ACI 224.1R-93 Causes, Evaluation and Repair of Cracks in Concrete Structures

ACI 224.1R-93 Causes, Evaluation and Repair of Cracks in Concrete Structures. 1.3.2  Thermal stresses-Temperature differences within a concrete structure may be caused by portions of the structure losing heat of hydration at different rates or by the weather conditions cooling or heating one portion of the structure to a different degree or at a different rate than another portion of the structure. These temperature differences result in differential volume changes. When the tensile stresses due to the differential volume changes exceed the tensile stress capacity, concrete will crack. The effects of temperature differentials due to different rates of heat dissipation of the heat of hydration of cement are normally associated with mass concrete (which can in- clude large columns, piers, beams, and footings, as well as dams), while temperature differentials due to changes in the ambient temperature can affect any structure. Cracking in mass concrete can result from a greater temperature on the interior than on the exterior. The temperature gradient may be caused by either the center of the concrete heating up more than the outside due to the liberation of heat during cement hydration or more rapid cooling of the exterior relative to the interior. Both cases result in tensile stresses on the exterior and, if the tensile strength is exceeded, cracking will occur. 1.3.8 Errors in design and detailing -The effects of improper design and/or detailing range from poor appearance to lack of serviceability to catastrophic failure. These problems can be minimized only by a thorough understanding of structural behavior (meant here in the broadest sense). Errors in design and detailing that may result in unacceptable cracking include use of poorly detailed reentrant comers in walls, precast members and slabs, improper selection and/or detailing of reinforcement, restraint of members subjected to volume changes caused by variations in temperature and moisture, lack of ade- quate contraction joints, and improper design of foun- dations, resulting in differential movement within the structure. Examples of these problems are presented by Kaminetzky (1981) and Price (1982). Reentrant comers provided a location for the con- centration of stress and, therefore, are prime locations for the initiation of cracks. Whether the high stresses result from volume changes, in-plane loads, or bending, the designer must recognize that stresses are always high near reentrant comers. Well-known examples are window and door openings in concrete walls and dapped end beams, as shown in Fig. 1.4 and 1.5. Additional properly anchored...