ACI 224.1R-07 Causes, Evaluation, and Repair of Cracks in Concrete Structures

ACI 224.1R-07 Causes, Evaluation, and Repair of Cracks in Concrete Structures. 1.3.4 Weathering—The weathering processes that can cause cracking include freezing and thawing, wetting and drying, and heating and cooling. Damage from freezing and thawing is the most common weather-related physical deterioration. Concrete may be damaged by freezing of water in the paste, in the aggregate, or in both (Powers 1975). Damage in hardened cement paste from freezing is caused by the movement of water to freezing sites and, for water in larger voids, by hydraulic pressure generated by the growth of ice crystals (Powers 1975). Aggregate particles are surrounded by cement paste, which prevents the rapid escape of water. When the aggregate particles are above a critical degree of saturation, the expansion of the absorbed water during freezing may crack the surrounding cement paste or damage the aggregate itself (Callan 1952; Snowdon and Edwards 1962). Concrete is best protected against freezing and thawing through the use of the lowest practical w/cm and total water content, durable aggregate, and adequate air entrainment. Adequate curing before exposure to freezing conditions is also important. Allowing the structure to dry after curing will enhance its freezing-and-thawing durability. Other weathering processes that may cause cracking in concrete are alternate wetting and drying, and heating and cooling. Both processes produce volume changes that may cause cracking. If the volume changes are excessive, cracks may occur, as discussed in Sections 1.3.1 and 1.3.2. 1.3.6 Poor construction practices—A wide variety of poor construction practices can result in cracking in concrete structures. Foremost among these is the common practice of adding water to concrete to increase workability. Added water has the effect of reducing strength, increasing settlement, and increasing drying shrinkage. When accompanied by a higher cement content to help offset the decrease in strength, an increase in water content will also mean an increase in the temperature differential between the interior and exterior portions of the structure, resulting in increased thermal stresses and possible cracking. In addition, by adding cementi- tious material, even if the w/cm remains constant, more shrinkage will occur because the paste volume is increased. Lack of curing will increase the degree of cracking within a concrete structure. The early termination of curing will allow for increased shrinkage at a time when the concrete has low strength. The lack of hydration of the cement, due to drying, will result not only in decreased long-term...