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1、 歐洲橋梁研究 在歐洲，一個共同研究的平臺隨著歐盟的發(fā)展誕生了。為了舉例說明歐洲的研究方法已經(jīng)被這種方式所替代，一個典型的案例就是有關(guān)英國后張法橋梁被討論的事，愛丁堡大學(xué)給出了這個關(guān)于用數(shù)字化脈沖雷達(dá)鑒定后張法混凝土橋梁孔隙的資源互補(bǔ)的案例。 引言 利用各研究團(tuán)體的研究成果去驗(yàn)證大量相關(guān)論據(jù)是所有研究領(lǐng)域所面臨的挑戰(zhàn)，這是為了能使研究和實(shí)踐更好地結(jié)合，具有針對性。此外，在歐洲，不容忽視的語言壁壘也是一個巨大的障礙,為了避免再次爆發(fā)類似于從1939年至1945的第二次世界大戰(zhàn)似的的歐洲內(nèi)戰(zhàn)，歐洲國家有了一個共同的政治目的，成立于20世紀(jì)60年代的歐共體便是基于這

2、一目的。它的成立帶有很強(qiáng)的政治動機(jī)，但英國并非這一組織的成員，因?yàn)楹芏嗟臍W洲國家視英國的利益為單純的經(jīng)濟(jì)利益，直到70年代，英國才加入由歐共體轉(zhuǎn)化而來的歐洲經(jīng)濟(jì)共同體。到90年代，歐洲經(jīng)濟(jì)共同體擴(kuò)展為歐洲國家聯(lián)盟，而此時的歐盟既有共同的政治目標(biāo)又有建立歐洲共同的貨幣體系的經(jīng)濟(jì)目標(biāo)。 隨著經(jīng)濟(jì)和政治的發(fā)展，土木工程，尤其是橋梁工程至今沒能形成某種統(tǒng)一陣線。這緣于英國和其他的歐洲國家迥異的大學(xué)教育培養(yǎng)體制，歐洲基金計劃如蘇格拉底計劃、大不列顛（英國）歐洲計劃等等。對改變這種局面發(fā)揮了獨(dú)特的作用，蘇格拉底計劃是以各成員國內(nèi)互派學(xué)生學(xué)習(xí)為基礎(chǔ)，而英國歐洲計劃主要是給予一些國家的學(xué)術(shù)機(jī)構(gòu)和工業(yè)伙伴以科

3、研援助；它通常是由一個工業(yè)國家牽頭。 關(guān)于知識的傳播，目前似乎已出現(xiàn)了2種非常不同的方式、英美主要集中于在相關(guān)期刊出版物上發(fā)表首要的研究成果，例如ASCE、LCE及其它期刊，而歐洲內(nèi)陸國家主要集中于在專門會議上展示其重要的研究成果，后者存在著局限性，新的研究成果的發(fā)布受到了限制。 另外，語言也是難以逾越的障礙，那些以英語作為強(qiáng)勢外語的歐洲內(nèi)陸國家積極參與各種國際會議，如德國、意大利、比利時、以及荷蘭和瑞士。然而，那些不以英語為強(qiáng)勢外語的歐洲國家對國際會議的參與并不積極，比如法國。 歐洲的研究 在歐洲，關(guān)于橋梁研究的方向基本可分為三種類型： 1、磚石結(jié)構(gòu)的拱橋 英國擁有大量的石拱橋，

4、某些地區(qū)有超過60%的公路橋?yàn)楣爬系氖皹?，這些拱橋當(dāng)初是為了馬車通行而建造的，但這種橋型在歐洲其它地區(qū)已比較少見，因?yàn)樗鼈冊诙?zhàn)中許多已被毀壞。 2、混凝土橋梁 從20世紀(jì)的50年代至70年代，在歐洲涌現(xiàn)了大量的混凝土結(jié)構(gòu)的橋梁，在那個時候，這種構(gòu)造被視為是免于維修的。歐洲也有大量的使用后張法建造的混凝土橋梁，但這種橋梁中的鐵制錨索套管會妨礙雷達(dá)對橋梁的檢測。這種問題僅是存在于法、英兩國。 3、鋼結(jié)構(gòu)橋梁 20世紀(jì)六七十年代，這種橋梁在英國遭受冷落，因?yàn)槿藗儼l(fā)現(xiàn)到這種橋需要維修保養(yǎng)。但它仍然被用于大跨徑橋梁和鐵路用橋，如今，隨著英國正在進(jìn)行的高速公路拓寬計劃的實(shí)施，這種橋梁又重新受到

5、了人們的青睞。 歐洲研究的活躍性 一個明確的信息表明專業(yè)人員在這個領(lǐng)域的研究工作正在歐洲興起，但并不代表這種研究方法已經(jīng)沒有了缺陷。 為了說明歐洲正在進(jìn)行這種形式的研究工作，愛丁堡大學(xué)給出了一個這種資源互補(bǔ)的案例：運(yùn)用數(shù)字化脈沖雷達(dá)對后張法建造的混凝土橋梁的孔隙進(jìn)行檢測。 后張法混凝土鐵路橋研究 Ove Arup和他的合作者對曼徹斯特的一座長160M的后張法分段預(yù)制施工建造的鐵路橋的上部結(jié)構(gòu)的長期穩(wěn)定性進(jìn)行了檢測和評估，這種技術(shù)已被運(yùn)用于城市輕軌系統(tǒng)。 特別需要關(guān)注的是后張力法施工的橋梁的完整性。物理檢查、非損傷雷達(dá)檢測及其它的研究方法均已被用來去調(diào)查橋梁中潛在的缺陷。 自從19

6、85年2月1日英國威爾士地區(qū)的一座名叫Ynys-y-Gwas橋梁突然坍塌以來。采用后張力法分段預(yù)制施工的橋梁在長期穩(wěn)定性方面受到關(guān)注。因?yàn)檫@種橋梁可能會毫無預(yù)兆地出現(xiàn)脆性斷裂，后張法鋼絞線在預(yù)制段搭接部位的防腐工作是影響這種類型橋梁長期穩(wěn)定性的主要因素。對容易發(fā)生脆性破壞處錨索套管中的沙漿孔隙的鑒定被認(rèn)為是防腐檢測中最為重要的步驟之一。 橋梁描述 總體布局 Besses o’ th’ Barn大橋始建于1969年，是一座三跨總長160m的后張法分段預(yù)制施工的混凝土鐵路橋。它的主跨跨徑90米，橫跨M62公路和巴利A665公路與Prestwick公路銜接，與A665公路最小橋下凈空高度為5.

7、18米，與M62公路的橋下凈空則大約為12.5米。 橋梁上部結(jié)構(gòu)由空心梯形混凝土箱梁組成。箱體橫截面高6.7m，寬4m。橋梁南部分引橋和中央主跨徑均采用這種長為1.27M的梯形混凝土箱梁結(jié)構(gòu)，后張法施工。這種箱形構(gòu)件的作用是支撐用來承受鐵軌和行車荷載的現(xiàn)澆混凝土懸臂梁。 大橋中跨和南部引橋跨徑均采用后張法裝配式預(yù)制結(jié)構(gòu)，這些后張法預(yù)制構(gòu)件構(gòu)件包括五種類型的預(yù)應(yīng)力張拉措施。 1、緣內(nèi)部的頂端和底部布置縱向錨索管道，管道內(nèi)的鋼筋束用沙漿封住； 2、分布在橫隔板部位的腹板兩側(cè)撓曲鋼筋束，鋼筋束被包裹在現(xiàn)澆混凝土內(nèi)； 3、橋梁跨中懸臂梁內(nèi)布置縱向鋼絞束； 4、在229mm寬的腹板內(nèi)側(cè)布置用

8、以提高抗剪切能力的豎直鋼絞束； 5、布置穿過冀緣底部用以支撐懸臂梁的橫向鋼絞束。 分段施工 使用分段預(yù)制施工體系是包工單位針對大橋南部和中央跨徑建議的備選方案之一，Current thinking認(rèn)為這種施工體系在對穿越構(gòu)件鉸接點(diǎn)處的鋼筋束沒有足夠的腐蝕措施時可能會導(dǎo)致整個結(jié)構(gòu)的脆性斷裂，最初的設(shè)計構(gòu)思是想采用現(xiàn)澆的混凝土結(jié)構(gòu)。 檢測和評估 檢測 各個階段的檢修工作貫穿在橋梁結(jié)構(gòu)所需的試驗(yàn)中，初期檢查主要記錄如下的一些明顯的缺陷： 1、在翼緣上表面的不合格的防水材料。 2、空心箱梁內(nèi)部300mm深度內(nèi)雨水的滲透情況。 3、鉸接縫和支座處有關(guān)排水裝置各種的問題。 4、中央跨徑

9、下端背面暴露出的縱向裂縫。 5、翼緣頂部預(yù)應(yīng)力構(gòu)件旁邊的縱向裂縫。 6、現(xiàn)澆混凝土表面大片的剝落部位和暴露在外的生銹的加強(qiáng)筋。 評估 參照最初的設(shè)計構(gòu)想，評估以下目標(biāo)項(xiàng)： 1、估算現(xiàn)存的承載能力。 2、挖掘原始設(shè)計資料中在結(jié)構(gòu)上的缺陷部分。 3、根據(jù)檢測出的問題作出判斷。 檢測和評估的結(jié)論 根據(jù)檢測和評估，仍然存在著一個主要的可疑因素，它就是埋藏在構(gòu)件里面的預(yù)應(yīng)力鋼絞線、電纜或桿件，雖然從結(jié)構(gòu)原理的角度進(jìn)行分析，這些假定均不成立。然而，它們一旦被腐蝕，就會對橋梁結(jié)構(gòu)的穩(wěn)定性帶來非常高的風(fēng)險，這個基本原理已經(jīng)得到證實(shí)。 對第一階段評估的全部內(nèi)容作如下處理： 1、進(jìn)行詳細(xì)是材

10、料試驗(yàn)，去判定隱藏在結(jié)構(gòu)內(nèi)部的情況，特別是予埋后張法鋼絞線的沙漿。 2、混凝土結(jié)構(gòu)耐久性試驗(yàn)。 修理不合格防水材料和混凝土表面的瑕疵。 檢測方法 非損傷雷達(dá)檢測 首期調(diào)查包括對預(yù)制構(gòu)件鉸接縫裂縫的勘測以及后張法鋼絞線管道中預(yù)應(yīng)力筋腐蝕和完整性的檢測。然而，對出現(xiàn)問題的嚴(yán)重性會很難做出判斷。這座橋梁一共有93個鉸接縫，平均每個鉸接縫有24根鋼絞線貫穿，也就是說，那兒大約有2200個要用來進(jìn)行檢測的部位。一個標(biāo)準(zhǔn)的鉸接縫橫端面，主梁內(nèi)部的24根鋼筋束很難被檢測到，因?yàn)槌算q接縫外，鋼絞線在施加預(yù)應(yīng)力之后又被現(xiàn)澆的混凝土所包裹。 顯然，鉸接縫處鋼筋束完全暴露在外是不切實(shí)際的，因此相比較而

11、言，用雷達(dá)檢測鋼筋束管道中的孔隙十分高效的，慶幸的是通過鉸接縫的放置鋼絞束的鋼制管道是斷開的，可以用雷達(dá)檢測管道槽內(nèi)的鋼絞束和孔隙。但是，仍然存在的問題是管道周圍的高密度的鐵元素會對雷達(dá)信號產(chǎn)生強(qiáng)烈的干擾，而實(shí)際上在這一寬102mm，150mm~800mm的混凝土區(qū)域內(nèi)正埋有許多高致密的鋼板。 雷達(dá)測試試驗(yàn) 有三家公司被邀請前來參觀和指揮試驗(yàn)研究工作，其中一家放棄，剩下的兩家被要求用兩周的時間去準(zhǔn)備試驗(yàn)和撰寫報告，再把他們的研究成果與物理勘探的結(jié)果進(jìn)行比較。 為了對比，選定了10個觀測孔，小孔是垂直向下鉆進(jìn)導(dǎo)管。這十個小孔中有幾個在鉸接縫處，還有幾個在充滿沙漿的管道處。為了便于使用內(nèi)徑表

12、面檢測儀，小孔直徑必須有25mm寬，結(jié)果顯示愛丁堡大學(xué)的研究成果的準(zhǔn)確度在60%左右。 雷達(dá)測繪為主 內(nèi)徑表面檢查儀檢驗(yàn)裂縫為輔 在完成對橋梁結(jié)構(gòu)雷達(dá)檢測后，再使用內(nèi)徑表面檢查儀來驗(yàn)證被預(yù)知的裂縫。結(jié)果顯示，在超過60%的案例中，雷達(dá)的檢測是準(zhǔn)確的。在其它幾個事例中，一些證據(jù)表明在管道上面的現(xiàn)澆混凝土層中發(fā)現(xiàn)了蜂窩狀的孔洞。 然而用內(nèi)徑表面檢查儀檢測時，很難判定裂縫的實(shí)際尺寸大小以及在錨索套管中延伸的距離，盡管這些裂縫僅占套管不到25%的部分。但事實(shí)上，大多數(shù)存在于沙漿表面與套管上部曲面邊界上的裂縫要比內(nèi)徑表面檢查儀所檢測到的要窄得多，（內(nèi)徑表面檢查儀的分辨率大約在9mm左右）。在少數(shù)幾

13、個案例中，在沙漿表面能明顯地看到預(yù)應(yīng)力鋼絞線，但沒有跡象表明有水滲透了進(jìn)去。而且使用內(nèi)徑表面檢查儀是不可能看到鋼絞線被腐蝕的情況。 數(shù)字雷達(dá)測試 這種測試方法是利用無線電頻率的雷達(dá)天線的穿透性。有以下幾個常用頻率：1GHZ，900MHZ，和500MHZ。最高的頻率能帶來最好的分辯率，但會減小其在混凝土中的穿透深度。最低頻率的穿透能力最強(qiáng)，但是分辯率最低。 雷達(dá)掃描到的數(shù)據(jù)被記錄到GSSISIR系統(tǒng)中，這種系統(tǒng)與雷達(dá)發(fā)射脈沖和記錄數(shù)據(jù)相對應(yīng)。雷達(dá)天線所收到的數(shù)據(jù)從模擬信號被轉(zhuǎn)換成了數(shù)字信號。這種轉(zhuǎn)換是使用一種16位的模擬信號變流器，它能使數(shù)據(jù)獲得相當(dāng)高的分辨率，以便用于后續(xù)的數(shù)據(jù)處理。這些

14、數(shù)據(jù)被顯示在一個高分辨率的彩色監(jiān)測器上。在可視化校對之后，將這些數(shù)據(jù)儲存在一個2.3千兆字節(jié)的磁帶上，用于后面的分析和處理。首先通過磁帶記錄下數(shù)字化雷達(dá)所掃描到的原始數(shù)據(jù)，再通過相關(guān)的設(shè)置和處理程序轉(zhuǎn)化為精確和可靠的數(shù)據(jù)。 沿著軌跡做特別的標(biāo)記，再通過記錄元件或天線把這個軌跡描述下來。 在大學(xué)實(shí)驗(yàn)室里進(jìn)行非破壞性試驗(yàn)后，將所有的數(shù)字記錄從微機(jī)上拷貝下來，（原始數(shù)據(jù)資料處理將會消耗35兆字節(jié)的內(nèi)存。）后張法分析需要運(yùn)用特殊的處理軟件。這種分析是通過變換的顏色和線形來顯示出特征點(diǎn)，也可以通過顏色的變換顯示相位的變化。除了這些能變換顏色的設(shè)備外，還可能使用用來過濾水平線和豎直線的特殊程序。用一個

15、大型熒屏測試器同時顯示原始數(shù)據(jù)和處理過的數(shù)據(jù)，從而就能很清晰地看到被處理過數(shù)據(jù)是在那些地方做過修改的，同時電腦顯示器將把反饋信息標(biāo)注到縱坐標(biāo)上。 一個更為先進(jìn)的軟件能夠顯示專用雷達(dá)脈沖掃描的區(qū)域，在單獨(dú)檢測錨索周圍狀況時，它是一種特別有價值元件。 調(diào)查研究結(jié)果的分析 在別處已經(jīng)有對調(diào)查結(jié)果非常充分的研究，數(shù)字化雷達(dá)繪圖的本質(zhì)就是將掃描到的被確認(rèn)有雙重相位位移的節(jié)點(diǎn)轉(zhuǎn)化為著色線條，從而缺陷部位即被診斷出來。 結(jié)論 1、一個關(guān)于橋梁研究平臺的雛形已經(jīng)在歐洲誕生。 2、雷達(dá)脈沖波檢測技術(shù)的運(yùn)用大大增加了對Besses o’ th’ Barn鐵路橋評估結(jié)論的可信度。 3、雷達(dá)勘察可以顯示

16、后張法鋼絞線孔道內(nèi)部的大部分缺陷區(qū)域。然而，即使運(yùn)用了極其一流的研究手段，也沒有跡象表明已發(fā)現(xiàn)受拉鋼絲的腐蝕原因。 Bridge research in Europe A brief outline is given of the development of the European Union, together with the research platform in Europe. The special case of post-tensioned bridges in the UK is discussed. In order to illustrate

17、the type of European research being undertaken, an example is given from the University of Edinburgh portfolio: relating to the identification of voids in post-tensioned concrete bridges using digital impulse radar. Introduction The challenge in any research arena is to harness the findings of di

18、fferent research groups to identify a coherent mass of data, which enables research and practice to be better focused. A particular challenge exists with respect to Europe where language barriers are inevitably very significant. The European Community was formed in the 1960s based upon a political w

19、ill within continental Europe to avoid the European civil wars, which developed into World War 2 from 1939 to 1945. The strong political motivation formed the original community of which Britain was not a member. Many of the continental countries saw Britain’s interest as being purely economic. The

20、1970s saw Britain joining what was then the European Economic Community (EEC) and the 1990s has seen the widening of the community to a European Union, EU, with certain political goals together with the objective of a common European currency. Notwithstanding these financial and political developm

21、ents, civil engineering and bridge engineering in particular have found great difficulty in forming any kind of common thread. Indeed the educational systems for University training are quite different between Britain and the European continental countries. The formation of the EU funding schemes —e

22、.g. Socrates, Brite Euram and other programs have helped significantly. The Socrates scheme is based upon the exchange of students between Universities in different member states. The Brite Euram scheme has involved technical research grants given to consortia of academics and industrial partners wi

23、thin a number of the states— a Brite Euram bid would normally be led by an industrialist. In terms of dissemination of knowledge, two quite different strands appear to have emerged. The UK and the USA have concentrated primarily upon disseminating basic research in refereed journal publications: AS

24、CE, ICE and other journals. Whereas the continental Europeans have frequently disseminated basic research at conferences where the circulation of the proceedings is restricted. Additionally, language barriers have proved to be very difficult to break down. In countries where English is a strong sec

25、ond language there has been enthusiastic participation in international conferences based within continental Europe —e.g. Germany, Italy, Belgium, The Netherlands and Switzerland. However, countries where English is not a strong second language have been hesitant participants }—e.g. France. Europea

26、n research Examples of research relating to bridges in Europe can be divided into three types of structure: Masonry arch bridges Britain has the largest stock of masonry arch bridges. In certain regions of the UK up to 60% of the road bridges are historic stone masonry arch bridges originally con

27、structed for horse drawn traffic. This is less common in other parts of Europe as many of these bridges were destroyed during World War 2. Concrete bridges A large stock of concrete bridges was constructed during the 1950s, 1960s and 1970s. At the time, these structures were seen as maintenance fr

28、ee. Europe also has a large number of post-tensioned concrete bridges with steel tendon ducts preventing radar inspection. This is a particular problem in France and the UK. Steel bridges Steel bridges went out of fashion in the UK due to their need for maintenance as perceived in the 1960s and 19

29、70s. However, they have been used for long span and rail bridges, and they are now returning to fashion for motorway widening schemes in the UK. Research activity in Europe It gives an indication certain areas of expertise and work being undertaken in Europe, but is by no means exhaustive. In or

30、der to illustrate the type of European research being undertaken, an example is given from the University of Edinburgh portfolio. The example relates to the identification of voids in post-tensioned concrete bridges, using digital impulse radar. Post-tensioned concrete rail bridge analysis Ove Aru

31、p and Partners carried out an inspection and assessment of the superstructure of a 160 m long post-tensioned, segmental railway bridge in Manchester to determine its load-carrying capacity prior to a transfer of ownership, for use in the Metrolink light rail system.. Particular attention was paid t

32、o the integrity of its post-tensioned steel elements. Physical inspection, non-destructive radar testing and other exploratory methods were used to investigate for possible weaknesses in the bridge. Since the sudden collapse of Ynys-y-Gwas Bridge in Wales, UK in 1985, there has been concern about t

33、he long-term integrity of segmental, post-tensioned concrete bridges which may be prone to ‘brittle’ failure without warning. The corrosion protection of the post-tensioned steel cables, where they pass through joints between the segments, has been identified as a major factor affecting the long-ter

34、m durability and consequent strength of this type of bridge. The identification of voids in grouted tendon ducts at vulnerable positions is recognized as an important step in the detection of such corrosion. Description of bridge General arrangement Besses o’ th’ Barn Bridge is a 160 m long, thre

35、e span, segmental, post-tensioned concrete railway bridge built in 1969. The main span of 90 m crosses over both the M62 motorway and A665 Bury to Prestwick Road. Minimum headroom is 5.18 m from the A665 and the M62 is cleared by approx 12.5 m. The superstructure consists of a central hollow trapez

36、oidal concrete box section 6.7 m high and 4 m wide. The majority of the south and central spans are constructed using 1.27 m long pre-cast concrete trapezoidal box units, post-tensioned together. This box section supports the in site concrete transverse cantilever slabs at bottom flange level, which

37、 carry the rail tracks and ballast. The center and south span sections are of post-tensioned construction. These post-tensioned sections have five types of pre-stressing: 1. Longitudinal tendons in grouted ducts within the top and bottom flanges. 2. Longitudinal internal draped tendons located al

38、ongside the webs. These are deflected at internal diaphragm positions and are encased in in site concrete. 3. Longitudinal macalloy bars in the transverse cantilever slabs in the central span . 4. Vertical macalloy bars in the 229 mm wide webs to enhance shear capacity. 5. Transverse macalloy bar

39、s through the bottom flange to support the transverse cantilever slabs. Segmental construction The pre-cast segmental system of construction used for the south and center span sections was an alternative method proposed by the contractor. Current thinking suggests that such a form of construction

40、can lead to ‘brittle’ failure of the entire structure without warning due to corrosion of tendons across a construction joint，The original design concept had been for in site concrete construction. Inspection and assessment Inspection Inspection work was undertaken in a number of phases and was l

41、inked with the testing required for the structure. The initial inspections recorded a number of visible problems including: 1、 Defective waterproofing on the exposed surface of the top flange. 2、 Water trapped in the internal space of the hollow box with depths up to 300 mm. 3、 Various drainage p

42、roblems at joints and abutments. 4、 Longitudinal cracking of the exposed soffit of the central span. 5、 Longitudinal cracking on sides of the top flange of the pre-stressed sections. 6、 Widespread sapling on some in site concrete surfaces with exposed rusting reinforcement. Assessment The subje

43、ct of an earlier paper, the objectives of the assessment were: 1、 Estimate the present load-carrying capacity. 2、 Identify any structural deficiencies in the original design. 3、 Determine reasons for existing problems identified by the inspection. Conclusion to the inspection and assessment Fol

44、lowing the inspection and the analytical assessment one major element of doubt still existed. This concerned the condition of the embedded pre-stressing wires, strands, cables or bars. For the purpose of structural analysis these elements、had been assumed to be sound. However, due to the very high f

45、orces involved,、a risk to the structure, caused by corrosion to these primary elements, was identified. The initial recommendations which completed the first phase of the assessment were: 1. Carry out detailed material testing to determine the condition of hidden structural elements, in particula

46、r the grouted post-tensioned steel cables. 2. Conduct concrete durability tests. 3. Undertake repairs to defective waterproofing and surface defects in concrete. Testing procedures Non-destructive radar testing During the first phase investigation at a joint between pre-cast deck segments the

47、observation of a void in a post-tensioned cable duct gave rise to serious concern about corrosion and the integrity of the pre-stress. However, the extent of this problem was extremely difficult to determine. The bridge contains 93 joints with an average of 24 cables passing through each joint, i.e.

48、 there were approx. 2200 positions where investigations could be carried out. A typical section through such a joint is that the 24 draped tendons within the spine did not give rise to concern because these were protected by in site concrete poured without joints after the cables had been stressed.

49、 As it was clearly impractical to consider physically exposing all tendon/joint intersections, radar was used to investigate a large numbers of tendons and hence locate duct voids within a modest timescale. It was fortunate that the corrugated steel ducts around the tendons were discontinuous throug

50、h the joints which allowed the radar to detect the tendons and voids. The problem, however, was still highly complex due to the high density of other steel elements which could interfere with the radar signals and the fact that the area of interest was at most 102 mm wide and embedded between 150 mm

51、 and 800 mm deep in thick concrete slabs. Trial radar investigations. Three companies were invited to visit the bridge and conduct a trial investigation. One company decided not to proceed. The remaining two were given 2 weeks to mobilize, test and report. Their results were then compared with ph

52、ysical explorations. To make the comparisons, observation holes were drilled vertically downwards into the ducts at a selection of 10 locations which included several where voids were predicted and several where the ducts were predicted to be fully grouted. A 25-mm diameter hole was required in ord

53、er to facilitate use of the chosen horoscope. The results from the University of Edinburgh yielded an accuracy of around 60%. Main radar survey, horoscope verification of voids. Having completed a radar survey of the total structure, a baroscopic was then used to investigate all predicted voids a

54、nd in more than 60% of cases this gave a clear confirmation of the radar findings. In several other cases some evidence of honeycombing in the in site stitch concrete above the duct was found. When viewing voids through the baroscopic, however, it proved impossible to determine their actual size o

55、r how far they extended along the tendon ducts although they only appeared to occupy less than the top 25% of the duct diameter. Most of these voids, in fact, were smaller than the diameter of the flexible baroscopic being used (approximately 9 mm) and were seen between the horizontal top surface of

56、 the grout and the curved upper limit of the duct. In a very few cases the tops of the pre-stressing strands were visible above the grout but no sign of any trapped water was seen. It was not possible, using the baroscopic, to see whether those cables were corroded. Digital radar testing The test

57、method involved exciting the joints using radio frequency radar antenna: 1 GHz, 900 MHz and 500 MHz. The highest frequency gives the highest resolution but has shallow depth penetration in the concrete. The lowest frequency gives the greatest depth penetration but yields lower resolution. The data

58、collected on the radar sweeps were recorded on a GSSI SIR System 10. This system involves radar pulsing and recording. The data from the antenna is transformed from an analogue signal to a digital signal using a 16-bit analogue digital converter giving a very high resolution for subsequent data proc

59、essing. The data is displayed on site on a high-resolution color monitor. Following visual inspection it is then stored digitally on a 2.3-gigabyte tape for subsequent analysis and signal processing. The tape first of all records a ‘header’ noting the digital radar settings together with the trace n

60、umber prior to recording the actual data. When the data is played back, one is able to clearly identify all the relevant settings —making for accurate and reliable data reproduction. At particular locations along the traces, the trace was marked using a marker switch on the recording unit or the an

61、tenna. All the digital records were subsequently downloaded at the University’s NDT laboratory on to a micro-computer.（The raw data prior to processing consumed 35 megabytes of digital data.） Post-processing was undertaken using sophisticated signal processing software. Techniques available for the

62、 analysis include changing the color transform and changing the scales from linear to a skewed distribution in order to highlight、突出certain features. Also, the color transforms could be changed to highlight phase changes. In addition to these color transform facilities, sophisticated horizontal and

63、vertical filtering procedures are available. Using a large screen monitor it is possible to display in split screens the raw data and the transformed processed data. Thus one is able to get an accurate indication of the processing which has taken place. The computer screen displays the time domain c

64、alibrations of the reflected signals on the vertical axis. A further facility of the software was the ability to display the individual radar pulses as time domain wiggle plots. This was a particularly valuable feature when looking at individual records in the vicinity of the tendons. Interpretati

65、on of findings A full analysis of findings is given elsewhere, Essentially the digitized radar plots were transformed to color line scans and where double phase shifts were identified in the joints, then voiding was diagnosed. Conclusions 1. An outline of the bridge research platform in Europe is

66、 given. 2. The use of impulse radar has contributed considerably to the level of confidence in the assessment of the Besses o’ th’ Barn Rail Bridge. 3. The radar investigations revealed extensive voiding within the post-tensioned cable ducts. However, no sign of corrosion on the stressing wires had been found except for the very first investigation. 出處： 出處：天工網(wǎng)