The Challenges of Modern Day Concrete Pavement Specs · The Challenges of Modern Day Concrete...

The Challenges of Modern Day Concrete Pavement Specs

Peter Taylor

The Perfect Pavement

• Sustainable• Safe• Smooth and creamy

2

The Perfect Material for Pavements

• Cost effective• Easy to build with• Get traffic on it fast• Unbreakable• Weather-proof• Sustainable• Resilient

3

The Perfect Specification

• You get paid after […] years

• Or we do a test that predicts life• Slump?• Strength?• Formation factor?

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Challenges to writing specifications

• “Get out of the way” Celik Ozyildirim, VA• “Protect the contractor from himself”

Maria Masten, MN• “Trust but verify” Proverb

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Why bother changing?

• Currently:• Focused on strength• Struggling to get durability• Wrestling with unintended

consequences

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Use of hoses to water gardens is prohibited


• Life is more complicated…

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1967 2017

No. of ingredients Cement, water, rock, sand, AEA

Add SCMs, admixtures, int. aggregates, limestone…

Opening Weeks Days (or hours)

Curing Weeks Days

De-icing Sand, NaCl Other chlorides, formates, acetates

Design life 20 years 100 years

Knowledge base In house Contracted out


• Reducing premature failures can add 20 years to predicted life – at no cost

• Agencies are demanding concrete that delivers on the promise• Prepared to pay for it

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The Dream

• Understand what makes concrete “good”• Have tests to measure the right things• Set appropriate limits• What about a near miss?• Write the spec• Prepare the mixtures to meet those specifications

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“The Reality Is…”

• Tradition!• Politics• Changing work force• Balancing risk• Understanding of the material• Understanding test limitations

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A Better Specification

• AASHTO PP84 published in March• Guide Specification• “Deemed to satisfy”• Avoids bonus discussion – that is local• Provisional = meaning we can

modify as we learn things

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Delivering concrete to survive it’s environment

PEM

• A program to make everyone’s life miserable

Require the things that matter

• Transport properties (everywhere)• Aggregate stability (everywhere)• Strength (everywhere)• Cold weather resistance (cold locations)• Shrinkage (dry locations)• Workability (everywhere)

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Transport properties (permeability)

• All deterioration mechanisms involve fluid movement

• Keep water out = longer life• Measurement has been difficult

• Boiled water• RCPT

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Transport Properties/Permeability (6.6)

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Section Property Specified Test Specified Value

Mixture Qualification

Acceptance Selection Details

6.6.1.1 Water to Cementitious

Ratio

— ≤0.45 or ≤0.50

Yes Yes

6.6.1.2 Formation Factor

Table 1 ≥500 or ≥1000

Yes Yes

6.6.2.1 Ionic Penetration, F

Factor

Appendix X2 25 mm at 30 yr

Yes, F Through ρ

Choose only one

Formation Factor

• Resistivity• Then calculate Formation Factor based on

• Moisture• Temperature• Geometry• Curing conditions• Ionic concentration of the

pore solution

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Formation Factor

• F = Resistivity (bulk)Resistivity (solution)

• Sample is fully saturated • Solution resistivity = ~0.01 kΩ•cm

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ASTM C1202 Classification Charge Passed (Coulombs) Resistivity (kΩ ∙ cm)a Formation Factor High >4,000 <5.2 520

Moderate 2,000–4,000 5.2–10.4 520–1,040 Low 1,000–2,000 10.4–20.8 1,040–2,080

Very low 100–1,000 20.8–207 2,080–20,700 Negligible <100 >207 20,700

Ionic Penetration

• Cs = Surface concentration• Cx = Depth concentration• Co = Base concentration

(At a given age)

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Aggregate Stability

• If aggregates expand = damage• D-Cracking• Alkali aggregate reaction

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Aggregate Stability (6.7)

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6.7.1 D Cracking T 161, ASTM C666

— Yes No —

6.7.2 Alkali Aggregate Reactivity

R 80 — Yes No —

D-Cracking

• Freeze thaw test• Local practice for limits

• Iowa pore index test• 4500 grams of ½ - ¾ inch material• Pressurized to 35 psi• Volume of water penetrating• 1 minute (large pore system) (primary) • 15 minutes

(capillary size pores) (secondary)

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ASR

• AASHTO R80

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Strength

• Strong enough • But not much more if you can avoid it• It comes along for the ride with durability

• Beware of shrinkage and heat

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Concrete Strength (6.3)

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6.3.1 Flexural Strength

T 97 600 psi Yes Yes Choose either or

both6.3.2 Compressive

StrengthT 22 4000 psi Yes Yes

Cold Weather

• Freeze-thaw• Entrained air

• De-icing salts• Sufficient SCM

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Hardened Cement Paste Freeze-Thaw Durability (6.5)




6.5.1.1 Water to Cementitious

Ratio

— 0.45 Yes Yes a

6.5.1.2 Fresh Air Content

T 152, T 196, TP 118

5 to 8% Yes Yes Choose only one

6.5.1.3 Fresh Air Content/SAM

T 152, T 196, TP 118

≥4 to 8%; ≤0.2

Yes Yes

6.5.2.1 Time of Critical Saturation

ASTM C1585 30 year Yes No a, b

6.5.3.1 Deicing Salt Damage

— 35% SCM Yes Yes

6.5.3.2 Deicing Salt Damage

M 224 Topical treatment

Yes Yes

6.5.4.1 Calcium Oxychloride

Limit

T 365 <0.15 g CaOXY/g

paste

Yes No

Choose only one

Saturated Freeze Thaw

• Saturated system• Water freezes and expands• Prevention

• Air void system• Prevent saturation (<85%)

Weiss


• How much air?• 5% behind the paver• Beware of air void stability


• How much air?• Track trends• Set action limits


• Spacing factor?• 0.008” is reasonable• It is not a step function• Relationship between total air and spacing

factor depends on AEA

Super Air Meter (SAM)

• AASHTO TP 118

32 Ley


• AASHTO TP 118

33 Ley


• Seals must be maintained• Operators must be trained

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Freeze Thaw (Time to Saturation)

Calcium oxychloride

• Calcium from cement• Chlorides from salts• Expands 31%• At 40°F

Sutter CaCl2 @ 40°F

Calcium oxychloride

• Measure with – Low temperature differential scanning calorimetry (LT-DSC)

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Calcium oxychloride

Calcium oxychloride

• Comparison between paste expansion and LT-DSC

Critical Properties

• Drying Shrinkage• Random cracking• Warping

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Reducing Unwanted Cracking Due to Shrinkage (6.4)

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6.4.1.1 Volume of Paste

— ≤25% Yes No

6.4.1.2 Unrestrained Volume Change

ASTM C157 420 με @ 28 Yes No

6.4.2.1 Unrestrained Volume Change

ASTM C157 360, 420, 480 με @ 91

Yes No

6.4.2.2 Restrained Shrinkage

T 334 Crack free @ 180

Yes No

6.4.2.3 Restrained Shrinkage

T 363 Σ < 60% f ʹr @ 7 Yes No

6.4.2.4 Probability of Cracking

Appendix X1 Yes No

Commentary

Quality Control Check

— — No Yes

Choose only one

Ring Test

• Assesses cracking risk• Starts immediately

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Dual Ring Test

• This ring can measure both expansion and contraction.

• At 7 days force a temperature gradient in the concrete.

Weiss

Probability of Cracking

• Assumes 60% restraint• C157 at 91 days

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Workability

• Not too wet• Not too dry

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Workability (6.8)

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Acceptance

6.8.1 Box Test Appendix X3 <6.25 mm, <30%

surface void

Yes No

6.8.2 Modified VKelly Test

Appendix X4 15–30 mm/root s

Yes No

Workability

• Need the right workability for the placement method

• Use for prequalification

Mixture proportioning matters!

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Traditional Mix Optimized Mix

VKelly

• Kelly ball test• Developed in the 1950s in US• Standardized in California DOT test• Comparable to slump test

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VKelly

• Measure initial slump (initial penetration)• Run vibrator for 36 seconds at 8000 vpm• Record depth every 6 seconds• Repeat• Plot on root time• Calculate slope = VKelly Index

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VKelly

• Same slump• Flags workable vs non-workable mixtures• Can leave out some cement with the right

gradation

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Box Test

• A test that examines:• Response to vibration • Filling ability of the grout (avoid internal voids)• Ability of the concrete to

hold an edge

52 Ley

Box Test

• The edges of the box are then removed and inspected for honey combing and edge slump

53 Ley

QC

• Should include• Unit weight• Calorimetry• Maturity• Strength development• Air void stability

• And a response…

• Risk management

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Uniformity

• Deliver the same mixture in every truck

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Calorimetry

• Calorimetry tells us about the chemistry of the system (Uniformity)

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Unit weight

• Uniformity• Affected by air and water

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Maturity

• Maturity readings• Sensors in the pavement determine in-place

maturity and thereby strength for opening

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40

50

60

70

80

90

100

110

0

100

200

300

400

500

600

700

800

900

1000

0 12 24 36 48 60

Slab

Tem

pera

ture

, F

Mat

urity

(°C-

Hrs)

Time(hrs)

Field Maturity (10/2/12)

Slab Temperature

Closing

• A framework is in place• The details need work

• Ruggedness• Limits• Correlation with life• Training

• Ongoing challenges• Tests take too long• Costs• Risk

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The Challenges of Modern Day Concrete Pavement Specs · The Challenges of Modern Day Concrete...

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