Cold-Weather Concreting

CIVE 570Spring 2012

ByDr. Adel El Kordi

ProfessorStructural Engineering

Department Faculty of EngineeringAlexandria University

Cold Weather

n Average daily temperature <5ºC (40ºF) for 3 successive days

n Stays <10ºC (50ºF) for more than ½ of any 24h period.

Effect of Freezing Fresh Concrete

n Up to 50% reduction of ultimate strength can occur if frozen —u Within a few hoursu Before reaching a strength of

3.5 MPa (500 psi)n Frozen only once at an early age —

u With curing nearly all strength can be restored

u Less resistance to weathering u More permeable

Relationship Between Initial Set and Casting Temperature

Relationship Between Final Set and Casting Temperature

Relationship Between Slump and Casting Temperature

Effect of Temperature on Strength Development

Effect of Low Temperatures on Strength

Effect of Cement Type on Early Age Strength in Cold Weather

Retaining Heat of Hydration

Heat of hydration is useful in winter concreting as itcontributes to the heat needed to provide a satisfactorycuring temperature; often without other temporary heatsources, particularly in more massive elements.

Cold-Weather Concreting

Methods to accelerate strength gain:n Type III or (HE) high-early-strength

cement.n Additional Portland cement (60 to 120

kg/m3).n Chemical accelerators

Durability of Non-Air-Entrained Concrete

Concrete that is not airentrained can suffer strength loss and internal as well assurface damage as a result of freezing and thawing.

Example of a concrete floor that was saturated with rain, snow, or water and then frozen, showing the need for air entrainment. This could also result in greater deflection of the floor and a surface that is less wear-resistant.

Durability of Non-Air-Entrained Concrete

Recommended Concrete Temperatures-Air-Entrained Concrete

Effect of Temperature of Materials on Concrete Temperatures

0.22(TaMa + TcMc) + TwMw + TwaMwa

0.22(Ma + Mc) + Mw + Mwa

T =

T = temperature of the freshly mixed concrete, °C (°F)

Ta, Tc, Tw, and Twa = temperature in °C (°F) of aggregates, cement, added mixing water, and free water on aggregates, respectively

Ma, Mc, Mw, and Mwa = mass, kg (lb), of aggregates, cementing materials, added mixing water, and free water on aggregates, respectively

Temperature of mixing water needed to produce heated concrete of required temperature.

Cooling After Protection

Section size, minimum dimensions, mm (in.)

Less than300 (12)

300 to 900(12 to 36)

900 to 1800 (36 to 72)

Over1800 (72)

28°C (50°F) 22°C (40°F) 17°C (30°F) 11°C (20°F)

To avoid cracking of the concrete due to sudden temperature change near the end of the curing period, ACI Committee 306 requires that the source of heat and cover protection be slowly removed. The maximum allowabletemperature drop during the first 24 hours after the end of the protection is given in Table

Checking Fresh Concrete TemperaturesControl Test

• Thermometer with a metal sensor suitable for checking fresh concrete temperatures.

Checking Hardened Concrete Temperatures – Control Test

Measuring Hardened concrete temperatures below the surface with a glass thermometer.

Temperature of Test Cylinders

Concrete test cylinders must be maintained at a temperature between 16°C and 27°C at the jobsite for up to 48 hours until they are taken to a laboratory for curing. For concrete mixtures with a specified strength of 40 MPa or greater, the initial curing temperature shall be between 20°C and 26°C

Recommended Duration of Temperature

Air-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

Recommended Duration of Temperature

Fully stressed, exposed, air-entrained concrete

Required percentage of standard-cured 28-day strength

Days at 10°C (50°F) Days at 21°C (70°F)

Type of portland cement

Type of portland cement

I or GU

II or MS

III or HE

I or GU

II or MS

III or HE

50 6 9 3 4 6 3

65 11 14 5 8 10 4

85 21 28 16 16 18 12

95 29 35 26 23 24 20

Cold-Weather Insulation, Aboveground

Thermal Resistance, R, for (10-mm Thick) Insulating Materials

Board and Slabs (m2·K)/W (°F·hr·ft2)/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 fill

Wood fiber, soft woods 0.231 3.33

Vermiculite 0.148 2.13

Insulating MaterialsHeat and moisture can be retained in the concrete by covering it with commercial insulating blankets. The effectiveness of insulation canbe determined by placing a thermometer under it and incontact with the concrete. If the temperature falls below the minimum required on Line 4 in Table 14-1, additional insulating material, or material with a higher R value, should be applied.

Insulating Concrete Forms (ICF)

Insulated Column Forms

High-density plywood

Rigid polystyrene

Rough plywood

With air temperatures down to 23°C, concrete was cast in this insulated column form.

Direct-Fired Heater

Indirect-Fired Heater

Hydronic Systems

Maturity ConceptMetric: M = (C + 10)

tInch-Pound: M = (F – 14)

twhereu M = maturity factor u = summation u C = concrete temperature, degrees

Celsiusu F = concrete temperature, degrees

Fahrenheitu t = duration of curing at temperature

C (F), usually in hours

Thermocouples and wiring at various depths in a caisson.

Automatic temperature recorder.