8 cold -weather concreting.pptx
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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
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
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
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
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.
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.
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
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