建筑材料的选择标准

文/ 加尼·瑞泽普（Ghani Razaqpur，加拿大） 译/ 王一帆

加尼·瑞泽普（Ghani Razaqpur，加拿大），加拿大工程院院士、南开大学环境科学与工程学院讲席教授

爱因斯坦说过，物质与能量可以互相转化。世界上所有事物都是由物质与能量构成的。我们触摸不到能量，但是我们能感受到它们，我们能看到光，听到声音，感到热量。物质则是可以碰触的，它就存在于我们周围，无时无刻无处不在，但我们却很少会去思考它。

假如我们能够用某种物质制作一样东西，那么那种物质就叫材料。利用各种各样的材料，我们可以修筑建筑、大坝、运河、铁路，也能生产手机、电脑、汽车等。材料与我们的生活息息相关，离开材料我们将举步维艰。

材料的分类

世界上有着数之不尽的材料，我们可以根据自己的需求用不同的方法将它们分类。其中一种是观察材料在自然状态下如何组合，这种方法叫作化学分类法。据此，我们将部分材料分类为金属，如金、银、铜、铁、锌等，它们通常是有光泽和坚硬的物质，并且能够导热导电。

还有一类材料是陶瓷，包含了氧化铝、二氧化硅、碳化硅、瓷、混凝土、玻璃等材料。它们都有很强的抗压能力，但是在受拉和弯折时十分易碎。例如，混凝土在抗压方面性能十分突出，但是，如果我们要用它制作柱子或横梁，则必须在混凝土里加入钢筋来增强它的抗拉力和抗扭力。

聚合物这一类中包含了塑料、橡胶、聚乙烯（PE）、聚苯乙烯（PS）、聚氯乙烯（PVC）、聚碳酸酯（PC）等。这类材料应用十分广泛，塑料制品在我们身边随处可见，如在笔、电脑屏幕、汽车、飞机等的生产中都需要塑料。聚合物的应用已经涵盖了我们生活的方方面面。

最后一类是复合材料，这类材料都是由至少两种上述材料组合而成的。现如今应用最广泛的两种复合材料是玻璃复合材料和碳复合材料。例如，复兴号动车组、C90飞机、C919飞机、部分火箭等都使用了碳纤维复合材料，因为碳纤维既轻巧又有很高的强度。

我们还有其他的分类方法。根据材料是天然的还是人造的，可以将其分类为天然材料和人工材料。例如，钢就是一种人工材料，因为自然界中没有钢。天然材料包含了如木头、泥、石头等材料。在古代，人们通常使用天然材料，因为天然材料可以随取随用。中国的长城就是由石头修筑的。用泥土很轻松地就可以造出砖块，改变木头和石头的形状也不需要很复杂的技术，这就是古人对材料的运用。如今我们创造了更多的新材料，它们也取代天然材料成为主流。

如今，我们在工程建设中使用了大量的人工材料。利用它们，我们建设了桥梁、道路、铁路、机场、隧道等交通工程，房屋、办公楼、公寓楼、工厂、发电站等建筑工程，还建设了水坝、港口、运河、堤堰等水利工程。

无数的材料被运用于工程建设中，其中最重要的几种天然材料为泥土、木材和石材，而最重要的几种人工材料为钢材、混凝土和沥青。在面对两种材料时我们该如何选择？为什么我们选择混凝土而不是钢材来建筑房屋？问题的答案是，我们遵循了重要的材料选择标准。

首要的选择标准是性能。例如，一块跳板应该具备什么性能？当一个跳水者站在上面时，它应该向下弯，当跳水者跳起时，跳板应该为其提供弹力，并且，它还应该具有足够的强度而不会折断。这就是对跳板的性能需求。其余的选择标准包括材料是否方便获得，使用该材料需要怎样的技术，还有材料的环保性能及造价——我们应该选用最经济的材料来达成目标。

对材料的选择取决于我们对材料的需求。没有最好的材料，也没有最差的材料。材料的好坏取决于它是否既安全又经济，还能满足特定的需求。

钢有许多特征，它的光泽、导电性和导热性属于物理学特性；它的硬度和弹性属于力学特性；它的元素构成、易腐蚀性属于化学特性。任意材料的全部特性都可以根据物理学、力学和化学来分类。材料的性能往往由它的一种或多种特性决定。特性不分好坏，只在于它能否满足使用者的需求。拥有万金油特性的材料是不存在的。工程师会选择一种材料，一定是因为这种材料的性能比其他材料更能满足需求，如造价、环保等。

我们怎么确定材料的特性呢？每制造出一种新材料，工程师们就会取一些样品，使用特殊的技术和设备针对它的特性进行测试。在测试了大量的材料之后，工程师们会把这种材料的特性及相关数据记录在专门的手册中，方便他们以后为项目选择最合适的材料。生产材料的公司一般都会自行测试并提供自家材料的性能数据。

混凝土

现在我们介绍一些材料运用的实例。沥青混凝土常用于道路建设。它由沥青与石粒混合，可以冷铺或热铺，最后再用压路机压实。当工程师选用某种沥青时，他会考虑它的各种特性，比如黏性、耐久性、延展性。同样，每当我们选择一种材料，我们都必须审视它的种种特性，看它是否符合我们的需求。

另一种材料是随处可见的混凝土，它主要由碎石、水泥和水构成。其实，混凝土是世界上除了水以外使用最多的材料。2018年，全世界共使用了370亿吨混凝土。中国的混凝土产量和使用量均占全世界的1/3。由于制造混凝土需要水泥，因此生产了大约40亿吨水泥。每生产1吨水泥都会产生超过1吨的二氧化碳——这是一种会导致气候变化的温室效应气体。

由于混凝土可以在任意地点制作成任意形状，所以在建设中我们不必一次就把整个建筑物建成。我们用混凝土建造出了很多伟大的桥梁，如世界上最长的桥梁——港珠澳大桥，世界上最高的桥梁——米洛高架桥。我们还用混凝土建造了很多伟大的建筑，如世界上最美丽的建筑之一多伦多市政厅、世界上最高的大楼迪拜塔。我们还建造出了中国的三峡大坝、美国的胡佛大坝、瑞士的韦尔扎斯大坝，还有无数的公路与运河。

和其他材料一样，混凝土有自己的优缺点。除了水泥，它的原料碎石、水都是天然易得的，它具有很强的抗压力，几乎能做成任意形状，并且造价低廉，生产和使用混凝土都不需要很高超的技能与技术，通常还有不错的耐久性。这些都是它的优点。

但是，混凝土需要时间来凝固，所以，使用混凝土的建造时间比钢材和木材都长；它容易开裂，特别是在干燥的气候环境里；它的抗拉力很弱，仅仅很小的拉力就可以让它断裂；它的能量吸收能力和延展性都很差。生产水泥能耗较高，同时产生大量二氧化碳。

如前所述，混凝土的抗拉性能很弱，所以不能单独应用于建造建筑或桥梁。而钢有着优秀的抗拉性能，因此，经过钢筋增强的混凝土可以同时拥有抗压性与抗拉性，这就是钢筋混凝土。它是一种非常优秀的复合材料，能在崩塌前吸收大量的能量，且强度高、造价低。

钢

钢在某种意义上也算是复合材料，但其实它是一种主要由铁和碳构成的合金。钢是现代社会中应用范围最广的工业材料之一。它的原材料是铁矿石。生产钢材的过程能耗很高，因此，同样会排出很多温室气体。

钢的生产过程很复杂，以下是比较重要的几步：把铁矿石分成小块，放入一种特殊的坩埚中，用火炉加热到1600—1700℃。熔化后的铁沉于坩埚底部，炉渣上浮。把分离出来的铁送入另一个炉中与碳和氧混合，最终的成品就是钢。把钢送到不同的工厂，就可以加工成各种零件与产品了。

钢具有很优秀的抗拉和抗压性，它有很强的韧性，在某些环境中十分耐久。钢在初次使用后是可以完全回收再利用的。钢结构的建筑工事的建造时间比混凝土框架要快得多，而且在建造时也不需要模板和临时主支撑。

但想要生产高质量的钢材、制作钢构件和构筑物都需要更高的技能与技术水平。在如海水、酸和高湿度这样的特定环境中，它会被锈蚀并失去耐久性。钢材造价相比混凝土和木头要更昂贵。生产钢材是高能耗的。钢结构建筑十分需要消防保护。

我们也用钢材建造过很多伟大的桥梁，如悉尼海港大桥、旧金山的金门大桥。我们建造过很多伟大的建筑，如位于芝加哥的约翰·汉考克中心、位于曼哈顿的世界贸易中心一号楼、位于伦敦的“小黄瓜”瑞士再保险公司大楼。

木材

最后是木材。木材是一种天然环保的材料。它能吸收二氧化碳，减少温室气体的排放。在不直接暴露在潮湿环境的情况下，木材可以持续使用很长时间，它还有合理的强度与硬度。天然木材无法用于建造大跨度构筑物，但经过人工改良的木材可以增加跨度。如今，木材已经不再用于建造行车用的桥梁了。

木材重量很轻，易于砍伐、搬运及组装。它是环保材料并且造价低廉。生产木材只需要消耗很少的能量，同时，还能吸收温室气体二氧化碳，减缓气候变化。

当然，木材也有缺点，当它暴露在过度干燥或潮湿的环境时容易腐朽；它的强度弱于钢材、混凝土和一部分聚合材料；它的防火性能不强，并且不能随处取用。

如今，木材主要用于修建房屋与一些特别的建筑物，如位于温哥华的里士满冬奥会椭圆运动场馆的木质屋顶，其设计遵从了“保暖屋顶”的设计理念。

如上所述，木材的防火性能不强。近期，举世闻名的、美丽的、极具历史意义的巴黎圣母院几乎毁于一场大火。虽然它主要是由不可燃的砖石砌筑，但它是由木质的屋顶所覆盖，木材燃烧，整个屋顶都崩塌落下。可见防火性是我们在为建筑挑选材料时需要慎重考虑的重要特性。

最后我要说，没有最好的材料，也没有最差的材料。性能符合要求的材料就是好材料。黄金具有很高的价值且价格昂贵，但我们不能用黄金造桥，也不能用黄金生产汽车和飞机，因为它的强度和硬度都不够。有一些材料之所以昂贵，是因为它不易获得，或者生产它消耗巨大。造价是在工程建造中一个重要的材料选择标准，我们都不想付出超过必要的成本。目前的研究主要专注于利用纳米技术和生物技术使材料更智能、更坚固、更便宜、更耐用。研究同时还致力于使材料可循环使用，并且更环保。

当我们将世界交接到你们年轻人的手中时，请让它变得更好。世界是一个封闭的系统，从外太空获取资源是不切实际的。被消耗的物质不会再次凭空出现。我们所拥有的都来自地球，如水和空气，这些都是无法被替代的资源。因此，你们应当学会像大自然一样进行循环利用与回收。大自然从不抛弃任何事物，一切都会被回收再利用。这是一个完整的循环，不停回到原点并再次向前。我们应当学习自然，努力地模仿它，回收利用所有的材料，让地球成为一个更好的地方。

Einstein said: you can change energy to matter or matter to energy.In this world, everything we know is made from matter or energy.We can’t touch energy, but we can feel it. For example, we can see light, we can hear sound, we can feel heat, etc. However, we can touch matter as it’s around us everywhere and every day. We don’t think about it too much!

If we can make something from some matter, we call the matter “Material”. From materials we build buildings, dams, canals, railways and produce phones, computers, cars, etc. Materials are very important, and we can’t live without them.

Materials Can Be Classified in Different Ways

There are millions of materials in this world that can be classified in different ways, depending on what you want and what the purpose of each is. One way to look at materials is to see how they are put together by nature. We call this the chemical structure of materials.In this way, we have materials that are metals like iron, steel, copper,zinc, gold, silver—generally shiny and hard, through which electricity and heat can travel.

There are also ceramics. These materials are inclusive to aluminum oxide, silicon dioxide, silicon carbide, porcelain, concrete, as well as glass. They are very strong when pushed upon but easy to break if pulled or twisted. For example, from concrete we build a lot of buildings. Concrete is very strong in compression, but we have to reinforce it by steel bars to give it resistance to tension.

Then we have polymers, like plastic, rubber, polyethylene (PE), polystyrene (PS), polyvinyl chloride (PVC), polycarbonate (PC). They are very important because almost everything we see around us is made from plastic. For example, it takes plastic to produce pens, computer screens, cars, airplanes, etc. Polymer is used in practically everything now.

Finally, composites. This means they are materials that are made from combinations of the above materials. The most important composites today are glass composites and carbon composites. Fuxing trains, C90 airplane, C919 airplane, many rockets and many other things are creations of carbon fiber composites as carbon fiber is light and strong.

We also classify materials in other ways. One of which is based on whether they are found naturally or under artificial conditions. We call them, under this guideline, non-engineered and engineered materials. For example, steel is an engineered material because we can’t get steel naturally. Non-engineered materials would be like wood, soil, stone, etc. In the old days, people preferred to use nonengineered materials because they are naturally available. The Great Wall of China is made from stone. People made bricks from soil easily, and shaped wood and stone without complicated technology. That was what people did before. Today, we create many more new materials, and we use more engineered materials than non-engineered ones.

Nowadays, we use a large amount of engineered materials in construction. We build transportation structures like bridges, roads,railways, airports, tunnels. We build building structures like houses,office buildings, apartment buildings, factories and power plants. We build hydraulic structures like dams, harbours, canals and dykes.

There are many materials used in construction, but the most important materials are soil, wood and stone as non-engineered materials, and steel, concrete, asphalt as engineered materials. How do we choose one material versus another? Why do we choose concrete to build some buildings but not steel? There are important material selection criteria as follows.

Performance is the number one criterion. For example, what is the performance we want from a diving board? It should go down when a diver stands on it, then a bounce should be given to him/her when he/she jumps from it. It should be strong enough to do this repeatedly without breaking. These are its performance requirements. The second criterion is material availability. The third criterion is the ability to properly use the material. The fourth criterion is not being harmful to the environment. The final criterion is the cost, as one should use the most economical material that can do the job.

So, choosing materials depends on what we want the material for.There is no such things as “the best material”, just like there is no such thing as “the worst material”. It just depends on whether it can safely and economically meet the specified need.

When we talk about steel, we often say that it’s shiny, it conducts heat and electricity. These are physical properties of steel. We also say that steel is strong and elastic. These are mechanical properties of steel. We say that steel is made from iron and carbon, steel rusts easily. These are chemical properties of steel. Properties of any material can be classified based on their physical, mechanical or chemical properties. Performance depends on one or more properties of a material. There is no good or bad property, this depends on what is required by the user. No single material can meet all performance requirements in every situation. Engineers choose a material because when compared to other materials, it may better satisfy the criteria.

How does one determine these properties? Every time a new material is produced, engineers take samples of it to the laboratory and test them for the property they are interested in, using special techniques and equipment. By testing many materials, engineers determine their properties and collect the data in special handbooks that they can later use to select the best material for a given job. Usually,companies that produce a material will test them and will provide information about the relevant properties of that material.

Concrete

Here are some examples of materials used in real construction.Asphalt concrete is used in road construction. It is made of asphalt mixed with stone particles, laid cold or hot, and compacted by a roller. When a certain asphalt type is selected by engineers, they consider several properties, such as: How well does it stick to the stone particles? (adhesion) Can it last a long time? (durability) Can it stretch without cracking or rupturing? (ductility) In a similar way,every time we engineers choose a material, we have to look at many properties of it to see if it will perform as we expect.

The other material is concrete, which is everywhere around us.Concrete is mainly made from stone, cement and water. In fact, no other material is used more than concrete in the world except water.In 2018, 37 billion metric tons of concrete was used globally. China produces and consumes one-third of all the concrete produced in the world. Since concrete is made from cement, approximately 4 billion tons of cement is needed to make that much concrete. Each ton of cement produces more than one ton of CO2, a greenhouse gas, contributing to the climate change.

Concrete can be cast in any shape and anywhere, so we don’t have to build the whole structure at once. From concrete we have built great bridges, like The Hong Kong-Zhuhai-Macao Bridge, the longest bridge in the world, and the Millau Viaduct, the tallest bridge.We build great buildings like the Toronto City Hall, one of the most beautiful buildings, and the Burj Khalifa in Dubai, the tallest tower in the world. And we build dams like the Three Gorges Dam in China, the Hoover Dam in USA, the Verzasca Dam in Switzerland. We build highways and canals.

Like all other materials, concrete has advantages and disadvantages.Raw materials of it, except cement, are natural and readily available.Concrete has high compressive strength. It can be made into practically any shape. The cost of concrete is competitive. It can be produced and placed with a relatively low level of skill and technology,and it’s usually durable. These are its advantages.

But it needs time to gain strength. It requires a longer construction time than steel or wood. It shrinks and cracks, especially in dry climates. It has low tensile strength, and cracks under low load. Cement production produces a lot of carbon dioxide, which is a greenhouse gas and contributes to climate change.

As is mentioned above, concrete is too weak when subjected to pulling (tension), so it cannot be used by itself to make buildings or bridges. But steel is very strong in resisting tension. So, concrete can be reinforced with steel bars or rods, then it can resist both push(compression) and pull (tension) forces. This type of concrete is called reinforced concrete. It is an excellent composite material and it can absorb a lot of energy before failure. It is also very strong and cost effective.

Steel

Steel is also in some sense a composite material, but it is actually an alloy of mainly iron and carbon. Steel is one of the most widely used industrial materials in the modern world. It is made from iron ore, which is extracted from the ground. A lot of energy is needed to make steel. Therefore, it is contributing in a significant way to greenhouse gas emissions.

Producing steel is a very time-consuming process, but here are some steps. First, we take iron ore from the mine, break it into small pieces, then put it in a furnace. Next, we heat it to around 1600-1700 degrees Celsius to melt it in a special crucible, with iron settling near its bottom and slag sitting on top of the iron. The two are separated and the iron is sent to another furnace where it is mixed with carbon and oxygen, and the final product is steel. Steel is then sent to mills to produce different steel shapes and products.

Steel has high tensile and compressive strength. It is very ductile,and very durable when not exposed to severe environments. Steel is completely recyclable after its initial use. Construction of steel frames requires much less time than similar concrete frames, and no forms or major temporary supports are needed during construction.

But it needs a higher level of skill and technology to produce good quality steel and fabricate steel components and structures. It corrodes and lacks durability when exposed to certain environments like seawater, acids and high humidity environments. It is generally more expensive than concrete and wood. Steel production is highly energy intensive and steel buildings need fire protection.

From steel we have built a lot of great bridges like the Sydney Harbour Bridge, and the Golden Gate Bridge in San Francisco. We have also built great buildings like the John Hancock Center in Chicago,the No.1 World Trade Center in Manhattan, and the Swiss Re Building (the Gherkin) in London.

Wood

And there is wood. Wood is a natural and environmental-friendly material. It stores carbon dioxide, so it reduces greenhouse gas emissions. When protected from humid conditions, it can last a very long time, and it has reasonable strength and stiffness. Natural woodcannot be used for very long span, but for longer spans engineered wood can be used. Wood is not currently used to build bridges for vehicular usage.

Wood is light, easy to handle and assemble, and easy to cut. It is environmentally friendly, and the cost is competitive. It requires low energy to produce, and as it stores the greenhouse gas CO2, it mitigates the climate change.

Wood has disadvantages too. It rots when exposed to excessive wetting and drying and high humidity. It is not as strong as steel, concrete and some polymeric materials, it is not very fire-resistant, and it is not readily available everywhere.

Wood is currently used to mainly build houses and some special buildings, like the roof of the Richmond Olympic Oval in Vancouver that is designed on the “warm roof ” principle.

As mentioned above, wood can’t resist fire. Recently, the famous,beautiful, and historic Notre Dame Cathedral in Paris was almost destroyed by fire. Although it was mainly made from bricks and stones, which did not get hurt by fire, it was covered by a wood structure. Once the wood burned, the whole roof fell apart and was destroyed. So, fire resistance is an important property that we should consider in selecting materials for buildings.

To finish, I can say that there is nothing as the best or worst material. Any material that can perform as required is good. You may know that gold is very valuable and expensive, but you cannot build a house with gold, you cannot make a car with gold, or make an airplane with gold, because gold is not strong and stiff enough. Some materials are expensive because they are not readily available or cost a lot to produce. Costs constitute a very important criterion for selection of materials in construction, as we don’t want to pay more than we have to. Research currently focuses on making materials smart,stronger, cheaper, and more durable by using nanotechnology and biotechnology. Research is also going on to make materials reusable and environment friendly.

After we leave this world to you, please keep it in a good shape. We have not kept it in as good of a shape as you should. You should reuse and recycle materials because the world is a close-circuit system. You cannot bring materials from space into earth. Once you use something, it cannot be replenished. Practically, there is nothing you can replace on the earth by something from another planet. Everything is just from this planet. You cannot bring water or air from another planet. So, you need to reuse and recycle as nature does.You will never see nature throwing something away, and everything gets recycled. It’s a full cycle, always goes back and forth. We should learn from nature and that’s what we should strive to imitate, make all materials recyclable and make this world a good place for everyone to live in.