What is Concrete Temperature Limit?

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Concrete Temperature limits are established to ensure that high quality concrete with specified requirements are produced. Various standards and specifications provide limits on temperature of concrete so as to achieve designated strength and durability. Concrete temperature limits changes based on the type of the elements and working conditions; hot or cold, windy or calm, dry or humid. This article highlights concrete temperature limits at different conditions based on different standards such as ACI Code, ASTM standards, and Australian Standard (AS).

Reasons to Set Concrete Temperature Limit

High initial temperature requires high water content to maintain required slump. High water content lead to lower strength, high permeability, greater shrinkage, lower air content at given air entraining admixture. If ambient temperature increase in combination with high initial temperature, then it reduces set time, higher early age strength, durability problem, thermal cracking in massive concrete structure. Lastly, low initial temperature lead to intolerable setting time in cool weather, influence rate of strength achievement, and reduce ultimate strength up to 50% if fresh concrete is frozen.

Concrete Temperature Limits

• ACI 301.5, ACI 301, and ASTM C94 set no temperature limit on fresh concrete in hot weather conditions if precautions are considered in proportioning, production, delivery, placing, and curing concrete. Nonetheless, ASTM C 1064-86 set a maximum limit between 26.7°C and 35°C. This can be modified in accordance with project specifications.
  

  Fig. 2: Reduction of concrete temperature in hot weather conditions

• For temperatures greater than 35°C, the use of ice-cold water is recommended to prevent formation of cracks at early stages.
• In hot weather conditions, AS 1379 requires that concrete temperatures at the point of delivery shall be within a range of 5°C to 35°C.
• Concrete placed & cured at a moderate temperature (15–25°C) will gain higher strength & durability than 35°C concrete.
• Curing concrete at high temperature accelerates curing and strength achievement, but provide lower maximum concrete strength.
• ACI 306 specify limits for concrete temperature in cold weather conditions.  These are provided in Table 1 and greatly related to the minimum dimensions of the structural member under consideration. Sections with smaller dimensions have higher minimum temperature requirements to account for less heat that can be retained.

Table 1 Minimum Concrete temperature limit during placement

Least dimensions of sections, cm	Minimum temperature of concrete as placed and maintained during the protection period, °C
Less than 30.48	12.77
30.48 to less than 91.44	10
91.44 to less than 182.88	7.22
Greater than 182.88	4.44

• Freezing of freshly poured concrete shall be prevented during the first 24 hours after concrete placement to allow concrete to achieve minimum strength of 3.45MPa otherwise irreparable loss in strength occurs.
• Whenever air temperature at the time of concrete placement is below 4.5°C and freezing temperatures within the first 24 hours after placement are expected, high initial temperature shall be established, protect concrete during placing, consolidating, and finishing to avoid delay of setting.
  

  Fig. 3: Limiting concrete temperature in cold weather conditions

• For structures that carry large loads at an early age, concrete must be maintained at a minimum of 10°C to accommodate stripping of forms and shoring and to permit loading of the structure.
• Drying placed concrete from low wintertime humidities, and heaters used in enclosures is a concern. It is advised to leave forms in place as long as possible, because they help distribute heat more evenly and help prevent drying of the concrete.
• If concrete is heated, sudden cooling needs to be prevented because it causes thermal cracking. This can be achieved by loosening the forms while maintaining cover with plastic sheeting, gradual decrease in heating inside an enclosure, or turning off the heat and allowing the enclosure to slowly equilibrate to ambient temperature.
• ACI 305 R-10 recommend that trial batches of concrete for any project shall be performed at a specified limited temperature or at maximum expected temperature of project site instead of temperature limit ranges from 20 to 30°C provided in ASTM C192/C192M.