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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。Contents:
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.
图1：塑料沉降裂缝的形成（初始和最终）
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 undulations
In 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.
图5：在槽和华夫饼干部分变化的裂缝。
它也可能发生在其他位置，例如间隔块下方。蘑菇柱的裂缝通常是水平的。它们通常也宽1毫米，可以过度整个部分。如果放置混凝土的亚基部或其他材料具有高吸收性（干燥土壤，永久形式），可以夸大沉降，再次钢筋布局后的裂缝。Prevention of Plastic Settlement Cracks in Concrete
The restraints that causeplastic settlement cracking在建筑中固有，一般不能避免。在设计细节阶段可以避免段深度的突然变化，但风险的主要减少是通过混合设计和混凝土的合适凝聚力来减少流血的。In simple terms prevention of plastic settlement cracks in concrete can be achieved by increasing the sand content. However, there is a limit to this at which the bleeding will increase. Very clean (marine-dredged) sand tend to assist water movement, so blending with ‘dirtier’ sand with a higher fines (<150 micron) content can be beneficial. A tendency to bleed largely depends on the properties of the cement.细度is a controlling factor, possibly because the finer particles hydrate more quickly thereby reducing the rate of sedimentation. Rich mixes are less prone than lean mixes. Pozzolanic additions may also help as they reduce water content and add to the fines. HighGGBS.内容应避免设置慢times allow for the bleeding to continue longer. Air entrainmentadmixturescan be used, the entrained air stabilizing the matrix and reducing the water movement. A similar effect is claimed for polypropylenefibres within concrete（英国协议委员会，1995）。
