilized – Cement, Lime, etc.

Soil classification systems such as the AASHTO and/or ASTM (Unified) Soil Classification Systems will help thepavement design engineer determine factors such as theCalifornia Bearing Ratio (CBR) or modulus of subgradereaction (k-value), but the engineer must be mindful of the preferred soil classification method for each projectbecause conversion between methods is not intuitive.

Subgrades

General guidelines for a good subgrade: A minimum CBR of 6 in the top 24 in. (610 mm) of subgrade provides an adequate working platform for construction, while limiting subgrade rutting under construction traffic to ½ in. (13 mm) or less.Typically, a specified percentage of compaction of 95percent (AASHTO T99) will provide an adequateworking platform for construction equipment and for excellent in-service performance of the subgradeportion of a concrete pavement structure.

Design subgrade for uniformity, not strength!

Subgrades

Obtain uniform support by controlling:Expansive soilsFrost-susceptible soils (frost heave)

PumpingWet Soils





Conference 3

Subgrades

What is frost heave?

Subgrades

Subgrades

Frost-susceptible soil types:

Subgrades

Controlling frost heave:Compaction and moisture controlDrainage, grade and water table elevationSelective Grading and MixingRemoval of silt pocketsProtection for utilities located in the subgradeNon-frost susceptible cover

Subgrades

Non-frost susceptible cover:

Subbases

AGAIN:Concrete pavement design thickness is relativelyinsensitive to support stiffness (modulus of subgradereaction), so it is improper engineering to make asubgrade/subbase stronger or thicker in an attempt todecrease concrete pavement thickness…





Conference 4

Subbases

Pumping of concrete pavements

Subbases

For pumping of a subbase to occur, several conditionsmust exist:

The pavement must have undoweled joints or joints with poor load transfer Water must be presentThe roadway must have fast moving, heavy loadsThe subgrade must be a fine-grained material or the subbasemust be an erodible material

Eliminating one or more of these casual factors shouldmitigate pumping.

Subbases

Do you need a subbase?Pavements that are expected to carry 200 trucks or fewer per day (or less than 1,000,000 18-kip (80 kN) ESAL’s over thecourse of the service life of the pavement) do not typicallyrequire a subbase to prevent pumping.

A subgrade soil that is naturally free draining typically will notpump.Subgrade soils with less than 45% passing a No. 200 (75μm)sieve and with a PI of 6 or less are adequate for moderatevolumes of heavy truck traffic without the use of a subbaselayer.

Subbases

Subbase Types:Unstabilized (granular) - 4 in. (100 mm) min. thicknessStabilized

Cement-StabilizedCement-Treated (CTB) - 4 in. (100 mm) min. thicknessLean Concrete (LCB) - 4 in. (100 mm) min. thickness

Asphalt-Stabilized (ATB) - 2 in. (50 mm) min. thickness

With any subbase type, it is possible to utilizewaste material (i.e., recycled concrete) asaggregate.

Subbases

Regardless of subbase type, the best results areobtained by:

Selecting materials that prevent pumpingSelecting materials that will not contribute to excessivedeflections and will remain stable over timeTreating the subbase surface (if necessary) to prevent bond tothe concrete pavementSpecifying material gradation that will ensure a reasonablyconsistent (and uniform) subbase across an individual projectBuilding the subbase to grade controls that foster a consistentconcrete pavement thickness and smoothness

Unstabi l ized Subbases

Unstabilized subbases must have: A 4 in. (100 mm) min. thickness A maximum particle size of no more than 1/3 the subbase thicknessLess than 15 percent passing the No. 200 (75μm) sieve An in-place density of 95 percent according to AASHTO T99

A Plasticity Index (PI) of 6 or less A Liquid Limit (LL) of 25 or less A L.A. abrasion resistance of 50% or less A grade tolerance of ± ½ in. (± 12 mm) by a 10 ft (3 m) straightedge A target permeability of about 150 ft/day (45 m/day), but no more than 350ft/day (107 m/day) in laboratory tests (free-draining, NOT PERMEABLE)

Of these, limiting the percent of fines passing the No. 200 (75μm)sieve is of utmost importance to creating a long-lastingunstabilized subbase that will prevent pumping.





Conference 5


Typical job control tolerances from the targetgradation for unstabilized subbases are:

±10% for materials 1 in. (25 mm) and larger.±8% for materials between 1 in. and No. 4 (25 mmand 4.75 mm).±5% for materials No. 4 (4.75 mm) and smaller.

The finished tolerance of the unstabilized subbaseshould be ±1/2 in. (± 12 mm) of the design profilegrade.


Be sure to consolidate to spec!


Be sure to consolidate to spec!


And at the proper thickness!

Stabi l i zed Subbases

Stabilized subbases generally refer to subbase materialsthat are bound by either cement or asphalt.The higher degree of support offered by a stabilized

subbase will not alter the required concrete pavementslab thickness appreciably, but it will add pumpingresistance and increase the overall strength of thepavement structure, spreading loads over larger areasand reducing strains.


On a cement-treated subbase, the loss of supportoccurs at a much slower rate. In this study it stabilizedaround one-half million load applications; even after 1million loads, joint effectiveness remains at a level of almost 80 percent.





Conference 6


Other benefits of stabilized subbases:

An excellent, stable all-weather construction platformBound subbase surfaces drain water quickly Aid in improving the final pavement smoothnessMinimize post-construction subbase consolidationMinimize intrusion of hard granular particles into thebottom of pavement jointsProvide an erosion resistant subbasePermit greater use of local materials

Stabi l i zed Subbases: CTB

Cement-treated subbases typically require: A minimum thickness of 4 in. (100 mm) About 2 to 5 percent cement, by weightGranular material per AASHTO M147 or Soil Classification Groups A-1, A-2-4, A-2-5, and A-3No more than 35 percent passing the No. 200 (75μm) sieve A Plasticity Index (PI) of 10 or less A maximum particle size of ¾ to 1 in. (19 to 25 mm) A grade tolerance of ± ½ in. (12 mm) by a 10 ft (3 m) straightedge

There is typically no strength requirement for cement-treatedsubbases (CTB) because a CTB is best controlled usingcompaction and/or density requirements. However, whenspecified, a target compressive strength range of 300 to 800 psi(2.1 to 5.5 MPa) is typical to ensure long-term durability torepeated cycles of wetting and drying or freezing and thawing,while keeping the layer from getting too stiff.

Stabi l i zed Subbases: CTB

Gradation for minimum cement requirements:

Stabi l i zed Subbases: LCB

Contain more cement than CTB but less thanconventional concrete. Appearance and consistency like conventionalconcrete, requiring consolidation by vibrationTypically uses relaxed concrete specificationsCan utilize recycled concrete or asphaltOften referred to as “econocrete”

Stabi l i zed Subbases: LCB

Typical LCB specifications include:Minimum thickness of 4 in. (100 mm)Compressive strength of 750 to 1,200 psi (5.2 to 8.3 MPa)Cement factors from 200 to 350 lb/yd3 (119 to 208 kg/m3)Slump of 1 to 3 in. (25 to 75 mm) Air content between 6% and 8%Maximum aggregate size of 1 to 2 in. (25 t o 50 mm) A grade tolerance of ± ¼ in. (± 6 mm) by a 10 ft (3 m) straightedge

Strength of a lean concrete subbase should be limited to 1,200 psi(8.3 MPa) or less to keep the subbase from getting too stiff,minimizing curling and warping stresses in pavement slabs. If thisstrength is exceeded, measures may need to be taken (i.e.,scoring joints into the lean concrete subbase) to mitigate thepotential problems.

Stabi l i zed Subbases: ATB

Typical ATB specifications include:Minimum thickness of 2 in. (50 mm) Asphalt cement content of about 4 to 4.5%

A maximum aggregate size of ¾ in. (75 mm) A watered or whitewashed surface before paving A grade tolerance of ± ¼ in. (± 6 mm) by a 10 ft (3 m)straightedge

It is often difficult to place an ATB to the desiredgrade, resulting in cost overruns on concrete.





Conference 7

Stabi l ized Subbases: Precaut ions

High friction

between thepavement and thesubbase.Increased curlingand warpingstresses in thepavement.

Stabi l ized Subbases: Precaut ions

Other means to reduce bond:

Permeable Subbases

Permeable subbases (subbases with a permeability of 350 ft/day(107 m/day) or greater in laboratory tests) have had a problematichistory in the field.The reasons include:

Instability as a construction platformLoss of support caused from aggregate breakdownEarly age cracking caused from penetration of concrete mortar into thesubbase voids during pavingIntrusion of fines from underlying layers into the permeable subbase voidsLoss of support caused from infiltration of the subgrade into the subbaseCost effectivenessVarious other overall field performance problems

Permeable Subbases

Loss of support due to aggregate breakdown:

Permeable Subbases

Loss of support due to infiltration:

Permeable Subbases

SR 22 – Loss of uniform support:





Conference 8

Permeable Subbases

SR 22 – …and loss of stability:

Permeable Subbases

Cost effectiveness:

Permeable Subbases

More than just initial cost…

Source: Pennsylvania Interstate System, Premature OGS Related Mid-Slab Cracking Repair NeedsReport

Repair Cost Distribution (2003)

$-

$1

$2

$3

$4

$5

$6

$7

$8

$9

1 2 3 4 5 9 10 12District

M i l l i o n

$ N e e

d e

dReplace Slab

6 Foot Patch

DBR

Permeable Subbases

Thus, permeable subbases are no longer recommended for concrete pavement structures.Free-draining subbases (subbases with apermeability between 50 and 150 ft/day (15 and46 m/day) in laboratory tests) and daylightedsubbases are the reasonable alternatives torapidly draining permeable subbases.

SR 22 Rep lac em ent Upc om ing Webi nars and Courses

Upcoming webinars and training courses include thefollowing;

Webinar #2, Texturing Concrete Pavements, January 31.Webinar #3, Introduction to the M-E PDG, February 18.

Webinar #4, Joint Layout and Design, March 4.Concrete Pavements 101, April 8-10, Tampa Florida.2008 Professor’s Seminar , June 16-19, Chicago, Illinois.

Please refer to the ACPA website atwww.pavement.comfor a complete listing of upcomingcourses for 2008.





Conference 9

T h a n k y o u !

Ques t ions?

Documents

A1_Subgrades & Subbases for Concrete Pavement_Rodden