Mass Concrete Thermal Control
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Transcript of Mass Concrete Thermal Control
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Mass Concrete Thermal Control
Ahmad Abu-Hawash, Iowa DOT
Dan Timmons, Jensen Construction
2009 Structures Design Construction Quality Workshop
April 1, 2009
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 2
What is Mass Concrete?• ACI: “any volume of concrete with
dimensions large enough to require thatmeasures be taken to cope with
generation of heat from hydration of thecement and attendant volume change tominimize cracking”
• Intentionally vague
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 3
Factors Affecting Mass Concrete• Concrete mix design: components
• Environmental condition: – Ambient temperature
– Concrete mix temperature
– Differential temperature
• Structural design: steel reinforcement
• Application: bridge elements• Least dimension: 3’, 4’, or 5’?
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 4
Mass Concrete Hydration• Significant heat is generated in the first few
days after placement• Expected to reach maximum temperaturewithin 1 to 3 days after placement
• Heat is trapped and can not escape quicklyresulting in: – Significant temperature difference: interior of
concrete is much hotter than its surface (>35°F)• Thermal Cracking
– Concrete mix getting too hot (>160°F)• Delayed Ettringite Formation (DEF)
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 5
Thermal Cracking• Thermal cracking develops when the tensile stress
exceeds the tensile strength of concrete: – Random map cracks in large foundation
– A series of vertical cracks in walls (widest near the base)
– Uniformly spaced cracks in beams (perpendicular to thelongest dimension)
• Mostly, a durability issue: easy pathways for air andwater
• In some severe cases, it may affect the structural
capacity
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Delayed Ettringite Formation (DEF)
• Development of unstable hydration• Long-term effect that may not show for
months or years after construction
• Can cause significant cracking
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 7
Thermal Control
• Objective: Eliminate thermal crackingby controlling temperature differentialand mix temperature (prior to, during
and after concrete placement)• Control measures should be evaluated
for costs vs. benefits
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 8
Thermal Control Measures
• Optimal concrete mix design• Insulation
• Concrete cooling before placement
• Concrete cooling after placement
• Use of smaller placements
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 9
Optimal Concrete Mix Design• Use low-heat cement such as Type II
• Use Class F fly ash and/or slag as a substitutefor a portion of the cement
• Use low water-to-cementitious materials ratio• Minimize the amount of cementitious
materials in the mix
• Use Larger and better graded aggregates• Limestone aggregates are better suited for
mass concrete
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 10
Insulation
• To control temperature differential: corevs. surface
• Has no significant effect on maximum
concrete temperature for placements of5’ or greater
• Typical R-value recommended: 2 to 4hr.ft².°F/Btu
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 11
Concrete Cooling before Placement
• Each 1°F of precooling is expected to reduce
the concrete temperature (after placement)by about the same amount
• Chilled water: about 5°F (100% subs.)
temperature reduction• Chipped or shaved ice: about 15°F to 20°F
(75% subs.) temperature reduction
• Liquid nitrogen (LN2): as low as 35°Freduction. Very effective but the mostexpensive option
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 12
Concrete Cooling after Placement
• Cooling pipes: – Non-corrosive piping embedded prior toconcrete placement
– Uniformly distributed: typically 1”Ø pipes@ 2’ to 4’ on center
– Removes heat from placed concrete bycirculating cool water from a nearby source
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 13
Smaller Placements
• Multiple lifts• Result in schedule delays and increased
cost due to additional effort for multiple
thermal control, and horizontal jointpreparation
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 14
Case Study: I-80 over Missouri River
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 15
Iowa Mass Concrete Specifications• Special Provision for Mass Concrete – Control of
Heat of Hydration – Mix design
• Cement: Type II, IP, or IS – min. 560 lbs
• Slag and Class F fly ash substitution• Maximum water to cementitious ratio = 0.45
– Thermal Control Plan (per 207.4R-05 ACI)
• Concrete temperature at placement: 40°F-70°F• Max. Concrete temperature after placement: 160°F
• Temperature differential: 35°F
• Temperature sensing and recording
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 16
I-80 over Missouri River
Thermal Control Plan
• Value engineering proposal by JensenConstruction to modify projectspecifications
• Proposed a thermal control plan basedon thermal modeling by CTL
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 17
Thermal Control Plan
• Concrete Mix• Maximum Concrete Temperature
• Temperature Difference Limit
• Cooling System
• Insulation
• Temperature Monitoring & Reporting
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 18
Concrete Mix• As developed by the supplier per
project specifications – Type IP-F Cement: 420 lbs
– Slag: 207 lbs – W/C ratio: 0.42
– Class V sand-gravel: 1586 lbs
– Limestone: 1322 lbs
– Air content: 6.5%
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 19
Maximum Concrete Temperature
•Initial concrete temperature based onseveral-truck rolling average: maximumof 85°F instead of 70°F
• Concrete temperature after placement:maximum of 160°F as per specifications
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 20
Temperature Difference Limit
• A compromise between the constantlimit and the performance-based limit
• Included a variable factor of safety
• More conservative in the early age butless conservative at the design strength
• Calculated specifically for the projectmix design
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 21
Temperature Difference Graph
0
10
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40
50
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80
90
100
0 1000 2000 3000 4000 5000 6000 7000
Compressive Strength**, psi
T e m p e r a t u r e D i f f e r e
n c e L i m i t * ,
° F
0
10
20
30
40
50
T e m p e r a t u r e D i f f e r e
n c e L i m i t * ,
° C
* Maximum allowable temperature difference between thetemperature sensor locations shown on Drawing Nos. 3 and 4.
** Actual compressive strength of the in-place concrete at the
surface, not the design strength.
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 22
Cooling System• Cooling piping system layout was developed
for each component (footing, stem, cap,…)
• River water was continuously circulated
through the cooling pipes until the insulationis removed
• Flow rate must be sufficiently high so that the
water does not heat by more than 2°F to 3°F
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 23
Typical Cooling System - Footing
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 24
Typical Cooling
System - Stem
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 25
Insulation• Used on top surfaces, over side forms and to
cover protruding reinforcing steel
• R-values in accordance with the Thermal
Control Plan• To remain in place throughout the monitoring
period but may be temporarily removed to
prepare for additional placements
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 26
Typical Insulation Blanket
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 27
Temperature Monitoring & Reporting
• To measure and report concretetemperatures at critical locations(center, surface,…)
• Two temperature sensors (a primaryand backup) at each location
• Data is recorded on an hourly basis
• Report of temperature data (max. anddifferential) is issued
T i l T t S
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 28
Typical Temperature Sensors
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 29
Typical Thermal Control Graph
0
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90100
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0 50 100 150 200 250 300 350Hours
T e m p e r a t u r e o r T e m p e r a t u
r e D i f f e r e n c e ,
° F
Air Temperature,
Center Temperature,
Top Surface Temperature,
Corner Temperature,
Side Surface Temperature,
Measured Temperature Difference, °F
TCP1b Temperature Difference Limi t, °F
TCP1b Footing #: Pier 7 Column final temps
Time/Date of Hour 0 :
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 30
Thermal Control Plan Elements
Pier Size* Date Cast
Cooling
Pipes Size* Date Cast
Cooling
Pipes
Least
Dim. Date Cast
Cooling
Pipes
Least
Dim. Date Cast
Cooling
Pipes
1 43' x 12' x 4'-6 10/20/2008 No 39' x 4' x 7' 12/4/2008 No 4' 12/4/2008 No 4' 1/23/2009 No
2 43' x 15' x 5' 11/19/2008 No 38' x 5' x 19' 1/9/2009 No 5' 2/18/2009 No 5' 3/20/2009 No
3 43' x 27' x 7'-3 10/30/2008 No 38' x 6' x 16' 11/21/2008 No 6' 1/23/2009 No 6' 2/25/2009 No
4 43' x 15' x 5' 11/4/2008 No 38' x 5' x 18' 12/10/2008 No 5' 3/5/2009 No 5'
5 43' x 19' x 6'-6 2/3/2009 No 38' x 5' x 20' 2/17/2009 No 5' 5'
6 46' x 18' x 5'-9 11/4/2008 No 38' x 8'-4 x 37' 11/19/2008 8'-4 1/6/2009 No 8'-4 1/22/2009 No
7 43' x 25'-6 x 9' 8/5/2008 Yes 38' x 7' x 35' 9/3/2008 Yes 7' 10/15/2008 Yes 7' 10/30/2008 No
8 77' x 39'-7 x 10'-6 12/30/2008 Yes 46' x 9' x 41' 2/12/2009 Yes 9' 9'
9 77' x 39'-7 x 10'-6 46' x 9' x 34' 9' 9'
10 43' x 19' x 5'-3 3/5/2009 No 38' x 8'-4 x 7' 3/13/2009 No 8'-4 8'-4
11 43' x 17' x 5'-9 1/20/2009 No 38' x 5' x 5' 1/28/2009 5' 2/20/2009 No 5' 3/6/2009 No
Footing Stem Columns Cap
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 31
Completion of Thermal Control• Hottest portion of concrete has reached and
begun to cool from its maximum temperature
• Concrete has reached and begun to cool from
its maximum temperature difference• At least 3 days has elapsed
• Difference between the hottest portion of
concrete and the average air temperature is< current difference limit
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April 1, 20092009 Structures Design Construction Quality WorkshopPage 32
Summary• The implementation of the Thermal
Control Plan saved money and kept theproject on schedule
• No thermal cracking in concrete wasreported
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A il 1 20092009 St t D i C t ti Q lit W k hP 33
References• Iowa DOT Standard Specifications and
Special Provisions
• Engineering Mass Concrete Structures,
November 2006 – PCA by J. Gajda andE. Alsamsam
• ACI 207.4R-05
• I-80 over Missouri RiverThermal Control Plan by CTL