The work described here was in part financed by the Swiss Federal Roads Office (research project AGB2012/010). We greatly acknowledge the financial support.

<ol>
	<li>Bertolini, L., Elsener, B., Pedeferri, P., Redaelli, E., Polder, R. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Corrosion of Steel in Concrete: Prevention, Diagnosis, Repair. 2nd edn. , (2013).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Development of an holistic approach to ensure the durability of new concrete construction. , (1997).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Condition+control+and+assessment+of+reinforced+concrete+structures+exposed+to+corrosive+environments.">Condition control and assessment of reinforced concrete structures exposed to corrosive environments.</a> F&#233;d&#233;ration Internationale du B&#233;ton (fib). , (2011).</li><li>Angst, U. 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Res. 39 (12), 1122-1138 (2009).</li><li>Breit, W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Zum+Ansatz+eines+kritischen+Chloridgehaltes+bei+Stahlbetonbauwerken.">Zum Ansatz eines kritischen Chloridgehaltes bei Stahlbetonbauwerken.</a> Beton- und Stahlbetonbau. (5), 290-298 (2011).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=European+Standard+EN14629:+Products+and+systems+for+the+protection+and+repair+of+concrete+structures+-+Test+methods+-+Determination+of+chloride+content+in+hardened+concrete.">European Standard EN14629: Products and systems for the protection and repair of concrete structures - Test methods - Determination of chloride content in hardened concrete.</a> European Committee for Standardization. , (2007).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=ASTM+C1152+Standard+Test+Method+for+Acid-Soluble+Chloride+in+Mortar+and+Concrete.">ASTM C1152 Standard Test Method for Acid-Soluble Chloride in Mortar and Concrete.</a> ASTM International. , (2012).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=SIA+269/2:2011+Erhaltung+von+Tragwerken+-+Betonbau.">SIA 269/2:2011 Erhaltung von Tragwerken - Betonbau.</a> Schweizerischer Ingenieur- und Architektenverein SIA. , (2011).</li><li>Draft, <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Draft+recommendation+for+repair+strategies+for+concrete+structures+damaged+by+reinforcement+corrosion.">Draft recommendation for repair strategies for concrete structures damaged by reinforcement corrosion.</a> Mater Struct. 27, 415-436 (1994).</li><li>Broomfield, J. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Corrosion of Steel in Concrete: Understanding, Investigation and Repair. 2nd edn. , (2006).</li><li>Richartz, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Die+Bindung+von+Chlorid+bei+der+Zementerh&#228;rtung.">Die Bindung von Chlorid bei der Zementerh&#228;rtung.</a> Zement-Kalk-Gips. 10, 447-456 (1969).</li><li>Vassie, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Reinforcement+corrosion+and+the+durability+of+concrete+bridges.">Reinforcement corrosion and the durability of concrete bridges.</a> Proc. Inst. Civ. Eng. Part 1. 76, 713-723 (1984).</li><li>Angst, U. 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Corros. 63 (12), 1147-1153 (2012).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Standard+Test+Method+for+Corrosion+Potentials+of+Uncoated+Reinforcing+Steel+in+Concrete.">Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steel in Concrete.</a> ASTM International. , (2015).</li><li>Planung, <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=SIA+2006:2013+Planung+Durchf&#252;hrung+und+Interpretation+der+Potenzialmessung+an+Stahlbetonbauten.">SIA 2006:2013 Planung Durchf&#252;hrung und Interpretation der Potenzialmessung an Stahlbetonbauten.</a> Schweizerischer Ingenieur- und Architektenverein SIA. , (2013).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=B3:+Merkblatt+f&#252;r+Elektrochemische+Potentialmessungen+zur+Detektion+von+Bewehrungsstahlkorrosion.">B3: Merkblatt f&#252;r Elektrochemische Potentialmessungen zur Detektion von Bewehrungsstahlkorrosion.</a> Deutsche Gesellschaft f&#252;r Zerst&#246;rungsfreie Pr&#252;fung DGZfP. , (2014).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=B2:+Merkblatt+zur+zerst&#246;rungsfreien+Betondeckungsmessung+und+Bewehrungsortung+an+Stahl-+und+Spannbetonbauteilen.">B2: Merkblatt zur zerst&#246;rungsfreien Betondeckungsmessung und Bewehrungsortung an Stahl- und Spannbetonbauteilen.</a> Deutsche Gesellschaft f&#252;r Zerst&#246;rungsfreie Pr&#252;fung DGZfP. , (2014).</li><li>Soylev, T. A., Fran&#231;ois, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quality+of+steel-concrete+interface+and+corrosion+of+reinforcing+steel.">Quality of steel-concrete interface and corrosion of reinforcing steel.</a> Cem. Concr. Res. 33 (9), 1407-1415 (2003).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=European+Standard+EN+12504-1:2009+-+Testing+concrete+in+structures.+Cored+specimens.+Taking,+examining+and+testing+in+compression.">European Standard EN 12504-1:2009 - Testing concrete in structures. Cored specimens. Taking, examining and testing in compression.</a> European Committee for Standardization. , (2009).</li><li>Andrade, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Test+methods+for+on-site+corrosion+rate+measurement+of+steel+reinforcement+in+concrete+by+means+of+the+polarization+resistance+method.">Test methods for on-site corrosion rate measurement of steel reinforcement in concrete by means of the polarization resistance method.</a> Mater Struct. 37 (273), 623-643 (2004).</li><li>Angst, U., Vennesland, &. #. 2. 1. 6. ;., Myrdal, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Diffusion+potentials+as+source+of+error+in+electrochemical+measurements+in+concrete.">Diffusion potentials as source of error in electrochemical measurements in concrete.</a> Mater Struct. 42 (3), 365-375 (2009).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=European+Standard+EN+14630:+Products+and+systems+for+the+protection+and+repair+of+concrete+structures+-+Test+methods+-+Determination+of+carbonation+depth+in+hardened+concrete+by+the+phenolphthalein+method.">European Standard EN 14630: Products and systems for the protection and repair of concrete structures - Test methods - Determination of carbonation depth in hardened concrete by the phenolphthalein method.</a> European Committee for Standardization. , (2006).</li><li>Gulikers, J., Andrade, C., Kropp, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Testing+and+Modelling+the+Chloride+Ingress+into+Concrete.">Testing and Modelling the Chloride Ingress into Concrete.</a> RILEM Proceedings PRO 19. , (2000).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=ASTM+C1084+Standard+Test+Method+for+Portland-Cement+Content+of+Hardened+Hydraulic-Cement+Concrete.">ASTM C1084 Standard Test Method for Portland-Cement Content of Hardened Hydraulic-Cement Concrete.</a> ASTM International. , (2013).</li><li>Boschmann K&#228;thler, C., Angst, U. M., Wagner, M., Elsener, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Image+analysis+for+determination+of+cement+content+in+concrete+to+improve+accuracy+of+chloride+analyses.">Image analysis for determination of cement content in concrete to improve accuracy of chloride analyses.</a> Cem Concr Res. , (2017).</li><li>Angst, U., Wagner, M., Elsener, B., Leemann, A., Nygaard, P. v. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Method+to+determine+the+critical+chloride+content+of+existing+reinforced+structures.">Method to determine the critical chloride content of existing reinforced structures.</a> VSS report no. 677. , (2016).</li><li>Lambert, P., Page, C. L., Vassie, P. R. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Investigations+of+reinforcement+corrosion:+Part+2+-+Electrochemical+monitoring+of+steel+in+chloride-contaminated+concrete.">Investigations of reinforcement corrosion: Part 2 - Electrochemical monitoring of steel in chloride-contaminated concrete.</a> Mater. Struct. 24 (143), 351-358 (1991).</li><li>Angst, U. M., Elsener, B., Larsen, C. K., Vennesland, &. #. 2. 1. 6. ;. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chloride+induced+reinforcement+corrosion:+Electrochemical+monitoring+of+initiation+stage+and+chloride+threshold+values.">Chloride induced reinforcement corrosion: Electrochemical monitoring of initiation stage and chloride threshold values.</a> Corros. Sci. 53 (4), 1451-1464 (2011).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> RILEM technical committee 235-CTC. , (2015).</li><li>Angst, U., Elsener, B., Larsen, C. K., Vennesland, &. #. 2. 1. 6. ;. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chloride+induced+reinforcement+corrosion:+Rate+limiting+step+of+early+pitting+corrosion.">Chloride induced reinforcement corrosion: Rate limiting step of early pitting corrosion.</a> Electrochim Acta. 56 (17), 5877-5889 (2011).</li><li>Boubitsas, D., Tang, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+influence+of+reinforcement+steel+surface+condition+on+initiation+of+chloride+induced+corrosion.">The influence of reinforcement steel surface condition on initiation of chloride induced corrosion.</a> Mater Struct. 48 (8), 2641-2658 (2015).</li><li>Pacheco, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Corrosion+of+steel+in+cracked+concrete+-+chloride+microanalysis+and+service+life+predictions.">Corrosion of steel in cracked concrete - chloride microanalysis and service life predictions.</a> TU Delft. , (2015).</li><li>Li, L., Sag&#252;&#233;s, A. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chloride+corrosion+threshold+of+reinforcing+steel+in+alkaline+solutions+-+Effect+of+specimen+size.">Chloride corrosion threshold of reinforcing steel in alkaline solutions - Effect of specimen size.</a> Corros. 60 (2), 195-202 (2004).</li><li>Angst, U., R&#248;nnquist, A., Elsener, B., Larsen, C. K., Vennesland, &. #. 2. 1. 6. ;. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Probabilistic+considerations+on+the+effect+of+specimen+size+on+the+critical+chloride+content+in+reinforced+concrete.">Probabilistic considerations on the effect of specimen size on the critical chloride content in reinforced concrete.</a> Corros. Sci. 53 (1), 177-187 (2011).</li></ol>

The authors have nothing to disclose.

The most critical steps for the success of the suggested experimental protocol to determine Ccrit are those including the measures taken to prevent false corrosion initiation and other steel bar end effects. In this regard, a variety of approaches were tested, among which the here reported protocol was found to yield the best results28. In further tests, this approach permitted decreasing the rate of false initiation to below 10%. On the one hand, this is owing to coating the border area of the exposed concrete surface with epoxy resin, which increases the length of transport of chlorides through the concrete to the steel bar ends considerably. On the other hand, replacing the original concrete around the steel bar at its ends with a dense, highly alkaline cementitious slurry significantly enhances the corrosion resistance in these areas. Such systems, i.e., coating the steel bar ends with a layer of a polymer-modified cementitious material, have proven successful also in other studies29,30.
Another important aspect is the criterion for corrosion initiation. This criterion is based on RILEM technical committee TC-235 that aimed to recommend a test method for the measurement of Ccrit in samples manufactured in the laboratory31. The rationale is that it is well known that onset of corrosion of unpolarized steel embedded in concrete might take place over a long period of time rather than a well-defined instant30,32. Steel may start corroding at relatively low chloride concentrations but if these are not able to sustain the corrosion process, repassivation will occur, which becomes apparent by a potential increase back to the initial passive level. Such depassivation-repassivation events are typically observed in similar studies30,33,34. The chloride concentration measured at a time of stable corrosion is more relevant for practice than the time at which the very first signs of potential deviations from the passive level become apparent. With the suggested criterion, Ccrit represents the chloride concentration at which corrosion initiates and also stably propagates.
A limitation of the method is that the samples are relatively small, which may have an influence on the results35,36. In order to counteract this, it is suggested to use a relatively high number of samples (ideally 10). The level of confidence depends on the statistical distribution of Ccrit in the actual test area. For more details in this regard, refer to reference36. An additional limitation is that the moisture conditions in the laboratory exposure may differ from those of an actual structure. Finally, the detection of corrosion initiation may be difficult in cases where the potential is generally negative, such as in slag cements or other sulfide containing binders.
To the best of our knowledge, this is the first method of Ccrit determination in engineering structures at a stage prior to corrosion initiation. In contrast to empirical experience from structures, which is by definition obtained after corrosion initiation, this method can be used to measure Ccrit for specific structures or structural members before corrosion degradation occurs; the results can thus be used to assess the risk of (future) corrosion and to predict the remaining time to corrosion initiation (service life modeling). Thus, this method has the potential to be used in material testing, similar to established methods used for mechanical testing (compressive strength, etc.)
The method is currently applied to a number of different concrete infrastructures in Switzerland. This will broaden the severely limited5 knowledge about statistical distributions of Ccrit in structures. Moreover, it will reveal the influence of different factors such as the age of structures, the construction materials used, etc., and thus provide important information for civil engineers and for decision-makers in infrastructure management.

Corrosion of steel in concrete, triggered by the penetration of chlorides through the concrete, is the most frequent cause of the premature degradation of reinforced and pre-stressed concrete structures, and thus presents one of the most important challenges in civil engineering1,2,3,4. Industrialized countries typically have a large inventory of aging concrete infrastructures, built in the second half of the last century, and thus with a history of several decades of exposure to marine climate or deicing salts used on roads. Being able to reliably assess the condition of these structures, i.e., the risk for corrosion, forms the basis for planning maintenance work and for infrastructure management, in general.
The established approach in engineering for managing chloride-induced steel corrosion in concrete is based on a chloride threshold value (also termed critical chloride content, Ccrit)1,5,6. According to this concept, corrosion initiation is thought to occur as soon as the chloride concentration in the concrete at the steel surface exceeds the Ccrit threshold. Thus, assessing the condition of existing structures and estimating the remaining service life typically rely on determining the chloride content at different depths in the concrete, particularly at the depth of the embedded reinforcing steel. A number of reliable and standardized methods exist to measure this chloride concentration in samples of concrete7,8. Comparing the results to Ccrit provides the basis for the assessment of corrosion risk, and planning the type and the extent of repair measures. However, this approach requires knowledge of Ccrit.
Different international standards and recommendations, as well as text books, stipulate values for Ccrit1,3,9,10,11. These are typically around 0.4% chloride by weight of cement, based on long-term experience or early studies12,13. However, it is well known that the actual resistance against Ccrit of a certain structure or structural member is strongly influenced by the materials used, by the age of the structure, and by exposure history and conditions1,5. Thus, it is generally accepted that experience from one structure should only be applied to other structures with caution.
Despite this, it is common engineering practice to use tabulated Ccrit values, independent of the actual structure. This can be explained by the huge scatter of Ccrit in the literature and by the lack of reliable methods to determine Ccrit on-site and in the laboratory5. The approach of using tabulated threshold values in durability assessments is in contrast to structural considerations in condition assessments of aging concrete structures. In the latter case, there exist a number of standardized test methods to determine mechanical properties, such as the strength of the materials in the structure (concrete, reinforcing steel), to be used in the calculations of the structural behavior.
In this work, an experimental protocol to determine Ccrit on samples taken from engineering structures is presented. The approach is based on drilling cores of reinforced concrete in parts of concrete structures where corrosion has not yet initiated. These samples are transferred to the laboratory where they are subjected to an accelerated corrosion test in order to study the conditions for corrosion initiation. The main advantage of the proposed method is that the samples stem from structures and thus exhibit real conditions concerning a number of parameters that are well known to greatly influence Ccrit and which cannot be representatively mimicked in laboratory-produced samples. This includes the type and the age of the concrete (young laboratory concrete versus mature site-produced concrete), the type and surface condition of the reinforcing steel used at the time of construction, and in general the properties of the steel-concrete interface14. Together with the accuracy of laboratory measurement methods, this approach permits the reliable determination of Ccrit for specific structures or structural members.
Application of the suggested protocol in engineering practice will - compared with the common practice of using a constant value for Ccrit - enhance the accuracy of condition assessments and the predictive power of models to analyze the remaining service life. The expected strong increase in repair works of our built infrastructure over the coming decades15 poses an urgent need for such an improvement in engineering of corroding infrastructures.

<table width="651">
	<colgroup>
		<col />
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="42">
			<td height="42" style="width:216px;height:42px;">Stranded wire</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td style="width:245px;">cross section at least 0.50 mm&#178;; ideally copper wire, tin plated</td>
		</tr>
		<tr height="63">
			<td height="63" style="width:216px;height:63px;">Self-tapping metal screw</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td style="width:245px;">any suitable self-tapping screw, typically of length 4-5 mm and diameter around 2.5 mm</td>
		</tr>
		<tr height="42">
			<td height="42" style="width:216px;height:42px;">Ring cable lug</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td style="width:245px;">suitable to connect screw and cable</td>
		</tr>
		<tr height="63">
			<td height="63" style="width:216px;height:63px;">SikaTop Seal-107</td>
			<td style="width:71px;">Sika</td>
			<td style="width:119px;"></td>
			<td style="width:245px;">two-part polymer modified cementitious waterproof mortar slurry</td>
		</tr>
		<tr height="21">
			<td height="21" style="width:216px;height:21px;">Epoflex 816 L</td>
			<td style="width:71px;">Adisa</td>
			<td style="width:119px;"></td>
			<td style="width:245px;">epoxy coating</td>
		</tr>
		<tr height="63">
			<td height="63" style="width:216px;height:63px;">Exposure tank</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td style="width:245px;">any suitable tank (e.g. rako box) with a lid;&#160; sufficiently large for exposing the samples</td>
		</tr>
		<tr height="63">
			<td height="63" style="width:216px;height:63px;">Reference electrode</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td style="width:245px;">Any stable reference electrode suitable for continuous immersion in sodium chloride solution</td>
		</tr>
		<tr height="21">
			<td height="21" style="width:216px;height:21px;">Tap water</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td style="width:245px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="width:216px;height:21px;">Sodium chloride</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td style="width:245px;"></td>
		</tr>
		<tr height="105">
			<td height="105" style="width:216px;height:105px;">Data logger</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td style="width:245px;">any device able to monitor the potentials of all samples vs. the reference electrode at the specified interval (input impedance &#62;10E7 Ohm)</td>
		</tr>
	</tbody>
</table>

experimental protocol to determine the chloride threshold value for corrosion in samples taken from reinforced concrete structures

The aging of reinforced concrete infrastructure in developed countries imposes an urgent need for methods to reliably assess the condition of these structures. Corrosion of the embedded reinforcing steel is the most frequent cause for degradation. While it is well known that the ability of a structure to withstand corrosion depends strongly on factors such as the materials used or the age, it is common practice to rely on threshold values stipulated in standards or textbooks. These threshold values for corrosion initiation (Ccrit) are independent of the actual properties of a certain structure, which clearly limits the accuracy of condition assessments and service life predictions. The practice of using tabulated values can be traced to the lack of reliable methods to determine Ccrit on-site and in the laboratory.
Here, an experimental protocol to determine Ccrit for individual engineering structures or structural members is presented. A number of reinforced concrete samples are taken from structures and laboratory corrosion testing is performed. The main advantage of this method is that it ensures real conditions concerning parameters that are well known to greatly influence Ccrit, such as the steel-concrete interface, which cannot be representatively mimicked in laboratory-produced samples. At the same time, the accelerated corrosion test in the laboratory permits the reliable determination of Ccrit prior to corrosion initiation on the tested structure; this is a major advantage over all common condition assessment methods that only permit estimating the conditions for corrosion after initiation, i.e., when the structure is already damaged.
The protocol yields the statistical distribution of Ccrit for the tested structure. This serves as a basis for probabilistic prediction models for the remaining time to corrosion, which is needed for maintenance planning. This method can potentially be used in material testing of civil infrastructures, similar to established methods used for mechanical testing.

Figure 7 displays typical steel potentials monitored during chloride exposure in the laboratory. Both examples show that the potential may drop significantly within a very short time, but that the corrosion process may not yet stably propagate, which becomes apparent through an increase of the potential towards its initial passive level. In this protocol, the time of corrosion initiation, i.e., the time at which exposure is stopped and Ccrit is determined, is defined by a marked potential drop followed by 10 days of negative potentials (see section 3.5.2 and the Discussion for more details).
It is common that it may take several months until stable corrosion initiation occurs. This also depends on the initial chloride content already present in the concrete when the samples are taken from the structures. In some cases of the experiments so far conducted, it took more than 1 year until corrosion initiated.
Figure 8 shows an example of Ccrit measured in 11 samples taken from a more than 40 years old road tunnel in the Swiss alps. All these samples were taken from within an area of 1 - 2 m2, thus presumably identically produced and exposed. In this example, the chloride content at the steel surface at the time of sampling was negligible. Additionally, the carbonation front was still far from the steel surface.
Figure 9 shows two examples where the steel potential decreased strongly upon exposure to the chloride-free solution. In one of these specific cases, it was during the subsequent (destructive) examination of the sample found that the concrete at the steel depth was already carbonated. Upon arrival of water at the steel surface, the corrosion process thus immediately started. In the other case, false corrosion initiation occurred at one of the steel bar ends.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/56229/56229fig1.jpg" /> 
Figure 1. Schematic Drawing of the Sample Taken from a Structure and Treated in the Laboratory: (a) concrete core with an embedded piece of reinforcing steel; (b) reducing the concrete cover on the exposed side and on the back side by water-cooled diamond cutting; (c) steel bar end protection by means of removing some concrete around the steel and replacing it with a dense cement paste/mortar and subsequent epoxy coating; and (d) epoxy-coating on lateral faces and on end zones of the exposed concrete surface. <a href="https://cloudflare.jove.com/files/ftp_upload/56229/56229fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/56229/56229fig2.jpg" /> 
Figure 2. Schematic Drawing of the Setup for the Corrosion Test in the Laboratory. This shows the placement of the samples in the exposure tank. Spacers are used to ensure contact to exposure solution from the bottom sample surface. All samples are connected to a data logger, measuring the potential of each sample vs. a reference electrode placed in the exposure solution. <a href="https://cloudflare.jove.com/files/ftp_upload/56229/56229fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/56229/56229fig3.jpg" /> 
Figure 3. Schematic Drawing of Possible Time-evolutions of the Steel Potential that Illustrates the Criterion for Corrosion Initiation. At point 1, a potential drop by less than 150 mV from the initial &#34;passive level&#34; occurs; at point 2, a potential drop by at least 150 mV occurs, which is followed by repassivation; at point 3, a potential drop of at least 150 mV occurs (within a max. of 5 days) and the achieved negative potential level is sustained over 10 days. At tini, withdraw the sample from the exposure solution. <a href="https://cloudflare.jove.com/files/ftp_upload/56229/56229fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/56229/56229fig4.jpg" /> 
Figure 4. Schematic Drawing Illustrating the Cutting and Splitting of the Concrete Sample after Detection of Corrosion Initiation. First, a &#34;trench&#34; is cut from the rear side, in parallel to the steel bar. By inserting a chisel or a similar tool, the trench can be used to split the sample as indicated by the arrows. <a href="https://cloudflare.jove.com/files/ftp_upload/56229/56229fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/56229/56229fig5.jpg" /> 
Figure 5. Photographs Illustrating Sample Analysis After Corrosion Initiation. (a) The two halves of the sample after splitting, and (b) a rust spot is visible at the steel surface after corrosion initiation. Photographs from different samples. <a href="https://cloudflare.jove.com/files/ftp_upload/56229/56229fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/56229/56229fig6.jpg" /> 
Figure 6. Schematic Drawing Illustrating the Sampling for the Chloride Analysis After Corrosion Initiation: (a) removal of epoxy-coated parts of the split concrete core (purple = cutting planes); (b) removal of the concrete cover down to 2 mm from the steel surface (purple = cutting plane); (c) grinding over a depth interval of +/-2 mm of the steel bar cover depth (red = sampled volume). <a href="https://cloudflare.jove.com/files/ftp_upload/56229/56229fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/56229/56229fig7.jpg" /> 
Figure 7. Representative Examples of Measured Potential vs. Time Curves. The typically pronounced potential drops that may be followed by a potential increase (repassivation) until stable corrosion initiation according to the suggested criterion finally initiates. (a) Shows a case where the potential stabilizes on the negative level, and (b) is an example where the potential continues decreasing over the studied period of 10 days. <a href="https://cloudflare.jove.com/files/ftp_upload/56229/56229fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" class="xfigimg" src="/files/ftp_upload/56229/56229fig8.jpg" /> 
Figure 8. Example of Ccrit Measured in 11 Samples Taken from Within a Small Concrete Area in a More than 40 Years Old Road Tunnel in the Swiss Alps. <a href="https://cloudflare.jove.com/files/ftp_upload/56229/56229fig8large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 9" class="xfigimg" src="/files/ftp_upload/56229/56229fig9.jpg" /> 
Figure 9. Examples of Marked Potential Decreases Immediately upon Exposure in Chloride-free Solution. In one case, the concrete at the steel depth was already carbonated, thus upon arrival of water at the steel surface, the corrosion process immediately started, leading to a sharp decrease in potential. In the other case, false corrosion initiation occurred at one of the steel bar ends, which here led to a more gradual potential decrease. <a href="https://cloudflare.jove.com/files/ftp_upload/56229/56229fig9large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Experimental Protocol to Determine the Chloride Threshold Value for Corrosion in Samples Taken from Reinforced Concrete Structures at JoVE.com

Experimental Protocol to Determine the Chloride Threshold Value for Corrosion in Samples Taken from Reinforced Concrete Structures

We propose a method to measure a parameter that is highly relevant for corrosion assessments or predictions of reinforced concrete structures, with the main advantage of permitting testing of samples from engineering structures. This ensures real conditions at the steel-concrete interface, which are crucial to avoid artifacts of laboratory-made samples.

The aging of reinforced concrete infrastructure in developed countries imposes an urgent need for methods to reliably assess the condition of these ...

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15.3K Views.  ETH Zurich. We propose a method to measure a parameter that is highly relevant for corrosion assessments or predictions of reinforced concrete structures, with the main advantage of permitting testing of samples from engineering structures. This ensures real conditions at the steel-concrete interface, which are crucial to avoid artifacts of laboratory-made samples.

Video: Experimental Protocol to Determine the Chloride Threshold Value for Corrosion in Samples Taken from Reinforced Concrete Structures

Laboratory Drop Towers for the Experimental Simulation of Dust-aggregate Collisions in the Early Solar System

For the purpose of investigating the evolution of dust aggregates in the early Solar System, we developed two vacuum drop towers in which fragile dust aggregates with sizes up to ~10 cm and porosities up to 70% can be collided. One of the drop towers is primarily used for very low impact speeds down to below 0.01 m/sec and makes use of a double release mechanism. Collisions are recorded in stereo-view by two high-speed cameras, which fall along the glass vacuum tube in the center-of-mass frame of the two dust aggregates. The other free-fall tower makes use of an electromagnetic accelerator that is capable of gently accelerating dust aggregates to up to 5 m/sec. In combination with the release of another dust aggregate to free fall, collision speeds up to ~10 m/sec can be achieved. Here, two fixed high-speed cameras record the collision events. In both drop towers, the dust aggregates are in free fall during the collision so that they are weightless and match the conditions in the early Solar System.

We present a technique to achieve low-velocity to intermediate-velocity collisions between fragile dust aggregates in the laboratory. For this purpose, two vacuum drop-tower setups have been developed that allow collision velocities between &#60;0.01 and ~10 m/sec. The collision events are recorded by high-speed imaging.

For the purpose of investigating the evolution of dust aggregates in the early Solar System, we developed two vacuum drop towers in which fragile dust ...

Detection and Recovery of Palladium, Gold and Cobalt Metals from the Urban Mine Using Novel Sensors/Adsorbents Designated with Nanoscale Wagon-wheel-shaped Pores

Developing low-cost, efficient processes for recovering and recycling palladium, gold and cobalt metals from urban mine remains a significant challenge in industrialized countries. Here, the development of optical mesosensors/adsorbents (MSAs) for efficient recognition and selective recovery of Pd(II), Au(III), and Co(II) from urban mine was achieved. A simple, general method for preparing MSAs based on using high-order mesoporous monolithic scaffolds was described. Hierarchical cubic Ia3d wagon-wheel-shaped MSAs were fabricated by anchoring chelating agents (colorants) into three-dimensional pores and micrometric particle surfaces of the mesoporous monolithic scaffolds. Findings show, for the first time, evidence of controlled optical recognition of Pd(II), Au(III), and Co(II) ions and a highly selective system for recovery of Pd(II) ions (up to ~95%) in ores and industrial wastes. Furthermore, the controlled assessment processes described herein involve evaluation of intrinsic properties (e.g., visual signal change, long-term stability, adsorption efficiency, extraordinary sensitivity, selectivity, and reusability); thus, expensive, sophisticated instruments are not required. Results show evidence that MSAs will attract worldwide attention as a promising technological means of recovering and recycling palladium, gold and cobalt metals.

Because of the importance and extensive use of palladium, gold and cobalt metals in high-technology equipment, their recovery and recycling constitute an important industrial challenge. The metal recovery system described herein is a simple, low-cost means for the effective detection, removal, and recovery of these metals from the urban mine.

Developing low-cost, efficient processes for recovering and recycling palladium, gold and cobalt metals from urban mine remains a significant challenge in ...

Experimental Protocol to Investigate Particle Aerosolization of a Product Under Abrasion and Under Environmental Weathering

The present article presents an experimental protocol to investigate particle aerosolization of a product under abrasion and under environmental weathering, which is a fundamental element to the approach of nanosafety-by-design of nanostructured products for their durable development. This approach is basically a preemptive one in which the focus is put on minimizing the emission of engineered nanomaterials' aerosols during the usage phase of the product's life cycle. This can be attained by altering its material properties during its design phase without compromising with any of its added benefits. In this article, an experimental protocol is presented to investigate the nanosafety-by-design of three commercial nanostructured products with respect to their mechanical solicitation and environmental weathering. The means chosen for applying the mechanical solicitation is an abrasion process and for the environmental weathering, it is an accelerated UV exposure in the presence of humidity and heat. The eventual emission of engineered nanomaterials is studied in terms of their number concentration, size distribution, morphology and chemical composition. The purpose of the protocol is to study the emission for test samples and experimental conditions which are corresponding to real life situations. It was found that the application of the mechanical stresses alone emits the engineered nanomaterials' aerosols in which the engineered nanomaterial is always embedded inside the product matrix, thus, a representative product element. In such a case, the emitted aerosols comprise of both nanoparticles as well as microparticles. But if the mechanical stresses are coupled with the environmental weathering, the experimental protocol reveals then the eventual deterioration of the product, after a certain weathering duration, may lead to the emission of the free engineered nanomaterial aerosols too.

In this article, an experimental protocol to investigate particle aerosolization of a product under abrasion and under environmental weathering is presented. Results on the emission of engineered nanomaterials, in the form of aerosols are presented. The specific experimental set-up is described in detail.

The present article presents an experimental protocol to investigate particle aerosolization of a product under abrasion and under environmental ...

Detecting the Water-soluble Chloride Distribution of Cement Paste in a High-precision Way

To improve the accuracy of the chloride distribution along the depth of cement paste under cyclic wet-dry conditions, a new method is proposed to obtain a high-precision chloride profile. Firstly, paste specimens are molded, cured, and exposed to cyclic wet-dry conditions. Then, powder samples at different specimen depths are grinded when the exposure age is reached. Finally, the water-soluble chloride content is detected using a silver nitrate titration method, and chloride profiles are plotted. The key to improving the accuracy of the chloride distribution along the depth is to exclude the error in the powderization, which is the most critical step for testing the distribution of chloride. Based on the above concept, the grinding method in this protocol can be used to grind powder samples automatically layer by layer from the surface inward, and it should be noted that a very thin grinding thickness (less than 0.5 mm) with a minimum error less than 0.04 mm can be obtained. The chloride profile obtained by this method better reflects the chloride distribution in specimens, which helps researchers to capture the distribution features that are often overlooked. Furthermore, this method can be applied to studies in the field of cement-based materials, which require high chloride distribution accuracy.

A protocol for obtaining a water-soluble chloride profile by using a high precision milling method is presented.

To improve the accuracy of the chloride distribution along the depth of cement paste under cyclic wet-dry conditions, a new method is proposed to obtain a ...

Installation Method to Enhance Quality Control for Fiber Reinforced Polymer Spike Anchors

Fiber reinforced polymer (FRP) anchors are a promising way to enhance the performance of externally-bonded FRPs applied to existing structures, as they can delay or even prevent debonding failure. However, a major concern faced by designers is the premature failure of the anchors due to stress concentration. Poor installation quality and preparation of the clearance holes can result in stress concentration that provokes this premature failure. This paper deals with an installation method that aims to reduce the impact of the stress concentration and to provide a proper quality control of the preparation of the drill hole. The method involves three parts: the drilling and cleaning of the holes, the smoothing of the hole edges with a customized drill bit, and the installation of the anchor itself, including the impregnation of the anchor dowel and its insertion. Anchor fans (the free length of the spikes) are then bonded to the external FRP reinforcement. For the end anchorage, and in the case of reinforcements with multiple plies, it is recommended that the anchor fan be inserted between two plies to assist the stress-transfer mechanism. 
The proposed procedure is complemented with a design approach for spike anchors, based on an extensive database. It is proposed that the design follow a number of steps, namely: selection of the anchor diameter and subsequent tensile strength of the connector (that is to say, the anchor before fanning out the free end), evaluation of the reduction in the tensile strength due to bending, provision of enough embedment to prevent slippage failure, and consideration of the number and spacing of anchors for a given reinforcement. In this sense, it should be noted that further research is needed in order to obtain a general expression for the contribution of spike anchors to overall bond strength of FRP reinforcements.

This manuscript presents a method for controlling the quality of installation for spike anchors designed to delay delamination of externally bonded fiber reinforced polymers. The protocol includes the preparation of the drill hole and the insertion process. The most influential parameters on the efficiency of the anchors are discussed.

Fiber reinforced polymer (FRP) anchors are a promising way to enhance the performance of externally-bonded FRPs applied to existing structures, as they ...

Methods of Ex Situ and In Situ Investigations of Structural Transformations: The Case of Crystallization of Metallic Glasses

We demonstrate the use of two nuclear-based analytical methods that can follow the modifications of microstructural arrangement of iron-based metallic glasses (MGs). Despite their amorphous nature, the identification of hyperfine interactions unveils faint structural modifications. For this purpose, we have employed two techniques that utilize nuclear resonance among nuclear levels of a stable 57Fe isotope, namely M&#246;ssbauer spectrometry and nuclear forward scattering (NFS) of synchrotron radiation. The effects of heat treatment upon (Fe2.85Co1)77Mo8Cu1B14 MG are discussed using the results of ex situ and in situ experiments, respectively. As both methods are sensitive to hyperfine interactions, information on structural arrangement as well as on magnetic microstructure is readily available. M&#246;ssbauer spectrometry performed ex situ describes how the structural arrangement and magnetic microstructure appears at room temperature after the annealing under certain conditions (temperature, time), and thus this technique inspects steady states. On the other hand, NFS data are recorded in situ during dynamically changing temperature and NFS examines transient states. The use of both techniques provides complementary information. In general, they can be applied to any suitable system in which it is important to know its steady state but also transient states.

Methods of Ex Situ and In Situ Investigations of Structural Transformations: The Case of Crystallization of Metallic Glasses

Here, we present a protocol to describe ex situ and in situ investigations of structural transformations in metallic glasses. We employed nuclear-based analytical methods which inspect hyperfine interactions. We demonstrate the applicability of M&#246;ssbauer spectrometry and nuclear forward scattering of synchrotron radiation during temperature-driven experiments.

We demonstrate the use of two nuclear-based analytical methods that can follow the modifications of microstructural arrangement of iron-based metallic ...

Multiscale Structures Aggregated by Imprinted Nanofibers for Functional Surfaces

Multiscale surface structures have attracted increasing interest owing to several potential applications in surface devices. However, an existing challenge in the field is the fabrication of hybrid micro-nano structures using a facile, cost-effective, and high-throughput method. To overcome these challenges, this paper proposes a protocol to fabricate multiscale structures using only an imprint process with an anodic aluminum oxide (AAO) filter and an evaporative self-aggregation process of nanofibers. Unlike previous attempts that have aimed to straighten nanofibers, we demonstrate a unique fabrication method for multiscale aggregated nanofibers with high aspect ratios. Furthermore, the surface morphology and wettability of these structures on various liquids were investigated to facilitate their use in multifunctional surfaces.

Presented is an easy method to fabricate nano-micro multiscale structures, for functional surfaces, by aggregating nanofibers fabricated using an anodic aluminum oxide filter.

Multiscale surface structures have attracted increasing interest owing to several potential applications in surface devices. However, an existing ...

Theoretical Calculation and Experimental Verification for Dislocation Reduction in Germanium Epitaxial Layers with Semicylindrical Voids on Silicon

Reduction of threading dislocation density (TDD) in epitaxial germanium (Ge) on silicon (Si) has been one of the most important challenges for the realization of monolithically integrated photonics circuits. The present paper describes methods of theoretical calculation and experimental verification of a novel model for the reduction of TDD. The method of theoretical calculation describes the bending of threading dislocations (TDs) based on the interaction of TDs and non-planar growth surfaces of selective epitaxial growth (SEG) in terms of dislocation image force. The calculation reveals that the presence of voids on SiO2 masks help to reduce TDD. Experimental verification is described by germanium (Ge) SEG, using an ultra-high vacuum chemical vapor deposition method and TD observations of the grown Ge via etching and cross-sectional transmission electron microscope (TEM). It is strongly suggested that the TDD reduction would be due to the presence of semicylindrical voids over the SiO2 SEG masks and growth temperature. For experimental verification, epitaxial Ge layers with semicylindrical voids are formed as the result of SEG of Ge layers and their coalescence. The experimentally obtained TDDs reproduce the calculated TDDs based on the theoretical model. Cross-sectional TEM observations reveal that both the termination and generation of TDs occur at semicylindrical voids. Plan-view TEM observations reveal a unique behavior of TDs in Ge with semicylindrical voids (i.e., TDs are bent to be parallel to the SEG masks and the Si substrate).

Theoretical calculation and experimental verification are proposed for a reduction of threading dislocation (TD) density in germanium epitaxial layers with semicylindrical voids on silicon. Calculations based on the interaction of TDs and surface via image force, TD measurements, and transmission electron microscope observations of TDs are presented.

Reduction of threading dislocation density (TDD) in epitaxial germanium (Ge) on silicon (Si) has been one of the most important challenges for the ...

Optimized Sealing Process and Real-Time Monitoring of Glass-to-Metal Seal Structures

Residual stress is an essential factor to keeping the hermeticity and robustness of a glass-to-metal seal structure. The purpose of this report is to demonstrate a novel protocol to characterize and measure residual stress in a glass-to-metal seal structure without destroying the insulation and hermeticity of sealing materials. In this research, a femto-laser inscribed fiber Bragg grating sensor is used. The glass-to-metal seal structure that is measured consists of a metal shell, sealing glass, and Kovar conductor. To make the measurements worthwhile, the specific heat treatment of metal-to-glass seal (MTGS) structure is explored to obtain the model with best hermeticity. Then, the FBG sensor is embedded into the path of sealing glass and becomes well-fused with the glass as the temperature cools to RT. The Bragg wavelength of FBG shifts with the residual stress generated in sealing the glass. To calculate the residual stress, the relationship between Bragg wavelength shift and strain is applied, and the finite element method is also used to make the results reliable. The online monitoring experiments of residual stress in sealing glass are carried out at different loads, such as high temperature and high pressure, to broaden functions of this protocol in harsh environments.

Key procedures to optimize the sealing process and achieve real-time monitoring of the metal-to-glass seal (MTGS) structure are described in detail. The embedded fiber Bragg grating (FBG) sensor is designed to achieve online monitoring of temperature and high-level residual stress in the MTGS with simultaneous environmental pressure monitoring.

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Artificial Thermal Ageing of Polyester Reinforced and Polyvinyl Chloride Coated Technical Fabric

Architectural fabric AF9032 has been subjected to artificial thermal ageing to determine changes of the material parameters of the fabric. The proposed method is based on the accelerated ageing approach proposed by Arrhenius. 300 mm x 50 mm samples were cut in the warp and fill directions and placed in a thermal chamber at 80 &#176;C for up to 12 weeks or at 90 &#176;C for up to 6 weeks. Then after one week of conditioning at ambient temperature, the samples were uniaxially tensioned at a constant strain rate. Experimentally, the parameters were determined for the non-linear elastic (linear piecewise) and viscoplastic (Bodner&#8211;Partom) models. Changes in these parameters were studied with respect to the ageing temperature and ageing period. In both cases, the linear approximation function was successfully applied using the simplified methodology of Arrhenius. A correlation was obtained for the fill direction between experimental results and the results from the Arrhenius approach. For the warp direction, the extrapolation results exhibited some differences. Increasing and decreasing tendencies have been observed at both temperatures. The Arrhenius law was confirmed by the experimental results only for the fill direction. The proposed method makes it possible to predict real fabric behavior during long term exploitation, which is a critical issue in the design process.

Here, we simulate accelerated thermal ageing of technical fabric and see how this ageing process influences the mechanical properties of the fabric.

Architectural fabric AF9032 has been subjected to artificial thermal ageing to determine changes of the material parameters of the fabric. The proposed ...

Experimental Protocol to Determine the Chloride Threshold Value for Corrosion in Samples Taken from Reinforced Concrete Structures

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