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Assessment and Rehabilitation of Existing Buildings and Bridges

Five days for engineers who know how to design new structures and now have to judge existing ones. The whole chain is covered, from the first walk-through to a written recommendation: deterioration mechanisms, investigation program, interpretation of test results, capacity verification, residual service life, repair and strengthening. Reinforced concrete buildings, structural steel buildings, bridges. Delivered in English.

Assessing a structure that already exists reverses the posture of design. On a new structure the engineer chooses conservative values because nothing can yet be measured. On an existing one the structure is there, available to be measured, and each measurement replaces a margin. The consequence runs through the five days: an element declared inadequate by a conservative model is a model to refine before it is a structure to strengthen.

What the participant can do on leaving

• identify a deterioration mechanism from its visible signature, and separate the mechanism from its symptom;
• build an investigation program in which every test answers a written question, and justify the sampling, which is also what keeps a testing budget under control;
• interpret non-destructive and destructive test results, with their scatter and their known traps;
• verify the capacity of a damaged member, concrete or steel, accounting for the loss of section, of bond and of ductility;
• rate an existing bridge and read the rating factor for what it states;
• select and justify a repair or strengthening strategy, including the option of doing nothing under monitoring, and write it up for a client.

Program

Half of the time is applied work, carried out on two case dossiers taken from real structures and released to the teams in stages: photograph sets, crack maps, survey drawings, laboratory sheets, inspection reports. Each team commits to a written answer before the next stage is opened. Releasing the dossier in stages is a deliberate device, not a convenience: anyone who sees the laboratory results at the same time as the photographs reasons backwards and never practices forming a hypothesis.

Day 1 Deterioration mechanisms and diagnosis

1. The assessment framework. What triggers an assessment and how the trigger governs the depth of the work. The compliance routes open for an existing building, and the decision chain that structures the week.
2. Reinforcement corrosion. Initiation and propagation as two distinct phases. Chloride ingress and carbonation. Marine aerosol, splash zones, high temperature, wetting and drying cycles, salt-bearing ground water. The weight of cover depth.
3. The other mechanisms. External and internal sulfate attack, alkali aggregate reaction, cracking of mechanical origin, differential settlement and aggressive ground. The discriminating feature of each.
4. Reading a structure. Damage mapping, structural against non-structural, and ranking of hypotheses. For the leading one, stating what would have to be observed to abandon it.

Day 2 Investigation and testing

1. Designing the program. The matrix linking a hypothesis to the technique able to confirm or kill it, and its reverse reading: what a technique cannot tell. Sampling to characterise or to find the worst case, which are not the same thing and do not support the same conclusion.
2. Testing for corrosion. Cover meters and radar, potential mapping, resistivity, corrosion rate, electrical continuity of the reinforcement. What each one measures, what governs its reliability, what invalidates it.
3. Testing the concrete. The rebound hammer and the correlation it requires, pulse velocity, impact echo, thermography. A gallery of traps, each drawn from a real report where a technique was read beyond what it can do.
4. Coring and laboratory. Core specification and the corrections applied, in-situ strength, carbonation depth, chloride profiling by depth increment, pull-off testing, sampling of reinforcement and residual section.

Day 3 Structural evaluation of concrete structures

1. From measurement to design value. Treatment of scatter, effect of sample size, in-situ value against characteristic value, and the level of knowledge acquired, which conditions the safety format.
2. Modeling an existing structure. Geometric survey and reinforcement survey, support conditions as built rather than as drawn, load history and past modifications.
3. Capacity of a damaged member. The three effects of corrosion taken separately. Pitting against general corrosion. Flexure, shear where the loss of stirrup section is decisive, punching.
4. Serviceability and residual service life. Deflection and cracking on a stiffness that has changed. Residual life stated as a prediction with its assumptions, and a sensitivity check on the assumption that moves the answer most.

Day 4 Steel structures and bridges

1. Existing steel. Identifying a grade from records, marking and sampling. Older grades, toughness and weldability, riveted, bolted and welded connections, and what each one closes off as a strengthening option.
2. Deterioration of steel. Uniform loss and pitting, crevice corrosion at faying surfaces, fatigue and detail categories, distortion-induced cracking, and the conditions that make brittle fracture possible.
3. Bridge inspection and condition. Inspection types and intervals, access constraints and their technical consequences, element-level defect catalog, and the parts systematically under-reported because they are hard to reach.
4. Bridge load rating. The rating factor and its terms, two levels answering two different questions, and the point at which the inspection enters the calculation.

Day 5 Repair, strengthening, integrated case

1. Strategy and decision. Doing nothing under monitoring as a legitimate decision, and the conditions that make it defensible. Maintenance, protection, repair, strengthening, restriction of use. The choice is made against the residual life required and the cost over the whole life.
2. Concrete repair and corrosion management. Breakout extent, treatment behind the bar, compatibility of the repair material, curing under high temperature. The incipient anode effect and how to avoid it. Cathodic protection, chloride extraction, realkalisation, and the maintenance obligation each one creates.
3. Strengthening systems. Section enlargement, bonded plates, externally bonded and near surface mounted polymer, external post-tensioning, added load paths. All judged on one grid, fire behavior included.
4. Integrated case and defense. The full chain carried by each team on the bridge dossier, then defended. The defense is judged on the chain of reasoning, not on the conclusion reached.

Three ideas the course undoes

• Corrosion does not only remove section. It also removes bond and the ductility of the bar. The third is the one that gets forgotten, and it is the one that turns a warned failure into an unwarned one.
• A chloride content says almost nothing. It is the shape of the profile over depth that separates chlorides that entered through the surface from those cast in, and only the first can be slowed by a surface treatment.
• Repairing without removing the cause moves the defect. A sound patch placed in contaminated concrete becomes cathodic relative to the concrete around it and corrodes its own edges. The next defect appears beside the repair, not in it.

Scope, and what is not covered

Covered: reinforced and prestressed concrete buildings, structural steel buildings, and road bridges of concrete and steel construction, for continued use, change of use, increased loading or extension of service life.

Not covered: seismic assessment and retrofit as a subject in its own right, which needs its own module and is touched here only where it constrains a strengthening choice; fire damage assessment, whose investigation logic differs; masonry and heritage structures; ground investigation and improvement, beyond the recognition of settlement-driven damage and of aggressive ground; forensic investigation for litigation, whose evidential requirements are not those of an engineering assessment; and the detailed design of the strengthening system beyond the sizing carried out in the workshops. The boundary is stated here so that it is agreed before the training rather than discovered during it.

Delivery and assessment

A positioning questionnaire opens the first morning, unmarked, kept and asked again on the last afternoon. It is also played online, with the right answers and a short explanation for each question. Progress is recorded through the written commitment each team makes at every stage of the two case dossiers, which is the main record, and through the final defense. Participants take away the session materials, the two dossiers with the reasoning that closed them, a checklist for planning an investigation, and the list of documents used. The course runs in-house at the client’s premises or as an open session, and the dossiers are adapted to the client’s own asset portfolio where the material can be supplied.

Reference framework

The codes in force in the country where the course is delivered, for buildings and for bridges, together with the documents from which they derive and to which practice refers: the American Concrete Institute documents on structural concrete and on the assessment and repair of existing concrete structures, the American Institute of Steel Construction specification, the American Society of Civil Engineers standard on the evaluation of existing buildings, the AASHTO manual for bridge evaluation, and the Federal Highway Administration technical documents on bridge strengthening and on fatigue of steel bridges. Test standards are cited method by method in the session materials.