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2009 Quality Concrete School2009 Quality Concrete School
IT’S HOT!!IT’S HOT!!
IT’S CHILLY!!IT’S CHILLY!!
SHRINKAGE!!SHRINKAGE!!
Detrimental Hot Detrimental Hot Weather ConditionsWeather Conditions
• High ambient temperature
• High concrete temperature
• Low relative humidity
• High wind speed
• Solar radiation
Hot Weather Effects Hot Weather Effects on Concreteon Concrete
• Increased water demand
• Accelerated slump loss
• Faster set
• Increased tendency for plastic cracking
• Difficulties controlling entrained air
• Increased potential for thermal cracking
PrecautionsPrecautions• Cool concrete • Cool concrete
ingredients • Reduce the time of
transport, placing and finishing
• Use sunshades, windscreens, fogging, or spraying to limit moisture loss during placing and finishing
Rule of ThumbRule of Thumb
• For every 10º F increase in concrete temperature:– You need 7 lbs (~ 1 gallon) of water per yd 3 to
maintain a given slump– You lose about 1 hr of setting time
– Oh yah, if you add that 1 gallon of water you will lose about 150 psi of compressive strength
Effect of Concrete Effect of Concrete Temperatures on StrengthTemperatures on Strength
BE COOL!BE COOL!
• More Rules of Thumb– Drop the water temp by 4º, concrete temp
drops 1º– Drop the aggregate temp by 2º, concrete
temp drops 1º
Evaporation of Evaporation of Surface Moisture Surface Moisture
from Concretefrom Concrete
Precautions to Minimize Precautions to Minimize Plastic Shrinkage CrackingPlastic Shrinkage Cracking
• Moisten aggregates• Cool aggregates and
mixing water• Dampen subgrade• Erect temporary
windbreaks and sunshades• Cover concrete• Fog slab immediately after
placing• Add plastic fibers
CONCRETE IN GRANTSCONCRETE IN GRANTS
• Brrrrrrrrrrrrrrrrrrr!!!!!!
• Check out WOW Café in Milan (Great Enchiladas and Chicken Fried Steak)
Cold Weather Cold Weather (ACI 306)(ACI 306)
• Average daily temperature <5ºC (40ºF) for 3 successive days
• Stays <10ºC (50ºF) for more than ½ of any 24h period
Effect of Freezing Effect of Freezing Fresh ConcreteFresh Concrete
• Up to 50% reduction of ultimate strength can occur if frozen —– Within a few hours– Before reaching a strength
of 3.5 MPa (500 psi)• Frozen only once at an early
age —– With curing nearly all
strength can be restored– Less resistance to
weathering – More permeable
Setting Times at Setting Times at Different TemperaturesDifferent Temperatures
Rule of ThumbRule of Thumb
• For every 10°C (18°F) reduction in concrete temperature, the times of setting of the concrete double
- thus increasing the amount of time that the concrete is vulnerable to damage due to freezing
Effect of Temperature on Effect of Temperature on Strength DevelopmentStrength Development
Effect of Casting Temperature Effect of Casting Temperature on Slumpon Slump
Relationship Between Temperature, Slump and Air Content
Retaining Heat of HydrationRetaining Heat of Hydration
Cold-Weather ConcretingCold-Weather Concreting
• Type III or HE high-early-strength cement
• Additional portland cement (60 to 120 kg/m3 or 100 to 200 lb/yd3)
• Chemical accelerators
Methods to accelerate strength gain:
Recommended Concrete TemperaturesRecommended Concrete Temperatures—Air-Entrained Concrete—Air-Entrained Concrete
Line ConditionThickness of sections, mm (in.)
Less than 300 (12)
300 to 900 (12 to 36)
900 to 1800 (36 to 72)
1Minimum temperature of fresh concrete as mixed for weather indicated.
Above -1°C (30°F)
16°C (60°F) 13°C (55°F)
2-18°C to -1°C (0°F to 30°F)
18°C (65°F) 16°C (60°F)
3Below -18°C
(0°F)21°C (70°F) 18°C (65°F)
4Minimum temperature of fresh concrete as placed and maintained.
13°C (55°F) 10°C (50°F)
NRMCA Heating CriteriaNRMCA Heating CriteriaFor plants seeking certification to supply concrete in subfreezing weather i.e., where concrete is placed regularly during sub-freezing weather, minimum heating capacity for water and/or aggregate of 15 boiler output horsepower per 100 cubic yard average daily cold weather production. (May be reduced to 10 bph if storage capacity permits round-the-clock operation of heating equipment.) One boiler horsepower = 33,500 BTU per hour transferred to the water.
• Protect from freezing
• Agitate if necessary
Admixtures
Recommended Duration Recommended Duration of Temperatureof TemperatureAir-entrained concreteAir-entrained concrete
Service category
Protection from early-age freezing
For safe stripping strength
Convent. concrete,
days
High-early strength concrete,
days
Convent.
concrete, days
High-early-strength concrete,
days
No load, not exposed, favorable moist-curing
2 1 2 1
No load, exposed, but later has favorable moist-curing
3 2
3 2
Partial load, exposed 6 4
Fully stressed, exposed
See next slide
Thermal Resistance, R, for (10-Thermal Resistance, R, for (10-mm [1-in.] Thick) Insulating mm [1-in.] Thick) Insulating
MaterialsMaterialsBoard and SlabsBoard and Slabs (m(m22·K)/W·K)/W (°F·hr·ft(°F·hr·ft22)/Btu)/Btu
Expanded polyurethane 0.438 6.25
Expanded polystyrene 0.277 4.0
Mineral fiberboard 0.204 2.94
Plywood 0.087 1.24
Loose fillLoose fill
Wood fiber, soft woods 0.231 3.33
Vermiculite 0.148 2.13
Cold-Weather Cold-Weather Insulation, Insulation,
AbovegroundAboveground
Insulating BlanketsInsulating Blankets
Thermal resistance of Thermal resistance of mineral fiber blanketmineral fiber blanket (50 to 70-mm [2 to 2.75-in.] thick) (50 to 70-mm [2 to 2.75-in.] thick)
1.2 1.2 (m(m22·K)/W·K)/W
7 7 (°F·hr·ft(°F·hr·ft22)/Btu)/Btu
Insulating Concrete Forms Insulating Concrete Forms (ICF)(ICF)
Insulated Column FormsInsulated Column Forms
High-density plywood
Rigid polystyrene
Rough plywood
Cooling After ProtectionCooling After Protection
Section size, minimum dimensions, mm (in.)Section size, minimum dimensions, mm (in.)
Less thanLess than
300 (12)300 (12)
300 to 300 to 900900
(12 to 36) (12 to 36)
900 to 900 to 18001800
(36 to 72)(36 to 72)
OverOver
1800 (72)1800 (72)
28°C 28°C (50°F)(50°F)
22°C 22°C (40°F)(40°F)
17°C 17°C (30°F)(30°F)
11°C 11°C (20°F)(20°F)
Maximum Temperature Drop in 24 HoursMaximum Temperature Drop in 24 Hours
EnclosuresEnclosures• Wood
• Canvas
• Tarpaulins
• Polyethylene Film
Heated EnclosureHeated Enclosure
Indirect-Fired HeaterIndirect-Fired Heater
Hydronic SystemsHydronic Systems
VideoVideo
Concreting on Ground
SHRINKAGE!!!SHRINKAGE!!!
Chemical and Autogenous Chemical and Autogenous ShrinkageShrinkage
Plastic ShrinkagePlastic Shrinkage
Volume ChangesVolume Changes
Shrinkage and CrackingShrinkage and Cracking
Minimize Shrinkage CrackingMinimize Shrinkage Cracking
LUNCHTIMELUNCHTIME