[PDF] - To Crack or Not to Crack? To Crack or Not to Crack? That is the PDF Document

SLIDE 1

10/10/2007 1

To Crack or Not to Crack? To Crack or Not to Crack? That is the Question That is the Question

Causes and Prevention Causes and Prevention

Presented by: Dave Frentress Presented by: Dave Frentress

Does Concrete Crack? Does Concrete Crack? Is it Dark at Night? Cracking Cracking

A recent NRMCA survey identified

A recent NRMCA survey identified cracking as the number one cracking as the number one complaint in the concrete industry. complaint in the concrete industry.

How many types of cracks are there?

SLIDE 2

10/10/2007 2

20 Causes of Cracks 20 Causes of Cracks

Physical Chemical Thermal Structural

After Hardening

Plastic Construction Thermal

Before Hardening

Types of Cracks

D Cracking Crazing Drying Shrinkage Shrinkable Aggregates Sulfate Carbonation Alkali Aggregate Rebar Corrosion Early Thermal Contraction Seasonal Variations Freeze Thaw Settlement Movement Creep Loads Plastic Settlement Plastic Shrinkage Traffic Vibration Sub grade Movement Formwork Movement Early Frost Damage

Identifying Causes of Cracks Identifying Causes of Cracks

Identify crack type

When did they show up?

When did they show up?

What type of pattern?

What type of pattern?

L k t L k t

Look at

Mix design

Mix design

Ambient conditions

Ambient conditions

Structure design & dimensions

Structure design & dimensions

Placing, finishing, curing

Placing, finishing, curing

Plastic Shrinkage Cracks Plastic Shrinkage Cracks

Show up before concrete is set

Typically disconnected

Typically disconnected -

not through full

not through full depth or thickness of structure depth or thickness of structure

Rapid evaporation

Rapid evaporation -

slow bleeding

slow bleeding

Weather

Hot Dry Windy

Hot Dry Windy

Cold

Cold -

slow set Temp Drop

slow set Temp Drop

Low bleed concrete

Low w/ c

Low w/ c HRWR HRWR

High air

High air Silica fume Silica fume

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10/10/2007 3

Plastic Shrinkage

Plastic Shrinkage Plastic Shrinkage Plastic Shrinkage Cracks Plastic Shrinkage Cracks Prevention Prevention

Fog mist

Evaporation retarder

“Man in a Can”

“Man in a Can” B i Ti B i Ti

Buying Time

Buying Time

Polypropylene Fiber

Wind or sun screens

Faster set

SLIDE 4

10/10/2007 4

Man in a can… Man in a can… Plastic Shrinkage Cracks Plastic Shrinkage Cracks Prevention Prevention

Faster set

Higher early strength

Cover/ protect concrete

Drying Shrinkage Cracks Drying Shrinkage Cracks

Show up 4 days to 8 weeks

Through entire structure at weak

Through entire structure at weak point point N l h i k N l h i k 1/ 2” 100’ 1/ 2” 100’

Normal shrinkage

Normal shrinkage - 1/ 2” per 100’ 1/ 2” per 100’

Aggravated by

High water content

High water content

Insufficient control joints

Insufficient control joints

Inadequate curing

Inadequate curing

SLIDE 5

10/10/2007 5

Factors Which Affect Factors Which Affect Shrinkage Cracking Shrinkage Cracking

Paste to aggregate ratio

W/ C ratio

Aggregate type and

Aggregate type and gradation gradation

Cement

Weather

Finishing practices

Drying Shrinkage Cracks Drying Shrinkage Cracks Prevention Prevention

Lower shrinkage mix

Larger Coarse Agg. size

Larger Coarse Agg. size

Lower water content

Max .45 w/ c

Max .45 w/ c

Proper control joints

Proper control joints Psst Psst -

they

they actually are cracks actually are cracks -

planned ones

planned ones

Per ACI

Per ACI

On the Plans!

On the Plans!

Proper curing

If you want Curing, you MUST specify it.

If you want Curing, you MUST specify it.

Bid Item

Bid Item

Batched into the load? Batched into the load? At the Plant… At the Plant…

SLIDE 6

10/10/2007 6

Added to the load! Added to the load! On the jobsite… What happens? What happens?

i l l i l l Moisture loss creates volume Moisture loss creates volume reduction and tensile forces, these reduction and tensile forces, these exceed the concretes ability to exceed the concretes ability to resist shrinkage and the concrete resist shrinkage and the concrete cracks. cracks.

Strength Results using W/C Values………… Strength Results using W/C Values…………

SLIDE 7

10/10/2007 7

Cracking of Concrete Due to Shrinkage Cracking of Concrete Due to Shrinkage

Original Length Original Length Unrestrained Shrinkage Unrestrained Shrinkage No Stress Developed No Stress Developed Restraints Cause Restraints Cause Tensile Stress Tensile Stress When Tensile Stress When Tensile Stress is Greater Than is Greater Than Tensile Strength... Tensile Strength... Concrete Cracks. 200 to 600 psi Tensile Stress Developed 200 to 600 psi Tensile Stress Developed 27 cubic feet 2 cubic feet are coming back out!

200 250 300 350 400

f Water

EXCESS WATER IN ONE CUBIC YARD OF CONCRETE WITH 500 POUNDS OF CEMENT

1 4 3 2

50 100 150 200 Pounds o

.25 .30 .35 .40 .45 .50 .55 .60 .65 .70 .75

Water/Cement Ratio

1

SLIDE 8

10/10/2007 8

Understanding Shrinkage Understanding Shrinkage

Even after hydration, concrete is a porous material

Why does concrete shrink Why does concrete shrink as it dries? as it dries?

Pores lose water due to

Pores lose water due to hydration and evaporation hydration and evaporation

As pores become less than

As pores become less than fully saturated, meniscus fully saturated, meniscus forms at the air forms at the air-

water

water interface due to surface interface due to surface tension tension

The surface tension of pore

The surface tension of pore solution which forms solution which forms meniscus also exerts inward meniscus also exerts inward pulling force on the side walls pulling force on the side walls

f the pore
f the pore

These forces in all pores in

These forces in all pores in range of 2.5 to 50 nm is range of 2.5 to 50 nm is primary cause of shrinkage primary cause of shrinkage

C P

C P

Thermal Shrinkage Cracks Thermal Shrinkage Cracks

Show up 1st or 2nd night

Seasonal temperature variations

High cement mass concrete

High cement mass concrete May or may not go through entire May or may not go through entire

May or may not go through entire

May or may not go through entire structure structure

Shatter or three point are frequent

Caused by

Rapid cooling of entire structure

Rapid cooling of entire structure

Differential cooling

Differential cooling

SLIDE 9

10/10/2007 9

Thermal Shrinkage Cracks Thermal Shrinkage Cracks Prevention Prevention

Reduce temperature rise

Less cement

Less cement -

more pozzolan

more pozzolan

Increase early age strength

A l t A l t

Accelerators

Accelerators

Reduce temperature loss or gradient

Cover or insulate

Cover or insulate

Beware Thermal shock!

Beware Thermal shock!

Crazing Crazing Crazing Cracks Crazing Cracks

Map cracking 1 to 7 days of age

Shrinkage of surface paste

Very fine

Very fine -

depth of < 1/ 8”

depth of < 1/ 8”

No impact on durability

No impact on durability p y p y

Rapid loss of surface moisture

Lack of curing

Lack of curing

High W/ C ratio (over a .45)

High W/ C ratio (over a .45)

Finishing bleed water into surface

Finishing bleed water into surface

“Blessing” the slab surface with water

“Blessing” the slab surface with water

“Jitterbugging”

“Jitterbugging”

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10/10/2007 10

What happens? What happens?

The paste shrinks more than the The paste shrinks more than the concrete, so when the concrete is concrete, so when the concrete is

ver worked the aggregate gets
ver worked the aggregate gets

pushed down leaving only paste near pushed down leaving only paste near pushed down leaving only paste near pushed down leaving only paste near the surface. It shrinks more than the surface. It shrinks more than the concrete below it, leaving a fine the concrete below it, leaving a fine map pattern of cracking. map pattern of cracking.

Crazing Cracks Crazing Cracks Prevention Prevention

Proper curing

Moderate slump or use admixtures

Don’t finish bleed water into concrete

Don’t sprinkle or spray water on the

Don’t sprinkle or spray water on the slab slab

Use surface retarders whenever it is

Use surface retarders whenever it is hot, dry and windy. Especially cool hot, dry and windy. Especially cool dry and windy dry and windy

Windy means + 5 MPH!

Curling Curling

Moisture Moisture Profile Profile Differential Differential Shrinkage Shrinkage

Concrete slabs curl due to differential moisture Concrete slabs curl due to differential moisture through the thickness, and the differing through the thickness, and the differing shrinkage response. shrinkage response.

Shrinkage Shrinkage Curling Curling Response Response Cracking Cracking Upon Loading Upon Loading

SLIDE 11

10/10/2007 11

Curling in Slabs Curling in Slabs

Differential volume change from

Differential moisture loss

Differential moisture loss

Temperature gradient

Temperature gradient

Prevention

Proper curing

Proper curing

Proper jointing

Proper jointing

Low water mix

Low water mix-

reasonable Cement content, max Agg.

reasonable Cement content, max Agg. sizes sizes

No poly vapor barriers

No poly vapor barriers

Prevent excessive bleeding

Prevent excessive bleeding

Shrinkage reducing admixture

Shrinkage reducing admixture

Evaporation Control/ retarder

Evaporation Control/ retarder

Jointing…Those Planned Cracks Jointing…Those Planned Cracks

Do not extend joint spacings

ver 1 1/ 2 times length

versus the width. 2 to 3 times the depth ¼ the depth minimum ¼ the depth - minimum

Exam ple: A four foot w ide slab the joints should not extend past 6 feet.

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10/10/2007 12

Compacting the Subgrade Compacting the Subgrade Base/subgrade Base/subgrade

SLIDE 13

10/10/2007 13

Base/subgrade Base/subgrade Joint Planning Joint Planning

Watch those Watch those corners corners Plan Joints Plan Joints -

prior to

prior to placement placement

SLIDE 14

10/10/2007 14

Joint Minimization, Joint Minimization, Crack “Elimination” Crack “Elimination”

Shrinkage Reducing Admixes

Warranty Slabs

Warranty Slabs

Cost savings on Joint stretching,

Cost savings on Joint stretching, maintenance. maintenance.

40 X 40 Portland, 50 X 50 in Seattle

40 X 40 Portland, 50 X 50 in Seattle

Performance of SRAs Performance of SRAs

Reduce shrinkage by as much as 80% at

Reduce shrinkage by as much as 80% at 28 days. 28 days.

Reduce ultimate shrinkage on the order of

Reduce ultimate shrinkage on the order of Reduce ultimate shrinkage on the order of Reduce ultimate shrinkage on the order of 25 to 50% . Reduce 25 to 50% . Reduce-

not eliminate…

not eliminate…

Allow up to 50 ft joint spacing with no

Allow up to 50 ft joint spacing with no cracking cracking – – Glacier Northwest Glacier Northwest (6” thick, proper practices, 3 day wet cure, (6” thick, proper practices, 3 day wet cure, etc.) etc.)

SRA job: Layout & Planning SRA job: Layout & Planning

SLIDE 15

10/10/2007 15

Greg’s baby No Restraint No Restraint Details Details

SLIDE 16

10/10/2007 16

EIC Winner EIC Winner SRA Impact on Curling SRA Impact on Curling

Joint Minimization, Joint Minimization, Crack “Elimination” Crack “Elimination”

TYPE K Komponent Cement

TYPE K Komponent Cement Shrinkage compensating Cement Shrinkage compensating Cement

15% Replacement of regular cement

Bi B Fl F d M Old N Bi B Fl F d M Old N

Big Box Floors Fred Meyer Old Navy

Big Box Floors Fred Meyer Old Navy

Parr Lumber

Parr Lumber

Additional Curing Requirements

Additional Curing Requirements

Operational Challenges

Operational Challenges-

Silo or Bags

Silo or Bags

SLIDE 17

10/10/2007 17

ORIGINAL LENGTH Potential Expansion Actual Expansion K FIBER REBAR FINAL LENGTH CONCRETE K-FIBER or REBAR

Figure 1

System-K with Fibers Type K with Rebar (#4 @ 24”ocew) Traditional Portland Concrete with Rebar (#4 @ 24”ocew)

Red = Portland Cement & Rebars Blue = Type K & Rebars Green= Type K & K Fibers-no Rebars Test slabs were: 50’ X 6’ by 6” thick

System-K Concrete has only 210 lf of joints

50,000 square foot slabs

Portland Concrete has 6270 lf of joints

OVER A MILE OF JOINTS ARE ELIMINATED

Figure 3

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10/10/2007 18

How Type K Works How Type K Works

Komponet System w/c 0.45 Hydration Conventional ODOT DECK Place w/c 0.40 Hydration w/c 0.25

Current DOT’s Using Type K Current DOT’s Using Type K

California DOT

Michigan DOT

Ohio DOT

Mix Design Mix Design

Cement

15 Percent type K

Standard Aggregates

W/ C ratio of 0.45 to 0.50

Slump of 5 to 6 inch

Standard admixtures

SLIDE 19

10/10/2007 19

Fred Meyers Distribution Center Chehalis Fred Meyer Distribution Center Chehalis, WA 60,000 square foot expansion 125’-157’ x 55’ joint spacing Costco Distribution Center Salt Lake City, UT 300,000 square feet 100’ x 200’ joint spacing

SLIDE 20

10/10/2007 20

Old Navy Old Navy Foam around Columns Foam around Columns Bars at Corner Bars at Corner

SLIDE 21

10/10/2007 21

Parr Lumber Parr Lumber Flatness Flatness Water Cure Water Cure

SLIDE 22

10/10/2007 22

Sofa Express

70,000 square feet Max Joint Spacing 130’ No Rebar After 5 weeks FF = 98.6 FL = 61.0 No cracks

Cracks Normally Occur at:

1. Mid-Panel 2. Columns in the Slab (notice no boxouts) 3. Perimeter Columns 4. Bollards 5. Re-entrant Corner 6. Plastic and Crazing

Type K Advantages: Type K Advantages:

No Drying Shrinkage

Easily Placed

Easily Finished

Lower maintenance Costs

No need for joints, dowels, etc.

Up to 50,000 square feet without joints

Length to width ratios increase

No Curling

Moisture is used

Moisture is used -

Vapor tests faster

Vapor tests faster

SLIDE 23

10/10/2007 23

Crack Control Crack Control -

Summary

Summary

Prepare subgrade properly

Prepare subgrade properly-

watch restraints

watch restraints

Specify and Order correct mix

Specify and Order correct mix-

don’t add water!

don’t add water!

Plan and cut proper control joints

Plan and cut proper control joints P t i ti P t i ti

Prevent excessive evaporation

Start curing early

Synthetic Fiber Reinforcement

Shrinkage Reducing Admixtures

Type K Cement