Frequently Asked Questions

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What is mass concrete?

Q. When is a structural section large enough to be defined as mass concrete? We're concerned about the potential for thermal cracking caused by heat of hydration in a structure that isn't a dam but is of fairly massive size.


A. ACI doesn't use specific size limits to define mass concrete. ACI 301-16 “Specifications for Structural Concrete” defines mass concrete as any volume of structural concrete in which a combination of dimensions of the member being cast, the boundary conditions, the characteristics of the concrete mixture, and the ambient conditions can lead to undesirable thermal stresses, cracking, deleterious chemical reactions, or reduction in the long-term strength as a result of elevated concrete temperature due to heat of hydration.

ACI 301-16 states that the only characteristic that distinguishes mass concrete from other concrete work is thermal behavior that may cause a loss of structural integrity and monolithic action. According to ACI 301-16, the maximum temperature in mass concrete after placement shall not exceed 160 °F (70 °C); and the maximum temperature difference between center and surface of placement shall not exceed 35 °F (19 °C).

To reduce thermal stresses and control cracking, the temperature of concrete should be controlled by:

- Materials selection, mainly the cementitious material content.

- Precooling of ingredients and postcooling of the concrete to lower temperature.

- Construction management, where efforts are made to protect the structure from excessive temperature differentials by knowledge of concrete handling, construction scheduling, and construction procedures.

For more information on mass concrete refer to section 8 of ACI 301-16, ACI 207.1R-05 “Guide to Mass Concrete”, 207.2R-07 “Report on Thermal and Volume Change Effects on Cracking of Mass Concrete”, and 224R-01 “Control of Cracking of Concrete Structures”.


References: ACI 301-16; ACI 207.1R-05; ACI 207.2R-07; ACI 224R-01

Topics in Concrete: cracking; curing; durability; mass concrete; placing

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