Concreting in Extreme Weather Conditions

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



WATER DEMAND AND SLUMP

Slump reduces as concrete temperature rises.Water demand rises for same slump as temperature rises


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


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.



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


REDUCE SURFACE

EVAPORATION


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



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.


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



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


RELATIONSHIP BETWEEN

TEMPERATURE , SLUMP AND AIR CONTENT



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


PRE-HARDENING PERIOD Fresh concrete should not be subjected to freezing

temperature before attaining sufficient strength. IS recommended pre-hardening periods are shown below


Freezing and ThawingDurability is

impaired.Fatigue stresses

in concrete

EFFECTS OF COLD WEATHER ON CONCRETE


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



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


AIR ENTRAINMENTEntrainment of air improves concretestrength in cold weather



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.


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


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.


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

temperature should be provided04/07/2023 34Plain and Reinforced Concrete - 1

INSULATION VALUE R FOR 1 IN THICK MATERIAL


INSULATION REQUIREMENT FOR 7 DAYS TEMPERATURE MAINTENANCE


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


INSULATING COLUMN FORM


ENCLOSURES

WoodCanvasTarpaulinsPolyethylene Film


HEATED ENCLOSURES


INDIRECTLY FIRED

HEATERS


HYDRONIC SYSTEM


Concreting in Extreme Weather Conditions

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