Report on Thermal and Volume Change Effects on Cracking of Mass Concrete

Description

This report is primarily concerned with evaluating the thermal behavior of mass concrete structures to control the cracking in members that occurs principally from thermal contraction with restraint. This report presents a detailed discussion of the effects of heat generation and volume changes on the design and behavior of mass concrete elements and structures, a variety of methods to compute heat dissipation and volume changes, and an approach to determine mass and surface gradient stresses.

It is written primarily to provide guidance for the selection of concrete materials, mixture requirements, and construction procedures necessary to control the size and spacing of cracks. The quality of concrete for resistance to weathering is not emphasized in recom-mending reduced cement contents; however, it should be understood that the concrete should be sufficiently durable to resist expected service conditions.

This report can be applied to most concrete structures with a potential for un acceptable cracking. Its general application has been to massive concrete members 18 in. (460 mm) or more in thickness; it is also relevant for less massive concrete members.

Mass concrete is defined in ACI 116R as: “any volume of concrete with dimensions large enough to require that measures be taken to cope with generation of heat from hydration of the cement and attendant volume change, to minimize cracking.” The most important characteristic of mass concrete that differentiates its behavior from that of structural concrete is its thermal behavior.

The generally large size of mass concrete structures creates the potential for large temperature changes in the structure and significant temperature differentials between the interior and the outside surface of the structure. The accompanying volume-change differentials and restraint result in tensile strains and stresses that may cause cracking detrimental to the structural design, the serviceability, or the appearance