a knowledge-based system

A KNOWLEDGE-BASED SYSTEM FOR CONCRETE BRIDGE INSPECTION

J. Brito, F. A. Branco and M. Ibañez

KEYWORDS: Bridges, durability, maintenance, inspection, in situ testing

SYNOPSIS Establishing periodic bridge inspection programmes to reduce maintenance and repair costs and maximize society benefits is a concern of the major bridge authorities. Nevertheless, bridge inspections are still performed according to local strategies and frequently their results depend a lot on the inspector's skill. To implement standardized inspections and to give technical support to the inspector, a knowledge-based system BRIDGE-1 - was developed within the EC supported research BRITE-EURAM project "Assessment of Performance and Optimal Strategies for Inspection and Maintenance of Concrete Structures Using Reliability Based Expert Systems". This interactive system, part of a larger expert system for bridge maintenance strategy (presently being developed), will supply the inspector, at the bridge site, with general information on the bridge, recommendations on diagnosis methods related with each particular defect detected in situ, probable causes, associated defects and pseudo-quantitative rating. Suggestions about the repair techniques to be used for each defect can also be obtained. Based on the bridge inspection data, the system will also prepare a standardized memo aid called Provisional Defect Report to be used at the headquarters.

RESEARCH SIGNIFICANCE The knowledge-based system presented in this paper is an useful tool to perform concrete bridge inspection. With this system, rationalized inspections, with expert technical information support, can be implemented at the bridge site, leading to better inspection results and better maintenance policies.

INTRODUCTION In most developed countries, the expansion of the road and railway systems has been slowing down in the last few decades and more emphasis is being laid in the maintenance and repair of the existing facilities. The resources that have been spent in the last years in keeping the existing bridges functionality up to the standards they were designed for, are not enough and there is a serious risk of loosing the investments of building them. As the maintenance budgets are always limited, this involves setting priorities and therefore defining maintenance strategies based on the real condition of each bridge obtained from periodic inspections. Initially, the inspections were performed at a random fashion, according to the news of a malfunctioning at a particular site. This situation frequently caused unnecessary costs because, at that stage, the inspected bridges already presented important deterioration and traffic would be significantly affected. The idea of setting inspection plans based on periodic visits to each bridge came forward and the theory of preventive repair was put into practice. However, these visits are made by personnel, in some cases without enough field experience, whose criteria is often subjective, leading to different evaluations of the same situation. The need for standardization resulted in the development of inspection manuals, in which each procedure at the bridge site is described and criteria for evaluation of the defects and repair costs is presented. With the development of the knowledge-based system presented here - BRIDGE-1, a step forward has been made to define standardized inspections and to give technical support to the inspector at the bridge site [1]. To perform this support, the system databases include standardized defects, causes, diagnosis methods and correlation matrixes between them, with expert information. As a complement to the inspection, the databases also include data from repair procedures, structural data on the bridge, environmental data to predict the deterioration process and data from previous inspections and repair work done. This system, to be used in a portable computer, will not perform decision-making which will be done at the headquarters following a more detailed analysis of all the data gathered and possibly a further number of laboratory tests. A computer program module BRIDGE-2, still being implemented, will then be used to perform a decision on the optimal repair / maintenance action to be performed [2].

INSPECTION STRATEGY The BRIDGE-1 functionality is based on a standardized inspection strategy. It consists of a periodic set of inspections with a fixed timetable, in which some flexibility is allowed to take into account a plausible global allocation of the inspection resources, complemented with special inspections when something serious is detected or suspected. Besides the periodic inspections, a bridge initial characterization will also be necessary, after the bridge is built and before it is put in service. Its main objectives are to confirm the design analysis and to create a "reference state" for all the future inspections [3]. This reference state may have to be altered after important repair, strengthening or deck widening has been performed in the bridge. The periodic inspection framework consists of two types of inspections [4]: current and detailed. The first one has a 15 month period and the second one replaces a current inspection in 5 year intervals. In the current inspections, serious defects are not supposed to be detected. For this reason, the inspection will be focused mainly on visual observation of the most exposed areas and, usually, special means of access and non-portable equipment will be dispensed with. The defects are registered, with their location in the cross section, and classified in terms of rehabilitation urgency. A report is prepared and sent to the periodic maintenance division with all the defects that need short or middle term action. The detailed inspection differs from the current inspection in that all details that are susceptible of raising future problems are investigated. It demands more human means and more specific equipment and it can lead to traffic impairment, therefore middle term planning is needed. Besides visual observation, non-destructive in situ tests are performed and eventually laboratory tests. At a detailed inspection, usually there is not a specific serious defect to be looked into, unless it has been detected previously in the current inspections [5]. During the detailed inspection or after the report has been made, a serious defect may be detected or suspected to exist. The affected areas must be subject to a thorough observation before any action is taken, and a structural assessment is then set forward. This type of assessment is very detailed but also limited in its scope, as usually only a few important problems are investigated. Besides all the diagnosis methods used in a detailed inspection, others may be used which are more specific, detailed and require non-portable equipment and specialized workmanship. Laboratory tests are frequently indispensable to complement the in situ information. This inspection is usually costly and involves traffic impairment at the bridge [5].

INSPECTION METHODOLOGY WITH BRIDGE-1 The Classification System To standardize the procedures at the inspection site, a defect classification system was prepared for BRIDGE-1. All the defects liable to be found in concrete bridges (totalling 94 entrances) were classified according to a geographical / functional / materials criteria in the following groups [6]: - A-A. SUPERSTRUCTURE GLOBAL BEHAVIOUR (4) - A-B. FOUNDATIONS / ABUTMENTS / EMBANKMENTS (9) - A-C. CONCRETE ELEMENTS (13) - A-D. REINFORCEMENT / CABLES (10) - A-E. BEARINGS (14) - A-F. JOINTS (11) - A-G. WEARING SURFACE (ASPHALT) / WATERTIGHTNESS (11) - A-H. WATER DRAINAGE (7) - A-I. SECONDARY ELEMENTS (15)

All the possible causes (direct or indirect) of these defects (117 entrances) were then classified according to a chronological criteria in the following groups [7]: - C-A. DESIGN ERRORS (28) - C-B. CONSTRUCTION ERRORS (26) - C-C. NATURAL ACCIDENTAL ACTIONS (10) - C-D. MAN-CAUSED ACCIDENTAL ACTIONS (6) - C-E. ENVIRONMENTAL ACTIONS (7) - C-F. NATURAL AGGRESSIVE FACTORS (11) - C-G. MAN-CAUSED AGGRESSIVE FACTORS (8) - C-H. LACK OF MAINTENANCE (8) - C-I. CHANGES FROM INITIALLY PLANNED NORMAL USE (13)

The in situ diagnosis methods used to detect or analyze the defects (81 entrances) were also classified according to the functioning principle and the type of results provided, in the following groups [8]:

- M.A. DIRECT VISUAL OBSERVATION (5) - M-B. MECHANICAL TECHNIQUES (25) - M-C. POTENTIAL DIFFERENCES MEASUREMENT (2) - M-D. MAGNETIC TECHNIQUES (3) - M-E. ELECTRICAL METHODS (3) - M-F. ULTRASONIC AND ELECTROMAGNETIC TECHNIQUES (5) - M-G. RADIOACTIVE METHODS (4) - M-H. ACOUSTIC TECHNIQUES (2) - M-I. THERMIC METHODS (3) - M-J. FORCE / DEFORMATION TECHNIQUES (15) - M-K. CHEMICAL INDICATORS (4) - M-L. FLUORESCENCE METHODS (1) - M-M. LOAD TESTS (4) - M-N. VIBRATION TESTS (5)

The repair techniques used to eliminate or prevent the defects listed above (69 entrances) were also classified in the same groups as the defects. In the present version of BRIDGE-1, only a prototype system was developed, limited to the main reinforced concrete corrosion related defects, listed below: - A-C01

Rust stain

- A-C07

Delamination / spalling

- A-C13

Crack over / under a bar

- A-D01

Exposed bar

- A-D04

Corroded bar

- A-D05

Bar with reduced cross-section

- A-D06

Broken bar

- A-E02

Obstruction due to rust in bearings

- A-E03

Broken retainer-bars

- A-E06

Corrosion in bearings

- A-E07

Deteriorated base plate / pot

- A-E08

Detachment / failure of anchor bolts / pins

- A-F05

Obstruction due to rust in joints

- A-F06

Corrosion in joints

- A-F07

Detachment / failure of anchorages

- A-F08

Loosening / failure of bolts / pins in joints

- A-I14

Deteriorated edge beams

For each of these defects, a defect form was prepared in order to be included in the inspection manual, as a complement to the system. Each of the forms includes the following information: short description of the defect, possible causes, possible consequences, inspection parameters to investigate and a defect rating (Fig. 1). The Correlation Matrixes To help the inspector in making decisions at the bridge, BRIDGE-1 has knowledgebased data introduced through correlation matrixes relating defects x causes, defects x diagnosis methods and defects x repair technics. Each of these matrixes is organized so that each line represents a defect and each column a possible cause ( or diagnosis method, or repair method). In the intersection of each line and column, representing the correlation between each defect and the other element, a classification was introduced representing the knowledge information. The criteria adopted for these classifications are presented in Tables 1 to 3. Defects Rating The budget for maintenance and repair of each bridge is frequently insufficient to handle all the defects that are liable to be found in the periodic inspections. Therefore, there is a need for a rating system in order to select the defects that need to be dealt with the highest priority. The rating criteria implemented in the system takes into account three basic aspects: Rehabilitation urgency; Importance to the structure´s stability; Volume of traffic affected. Each defect is classified by the inspector, according to the classification presented in Table 4 [9], and the corresponding points are considered by the system to obtain a global rating of the defect. The defects are then included in groups of priority of action according to the number of points assigned to each one. Some of the information needed to rate the defects is a characteristic of each bridge and is included in its databases.

OPERATING THE BRIDGE-1 SYSTEM General Procedures

To use the system at the bridge site, its fixed databases must already be loaded. These include general information on the bridge, the classification system, the correlation matrixes and the defects rating system [10]. The general information database contains some short but thorough information divided in the following items: bridge site; design information; budget information; traffic information; strength information; load information; deterioration information; factors that model the repair costs; cross-section information. This information is the "identity card" of the bridge [1]. When a new defect is found in a cross-section, the inspector initially fills in the data concerning that cross-section. The type of the cross-section (deck or column) is identified and a code name is assigned to it. To help the inspector to make decisions about the defects detected during the inspection, the system is then used as a memo aid giving hints as to what should be done. The inspector selects the defect that has just been detected and has access to the following standard help: 1.- Diagnosis Methods 2.- Probable Causes 3.- Associated Defects 4.- Related Repair Techniques Option 1 will list all the diagnosis methods correlated with the defect found, separating the high from the low correlated. Option 2 will act similarly concerning the probable causes of the defect. Option 3 gives a measure of the probability of finding other defects in the bridge, taking into account that the selected one has been detected. This correlation measure between defects k and j, CIkj, is calculated using the correlation matrix between defects and their causes by [11]: N

C I k j= ∑ c k i cj i i= 1

For each defect detected (defect k), the system reads the correspondent line in the matrix. Every time it finds a number different from zero cki, it travels along the correspondent cause column. Every time it finds a number different from zero cji, it adds the value cki cji to the correlation measure of the defect in line j.

Option 4 will suggest which are the usual repair methods for the defect detected and the associated repair parameters, to be defined at the bridge site, to perform a repair cost decision at the headquarters. Using BRIDGE-1, the inspector is also able to record the results of the inspection in the Provisional Inspection Database as a Provisional Defect Report. This will then be used to help in the preparation of the definitive inspection report done at the headquarters. The system will require the detected defect's code, the diagnosis method(s) used and the probable cause(s) as described before. Comments can also be added. Example of the System's Use In this example, the inspector is supposed to be at a bridge of which all the general information has been recorded previously. He has detected several defects in cross-sections, already in the database, and wants to record them in a Provisional Defect Report. Some of the screens presented to him by the system are reproduced in Figs 2 to 8 showing an example of the current procedures: - Out of the possible defects in cross-section deck A-2, the inspector selects the defect A_D05 (Fig.2). - The system suggests several diagnosis methods to detect / analyze defect A_D05. The inspector selects, among the highest recommended ones: M_A01 and M_K01 (Fig.3). - The system suggests possible causes for the defect A_D05. The inspector selects among the most probable ones: C_14, C_A24, C_F01, C_F02 and C_A26 (Fig.4). - To define the importance of the defect, the inspector classifies the defect A_D05: a) with 3 in terms of Rehabilitation Urgency (reddish rust connected with carbonation induced corrosion) (Fig.5). b) with A in terms of Importance to the Structure's Stability (defect found in the deck) (Fig.6). c) with k=0 in terms of the degree of obstruction to normal traffic caused by the defect (the defect does not impair the traffic at all) (Fig.7). - If the inspector wants to analyze possible repair techniques for the defect, the system will suggest several methods, divided in groups according to their current applicability (Fig. 8).

At the end, the inspector can also add further comments concerning the defect A_D05 found in cross-section deck A-2 in a commentary screen. All this information will go into the Provisional Report.

CONCLUSIONS The BRIDGE-1 system is a first step towards a fully knowledge-based inspection system, helping in the standardization of the inspection techniques and acting as an useful tool to bridge inspectors. The information it creates will, in the future, be used by the BRIDGE-2 system in which the decision system concerning the optimal maintenance / repair strategy is being implemented. At a more advanced stage, the system will have expert knowledge (in terms of flowcharts) to allow it to indicate the best repair technique for the defect, instead of just suggesting techniques. To do that, the system will ask a set of parameters that characterize the defect and, with them, the best repair method will be pointed out. Some of these parameters, defined by the inspector, will allow an estimation of the costs of the selected technique. To test the prototype of BRIDGE-1 system, only the corrosion correlated defects in reinforced concrete bridges were implemented at its first stage. Nevertheless, the correlation matrix for all the other defects has already been defined and will be soon implemented to obtain a global system for concrete bridge inspection.

ACKNOWLEDGEMENTS This paper presents part of the results of the EC supported research project BRITE/EURAM P3091 "Assessment of Performance and Optimal Strategies for Inspection and Maintenance of Concrete Structures Using Reliability Based Expert Systems". The partners in this project are: Instituto Superior Técnico ( Lisboa, Portugal) / LABEIN ( Bilbao, Spain) / CSR ( Aalborg, Denmark) / Jahn Ingenieurbureau ( Hellevoetsluis, Holland) / University of Aberdeen, (Aberdeen, UK)

Part of the research work was developed within the PhD thesis of one of the co-authors, at Instituto Superior Técnico and CMEST - Structures Research Center of the Technical University of Lisboa.

REFERENCES [1] -

de Brito, J.; Branco, F.A., "Proposal for the BRIDGE1 Module", BRITE-EURAM Report T4.3-01, 1991, Lisboa. [2] - de Brito, J.; Branco, F.A., "Proposal for the BRIDGE2 Module", BRITE-EURAM Report T4.3-05, 1992, Lisboa. [3] - Ministère des Transports - Direction des Routes et de la Circulation Routière, "Instruction Technique pour la Surveillance et l'Entretien des Ouvrages d'Art", 1979, Paris. [4] - Andrey, D., "Maintenance des Ouvrages d'Art: Méthologie de Surveillance", PhD Thesis, École Polytechnique Fédérale de Lausanne, 1987, Lausanne. [5] - de Brito, J.; Branco, F.A., "A Decision System for Bridge Management". Proc. 14th IABSE Congress, pp.597,1992, New Dehli. [6] - de Brito, J.; Branco, F.A., "Classification of Anomalies in Concrete Bridges". BRITE-EURAM Report T2.1-01, 1990, Lisboa. [7] - de Brito, J.; Branco, F.A., "Classification of Possible Causes of Anomalies in Concrete Bridges", BRITE-EURAM Report T2.1-02, 1990, Lisboa. [8] - de Brito, J.; Branco, F.A., "Classification of Diagnosis Methods for Concrete Bridges", BRITE-EURAM Report T3.1-01, 1991, Lisboa. [9] - McClure, R.; Hoffman, G., " The Pennsylvannia Bridge Management System", Bridge Management Conference, 1990, Guildford. [10] - Ibañez, M., "User's Manual for BRIDGE1", BRITE-EURAM Report T4.3-08, 1992, Bilbao. [11] - Sørensen, J.; Thoft-Christensen, P., "Final Modelling of Inspection Strategy and Decision System for Concrete Bridges", BRITE-EURAM Report T3.3-07, 1991, Aalborg.

READERSHIP

Structural engineers in general, and, in particular, engineers involved in bridge management systems or in inspection and maintenance of deteriorated concrete structures.

BIOGRAPHIC NOTES Jorge de Brito graduated in Civil Engineering and got is MASc degree at IST - Technical University of Lisbon, Portugal, where he is presently developing his PhD Thesis, as a research assistant. His research work deals with deterioration, rehabilitation and management of concrete structures. Fernando A. Branco is associate professor of civil engineering at IST - Technical University of Lisbon, Portugal, and director of the University Structures Research Center (CMEST). He is national representative in the European Permanent Committee for Experimental Mechanics (PCSA) and in the International Union of Testing and Research Laboratories (RILEM). His primary research interests deal with the structural behaviour of bridges and special structures. Milagros Ibañez is a specialist on artificial intelligence, working in IDEIA Department , at LABEIN (Bilbao Research Center). She is involved in research of advanced software applications, related to national and European research programs. Previously, she has been doing research on artificial intelligence techniques for two years at the University of Philadelphia.

LIST OF TABLES

TABLE 1 - Correlation of causes versus defects TABLE 2 - Correlation of diagnosis methods versus defects TABLE 3 - Correlation of repair methods versus defects TABLE 4 - Defects rating

LIST OF FIGURES

FIG. 1 - Defect form FIG. 2 - Defect list FIG. 3 - Suggested diagnosis methods FIG. 4 - Suggested causes FIG. 5 - Classification of defect in terms of rehabilitation urgency FIG. 6 - Classification of defect in terms of importance to the structure's stability FIG. 7 - Classification of defect in terms of affected traffic FIG. 8 - Suggested repair techniques

0 - NO CORRELATION - no relation whatever (direct or indirect) between the defect and the cause. 1 - LOW CORRELATION - indirect cause of the defect connected only with the early stages of the deterioration process; secondary cause of the deterioration process and not necessary for its development. 2 - HIGH CORRELATION - direct cause of the defect associated with the final stages of the deterioration process; when the cause occurs, it is one of the main causes of the deterioration process and is indispensable to its development.

0 - NO CORRELATION - no relation whatever (direct or indirect) between the defect and the repair technique. 1 - LOW CORRELATION - preventive repair aiming at eliminating the cause or causes of the defect but not the deterioration. 2 - HIGH CORRELATION - defect repair aiming at eliminating the deterioration of the area in which the defect was detected.

0 - NO CORRELATION - no relation whatever (direct or indirect) between the defect and the diagnosis method. 1 - LOW CORRELATION - the diagnosis method may be useful as a second choice to a high correlation method when this last one can not be performed or gives inconclusive results; it may also be useful to give some secondary information on the extent or cause of the defect. 2 - HIGH CORRELATION - the diagnosis method is, in principle, indispensable to the inspection of the defect; it provides essential information on the extent, degree and cause of the defect; it may be replaced by a low correlation method if, for some reason (lack of equipment, workmanship, time, etc.), it can not be performed; its use does not invalidate the use of other methods if more detailed information is thought necessary.

CRITERIA

CLASSIFICATION

Rehabilitation Urgency

POINTS

0 immediate action required 30 1 short-term (6 months) action required 25 2 medium-term (15 months) action required 15 3 long-term action required 5 ___________________________________________________________________________ Importance to A structural defect in main structural elements 40 the Structure's B semi-structural defect in main or secondary structural elements 25 Stability C non-structural defect 15 ___________________________________________________________________________ Volume of α t.v. x d.l. x k ≥ 15.000 vehicle km / day 30 Traffic Affected β 15.000 vehicle km / day > t.v. x d.l. x k ≥ 3.000 vehicle km / day 20 γ t.v. x d.l. x k < 3.000 vehicle km / day by the Defect 10 t.v. - average daily traffic volume over the bridge (in both directions) [vehicle / day] d.l. - detour length caused by the total disruption of the bridge [km] k - degree of obstruction to normal traffic caused by each defect

BRIDGE INSPECTION Cross-section: deck A-2 Choose a defect detected on the bridge:

→

More ↓

______________________________________________ A_C01 Rust stain A_C07 Delamination / spalling A_C13 Crack over / under a bar A_D01 Exposed bar A_D04 Corroded bar A_D05 Bar with reduced cross-section A_D06 Broken bar A_E02 Obstruction due to rust in bearings A_E03 Broken retainer-bars A_E06 Corrosion in bearings ______________________________________________

BRIDGE INSPECTION DEFECT: A_D05 Bar with reduced cross-section CROSS-SECTION: deck A-2 TYPE OF INSPECTION: Current Inspection Please select the Diagnosis Methods you used to conclude the defect: ________________________________________________________________ M_A01 Unaided direct visual observation M_C01 Galvanic cell test M_K01 Phenolphtalein M_A02 Using binoculars, micrometer, camera or video equipment M_A04 Using special means of aerial access M_A05 Underwater / on water M_K02 Silver nitrate M_K03 Rapid chloride test ________________________________________________________________

BRIDGE INSPECTION DEFECT: A_D05 Bar with reduced cross-section CROSS-SECTION: deck A-2 TYPE OF INSPECTION: Current Inspection Please select the probable Causes of the defect: ________________________________________________________________ C_A14 Insufficient reinforcement / prestressing design cover C_A24 Drainage directly over concrete, joint, bearing or anchorage C_B09 Deficient concrete compaction / curing C_B11 Inaccurate reinforcement / prestressing positioning / detailing C_F01 Water (wet / dry cycles) C_F02 Natural carbon dioxide C_F03 Salt / salty water (chlorides) C_G01 Water (man-caused) C_G02 Man-caused carbon dioxide C_G03 Man-caused deicing salts C_A20 Excessive exposed areas in structural elements / faulty geometry C_A23 No prevision of a minimum inclination in quasi-horizontal surfaces C_A25 Other drainage design faults C_A26 Lack of waterproofing membrane C_A28 Incomplete / contradictory / overcompact drawings C_B01 Wrong interpretation of the drawings ________________________________________________________________ More ↓

BRIDGE INSPECTION DEFECT: A_D05 Bar with reduced cross-section REHABILITATION URGENCY: 0. 1. 2. 3.

Mainly black rust in areas of maximum moments with a local loss over 3% Mainly black rust in areas of maximum moments with a local loss under 3% Predominantly black rust in intermediate areas Predominantly reddish rust __________________________ OPTION [0 TO 2] . . __________________________

BRIDGE INSPECTION DEFECT: A_D05 Bar with reduced cross-section IMPORTANCE TO THE STRUCTURE'S STABILITY: A. Reinforcement in the deck, main beams, columns, abutments or foundations C. Reinforcements in the auto-safes, parapets, sidewalks surface and approach slabs

__________________________ OPTION [A TO C] . . __________________________

BRIDGE INSPECTION DEFECT: A_D05 Bar with reduced cross-section AVERAGE DAILY TRAFFIC OVER THE BRIDGE: 20.000 vehicles DETOUR LENGTH: 5,0 km VOLUME OF TRAFFIC AFFECTED BY THE DEFECT: k - degree of obstruction of normal traffic over the bridge caused by the defect

__________________________ k VALUE [0.0 TO 1.0] . . __________________________

BRIDGE INSPECTION DEFECT: A_D05 Bar with reduced cross-section CROSS-SECTION: deck A-2 TYPE OF INSPECTION: Current Inspection The related repair techniques for the defect are: ________________________________________________________________ (A)

HIGH CORRELATION

1. R_D01 Concrete Patching (with reinforcement / prestressing cleaning) 2. R_D02 Concrete Patching (with reinforcement / prestressing splicing / replacement) (B)

LOW CORRELATION not specified

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