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Diseño de pavimentos de concreto: de la teoría a la práctica

TALLER DE DISEÑO DE PAVIMENTOS DE

CONCRETO

November 7, 2014

Robert Rodden, P.E.

Senior Director of

Pavement Technology

We Start from the Ground Up

Subgrade Characteristics

• Goal is uniform support, so control:• Expansive soils

• Frost-susceptible soils

• Pumping

• Wet soil

• Varieties:• Unstabilized

• Stabilized• Cement-Treated

• Lime-Stabilized

Subgrade Design

• During construction concerned with:• Moisture content

• Compaction effort

• Working platform

• Structural design concerned with:• Thickness of any treatment

• Strength (bearing capacity) OR• California bearing ratio (CBR)

• Stiffness (resistance to deformation)• Resistance value (R-value)

• Resilient modulus (MR)

California Bearing Ratio (CBR)

• Bearing capacity relative to well-graded crushed stone

Resistance Value (R-value)

• Resistance to deformation; ratio of transmitted lateral pressure to applied vertical pressure

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Resilient Modulus (MR)

• Elastic modulus (e.g., stress-strain) under repeated axial cyclical stress

Go-to References on Soils/Subgrade

Then We Might Add a Subbase(or more)

Subbase Characteristics

• Goals are:• Erosion resistance

• Uniform support

• Varieties:• Unstabilized

• Stabilized• Cement-Treated (CTB)

• Asphalt-Treated (ATB)

• Lean Concrete (LCB)

• …Drainable

Subbase Design

• During construction concerned with:• Moisture content

• Compaction effort

• Working platform

• Structural design concerned with:• Thickness and # of layers

• Drainability

• Stiffness (resistance to deformation)• Modulus of elasticity (E)

• Resilient modulus (MR) (unbound)

Drainability

Permeable subbase:Permeability of 350 ft/day (107 m/day)

Problems included unstable construction platform, aggbreakdown in service, infiltration of fines, etc.

Free-draining subbase:Permeability of 50-150 ft/day (15-46 m/day)

in laboratory tests

Can be unstabilized or stabilized

Most agencies have already moved away from permeable in favor of free-draining

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Performance of Drainable Layers

NCHRP 1-34 projects investigated long-term performance of permeable subbases

1-34D used LTPP SPS 2 sites located across the U.S. and exposed to widely varying environments, levels of traffic, soil types, concrete thicknesses & strengths, lane widths, subbases, etc.

Experimental Data

Conclusions From 1-34D Study

Subbase stiffness matters more than drainage for concrete pavement performance

There is an optimal stiffness for subbases (not too stiff, not too flexible)

Although excess moisture and poor drainage has been shown to be detrimental to pavement performance in the past, current designs are less susceptible to moisture damage (thicker sections, improved materials, widespread use of dowels, etc.)

The US is Moving towards Free-Draining

Support = Subgrade + Subbase

Plate Theory Combines Support into One

Layered elastic theory inappropriate because stiffness differential

Plate theory combines all support (modulus + thickness of each layer) into a composite known as modulus of subgrade reaction (k-value)

Essentially a spring constant (Hooke’s)

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Composite k-value is IterativeConcrete

Asphalt | 6” (150 mm) | 350 ksi (2.4 GPa)

Aggregate | 6” (150 mm) | 30 ksi (207 MPa)

Soil | CBR = 3

Soil | MRSG = 4,118 psi(28 MPa)

Aggregate | 6” | 30ksi

+ 100 psi/in.(27 MPa/m)

= 244 psi/in.(66 MPa/m)

244 psi/in.(66 MPa/m)

+ 244 psi/in.(66 MPa/m)

Asphalt | 6” | 150 ksi

= 388 psi/in.(105 MPa/m)

Add a layer at a time until the composite k-value

immediately beneath the concrete is determined

Note on Subgrade/Subbase Support

Overzealous engineering of a roadbed could have a negative effect once all loads are considered.

Increasing k-value Doesn’t Greatly Decrease the Required Thickness

Concrete pavement design thickness is relatively insensitive to support stiffness (modulus of subgrade reaction), so it is improper engineering to make a subgrade/subbase stronger or thicker in an attempt to decrease concrete pavement thickness…

Analyses conducted in StreetPave

Embedded Steel (if needed)

Dowel Bar Characteristics

Goals are:Faulting resistance

Load transfer

VarietiesShape: Round, plate, etc.

Material: Steel, MMFX, FRP, etc.

Corrosion: Epoxy, zinc, plastic, etc.

Dowel Bar Design

• During construction concerned with:• Alignment and location

• Structural design concerned with:• There or not?

• …But doweldesign is:• Size

• Spacing

• Embedment depth

• Shape

• Material

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Tiebar Considerations

Goals are:Keeping joint tight

NOT FOR LOAD TRANSFER

Varieties:Deformed, round bar

Material: Steel, MMFX, FRP, etc.

Corrosion: Epoxy, plastic, etc.

Tiebar Design

• During construction concerned with:• Alignment and location

• Structural design concerned with:• Edge support (e.g., tied shoulder)

• …But tiebar design really is:• Size

• Spacing

• Embedment depth

• Material

Concrete

Concrete Considerations

Goals are:Uniformity and consistency

Workability until placed

Ability to be consolidated

Ability to hold edge if slipforming

Finishability

Required strength/stiffness

Durability

Varieties:Too many to discuss

Concrete Design

• Structural design concerned with:• Strength (resistance to cracking)

• Flexural (modulus of rupture)

• Compressive

• Stiffness (resistance to deformation)• Modulus of elasticity (E)

• Property altering additions• Fibers

• Chemical admixtures

• Mineral admixtures (SCMs)Focus in design is hardened properties such as strength and stiffness. Many of the chemical and mineral admixtures also alter fresh concrete properties.

L/3Span Length = L

d=L/3

Placing, Consolidating, Forming, Finishing, Texturing

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Placing, Consolidating, Forming, Finishing, Texturing Considerations

Goals are:Consistency/uniformity

Smoothness

Varieties:Too many to cover!

Placing, Consolidating, Forming, Finishing, Texturing Design

• During construction concerned with:• EVERYTHING!

• Structural design concerned with:• Thickness

• ???

Can Finish/Texture Affect Design?

Curing

Curing Characteristics

• Goals are:• Prevent (or replenish) loss of moisture

• Maintain a favorable temperature

• Varieties:• Initial cure

• Evaporation retarder

• Misting or fogging

• Final cure• Membrane-forming compounds

• Insulating blankets

• Electric blankets, linseed oil, plastic sheets, wet covers, etc.

Curing Design

• During construction concerned with:• Application rate

• Uniformity of application

• Structural design concerned with:• Strength gain

Age (days)

0

25

50

75

100

125

150

37 28 90 180

Co

mp

ress

ive

Str

engt

h(%

of 2

8-d

ay M

oist

Cu

red) Moist-Cured Entire Time

In Air After 7 days

In Air After 3 days

In Air Entire Time

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Jointing (Sawcutting)

Concrete Shrinks!

Drying Shrinkage

Hydration Uses Water

Thermal Shrinkage

Hot then Cold

HOT AT SET∆ ∗ ∆ ∗

ChemicalShrinkage

COOLED OFF

Shrinkage + Restraint = CRACKS!?!

HOT AT SET, HIGH MOISTURE, UNHYDRATED CEMENT

COOL, DRY, HYDRATED CEMENT

TEFLON | No Friction/Restraint

If no restraint

With restraint

Subgrade/Subbase | Restraint

What Happens without Joints?

Without joints, natural transverse & longitudinal cracking would form about like this…

40-80 ft

(12-24 m)

15-20 ft

(4.6-6.1 m)

So We Joint to Control Cracks

We place joints at a slightly shorter spacing to prevent natural cracking…

Jointing Characteristics

• Goals are:• Control cracking

• Isolate opposing axes of movement

• Varieties:• Crack control

• Contraction

• Construction

• Isolation

• Doweled

• Tied

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Jointing Design

• During construction concerned with:• LOCATION and alignment

• Sawcut depth

• TIMING! TIMING! TIMING!

• Structural design concerned with:• Spacing

• Skew (uncommon) Sealing(if required)

Sealing Characteristics

• Goals are:• Minimize infiltration of:

• Water and/or

• Incompressibles

• Varieties:• Field Poured:

• Hot-poured

• Silicone

• Two-component cold poured

• Preformed

Field Poured Sealant

Preformed Seal

Sealing Design

• During construction concerned with:• Cleanliness of joint

• Uniformity/Consistency

• Shape factor

• Recess

• Structural design concerned with:• Moisture in structure

Open it Up to Traffic…

… and See What Happens.

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Performance Metrics

Design Life

 ‐

 5,000

 10,000

 15,000

 20,000

 25,000

 30,000

 35,000

10 20 30 40

Number of Trucks over Design Life 

(growth = 2%/yr)

Design Life

100/day

250/day

500/day

Cracking Should Location Impact Design?

Historical thought: Cut joints short enough to control cracking due to environmental loading and then make thickness enough to support local applied load spectrum

Reality: Materials (e.g., coefficient of thermal expansion of agg, shrinkage potential of concrete, etc.), ambient environment (seasonal rainfall, temperature drop from set temperature, etc.), and other factors impact performance… so we’ve just been conservative with our factor(s) of safety to account for this and end up with concrete pavements that last longer than the design life!

Cracking Modes in JPCP

L/3Span Length = L

d=L/3

To combat, thinking is to make support

stronger to resist deflection.

Cracking Modes in JPCP

Edge support is lost!

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Cracking Modes in JPCP Cracking Modes in JPCP

Faulting Do You Need a Subbase?

General rules on when to use 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 the course of the service life of the pavement) do not typically require a subbaseto prevent pumping.A subgrade soil that is naturally free draining typically will not pump.Subgrade soils with less than 45% passing a No. 200 (75 μm) sieve and with a PI of 6 or less are adequate for moderate volumes of heavy truck traffic without the use of a subbase layer.

… so really more to prevent pumping than for structure!!

Preventing Pumping

Pumping of subgrade/subbase requires:

1. Undoweled joints or joints w/ poor load transfer

2. Water3. Fast moving, heavy loads 4. Fine-grained material in subgrade

or the subbase must be an erodible material

Eliminate casual factors to mitigate pumpingIncluding subbase &/or doweled pavement are other safety factors

Gross Measure (e.g., IRI, serviceability)

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Other Distresses

Few More Things to Know Before We Jump Into Design…

Is Pavement Design “an Exact Science”?

NCHRP 1-26 Phase II Final Report

“The new pavement will be built in the future, on subgrades often not yet exposed or accessible; using materials not yet manufactured from sources not yet identified; by a contractor who submitted the successful "low dollar" bid, employing unidentified personnel and procedures under climatic conditions that are frequently less than ideal.”

… So We Add Factor(s) of Safety

Reliability (scales traffic, distress, etc.)

Overall deviation (model agreement w/local performance)

Loss of support (model agreement w/local perf.)

Input levels (correlations to what matters)

Rounding (design to mm, round up some and then contractor builds thicker than necessary to ensure pay)

Beware of Bear Traps

Over conservative inputs

Nonsensical inputs

Poor relationships

Fudge factors

Assigning improper values can create over-conservative designs… junk in = junk out

Concrete Strength

Use average, in-field strength for design

(not min specified)

L/3Span Length = L

d=L/3

If specify minimum flexural strength at 28-day of 550 psi (3.79 MPa) & allow 10% of beams to fall below minimum:

STEP 1

Estimate SDEV:

9% for typical ready mix.

SDEV = 550 * 0.09 = 50 psi

= 3.79 * 0.09 = 0.34 MPa

STEP 2

S’c design = S’c minimum + z * SDEV

S’c design = 550 psi + 1.282 * 50 psi

= 3.79 MPa + 1.282 * 0.34 MPa

S’c design = 614 psi (4.23 MPa)

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ESAL = # of 18 kip (8,165 kg) equivalent single axles needed to cause same “response”

Because pavement responses are different for concrete and asphalt, ESALs are different for the same exact traffic loading… ESAL ≠ traffic

ESALs depends on thickness, among other things

Flexible ESALs generally about 1/3 less than rigid ESALs for highway-type traffic; NEVER COMPARE RIGID & FLEXIBLE ESALs

Equivalent Single Axle Loads (ESALs) Equivalent Single Axle Loads (ESALs)

15,400 kg 15,400 kg

10,900 kg5,400 kg

34 kPa 207 kPa

Axle Load (kips) Flexible LEF Rigid LEF

10 0.102 0.082

18 1.00 1.00

34 11.3 12.9

Convert to ESAL w/Load Equivalency Factor (LEF)…

Traffic Characterization

Traffic engineers use many different methods:Avg. Daily Truck Traffic (ADTT)

Avg. Daily Traffic (ADT) x %trucks

Avg. Annual Daily Truck Traffic (AADTT)

Avg. Annual Daily Traffic (AADT) x %trucks

Pavement engineer just wants trucks/day in design lane, but we still break traffic down in different ways:

Equivalent single axle loads (ESALs) -> not JUST traffic

Axle load spectrum data -> axle type x axle weight x # reps

Vehicle type -> vehicle axle type(s) & weight(s) x # reps

Truck factor -> trucks/day in design lane x ESALs/truck

Local Calibration

Thank you.Questions? FEEDBACK!

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Robert Rodden, P.E.

Senior Director of Pavement Technology

American Concrete Pavement Association

rrodden@acpa.org | 847.423.8706