7.6.1. Maintenance, inspection
The maintenance, inspection and assessment are important elements of preservation of value, seen from the economy as a whole.
Regular inspections of bridges can detect possible defects which can be caused due to the accident or due to the long-term actions of chemicals or environmental effects.
The chemicals and environmental effects can be partially predicted for each component of the structure, which has its life time. On the base of this knowledge it is possible to create maintenance plan for every part of the structure.
The maintenance of structures can prevent their further deterioration.
The inspection, assessment and upgrading of structures are engineering disciplines which are gaining significance both from a technical and economic point of view.
The codification of assessment procedures for existing structures is representing a new field of development. New non-destructive methods which are developed are most suitable and efficient for the assessment of bridges and other concrete structures e.g. concerning corrosion of the reinforcement, detection of faults in concrete members, location of ducts and the detection of fractures of prestressing steel.
7.6.2. Valuation of existing structures
New methods of evaluation, repair and strengthening which are presented yet seem to be accepted by custom.
The reasons are to be found in the higher demands concerning the variability of bridges use, the limited durability of bridges and increased safety requirements. The people responsible for design, execution and use need design rules for durability. Insufficient job site quality control procedures and non-existing and/or insufficient maintenance and inspection during use need to be covered by Design Concepts for Durability, both for the design of new structures and for the repair of damaged structures.
The codification of assessment procedures for existing structures is a new field of development. Chapter 8 of the draft ISO 2394, Reliability of Structures, is especially devoted to the assessment of existing structures and may by regarded as the first international model code in this field.
The analysis and assessment of an existing structure follows the same basic principles as the assessment of a structure under design. In the context of this code, this means that probabilistic models should be produced for the basic variables and that mechanical models should be made for the loads and structural properties.
ISO 2394 treats these problems within the framework of probabilistic methods.
For application in everyday practice, the results of those methods need to be translated into standard load and resistance factor procedures.
If a structure is to be reconstructed, an assessment must be mandatory. The dimensions and properties of the present structure must be known in order to be able to make an analysis of the reconstructed building.
Many bridges have been designed for traffic loads that are much smaller than the present day traffic loads. Many engineers have the feeling that no further investigation is required if the bridge does not show signs of distress. This is a dangerous way of reasoning. It should be at least verified that the structure is of such a type that it indeed will show signs of distress in due time.
When there is obvious damage to a structure, a structural appraisal is clearly required. The difficult point is to indicate the demarcation between ‘harmless deterioration’ and
‘deterioration demanding further investigation’.
In cases where a structure has been loaded by some extreme load, e.g. earthquake or fire, there is a need for further investigation, even if this particular structure does not show any visible damage at first sight.
A routine inspections according to the maintenance plan can be relatively simple, e.g.
looking at appearance aspects only. If the result is within the predefined limits, the assessment is that the structural capacity is still sufficient. If the result is outside the predefined limits, a further investigation might follow, or it might be decided to repair the structure immediately.
The highest priority needs for bridge inspection, for instance, are a global bridge evaluation, the rapid assessment of asphalt covered bridge decks, the evaluation of unknown foundations, the evaluation of prestressing steel in prestressed concrete, and integration of the nondestructive evaluation into bridge management systems.
7.6.3. Nondestructive methods
The following NDT methods are most suitable and efficient for the assessment of bridges and other concrete structures.
Corrosion of reinforcement, concrete cover and potential mapping
The state of corrosion of the reinforcement over an extensive area is assessed using a computer-aided concrete cover measuring instrument. In addition, the potential monitoring method is used to determine the potential difference between the reinforcement and a reference electrode over an extensive area in a previously defined grid. In the areas found to have a particularly low potential difference, a chloride section of the concrete is additionally analyzed. By comparing these 3 parameters the areas in which there is already active corrosion or which are highly likely to develop corrosion can be localized. This method is applied especially to bridges after removing the old carriageway surfacing or the old waterproofing course.
The condition for application of the potential monitoring method is that the concrete must possess sufficient conductivity. The concrete must be a conductive electrolyte. The question to be asked here is whether or not the concrete surface needs to be pre-wetted before performing the measurement. The concrete does not have to be pre-moistened if it is permanently in contact with the atmospheric air. The air humidity usually ensures a sufficient level of moisture in the void system of the concrete.
Ultrasonics and impact echo
One of the principal objectives in the development of NDT-CE (Non Destructive Testing in Civil Engineering) techniques is a reliable assessment of the integrity or the detection of defects in concrete members. Such inspections could in the past only be solved by means of radiography, for a concrete thickness less than 0,6 m, or by partially destructive methods. The Ultrasonic Pulse-Echo or Pulse-Velocity methods are the right tools for this type of detection.
When the load capacity of a concrete unit is in doubt, it is desirable to know where air voids, gravel pockets and areas of low concrete strength are located exactly. Ultrasound measurements were used to identify areas of low quality concrete in columns more than 800 mm thick by correlating pulse velocity with the compressive strength of sample cores drilled.
It is shown that grid measurements and 3D visualisation are essential tools to obtain an easy-to-read picture of the interior of the columns. The Impact-Echo method is particularly suitable
Defective areas in concrete are traceable by radar. The radar beam – an electromagnetic signal – penetrates the concrete and is reflected at the interface with the defective area. Not only defective areas but also the prestressing tendons which usually have a concrete cover of at least 100 mm can be localized by radar. The electromagnetic pulse is reflected by the reinforcement and the tendon. The backscattered signals are received and displayed in dependence on the position of the antenna. The graphic recording of the radar signal shows the tendons as typical reflection hyperbolas. The main advantage of radar compared to other conductive detectors is its capability to detect deeply embedded metal sheathings behind the normal reinforcement.
fractures of prestressing steel: Remanent magnetism
Before repairing a prestressed concrete structure the prestressing tendons should also be examined to check whether they are intact. Opening of the grouted prestressing tendons always involves the risk of closing them reliably in order to avoid an area of corrosion with anodic polarization. A grouted prestressing tendon should not be inspected by random sampling, but must be checked over its entire length, at least in the areas subjected to extreme loading, using a nondestructive method. A method based on remanent magnetism has been developed. The testing equipment has been continually improved and allows the detection of failures with high accuracy, it can be mounted easily and offers efficient application.
The main field of repair strategies are related to the design of corrosion protection for ordinary reinforcement.
New class of corrosion inhibitors for reinforced concrete structures has been developed.
The inhibitor can be applied on the surface of the concrete structure by brush, spraying or flow coating and will penetrate deep into the concrete to protect the reinforcing bars against corrosion or to stop corrosion without any need for the concrete to be removed. The inhibitors are based on amino alcohols (Ama) or are aqueous mixtures of partly neutralised AMAs.
Rehabilitation of poorly grouted tendons
The localization of metal sheathings which are incompletely grouted using nondestructive testing methods is only possible by radiographic testing and subsequent and elaborate image processing. This method is not suitable to be used over an extensive area. Nevertheless, it is helpful to gain an insight into poorly grouted metal sheathings using a partially destructive method, i.e. by tapping and opening the metal sheathing and inspecting it with an endoscope.
These inspections focus on areas in which the probability of defective parts is particularly high, for example at the anchorage points or at the high and low points of the tendons.
The size of the void can then be determined at the tapping points by means of vacuum measurement and, using the vacuum, the void can be grouted with injection grouting resin or cement paste.
7. Bibliography
[1.1] ENV 1992-2 1996 Eurocode 2: Design of concrete structures Part 2:
Concrete Bridges
[1.2] ENV 1998-2 1994 Eurocode 8: Design provisions for earthquake resistance of structures Part 2: Bridges
[1.3] CEN/TC 229/WG1/TG14 BRIDGE ELEMENTS
[1.4] prEN 13369 1999 Common rules for precast concrete products.
[3.1] Rossner, Brücken aus Spannbeton-Fertigteilen, 1988, Ernst & Sohn, Berlin [3.2] Vorläufige Richtlinien für Straßen- und Wegebrücken aus Spann- und
Stahlbeton-Fertigteilen (R FT-Brücken) 1979, Forschungsgesellschaft für das Straßenwesen
[4.1] Bridge Design for Durability. A TRL Publication Incorporating Methods of Achieving Continuity in
[4.2] Composite Concrete Bridge Decks by Brian P. Pritchard.
Prestressed Beam Integral Bridges by Dr. Edmund C. Hambly and Bruce Nicholson (PCA/U.K. publication)
[4.3] Continuity of Bridge Decks using Precast Beams by Dr H. P. J. Taylor (Chairman PCA/U.K.)
[4.4] Serviceability Limit State Aspects of Continuous Bridges using Precast Concrete Beams by L. A. Clarke and I. Sugie (PCA/U.K. publication)
[5.1] fib-Guide to good practice (bulletin No. 9, Guidance to good bridge design, July 2000), chapter 1.4.2.
[6.1] RILEM Report 14 Durability Design of Concrete Structures Edited by A. Sarja and E. Vesikari 1996
[6.2] fib-bulletin 3 Structural Concrete
Textbook on Behaviour, Design and Performance Volume 3
[6.3] EUROCODE 8-2 DAN
[6.4] Association Française de Génie Parasismique (AFPS) guide, 1992
[6.5] Ponts Courants en Zone Sismique - Guide de Conception – January 2000, SETRA and SNCF
[6.6] PS92 seismic engineering regulations
[6.7] Association Française de Génie Parasismique (AFPS) recommendations, 1990