comment is a key material in constr

comment is a key material in construction. it consists of a very fine powder. When water is added to cement, a chemical reaction occurs, and the cement begins to set - it starts to become solid. the most widely used cement-based material is concrete, which is made from cement, fine aggregate , coarse aggregate (gravel) and water. After concrete has set, it needs time to reach its structural strength-the strength needed to perform effectively. Generally, engineers' consider that this strength is reached after 28 days- a point called 28-days strength.

Concrete mix designs, when are specified by engineers, state the proportions of cement, fine aggregate and coarse and aggregate to be used for specific structures. For example, a 1:2:4 (one-two-four) mix consists of one part cement, two parts fine aggregate and four parts coarse aggregate. For mixing precise quantities - known as batching - proportions are measured by weight. Mix designs also specify the water-cement ratio-the amount of water added relative to the amount of cement used. Excess water reduces the strength of concrete, so the quantity of water is kept to a minimum. But as drier concrete is more difficult to work with, an additive (added chemical substance) called a plasticizer is often used. This helps the concrete to flow more easily. Other additives can also be be used-for example, a retarder may be added to delay setting, which gives workers more time to pour (place) the concrete.
Reinforced concrete (RC) structures contain steel bars. Steel reinforcement is needed mainly because concrete is weak in tension - that is, bad at resisting stretching forces. As steel is strong in tension, reinforcing bars overcome this weakness.
In order, to form the different parts of structures, formwork - sometimes also called shuttering - is used. This consists of moulds of the required size and shape, made from steel or timber, which are used to contain the concrete until it has set.
When wet concrete is cast (placed) in its final position, it is called in-situ concrete. Instead of being cast in-situ, reinforced concrete elements can also be precast - cast at a factory - then delivered to the construction site ready for assembly. Sometimes, precast concrete is also before the concrete is poured. The bars are then held in tension while wet concrete is poured around them. After the concrete has fully set, the bars become 'trapped' in tension. This increases the concrete's ability to resist bending forces.

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评论是一种重要的建筑材料。它由一个非常细的粉末组成。当水被添加到水泥，化学反应发生，水泥开始设置-它开始变得坚实。最广泛使用的水泥基材料是混凝土，它是由水泥、细骨料、粗骨料（碎石）和水。混凝土后，它需要时间来达到它的结构强度所需要的力量来有效地执行。一般来说，工程师们认为，这种强度是在28天之后达到的--一个叫做28天强度的点，在工程师指定的时候，状态的水泥，细骨料和粗骨料的比例要用于特定结构。例如，一个1:2：4（一二四）混合料由一部分水泥、2部分细骨料和四部分粗骨料组成。用于混合精确的量，称为配料比例由重量来衡量。混合设计也指定水灰比水泥用量的增加，相对量的水泥。过量水降低混凝土强度，所以水的量被保持在最低限度。但是随着干燥混凝土的工作难度更大，一种称为增塑剂的添加剂（添加化学物质）常被使用。这有助于混凝土更容易地流动。其他添加剂也可用于例如，一个减速器可能被添加到延迟设置，这给工人更多的时间来倒（地方）的混凝土。钢筋混凝土结构钢筋混凝土结构。钢筋是必要的，主要是因为混凝土在拉力下是很弱的，这是不好的。由于钢是在拉力强，加强筋克服这个弱点。
秩序，形成结构的不同部分，模板-有时也被称为模板-使用。这是由所需的大小和形状的模具，由钢或木材制成，这是用来包含混凝土，直到它已设定。现浇钢筋混凝土构件，而不是在原地，也可以在工厂预制，然后将其交付给施工现场，准备装配。有时，预制混凝土也在浇筑混凝土之前。在潮湿的混凝土浇筑的时候，这些钢筋也被拉了。在混凝土有充分的设置后，钢筋被“困”了。这增加了混凝土抵抗弯曲力的能力。