Reading time:1 minute混凝土中的塑料沉降裂缝在混凝土的初始设置期间形成。塑性沉降裂缝的机理，外观和预防在下面讨论。
混凝土塑料沉降裂缝机理如果混凝土中固体的沉降可以在没有障碍的情况下自由地举行，则铸件混凝土的深度和体积将减少，但没有开裂。但是，对这种运动的任何限制，例如加固，可导致塑料沉降裂缝。与防止进一步向下运动的那些相比，固体继续沉降的情况下，混凝土将“突破其背部”，并且在表面迫使张力时出现撕裂。裂缝可能在反映加强布局的常规间隔处开发。混凝土中的裂缝often occur in conjunction with voids under the bars as shown in Figure 1. Figure 1(a) shows initiation and Figure 1(b) the condition after a few hours. These crescent-shaped voids may initially be filled with bleed water. The region of bond between the bar and concrete is thus reduced.
Appearance of Plastic Settlement Cracks in Concrete塑料沉降裂缝的视觉外观The most common restraint in slabs is from thereinforcement.The cracks occur on the top surface and usually follow the line of the uppermost bars, giving a series of parallel cracks; there may also be shorter cracks at right angles over the bars running in the opposite direction. Plastic settlement cracks are typically 1 mm wide and usually run from the surface to the bars (see Figures 2 and 3). Thesettlementmay also result in visible undulations on the concrete surface, with the high points over the top reinforcing bars.
Fig. 2: General Plan View of Cracks
Fig. 3: Section showing undulationsIn some cases where the bars in the top layer of reinforcement are close together, the whole surface layer of the concrete may be ‘suspended’ on the reinforcement while the concrete below settles. This can lead to a horizontal discontinuity beneath the bars, resulting in a loss of bond and with time delamination of concrete cover that protects the reinforcing steel against corrosion. Unlike硬化混凝土裂缝, due to overloading for instance, these cracks form at a very early age and pass through the cement paste and do not pass through aggregate particle pieces. The path is therefore more tortuous. This form of crack can be potentially serious as it passes longitudinal with the reinforcement and extends to the steel, negating the resistance to corrosion provided by the concrete.Fine crackscan occur in relatively narrow formed surfaces such as columns. The concrete may arch between the containing form faces. Settlement below the restrained concrete results in a crack being formed, generally coinciding with the links. It is sometimes possible for plastic settlement cracks to form on a vertical face where reinforcement has restricted the free flow of concrete within the formwork. In such cases it is possible that the cracks are formed between the lines of the reinforcement. The concrete can also be supported by theformworkface. This causes restraint to the concrete between connected members and is especially evident where changes in section cause differential settlement, the concrete in the deeper section settling more than the shallower section resulting in a crack. This is noticeable in the transition between a flared柱子头部（蘑菇）和普通柱，以及在槽中的瓦夫饼板中，在网上发生更多的沉降比相对薄的法兰（见图4和5）。裂缝可能穿过法兰并看起来类似于shrinkage cracks.
Fig. 4: (a) Arching near the top of column, cracking coincides with links. (b) Cracks at change of section.