Nowadays, with the continuous increase of real estate and bridge construction, the use of concrete is becoming more and more extensive. Although concrete is the most commonly used building material in contemporary society, the application process is not only time-consuming, but also produces A large amount of construction waste goes against the concept of green development. Nowadays, 3D printing concrete technology, which combines 3D printing with concrete technology, is not only efficient and environmentally friendly, but will also become an important turning point in the history of concrete development.
3D printed concrete technology is a new type of concrete moldless molding technology. Its main principle is to use computers to conduct 3D modeling and segmentation of concrete components to generate three-dimensional information, and then pass the prepared concrete mixture through The extrusion device, according to the set program, is mechanically controlled and extruded from the nozzle for printing, and finally the concrete component is obtained. This technology not only has the advantage of self-compacting concrete that does not require vibration, but also has the advantage that sprayed concrete facilitates the manufacture of complex components.
Compared with other concretes, ultra-high performance concrete is very different in raw material selection. The main manifestations are as follows:
(1) In terms of cementitious materials: UHPC has a very low water-cement ratio, generally between 0.14-0.27, and a large amount of cement, generally between 700-1010Kg/m3 About; use silica fume with smaller particle size and spherical shape. According to relevant information, the mass ratio of silica fume to cement in UHPC is about 1:4. At this ratio, the calcium hydroxide produced by complete hydration of cement can be completely consumed by the pozzolanic reaction of silica fume.
(2) Aggregate: UHPC does not use coarse aggregate, and at the same time, quartz sand with similar mechanical properties to the slurry is selected as fine aggregate. In this way, there is no skeleton restriction effect of coarse aggregate, and the pores and micro-cracks between the interface between the slurry and the aggregate caused by shrinkage can be greatly reduced.
(3) In terms of admixtures: UHPC can obtain ultra-low water-cement ratio and porosity, which is inseparable from the support of concrete admixture technology. Traditional water-reducing agents such as naphthalene series are increasingly unable to meet engineering needs due to limitations in technical performance. Polycarboxylate water-reducing agents, as a new generation of high-performance water-reducing agents that have attracted much attention, enable the achievement of ultra-low water-binder ratios. became a reality. The water reduction rate of polycarboxylate water reducing agent can reach more than 25%-40%, with excellent slump retention performance. It can also effectively improve the workability of concrete, giving concrete higher strength and excellent durability. It is suitable for various high-strength, Formulation of high performance concrete. In addition, commonly used admixtures include defoaming agents, shrinkage reducing agents, etc.
Analysis of the composition of soil:
(1) Ordinary Portland cement may not meet the requirements of 3D printing in terms of strength, setting time, etc., so the composition of the cement can be changed. Mineral composition, fineness of clinker, etc. For example, sulphoaluminate cement or aluminate modified Portland cement can be used to achieve shorter setting time and higher early strength.
(2) 3D printing concrete technology is realized through nozzles, and appropriate aggregate particle size needs to be used to meet the nozzle requirements. If the particle size of the aggregate is too large, it will clog the nozzle; if the particle size is too small, the specific surface area of the slurry required to wrap the aggregate will be large, there will be more slurry, the hydration rate will be fast, and the heat of hydration per unit time will be high, which will affect various properties of the concrete. of deterioration.
(3) The new concrete that meets the requirements of 3D printing is different from traditional concrete. Its mix ratio cannot be determined by traditional water-cement ratio, sand rate, etc. In order to achieve the ideal state of printed concrete, such as high strength, high durability, good workability, suitable setting time, good pumpability and buildability, the theory needs to be improved from a new perspective.
(4) As an essential component of modern concrete, admixtures are an important method and technology for concrete modification. 3D printed concrete must have better rheology for easy extrusion and can quickly solidify in the air to prevent the structure of the printed concrete from being damaged by its own gravity. The maximum particle size of the aggregate will become smaller and its shape will be closer to a circle. shape, and ultimately need to solve the problem of condensation between layers and improve early strength, which require new concrete admixtures to solve.
At present, 3D printing concrete technology has become increasingly mature. It can easily print high-cost curved buildings that are difficult to build by other methods, and has been used in many countries. Coupled with its obvious high efficiency, low carbon, green and other characteristics, it will bring obvious changes to the environmental protection, construction, ready-mixed concrete and other industries in the future.
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