Technical Questions

ACI Committees, Membership, and Staff have answered common questions on a variety of concrete related topics.

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 Concrete Terminology 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 PRC-207.1-21states 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 SPEC-301-20, 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 SPEC-301-20 “Specifications for Concrete Construction”, ACI PRC-207.1-21 “Mass Concrete—Guide”, ACI PRC-207.2-07 “Report on Thermal and Volume Change Effects on Cracking of Mass Concrete”, and ACI PRC-224-01 “Control of Cracking of Concrete Structures”.


References: ACI 301-20; ACI PRC 207.1-21; ACI 207.2R-07; ACI 224R-01; ACI PRC-207.4-20

Topics in Concrete: cracking; Curing of Concrete; Durability of Concrete; mass concrete; Placing Concrete

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