National Academies Press: OpenBook

Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements (2006)

Chapter: Chapter 2 - Condition Evaluation of Superstructure Elements

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Suggested Citation:"Chapter 2 - Condition Evaluation of Superstructure Elements." National Academies of Sciences, Engineering, and Medicine. 2006. Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements. Washington, DC: The National Academies Press. doi: 10.17226/13934.
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Suggested Citation:"Chapter 2 - Condition Evaluation of Superstructure Elements." National Academies of Sciences, Engineering, and Medicine. 2006. Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements. Washington, DC: The National Academies Press. doi: 10.17226/13934.
×
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Suggested Citation:"Chapter 2 - Condition Evaluation of Superstructure Elements." National Academies of Sciences, Engineering, and Medicine. 2006. Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements. Washington, DC: The National Academies Press. doi: 10.17226/13934.
×
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Suggested Citation:"Chapter 2 - Condition Evaluation of Superstructure Elements." National Academies of Sciences, Engineering, and Medicine. 2006. Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements. Washington, DC: The National Academies Press. doi: 10.17226/13934.
×
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Suggested Citation:"Chapter 2 - Condition Evaluation of Superstructure Elements." National Academies of Sciences, Engineering, and Medicine. 2006. Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements. Washington, DC: The National Academies Press. doi: 10.17226/13934.
×
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Suggested Citation:"Chapter 2 - Condition Evaluation of Superstructure Elements." National Academies of Sciences, Engineering, and Medicine. 2006. Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements. Washington, DC: The National Academies Press. doi: 10.17226/13934.
×
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Suggested Citation:"Chapter 2 - Condition Evaluation of Superstructure Elements." National Academies of Sciences, Engineering, and Medicine. 2006. Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements. Washington, DC: The National Academies Press. doi: 10.17226/13934.
×
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Suggested Citation:"Chapter 2 - Condition Evaluation of Superstructure Elements." National Academies of Sciences, Engineering, and Medicine. 2006. Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements. Washington, DC: The National Academies Press. doi: 10.17226/13934.
×
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Suggested Citation:"Chapter 2 - Condition Evaluation of Superstructure Elements." National Academies of Sciences, Engineering, and Medicine. 2006. Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements. Washington, DC: The National Academies Press. doi: 10.17226/13934.
×
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Suggested Citation:"Chapter 2 - Condition Evaluation of Superstructure Elements." National Academies of Sciences, Engineering, and Medicine. 2006. Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements. Washington, DC: The National Academies Press. doi: 10.17226/13934.
×
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3Introduction Condition evaluation or bridge inspections are performed by every bridge owner as mandated by the National Bridge Inventory Program (NBIP). This program resulted in the development of the National Bridge Inspection Standards (NBIS), which prescribe minimum requirements for the inspection of highway bridges on public roads. The visual inspection method is by far the most common form of non- destructive evaluation (NDE) technique used to satisfy the NBIS requirements. The AASHTO Manual for Condition Evaluation of Bridges outlines five different types of bridge inspections: 1. Initial Inspection—Upon the completion of a new bridge structure, this inspection is performed to obtain all struc- ture inventory and appraisal data and to determine the baseline structural conditions and identify current or potential problem areas. 2. Routine Inspection—This inspection is performed on a regular interval of time (usually every 2 years), in accor- dance with the requirements prescribed by the NBIS, to determine the physical and functional condition of the bridge and to identify changes since the last inspection. 3. Damage Inspection—Damage resulting from environ- mental or human actions triggers this inspection, whose primary goal is to identify the need for further action. 4. In-Depth Inspection—This inspection focuses on certain sections of the bridge structure to investigate deficiencies not detected during Routine Inspection. 5. Special Inspection—This inspection is conducted to monitor a single known defect or condition. Routine Inspections are performed on a regular basis and provide a good history of the superstructure elements. The other inspections are performed only when triggered by a specific event. The Routine Inspection indicates the onset of or ongoing corrosion on a superstructure element based on the results of the visual survey; however, it does not provide sufficient information for planning purposes. In addition to collecting visual information, a few agencies collect concrete samples to analyze the distribution of chloride ions in con- crete, measure the quantity of damage, or conduct a half-cell potential survey to determine the distribution of active sites where corrosion may be occurring. The results, in conjunc- tion with the visual survey results, are used to determine the condition rating of the bridge element. In general practice, when the Routine Inspection identifies that the condition of the structure has degraded sufficiently to require a repair, a more detailed inspection is performed to prepare construc- tion documents. The primary goal of this detailed inspection is to determine the quantity of repairs, the type of repairs, and the need for preventive measures. The scope of such inspec- tions varies from state to state and agency to agency depend- ing on local needs and available resources. Some agencies are more sophisticated than others and use one or more of the newer technologies available to do an inspection, model the damage to predict future progression, identify the repair, identify applicable corrosion mitigation technologies, and perform life cycle cost analysis. This strategy puts a significant financial burden on the owner because it is does not allow the owner to manage the inventory cost-effectively. The owner is always responding to an urgent need for repairs, and the superstructure elements are often repaired when they have experienced significant damage. If a preventive strategy were to be used, the cost of repairs and the overall life cycle cost of the structure could be reduced. To implement such a strategy, a modeling tool is required that would allow the owner to estimate the future performance of the structure. An inspection protocol is proposed herein that uses such modeling and also minimizes the amount of survey work to be performed. In addition to modeling the expected future damage, this protocol proposes a method of quantifying the future propensity for corrosion by using an index that C H A P T E R 2 Condition Evaluation of Superstructure Elements

represents the distribution of chloride ions in the concrete. This index can be used to identify corrosion mitigation technologies that are most suited for arresting corrosion in the future. Development of an Evaluation Protocol An optimal approach to superstructure maintenance would be to modify the extent of the Routine Inspection to provide additional information that can be used for model- ing and developing a Susceptibility Index (SI) with minimal cost and perform the In-Depth Inspection only when neces- sary. The additional information from the Routine Inspection can be used to plan and prioritize necessary prevention and repairs. The In-Depth Inspection can be used to collect data necessary to develop the construction documents. Routine Inspections, as generally conducted by state and local agencies, do not provide sufficient information to cor- rectly identify the corrosion status of the subject superstruc- ture element. It mostly documents the progression of ongoing corrosion-induced damage based on the observation of telltale signs such as rust staining, cracking, delamination, and spalling of concrete on concrete surfaces with no surface treatment and sometimes the presence of chloride ions. On bridge decks that have an asphalt overlay, the initiation and progression of corrosion is not visible, and the Routine Inspection does not provide reliable information. If initiation of corrosion has been observed during Routine Inspection, but sufficient corrosion-induced damage has not yet occurred to necessitate a repair, then it is prudent for the owner to try and determine approximately when the structure would need major construction to restore or maintain its full functional- ity. Also, when a structure is scheduled for an expansion or an upgrade to meet present codes, it seems appropriate to iden- tify the future performance of the existing sections. If the existing sections are expected to suffer from corrosion- induced damage in the future, a corrosion mitigation system may be installed during the expansion or the upgrade to obtain the desired service life. To obtain a reasonable estimate of the future progression of corrosion-induced damage, at a minimum, the knowledge of chloride ion distribution, clear concrete cover, and quan- tity of damage in the form of delamination and spalling is necessary. With this information, a diffusion model can be used to estimate the increase in concrete damage as a function of time. A different approach would be to document the pro- gression of damage with time and to fit a cumulative Weibull distribution curve to the data to predict future progression of damage. In addition to estimating future damage, the model could calculate SI based on the distribution of chloride ions in areas that are presently not damaged. This index would reflect the susceptibility of the undamaged areas to corrosion in the future and allow the owner to effect a corrosion pre- vention strategy. The corrosion process occurs in two phases, corrosion ini- tiation and propagation. The rate of corrosion during the ini- tiation phase is low and can increase exponentially with time during the propagation phase depending on the various parameters that control its rate, such as temperature and its variation, availability of oxygen, conductivity of concrete, humidity, ingress of chloride ions, and formation of a macro- cell. Therefore, if the corrosion process is allowed to continue to occur unabated, the rate of damage development will increase with time. This generally results in increase in cost as the quantity of repairs increases and often will require a major construction effort. With the increasing rate of corrosion and quantity of damage, the pool of suitable corrosion mitigation systems diminishes, more aggressive and costly corrosion sys- tems become necessary, and replacement of the concrete ele- ment becomes a more viable option. During the early stages of corrosion, fewer repairs and less costly corrosion mitiga- tion systems can be used, which likely reduces the life cycle cost of the structure. Use of a protocol with modeling dam- age and calculation of the SI would help identify options for implementing a prevention regime rather than the usual respond-to-urgency approach. For bridge superstructures exposed to adverse environ- ments, the question is not if corrosion will occur, but when it will occur and when a major repair will be required. Two types of corrosion condition surveys are proposed: the “Preliminary Corrosion Condition Evaluation” (PCCE) and the “In-Depth Corrosion Condition Evaluation” (In-Depth Inspection). The PCCE should be performed in conjunction with Routine Inspection, but not at the 2-year frequency. It should be per- formed after the first signs of corrosion initiation are observed during a Routine Inspection, preferably during the subsequent inspection. The results of the first PCCE are then used to deter- mine future action. Depending on the need and available resources and the results of the modeling performed with the data collected during the PCCE, several different actions can be taken. The results can be used to identify and install an appropriate corrosion mitigation system to arrest future corrosion-induced damage, or they can be used to just monitor the health of the structure and to optimally schedule the repair. If the PCCE is used to monitor the health of the structure for the purpose of determining the optimal time to repair, more than one PCCE may be performed. For example, if the first PCCE suggests that corrosion initiation will not occur for 20 years, then another PCCE is not necessary for another 10 to 15 years. When the next PCCE should be performed is dependent on the type of structure, the wearing surface (bare concrete or asphalt overlay with or without membrane), and the presence of epoxy-coated rebar. The inclusion of the PCCE in the Routine Inspection should in no way impact the 4

Routine Inspection schedule, and the PCCE should be included in the Routine Inspection only as necessary. The scope of work for the PCCE will depend on the struc- ture, the owner’s needs, and the available resources. The pres- ence of an asphalt overlay with or without a membrane and the presence of epoxy-coated rebar will significantly impact the scope of work. A discussion of the recommended scope of work for each instance is presented later in this chapter. When a bridge superstructure is already slated for repairs, based on either Routine Inspection or the PCCE, an In-Depth Inspection needs to be performed to obtain accurate repair quantities, determine the type of repairs (full depth, partial depth, or type of crack repair), ascertain the need for corro- sion mitigation, and select a corrosion mitigation system for the structure. In planning the repair of a bridge superstruc- ture element, it is important to examine the areas that have not yet deteriorated and will not be repaired. If the undam- aged areas have a propensity for corrosion, some form of cor- rosion mitigation system will be required to ensure that corrosion in these areas does not reduce the service life of the concrete element. In addition, corrosion may be accelerated by the coupling of repaired areas and adjacent chloride contaminated areas. Under such circumstances, a corrosion control system will be necessary to stop or control corrosion in the undamaged areas to obtain the desired service life. Additional tests are also performed in this evaluation to verify the existence of any other concrete deterioration processes—such as freeze-thaw damage, alkali-silica reactiv- ity (ASR), and ettingrite formation—and to ascertain the compatibility of potential corrosion control systems. For example, if freeze-thaw damage or ASR is present, then the repair and control system must include a mechanism to con- trol these deterioration processes in addition to corrosion. The knowledge of electrical continuity will be required if cathodic protection is to be considered for corrosion control. Data collected during previous PCCE can be used in the In- Depth Inspection, although some of the data will need to be augmented. The damage survey will need to be conducted with a higher level of accuracy to obtain reasonable quanti- ties for construction. This inspection or evaluation should not be performed any earlier than 2 years from the actual con- struction because damage quantities can change, especially in the later stages of the corrosion process. The following sections describe different test methods and techniques used in each type of evaluation. Test Methods and Test Techniques Numerous test methods and test techniques are available for use in evaluating bridge superstructures. Following is a dis- cussion of these methods and techniques. Applicability and limitations of each test method or technique are identified. Visual Survey A visual survey is conducted to observe and document the overall condition of the structure under investigation. This survey is a vital part of the evaluation because the use of subsequent test procedures depends on the visual assess- ment of the structure. The inspection should follow an orderly progression over the structure so that no section of the deck or superstructure is overlooked. A visual survey should be conducted in accordance with ACI 201.1R-92, “Guide for Making a Condition Survey of Concrete in Service.” The following deteriorations are typically found on bridge superstructures and should be documented: • Cracking, • Spalling, • Scaling, • Rust staining, • Efflorescence, and • Patching or existing repairs. The visual survey can be performed at various levels of accuracy. The highest level of accuracy would require docu- mentation of each occurrence of deterioration and its exact dimensions. This would require significant time and labor resources. It is often not necessary to document deterioration to that level of accuracy. However, the following minimum documentation is recommended: • Type and frequency of cracking. The type of cracking pro- vides clues as to the cause of it. The frequency can be clas- sified by using a rating scheme—such as very slight, slight, moderate, severe, and very severe—or can be documented as density (i.e., total length of cracks per unit of area). The density measurement of cracks can be limited to cracks exceeding certain width criteria, and the crack survey can be limited to a select section of the superstructure. It would be impractical to perform such a survey on the entire bridge deck or all beams and girders. This information becomes more relevant and results in a better estimate of quantities if crack repair in the form of routing or injection will be necessary. • Location and dimension of spalls. Because all spalls, along with delaminated areas, will need to be repaired, accurate estimates of quantities are essential for preparing contract documents. • Location of scaling. This should include an estimate of impacted area. • General location of rust staining. This will give an idea of what parts are corroding. • Locations and dimensions of all existing repairs or patches. 5

Photographic and video documentation of visual survey significantly improves the quality of the data and allows a review of the damage in more detail at a later date. Delamination Survey A typical form of deterioration induced by corrosion of reinforcing steel is cracking and delamination of the concrete. A delamination is a separation of concrete planes resulting from tensile failure. Depending on the ratio of cover to bar spacing, the fracture planes will either form cracks or cause a delamination parallel to the surface. The “Diagnosis of Dete- rioration in Concrete Structures” states that cracks are the likely results of a ratio of cover/bar diameter equal to 1, larger cracks and risk of delamination when cover/bar diameter equals 2, and delamination when cover/bar diameter equals or exceeds 3 [4]. The size of the delamination generally increases with time due to continuation of the corrosion process, freeze-thaw cycles, and impact of traffic. Upon attainment of critical size, a delamination will result in a spall. Upon reaching a critical quantity, delaminations can impact the structural integrity of the concrete element. Several different techniques and types of equipment are presently available to detect delaminations. All of these tech- niques are based on the propagation or reflection of energy. Mechanical impact energy is used by techniques such as sounding and impact-echo. Acoustic energy is used by ultra- sonic pulse velocity. Thermal and electromagnetic energy are used by infrared thermography and ground-penetrating radar, respectively. Sounding Perhaps the most commonly used and inexpensive method for determining the presence and extent of delaminations is sounding. Depending upon the orientation and accessibility of the concrete surface, sounding can be performed with a hammer, steel rod, or a chain. The concrete is struck with a hammer or rod, or a chain is dragged across a horizontal sur- face. Good concrete with no delaminations produces a sharp ringing sound; delaminated areas emit a dull, hollow sound. ASTM C 4580-86, “Standard Practice for Measuring Delam- inations in Concrete Bridge Decks by Sounding,” governs this test procedure. For bridge decks, a chain is dragged along the concrete surface to locate delaminated areas. Edges of delam- inated areas are then defined using a steel rod or hammer. Delaminated areas are outlined on the concrete surface, measured, and recorded on drawings with reference to the survey grid coordinates. As delaminations are generally irreg- ular in shape, the irregular shape is enclosed in a rectangular or a triangular shape, which is documented. During concrete repairs, regular geometric sections of concrete are removed and repaired because of the increase in efficiency and reduc- tion of costs. This technique depends on operator judgment and is prone to operator errors. Operator fatigue and background noise can reduce the accuracy and the speed of the survey. Often times, the sounding technique is combined with the use of sand and vibration of the concrete surface. On bridge decks when ambient noise is high, sand can be broadcast on the sur- face of the deck. The movement of the sand when a hammer is struck on the concrete surface is used to detect delamina- tion. Sand particles on a delaminated surface will bounce up because of the rebound from the delaminated plane. If sand is not available, the operator can place his or her palm on the concrete surface and feel the rebound when the hammer is struck on the concrete surface. Impact Echo Impact echo is a process in which a mechanical impact pro- duces a stress wave in a material. This stress wave travels through the material until a discontinuity is encountered. At this discontinuity, a portion of the stress waves are reflected. For any isotropic and elastic media, there are three modes of stress wave propagation: dilatational, distortional, and Rayleigh waves. Dilatational waves (P-waves, or compression waves) produce particle motion that is parallel to the direc- tion of stress wave travel. Distortional (S-waves, or shear waves) produce particle motion that is perpendicular to the direction of stress wave travel. Rayleigh waves (R-waves) travel near the surface of the material in a retrograde ellipti- cal motion. Typically, a mechanical impact is produced by striking a small (diameter less than 2 inches) metal sphere on the surface of the material to be tested. This impact produces stress waves that propagate in all directions.As these stress waves propagate into a material, reflections occur at interfaces between two materials of differing acoustic impedance. A transducer, located at the surface near where the impact initiated, can receive these reflected stress waves. By measuring the time dif- ference between the initial impact and the receipt of reflected stress waves from a boundary, the depth to that boundary can be estimated. It is important to have a basic understanding of the velocity of the stress wave traveling through the material. A typical P-wave velocity of 9,800 to 14,800 feet per second is assumed for concrete (depending on the proportions and specific gravities of the constituents). This value can also be measured by performing impact echo where the depth to a boundary or thickness is already known. The resolution and depth of penetration are controlled by the impact duration or contact time between the metal impactor and the surface of the material. The impact dura- tion is proportional to the diameter of the impactor; 6

shorter duration impacts are produced with impactors of smaller diameter. In addition, the frequency of the stress wave is proportional to the impact duration; an impact of shorter duration produces a stress wave of higher fre- quency. Thus, an impactor of small diameter will produce a higher-frequency stress wave. A higher-frequency stress wave will provide greater resolution but less depth of pen- etration. Impact echo can be a useful technique given the proper scenario. The surface texture of the material to be tested is important to consider. Although impact echo can be used to perform analysis on hot-mix asphalt (HMA) pavements, because of the typically rough surface texture, the useful- ness of this technique in this application may be limited. Impact echo, however, has been employed as a useful tech- nique to monitor the condition of various concrete struc- tures [5, 6]. Ultrasonic Pulse Velocity Pulse velocity works in much the same manner as impact echo. The primary difference is in the stress wave that is applied. While impact echo employs a stress wave (result- ing from a mechanical impact), ultrasonic pulse velocity uses a high-frequency (greater than 20 kHz) sound wave emitted from a piezoelectric transducer. The pulse velocity method employs two transducers, one to send and one to receive the sound wave. Using the two transducers, the travel time is calculated. If the thickness of the material is known, the travel time can be converted into velocity and compared with typical values (sound wave speed of approximately 12,100 feet per second for concrete). By comparing the calculated velocity with typical values, a determination can be made about the condition of the material under study. Several factors may affect the accuracy of the ultrasonic pulse velocity method [7]. Because proper contact between the transducers and the surface of the material being tested is critical, a couplant is used to ensure that there is not an air gap between the surface of the material and the transducer. The temperature and the moisture content of the concrete may also play a small role in affecting the travel time of the ultra- sonic pulse. In addition, the minimum spacing between the transducers (i.e., the thickness of material being tested) should be greater than approximately 4 to 6 inches. This value will depend upon the velocity of the ultrasonic pulse in the material and the frequency of vibration of the piezoelectric material in the transducer. As with impact echo, there may be some difficulties in using ultrasonic pulse velocity on an HMA surface because of the rough surface texture typical of HMA layers and the dif- ficulty of establishing a proper contact with the surface. Infrared Thermography Infrared thermography is based on the principle that defects within a material will alter the way heat flow is dissi- pated at the surface of that material [8, 9]. These changes in surface temperature can be measured to locate and possibly determine the quantity of subsurface defects. Heat flow will occur when the temperature of the material differs from the temperature of its surroundings. A common practice when using infrared thermography on large structures is to perform the testing early in the morning while the sun is heating a structure or just after dark when the energy from the sun is being released into the environment. Since concrete structures typically involve large areas, natural sources of heat (such as the sun) can be useful in performing infrared thermography. However, this form of passive heating tends to reduce the ability to resolve the dimensions of spe- cific defects since radiation from the sun is not very strong and thus it takes a significant amount of time to provide enough heat to flow through a large structure [10]. Several factors can influence the accuracy of infrared thermography outside the laboratory setting, including surface texture, wind speed, and surface moisture. Each of these factors can influ- ence the way that heat is adsorbed or dissipated. Ground-Penetrating Radar Ground-penetrating radar (GPR) operates by directing electromagnetic waves toward an object of interest. In this technique, as the electromagnetic waves pass through the object, energy is reflected at boundaries between two materi- als possessing differing values of the dielectric constant (i.e., real part of the complex permittivity). While some energy is reflected at each boundary, the remainder continues propa- gating into the object. Typical dielectric constant values seen in civil engineering applications include air = 1, concrete = 8 − 12 (depending on material properties and proportions of constituents), and water = 81. More energy is reflected at an interface between two materials where the difference in the dielectric constant is greatest. Thus, it is easier to detect (more energy is reflected) a defect such as a water-filled delamina- tion (water/concrete interface) rather than a defect such as an air-filled delamination (an air/concrete interface). In addi- tion, less energy is reflected back from the air-concrete inter- face because of the similar dielectric constants, and more energy is available to travel inside the concrete. Typical frequency ranges of operation for GPR are from 80 MHz to 2 GHz, with the higher-frequency antenna being more suited to bridge deck studies. An antenna of higher fre- quency will result in greater resolution (i.e., ability to differ- entiate between objects), but less depth of penetration. Research studies have shown that GPR can be used in certain 7

instances to monitor subsurface condition of bridge decks. Some researchers used GPR to locate defects and reinforce- ment bars [11, 12]. The propagation of GPR through concrete is not com- pletely understood, and many factors impact its propagation and reflection of the signal. At present, GPR is primarily used to measure slab thickness and location of embedded metals; all other applications for detecting cracks, delaminations, chlorides in concrete, etc., are qualitative comparisons only. Cover Depth Measurements The thickness of concrete cover over reinforcing steel has a significant influence on the time to initiation of corrosion when chloride ions are diffusing into the concrete element from the environment. Shallow cover on a structure will lead to more rapid accumulation of chloride ions at the steel depth in excess of the threshold required to initiate corrosion and subsequently results in faster development of concrete damage. The location of a reinforcement bar and its depth of cover can be obtained nondestructively by using a pachometer or a covermeter. These devices measure variations in magnetic flux caused by the presence of reinforcement bars to locate their presence and depth. In general, detection of the rein- forcement is very accurate given proper usage by the opera- tor. Some pachometers or covermeters provide an estimation of the bar size; however, they may have an error of up to one bar size. Measurements of the cover depth are generally more accurate when the structure is lightly reinforced. As an alter- native method, small holes may be drilled into the concrete to measure the cover. This method can be more accurate, but it also introduces defects into the structure. Commercially available covermeters are usually compact with single-element, hand-held probes and allow easy access to structural elements. They are useful for locating and deter- mining the cover over individual reinforcement bars. How- ever, they can be time consuming when trying to determine location and cover depth over large areas. Chloride Ion Content Analysis Chloride ions are the primary cause of reinforcing steel corrosion. The primary sources of chloride ions are chloride- bearing admixtures used during construction, chloride- contaminated constituents (water or aggregate) used during construction, deicing salts applied to surface of structure, and air-borne chlorides and direct exposure to sea water in marine environments. It is generally accepted that corrosion of reinforcing will only occur once a threshold value of chloride ion content adjacent to the bars is reached. It is generally given that this threshold value is approximately 0.025% to 0.033% by weight of concrete. Hence, it is important to determine the chloride ion distribution in a structure under investigation to be able to determine its susceptibility to corrosion. Chloride profiles (chloride concentration versus depth from the surface) pro- vide valuable information on the source of the chloride ions and the apparent rate of diffusion of the chloride ions in the concrete. The rate of diffusion can be used to calculate when the chloride ion concentration at the steel/concrete interface will exceed the threshold required to initiate corrosion, if it has not already exceeded it. The chloride content in concrete can be determined through analysis of powdered concrete samples. Samples can be collected on-site at different depths up to and beyond the depth of the reinforcing steel using a hammer drill. Extreme care should be exercised to avoid inadvertent con- tamination of the samples. Alternatively, cores can be col- lected and powdered samples can be obtained at different depths in the laboratory. Chloride ions in concrete exist in two forms, chemically bound and soluble in the concrete pore water. The chloride ion content of concrete is usually measured in the laboratory using wet chemical analysis. The total chloride (or acid solu- ble) test method measures the sum total of all chemically bound and free chloride ions in the concrete. The water solu- ble test method measures only the free ions soluble in pore water. The water soluble chloride ions are linked to the initi- ation of corrosion. Because the water soluble test method is not very accurate or repeatable, the general practice is to use the acid soluble test method. Most researchers have used the acid soluble test method and reported varying threshold val- ues for corrosion initiation depending on design of the con- crete mix. It has been reported by several researchers that the chloride to hydroxide ratio is more important than the actual concentration of the chloride ions. However, in practice it is not easy to determine the chloride by hydroxide ratio. Although laboratory testing is most accurate, it is also time consuming. It often takes weeks to produce usable results. As a result, field test kits have been developed that allow more rapid determination of chloride levels on-site. All field test kits use the acid method for analysis; they are typically not as accurate as laboratory analysis, but they do provide good correlation with the laboratory test method. Therefore, a correction factor must be applied depending on the type of field test kit used. Results of the chloride content analysis are reported as either percentage chloride by weight of concrete, parts per mil- lion (ppm) of chloride ions, percentage chloride per weight of cement, or weight of chloride per volume of concrete. Express- ing the percentage of chloride per weight of either cement or concrete or the weight of chloride per volume of concrete requires the knowledge of the cement content (typically 657 lb/yd3) or a unit weight (typically 3,915 lb/yd3). 8

Electrical Continuity Testing Continuity testing is performed to determine if various metallic objects (usually reinforcing bars) within the concrete are in direct contact, or electrically continuous, with each other. This type of testing is needed for the following three reasons: • Results of this test are needed prior to conducting the cor- rosion potential survey and rate of corrosion tests. • Direct contact between reinforcing steel and other metals (e.g., aluminum or galvanized steel) can lead to corrosion due to dissimilar metals and the presence of electrical con- tinuity supports the formation of macrocells. • The state of electrical continuity of all embedded metals must be known when considering cathodic protection as a long-term protection option. The electrical interaction of embedded reinforcing bars and external metallic components of bridge superstructures influence the results of all electrical tests run during a con- dition survey. The corrosion potential survey is particularly sensitive to continuity because all the reinforcing steel within a given area must be continuous. If a ground con- nection is made to a bar or other metallic component that is electrically isolated from the reinforcing bar in the survey area, the readings will essentially provide remote corrosion potential measurements of the isolated ground and are therefore meaningless. Normally, steel chairs, direct contact at intersection points, and wire ties provide good electrical continuity throughout cast-in-place sections of a bridge. Precast con- crete bridge members also typically exhibit good electrical continuity. However, electrical continuity should always be verified during a condition survey. Continuity across expansion joints, between scuppers and reinforcing bars, and between railings and reinforcing bars is always suspect and requires verification. Any metallic component can be used as the ground location for testing if it is electrically continuous to the reinforcing bars being tested. During the survey planning stage, proposed potential grid map locations should be laid out to avoid spanning obvious dis- continuities. Theoretically, when epoxy-coated reinforcing steel is encountered during evaluation of a structure, every bar should be electrically isolated (i.e., electrically discontinu- ous). However, previous experience in testing these structures has shown that the degree of electrical discontinuity can range from partial to complete depending on the structure and construction practice. Therefore, before conducting elec- trical tests on structures containing epoxy-coated reinforcing steel, electrical continuity testing should be performed. Corrosion Potential Survey Because corrosion is an electrochemical process, the electrical potential is a parameter that can be measured to indicate the state of the corrosion process. The corrosion potential of a reinforcement bar provides an indication of the status of corrosion at the measurement site at the time of measurement. A potential difference is measured between a half-cell that is placed on the surface of the con- crete structure and a reinforcing bar acting as a ground. These values are compared with empirical values to deter- mine the relative probability of corrosion activity. A sur- face map of potentials can be created by performing multiple measurements on the concrete surface following a grid pattern. Corrosion potentials are most often performed using a copper-copper sulfate (Cu-CuSO4) half-cell. In addition, silver- silver chloride (Ag-AgCl) or graphite cells can be embedded as permanent reference cells. The Cu-CuSO4 half-cell is popular because copper is easily maintained at a standard potential over a wide range of operating conditions and because copper sulfate and distilled water are easily obtained. Corrosion potential surveys are preferably carried out on a regular interval grid such that they can be plotted to create equipotential contour maps. Difficulties in half-cell place- ment, variations in chloride distribution, and changes in tem- perature and moisture content affect the half-cell potentials over a wide area. Therefore, evaluating a large number of closely spaced half-cell potentials is necessary. Each test location to be surveyed must have a unique ground location if the underlying reinforcing steel is not elec- trically continuous. If continuity testing has verified the elec- trical continuity, a common ground location can be used for several potential measurements. The size and layout of the half-cell potential survey areas are dictated by the recording memory of the multimeter (if so equipped) and/or the phys- ical layout of the structure. Typically, each span is mapped separately such that a map of each span can be individually printed. The ground location is established by exposing a por- tion of reinforcing bar and drilling a three-sixteenths inch diameter hole into it. A self-tapping screw can then be inserted and a test lead from the multimeter can be clipped to the screw for a secure connection. The following guidelines have been developed for evaluat- ing Cu-CuSO4 half-cell potentials of uncoated reinforcing bars in concrete (see ASTM C-876, “Standard Test Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete”): • A 90% probability of no corrosion activity on the reinforc- ing bar at the time of measurement exists if the half-cell potential are less negative than −0.200 V. 9

• An increasing probability of corrosion activity exists if the half-cell potential falls between −0.200 V and −0.350 V. This probability depends on factors such as chloride con- tent at the reinforcing bar level, moisture content of the concrete, temperature, etc. Typically, values within this range are said to have an uncertain probability of corro- sion activity. • A 90% probability of corrosion activity on the reinforcing bar at the time of measurement exists if the half-cell poten- tials are more negative than −0.350 V. Comparing differences in half-cell potentials across a structure or in an area of a particular concrete member is more indicative of the probability of corrosion activity. For example, a 5 ft2 section of deck that has potentials that vary by 100 mV between readings is more active than a similar sec- tion with a 30-mV reading variation. An important application of the corrosion potential survey is to develop a historical record of corrosion potentials for a given structure. If the potential survey is conducted at a reg- ular time interval, variations in the potentials with respect to time can indicate if the corrosion activity of the steel is increasing or if the total area of steel showing active potentials is increasing. It is important to point out that the corrosion potential is an indicator of the corrosion process and not a measurement of the corrosion rate. Corrosion rate is a function of many parameters, such as temperature, potential difference of an open circuit, concrete resistivity, ratio of anodic and cathodic areas, and rate of diffusion of oxygen to cathodic areas. Cor- rosion potentials can be measured using any high-quality multimeter (similar to that used for electrical continuity) and recorded manually. In addition, there are several commercial multimeters that have the ability to store readings for later analysis. There are also half-cell arrays that allow for testing at multiple points simultaneously using a single multimeter. These multiple array systems allow large areas (such as bridge decks) to be tested rapidly. Corrosion Rate Measurement Techniques for measuring the corrosion rate of rein- forcement in concrete have been developed in recent years. These techniques provide information on the rate at which steel is being oxidized. The higher the rate, the sooner con- crete cracking and spalling will appear at the surface. This information is very useful in estimating the time to addi- tional damage and in selecting cost-effective repair and long-term protection systems. Although the techniques have been used in electrochemical laboratories for decades, field application of the equipment and methods are rela- tively recent developments. Corrosion rate devices apply a small voltage or current per- turbation to the reinforcement, and the corresponding cur- rent or voltage response is measured. The data are then manipulated using the Stern-Geary equation to derive the rate of corrosion. From a field test standpoint, one of the dif- ficulties is in determining the area of reinforcement tested. This is important since corrosion rate is defined in terms of the corrosion current per unit area of reinforcement. The magnitude of the corrosion current measured is a direct indi- cation of how fast corrosion is occurring on the reinforce- ment surface. High currents indicate a high corrosion rate, and vice versa. The measurement of the corrosion rate is only valid for the particular conditions at the time of measurement. For exam- ple, ambient temperatures vary throughout the year and con- crete moisture may also vary with time; these changes result in a corresponding change in the corrosion rate. Therefore, prediction of future corrosion activity must also include an evaluation of the dynamic environmental effects. The most active corrosion or highest corrosion rates may not be occur- ring at the time of the field study. Continuous or intermittent monitoring over a period of time provides a more accurate appraisal of the corrosion rate. Corrosion rate testing (as described above) is only valid for conventional bare (i.e., uncoated) steel. No data interpreta- tion procedures have been developed for epoxy-coated or gal- vanized reinforcement. Several commercial instruments have been developed to measure corrosion rate; each varies in the interpretation of the results from the respective instrument. Petrographic Analysis Several quality control issues and deterioration processes in concrete cannot be easily identified by nondestructive means. They, however, can be more easily discerned by col- lection of a core and examined petrographically. Petrographic analysis consists of inspection of a freshly fractured and pol- ished concrete surface with the unaided eye and by micro- scopic examination. Petrographic examination is often supplemented with chemical analysis, X-ray diffraction analysis, and scanning electron microscopy. Information obtained during a petrographic analysis may include • Condition of material; • Causes of inferior quality; • Identification of distress or deterioration caused by chloride- induced corrosion, carbonation, ASR, freeze-thaw cycles, etc.; • Probable future performance; • Compliance with project specifications; • Degree of cement hydration; 10

• Estimation of water-cement ratio and unit weight; • Extent of paste carbonation; • Presence of fly ash and estimation of amount of fly ash; • Evidence of sulfate and other chemical attack; • Identification of potentially reactive aggregates; • Evidence of improper finishing; • Estimation of air content and percentage of entrained ver- sus entrapped air voids; • Evidence of early freezing; and • Assessment of the cause of cracking. Selection of Tests for Corrosion Condition Evaluation When resources permit, all tests listed previously should be performed during each evaluation. However, if it is necessary to reduce the scope of work, the selection of tests should be based on the primary goal of the survey. The primary goal of the PCCE is to predict the future progression of damage and calculate SI, whereas the primary goal of the In-Depth Inspection is to obtain sufficient data to prepare construction documents and calculate the SI. Following is a brief discus- sion on a methodology of selecting test procedures. The visual and the delamination survey must be performed regardless of the goal of the corrosion condition survey; the degree of accuracy for the visual survey should remain the same. However, the extent of the delamination survey may be varied depending on the survey goals. For a PCCE survey, the delamination survey can be performed in representative test areas and not over the entire surface of the superstructure element. The test areas are selected so that they are represen- tative of all variations in the structure because variations in the concrete condition, exposure, or construction quality may result in variation of durability of the concrete. During an In-Depth Inspection, a high degree of detail for the visual and delamination surveys is required to derive repair quantities. Because the visual survey by itself is not suf- ficient to obtain total repair quantities, the delamination sur- vey must also be performed. Most often, repair quantities are underestimated during the repair design process, and the project cost increases once construction is initiated. A general rule of thumb is to double the repair quantities identified during a visual and a delamination survey. During construc- tion, the contractor will need to remove additional concrete adjacent to previously identified damaged areas. Good con- crete repair practice requires that additional concrete adjacent to the damaged area be removed until no corrosion is observed on the reinforcing bars to account for the fact that the process of delamination has just been initiated adjacent to the damaged area but has not reached completion. Such areas are often hard to identify during the delamination survey and will need to be accounted for. Another cause for the increase in quantities is that during construction it is often more effi- cient to join one or more adjacent damaged areas, enlarge them, or regularize the geometry of the damage area. All of these factors increase the quantity of repair. Another option is for the contractor to estimate the exact repair quantities during the construction; the owner will have representatives on-site during construction to verify these quantities. This option has the advantage of closing the structure down only once during the construction, but it does not provide the owner with a good estimate on the total cost of the project because the cost is determined as the project progresses. Care must be exercised if delamination testing is to be per- formed on treated bridge decks (i.e., decks with overlays, membranes, or some kind of surface treatment). Delamina- tion surveys performed on concrete overlays identify not only the delaminations in the concrete slab but also the disbondment of the overlay from the concrete slab. In such cases, cores must be collected in areas identified to be delam- inated to ascertain if the delamination is at the overlay- concrete interface or in the concrete slab. As sounding surveys cannot be performed on asphalt overlays, other non- destructive test methods have to be used. To date, none of the nondestructive test methods have proved efficient and/or accurate with asphalt overlays. Whenever asphalt overlays are to be removed for replacement, a delamination survey should be performed on the exposed deck. The same is true with all other overlays, such as thin concrete-epoxy, epoxy, and waterproofing membranes. Sounding surveys can be performed on concrete slabs treated with certain types of sealers and paints. The chloride ion content analysis should also be performed during each PCCE and In-Depth Inspection. The most effi- cient way to perform chloride ion content analysis is to collect 2-inch-diameter cores during field evaluation and collect powdered concrete samples from the cores at various depths in the laboratory and analyze the powdered concrete samples for total chloride ion content. If the data from the cores are to be used as input for a diffusion model, chloride ion content analysis should be performed at least at six depths from the core. The core method affords full control on the quality of the sample and reduces contamination of the sample under field conditions. It is a lot easier and faster to collect a core than to collect powdered concrete sample from six depths from a given location. In addition to chloride ion content analysis, the core can be used for carbonation testing. There are two ways to collect powdered concrete samples from the core. One method uses a drill press to collect powdered samples from the side of the core from various depths. The other method uses a grinding wheel to powder the entire cross-section of the core at the subject depth. These methods are discussed in detail in Chapter 8. 11

The clear concrete cover information needs to be obtained only once from the structure. As clear concrete cover is not expected to change with time, it does not have to be collected during subsequent evaluations. The measurement of clear con- crete cover is not laborious and can be efficiently collected using covermeters. The sampling size of the clear concrete cover should be as large as possible; at a minimum, 30 meas- urements must be obtained. Clear concrete cover may vary from one individual element to another due to construction practice. For example, it may vary from one span of the deck to another due to change in crew or construction practice. Con- crete elements that are precast by a single manufacturer gener- ally tend to have similar cover. Field data suggest that cover measurements generally satisfy the requirements of a normal distribution [13]. For beams and girders, it is necessary to con- duct clear concrete cover measurement because of the possible low cover over the stirrups. When concrete cover is lower than 0.5 inch, the carbonation front reaches the steel-concrete inter- face quickly and steel starts to corrode. The thin cover over the reinforcing steel is easily delaminated and spalled. On bridge structures, carbonation testing is most relevant on older structures, especially those built in the first half of the twentieth century. Carbonation testing can be performed on the cores collected for chloride ion content analysis, although not all cores need to be tested. Carbonation testing can also be performed in the field in small 0.5- to 0.75-inch- diameter holes. The holes can be drilled approximately 1 inch deep, and carbonation testing can be performed in the holes by spraying phenolphthalein solution. If conducted in the field, carbonation testing should be performed at three to five locations. Electrical continuity testing must be performed if half-cell potential and/or corrosion rate testing is to be performed. It should also be performed if application of a cathodic protec- tion system or electrochemical chloride extraction is a viable option for corrosion control on the subject superstructure element. Half-cell potential and corrosion rate measurement are conducted to ascertain the state of corrosion in presently undamaged areas. Whether corrosion has initiated or not can be determined by evaluating the magnitude of the half-cell potential. Corrosion rates provide information on the rate at which corrosion is occurring at the time of the measurement at the location of the measurement. Both of these tests pro- vide additional information on the propensity for corrosion in undamaged areas. They are often used to substantiate the conclusions reached based on results of visual, delamination, cover, and chloride surveys. Petrographic analysis must be performed during an In- Depth Inspection and prior to selecting a corrosion control system. This test provides information on the quality and inherent deterioration mechanism in the concrete material. If the constituents of concrete make it susceptible to freeze- thaw, ASR, or other concrete deterioration processes, then these susceptibilities must be taken into account in the selec- tion of the corrosion control system. The presence of one or more of these deterioration processes may impact the overall effectiveness of the corrosion control system. When epoxy-coated rebars are used in the construction, it is necessary to know the condition of the epoxy. Epoxy-coated rebars should be collected in cores. In the laboratory, the epoxy-coated rebars are extracted from the concrete cores, and the condition (visual rating) of the epoxy-coated rebars must be documented along with adhesion of the coating and the number of defects or damages in the coating that expose the reinforcing steel to the environment. 12

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Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements Get This Book
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TRB's National Cooperative Highway Research Program (NCHRP) Report 558: Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements examines step-by-step procedures for assessing the condition of corrosion-damaged bridge elements. It also explores procedures that can be used to estimate the expected remaining life of reinforced concrete bridge superstructure elements and to determine the effects of maintenance and repair options on their service life. NCHRP Web-Only Document 88 contains the data used in the development and validation of the service life model described in NCHRP Report 558. Also, the computational software (Excel spreadsheet) for the service life estimation process is available.

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