DURABILITY

Standards - validating:

history,

consistency &

compliance.

DURABILITY – why do we care?

most production sacrifices quality for economy?

design life vs. cost-sensitivity:– shelf-life beer: $cost ≅≅≅≅ low % of annual income,

– cost of motor vehicles / income decreasing: lower-end product has 3 yr warrantee and 6 yr depreciation,

– housing/accommodation approx. ⅓ net annual income

– cost sensitivity ↑: item cost / income ↑

why do we care?design life:

– residential - 40-60 years

– non-residential structures - 40-60 years (best ROI considering NPV)

– engineering structures — 100 years:

– powerful customers: few & well informed,

– high item cost / income,

durability:– acceptable performance and serviceability in relation to

economic ramifications of repair/replacement.

what do we need to achieve?

reinforcement:– minimise/delay corrosion to ensure integrity and

serviceability over specified time-frame

concrete matrix:– tensile region – protect reinforcement and retain

development bond,

– compression region – maintain cross-section and strength.

content and program:2 other speakers this evening:

Tom Glasby and, Reuben Barnes:– both will contribute best-practice methods of specifying

general concrete structures,

Reuben (our keynote speaker)– has come to present from WA,

– will update us with non-destructive testing methods for assessing both:• specification compliance and,

• ageing performance/characteristics.

the basics:this section of my presentation:

– assumes that you have either forgotten or never learnt the basics of the degradation of concrete structures,

– but that you will know where to find the info if so inspired,

main influences on concrete durability requirements:

– most of these aspects can affect concrete both before and after it is hardened,

– design, specification, supply and construction all affect the integrity of a concrete element,

•serviceability•abrasion,•and loading

•moisture,•temperature,•durability•chemical attack,

the basics:most influential aspects (considering adequate duty-of-carewas taken in design and construction) are:

– moisture,– chemical attack,

in relation to:– corrosion of reinforcing steel &,– degradation of concrete matrix

steel corrosion is typically the quickest and bulk of codedurability clauses are dedicated to this.surprising how many people think that cover preventscorrosion by preventing the ingress of water.

constantly remind your friends (at BBQs etc.) concretetypically has some degree of permeability to gases and ormoisture vapour.

electrochemical process with 2 essential reactions:– anodic and cathodic,

anodic dissolution process is represented by:Fe = Fe2+ + 2e-

in conditions where in pore solution (the moisture in concrete voids) :

– 02 is plentiful &,– pH ≥ 12

wonderful passivation layer forms around the reo, & thusanodic rate is very low.

2 conditions destroy this passivation layer:

– Carbonation,– exposure to Chloride ions (Cl-)

Carbonation:– reduction/neutralisation of pH due to ingress of CO2

from atmosphere.

– over time a Carbonation ‘front’ moves towards the reo and dissolves the passivation layer.

– Fe2+ ions move away from reo and cross-section reduces,

Chloride attack:Penetration of chloride ions (ionization):

– most typically from sea water,

– cause corrosion even in high pH environ,

– localised breakdown and pitting of the reinforcing steel,

– rapid anodic dissolution and fast corrosion

corrosion:oxygen - acts as the fuel for the corrosion reaction.moisture - as the connection/conduit (corrosionwould completely stop without it).most texts simplify the corrosion process into 2 phases:

– initiation and propagation,– initiation period - the time before corrosion starts to occur,– propagation phase – changes affect serviceability &further

life-span.

corrosion:Carbonation relates solely to the initiation phase.

Cl-, in conjunction with O2 & H20 induce shorterinitiation and propagation periods.

present-day durability requirements :our standards set out the minimum requirements:

– AS 2159-1995 Piling

– AS 3600-2001 Concrete structures

– AS 5100.4-2004 Bridge design - concrete

AS 3600 Concrete structures:– design life = 40-60years.

– recognises corrosion as most rapid and frequent failure,

– limited range of environs,

– limited no. of durability failure types, i.e. corrosion of reo and abrasion,

– refers professionals to other publications for more onerous requirements: eg. joint CCAA/CIA Durable Concrete Structures.

historical background:paper: Ho and Lewis (CSIRO Melb),“ The Specification of Concrete for Reinforcement

Protection”, US Cement and Concrete Research, Vol. 18, 1988,

– based on a CCAA sponsored research,

– research and findings are enduring,

– the main reference for current corrosion requirements per standards,

– paper is available from our CCAA national library.

research - Ho and Lewis :3 parameters typically associated with durability:

1. concrete strength, 2. w-c ratio, 3. cement content,

these factors are vital but…similar strength concretes can havevastly different performance characteristics wrt corrosion,additional considerations are needed for corrosion to control theinfiltration of:

for corrosion, the most important concrete properties are:1. enduring/remaining alkalinity:

- assessed by checking carbonation,2. pore structure:

- measured by water sorptivity (permeability),

O2,

H20

CO2,

Cl-,

good quality concrete:

is represented by:– low values of both Carbonation & water sorptivity,

– high 7 day strength (more critical than 28-day for sorptivity) & thus:

good curing is essential– water reducers are extremely advantageous wrt w/c ratio

and 7-day strength

Sorptivity:

– related to the rate of water penetration

– moisture the main vehicle for transporting chloride ions

– depth and quality of cover reo restricts moisture front from reaching reo,

– Ho & Lewis relate ‘wetting periods’ to exposure conditions e.g.:• moderate exposure (A2) – ½ day continuously wetted,

• severe exp (B1) – 2 days continuously wetted

Carbonation:

– carbonation also exasperates the effects of chlorides,– worst with a relative humidity 45-65%,– above 65% too saturated,– below 45% too little moisture for dissolution & reaction,– Ho & Lewis correlated that carbonation rate < 4mm/y0.5

was required for:• 30mm cover to provide,• an initiation period of 40 years.

– corrosion initiates 6mm ahead of carbonation front (15mm if high free-chloride concentration),

– in dryer exposure conditions (or away from the coast) shorter initiation period can be accepted as propagation is slower.

water sorptivity and carbonation rates:

• Ho & Lewis provide recommended values to accommodate various exposure conditions & cover levels:

Ho & LewisCarbonation front =

No. (years) x C’a (mm/y0.5)

..as corrosion can occur ahead of carbonation front:

Required cover =

Design life (years) x C’a (mm/y0.5) + const.

where const = 6mm (B1 exposure),

= 15mm (≥ B2 exposure)

Australian standards – comparative review:

current applicable standards:– AS 2159-1995 Piling– AS 3600-2001 Concrete structures– AS 5100.4-2004 Bridge design – concrete

plus:– DR 05252- May 2005 Concrete structures

• for public comment until end of July

durability consideration:– time-line,– knowledge,– consistencies / deviation.

Notes:•In general, changes/amendments are consistent with a knowledge time-line,

•AS 5100 & DR 05252 do not tabulate class reductions for plain members – these can contain reo and/or embedded fixtures

•A1 classification not included in AS 5100 - increased min. requirements for 100 yr design life.

•Salt-rich arid changed from Class C in AS3600, to Class U in DR 05252 – recognition of variability/severity of effects

•Increased requirement for non-res internal environ in DR 05252 – providing increased ROI

•Additional C2 classification increases cover requirements in splash zone

Notes:•Min 7 day curing in AS 5100 accommodates 100 yr design life,

•All other aspects consistent

Notes:•Difficult to correlate AS 2159 piling code,

•Improved chloride consideration in AS 5100 for 100 year,

•DR 05252 provides for more detailed classification for sulphate levels

Notes:•Min 32 MPa for reinfconcrete in AS2159

•Additional 10 mm cover in AS5100 for A classifications – provides additional 60 years at carbonation rate of 4.0mm/y0.5,

•Additional 5 mm cover in AS5100 for A classifications – reduced initiation period permissible

•Min. 50MPa in AS5100 for C class,

• 15mm additional cover for class C2 in DR 05252

Notes:•Not directly referenced in AS2159,

•All other codes the same,

• ‘-ve’ value indicates reduced cover,

• cover and quality requirements as tabulated include allowance for above

Conclusions:• Aust Standards set out the minimum requirements for

common environmental exposure,

• numerous relevant references are provided in commentary supplements,

• water sorptivity and carbonation are the key performance indicators for durability,

• concrete quality (in terms of durability) can vary significantly wrt strength,

• level of sorptivity:– minimises the ingress of reactive chemicals (without which

corrosion would note occur),

– affects both corrosion initiation and propagation periods

– is reliant on adequate curing and 7- day strength

Conclusions:• Durability requirements as incorporated in our

Australian Standards are:– based on sound enduring research:

• As referenced by AS3600

• undertaken in AUS by CSIRO,

• equations closely replicate current requirements in AS3600 and AS5100

– very uniform across relevant standards allowing for 40-60 & 100 year design life,

– chronological variations occur as standards are revised to incorporate the most recent and developed knowledge,

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