Post on 28-Oct-2014
CONCRETING IN EXTREME WEATHER CONDITIONS
Mansoor Azam Qureshi
NUST
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EXTREME WEATHER Weather conditions at a jobsite—hot or
cold, windy or calm, dry or humid—may be vastly different from the optimum conditions assumed at the time a concrete mix is specified, designed, or selected, or from laboratory conditions in which concrete specimens are stored and tested. Weather extremes may be:-Hot WeatherCold Weather
HOT WEATHER CONCRETING
High ambient temperature High concrete temperature Low relative humidity High wind speed Solar radiation
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AFFECTS OF HOT WEATHER ON CONCRETE
Increased water demand Accelerated slump loss Faster setting Increased tendency for plastic cracking Prompt and early curing Difficulties controlling entrained air Increased potential for thermal cracking High temperature causes loss in strength loss in
long run
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WATER DEMAND AND SLUMP
Slump reduces as concrete temperature rises.Water demand rises for same slump as temperature rises
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ADDITION OF WATER Decreased strength from higher water to
cement ratio Decreased durability due to cracking Increased permeability Non uniform surface appearance Increased tendency for drying shrinkage Reduced abrasion resistance from
tendency to sprinkle water during finishing
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EFFECT OF CONCRETE TEMPERATURE ON SETTING TIME
High temperatures of freshly mixed concrete increase the rate of setting and shorten the length of time within which the concrete can be transported, placed, and finished. Setting time can be reduced by 2 or more hours with a 10°C (18°F) increase in concrete temperature.
THUMB RULE 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
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AFFECTS OF HIGH TEMPERATURE ON STRENGTH
Early strength up to 4 to 5 days is better at higher temperature but lower at 28 days and beyond.
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COOL CONCRETE
Liquid nitrogen added directly into a truck mixer at the ready mix plant is an effective method of reducing concrete temperature for mass concrete placements or during hot-weather concreting.
TEMPERATURE CONTROL OF MATERIALS
Concrete temperature reduces by 1 degree for 4 degree temp drop in water or 2 degree temp drop in
aggregates.
A crusher delivers finely crushed ice to a truck mixer
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REDUCE SURFACE
EVAPORATION
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1. Low air temperature2. High concrete temperature3. Low humidity 4. High wind speed
CONTROL OF PLASTIC SHRINKAGE
Moisten aggregates Cool aggregates and
mixing water Dampen sub grade Erect temporary
windbreaks and sunshades
Cover concrete Fog slab immediately
after placing Add plastic fibers
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CURING AND PROTECTION
Loosened formwork as soon as practical without damage to the concrete.
Curing water should not be more than about 11°C (20°F) cooler than the concrete to prevent thermal cracking.
In hot weather, continuous moist curing for the entire curing period is preferred.
Moist-cured surfaces should dry out slowly after the curing period to reduce the possibility of surface crazing and cracking.
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Use materials and mix proportions that have a good record in hot-weather conditions
Cool the concrete or one or more of its ingredients
use a concrete consistency that allows rapid placement and consolidation
Reduce the time of transport, placing and finishing as much as possible
Schedule concrete placements to limit exposure to atmospheric conditions, such as at night or during favorable weather conditions .
consider methods to limit moisture loss during placing and finishing, such as sunshades, windscreens, fogging, or spraying
PRECAUTIONS
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PRECAUTIONS Apply temporary moisture-retaining films after
screeding For unusual cases in hot weather and where careful
inspection is maintained, a retarding admixture may be beneficial in delaying the setting time, despite the somewhat increased rate of slump loss resulting from their use.
NOTE: Which precautions to use and when to use them will depend on: the type of construction; characteristics of the materials being used; and the experience of the placing and finishing crew in dealing with the atmospheric conditions on the site.
COLD WEATHER Average daily temperature <5ºC (40ºF) for 3 successive
days Stays <10ºC (50ºF) for more
than ½ of any 24h period
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EFFECTS OF COLD WEATHER ON CONCRETE Delay in setting
and hardening.Setting gets
stopped below -10 C.
Setting times increase appreciably.
Slow progress of work.
Delay in removal of formwork
SETTING TIME - THUMB RULE
For every 10°C (18°F) reduction in concrete temperature, the times of setting of the concrete double
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RELATIONSHIP BETWEEN
TEMPERATURE , SLUMP AND AIR CONTENT
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EFFECTS OF COLD WEATHER ON CONCRETE-1 Freezing of Fresh
Concrete.Stop hydration of
cementExpansion of
concrete causing disruption of crystallization
Ice crystal formations occur as unhardened concrete freezes.
EFFECTS OF FREEZING ON FRESH CONCRETE-2
Up to 50% reduction of ultimate strength can occur if frozen Within a few hoursBefore 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 permeable04/07/2023 22Plain and Reinforced Concrete - 1
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PRE-HARDENING PERIOD Fresh concrete should not be subjected to freezing
temperature before attaining sufficient strength. IS recommended pre-hardening periods are shown below
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Freezing and ThawingDurability is
impaired.Fatigue stresses
in concrete
EFFECTS OF COLD WEATHER ON CONCRETE
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PRECAUTIONS Insulation and utilization of heat of
hydration. Selecting suitable type of cement. Heating of materials. Admixtures of anti-freezing materials. Electrical heating of concrete mass. Use of air entraining agents
REMEDIAL MEASURES
Protect Concrete
Cover concrete to retain heat of hydration for maintaining concrete temperature
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SELECTING SUITABLE CEMENT
Early-age compressive-strength relationships for Type I and Type III portland cement concretes mixed and cured at 4°C (40°F) compared to 23°C (73°F)
ACCELERATE STRENGTH GAIN
Type III or HE high-early-strength cement
Additional portland cement (60 to 120 kg/m3 or 100 to 200 lb/yd3)
Chemical accelerators
Protection by Windbreak and high Strength
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AIR ENTRAINMENTEntrainment of air improves concretestrength in cold weather
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MINIMUM LAYING TEMPERATURES FOR AIR ENTRAINED CONCRETE
ACI 306R-88
HEAT MATERIALS
Heating aggregates, cement and mixing water will raise the temperature of fresh concrete to combat heat loss in cold weather.
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PROTECTION OF FRESH CONCRETE
Concrete must be maintained at specified temperature for periods
given below.
Cooling of concrete after protection period should be gradual and temperature drop in first 24 hrs should not exceed values given below
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CONCRETING ON GROUND The ground must be thawed before concreting. Hydration will furnish some curing heat only. Enclosure is required and insulating blanket
may be sufficient. Vented heater may be required in enclosed
area. Concrete should not be placed on frozen sub
grade or backfilled with frozen fill. recommended temp must be maintained.
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ABOVE GROUND CONCRETING
Portable heater can be used to heat underside of floor slab thus obviating need for higher strength of concrete.
Enclosure must be constructed. Direct fired heater without venting can be
used. Formwork can be preheated before placing
concrete. Enough insulation to maintain curing
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INSULATION VALUE R FOR 1 IN THICK MATERIAL
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INSULATION REQUIREMENT FOR 7 DAYS TEMPERATURE MAINTENANCE
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INSULATING BLANKETS
Thermal resistance of mineral fiber blanket (50 to 70-mm [2 to 2.75-in.] thick)
1.2 (m2·K)/W
7 (°F·hr·ft2)/Bt
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INSULATING FORM WORK
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INSULATING COLUMN FORM
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ENCLOSURES
WoodCanvasTarpaulinsPolyethylene Film
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HEATED ENCLOSURES
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INDIRECTLY FIRED
HEATERS
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HYDRONIC SYSTEM
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