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 Corrosion y Proteccion Ingeneria, S.C. Rio Nazas 6. Cuernavaca, Morelos. Mexico. 62290.
Carlos Lopez Andrade, Cecil Knight, PEMEX Refinacion. Torre Ejecutiva, Marina Nacional 329, Col. Huasteca, México, Distrito Federal, C.P. 11311.
Jorge A. Ascencio, and Lorenzo Martínez* Instituto de Ciencias Físicas, Universidad Nacional Autonoma de Mexico, Ave Universidad 1001, Col. Chamilpa, Cuernavaca, Morelos. CP 62210. *
Also at Corrosion y Proteccion Ingenieria SC
ABSTRACT This work serves to report on the results of the corrosion evaluations produced by a PEMEX initiative to rehabilitate the reinforced concrete structures in their facilities that are used to supply gas and fuel oil to important coastal regions of Mexico including Mazatlan and Salina Cruz. A general assessment is presented showing the concrete repair needs and corrosion prevention requirements for the steel reinforcement in concrete beams, slabs and piles of these structures. A diagnosis was performed employing different test methods including visual survey, sonic hammering, sclerometer concrete strength measurements, corrosion potential measurement, concrete electrical conductivity, and chloride penetration. The results of the study are discussed, as well as the recommendations in terms of repairs to be performed and protection to be used. The general recommendation delivered was to mitigate corrosion activity through cathodic protection for the reinforcing steel and protective coatings. The cathodic protection recommended was mainly embedded distributed and discrete sacrificial anodes.
1. INTRODUCTION In recent years, corrosion analysis and control in concrete structures has been of concern of public and private sectors due to the very high costs associated with the repair of deteriorated concrete structures. These structures are often exposed to the aggressive surrounding environments and heavy workloads which only aggravate the situation further. Many factors influence the extent that corrosion is harmful to these structures including: the quality of the concrete, the presence of chlorides, carbonation of the concrete, and the availability of moisture. The failure of understanding the possible effects of corrosion on infrastructure can, at times have catastrophic results. Failures in recent years have reinforced the importance of corrosion and that concrete structures are not immune to its damaging effects. PEMEX management had the initiative to evaluate and to rehabilitate reinforced concrete structures, particularly those dedicated to the gas and fuel oil supply of important regions of the Mexican coastline. This report details the evaluations performed to structures on the Mexican pacific coast area including infrastructure in Mazatlan and Salina Cruz. This paper lists a series of procedures that were used to evaluate the structures and the status of the steel corrosion in within these structures; particularly the inspections of hotspots, identification of areas affected by performing various tests, measuring and sampling of the structure. The testing procedures, standards and criterion were in compliance with ASTM International (American Society for Testing and Materials), NACE International (National Association of Corrosion Engineers) and ICRI (International Concrete Repair Institute).
2. EVALUATION a) Document evaluation In order to understand the integrity of the concrete structures, existing documentation was collected including drawings, details, construction specifications, previous repairs completed, and historical information for operation of these structures when available for the previous 12 months. The operating condition and integrity of the structures were assessed through direct and indirect inspections. b) Identification of areas with visible defects During this stage, a preliminary study of the structure was conducted and its general characteristics and location is noted. The objective was to make a preliminary assessment, which provides information to determine the extent and type of studies to be carried out in the following stages of the examination process. Also considered regarding the areas for assessment was noting the activities and use of areas as well as the operations on that structure and the local environment. A visual inspection was completed to find areas with corrosion problems (as is illustrated in figure 2a). The technicians searched for areas with concrete damage, rust staining, and cracks, which are caused by the corrosion of the reinforcement. These areas were identified and qualitative characteristics and possible causes noted. Corrosion problems had typically manifested in those parts of the structure with seawater exposure. An initial assessment on safety and durability of the structure was made.
a
b
c
Figure 1. Examples of the visual and physical inspection procedures: a) recognition of the area, b) acoustic detection and c) impact test.
c) Identification of areas with hidden defects Detection of hidden corrosion defects by sounding Initially, the effects of the damage by corrosion in steel may not seen at a glance, since the delamination happens internally and does not manifest on the exterior surface of the structure. When a delamination exists, the space between the solid and loose concrete produces a particular sound that allows identification of the affected area. This technique, according to the ASTM Standard D4580[1] and combined with measurements of potential and resistivity, allows identification of active corrosion that are not visible from the surface. This acoustic inspection technique was applied, as it is shown in figure 1b, with impact on the surface of the concrete. Physical impact test of concrete structures to determine its resistance Reinforced concrete has excellent resistance to compression. However, corrosion of the reinforcing steel while causing reduced steel section, also has the added deficit of creating corrosion products of a greater volume that create tensile forces greater than the tensile strength of concrete. This causes fractures in the concrete cover. The mechanical strength of the steel decreases linearly with the section reduction, but also tension and fatigue resistance properties can be reduced 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/cm2. One method to asses concrete quality is by its resistance to force: it is an indicative value to check the load capacity and the durability of concrete structures. The resistance to force was performed with a test hammer, making physical impacts on the concrete as illustrated in figure 1c and according to the described in the ASTM standard C805.[2] d) Electrochemical assessment tests Electric continuity verification of steel reinforcement Electrical continuity within the reinforcing steel grid is required for the selection of future corrosion mitigation methods. Electrical continuity is determined by exposing reinforcing steel at several locations and tested using a high impedance multi-meter. Electrical continuity is also checked at the locations where there is exposed steel. As per ACI 222R-01 Standard in Section 4.3.1.6a, if the potential
difference between the reinforcing bars is less than one milli-volt, then the reinforcing steel is deemed electrically continuous. Half-cell corrosion potential measurements Half-cell corrosion potential measurements are done as a standard measurement that indicates the status of reinforcing bar corrosion (passive or active). Measurement are carried out using a standard copper/copper-sulphate (Cu/CuSO4) reference electrode (CSE), connected to a high impedance multimeter as per ASTM Standard C-876[1]. Half-cell values are interpreted as specified in the standard defining different states of corrosion for steel in concrete. These values allow you to estimate if the steel is passive or active by indicating the probability of reinforcing steel corrosion inside the concrete, and is usually performed on a pre-established grid of test points; in this case 1.5 meters using a CSE. This method is shown in figure 2a. A variability in the readings would indicate the potential for corrosion between two areas (more anodic vs. more cathodic). The corrosion potential determination technique has proved to be a useful method to locate areas of active corrosion in structures, and large areas can be tested in a relative short timeframe. Because the technique is not accurate in determining the amount of corrosion, other complimentary techniques must also be considered.
a
b
c
Figure 2. Examples of the electrochemical methods: a) verification of the electrical continuity, b) resistivity analysis and c) concrete core 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 is therefore an estimate of how protective the concrete is to the embedded steel. For low resistivity concrete, the ion transfer through the specific media is easier and a higher likelihood of corrosion exists. The samples are tested in wet and dry conditions. Additionally, testing concrete resistivity provides useful information that can be used for cathodic protection design and provides understanding as to whether the system chosen for rehabilitation is compatible with the existing concrete, which is done on site as is shown in figure 2b. For example, concrete with less than 15,000 ℌ-cm resistivity is ideal for galvanic protection of reinforcing steel. The electrical resistivity is a material property and corresponds to the reciprocal of its conductivity; the unit of measure is kℌ-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 paste and the presence of salts dissolved in the aqueous phase. It is based on variables such as cement type; inorganic additives; the
water/cement ratio; the porosity of the structure; among others. Since the resistivity is one of the factors that control the rate of steel corrosion in concrete, there is increased interest in determining this intrinsic property of the concrete. According to tests and analyses carried out concretes particularly saturated with chloride and water, normally have at the order of 10kΩ-cm resistivity. The purpose of this test is to determine the resistivity of the reinforced concrete on site, to locate areas in which the steel may not be passive and therefore susceptible to corrosion if sufficient oxygen and humidity conditions exist. Electrical resistivity measurements are made using the method known as "Wenner" or "4 tips". For this evaluation, a device of own design, connected to a Nilsson Model 400 resistance meter was used. 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 is taken from standard SP0308-2008[2], relating to "methods of inspection for the evaluation of corrosion in reinforced concrete structures" of NACE International. e) Sampling and lab analysis Sampling of concrete core for analysis Core extraction is completed using a wet coring machine with diamond core barrel. Special care is taken in the management of samples to avoid contamination as per standard ASTM C42[3]. The core samples must have a minimum diameter of 5 cm to 7.5 cm depending on the size of aggregate, and are cut at different depths. The cores are for chloride and carbonation testing. They are taken not only in areas where there is deterioration, but also where concrete appears to be in good condition. The use of a rebar locator allows the samples to be taken in close proximity to the reinforcing steel and to measure the depth of cover for correlation of the test to the actual depth of the steel. The sampling is seen in Figure 2c. Evaluation of chloride penetration With reinforced concrete in permanent or intermittent contact with sea water, sulfates and chlorides can penetrate by osmosis, capillary action and diffusion. Chlorides initiate corrosion by breaking down the naturally protective oxide layer on the steel. Sulfates damage concrete by causing expansive compounds in the cement paste which further facilitates the penetration of the chlorides and corrosion of the steel. Any damage facilitates further contamination and the process is accelerated. The chloride ion content can be measured as a percentage by weight of the concrete as per AASHTO T260 Standard. The chloride ion content in percent by weight of concrete is measured using the
‘Revised SHRP Chloride Analysis Procedure’. Chloride ion content below 0.025% by weight of concrete is considered below the threshold for chloride induced corrosion of the reinforcing steel. If the chloride levels climb above this threshold, the reinforcing steel may lose its passive layer and could initiate corrosion activity. Evaluation of concrete carbonation The carbonation of concrete occurs when the pH of the concrete, normally between 12 and 13, reduces to pH between 9 and 10. When this occurs, the layer of iron oxide or passive layer naturally formed in high pH concrete disappears, thus allowing corrosion to more readily occur. Concrete core samples are broken in half in the laboratory and the freshly exposed concrete is sprayed with a solution 0.15 % phenolphthalein in ethanol. Where the indicator solution is pink or purple in color correspond to a pH of 10 or higher, indicating sufficient alkalinity to maintain the passive oxide layer. Where the solution does not change color indicates a pH of 10 or less, which indicates carbonated concrete.
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 and South Tie-up Docks The North and South Tie-up Docks are mooring structures where vessels are supported. These docks are similar structures and differ only on the upper surface. The evaluation concentrated on the front faces, which receive the pressure of workload. Visually both structures show similar levels of deterioration, however there were some other noted differences. The North Dock showed a certain amount of delamination area: more than 20% of the total surface of the concrete which was more concentrated at the bottom of the front face. Corrosion potential readings indicated corrosion activity in 30% of the readings and 60% of the readings were in the uncertainty range. The chloride content tests showed over threshold amounts at 13mm deep. This would indicate that the steel near the surface would be in danger of being in a corrosive environment. The carbonation tests indicated shallow carbonation depths, at just 8 mm. It should be remembered that the threshold level for chlorides in carbonated concrete is much less than in normal concrete. South Dock showed much less delaminated area: only 10 per cent of the total of the concrete surface. The corrosion potentials in the front face of the dock indicate small areas of corrosion activity with 70% of readings in the low probability range. Chloride content tests showed over threshold amounts at 13mm. The carbonation found in the sample indicated a low level of carbonation (8 mm).
It was determined from the results from the North and South faces as well as the underside of the Operations Platform that the corrosion activity does not appear to be generalized beyond the zones of deterioration. Preventative measures for future chlorides should be considered to maintain the condition of the structure. Operations Platform The Operations Platform is the most important structure at the site because it is the structure that supports the main fuel transport infrastructure. The north and south faces of the Operations Platform had similar testing results. Both showed low levels of delaminated concrete, 6-8%, and on either face, the corrosion potentials indicated 80-87% of the area to have corrosion activity. Samples from the south face showed the presence of chlorides at two tested depths to the depth of the reinforcing steel. From this information the main problem issue is the delaminated concrete areas and concrete immediately adjacent. The topside of the Operating Platform displays the highest level of deterioration of all the structures with nearly 40% of the total area delaminated. This quantity is consistent with corrosion potential measurements of which 40% show a high probability of active corrosion, with another 50% of the readings in the range of uncertainty. In the two core samples obtained from this area, chlorides were found down to a depth of 50 mm, and 89 mm in depth. The samples also have levels of carbonation between 13 mm and 16 mm. This indicates generalized corrosion activity and a distinct possibility of damage in the future, beyond the current delaminated areas. The underside of the Operating Platform visually displays the lowest degree of deterioration. Corrosion potential measurements showed 30% of the readings in active corrosion range with the remaining readings in the uncertain range. The samples taken from this area showed presence of chlorides at a depth of 13 mm along with carbonation depth to 19 mm. These results indicate a status of active corrosion, but that at this point, only affects shallow steel. However, with further exposure to chlorides over time, corrosion damage to the structure may become more prevalent. Summary The structures evaluated showed damage caused by chloride exposure and carbonation of the concrete, however the level of deterioration in each area is different and may require different solutions. Before installing any corrosion protection system, delaminated and damaged concrete should be removed and repaired. In general, the PEMEX Mazatlan dock structure is in relatively good condition. However, the deteriorated areas should be addressed in order to ensure a longer useful life for the structure.
Salina Cruz For the study and analysis of reinforced concrete structures that form Pier 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 in dimensions and reinforcement details. The Operations Platform and Access Bridge are independent structures which differ structurally. The current conditions were assessed for the movement gateway, the gateway platforms, and frameworks and concrete joints that serve to support 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 the problems and recommendations of these structures were studied in particular. Analysis of chlorides The chloride content (Percent Cl- by weight of concrete) was, 82% of the examined samples show chloride levels higher than threshold. It can be concluded that all structures are experiencing corrosion problems due to chlorides in concrete where in some cases the corrosion has progressed to the point of delaminations. Only the testing of the east face of the operations platform results show low content of chloride, since this structure is new or it was recently rehabilitated. The chloride ions percentages decrease with deeper samples, confirming 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, we can assume that the corrosion processes will continue and increase over time if left unchecked. Analysis of Cu/CuSO4 potentials The measured corrosion potentials show values with active corrosion. It is noted a direct relationship between areas with significant damage and their respective potential measurements. From the corrosion potential measurements obtained from the operations platform, 96% correspond to high probability of corrosion activity, which coincides with the identified damages. This ratio is similar to the measurements obtained showing 85% of readings of low risk of corrosion for the newly repaired east face. The access bridge showed only 15% of corrosion potentials were in the active corrosion range, all located at the start of the access where signs of corrosion are found on both the bridge and the nearby structures (pipe racks). Overall 70% of the areas show high risk of corrosion and only 15% shows a low risk; therefore corrosion protection measures should be implemented to prevent further deterioration. The West Tie-up Docks 1 and 2 showed active corrosion potentials; 90% and 83% respectively, which indicate a requirement for corrosion protection of these structures. The West Tie-up Docks 3 and 4 had values with low active corrosion potentials (82 per cent each), matching the observed damages. The East tie up
docks 1 and 2, similar to west counterparts, showed high potential risk of corrosion; 90 % and 100 % respectively, which also show a requirement for corrosion protection of these structures. The East Tie-up Docks 3 and 4 had low risk potentials with potentials of 61% each, which coincide with the observed damages. The structures located at the end of the Pier 9 consist of 3 mooring platforms, 9 gateway heads, and the walkways that join them. Corrosion potential measurements from these structures show a high percentage (97%) with active corrosion which coincides with the observed deterioration of the elements. Rehabilitative solutions will be required for all these structures. Summary These evaluated structures are located in a marine environment and have been affected by the waters of the Pacific Ocean. Chlorides penetrating the concrete have affected the structural elements of reinforced concrete, some of them showing advanced corrosion-related deterioration and in some cases, the structural integrity is compromised. This is illustrated in Figure 3, which shows examples of the different identified damages. The main objective of this assessment was to sample and measure, in order to determine and define the current state of the different elements of the structure. The multidisciplinary analysis of the obtained information has lead to the creation of findings with sufficient technical support to define the scope, urgency, methods or systems and materials for rehabilitation.
Figure 3. Examples of visible damages on: a) walkways, b) platforms, c) concrete frames and d) structural supports.
From the information and analysis of the gathered data, it can be established that: •
The reinforced concrete structures that make up Pier 9 generally show problems associated with severe corrosion activity. Corrosion protection is required to extend the useful life of these structures and streamlining these petroleum facilities.
•
Because structures are in an aggressive marine environment, it was recommended to take actions for their rehabilitation to prevent unsafe conditions that can induce risks for staff, facilities and environment.
•
Some structures show deterioration in localized areas; however the readings of corrosion potential and the percentage of chlorides show it necessary to control corrosion throughout the entire structure.
•
There are structures as the located at the end of Pier 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.
•
Chloride ions are responsible for the loss of the steel reinforcement due to corrosion. Carbonation does not present a problem.
•
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 and repairs without achieving control of the corrosion problem and rather have aggravated it further. As all the control systems and protection against corrosion, require the repair of 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 existing corrosion problems.
4. GENERAL CONCLUSIONS AND SUGGESTED ACTIONS In case of Mazatlan, we can conclude that the evaluated structures are in relatively good conditions, with low risk corrosion potentials, and just a few parameters that require attention to control corrosion processes. The Salina Cruz case involves bigger issues. The structures have advanced corrosion deterioration in most of the structures with evident concrete damages which, in some areas, put at risk the continuous operation of the installation, including the support of the own structures. For this dock, repair and replacement 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 as general rehabilitation strategy. Deteriorated concrete must be removed and replaced in all areas where signs of damage have occurred. These repairs should be performed into sound concrete until the reinforcing steel exhibits no signs of corrosion. For both docks systems (Mazatlan and Salina Cruz) proposed rehabilitation actions and suggested strategies include: cleaning procedures, installation of alkali-activated discrete and distributed zinc anodes depending of the configuration, also the application of protective coatings against chlorides and carbonation as a polymer coating or hisolids epoxy coating. In conclusion, we would also like to stress the necessity of this kind of evaluation of structures exposed to aggressive environments such as maritime terminals and others 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. Acknowledgments The authors would like to thank Eduardo Gonzalez and Martin Beaudette of Vector Corrosion Technologies for their help with the technical evaluations.
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