EXTREME WEATHER CONCRETING

Presented by- Anuj Agarwal

Hot Weather Concreting

• Any operation of concreting done at atmospheric temperatures above 40°C or any operation of concreting ( other than steam curing) where the temperature of concrete at time of its placement is expected to be beyond 40°C.

EFFECTS OF HOT WEATHER ON CONCRETE

• Accelerated Setting - High temperature increases the rate of setting of the concrete. The duration of time during which the concrete can be handled is reduced. Quick stiffening may necessitate undesirable retempering by addition of water. It may also result in cold joints.

• Reduction in Strength -High temperature results in the increase of the quantity of mixing water to maintain the workability with consequent reduction in strength.

• Increased Tendency to Crack - Either before or after hardening plastic shrinkage cracks may form in the partially hardened concrete due to, rapid evaporation of water.

• Rapid Evaporation of Water During Curing Period - It is difficult to retain moisture for hydration and maintain reasonably uniform temperature conditions during the curing period.

• Difficulty in Control of Air Content in Air-Entrained Concrete - This adds to the difficulty of controlling workability. For a given amount of air-entraining agent, hot concrete wilI entrain less air than concrete at normal temperatures.

EFFECTS OF HOT WEATHER ON CONCRETE PROPERTIES

• EFFECTS ON COMPRESSIVE STRENGTH: Concretes mixed, placed and cured at elevated temperatures normally

develop higher early strength than concretes produced and cured at normal temperatures, but at 28 days or later the strengths are lower.

• EFFECTS ON WORKABILITY AND WATER DEMAND For maintaining the same workability, the quantity of water in the

concrete mix has to be increased as the concrete temperature increases. If the amount of water remains unchanged, theri conversely, there will be loss of workability of concrete as the temperature increases.

• EFFECTS ON SHRINKAGE: In the hot weather, whenever the rate of evaporation of water from the

concrete mix is greater than the rate at which water rises to the surface of freshly placed concrete ( bleeding ), plastic shrinkage cracking will usually occur. High concrete temperature, high air temperature, high wind velocity and low relative humidity, or combinations thereof, cause rapid evaporation which significantly increases the likelihood of occurrence of plastic shrinkage cracking.

RECOMMENDED PRACTICES AND PRECAUTIONS

• TEMPERATURE CONTROL OF CONCRETE INGREDIENTS The most direct approach to keep concrete temperature down is by

controlling the temperature of its ingredients. Thus, in hot weather all available means shall be used for maintaining these materials at as low temperatures as practicable. The aggregates can be protected from direct sunrays by erecting temporary sheds over the aggregate stock pile. Water can also be sprinkeled on to the aggregate before using them in concrete. Other methods are use of ice as mixing water, cooling with nitrogen and cooling trucks.

• TEMPERATURE OF CONCRETE AS PLACED. In hot weather, wherever necessary, the ingredients of concrete should be cooled to

the extent necessary to maintain the temperature at the time of placing below 40 C.

• PRODUCTION AND DELIVERY. Temperatures of aggregates, water, and cement shall be maintained at the lowest

practical levels so that the temperature of the concrete is below 40% at the time of placement.

• PLACEMENT, PROTECTION AND CURING Placement and Finishing- Forms, reinforcement, and sub grade shall be sprinkled

with cool water just prior to placement of concrete. Continuous curing is important because the volume changes due to alternate wetting and drying promote the development of surface cracking. When temperature conditions are critical, concrete placement may be restricted to the evenings or night when temperatures are lower and evaporation is less.

COLD WEATHER CONCRETING

• Cold Weather Concreting - Any operation of concreting done at about 5°C atmospheric temperature or below.

EFFECTS OF COLD WEATHER ON CONCRETE

• Delayed Setting - When the temperature is falling to about 5°C or below, the development of concrete strength is retarded compared with the strength development at normal temperatures. The hardening period, necessary before the removal of forms is thus increased and the experience from concreting at normal temperature cannot be used directly.

• Freezing of Concrete at Early Ages - When concrete is exposed to freezing temperature, there is the risk of concrete suffering irreparable loss of strength and other qualities, that is, permeability may increase and the durability may be

impaired.

• Repeated Freezing and Thawing of Concrete - If concrete is exposed to repeated freezing and -thawing after final set and during the hardening period, the final qualities of the concrete may also be impaired.

• Stresses Due to Temperature Differential - It is a general experience that large temperature differentials within the concrete member may promote cracking and have a harmful effect on the durability. Such differentials are likely to occur in cold weather at the time of removal of form insulations.

EFFECTS OF COLD WEATHER ON CONCRETE PROPERTIES

• EFFECTS ON COMPRESSIVE STRENGTH Concretes mixed, placed and cured at temperatures lower than the normal

develop strength at a slower rate. However, this slower development of initial strength does not necessarily impair the long term strength and adequately protected concrete in winter would have the long term strength not significantly lower than at normal temperatures.

• EFFECT OF FREEZING ANDTHAWING CYCLES ON HARDENED

CONCRETE. Damages due to freezing and thawing is usually measured in terms of

lowering of dynamic modulus of elasticity of concrete.

• EFFECTS OF FREEZING OF FRESH CONCRETE Freshly placed concrete is vulnerable to freezing temperatures

so much so that if water in fresh concrete is allowed to be frozen, irreparable damage to the quality of concrete and permanent lowering of compressive strength can occur.

RECOMMENDED PRACTICES AND PRECAUTIONS

• TEMPERATURE CONTROL OF CONCRETE INGREDIENTS. The temperature at the time of setting of concrete can be raised by

heating the ingredients of the concrete mix. The temperature of the water should not be more than 65 C.

• USE OF INSULATING FORMWORK. A fair amount of heat is generated during hydration of cement such heat

can be gainfully conserved by having insulating formwork covers.

• PROPORTIONING OF CONCRETE INGREDIENTS. Since the quantity of cement in the mix affects the rate of increase in

temperature, an additional quantity of cement can be used. High Alumina Cement is generally prefered.

• PLACEMENT AND CURING. Before placing the concrete all snow must be removed.

• DELAYED REMOVAL OF FORMWORK. Since rate of strength is slow formwork and props have to be kept in place for a longer time.

• USE OF PRECAST MEMBERS.

REFRENCES• IS: 7861 (part I) 1981• IS: 7861 (part II) 1981• CONCRETE TECHNOLOGY – M.L. GAMBHIR

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