ENGINEERING THE REHABILITATION OF REINFORCED CONCRETE STRUCTURES IN GAS AND FUEL OIL DISTRIBUTION DOCKS OF THE PACIFIC COAST OF MEXICO.
Jorge Canto,1 Edgar Maya,1 Lorenzo M. Martinez-dela-Escalera,1 (1)Corrosi贸n y Protecci贸n Ingenier铆a, S.C;
Carlos Lopez Andrade,2 Cecil Knight,2 (2) Petroleos Mexicanos.
Jorge A. Ascencio3, and Lorenzo Mart铆nez2,3 (2) Universidad Nacional Autonoma de Mexico.
ABSTRACT. This work is dedicated to report the engineering developments produced by an initiative to rehabilitate the reinforced concrete structures of PEMEX dedicated to the gas and fuel oil supply of important regions of the Mexican coastal regions including Mazatlan, Acapulco and Salina Cruz. A general assessment is presented showing the apparent needs of repairs and corrosion prevention investments associated to the steel reinforcement in concrete beams, slabs and pilots of the docks. A diagnosis was performed employing visual, sonic hammering, sclerometer concrete strength measurements, rebar to concrete electrolyte potentials, concrete electrical conductivity, and chloride penetration. The results of the study are discussed as well as the consequences in terms of repairs to be performed. We show an approach to mitigate corrosion activity through applying cathodic protection and protective coatings to reinforced concrete steel. The design of cathodic protection was developed based on distributed and discreet sacrificial anodes.
1. INTRODUCTION.In the recent years the corrosion analysis and control in concrete structures, has been of concern of the various agencies both public and private, by the very high cost that can be caused degradation of these structures of concrete, the aggressiveness of the surrounding environment and workloads to that are subjected. The factors influencing the intensity of the process corrosion are the quality of the concrete, the presence of pollutants, etc. In addition, moisture plays a role highlighted so of not to carry out measures appropriate verification and corrosion control may result in structural failures of these structures. In fact there are multiple reports about big troubles derived from the concrete corrosion, as the bridge break down in Concord, North Caroline, where important signs of corrosion were found in the metal balks of the walking bridge producing injuries to more than 100 persons [], the impact of this kind of corrosion to multiple structures is well studied. The present document lists a series of procedures that will make it possible to monitor the status of the steel corrosion in the elements of particular by inspections of hotspots and identification of areas affected by performing various testing, measurement and sampling of the structure. This also provides information detailed work about the experience in common problem identified in the reinforced concrete structures of the evaluated docks. Thus also, it is worth noting that the procedures are based on standards and criteria of the ASTM International (American Society for Testing and Materials), NACE International ( National) Association of Corrosion Engineers ) and the ICRI (International Concrete Repair Institute) Since these documents play an important role in the infrastructure of the information oriented design, manufacturing and trade in the economy world. In this way, authorities of PEMEX had the initiative to evaluate and to rehabilitate the reinforced concrete structures, particularly those dedicated to the gas and fuel oil supply of important regions of the Mexican coastal regions. This work is dedicated to report the engineering developments produced by an initiative to rehabilitate the reinforced concrete structures of PEMEX dedicated to the gas and fuel oil supply of important regions of the Mexican pacific coast including docks from Mazatlan, Acapulco and Salina Cruz.
2. INVOLVED PROCEDURES. a) Evaluation documentary analysis of integrity, flat constructive, records of failures and repairs and relevant general documentation for corrosion control and integrity of frames of corrosion in particular hotspots strengthened spring. In order to scan of the integrity of the structures of concrete, both studies existing information was collected prior of drawings, details, constructive specifications and the repairs made, historical information for operation of these structures, at least in the past 12 months, as well as of the conditions of operation and status of mechanical integrity of the same as can be assessed through direct inspections or by assessments indirect.
The information considered for their collection lists: 1. Flat constructive; 2. Armed detailed drawings and specifications; 3. Flat structural; 4. Documentation repairs and previous studies. b) Inspection and recognition of the structures to assess, identification of areas affected by corrosion, visible During this stage occurs a preliminary study of the structure and its characteristics as location in general. The objective of this study was make a first assessment, which decided the extent and type of studies carried out in the following stages of the examination process. At this stage we confined ourselves to recognize the area and to establish the functioning of the structure to verify, carrying out a thorough inspection of the same structure and its environment, noting the activities and use of areas. Since the corrosion problems manifest themselves in those parts of the structure where accumulations of water occur or leaks in this second stage was a visual inspection to find areas with corrosion problems (as is illustrated in figure 2a). These are detected to see the areas where there are oxide drainage or areas with cracks or cracks, which cause corrosion of the structures of reinforcement in concrete. Identified qualitative characteristics of the materials and were located symptoms and injuries as well as their possible causes. An initial assessment on safety and durability of the structure was made.
Figure 1. Examples of the visual and physical carried out procedures: a) recognition of the area, b) acoustic detection and c) impact test.
c) Identification of areas of hidden, corrosion by physical evidence of evaluation Detection of hidden, corrosion areas by sampling sound Initially the effects of the damage by corrosion in steel booster not seen at a glance, since the effect of the dellamination happens internally and not manifests immediately on the surface exterior of the structure. This acoustic inspection techniques were applied, as it is shown in figure 1b, to basis of impact on the surface of the concrete, so that when there is a dellamination of the structure, the space within between a specific firm and detached, produces a particular sound that allows you to identify and define the affected area. This technique, according to the ASTM standard D4580[1] and combined with measurements of potential and resistivity, allows identify active corrosion still not they signify sites. Physical impact test of concrete structures to determine its resistance The elements of the reinforced concrete (rod, cement, portland, sand, gravel and water) confer excellent properties to such material, such as resistance to
compression and tension. However, with the strengthening, bars corrosion steel section is reduced and the particular dilates and fracture, so lost tensile strength and fatigue, generalizing progressively the corrosive process. The mechanical ability of the steel decreases linearly with the section reduction, but also stretching and fatigue resistance properties are most sensitive to corrosion, so they can reduce substantially with small losses of section. Resistance to the compression of the concrete corresponds to the maximum stress (general rupture) of axial compression in kg/cm 2 can support an element of concrete. Concrete quality measured is primarily by its resistance to pressure, because it is an indicative value to check the capacity load and the durability of concrete structures. The test rebound in particular to determine its resistance, is performed with a test hammer, making physical impacts on the concrete as it is illustrated in figure 1c and according to the described in the ASTM standard C805.[2] d) Electrochemical assessment tests the integrity of the structures of concrete Electric continuity verification of steel reinforcement of the concrete. The verification of the electrical continuity between various elements of the steel reinforcement of a structural component allows the determination of insulating oxide between these elements. Verification involves locate the armor or part of it, to achieve a good electrical connection and determining if there is electrical continuity in the reinforced concrete steel. This is also a precursor to the measurement of potential corrosion, because it provides the contact points needed to these measurements. This verification is done by connecting a multimeter between points of contact. Polarization potential measurement on concrete structures using Cu/CuSo4 cell reference. The electrochemical potential steel corrosion (ECORR) concrete is a parameter that indicates the status of corrosion reinforcing bar (passive or active). ECORR measurement shall be carried out using a standard copper/copper sulphate (Cu/CuSO4) reference electrode saturated (CSE), connected to a high impedance input, multimeter According to standard ASTM C-876. [1] ECORR values are interpreted as specified in the standard ASTM C-876, defining different States of corrosion for steel in concrete. These values allow you to set if the armor is found in passive. Because corrosion is an electrochemical process, this generates a voltage and current in the same way as the makes a battery. The part of the steel that is corroded can be considered as half of the cell or battery called anode. Coupling this anode with a standard cathode, the voltage measurement may be compared with known values that provide an indication of the extent of corrosion. It is called media cell because it is the other half of the electric cell or cathode. The average cell is commonly made by a copper (bar) electrode on an electrolyte composed of one solution containing its own ions (copper sulfate). Sometimes it is used a silver within a solution to the same metal electrode. When the average cell is connected with the steel reinforcement embedded within the concrete full electric cell generates a voltage difference due to the difference between metals (copper, steel) that occupy different sites electrochemical series. When this cell is moved through of the concrete surface there is a change in the cell due to the
a
b Figure 2. Examples of the potential measurement on concrete.
differences of the steel conditions within the matrix of concrete surface conditions. This test indicates the probability of reinforcing steel corrosion inside the concrete, and usually occurs by performing actions on a mesh of points with a separation previously established; this will be established a separation between 1.5 points metro, and was used as a reference Standard Copper/copper sulphide electrode. If steel within concrete is on the same media, and there is corrosion, then we should expect a difference of potential greater than - 200 mV, which corresponds to the potential difference between the two average cells. If instead the concrete is polluted, then the solution will have different amounts chlorides in different areas and some parts of the steel could etch. The average cell technique displays different values of potential for corrosion areas contaminated with chlorides. This produces a difference of potential between the two largest average cells. The corrosion potential determination technique has proved to be an effective method to locate the armor with corrosion in the structures when a big sampling process is considered. And while it is not accurate about the intensity of the corrosion process, other complimentary techniques must besides considered. This method is shown in figure 2a.
Figure 3. Examples of the specialized methods: a) verification of the electrical continuity, b) resistivity analysis and c) concrete cores removal.
Electrical resistivity evaluation of the concrete The resistivity is a measurement of the ability of the concrete to support the passage of ions and it lets us to know how protected to the steel reinforcement immersed or embedded in it. For low resistivity values, the ion transfer through
the specific media is better. Test of concrete resistivity allows knowing if the system chosen for rehabilitation is compatible with the existing concrete, which is done in site as is shown in figure 2b. The samples are tested on wet and dry conditions. For example, a concrete with less than 15,000 Ω-cm resistivity is ideal for protection as galvanic strengthening. The electrical resistivity is a property of each material, and corresponds to the reciprocal of its conductivity; the unit of measure is kΩ-cm or ω-m. Largely resistivity depends of the degree of saturation of the pores of the concrete, and to a lesser degree of the hydration of the dough and the presence of salts dissolved in the aqueous phase. It is based on variables such as cement type; those additions inorganic; the water/cement ratio; the porosity of the structure; between others. Since the resistivity is one of the factors that control in concrete, steel corrosion rate currently it has increased interest in determining this intrinsic property of the concrete. According to tests and analyses carried out, could establish that the particular saturated with water, normally have at the order of 10kΩ-cm resistivity. The purpose of this test is to determine the resistivity power of the reinforced concrete on site, to locate areas in which the armor could not be passive and therefore susceptible to etch is if you give oxygen and humidity conditions. Electrical resistivity measurements are made using the method known as "Wenner"or"4 tips". For this, we use a device of own design, connected to a meter of resistance type Nilsson model 400. The electrical resistivity is a property of each material and corresponds to the reciprocal of its conductivity; your unit of measure is the ohm-cm. It is one of the parameters that control the speed of steel corrosion. As a general criterion from the point of view of risk of corrosion, resistivity concrete levels can be considered as follow: Resistivity (kΩ-cm)
Corrosion risk
Higher than 20 Between 20 to 10
Small risk Moderate
Lower than 10
High
Table 1. Corrosion risk in function of electric resistivity.
This criterion taken from standard SP0308-2008 [2], relating to "methods of" "Inspection for the evaluation of corrosion in reinforced concrete structures" of NACE International. Sampling of concrete core for analysis. Cores extraction is through a machine pedestal employing a wet drill purified diamond drill, taking special care in the management of samples to avoid contamination and following the standard ASTM C42[3]. The kernels must have a minimum diameter of 5.0 cm to 7.5 cm, and are cut at different depths, since they serve as specimens for the study of pH, chloride and carbonation penetration. El resulting dust is also collected for analysis. These cores are taken not only in areas where there is further deterioration, but also where concrete appears good conditions. Since samples vary with distance to the steel of booster, should be taken as close as possible to this using a detector or metals Locator. The standard C42 ASTM test method covers the procurement, preparation and cores drilled concrete test for its analysis. The method is exemplified in figure 2c.
Evaluation of chloride penetration In the reinforced concrete in permanent or intermittent contact with sea water, sulfates and chlorides of calcium, magnesium and alkali, are inseparable way, they can penetrated by porosity, osmosis, capillary action and dissemination. The sulfates attack to the concrete forming causing expansive compounds of damages, which facilitates the penetration of the chlorides and corrosion of the armor. Expansive corrosion products lead to loss adherence and the internal damaging joined the external produced by sulfates, increasing and accelerating both actions. These phenomena in principle may have successively, starting with the dissemination of the ion chloride Ionic RADIUS is less than sulfate, so it spreads faster, ending with occur simultaneously with overlay cause and effect. The content of the ion chloride as a percentage by weight of the concrete is measured using the "revised procedure SHRP for" "chlorides analysis". A threshold 0.025 % by weight of concrete, limit of agreement to NACE International, is used for the analysis of results present chloride ion. Content below 0.025 % threshold for specifically, in particular non-carbonated, weight is below the limit which induced chlorides in steel corrosion occur in reinforcement. If the level of chlorides is over this limit, the steel, reinforcement can lose the passive layer and initiating the process of corrosion. Quantification of carbonation of concrete The Carbonation concrete occurs when the pH of the concrete (usually between 12 and 13), reduces values between 9 and 10. When this happens, the layer of oxide formed naturally (passive layer) of steel booster disappears, giving home corrosion. Using cores concrete, decision is determines the depth of carbonation. These cores are broken in the laboratory and immediately exposed part is sprayed with a solution 0.15 % of phenolphthalein ethanol. Those parts where you get one pink or purple, coloration corresponds to a pH of 10 or higher, indicating not carbonation. Parties which do not change the color of the solution applied (colorless) show a less than 10 pH and probably are carbonated.
3. RESULTS OF SITE ANALYSIS. Mazatlan. The study of this project was divided into 3 structures: 1. North tie up dock 2. South tie up dock 3. Operations Platform (North face, South face, Top surface and Bottom structure) North and south docks are similar structures and are just different on the upper surface, from these docks assessed only the front face, which receives the pressure of workload. Operations platform The platform of operations is the most important structure because it is the point that hosts the main infrastructure in the transport of fuel. The north and south faces of the platform for operations have similar trends in the results of testing. Both show low levels of rolling concrete, 6-8 %, similarly, the corrosion
potentials indicate a probability of 80-87 % to have corrosion activity. The cylindrical concrete samples from the South face of the platform for operations and that were collected from the depth of the steel reinforcement, indicates the presence of chlorides at two levels of test. From this information is based the assumption that the main problem is the corrosion in the delaminated concrete zones and around these areas. The upper area of the Operating Platform, displays the further deterioration of all the structures with nearly 40% of the total area; areas of rolling are the most worrying points on the surface. The quantities of rolling they are consistent in all testing. Measurements of potential, the results indicate that 40 % of these there is a high probability of active corrosion, while 50 % of the readings are in the range of uncertainty. In the two samples obtained from this area chlorides were found down to a depth of 50 mm (2.5 "), with a sample indicating a penetration to 89 mm (3.5 inch) in depth. The samples also have levels of carbonation between 13 mm and 16 mm (½ to 5/8 "). This indicates a probable generalized corrosion activity and the possibility of damage in the future, beyond the current delaminated areas. The Lower Structure below the Operating Platform, visually displays the lowest degree of deterioration. In the taken corrosion potential, 30 % of the readings indicate that there is an active corrosion and remaining readings are in the range of uncertain. The concrete cylinder taken from this area shows presence of chlorides at a depth of 13 mm (½ ") and a 19 mm carbonation depth (¾"). The results of the depth of carbonation and chloride penetration indicate a status of active corrosion, but that at this point, only concerns superficial steel. However progressive pollution from chlorides, with the time could cause future damage by corrosion in structure. North and South tie up docks The North and South tie up docks are mooring structures where vessels are supported. Visually both structures show similar levels of impairment, however there are some differences. The North dock showed a certain amount of delamination area, of more than 20% of the total surface of the concrete, more concentrated at the bottom of the frontal face. Corrosion potential relevant readings indicate the existence of corrosion active in 30 % of these and 60 % are in the area of uncertain. The chloride content tests only show them at a level of ½ ". This may indicate that the steel surface would be in danger of having a corrosion stage; however it also could indicate that the collected sample was not in an area with corrosion activity. The carbonation tests indicate a succinct carbonation level of carbonation, at just 8 mm. It should be remembered that the limit level for chlorides in carbonated concrete is much less that in normal concrete. In the South Pier found a delaminated area much lower, only 10 per cent of the total of the concrete surface. The potential in the front face of the dock indicate small activity of corrosion with 70% of readings in the low probability range. Chloride content tests correspond to the level of 13 mm (½ "). The carbonation found in the sample indicates a low level of carbonation (8 mm), similar results
to the found in the North and South faces, also to the bottom structure of the Operations Platform; the corrosion activity does not appear to be generalized beyond the zones active deterioration. Future prevention chlorides may be worthy of consideration. Summary Structures evaluated shows damage and caused by the action of rolling areas corrosive, however the level of deterioration in each area are different and they require different solutions for each particular item. Before installing any corrosion protection system, delaminated must be withdrawn, as well as the concrete showing damage to be repaired. In general the PEMEX in Mazatlan dock structure is in regular to good conditions. However, it shows some deterioration zones that should considered to treat in order to ensure a longer useful life for the structure.
Salina Cruz For the study and analysis of structures of reinforced concrete that form the wharf 9 of the Maritime Terminal Salina Cruz, are divided into the following areas: 1. East Tie up Docks 1 and 2 2. East Tie up Docks 3 and 4 3. West Tie up Docks 1 and 2 4. West Tie up Docks 3 and 4 5. Operations Platform 6. Access Bridge 7. Concrete Frames and Support Joints, 8. Rear Docks 9. Gateways and Platforms The East Tie up Docks 1, 2, 3 and 4, as well as the West Tie up Docks 1, 2, 3 and 4, are similar structures both its dimensions and characteristics of reinforcement armed. The Operations Platform and Access Bridge are independent structures and also different in their characteristics of armed and structural work. The current conditions were together assessed for the movement gateway, the gateway platforms, and frameworks and concrete joints that serve as support to the entire pipelines network of this dock; from these structures a severe problem of corrosion can be identified, which greatly affects the operation integrity; subsequently it is studied in particular the problems and recommendations of such structures. Analysis of chlorides In agreement to the corresponding annex to the chlorides contents (CL % by weight of) (concrete), 82 % of the examined samples show chlorides levels, by higher than the level regarded as limit (0.03 % by concrete weight ACI 222). We can say that all the evaluated items show corrosive activity from the chloride content terms. In some cases the corrosive process shows progress finding structures in where the concrete around the steel was lost due to the generated from oxides. The exception is the east face of the operations platform, whose results show low content of chloride, since this structure is new or it was recently rehabilitated.
The chloride ions percentages decrease when the samples are taken deeper, this result confirms that chlorides have been deposited on the outside of the different structures and eventually penetrate the concrete allowing this process through its permeability. Taking into account the results of chloride content in general, we can ensure that the process of being developed there corrosion is important and increase its action over time if left unchecked. Analysis of Cu/CuSO 4 potentials The measured corrosion potentials, in relation to the average reference cell Cu/CuSO 4 show values with active corrosion and severe damage risk. It is noted a direct relationship between areas with significant damages and their respective potentials measurements. The corrosion potential obtained from the operations platform corresponds to 96 % with a high-risk of corrosion, which coincides with the identified damages; in the same way in this structure measurements were obtained showing a 85 % of readings showing low risk of corrosion there for the east face (new wall). The access bridge displays 15 % of potential with severe corrosion, all located in the beginning of the access where also corrosion is found both on the bridge in the nearby structures (pipe racks). 70 % of the potentials show high risk of corrosion and only 15 % shows a low risk, so we recommend that the corrosive process in this extension must be control by the implementation of tasks. While the West tie-up docks 1 and 2 show high risk corrosion potentials; 90 % and 83 % respectively, then this merits a control in the extension of these structures. The West tie up docks 3 and 4 had values with low risk (82 per cent each), matching to the observed damages. The East tie up docks 1 and 2, similarly to the previous ones, show high potential risk of corrosion; 90 % and 100 % respectively, which merits a control in whole extension of these structures. The East tie up docks 3 and 4 had low risk potentials with potentials of 61%t each, which coincide with the observed damages. The structures located at the end of the wharf # 9 structures are formed by 3 mooring platforms, 9 gateway heads, and the movement to gateway that join them. Potentials obtained values from these structures show a high percentage (97 %) with a severe corrosion which coincides with the observed deterioration of the elements, so it is required a rehabilitation process on 100 % of these structures. Summary These evaluated structures, are located in marine environment, so have been affected by the waters of the Pacific Ocean, which because its high chlorides content when these come into contact with the concrete and penetrate it, they affect the structural elements of reinforced concrete, some of them show advanced corrosion processes and in some cases, the structural integrity is compromised as in front (walls apron) screens. This illustrated in figure 3, which shows examples of the different identified damages.
The main objective of this assessment is to have measurements, allowing for through its interpretation and data crossing, define the current state of conservation of the different elements of the evaluated structure. The multidisciplinary analysis of the obtained information should lead to the generation of findings with sufficient technical support to define the scope, urgency, methods or systems and materials for rehabilitation strategy named structure.
Figure 4. Examples of visible damages on: a) movement gateways, b) platforms, c) concrete frames and d) structural supports.
The analysis of the results showed in this case, its associated data, as well as of the visual inspection and tests carried out on the spot, it can be establish that: • Structures of reinforced concrete, belonging to Wharf # 9 generally show problems risks are high severe corrosion, so is required to work for controlling the corrosive process, extend the useful life of these structures and streamlining these petroleum facilities. •
Because structures are in an aggressive marine environment, it is recommended to make actions for the rehabilitation to prevent unsafe conditions that can induce risks for staff, facilities and the own environment
•
Some structures show corrosion evidences fissures and delaminated zones in localized areas; however the readings of corrosion potential and the percentage of chlorides brought the corrosive process this asset, so it becomes necessary to control this process throughout the area structure.
•
Chloride ions are the responsible for the loss of the steel reinforcement, pasivation of the corrosive process with the presence of steel trigger completely discovered. The despasivation by carbonation, does not present problems.
•
The external source of chlorides comes from the marine environment and not from the used materials in the production and/or pouring of concrete. Measurements indicate higher contents of chlorides in the surface than for the deeper sections.
•
Some structures have received maintenance, repairs without achieving control the problem of existing corrosion and rather has strengthened the problem; so it is requires to adopt instructions that include control systems. As all the control systems and protection against corrosion, require jointly repair affected concrete, it is expected that the implementation of each of the recommended strategy systems of rehabilitation, follow the recommendations given by the International Repair of Concrete Institute (ICRI), for controlling the existent corrosive process.
•
There are structures as the located at the end of spring # 9 (2 mooring platforms, 9 heads and the movement gateways), as well as frameworks and ducts that media joints run parallel to the bridge access; which show serious deterioration due to corrosion that calls for their immediate rehabilitation process. There are actions in respect of rehabilitation strategy.
4. GENERAL CONCLUSIONES AND SUGGESTED ACTIONS In case of Mazatlan, we can conclude from the evaluated structures, that they are on relatively good conditions, with presence of low risk corrosion potentials, and just a few parameters that require attention to control corrosion processes. The Salina Cruz case involved bigger problems because the structures have corrosion advanced processes in most of the structures, with evident concrete damages, which in some areas put in risk the continuous operation of the installation, including the support of the own structures. For this dock, the necessity of substitution of concrete and steel segments is proposed because the significant recognized damage to the structures. As it was mentioned above, the inclusion of systems of control and protection against corrosion of the steel reinforcement of these structures, involves repairing the concrete that is around the armor as general rehabilitation strategy. Needed, and we underline in this sense, the concrete showing signs of fractures, cracks and/or delaminated zones should be withdrawn as part of the repair; likewise this repair of concrete shall be kept until where notice than steel of reinforcement not sample oxidation. For both docks systems (Mazatlan and Salina Cruz) we proposed rehabilitation actions and we suggested next possible strategies: cleaning procedures, installation of discrete and distributed zinc anodes (as the Vector Corrosion products) depending of the configuration (which were punctually determined for each), also the application of protective coatings against chlorides and carbonation as a Polymer Coating or Hi Solids Epoxi Coating.
In general we can also conclude the necessity of increase the efforts of this kind of studies in structures exposed to aggressive environments as maritime terminals and some other exposed to high concentrations of chlorides. However the own development of the complete methodology opens the perspective to be implemented in the diagnostic and specific proposals for the corrosion processes control.
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