Investigation on the use of iron and steel for restoration purposes during 19th and 20th century

Hui-Yin Lee INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Hui-Yin Lee INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Spain l 2008 DECLARATION Name: Hui-Yin Lee Email: [email protected] ID nº: 214274530 Title of the Msc Dissertation: INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Supervisor(s): Pere Roca I Fabregat Year: 2008 I hereby declare that the MSc Consortium responsible for the Advanced Masters in Structural Analysis of Monuments and Historical Constructions is allowed to store and make available electronically the present MSc Dissertation. University: Technical University of Catalonia (UPC) Date: 17h July 2008 Signature: Acknowledgement I am deeply indebted to my supervisor Prof. Pere Roca I Fabregat, who is always positive and helpful for his students. I would like to express my gratitude to all those who gave me the possibility to complete this thesis. I want to thank the Erasmus Mundus Programmr for giving me this opportunity to particiapte in this Master and financial support. Especially, I would like to give my special thanks my family for their support all the way. I am thankful to all my family members for their thoughtfulness and encouragement. " At last but not least, I would like to thanks all of the friends who I made in Europe for all the good time we shared. Hui-Yin Lee Abstract Since the earliest times, wrought iron cramps and dowels were used in the traditional masonry structures to secure stones which might be prone to movement or displacement. In the period between the late 19th century and the early 20th century, masonry-clad buildings are exploded to use. However, due to the porous nature of the mortar and the inconsistent fill around the steel members, the protective oxide film is lost over time, resulting in corrosion of the steel framing and other embedded metals. The metals which are used for historical construction are cast iron, steel and wrought iron. The causes of metal deterioration are corrosion, mechanical breakdown, weathering and connection failure. The traditional restorations are repair, replacement and jointing method. And new, a new method, cathodic protection, is also applied on historical building restoration. New materials began to be used for replacing the original materials, The most common have been aluminum, epoxies, reinforced polyester, glass fiber-reinforced concrete and titanium. The first cathodic protection system for stone clad steel framing was installed in 1991. Cathodic protection (CP) prevents corrosion by converting all of the anodic sites on the metal surface to cathodic sites by supplying electrical current from an alternate source. Different from the traditional way, CP method does not to remove the masonry and install new masonry back. According to Roberto Pane, the extreme variety of cases and the need for safeguarding a large amount of monuments shows that restoration can not be constrained within stiff limits .As a conclusion of this studying, due to the former experience, the Barcelona Cathedral’s façade can be considered to use the CP method for restoration blend with the old with regard to size, scale and appearance. INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 1 Table of contents 1. Introduction................................................................................. 5 1.1 Motivations ................................................................................ 7 1.2 Objective................................................................................... 8 2. Metal .......................................................................................... 9 2.1 The Varieties of metal for historical constructions............................ 9 2.2 Cast iron.................................................................................... 9 2.2.1 Varieties of cast iron................................................................10 2.2.2 Grey cast iron.........................................................................12 2.2.3 White cast iron .......................................................................13 2.2.4 Malleable ...............................................................................14 2.2.5 Historical Use .........................................................................14 2.2.6 Structural use.........................................................................16 2.3 Steel........................................................................................18 2.3.1 Steel Classification Systems .....................................................20 2.3.2 Historical Use .........................................................................21 2.4 Wrought iron.............................................................................22 2.4.1 Historical Use .........................................................................23 3. Causes of Metal Deterioration........................................................25 3.1 Corrosion..................................................................................25 3.1.1 Corrosion Process....................................................................26 3.1.2 Corrosion as an Electrochemical Process ....................................26 3.1.3 Types of Corrosion ..................................................................27 3.1.3.1 Uniform corrosion.................................................................27 3.1.3.2 Pitting corrosion...................................................................28 3.1.3.3 Crevice or concentration cell corrosion....................................29 3.1.3.4 Galvanic corrosion................................................................29 3.1.3.5 Other corrosion forms...........................................................30 3.2 Mechanical breakdown................................................................32 3.2.1 Abrasion ................................................................................32 3.2.2 Fatigue..................................................................................32 3.2.3 Creep ....................................................................................33 3.2.4 Fire .......................................................................................33 3.2.5 Overloading............................................................................33 3.3 Weathering ...............................................................................33 3.4 Connection Failure .....................................................................34 4. Methods for repair and rehabilitation of metal .................................35 4.1. Repairs....................................................................................36 4.1.1 Repair techniques used for cast iron..........................................36 4.1.1.1 Welding ..............................................................................36 4.1.1.2 Dowels................................................................................37 INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 2 4.1.1.3 Metal stitching .....................................................................37 4.1.1.4 Adhesives............................................................................38 4.1.2 Repair techniques used for wrought iron and steel ......................38 4.2 Replacement.............................................................................39 4.2.1 Cast iron................................................................................39 4.2.2 Wrought iron and old types of steel...........................................40 4.2.3 Jointing methods.....................................................................40 4.3 Substitute Materials ...................................................................41 4.3.1 Cast aluminum .......................................................................41 4.3.2 Epoxies..................................................................................42 4.3.3 Reinforced polyester................................................................42 4.3.4 Glass fiber-reinforced concrete .................................................43 4.3.5 Titanium ................................................................................43 4.4 Cathodic Protection techniques....................................................44 4.4.1 History ..................................................................................44 4.4.2 Theoretical principles...............................................................44 4.4.3 CP system types .....................................................................45 5. Case Study.................................................................................49 5.1 Buried metal cramp conservation in the Inigo Jones Gateway, Chiswick House grounds,London(Keith Blackney and Bill Martin) ...........49 5.1.1 Survey...................................................................................50 5.1.2 Repair strategy.......................................................................51 5.1.3 Conclusion .............................................................................53 5.2 Barcelona Cathedral...................................................................54 5.2.1 The façade .............................................................................54 5.2.2 Current condition of the façade.................................................55 5.2.3 Repair ...................................................................................56 5.2.4 Recommendation....................................................................56 6. Conclusion..................................................................................57 Reference.......................................................................................58 INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 3 List of Figures Figure 1 The metallic cramp.......................................................5 Figure 2 The Brooklyn Museum ..................................................5 Figure 3 The puddling process of smelting iron.........................23 Figure 4 A corroded wrought iron chain.....................................25 Figure 5 A corroded steel.........................................................25 Figure 6 Electrochemical Process ............................................26 Figure 7 Uniform corrosion ......................................................32 Figure 8 Pitting corrosion.........................................................32 Figure 9 Galvanic corrosion......................................................32 Figure 10 Intergranular corrosion .............................................32 Figure 11 1.Drilling the lock hole pattern 2.Installing the Lock 3.Start installing stitching studs 4.Completed repair is ground flush ........38 Figure 12 This lightweight fiberglass column at street level sustained damage from impact within a few years of installation. Photo: Building Conservation Associates..........................................................42 Figure 13 Modern use of titanium to replace steel strengthening ..43 Figure 14 Schematic of SACP and ICCP Systems ........................46 Figure 15 The Inigo Jones gateway...........................................49 Figure 16 Cathedral................................................................54 Figure 17 The rust cramp ........................................................55 Figure 18 The rust cramp ........................................................55 Figure 19 The crack on the pinnacle due to the metal corrosion....55 Figure 20 The corrosion of the metal elements...........................55 Figure 21 The corrosion of the steel components........................56 Figure 22 The support of the dome...........................................56 INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 4 List of Tables Table 1 Carbon contents of cast iron, steel and wrought iron..........9 Table 2 Varieties of historical cast iron.......................................11 INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 11 Table 2 Varieties of historical cast iron Type of cast iron Nominal composition (% by weight) Microstructure Physical properties Uses Notes Grey C 3.4, Si 1.8, Mn 0.5 Graphite in flake form in an iron matrix. Flakes form discontinuities. Strong in compression. Relatively weak in tension. Good resistance to corrosion. Easily machined and cut. Very large castings practicable. Main form of cast iron used in construction, for columns, beams, decorative panels etc., as well as machinery. Historic cast iron nearly all grey iron. Little used in construction today except for pipes, pipe fittings, manhole covers, etc. White C 3.4, Si 0.7, Mn 0.6 No free graphite. Carbon combined with iron as hard carbides. Low equivalent carbons. Low silicon content. Very hard and very brittle. Machined by grinding only. Surfaces needing high resistance to abrasion. Virtually irrelevant to construction industry. Malleable C 2.5, Si 1.0, Mn 0.55 Made by prolonged heat treatment of white iron castings. Carbides transformed into graphite in nodular form with few discontinuities in iron matrix. Very strong in tension as well as compression, with good ductility. Hinges, catches, step iron and similar castings of limited size. Decorative panels of fragile design. Likely to be superseded by ductile iron which has similar properties and can be cast in a wide range of section thickness. INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 12 2.2.2 Grey cast iron Silicon is essential to making grey cast iron as opposed to white cast iron. When silicon is alloyed with ferrite and carbon in amounts of about 2 percent, the carbide of iron becomes unstable. Silicon causes the carbon to rapidly come out of solution as graphite, leaving a matrix of relatively pure, soft iron. Weak bonding between planes of graphite leads to a high activation energy for growth in that direction, resulting in thin, round flakes. This structure has several useful properties. The metal expands slightly on solidifying as the graphite precipitates, resulting in sharp castings. The graphite content also offers good corrosion resistance. Graphite acts as a lubricant, improving wear resistance. The exceptionally high speed of sound in graphite gives cast iron a much higher thermal conductivity. Since ferrite is so different in this respect (having heavier atoms, bonded much less tightly) phonons tend to scatter at the interface between the two materials. In practical terms, this means that cast iron tends to “damp” mechanical vibrations (including sound), which can help machinery to run more smoothly. All of the properties listed in the paragraph above ease the machining of grey cast iron. The sharp edges of graphite flakes also tend to concentrate stress, allowing cracks to form much more easily, so that material can be removed much more efficiently. Easier initiation of cracks can be a drawback once an item is finished, however: grey cast iron has less tensile strength and shock resistance than steel. It is also difficult to weld. INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 13 Grey cast iron's high thermal conductivity and specific heat capacity are often exploited to make cast iron cookware and disc brake rotors. 2.2.3 White cast iron With a lower silicon content and faster cooling, the carbon in white cast iron precipitates out of the melt as the metastable phase cementite, Fe3C, rather than graphite. The cementite which precipitates from the melt forms as relatively large particles, usually in a eutectic mixture where the other phase is austenite (which on cooling might transform to marten site). These eutectic carbides are much too large to provide precipitation hardening (as in some steels, where cementite precipitates might inhibit plastic deformation by impeding the movement of dislocations through the ferrite matrix). Rather, they increase the bulk hardness of the cast iron simply by virtue of their own very high hardness and their substantial volume fraction, such that the bulk hardness can be approximated by a rule of mixtures. In any case, they offer hardness at the expense of toughness. Since carbide makes up a large fraction of the material, white cast iron could reasonably be classified as a cermet. White iron is too brittle for use in many structural components, but with good hardness and abrasion resistance and relatively low cost, it finds use in such applications as the wear surfaces of slurry pumps, shell liners and lifter bars in ball mills and autogenously grinding mills, balls and rings in coal pulverizes and balls for rolling-element bearings and the teeth of a backhoe's digging bucket. Although the latter two applications would normally use high quality wrought high-carbon martens tic steels and cast medium-carbon martens tic steels respectively. It is difficult to cool thick castings fast enough to solidify the melt as white cast iron all the way through. However, rapid cooling can be INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 14 used to solidify a shell of white cast iron, after which the remainder cools more slowly to form a core of grey cast iron. The resulting casting, called a “chilled casting”, has the benefits of a hard surface and a somewhat tougher interior. White cast iron can also be made by using a high percentage of chromium in the iron; Cr is a strong carbide-forming element, so at high enough percentages of chrome, the precipitation of graphite out of the iron is suppressed. High-chrome white iron alloys allow massive a casting to be sand cast, i.e., a high cooling rate is not required, as well as providing impressive abrasion resistance. 2.2.4 Malleable Malleable iron starts as a white iron casting, that is then heat treated at about 900 °C. Graphite separates out much more slowly in this case, so that surface tension has time to form it into spheroidal particles rather than flakes. Due to their lower aspect ratio, spheroids are relatively short and far from one another, and have a lower cross section vis-à-vis a propagating crack or phonon. They also have blunt boundaries, as opposed to flakes, which alleviates the stress concentration problems faced by grey cast iron. In general, the properties of malleable cast iron are more like mild steel. There is a limit to how large a part can be cast in malleable iron, since it is made from white cast iron. 2.2.5 Historical Use Because cast iron is comparatively brittle, it is not suitable for purposes where a sharp edge or flexibility is required. It is strong under compression, but not under tension. Cast Iron was first invented in China, and poured into molds to make weapons and figurines. Historically, its earliest uses included cannon and shot. In England, the ironmasters of the Weald continued producing these INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 15 until the 1760s, and this was the main function of the iron industry there after the Restoration, though probably only a minor part of the industry there earlier. Cast iron pots were made at many English blast furnaces at that period. In 1707, Abraham Darby patented a method of making pots thinner and hence cheaper than his rivals could. This meant that his Coalbrookdale Furnaces became dominant as suppliers of pots, an activity in which they were joined in the 1720s and 1730s by a small number of other coke-fired blast furnaces. The development of the steam engine by Thomas Newcomen provided a further market for cast iron, since this was considerably cheaper than the brass of which the engine cylinders were originally made. A great exponent of cast iron was John Wilkinson, who amongst other things cast the cylinders for many of James Watt's improved steam engines until the establishment of the Soho Foundry in 1795. Cast iron bridges The major use of cast iron for structural purposes began in the late 1770s when Abraham Darby III built the Iron Bridge, although short beams had been used prior to the bridge, such as in the blast furnaces at Coalbrookdale. This was followed by others, including Thomas Paine, who patented one; cast iron bridges became common as the Industrial Revolution gathered pace. Thomas Telford adopted the material for his bridge upstream at Buildwas, and then for a canal trough aqueduct at Longdon-on-Tern on the Shrewsbury Canal. It was followed by the spectacular Chirk Aqueduct and the breath-taking Pontcysyllte Aqueduct, both of which remain in use following recent restorations. Cast iron beam bridges were used widely by the early railways, such as the Water street bridge at the Manchester terminus of the Liverpool and Manchester Railway. INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 16 However, problems arose when such a bridge collapsed shortly after opening in 1846. The Dee bridge disaster was caused by excessive loading at the centre of the beam by a passing train, and many similar bridges had to be demolished and rebuilt, often in wrought iron. The bridge had been under-designed, being trussed with wrought iron straps, which were wrongly thought to reinforce the structure. Nevertheless, cast iron continued to be used for structural support, until the Tay Rail Bridge disaster of 1879 created a crisis of confidence in the material. Further bridge collapses occurred, however, culminating in the Norwood Junction rail accident of 1891. Thousands of cast iron rail under-bridges were eventually replaced by steel equivalents. 2.2.6 Structural use Cast iron has been used for centuries, and was used occasionally in architecture in the pre-modern period. It was in eighteenth century Britain that new production methods first allowed cast iron to be produced cheaply enough and in large enough quantities to regularly be used in large building projects. One of the first important projects was The Iron Bridge in Shropshire, a precedent setting structure made almost entirely of cast iron. However, it was grossly over-designed, and the makers (principally Abraham Darby) suffered financially as a result. The quality of the iron used in the bridge is not high, and nearly 80 brittle cracks are visible in the present structure. Later designers and engineers, such as Thomas Telford improved both the design and quality of the material in his bridges (for example, at Buildwas upstream of Coalbrookdale) and aqueducts (such as the world-famous Pontcysyllte Aqueduct in North Wales. Cast iron has some architectural advantages, and some weaknesses. It is strong in compression and its strength and stiffness deteriorate when subjected to the high heat of a fire. In the early era of the INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 17 industrial revolution it became popular to use cast iron in factory construction, in part owing to the misconception that such structures would be "fireproof." Cast iron was strong enough to support the heavy machinery but was vulnerable to the frequent fires that would occur in such factories. William Strutt pioneered this innovation, building a number of industrial buildings using cast iron supports. However, there were numerous building collapses caused by brittle fracture of large cast iron beams, where the bottom side of the beam was in tension, often from defects such as blow holes within the beams. Internal defects like blowholes were common in large beams. Cast iron was also used widely in bridge construction for the new railway system, sometimes with horrific results, especially when cast iron girders were used instead of arches. The first use was at the Water Street terminus of the Liverpool and Manchester Railway in 1830 to a design by William Fairbairn, a successful design which was demolished about 1900 owing to the widespread concern about cast iron under bridges on the rail network in Britain. To build longer spans, wrought iron trusses were added to strengthen the structure by Robert Stephenson over the river Dee. Such trussed bridges had to be demolished following the Dee bridge disaster of 1847. This led to the use of wrought iron composite beams formed by riveting sheets together, and then steel rolled beams when steel became available in the late 1860s and 1870s. Cast iron continued, however, to be used in railway under bridges, and there were a number of serious failures involving loss of life. The most serious accident occurred in 1879 with the Tay Bridge disaster when the centre part of the bridge collapsed in a storm as an express train was passing over. The whole train was lost with more than 75 passengers and crew. The weakest parts of the bridge were cast iron lugs holding tie bars in place, and cast iron in new bridges was effectively abandoned after the disaster. Most small INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 18 cast iron beam structures were demolished and replaced after the Norwood Junction rail accident of 1891. 2.3 Steel Generally, carbon is the most important commercial steel alloy. Increasing carbon content increases hardness and strength and improves hardenability. But carbon also increases brittleness and reduces weldability because of its tendency to form martensite. This means carbon content can be both a blessing and a curse when it comes to commercial steel. And while there are steels that have up to 2 % carbon content, they are the exception. Most steel contains less than 0.35 % carbon. To put this in perspective, keep in mind that’s 35/100 of 1%. Now, any steel in the 0.35 to 1.86 % carbon content range can be hardened using a heat-quench-temper cycle. Most commercial steels are classified into one of three groups (Bob Capudean, 2003): 1. Plain carbon steels 2. Low-alloy steels 3. High-alloy steels Plain Carbon Steels These steels usually are iron with less than 1 percent carbon, plus small amounts of manganese, phosphorus, sulfur, and silicon. The weldability and other characteristics of these steels are primarily a product of carbon content, although the alloying and residual elements do have a minor influence. Plain carbon steels are further subdivided into four groups: 1. Low INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 19 2. Medium 3. High 4. Very high Low. Often called mild steels, low-carbon steels have less than 0.30 % carbon and are the most commonly used grades. They machine and weld nicely and are more ductile than higher-carbon steels. Medium. Medium-carbon steels have from 0.30 to 0.45% carbon. Increased carbon means increased hardness and tensile strength, decreased ductility, and more difficult machining. High. With 0.45 to 0.75% carbon, these steels can be challenging to weld. Preheating, post heating (to control cooling rate), and sometimes even heating during welding become necessary to produce acceptable welds and to control the mechanical properties of the steel after welding. Very High. With up to 1.50 % carbon content, very high-carbon steels are used for hard steel products such as metal cutting tools and truck springs. Like high-carbon steels, they require heat treating before, during, and after welding to maintain their mechanical properties. Low-alloy Steels When these steels are designed for welded applications, their carbon content is usually below 0.25 % and often below 0.15 %. Typical alloys include nickel, chromium, molybdenum, manganese, and silicon, which add strength at room temperatures and increase low-temperature notch toughness. These alloys can, in the right combination, improve corrosion resistance and influence the steel’s response to heat treatment. But the alloys added can also negatively influence crack susceptibility, so INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 20 it’s a good idea to use low-hydrogen welding processes with them. Preheating might also prove necessary. This can be determined by using the carbon equivalent formula, which we’ll cover in a later issue. High-alloy Steels For the most part, we’re talking about stainless steel here, the most important commercial high-alloy steel. Stainless steels are at least 12 % chromium and many have high nickel contents. The three basic types of stainless are: 1. Austenitic 2. Ferritic 3. Martens tic Martens tic stainless steels make up the cutlery grades. They have the least amount of chromium, offer high harden ability, and require both preand post heating when welding to prevent cracking in the heat-affected zone (HAZ). Ferritic stainless steels have 12 to 27 % chromium with small amounts of austenite-forming alloys. Austenitic stainless steels offer excellent weld ability, but austenite isn’t stable at room temperature. Consequently, specific alloys must be added to stabilize austenite. The most important austenite stabilizer is nickel, and others include carbon, manganese, and nitrogen. 2.3.1 Steel Classification Systems Before we look at a couple of common steel classification systems, let’s consider one more high-carbon metal, cast iron. The carbon content of cast iron is 2.1% or more. There are four basic types of cast iron: INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 27 Considering the sketch of a water droplet, the oxidizing iron supplies electrons at the edge of the droplet to reduce oxygen from the air. The iron surface inside the droplet acts as the anode for the process(Ebbing,1990) Fe(s) -> + Fe2+(aq) + 2eThe electrons can move through the metallic iron to the outside of the droplet where O2(g) + 2H2O(l) + 4e- -> 4OH-(aq) Within the droplet, the hydroxide ions can move inward to react with the iron(II) ions moving from the oxidation region. Iron(II) hydroxide is precipitated. Fe2+(aq) + 2OH-(aq) -> Fe(OH)2(s) Rust is then quickly produced by the oxidation of the precipitate. 4Fe(OH)2(s) + O2(g) -> 2Fe2O3 •H2O(s) + 2H2O(l) The rusting of unprotected iron in the presence of air and water is then inevitable because it is driven by an electrochemical process. 3.1.3 Types of Corrosion 3.1.3.1 Uniform corrosion Uniform attack appears as an even layer of rust on the steel surface. This is the most common form of corrosion that is found in perimeter steel of masonry clad steel framed buildings. Uniform corrosion (see Figure 7) is generally due to electrochemical reactions, which occur from the presence of oxygen and moisture. Under certain conditions the water or moisture that is in contact with the steel, may have extremely low values of pH due to acidic pollutants from rainfall. In fact acid rain pH surveys have shown that acid precipitation at a pH of INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 28 2 is not uncommon, especially during the initial period of snow or rain. Carbonation, another contributor to corrosion, is a process by which carbon dioxide enters into the masonry and reacts with the steel. Carbon dioxide combines with the pore water in the mortar to form carbonic acid, which reduces the pH of the mortar to approximately 8 or 9. At these levels the protective oxide film is no longer stable and with adequate supply of oxygen and moisture corrosion will start. The penetration of masonry by carbonations is a slow process, the rate of which is determined by the porosity and permeability of the mortar. 3.1.3.2 Pitting corrosion Pitting corrosion (see Figure 8), or pitting, is a form of extremely localized corrosion that leads to the creation of small holes in the metal. The driving power for pitting corrosion is the lack of oxygen around a small area. This area becomes anodic while the area with excess of oxygen becomes cathodic, leading to very localized galvanic corrosion. The corrosion penetrates the mass of the metal, with limited diffusion of ions, further pronouncing the localized lack of oxygen. The mechanism of pitting corrosion is probably the same as crevice corrosion. The mechanism of pitting corrosion is supposed by some that gravitation causes downward-oriented concentration gradient of the dissolved ions in the hole caused by the corrosion, as the concentrated solution is denser. This however is unlikely. The more conventional explanation is that the acidity inside the pit is maintained by the spatial separation of the cathodic and anodic half-reactions, which creates a potential gradient and electro migration of aggressive anions into the pit. This kind of corrosion is extremely insidious, as it causes little loss of material with small effect on its surface, while it damages the deep INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 29 structures of the metal. The pits on the surface are often obscured by corrosion products. Pitting can be initiated by a small surface defect, being a scratch or a local change in composition, or damage to protective coating. Polished surfaces display higher resistance to pitting. 3.1.3.3 Crevice or concentration cell corrosion The process is similar to pitting corrosion except that it occurs in the very narrow gaps between faying surfaces where the solution concentrates by capillary or other action. Some moisture evaporates and the remaining solution trapped in the crevice becomes stagnant and increasingly corrosive. Crevice corrosion is often brought about by poor sealing/protection practices when applying/rinsing off acid etches cleaning solutions prior to painting. If dust containing salt is allowed to remain trapped in joints hygroscopic action will eventuate in a corrosion cell. Water staining found between bundled and stored aluminum sheets is a form of crevice corrosion. 3.1.3.4 Galvanic corrosion Galvanic corrosion (see Figure 9) is an electrochemical process in which one metal corrodes preferentially when in electrical contact with a different type of metal and both metals are immersed in an electrolyte. Conversely, a galvanic reaction is exploited in primary batteries to generate a voltage. A common example is the carbon-zinc cell where the zinc corrodes preferentially to produce a current. The lemon battery is another simple example of how dissimilar metals react to produce an electric current. When two or more different sorts of metal come into contact in the presence of an electrolyte a galvanic couple is set up as different INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 30 metals have different electrode potentials. The electrolyte provides a means for ion migration whereby metallic ions can move from the anode to the cathode. This leads to the anodic metal corroding more quickly than it otherwise would; the corrosion of the cathodic metal is retarded even to the point of stopping. The presence of electrolyte and a conducting path between the metals may cause corrosion where otherwise neither metal alone would have corroded. Even a single type of metal may corrode galvanic ally if the electrolyte varies in composition, forming a concentration cell. A common example of galvanic corrosion is the rusting of corrugated iron sheet, which becomes widespread when the protective zinc coating is broken and the underlying steel is attacked. The zinc is attacked preferentially because it is less noble, but when consumed, rusting will occur in earnest. With a tin can, the opposite is true because the tin is nobler than the underlying steel, so when the coating is broken, the steel is attacked preferentially. A rather more spectacular example occurred in the Statue of Liberty when regular maintenance in the 1990s showed that galvanic corrosion had taken place between the outer copper skin and the wrought iron support structure. Although the problem had been anticipated when the structure was built by Gustave Eiffel to Frédéric Bartholdi's design in the 1880s, the insulation of shellac between the two metals failed over a period of time and resulted in rusting of the iron supports. The renovation replaced the original insulation with PTFE (Polytetrafluoroethylene). 3.1.3.5 Other corrosion forms Intergranular corrosion (see Figure 10): attacks the grain INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 31 boundaries within the metal structure when the chemical differences between grains and the grain boundaries react with each other in the presence of an electrolytic solution. Usually occurs in areas where end grain is exposed, rivet holes for example. Exfoliation corrosion, where grain layers are lifted away, is an advanced form. Stress corrosion cracking: occurs when high internal tensile stresses induced in the metal during manufacture are allowed to remain i.e. the metal is not properly stress relieved. A high permanent stress can also be introduced during structure assembly if any of the welding or fastening processes are incorrectly performed. If this residual stress condition or a stress condition introduced during normal operation exists in association with pitting or other corrosion multi-branched internal cracking may occur within the metal. Filiform corrosion: occurs in high relative humidity conditions under paint coatings such as polyurethane finishes usually because contamination traces were not completely removed from the substrate or a treatment process was not allowed to fully complete before applying the next coating. Fretting or friction corrosion occurs when vibration or in-flight loads causes oscillatory movement between faying surfaces. A form of chemical corrosion will occur if incompatible surface coatings come in contact; for example methyl ethyl ketone [used as a solvent in a number of fabric cements] will attack zinc chromate metal primers leaving the metal open to corrosion under the fabric. Also organic acids are present in bird and animal wastes so keep mice out of the airframe (and by the way you've no idea the damage that mouse urine can do to electronics); and if you operate from a INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 32 cattle paddock clean off any 'meadow mayonnaise' that accumulates. Figure 7 Uniform corrosion Figure 8 Pitting corrosion Figure 9 Galvanic corrosion Figure 10 Intergranular corrosion 3.2 Mechanical breakdown Metal architectural elements can also fail from purely physical causes such as abrasion or a combination of physical and chemical attack, such as weathering and stress corrosion cracking. 3.2.1 Abrasion Abrasion is the erosion of the metal caused by the impact of dust, san grit, sleet, rain and hail, or by rubbing with another architectural or human element. Abrasives can encourage corrosion by removing the protective corrosion deposits from the metal surface. Abrasion is a critical problem with metal flashings and valleys used on slate roofs. As the slate deteriorates, particles break off and are washed down the valleys, causing erosion. 3.2.2 Fatigue Fatigue is failure of a metal by the repeated application of cyclic stresses below the elastic limit-the greatest stress a material can withstand without permanent deformation after removal of the load. It results from a gradual or progressive fracture of the crystals. It has INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 33 been estimated that approximately 90% of the structural failure of metal railroad bridges are fatigue failures which develop late in the life of the structure. 3.2.3 Creep Creep is the continuous flow or plastic deformation of soft metals under sustained stressing, sometimes under relatively high temperatures. It tends to distort the metal, causing thin areas that may failure and a lack of stain hardening. The stressing may be related to thermal expansion and contraction and the weight of the metal. 3.2.4 Fire Fire can cause unprotected metal framing members to become plastic and fail rapidly. Structural iron or steel that has survived a fire without deformation is usually safe to reuse, but any questionable member should be load tested 3.2.5 Overloading Overloading is the stressing of a metal member beyond its yield point so that permanent deformation, fracturing, or failure occurs. The member can fail through the application of static loads, dynamic loads, thermal stresses and settlement stresses either singly or combination. Buckling is a form of permanent deformation from overloading which is usually caused by excessive weight but can also be caused by thermal stresses. Members can also be overloaded if their support is removed and loads are redistributed to other members which can become overstressed and deformed. 3.3 Weathering A metal architectural element subjected to the weather is exposed INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 34 to various chemical and physical agents at one time. The result is a kind of synergism where the total effect is greater than the sum of the individual effects. For instance, the rate of corrosion accelerates with increases of temperature, humidity and surface deposits of salts, dirt and pollution. 3.4 Connection Failure The failure of the connections of metal architectural elements, especially structural members, can also be caused by a combination of physical and/or chemical agents. The most common type of connections used for metal structural elements of historic buildings including bolting, riveting, pinning and welding. These connections can fail through the overloading, fatiguing or corrosion of the connectors. Common examples of this type of failure include the corrosion, usually by the galvanic effects, of nails, bolt heads, rivets and areas covered by fastening plating. The effective cross-sectional area of the connections is often reduced by corrosion, making the connectors more susceptible to stress failure. INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 35 4. Methods for repair and rehabilitation of metal Common problems encountered today with metal construction include badly rusted or missing elements, impact damage, structural failures, broken joints, damage to connections, and loss of anchorage in masonry. Oxidation, or rusting, occurs rapidly when metal is exposed to moisture and air. The minimum relative humidity necessary to promote rusting is 65%, but this figure can be lower in the presence of corrosive agents, such as sea water, salt air, acids, acid precipitation, soils, and some sulfur compounds present in the atmosphere, which act as catalysts in the oxidation process. Rusting is accelerated in situations where architectural details provide pockets or crevices to trap and hold liquid corrosive agents. Furthermore, once a rust film forms, its porous surface acts as a reservoir for liquids, which in turn causes further corrosion. If this process is not arrested, it will continue until the iron is entirely consumed by corrosion, leaving nothing but rust (Margot Gayle David and John G. Waite, 1998). When the historical structures need to restore or renovate, the question arises of the whether damaged or broken elements can or should be repaired. The restoration techniques are directly linked to both the types of material used and the question of whether the intention is to simply to restore the appearance of the structure or to ensure that actual forces can be accommodated. This chapter will indicate the methods that can be considered for use on cast iron, wrought iron and steel. If it is necessary to replace elements attention will be also be paid to the availability of historic types of iron and steel and to when modern materials be used(G.G Nieuwmeijer & G.J. Arends, 2003). INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 36 When a structure that has a high value of history is only slightly damaged it is usually possible to return it to its original condition and thus to retain a consistent appearance and to show how it was constructed. However, when the damage is more serious some elements will have to be replaced. Depending on the historic and cultural value and the budget, either the original materials and construction methods will be used or it will be necessary to resort or the use of modern materials and modes of connection. However, the new construction must be blend with the old with regard to size, scale and appearance. 4.1. Repairs 4.1.1 Repair techniques used for cast iron The most important methods of repair are welding, using dowels, metal stitching and using adhesives. The degree to which forces can be transferred depends on the method used. 4.1.1.1 Welding Owing to the high carbon content it is different although not impossible to weld cast iron. This method requires great skill. A distinction must be made between hot and cold-welding. With hot welding, the cast iron must be slowly preheated up to 450℃-600℃. An electrode with a cast iron or carbon rich steel core must be used for the welding and to avoid stresses the welded element must be allowed to cool slowly. The bigger the cast iron element, the more difficult it is to weld it. Preferably the preheating must be done in a furnace but it may also be done in situ. For cold welding the cast iron element is either cold or only slightly preheated. The electrode used must have a different composition from that of the parent material and is usually rich in nickel. The weld INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 43 4.3.4 Glass fiber-reinforced concrete Glass fiber-reinforced concrete, known as GFRC, is similar to fiberglass except that a lightweight concrete is substituted for the resin. GFRC elements are generally fabricated as thin shell panels by spraying concrete into forms. Usually a separate framing and anchorage system is required. GFRC elements are lightweight, inexpensive, and weather resistant. Because GFRC has a low shrinkage coefficient, molds can be made directly from historic elements. However, GFRC is very different physically and chemically from iron. If used adjacent to iron, it causes corrosion of the iron and will have a different moisture absorption rate. Also, it is not possible to achieve the crisp detail that is characteristic of cast iron. 4.3.5 Titanium Titanium may also be connected directly with metals and alloys such as stainless steels which are galvanically compatible when in their passive condition in a specific environment. These alloys may become activated by local corrosion or pitting but the added effect of coupling to titanium is small. The primary consideration must be to ensure that the alloy selected is appropriate for the service environment. Figure 13 Modern use of titanium to replace steel strengthening INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 44 4.4 Cathodic Protection techniques 4.4.1 History Traditional methods of repair for masonry clad steel framed buildings, which consist of removing the masonry, treating the steel with a protective coating or paint, providing new mortar encasement and installing new masonry might be both an expensive and impractical option. Cathodic protection (CP), a corrosion control method which was presented by Sir Humphrey Davy in 1824 as a series pf papers to the Royal Society describing how CP could be used to prevent the corrosion of copper sheathing in the wooden hulls of British naval vessels. Since then it has been applied to many other areas, including marine application and for the preservation of buried underground structures such as pipelines and tanks. In 1991 the first cathodic protection system for stone clad steel framing was installed by Taywoods Engineering on the entrance colonnade of Dublin’s 1019 College pf Science, now the Irish Prime Minister’s residence. In 1995, English Heritage applied cathodic protection to corroded ferrous cramps of the Inigo Jones gateway, which stands in the parkland surround Chiswick Housein London, England. Now this same technology is being applied to steel-frame structures in the UK and North America. 4.4.2 Theoretical principles Cathodic protection prevents corrosion by converting all of the anodic (active) sites on the metal surface to cathodic (passive) sites by supplying electrical current (or free electrons) from an alternate source. Usually this takes the form of galvanic anodes which are more active INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 45 than steel. This practice is also referred to as a sacrificial system, since the galvanic anodes sacrifice themselves to protect the structural steel or pipeline from corrosion. In the case of aluminum anodes, the reaction at the aluminum surface is: 4Al => 4AL+++ + 12 e- (four aluminum ions plus twelve free electrons) and at the steel surface, 3O2 + 12e- + 6H20 => 12OH- (oxygen gas converted to oxygen ions which combine with water to form hydroxyl ions) As long as the current (free electrons) is arriving at the cathode (steel) faster than oxygen is arriving, no corrosion will occur. 4.4.3 CP system types Cathodic protection may be achieved by two somewhat different techniques, although the principles described above hold true for both. These are sacrificial anode cathodic protection and impressed current cathodic protection. Sacrificial Anode Cathodic Protection Sacrificial anode cathodic protection (SACP) system uses sacrificial anodes (made of zinc, aluminum, magnesium or alloys of these) which corrode preferentially to the metalwork. These are placed in close proximity to the corroding metalwork and are electrically connected to it. As the sacrificial anode corrodes it generates a current that passes through the building material to provide INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 46 protection to the embedded metal work. The current is conducted ionically though the building material by means of pore water contained within it. Impressed Current Cathodic Protection Impressed current cathodic protection (ICCP) systems are commonly used where there are increased current requirements for corrosion protection, where the driving voltage is greater than what can be obtained with galvanic systems and where there may be a need for detailed control of the system. ICCP systems are extremely accurate and can be controlled to deliver optimal protective current to the structure. Anodes are connected to a DC power supply to deliver low-voltage electric currents and are controlled by strategically placed reference electrodes. These reference electrodes are connected to a monitoring system that observes and verifies the system performance. Whether applied to reinforced concrete, steel frame structures or other structures subject to corrosion, CP provides all the products and engineering support needed to design optimal ICCP systems. Figure 14 Schematic of SACP and ICCP Systems INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 47 As the ICCP systems forces the metal to become cathodic, and therefore protected, the other side of the CP circuit becomes anodic and therefore more corrosive. Materials for these current emitters (or anodes) therefore have to be highly resistance to corrosion. A variety of commercial anodes are available for use with ICCP systems: Carbon-loaded coatings can be used over shout periods although their life is normally restricted to 10 to 15 years. For extended use, embedded titanium anodes, coated with precious metals or conductive ceramic coatings, are normally used. Conductive coating systems are either based on thermally sprayed zinc or conductive paint. The organic-based conductive paint is generally water-based and loaded with graphite to provide a current pathway. Conductive, connections, surface-applied overlays are also available and these normally contain carbon fibers and/or graphite in sufficient quantities to provide a continuous conductive pathway for current. The resulting coating is lightweight and has mechanical and thermal properties similar to that of concrete. However, coatings and overlays are not normally used on heritage structures due to aesthetic constraints. For titanium-based anodes: titanium on its own is unsuitable as it readily oxidizes in the air and especially when acting as an anode. It builds up an electrically resistive barrier. Platinum, or a conductive ceramic mixed metal oxide (MMO) coating is therefore applied to substrates to give a low resistance layer to emit the current. Platinum is expensive but the MMO coatings are cheaper and those coated anodes are now available as mesh, ribbon or wire. These are suitable for supplying high currents (up to 110mA/m2) and have a reported lifetime on excess of 75 years. Mesh is typically used to provide INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 48 protection to large areas of reinforcement in concrete, whereas ribbon and wire may be used in the joints between stones and bricks to provide protection to horizontal and vertical iron and steel members. However, this protection may not be provided in the few cases where in very strong reducing acid conditions (e.g. in concentrated sulphuric and hydrochloric acids and hydrofluoric acid) the oxide film on titanium is attacked and cannot be maintained or restored. Stand-alone (or discrete) anodes, comprising MMO coated titanium rods, are also available. These are also suitable for supplying large currents (800mA/ m2) and have an estimated life of 300 years. There are often more expensive than the surface mounted MMO coated anodes. The discrete anodes may be linked together using titanium wire and are commonly used where deeply embedded metalwork is to be protected or where surface-mounted systems are inappropriate. The discrete anodes are placed in pre-drilled holds on either side of the embedded metalwork using either a carbon loaded backfill or cementicious grout to fix them in. The titanium connection wires are hidden in the joints and repointed. The design of a CP system should also take into account many other factors including: 1. The surface area of iron or steel to be protected. 2. The resistivity of the masonry or stonework. 3. The distance and uniformity of the anodes to the embedded metalwork. 4. The service lifetime of the CP systems. 5. The aesthetic of the building. INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 49 5. Case Study 5.1 Buried metal cramp conservation in the Inigo Jones Gateway, Chiswick House grounds,London(Keith Blackney and Bill Martin) The Inigo Jones Gateway which is in the parkland surrounding Chiswick House, and English Heritage property in London, has been at least three different restoration attempts. This stone gateway was designed by Jones in 1621 and the flanking walls were most probably added in 1738. Figure 15 The Inigo Jones gateway By 1990 it was considered to be in such poor condition that action needed to be taken to address the main causes and symptoms of deterioration, something which the previous interventions had INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 50 signally failed to achieve. These interventions had only served to compound the decay mechanisms. One of the most serious threats to the stability of structure, in common with so many historic buildings, was the corrosion of metallic cramps embedded in the masonry. The accepted practice of cutting out or dismantling to remove such fixings was not considered appropriate and instead a method of nondestructive electrochemical rehabilitation, ICCP, was explored and developed for use on the structure. 5.1.1 Survey In 1990 a survey was commission by English Heritage to assess the condition of the gateway and to provide suggestions for remedial treatment. The survey identified three main forms of decay affect the Bath stone structure. 1. The decayed surface of the Bath Stone gateway. The build-up of dirt and the deposited material on the wall had resulted in the formation of skins of calcium sulphate, the product of the reaction of the calcium carbonate of the Bath stone with the sulphur dioxide and trioxide in the urban atmosphere. 2. Erosion of the stones. Hard repair mortars had been used in areas of previously deteriorated work. Because of the impermeability of these repairs and the difference in density between them and the surrounding stone, the soluble salts present in the stone at the time of treatment, were either entrapped behind the patches, where they acted on the underlying stone to produce zones of deeply-seated erosion, or they acted on the stone surfaces at the edges of the repairs. 3. Iron cramps rusting. Impulse radar (IR) was selected to be the survey method. IR INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 51 indicated that not only the location of metal fixing but it would also give a good indication of their condition and effect on surrounding masonry. The Inigo Jones gateway was divided into horizontal survey lines along joints between ashlar and where possible through the center of each stone. The results of using of IR were as following: - precise dimensional relationships of the structure - a range of fixings, cramps and dowels relationship of core to block formation - locations and conditions of cramps - type and orientation of cramps - cramps depths were also given - non-metallic fixings and areas of non-metallic wire armature - Micro-cracks in the masonry - block depths 5.1.2 Repair strategy 1. ICCP installation English Heritage considered that in principle it would be possible to cathodically protect wrought iron, the main metallic material for cramps and dowels, and that brick and limestone would carry sufficient moisture to enable the even distribution of protection current. Also a system of keyhole surgery was advised to make the cable-to-iron connection. The criteria for this system were: - To be electrically can mechanically sound - To connect a small diameter wire to an embedded cramp at INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 52 230mm depth using a working envelope of a 10mm diameter hole. - To be suitable for use (even if some further development were needed) during a large ICCP installation. After a site testing, the redesign used information gained from the various site surveys which suggested that cracking and damage to the stonework was localized mainly, not unexpectedly, around the cramps located close to the surface. Five cramps on the south elevation and nine on the north elevation were selected to take the ICCP treatment. Monitoring would be achieved by placing five reference electrodes at selected locations. Two-pieces anodes would be placed on either side of the cramps. The DC negative and positive connections would be in a ring main configuration with cramp and anode connections being insulated with double layer mastic filled heat-shrink sleeving. Individual reference electrode cables were taken back to the monitoring panel on the transformer rectifier, sited in the house. 2. Repair to deteriorated render and repairs to stone works. This part included replacements of a small number of selected stones and rebuilding of the previously collapsed section beneath the pediment and the repointing od the holes and chases resulting from the ICCP installation. 3. Cleaning, consolidation and sheltercoating of the stone surfaces. The cleaning of the stone and render surfaces was accomplished using a Microparticle system from Jackson and Cox Ltd. This is a dry air-abrasive method using aluminum oxide particles with a range of 17-20 microns. The blasting media is INVESTIGATION ON THE USE OF IRON AND STEEL FOR RESTORATION PURPOSES DURING 19TH AND 20TH CENTURY Erasmus Mundus Programme ADVANCED MASTERS IN STRUCTURAL ANALYSIS OF MONUMENTS AND HISTORICAL CONSTRUCTIONS 59 · Margot Gayle David and John G. Waite, Metals in america's historic buildings,1998 · Recognizing Metals and their Corrosion Products,Bart Ankersmit, Martina Griesser-Stermscheg, Lyndsie Selwyn, and Susanne Sutherland, 2008 · Steven F. Daily, Steven D., Somerville, P.E., Using Cathodic Protection to Control Corrosion of Masonry Clad Steel Framed Buildings, Corrpro Companies, Inc., 2007 · The Ohio Department of Natural Resource Division of Water, Dam Safety: Problems with Metal Materials,2001