第七章混凝土結(jié)構(gòu)

1、Reinforced concrete鋼筋混凝土Reinforced concrete is concrete in which reinforcement bars ("rebars"), reinforcement grids, plates or fibers have been incorporated to strengthen the concrete in tension. It was invented by French gardener Joseph Monier in 1849 and patented in 1867. The term Ferro

2、Concrete refers only to concrete that is reinforced with iron or steel. Other materials used to reinforce concrete can be organic and inorganic fibers as well as composites in different forms. Concrete is strong in compression, but weak in tension, thus adding reinforcement increases the strength in

3、 tension. In addition, the failure strain of concrete in tension is so low that the reinforcement has to hold the cracked sections together. For a strong, ductile and durable construction the reinforcement shall have the following properties:鋼筋混凝土就是混凝土中應(yīng)用鋼筋、鋼筋網(wǎng)、鋼板、纖維共同作用來承受拉力。1849年，一位名叫Joseph Monier

4、 的法國園丁發(fā)明了鋼筋混凝土，并于1867年申請專利。術(shù)語中所指的鋼筋混凝土僅指利用鑄鐵或者鋼進行增強的混凝土。此外一些有機纖維、無機纖維或者很多形式的復(fù)合材料也可用來增強混凝土?；炷恋目箟耗芰軓?，但是抗拉能力很弱，因此這些增強措施提高了混凝土的抗拉能力。另外因為混凝土的抗拉能力很弱，所采用的增強措施必須具備將混凝土破碎的部分連接起來的功能。一個結(jié)構(gòu)堅固、延展性好、耐久的混凝土結(jié)構(gòu)需要具備以下的性能：· High strength · High tensile strain · Good bond to the concrete · Thermal

5、compatibility · Durability in the concrete environment · 強度高· 抗拉剛度高· 混凝土之間粘結(jié)性能好· 溫度相容性· 耐久性好In most cases reinforced concrete uses steel rebars that have been inserted to add strength.在大多數(shù)情況下利用在混凝土中配置鋼筋的方式來提高強度。Use in construction在建筑中的應(yīng)用Concrete is reinforced to give it

6、 extra tensile strength; without reinforcement, many concrete buildings would not have been possible.混凝土的增強鋼筋主要是提高混凝土的抗拉強度；正是混凝土采用鋼筋才使得許多混凝土建筑成為可能。Reinforced concrete can encompass many types of structures and components, including slabs, walls, beams, columns, foundations, frames and more.鋼筋混凝土包括許多

7、種類的結(jié)構(gòu)和構(gòu)件，例如樓板、墻，梁，柱，基礎(chǔ)，框架等等。Reinforced concrete can be classified as precast or cast in-situ concrete.鋼筋混凝土可以歸類為預(yù)制混凝土和現(xiàn)澆混凝土。Much of the focus on reinforcing concrete is placed on floor systems. Designing and implementing the most efficient floor system is key to creating optimal building structures.

8、 Small changes in the design of a floor system can have significant impact on material costs, construction schedule, ultimate strength, operating costs, occupancy levels and end use of a building.工程中主要承擔(dān)荷載的鋼筋混凝土構(gòu)件是樓蓋系統(tǒng)。設(shè)計并施工建造最有效的樓蓋系統(tǒng)是達到理想的建筑結(jié)構(gòu)的關(guān)鍵。樓蓋設(shè)計的一個小小的改變會對材料費，施工進度，強度極限，生產(chǎn)費用，居住水平，和建筑的最終用途產(chǎn)生很大的影

9、響。Behavior of reinforced concrete鋼筋混凝土的性能Materials材料Concrete is a mixture of of coarse (stone or brick chips) and fine (generally sand) aggregates with a binder material (usually Portland cement). When mixed with a small amount of water, the cement hydrates form microscopic opaque crystal lattices e

10、ncapsulating and locking the aggregate into a rigid structure. Typical concrete mixes have high resistance to compressive stresses (about 4,000psi (28MPa); however, any appreciable tension (e.g., due to bending) will break the microscopic rigid lattice, resulting in cracking and separation of the co

11、ncrete. For this reason, typical non-reinforced concrete must be well supported to prevent the development of tension.混凝土由是粗的（石頭或碎磚塊）和細(xì)的（普通的沙子）骨料與膠凝材料（通常是硅酸鹽水泥）混合制成的。當(dāng)摻入少量水，水泥與水化合形成微小的不透明的晶體，將整體封閉成一個穩(wěn)定的結(jié)構(gòu)?；炷粱旌衔锏牡湫吞卣魇怯泻軓姷氖軌耗芰Γù蠹s是28MPa）;然而，任何拉力（由于拉力造成的彎曲）都會打破微觀的晶格從而導(dǎo)致混凝土裂縫和分離。也是由于這個原因，無鋼筋的素混凝土主要是防止受拉

12、破壞。If a material with high strength in tension, such as steel, is placed in concrete, then the composite material, reinforced concrete, resists not only compression but also bending and other direct tensile actions. A reinforced concrete section where the concrete resists the compression and steel r

13、esists the tension can be made into almost any shape and size for the construction industry.像復(fù)合材料，鋼筋混凝土等有很強的抗拉能力的材料，他們不僅能夠抗壓抗拉還能夠抗彎或者是抵抗其他形式的拉力。鋼筋和混凝土分工協(xié)作，混凝土抗壓，鋼筋抗拉，在建筑行業(yè)中鋼筋混凝土的形式可以被制成任何形狀和任何大小。Key characteristics :Three physical characteristics give reinforced concrete its special properties.主要性能：

14、三個使鋼筋混凝土有獨特性能的物理特征First, the coefficient of thermal expansion of concrete is similar to that of steel, eliminating large internal stresses due to differences in thermal expansion or contraction.第一，混凝土的溫度膨脹系數(shù)與鋼筋相近，消除了由于溫度膨脹系數(shù)不同導(dǎo)致過大的應(yīng)力和收縮。Second, when the cement paste within the concrete hardens this

15、conforms to the surface details of the steel, permitting any stress to be transmitted efficiently between the different materials. Usually steel bars are roughened or corrugated to further improve the bond or cohesion between the concrete and steel.第二，水泥漿依附在鋼筋表面，依據(jù)鋼筋的螺紋形狀而硬化，使得任何形式的應(yīng)力能夠在不同材料間高效的傳遞。通

16、常鋼筋是粗糙的或者有螺紋的，這樣可以進一步增大混凝土和鋼筋之間的粘結(jié)力。Third, the alkaline chemical environment provided by the alkali reserve (KOH, NaOH) and the portlandite (calcium hydroxide) contained in the hardened cement paste causes a passivating film to form on the surface of the steel, making it much more resistant to corro

17、sion than it would be in neutral or acidic conditions. When the cement paste exposed to the air and meteoric water reacts with the atmospheric CO2, portlandite and the Calcium Silicate Hydrate (CSH) of the hardened cement paste become progressively carbonated and the high pH gradually decreases from

18、 13.5 12.5 to 8.5, the pH of water in equilibrium with calcite (calcium carbonate) and the steel is no longer passivated.第三，存留的堿性物質(zhì)（KOH，NaOH）和水泥漿中含有的氫氧鈣石形成了堿性的化學(xué)環(huán)境，無形中在鋼筋的表面形成了一層鈍化膜，使得鋼筋比在中性環(huán)境和酸性環(huán)境中更加抗腐蝕。當(dāng)水泥漿被暴露在空氣中時，大氣水和空氣中的CO2相互作用使得硬化的水泥漿中的氫氧鈣石和水化硅酸鈣含碳量逐漸增加，pH值逐漸從13.5 12.5減少到8.5，水和碳酸鈣維持的恒定pH值開始變化，

19、鋼筋也不再是鈍化的。As a rule of thumb, only to give an idea on orders of magnitude, steel is protected at pH above 11 but starts to corrode below 10 depending on steel characteristics and local physico-chemical conditions when concrete becomes carbonated. Carbonation of concrete along with chloride ingress a

20、re amongst the chief reasons for the failure of reinforcement bars in concrete.一般說來，當(dāng)混凝土碳化時，根據(jù)鋼筋的性能和當(dāng)?shù)氐奈锢砘瘜W(xué)條件，鋼筋在pH大于11時處于保護狀態(tài)，但pH小于10時開始腐蝕。碳化混凝土被氯離子侵蝕是混凝土中的鋼筋被腐蝕的最主要原因。The relative cross-sectional area of steel required for typical reinforced concrete is usually quite small and varies from 1% for m

21、ost beams and slabs to 6% for some columns. Reinforcing bars are normally round in cross-section and vary in diameter. Reinforced concrete structures sometimes have provisions such as ventilated hollow cores to control their moisture & humidity. 相對來說，一般的鋼筋混凝土對鋼筋截面面積要求非常小，通常只有大多數(shù)的梁、板配筋率要求為1%，柱配筋率

22、要求為6%。鋼筋通常是直徑不同的圓鋼。為了控制合理的濕度，鋼筋混凝土結(jié)構(gòu)有時會對通風(fēng)口有要求。Distribution of concrete (in spite of reinforcement) strength characteristics along the cross-section of vertical reinforced concrete elements is inhomogeneous article "Concrete Inhomogeneity of Vertical Cast-In-Situ Elements In Frame-Type Building

23、s"依據(jù)文章“框架結(jié)構(gòu)中豎向現(xiàn)澆構(gòu)件混凝土的不均勻性”我們可以了解到，豎向鋼筋混凝土構(gòu)件雖然得到了增強，但在內(nèi)部沿著鋼筋，混凝土的應(yīng)力分布也是不均勻的。Reinforcement and terminology of Beams梁的增強和概念A(yù) beam bends under bending moment, resulting in a small curvature. At the outer face (tensile face) of the curvature the concrete experiences tensile stress, while at the inn

24、er face (compressive face) it experiences compressive stress.梁在彎矩的作用下會產(chǎn)生一個小的彎曲變形。 在彎曲外表面（拉伸面）混凝土產(chǎn)生拉應(yīng)力，在彎曲內(nèi)表面（壓縮面）混凝土產(chǎn)生壓應(yīng)力。A singly-reinforced beam is one in which the concrete element is only reinforced near the tensile face and the reinforcement, called tension steel, is designed to resist the tens

25、ion.單筋梁是指只在混凝土構(gòu)件受拉區(qū)設(shè)置抗拉鋼筋，用來抵抗拉力。A doubly-reinforced beam is one in which besides the tensile reinforcement the concrete element is also reinforced near the compressive face to help the concrete resist compression. The latter reinforcement is called compression steel. When the compression zone of a

26、concrete is inadequate to resist the compressive moment (positive moment), extra reinforcement has to be provided if the architect limits the dimensions of the section.雙筋梁是指混凝土梁既設(shè)置抗拉鋼筋來抵抗拉力，也在受壓區(qū)設(shè)置抗壓鋼筋。當(dāng)構(gòu)件受壓區(qū)混凝土不足以抵抗彎矩（正彎矩）所產(chǎn)生的壓力時，且構(gòu)件的設(shè)計截面尺寸受到了限制，那就需要額外的鋼筋增強。An under-reinforced beam is one in which

27、the tension capacity of the tensile reinforcement is smaller than the combined compression capacity of the concrete and the compression steel (under-reinforced at tensile face). When the reinforced concrete element is subject to increasing bending moment, the tension steel yields while the concrete

28、does not reach its ultimate failure condition. As the tension steel yields and stretches, an "under-reinforced" concrete also yields in a ductile manner, exhibiting a large deformation and warning before its ultimate failure. In this case the yield stress of the steel governs the design.適筋

29、梁是梁受拉區(qū)的抗拉能力比受壓區(qū)的混凝土和抗壓鋼筋的抗壓能力弱的梁（受拉區(qū)適筋）。當(dāng)混凝土構(gòu)件的彎矩逐漸增加，抗拉鋼筋屈服，此時混凝土并沒有達到極限破壞條件。隨著抗拉鋼筋的屈服和拉伸，“適筋”混凝土也以一種韌性破壞的方式屈服，并在最終破壞前有較大形變，產(chǎn)生預(yù)兆。所以，在設(shè)計時應(yīng)該主要考慮鋼筋的屈服強度。An over-reinforced beam is one in which the tension capacity of the tension steel is greater than the combined compression capacity of the concrete a

30、nd the compression steel (over-reinforced at tensile face). So the "over-reinforced concrete" beam fails by crushing of the compressive-zone concrete and before the tension zone steel yields, which does not provide any warning before failure as the failure is instantaneous.超筋梁是抗拉鋼筋的抗拉能力比混凝

31、土和抗壓鋼筋的抗壓能力強的梁 (在受拉區(qū)超筋)。在受拉區(qū)鋼筋屈服前，超筋梁受壓區(qū)的混凝土就發(fā)生破壞，破壞非常突然，沒有任何預(yù)兆。A balanced-reinforced beam is one in which both the compressive and tensile zones reach yielding at the same imposed load on the beam, and the concrete will crush and the tensile steel will yield at the same time. This design criterion

32、is however as risky as over-reinforced concrete, because failure is sudden as the concrete crushes at the same time of the tensile steel yields, which gives a very little warning of distress in tension failure.少筋梁是指受拉區(qū)和受壓區(qū)在同一荷載作用下達到屈服強度，混凝土突然破壞，抗拉鋼筋也同時破壞。然而這種設(shè)計標(biāo)準(zhǔn)非常危險，混凝土和抗拉鋼筋同時破壞，在破壞前幾乎沒有任何預(yù)兆。Steel-

33、reinforced concrete moment-carrying elements should normally be designed to be under-reinforced so that users of the structure will receive warning of impending collapse.重要的鋼筋混凝土構(gòu)件通常都設(shè)計成適筋梁形式，這樣在即將破壞前會有預(yù)兆。The characteristic strength is the strength of a material where less than 5% of the specimen sh

34、ows lower strength.強度是材料的力學(xué)性能，通常取值低于材料的試件試驗強度的5%。The design strength or nominal strength is the strength of a material, including a material-safety factor. The value of the safety factor generally ranges from 0.75 to 0.85 in Allowable Stress Design.材料強度包括材料強度的計值或標(biāo)準(zhǔn)值以及安全系數(shù)。在許用應(yīng)力設(shè)計中，安全系數(shù)的值通常為0.75到0.85

35、。The ultimate limit state is the theoretical failure point with a certain probability. It is stated under factored loads and factored resistances.極限狀態(tài)是理論上的破壞點。它考慮到了荷載和抗力的因素。Common failure modes of steel reinforced concrete鋼筋混凝土常見的破壞形式Reinforced concrete can fail due to inadequate strength, leading t

36、o mechanical failure, or due to a reduction in its durability. Corrosion and freeze/thaw cycles may damage poorly designed or constructed reinforced concrete. When rebar corrodes, the oxidation products (rust) expand and tends to flake, cracking the concrete and unbonding the rebar from the concrete

37、. Typical mechanisms leading to durability problems are discussed below.鋼筋混凝土由于強度不足和耐久性差會發(fā)生機械破壞。腐蝕、凍融都可能會導(dǎo)致設(shè)計有缺陷或者正在建設(shè)中的鋼筋混凝土結(jié)構(gòu)破壞。當(dāng)鋼筋受到侵蝕，氧化物（銹）的面積增加并開始剝落，導(dǎo)致混凝土開裂并使鋼筋與混凝土剝離。下面將討論典型的機械破壞所導(dǎo)致的耐久性問題。Mechanical failure :Cracking of the concrete section can not be prevented; however, the size of and locat

38、ion of the cracks can be limited and controlled by reinforcement, placement of control joints, the curing methodology and the mix design of the concrete. Cracking defects can allow moisture to penetrate and corrode the reinforcement. This is a serviceability failure in limit state design. Cracking i

39、s normally the result of an inadequate quantity of rebar, or rebar spaced at too great a distance. The concrete then cracks either under excess loading, or due to internal effects such as early thermal shrinkage when it cures.機械破壞：混凝土截面開裂是不能避免的，但是可以通過技術(shù)手段來控制裂縫的大小和位置，比如配筋、設(shè)置伸縮縫、養(yǎng)護方法或混凝土的配合比設(shè)計。裂縫會導(dǎo)致水氣

40、侵入并侵蝕鋼筋。這就是極限狀態(tài)設(shè)計中的正常使用極限狀態(tài)破壞。產(chǎn)生裂縫通常是由于配筋不足、鋼筋間距過大造成的?；炷猎诤奢d過大時會產(chǎn)生裂縫，混凝土的內(nèi)部影像比如混凝土硬化時熱收縮也會產(chǎn)生裂縫。Ultimate failure leading to collapse can be caused by crushing of the concrete, when compressive stresses exceed its strength; by yielding or failure of the rebar, when bending or shear stresses exceed the

41、 strength of the reinforcement; or by bond failure between the concrete and the rebar.導(dǎo)致混凝土最終破壞的原因可能有以下幾種：壓力超過抗壓極限；鋼筋屈服或破壞；彎曲應(yīng)力或剪切應(yīng)力超過鋼筋混凝土承載極限；鋼筋和混凝土之間的粘結(jié)失效破壞。Carbonation 碳化作用Carbonation, or neutralisation, is a chemical reaction between carbon dioxide in the air with calcium hydroxide and hydrated

42、 calcium silicate in the concrete. The water in the pores of Portland cement concrete is normally alkaline with a pH in the range of 12.5 to 13.5. This highly alkaline environment is one in which the embedded steel is passivated and is protected from corrosion. According to the Pourbaix diagram for

43、iron, the metal is passive when the pH is above 9.5.5 The carbon dioxide in the air reacts with the alkali in the cement and makes the pore water more acidic, thus lowering the pH. Carbon dioxide will start to carbonate the cement in the concrete from the moment the object is made. This carbonation

44、process will start at the surface, then slowly move deeper and deeper into the concrete. The rate of carbonation is dependent on the relative humidity of the concrete - a 50% relative humidity being optimal. If the object is cracked, the carbon dioxide in the air will be better able to penetrate int

45、o the concrete. When designing a concrete structure, it is normal to state the concrete cover for the rebar (the depth within the object that the rebar will be). The minimum concrete cover is normally regulated by design or building codes. If the reinforcement is too close to the surface, early fail

46、ure due to corrosion may occur. The concrete cover depth can be measured with a cover meter. However, carbonated concrete only becomes a durability problem when there is also sufficient moisture and oxygen to cause electro-potential corrosion of the reinforcing steel.碳化作用和中和作用是空氣中的二氧化碳和混凝土中的氫氧化鈣、含水硅

47、酸鈣之間的化學(xué)反應(yīng)。硅酸鹽混凝土氣孔中的水分通常呈堿性，pH值在12.5至13.5之間。這種高堿性環(huán)境會使埋置的鋼筋鈍化，防止鋼筋腐蝕。根據(jù)鐵的電位-pH圖，在pH值高于9.5時，金屬是鈍化的?？諝庵械亩趸寂c水泥中的堿性物質(zhì)作用，會降低氣孔中水的pH值。一旦有條件，二氧化碳會充滿水泥的氣孔。這種碳化過程從混凝土的表面開始，然后慢慢的深入混凝土中。碳化速率依賴于混凝土的相對濕度，相對濕度50%是最佳條件。一旦混凝土某處產(chǎn)生破裂，空氣中的二氧化碳就能順利的進入混凝土內(nèi)部。設(shè)計混凝土結(jié)構(gòu)時，通常都會設(shè)計混凝土保護層來保護鋼筋（整個構(gòu)件內(nèi)部的鋼筋）。設(shè)計或施工規(guī)范都規(guī)定了最小的混凝土保護層厚度。如

48、果鋼筋距離外表面過近，鋼筋可能會很快被腐蝕。混凝土保護層厚度可以用保護層厚度測定儀進行測量。然而，當(dāng)水分和氧氣含量很高時鋼筋會發(fā)生電離腐蝕，混凝土碳化就成了一個耐久性問題了。One method of testing a structure for carbonation is to drill a fresh hole in the surface and then treat the cut surface with phenolphthalein indicator solution. This solution will turn pink when in contact with a

49、lkaline concrete, making it possible to see the depth of carbonation. An existing hole is no good because the exposed surface will already be carbonated.檢測一個結(jié)構(gòu)的碳化程度的方法是在表面鉆一個小洞，在小洞的切面用酚酞指示劑檢測。弱指示劑顯示粉色，則說明下層的混凝土呈堿性。鉆出的小洞對混凝土是不利的，暴露在空氣中的部分將很快被碳化。Chlorides 氯化物Chlorides, including sodium chloride, can p

50、romote the corrosion of embedded steel rebar if present in sufficiently high concentration. Chloride anions induce both localized corrosion (pitting corrosion) and generalized corrosion of steel reinforcements. For this reason, one should only use fresh raw water or potable water for mixing concrete

51、, insure that the coarse and fine aggregates do not contain chlorides, and not use admixtures that contain chlorides.在高濃度的氯化物包括氯化鈉中，會加快腐蝕配置的鋼筋。氯離子會導(dǎo)致局部腐蝕（點狀腐蝕）和鋼筋的腐蝕。因此，混合混凝土需要用未加工的水或者是飲用水，并確保粗骨料細(xì)骨料不含有氯離子，也不要加入帶有氯離子的外加劑。It was once common for calcium chloride to be used as an admixture to promote ra

52、pid set-up of the concrete. It was also mistakenly believed that it would prevent freezing. However, this practice has fallen into disfavor once the deleterious effects of chlorides became known. It should be avoided when ever possible.氯化鈣作為一種外加劑，通常用做加速混凝土的初凝。也經(jīng)常被誤認(rèn)為能夠抗凍。然而，當(dāng)發(fā)現(xiàn)氯化物的腐蝕作用時這種觀念也被推翻了。要盡一

53、切可能的阻止氯化物的存在。The use of de-icing salts on roadways, used to reduce the freezing point of water, is probably one of the primary causes of premature failure of reinforced or prestressed concrete bridge decks, roadways, and parking garages. The use of epoxy-coated reinforcing bars and the application o

54、f cathodic protection has mitigated this problem to some extent. Also FRP rebars are known to be less susceptible to chlorides. Properly designed concrete mixtures that have been allowed to cure properly are effectively impervious to the effects of deicers.除冰鹽通常被用在路面上，因為除冰鹽可以降低水的凝固點，這也是鋼筋混凝土和預(yù)應(yīng)力混凝土橋

55、面，道路，停車場過早被破壞的主要原因之一。使用環(huán)氧樹脂涂覆鋼、陰極保護能夠一定程度上減輕這種現(xiàn)象。另外FRP鋼筋受氯離子的影響很小。設(shè)計合理的混凝土拌合物也可以有效的防止氯離子的侵蝕。Another important source of chloride ions is from sea water. Sea water contains by weight approximately 3.5wt% salts. These salts include sodium chloride, magnesium sulfate, calcium sulfate, and bicarbonates.

56、 In water these salts dissociate in free ions (Na+, Mg2+, Cl-, SO42-, HCO3-) and migrate with the water into the capillaries of the concrete. Chloride ions are particularly aggressive for the corrosion of the carbon steel reinforcement bars and make up about 50% of these ions.氯離子的另一個主要來源是海水。海水重量的3.5

57、 %是鹽。這些鹽包含氯化鈉，硫酸鎂，硫酸鈣和碳酸氫鹽。在水中，這些鹽分是游離狀態(tài)的，以Na+, Mg2+, Cl-, SO42-, HCO3-離子形式存在，隨著水進入混凝土內(nèi)部。氯離子占這些離子總數(shù)50%對碳化鋼筋的腐蝕效果非常強。In the 1960's and 1970's it was also relatively common for Magnesite, a chloride rich carbonate mineral, to be used as a floor-topping material. This was done principally as a levelling and sound attenuating layer. However it is now known that when these materials came into contact with moisture it produced a weak solution of hydrochloric acid due to the presence of chlorides in the magnesite. Over a period of time (typically decades) the solution caused cor