Durability Flashcards
Fresh state of concrete
- lasts 2-4 hours
- setting locks defects in microstructure
- curing prevents water escaping so reaction can take place
microstructure influenced by:
- capillary porosity determined by w/c ratio and curing
- bleeding and settlement causing voids and channels
- segregation of very wet mixes
- poor compaction = excessive voids
- poor placing and handling = segregation
Factors controlling hardening:
- dispersion of cement grains within paste
- temperature of concrete (low temp = slow dev. but high strength)
- quality of moisture curing during first few days
Low W/CM
- low capillary porosity
- small pores poorly connected
- low permeability
- high strength
High W/CM
- high capillary porosity
- large pores well connected
- high permeability
- low strength
Hardened State
- heterogenous material due to micro and macro variations
- durability controlled by quality of cover concrete
ITZ
Interfacial Transition Zone
- high proportion of calcium hydroxide and ettringite
- high porosity compared to bulk paste
Reasons for Failure
- structural collapse
- foundation failure
- accidents
- inadequate durability
Intrinsic Durability
- macro defects: cracking, compaction voids, delaminations
- meso defects: capillary porosity, bleed lens and channels
- micro defects: ITZ, gel pores
entrained air bubbles
- deliberate
- good for freeze thaw - allows for expansion
- can be used to reduce material quantity
entrapped air void
want to AVOID
service life predictions
- expected to be maintenance free for 50 years (100 for bridges)
- in reality, exposed to more severe environments often need repair in 15-25 years
pore solution or external water/solution
- corrosion
- alkali aggregate reaction
- sulfate attack
- acid attack
- frost attack/salt scaling
high permeability
- easy for chloride ion to get to steel
- capillary pores connected
porosity of concrete
- compaction pores
- entrained air
- capillary pores
- gel pores
important in concrete since it is a brittle material
compaction pores
- 1-10 mm
- affects strength and durability
entrained air
- 2 - 0.3 mm
- affects workability and strength
capillary pores
10nm - 100micron
- affects strength and durability
gel pores
2-10 nanometres
- affects shrinkage and creep
concrete microstructure has three major types of defects
- macro defects: major cracking in material
- meso defects: capillary porosity
- micro defects: interfacial transition zone (ITZ)
enhanced microstructure produced by the following
- reduced cracking: better site practice and good design
- reduced porosity: mix design, use of SCM
- better ITZ: SCM
beneficial effects of SCM
- micro-filler effect
- increased CSH
- wall effect
- pore blocking
Pozzolanic effect
- hydration of calcium silicates in Portland cement produces calcium hydroxide (weak and porous)
- reaction of silica fume with calcium hydroxide produces increased quantity of cementitious material
ASR
Alkali-Silica Reaction
- reaction between alkalis and siliceous rocks
- products may cause abnormal expansion and cracking
- affects all types of structures
alkalis
- sodium
- potassium
siliceous rocks or minerals
- opaline chert
- strained quartz
- acidic volcanic glass
ASR mechanism
- if silica reactive it may be “attacked” by OH- and then Na+ and K+
- forms alkali-silica gel composed of Na, K, and Si
- gel absorbs water from surrounding cement paste and expands
- causes internal stresses and eventually leads to cracking
Requirements for ASR
- reactive silica
- sufficient alkali
- sufficient moisture
Alkalis in portland cement
- represent a small proportion of the cement
- most end up in pore solution and associated OH sufficient to produce a pH in range of 13.2 to 14.0
Solutions to alkali in cement
- can add microsilica/silica fume (SCMs) to react with OH-
- change aggregate (can be difficult)
Effect of relative Humidity on ASR
little significant expansion if relative humidity maintained below 80%
thickness of ITZ
20-40 microns
four states of corrosion possible for RC
- passive state (steel embedded in uncontaminated corrosion)
- pitting corrosion (chloride-induced)
- general corrosion (carbonation-induced)
- active, low corrosion (saturated concrete)
need following conditions for corrosion:
- reactive metal
- oxidising agent
- moisture
- electrolyte that allows easy ionic movement
oxidation of iron
- oxidises and hydrates to form a range of ferrous and ferric compounds
- these products may occupy up to 6 times the original volume of the metallic iron
- significant expansion pressures cause cracking in concrete
Chloride-induced corrosion
- presence of salt and water creates the right conditions for rapid corrosion rates - generates pits and expansive rust
- minimum concentration of chlorides required to disrupt passive oxide film on embedded reinforcement
Carbonation-induced corrosion
- occurs in relatively dry environments and corrosion rates moderate
- usually aesthetic damage
- rates of more than 1mm/year only occur in very poor quality concrete
parameters influencing corrosion damage
- geometry of structural elements
- cover depth
- moisture condition of concrete
- age of structure
- presence of cracking
- service stresses in concrete
Chloride-induced damage
- deep pitting of reinforcement
- significant cracking and spalling
- eventually affects structural integrity
- costly to repair if ignored until damage is obvious
carbonation-induced damage
- general corrosion with little pitting
- minor cracking and rust staining
- mostly affects aesthetics, not integrity
- can be repaired relatively cheaply provided not left too late
Designing for durability, need to consider:
- structural interactions
- environmental conditions
- quality of construction
- level of durability required
most common durability design errors
- underestimating environmental exposure
- inadequate cover
- reluctance to use SCM’s in concrete
- no allowance for cracking of concrete during service
- lack of enforcement of curing specifications
- complete lack of supervision
Visual evidence of corrosion
- rust stains
- cracking along reinforcing
- spalling of cover
visual evidence of ASR
- expansive map cracking
- restrained cracks following reinforcing
Visual evidence of shrinkage
- defined cracks after 3-6 months drying
- excessive displacemetn
Visual evidence of chemical damage
- leaching of concrete surface
- salt deposits
- spalling expansion
Visual evidence of fire damage
- surface discolouration
- softening
- micro-cracking
Visual evidence of structural damage
- major cracking in areas of high stress
- localised crushing
- deflections
Delaminations
internal cracks or voids that run parallel to the concrete surface - therefore hard to detect
- particularly unwanted on bridge decks as defect is aligned with direction of principle stress
Delamination survey
- chain drag
- acoustic methods
- hammer
types of delamination
- poor finishing (bleed water rises and forms a lens)
- corrosion induced (severe exposure, closely spaced reinforcing)
- ASR expansion-induced (heavily stressed, long-span bridge decks)
Cover survey
- locates position and depth of reinforcing steel in concrete
- cover to reinforcing one of the most important factors controlling durability
- reduction of cover from 50mm to 30mm may reduce the time to corrosion from 50 to 10 years
Chloride testing
- determines % chloride per mass concrete at particular depth
<0.4% = low risk
0.4-1.0% = moderate risk
>1% = high risk
carbonation depth
- carbonation occurs under relatively dry conditions so carbon dioxide can diffuse
- carbonation-induced corrosion occurs at low covers, exposed to fluctuating moisture levels
Rebar Potentials
- measures thermodynamic risk of corrosion
- cannot evaluate rate
- mostly used for chloride-induced corrosion of decks
- use in combination with other tests
Resistivity Testing
- concrete resistivity controls the rate at which steel corrodes in concrete once favourable conditions for corrosion exist
- high resistivity = low travel from anode to cathode = low rate of corrosion
factors influencing resistivity
- moisture content
- permeability
- ionic concentration
Corrosion Rate
- only reliable method of measuring corrosion activity in concrete
- fluctuate widely depending on environment - need to take more than one measurement
- testing is time consuming
- requires experience/knowledge
- results are definitive
Cover survey reliability affected by:
- reinforcement of deep covers (usually reliable to within 5mm, but less reliable for particularly deep sections)
- areas of closely spaced bars
- measuring different bar sizes and types
- interference from magnetic material
Rebar Potential limitations
- interpretation of plots take experience
- delaminations may disrupt potential fields
- effect of environment and cover depth
- does not measure corrosion rate
Limitations of resistivity
- affected by carbonation and wetting
- avoid surface conductive layers
- do not measure in vicinity of steel
- unstable reading on dry concrete
- a complimentary test
Corrosion rate applications
- generally used once areas of corrosion activity have been identified
- requires multiple readings over course of year to develop estimated corrosion rates
advantages of corrosion rate measurements
- provide definitive assessment of corrosion
- particularly useful at start of corrosion process
- predictions of future damage can be made
3 main transport mechanisms
- sorption (capillary suction)
- permeation (hydraulic conductivity)
- diffusion
rate of ingress dependent on
- microstructure
- environment
- service stresses
- internal reactions
Durability Index Tests
- absorption
- permeation
- diffusion
absorption test
- water sorptivity test
- useful for assessing cover concrete quality as affected by curing
permeation test
- oxygen permeability test
- able to assess gaseous permeation/diffusion through concrete - used for carbonation
diffusion test
- bulk diffusion or resistivity test
- able to assess resistance of concrete to ionic diffusion of chlorides
What is sorptivity?
- measure of unsaturated flow of fluids into concrete
- assesses capillary forces that result in fluid being drawn into pore structure
- near surface effect
- simple to do in lab, but not in field
problems with field sorptivity test
- don’t know the saturation levels
- different saturations at different sites
review of sorptivity test
- only relevant for near-surface properties
- sorptivity not constant with time
- moisture content and sampling conditions important
Diffusion
- motion of molecules of water causes molecules to move from regions of high to low concentration
- equilibrium when molecules in equal concentration in all regions
Bulk Diffusion Test
- typical test length 35-40 days
- might extend to 90 days for concretes with lower diffusion rates
Review of Bulk Diffusion test
- measures “apparent diffusion” which does not address binding of ions
- fairly long term test
- test uses saturated sample to avoid surface absorption
- fairly complex analysis
Rapid Chloride Resistance Tests
- determines non-steady state diffusion coefficient
- good for comparative testing of different concrete mixes
permeation through concrete
- rate of mass flow proportional to pressure gradient across concrete
- resistance to flow given by coefficient of permeability
- coefficient not intrinsic to the material - affected by test method
- conditioning of sample very important
chlorides at steel cause:
cracking
carbonation causes:
rust staining
insufficient cover causes:
spalling
poor quality concrete causes:
delamination
Patch repair check list
- locate embedded post-tensioning
- conduct structural review before removing significant concrete
- exposed corroded rebar should be undercut to ensure adequate coverage and bond with new concrete
- full circumference of exposed bar should be cleaned
- loose rebar should be tied to other secure bars
- of more than 25% bar cross section lost, structural review
- edges of patch should be straight and square with surface to ensure maximum integrity of the patch
“Anodic Ring” Effect
- formation of new anode adjacent to patch
- patch just delays or moves corrosion
Improving performance of patch repairs
- treat surface
- treat steel rebar
- install discrete sacrificial anode
- install galvanic anode
options to treat surface
- membranes
- dense overlay
- penetrating sealer
options to treat steel rebar
- epoxy coating
- zinc painting
effect of treating steel rebar
- electrically isolates steel in the patch and prevents becoming cathode
- reduces risk of incipient anode formation
- don’t need to protect this part
effect of installed galvanic anode
- how far protection extends depends on resistivity of concrete
- provides cathode protection to steel in vicinity of patch
- can be counterproductive if have good concrete with high resistivity
sacrificial anode
- less noble metal connected to steel reinforcement
- confers protection through sacrificial corrosion
- only suitable when resistivity of system low
impressed current
- force steel to act as cathode using applied current
- slotted system
- titanium anode mesh: add at start and turn on when needed
requirements for impressed current
- electrical continuity of reinforcement
- concrete conductivity
- no short circuits
- direct current supply
- impressed current anode
- electrolyte
electrochemical chloride extraction
- DC power supply between rebar and temporary anode outside of concrete
- forces Cl- ions away from rebar and to solution at anode
- only done for brief period of time (weeks)
- some popularity but mixed results
- decreased effectiveness of remaining concrete - easier to penetrate
FRP wrapping
- works best for circular cross-sections
- prevents chloride penetration
NZ aggregates causing ASR
- Waikato River Sands
- Taranaki andesite
DEF
Delayed Ettringite Formation
- delayed formation of mineral ettringite - a normal product of early cement hydration
- result of high early temperatures (above 70 - 80 degrees)
- water from external source required
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