Early Volume Changes and Cracking - Nứt bê tông có thể xảy ra trước khi làm cứng, tức là khi bê tông đạt đến giai đoạn mà trong đó nó không phải là nhựa và, do đó, không có thể chứa những thay đổi khối lượng đầu. Theo đó, các vết nứt kết quả được gọi là 'prehardening vết nứt' hoặc 'vết nứt nhựa ". Nói chung, trước khi làm cứng các vết nứt, nếu xảy ra, phát triển một vài giờ sau khi bê tông đã được đặt và hoàn thành. Các cơ chế có liên quan có. | Chapter 5 Early Volume Changes and Cracking . INTRODUCTION Cracking of concrete may occur before hardening . when the concrete reaches the stage in which it is not plastic any more and therefore cannot accommodate early volume changes. Accordingly the resulting cracks are known as prehardening cracks or plastic cracks . Generally pre-hardening cracks if occurring develop a few hours after the concrete has been placed and finished. The mechanisms involved may be different and accordingly distinction is made between plastic shrinkage cracks and plastic settlement cracks . . PLASTIC SHRINKAGE When the fresh concrete is allowed to dry contraction takes place. This contraction in the pre-hardening stage is known as plastic shrinkage and is to be distinguished from shrinkage in the hardened stage which is known as drying shrinkage see Chapter 7 . Plastic shrinkage may cause cracking during the first few hours after the concrete has been placed usually at the stage when its surface becomes dry. Such cracks are characterised by a random map pattern Fig. A but sometimes they develop as diagonal cracks at approximately 45 to the edges of the slab Fig. B . At other times the cracks may develop along the reinforcement particularly when the reinforcement is close to the surface. Copyright 1993 E FN Spon Fig. . Typical plastic cracking in a concrete slab. The width of the cracks varies and may reach a few millimeters. Similarly their length varies from a few millimeters to 1 m and more. Usually the cracks taper rapidly from the top surface but in extreme cases a crack may penetrate the full depth of the slab. Copyright 1993 E FN Spon Fig. . Schematic description of early age shrinkage of concrete with time. Adapted from Ref. . The drying and the associated plastic shrinkage of fresh concrete is schematically described in Fig. . Four stages are distinguishable. Stage I Rate of bleeding is greater than the rate of drying. Consequently the surface of .