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