Concrete is a man-made composite the major constituent of which is natural aggregate, such as gravel and sand or crushed rock. Alternatively artificial aggregates, for example, blastfurnace slag, expanded clay, broken brick and steel shot may be ud where appropriate. The other principal constituent of concrete is the binding medium ud to bind the aggregate particles together to form a hard composite material. The most commonly ud binding medium is the product formed by a chemical reaction between cement and water. Other binding mediums are ud on a much smaller scale for special concretes in which the cement and water of normal concretes are replaced either wholly or in part by epoxide or polyester resins. The polymer concretes known as resin-bad or resin-additive concretes respectively are costly and generally not suitable for u where fire-resistant properties are required but they are uful for repair work and other special applications. Resin-bad concretes have been ud, for example, for precast chemical-resistant pipes and lightweight drainage channels. This ction deals only with normal concretes in which cement and water form the binding medium. In its hardened state concrete is a rock-like material with a high compressive strength. By virtue of the ea with which fresh concrete in its plastic state may be moulded into virtually any shape it may be ud to advantage architecturally or solely for decorative purpos. Special surface finishes, for example, expod aggregate, can also be ud to great effect. Normal concrete has a comparatively low tensile strength and for structural applications it is normal practice either to incorporate steel bars to resist any tensile forces (reinforced concrete) or to apply compressive forces to the concrete to counteract the tensile forces (prestresd concrete). Concrete is also ud in conjunction with other materials, for example, it may form the compression flange of a box ction the remainder of which is steel (composite construction). Concrete is ud structurally in buildings for foundations, columns, beams and slabs, in shell structures, bridges, wage-treatment works, railway sleepers, roads, cooling towers, dams, chimneys, harbours, off-shore structures, coastal protection works and so on. It is ud also for a wide range of precast concrete products which includes concrete blocks, cladding panels, pipes and lamp standards.谐音小故事 农产品开发 The impact strength, as well as the tensile strength, of normal concretes is low and this can be improved by the introduction of randomly orientated fibres into the concrete mix. Steel, polypropylene, asbestos and glass fibres have all been ud with some success in precast products, for example, pipes, building panels and piles. Steel fibres also increa the flexural strength, or modulus of rupture, of concrete and this particular type of fibre-reinforced concrete has been ud in ground paving slabs for roads where flexural and impact strength are both important. Fibre-reinforced concretes are however esntially special-purpo concretes and for most purpos the normal concretes described in this book are ud. 小动物怎么叫 In addition to its potential from aesthetic considerations, concrete requires little maintenance and has good fire resistance. Concrete has other properties which may on occasions be considered less desirable, for example, the time-dependent deformations associated with drying shrinkage and other related phenomena. However, if the effects of environmental conditions, creep, shrinkage and loading on the dimensional changes of concrete structures and structural elements are fully appreciated and catered for at the design stage no subquent difficulties in this respect should ari.香菇油菜 富县高级中学 A true appreciation of the relevant properties of any material is necessary if a satisfactory end product is to be obtained and concrete, in this respect, is no different from other materials. | 混凝土是人造复合材料，其主要组成材料为天然集料，如砾石和砂或碎石。或者人造集料，如高炉矿渣、陶粒、碎砖和钢丸也可在适当的地方使用。混凝土的另一主要组成材料是用于粘结集料颗粒以形成坚硬的复合材料的粘结剂。最常用的粘结剂是水泥和水之间通过化学反应生成的产物。其它的粘结剂以小得多的剂量用于特殊混凝土，此时普通混凝土中的水泥和水被环氧树脂或聚酯树脂完全或部分取代。这些被称为树脂基或填加树脂混凝土相对昂贵，通常不适用于要求有防火性能的地方，但它们可用于修补工程和其它特殊用途。例如，树脂基混凝土已被用于预制抗化学侵蚀管和轻质排水道。此节仅讲述由水泥和水生成粘结剂的普通混凝土。 混凝土在硬化状态是一种象岩石一样的具有高抗压强度的材料。由于塑性状态的新拌混凝土很容易浇注进实际的任何形状里，所以它可用于建筑上或仅用于装饰目的。特殊的表面光度，例如，裸露的集料，也能被有效地利用。华山儿童公园 普通混凝土抗拉强度相对较低，在实际结构应用中或加入钢筋抵抗拉力（钢筋混凝土），或对混凝土施加压力以抵消这些拉力（预应力混凝土）。混凝土也用于与其它材料的接合，例如，它可形成槽钢的受压翼缘，而其余部分为钢（组合结构）。混凝土在建筑结构中用于基础、柱、梁和板，在薄壳结构中用于桥、污水处理工程、铁路枕木、公路、冷凝塔、大坝、烟囱、港口、近海结构、海防工程等。它也大范围用于预制混凝土产品，包括混凝土砌块、围护板、管和电杆。 个人所得税怎么计算 普通混凝土的抗冲击强度与抗拉强度一样较低，但可向混凝土拌和物中填加杂向纤维来改善。钢、聚丙烯、石棉和玻璃纤维都已成功用于浇注制品，如管材、建筑物护墙板和桩。钢纤维也会增加混凝土的抗弯强度或断裂模量，这种纤维增强混凝土已被用于抗弯强度和抗冲击强度都很重要的公路路面板。然而纤维增强混凝土本质上是特殊混凝土，对于大多数用途是采用此书叙述的普通混凝土。 混凝土除了美学方面的可能考虑外，几乎不需要维护，并有较好的耐火性。混凝土还有其他特性可能经常被认为不够理想，如，与干缩有关的随时间变化的变形和其它相关现象。若完全估计了环境条件、徐变、收缩和加荷对混凝土结构和构件的尺寸改变的影响，并在设计阶段采取对策，那么在这方面就不会出现继发的困难。 若想获得满意的成品，必须正确评价材料的相应性能，这一点，混凝土与其它材料没什么不同。 | 人行道英语
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