The rapid growth from 1945 onwards in the prestressing of concrete shows that there was a real need for this high-quality structural material. The quality must be high because the worst conditions of loading normally occur at the beginning of the life of the member, at the transfer of stress from the steel to the concrete.?Failure is therefore more likely then than later, when the concrete has become stronger and the stress in the steel has decreased because of creep in the steel and the concrete, and shrinkage of the concrete.?Faulty members are therefore observed and thrown out early, before they enter the structure, or at least before it becomes inconvenient and expensive to remove them.

自1945年以來(lái)，預(yù)應(yīng)力混凝土的迅速發(fā)展表明了對(duì)于高質(zhì)量的結(jié)構(gòu)材料的實(shí)際需要。結(jié)構(gòu)必須具有高質(zhì)量，這是因?yàn)榻Y(jié)構(gòu)構(gòu)件承受荷載的最不利情況通常發(fā)生在它的壽命的初始階段，即發(fā)生在將鋼筋應(yīng)力傳遞給混凝土的時(shí)候。因而破壞更易于在這時(shí)發(fā)生，而不是發(fā)生在以后混凝土強(qiáng)度有所提高，同時(shí)由于鋼筋和混凝土的徐變以及混凝土的收縮而使鋼筋應(yīng)力有所降低的時(shí)候。所以，應(yīng)該在構(gòu)件成為結(jié)構(gòu)物的組成部分之前，或者至少在撤換它已很不方便而且耗資較高之前，及早檢查出不合格的構(gòu)件并予以報(bào)廢。

The main advantages of prestressed concrete in comparison with reinforced concrete are:

(a) The whole concrete cross-section resists load. In reinforced concrete about half the section, the cracked area below the neutral axis, does no useful work. Working deflections are smaller.

(b) High working stresses are possible. In reinforced concrete they are not usually possible because they result in severe cracking which is always ugly and may be dangerous if it causes rusting of the steel.

(c) Cracking is almost completely avoided in prestressed concrete.?

與鋼筋混凝土相比，預(yù)應(yīng)力混凝土的主要優(yōu)點(diǎn)是：

①整個(gè)混凝土截面承受荷載。而在鋼筋混凝凝土中約有一半的截面，即中性軸以下的開裂面，不起作用。使用階段的撓度較小。

②可以承受很高的工作應(yīng)力。而在鋼筋混凝土中，這通常是不可能的，因?yàn)?/span>這樣導(dǎo)致構(gòu)件嚴(yán)重開裂，開裂會(huì)影響美觀，而且如果造成鋼筋生銹的話，還會(huì)發(fā)生危險(xiǎn)。

③預(yù)應(yīng)力混凝土幾乎可以完全避免出現(xiàn)裂縫。

The main disadvantage of prestressed concrete is that much more care is needed to make it than reinforced concrete and it is therefore more expensive, but because it is of higher quality less of it needs to?be used. It can therefore happen that a solution of a structural problem may be cheaper in prestressed concrete than in reinforced concrete, and it does often happen that a solution is possible with prestressing but impossible without it.

預(yù)應(yīng)力混凝土的主要缺點(diǎn)是，制造時(shí)需要比鋼筋混凝土更加細(xì)心地操作和管理，因而成本較高；但由于其性能較好，所以預(yù)應(yīng)力混凝土構(gòu)件的尺寸較小。因此會(huì)遇到這樣的情況：一個(gè)結(jié)構(gòu)問題如果采用預(yù)應(yīng)力混凝土就可能比采用鋼筋混凝土所花費(fèi)的成本更低；而且通常還會(huì)出現(xiàn)這樣的情況：只有采用預(yù)應(yīng)力才能得到解決，而不加預(yù)應(yīng)力就無(wú)法解決。

Prestressing of the concrete means that it is placed under compression before it carries any working load. This means that the section can be designed so that it takes no tension or very little under the full design load. It therefore has theoretically no cracks and in practice very few. The prestress is usually applied by tensioning the steel before the concrete in which it is embedded has hardened. After the concrete has hardened enough to take the stress from the steel, some of the stress is transferred from the steel to the concrete. In a bridge with abutments able to resist thrust, the prestress can be applied without steel in the concrete. It is applied by jacks forcing the bridge inwards from the abutments. This method has the advantage that the jacking force, or prestress, can be varied during the life of the structure as required.

對(duì)混凝土施加預(yù)應(yīng)力，指的是在混凝土承受任何使用荷載以前，使它受到壓縮。這意味著混凝土截面可以設(shè)計(jì)得使它在全部設(shè)計(jì)荷載作用下，不承受拉應(yīng)力，或只承受很小的拉應(yīng)力。因此，從理論上說(shuō)，預(yù)應(yīng)力混凝土構(gòu)件沒有裂縫，而實(shí)際上也只有非常少的裂縫。預(yù)應(yīng)力一般是在埋置鋼筋的混凝土完全硬化之前，通過張拉鋼筋來(lái)施加的。在混凝土已經(jīng)硬化得足以承受由鋼筋傳來(lái)的應(yīng)力以后，此應(yīng)力的一部分就可以由鋼筋傳遞到混凝土上去。在橋臺(tái)能承受推力的橋梁中，橋體混凝土中沒有鋼筋也能施加預(yù)應(yīng)力。這種預(yù)應(yīng)力是用千斤頂施加的，千斤頂迫使橋體受到自橋臺(tái)向中心的力。這種方法的優(yōu)點(diǎn)是在結(jié)構(gòu)的使用壽命期間，千斤頂?shù)淖饔昧?，或者說(shuō)預(yù)應(yīng)力的大小可以按照需要進(jìn)行調(diào)整。

In the ten years from 1950 to 1960 prestressed concrete ceased to be an experimental material and engineers won confidence in its use.?With this confidence came an increase in the use of precast prestressed concrete particularly for long-span floors or the decks of motorways. Wherever the quantity to be made was large enough, for example in a motorway bridge 500 m long, provided that most of the spans could be made the same and not much longer than 18 m, it became economical to use factory-precast prestressed beams, at least in industrial areas near a precasting factory. Most of these beams are heat-cured so as to free the forms quickly for reuse.

在1950年至1960年的10年中，預(yù)應(yīng)力混凝土不再是一種試驗(yàn)性材料，工程師們?cè)趹?yīng)用這種材料方面獲得了人們的信任。這種信任使預(yù)制的預(yù)應(yīng)力構(gòu)件的應(yīng)用日益廣泛，尤其是在大跨度樓板或公路橋面構(gòu)件方面。凡是所需制作的構(gòu)件的數(shù)量達(dá)到一定程度，例如在一座500 四長(zhǎng)的公路橋上，如果大多數(shù)跨度可以做成同樣大小，并且不大于18 四的話，采用在工廠中預(yù)制的預(yù)應(yīng)力梁是經(jīng)濟(jì)的，至少在預(yù)制廠附近的工業(yè)區(qū)內(nèi)是如此。這類梁大多數(shù)是用加熱法養(yǎng)護(hù)，以便能夠很快脫模，使模板能夠被盡快地重復(fù)使用。

In this period also, in the United States, precast prestressed roof beams and floor beams were used in many school buildings, occasionally 32 m long or more. Such long beams over a single span?could not possibly be successful in reinforced concrete unless they were cast on site because they would have to be much deeper and much heavier than prestressed concrete beams. They would certainly be less pleasing to the eye and often more expensive than the prestressed concrete beams. These school buildings have a strong, simple architectural appeal and will be a pleasure to look at for many?years.

在這個(gè)時(shí)期內(nèi)，美國(guó)的許多學(xué)校建筑也采用預(yù)制的預(yù)應(yīng)力屋頂梁和樓板梁，它們有時(shí)可達(dá)32m或者更長(zhǎng)一些。這樣長(zhǎng)的單跨梁除非現(xiàn)場(chǎng)澆筑，否則用鋼筋混凝凝土是不可能取得成功的，因?yàn)殇摻罨炷亮旱牧焊吆椭亓慷紩?huì)遠(yuǎn)遠(yuǎn)大于預(yù)應(yīng)力混凝土梁。與預(yù)應(yīng)力混凝土梁相比，它們當(dāng)然會(huì)顯得不夠美觀，而且成本也往往更高一些。這些學(xué)校建筑要求具備堅(jiān)固和簡(jiǎn)樸的建筑感染力，并且在許多年的時(shí)間內(nèi)都會(huì)使人們看上去覺得賞心悅目。

The most important parts of a precast prestressed concrete beam are the tendons and the concrete. The tendons, as the name implies, are the cables, rods or wires of steel which are under tension in the concrete. Before the concrete has hardened ( before transfer of stress) , the tendons are either unstressed ( post-tensioned prestressing) or are stressed and held by abutments outside the concrete ( pre-tensioned prestressing ). While the concrete is hardening it grips each tendon more and more tightly by bond along its full length. End anchorages consisting of plates or blocks are placed on the ends of the tendons of post-tensioned prestressed units, and such tendons are stressed up at the time of transfer, when the concrete has hardened sufficiently. In the other type of prestressing, with pretensioned tendons, the tendons are released from external abutments at the moment of transfer, and act on the concrete through bond or anchorage or both, shortening it by compression, and themselves also shortening and losing some tension.

預(yù)制的預(yù)應(yīng)力混凝土梁最重要的部分是預(yù)應(yīng)力筋和混凝土。預(yù)應(yīng)力筋，顧名思義就是在混凝土中承受拉力的鋼纜、鋼筋或鋼絲束。在混凝土硬化以前（即在傳遞應(yīng)力以前）預(yù)應(yīng)力筋或者是未張拉（后張法），或者是已經(jīng)進(jìn)行了張拉并固定在混凝土外的合座上（先張法），隨著混凝土的逐步硬化，它通過沿其全長(zhǎng)的粘結(jié)力將預(yù)應(yīng)力筋握裹得越來(lái)越緊。端部錨具，包括錨板或錨塊，設(shè)置在后張預(yù)應(yīng)力構(gòu)件的預(yù)應(yīng)力筋的兩端。在混凝土已經(jīng)硬化到一定程度時(shí)，對(duì)這些預(yù)應(yīng)力筋進(jìn)行張拉，將預(yù)應(yīng)力傳遞給混凝土。在另外一種預(yù)加應(yīng)力的方法中，先張拉預(yù)應(yīng)力筋，待預(yù)應(yīng)力筋從外部臺(tái)座上放松時(shí)，通過黏結(jié)力或錨固力，或者兩者兼而有之地將應(yīng)力傳遞給混凝土，使混凝土受壓變短，同時(shí)鋼筋本身也縮短，因而損失一部分拉力。

Further shortening of the concrete (and therefore of the steel) takes place with time. The concrete is said to creep. This means that it shortens permanently under load and spreads the stresses more uniformly and thus more safely across its section. Steel also creeps, but rather less. The result of these two effects (and of the concrete shrinking when it dries) is that prestressed concrete beams are never more highly stressed than at the moment of transfer.

隨著時(shí)間的增加，混凝土（同時(shí)還有鋼筋）還會(huì)產(chǎn)生進(jìn)一步的縮短。混凝土的這種現(xiàn)象被稱為徐變。徐變意味著在荷載作用下，混凝土會(huì)永久性地縮短，應(yīng)力在截面上會(huì)分布得更加均勻，因而也更安全。鋼筋也產(chǎn)生徐變，但徐變量很小。這兩種作用（還有混凝土干燥時(shí)的收縮）的結(jié)果是：預(yù)應(yīng)力凝土梁中的應(yīng)力永遠(yuǎn)不會(huì)高于傳遞預(yù)加應(yīng)力時(shí)所受到的應(yīng)力。

The factory precasting of long prestressed concrete beams is likely to become more and more popular in the future, but one difficulty will be road transport. As the length of the beam increases, the lorry becomes less and less manoeuvrable until eventually the only suitable time for it to travel is in the middle of the night when traffic is at a minimum. The limit of length for road transport varies with the traffic in the district and the route, whether the roads are straight or curved.?Precasting at the site avoids these difficulties; it may be expensive, but it has often been used for large bridge beams.

將來(lái)，在工廠里預(yù)制預(yù)應(yīng)力混凝土長(zhǎng)梁可能會(huì)愈來(lái)愈普遍，但是路途運(yùn)輸將成為一個(gè)難題。隨著梁的長(zhǎng)度的增加，運(yùn)輸車輛將變得愈來(lái)愈不容易操縱駕駛，以至于不得不將運(yùn)輸構(gòu)件的最適宜時(shí)間安排在半夜交通量最少的時(shí)候。不管道路的曲直如何，道路運(yùn)輸對(duì)梁的長(zhǎng)度的限制是隨地區(qū)交通量和道路交通量而有所不同的。現(xiàn)場(chǎng)預(yù)制可以避免這些困難；盡管這樣做成本可能高一些，但是這種方法已經(jīng)常應(yīng)用于制造大跨度橋的梁。