为了使混凝土能够符合施工现场要求，混凝土在运输过程中必须保持一定的流塑性，以便可以浇注成任意的形状已达到施工要求。混凝土是一种由多种材料组成的流体，因此其具有一般流体的特性。流体主要有以下特征：
In order to make the concrete meet the requirements of the construction site, the concrete must maintain a certain flow plasticity during transportation, so that it can be poured into any shape to meet the construction requirements. Concrete is a fluid composed of many materials, so it has the characteristics of general fluid. Fluid mainly has the following characteristics:
①在静止状态下流体仅仅能够承受法向应力，即静压强；而在运动状态下流体可以同时具有法向应力和切向应力的作用；
① In the static state, the fluid can only bear the normal stress, that is, the static pressure; In the moving state, the fluid can have the effect of normal stress and tangential stress at the same time;
②流体的形状不固定，在剪切力的作用下可以发生形状的变化，当外力消失时只能够停止变形，而不能恢复流体原有的形状；③使流体产生变形所需的剪切力非常小，所以流体很容易根据容器的空间来改变自身的形状。
② The shape of the fluid is not fixed, and it can change under the action of shear force. When the external force disappears, the deformation can only be stopped, but the original shape of the fluid cannot be restored; ③ The shear force required to deform the fluid is very small, so the fluid can easily change its shape according to the space of the container.
正是由于流体具有以上特点，所以在流体静止时不能承受切向应力，在运动时，则会对相邻两层流体间的相对运动有抵抗性，这种抵抗分子间相对运动的力被称为流体间的粘性力。这种抵抗两层流体间相对滑动的速度的性质，称为粘性。
It is precisely because the fluid has the above characteristics that it cannot bear the tangential stress when the fluid is stationary, and it will resist the relative motion between adjacent two layers of fluid when it is moving. This force against the relative motion between molecules is called the viscous force between fluids. This property of resisting the relative sliding velocity between two layers of fluid is called viscosity.
粘性的大小取决于流体的种类，不同的流体，其粘性是不同的，而且经过实验表明，流体间的这种粘性力的大小与流体自身具有的粘性和流体分子之间的相对速度成正比，当流体的粘性较小（如空气和水的粘性都很小）时，流体之间相对运动速度不大时，流体分子间的粘性力相比于其他外力可忽略。
The size of viscosity depends on the type of fluid. Different fluids have different viscosities, and experiments show that the size of this viscous force between fluids is directly proportional to the viscosity of the fluid itself and the relative velocity between fluid molecules. When the viscosity of the fluid is small (such as the viscosity of air and water is small), and the relative velocity between fluids is small, the viscous force between fluid molecules can be ignored compared with other external forces.
流体是分层流动的，层与层之间因速度不等而会产生相对运动，速度快的流层带动速度慢的流层，反之，速度慢的流层会阻止速度快的流层。带动力与阻力是一对大小相等、方向相反的作用力，这就是流层之间的内摩擦力（或黏性摩擦力）。流体的黏性是阻止发生剪切变形的一种特性，而内摩擦力则是黏性的动力表现。
The fluid flows in layers, and the relative motion between layers will occur due to different speeds. The fast flow layer will drive the slow flow layer, on the contrary, the slow flow layer will prevent the fast flow layer. The driving force and resistance are a pair of equal and opposite forces, which is the internal friction (or viscous friction) between the flow layers. The viscosity of fluid is a characteristic of preventing shear deformation, and the internal friction is the dynamic performance of viscosity.
混凝土搅拌机的搅拌筒旋转与之相似，搅拌筒内的混凝土拌合料在搅拌筒旋转带动下，圆周外层部分拌合料被螺旋叶片推动沿筒壁上升，到达某处后由于重力和搅拌离心力的共同作用，从高点斜抛下来与其他混凝土混合；半径r内的混凝土拌合料在螺旋叶片的带动下做圆周运动，外层流体流速快，越靠近轴线处流速越小。
The rotation of the mixing drum of the concrete mixer is similar. Driven by the rotation of the mixing drum, the concrete mixture in the mixing drum is driven by the spiral blade to rise along the drum wall. After reaching a certain place, due to the combined action of gravity and mixing centrifugal force, it is thrown down obliquely from the high point to mix with other concrete; The concrete mixture within radius r moves in a circular motion driven by the helical blades. The outer fluid flow rate is fast, and the closer it is to the axis, the smaller the flow rate.
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