T3-5: Prognosis for the Structure Flashcards
(31 cards)
What are five key things to determine in the evaluation of a concrete structure?
- Deterioration mechanisms
- Causes of deterioration
- Likely future performance
- Repair possibilities
- Extent of repair
What does the assessment of carbonation entail (x2)?
- Assessment of residual life
- Carbonation and cover depth analysis
What does this equation represent?
Carbonation depth
NB. equation assumes that the rate of advance of the carbonation front is diffusion controlled
[NAQ] example carbonation predictions
[NAQ] carbonation depth vs age of structure graph
Why are histograms of carbonation and cover depth frequency superimposed?
Because any overlap can be assessed, and a probability of corrosion estimated
Does higher rainfall lead to more/less/no difference in terms of carbonation rate, and why?
What happens if it is very dry?
NB. water is needed for carbonation to occur, just ‘extremes’ slow it down
Less
- e.g. outside surface of structures
- excessive moisture (saturation) blocks pores; slowing carbonation
- if very dry, also leads to slower carbonation; not enough moisture for chemical reactions to occur
Describe the programme of the assessment of chloride profiles (x6 steps)
- Best-fit curve through points
- Establish surface chloride concentration
- Establish background chloride concentration
- Establish chloride diffusion coefficient
- Calculate chloride profiles at different ages
- Determine when initiation concentration reaches reinforcement
What does this equation represent?
It is the chloride diffusion modelled in accordance with Flick’s 2nd law
[NAQ] graph(s) showing chloride content results/ chloride prediction
What method is used to measure corrosion rate?
What is measured?
What sensors are used?
Linear polarisation resistance
- measures corrosion rates by analysing the relationship between the potential and current generated between electrodes in a corrosive environment
- embedded probes in the concrete
- demountable devices
[NAQ] sketches showing process of measuring corrosion rate
For modelling corrosion rate from linear polarisation resistance, what needs to be assumed?
A typical corrosion current, based on prevailing environmental conditions
Name different sensors/ methods used for monitoring a structure (x11)
- Embedded half-cells
- Resistivity probes
- Corrosion probes
- Linear polarisation
- Precipitation detectors
- Internal relative humidity (RH) probes
- Temperature sensors
- Impact/deflection sensors
- Crack movement
- Tilt/lean sensors
- Video surveillance
What problem can occur that can make at-risk areas not visible?
Half-joint issues
Describe half-joint issues
What do they arise from?
What is concealed?
What happens to the joint area?
- Arise from moisture accumulation, and corrosion of reinforcement, leading to concrete deterioration
- Concealed bearing
- At-risk areas not visible
- Corrosion risk masked by uncorroded steel
- Sampling is complicated by congested reinforcement layout
- Joint area becomes a salt trap
- Deteriorated state needs to be reassessed
Which of these bridge deck support arrangements (half-joints) is likely to cause the biggest and smallest maintenance challenge?
Most = A:
- No physical access for inspection or maintenance of the bearing shelf
- High risk of hidden deterioration
Least = D:
- Clear access to bearings and half-joint zones
Name different types (x8) of issues with half joint inspection
- Poor inspectability
- Movement joint failure
- Salt water build up
- Coastal exposure
- Corrosion risk
- Winter maintenance
- Concrete protection
- Corrosion fuels
What is a hidden defect in asphaltic plug joints?
May conceal substantial leakage beneath
What are half-joint issues/ hidden defects, shown by I-R optic endoscope viewers (x2)?
- Salt water build-up
- Active corrosion
As well as I-R fibre-optic endoscope viewers, what are three other inspection methods
-
Long, directional cores (Ferroscan/radar)
- half-cell “down the hole” surveys - EMR corrosion excitation surveys
-
Sonic pulses (shown)
- e.g. the Doktor equipment
- acoustic emission
- embedded monitoring probes
- re-analysis in corroded weakened state
Name one thing that can be done to minimise risk of half-joint issues
Install a surface protection system to stop chloride from getting into the concrete
[NAQ] post-tensioned bridge beams sketches, showing leakage path, gap in duct and trapped air pocket
Name different types (x8) of issues with duct inspection
- Poor inspectability
- Movement joint failure
- Salt water build up
- Coastal exposure
- Corrosion risk
- Winter maintenance
- Concrete protection
- Corrosion fuels
