42907 Design for Durability

How 42907 is assessed

The four weights come from the subject's own assessment overview for this offering and sum to 100. The official subject record publishes no assessment detail at all, so every weight, task name and date here is taken from the subject site and the lecture materials rather than from the handbook. Three things need confirming before each deadline. The subject's pages give different answers for when three of the four tasks are due, and the assignment page is the one the submission portal enforces. The two quizzes are described on the lecture slides as timed windows of twenty four hours, with stated coverage that differs between sources. And the second part of the design project is called individual on one page and a group submission on another. No pass condition attached to any individual task appears in the materials available.

Current 42907 dates

Dates are as published in the subject's own assessment overview, schedule and assignment pages for this offering, which do not fully agree with one another. Confirm exact deadlines and submission settings in the live LMS.

What 42907 covers

Design for Durability asks one question in four settings: how thick and how good does the concrete cover have to be for this member to survive its design life. The guide opens on the material itself, the cement paste and the pore network that carries every aggressive species inward, then separates durability from design life and service life and sets out the exposure classification that turns a location into a pair of numbers.

The four deterioration processes follow in the subject's own order: chloride induced corrosion and its service life model, carbonation induced corrosion, sulfate attack in aggressive ground, and alkali silica reaction. Cements and supplementary binders sit between them, because the binder choice moves several of those processes in opposite directions.

The last part turns to cracking, shrinkage and the restrained early age cracking of a deck slab, and the final chapter assembles the whole procedure as the design report runs it.

It is a postgraduate subject on the durability design of concrete structures, targeting a design life of 50 years for buildings and 100 years for infrastructure as the Australian Standards require.

The teaching runs from durability and the design life concept, through environmental loads and exposure classification, the corrosion of steel in concrete by chloride ingress and by carbonation, the corrosion of concrete by sulfate attack and alkali silica reaction, and the role of concreting materials, to design from a standard and design from first principles, specifications and quality control, and the control of restrained shrinkage induced early age cracking.

It is a calculation subject as well as a materials subject: the chloride model is an error function solution of Fick's second law and the carbonation model is a root of time front law, and both are run from published performance data to produce a concrete cover in millimetres.

The assessment is built around a single bridge design project rather than an examination, so the marks go to designing each structural member for its own micro environment and then defending that design against the prescriptive requirements of the Standard.

Worked example · free

Designing the minimum cover for a permanently submerged member over one hundred years

Key terms

Design life

The period for which a structure or member is intended to remain fit for its purpose, with minor maintenance. The Australian concrete structures standard targets 50 years and the bridge design standard 100 years, and the design project runs at a hundred.

Exposure classification

A code assigned to a concrete surface from its environment, which then fixes a minimum characteristic strength, a minimum curing period and a required cover. Six graded rungs run from A1 up to C2, and a seventh code covers an environment the table never names.

Apparent diffusion coefficient

The coefficient used to model chloride ingress in concrete, absorbing the porosity, the interactions between ions in the pore water, the binding capacity of the matrix and the ageing of all three. No reliable model predicts it from mix design, so it is measured or taken from published data.

Chloride threshold

The total chloride content at the steel at which the passive film fails and corrosion can begin. This subject designs to 0.4 per cent of cement mass, which converts to about 0.065 per cent of concrete mass at a binder content of 400 kilograms per cubic metre.

Carbonation coefficient

The single parameter of the carbonation front law, in millimetres per root year. A value of 2.5 puts the front at 2.5 millimetres after one year and 25 millimetres after a hundred.

Initiation period

The time before the steel depassivates, during which nothing is corroding and the cover is doing its work. In the severe marine classes this subject requires the initiation period alone to reach the design life.

Propagation period

The time after depassivation during which the steel actively corrodes. A carbonation design may count it up to the point where the cover cracks, and it is calculated as the corrosion depth for cracking divided by the corrosion rate.

Supplementary cementitious material

An industrial by product used as part of the binder, chiefly fly ash, ground granulated blast furnace slag or silica fume. It reduces chloride ingress, sulfate expansion and alkali silica expansion, and increases carbonation depth.

Delayed ettringite formation

An expansive reaction occurring at late ages in concrete that is already rigid, cracking and spalling it. High curing temperature is its principal trigger, which is why the bridge standard caps concrete temperature during curing.

42907 FAQ

Questions students ask in each 42907 chapter

Why does the water to cement ratio matter so much for durability?

Because the mixing water that is not consumed by hydration leaves capillary pores behind, and the more of it there is the more those pores interconnect into a continuous path through the cover. The measured permeable voids of this subject's own data nearly double, from 8.42 to 16.22 per cent, as the ratio rises from 0.42 to 0.63. Every ingress model later in the guide takes its coefficient from that pore network, so the ratio decided at batching sets the durability of the member for its whole life. Chapter 01 · Concrete basics and transport properties

What is the difference between design life and service life in practice?

Design life is what you aim at and service life is what you get. A bridge member has a 100 year design life because the bridge code was written to deliver one; whether it achieves a 100 year service life depends on whether the deterioration processes were correctly identified and resisted. A predicted service life shorter than the design life is precisely the finding a durability report exists to produce, and it should be reported as a shortfall in years rather than hidden. Chapter 02 · Durability, design life and service life

Why is permanently submerged concrete classified milder than tidal concrete?

Because corrosion is an electrochemical process that needs oxygen at the cathode as well as chloride at the anode. A permanently submerged surface is saturated, so oxygen has to reach the steel by diffusion through water rather than through air, and the supply is starved. Corrosion is therefore slow even after the steel depassivates. Severity peaks where wetting and drying cycle, because each cycle delivers salt by absorption on the wet half and oxygen through air filled pores on the dry half. Chapter 03 · Environmental loads and exposure classification

Why is the chloride threshold written on total chloride rather than free chloride?

Because binding is not permanent. A large bound reservoir can be released when the pore solution chemistry changes, and carbonation of the surface layer in particular releases it. A criterion written on free chloride alone would pass a member that is holding that reservoir and waiting to let it go. The practical consequence is that the acid soluble laboratory result, not the water soluble one, is the figure you compare with the threshold. Chapter 04 · Chloride transport and corrosion initiation

Why does cover appear squared in the time to initiation?

Because the diffusion front advances as the square root of time, so inverting the relationship for time puts the depth in as a square. The practical consequence is that small cover gains are worth more than they look: ten per cent more cover buys about twenty one per cent more time. The subject pairs this with a field observation, that lack of adequate cover is the overwhelming cause of premature corrosion, which makes site attention to cover a cheaper lever than a better binder. Chapter 05 · Service life design for chloride ingress

Why does carbonation need a particular humidity rather than simply as little water as possible?

Because it needs two incompatible things. Carbon dioxide diffuses through air filled pores, so a saturated network chokes it, but the reaction itself happens in the pore solution, so a bone dry network gives it nowhere to occur. The subject places the optimum at about 65 per cent relative humidity and states it twice. That is the front; the corrosion that follows peaks somewhere around 90 to 95 per cent, which is why a member that cycles between the two is the dangerous case. Chapter 06 · Carbonation induced corrosion of reinforcement

Do supplementary binders always improve durability?

No, and treating them as uniformly good is a shipped defect waiting to happen. Across the six mechanisms this chapter tracks, four improve and two get worse: carbonation depth increases and early age strength falls. The four that improve are chloride ingress, sulfate expansion, alkali silica expansion and the heat of hydration. A binder choice that is right for a submerged member can therefore be the wrong choice for a sheltered one on the same structure. Chapter 07 · Cements and supplementary cementitious materials

Why can cover not protect against sulfate attack the way it protects against corrosion?

Because nothing is happening to the steel. The paste itself is being destroyed, throughout the thickness the sulfate has reached, so adding cover simply adds more material to be attacked. That is why the whole prevention list is aimed at permeability and mix composition rather than at geometry, and why the design project asks you to design the footings for aggressive ground by the Standard rather than by modelling a front. Chapter 08 · Sulfate attack, DEF and aggressive soils

Can the reaction be stopped once it has started?

No. The guidelines state that there is nothing is known that will halt alkali aggregate reaction in practice once it has begun, and that remedial work is aimed at appearance and at keeping the member structurally competent. That is the sharpest contrast in the subject: corrosion can be arrested by cathodic means or by removing the chloride, whereas this has to be designed out before the concrete is batched. Chapter 09 · Alkali silica reaction and its prevention

Why is bleeding treated as a durability topic rather than a finishing one?

Because four of its six listed consequences bear directly on durability: a weak top layer, reduced bond to the aggregate, reduced bond to the reinforcement and increased permeability. The last of those is the connection to every ingress model in the guide, and channelled bleeding is worse still, because fast moving bleed water can leave continuous channels through the paste. A cover designed as a pore network has been compromised before the concrete has even set. Chapter 10 · Concrete cracking and shrinkage

Why is this called early age cracking rather than simply shrinkage cracking?

Because of the timing of the race. Shrinkage begins as soon as the concrete meets an unsaturated environment, which can be within hours, while tensile strength takes days to develop. The dangerous window is the one where the stress has arrived and the strength has not, and it closes as the concrete matures. A member that survives its first weeks is very much less likely to crack from restrained shrinkage later. Chapter 11 · Restrained shrinkage and early age cracking

How are the forty marks actually distributed?

Per member, and identically for each of the four. One mark for assessing the environmental loads and determining which governs; seven for the concrete specification together with the first principles models for chloride and, where it applies, carbonation, plus the aggressive ground design for the footings; and two for confirming the governing load, comparing the first principles result with the Standard and giving the final design. That is ten marks a member and forty in total. Chapter 12 · The service life design report: method and marks

How to prepare for the assessments

Treat 42907 as a design procedure you rehearse rather than a syllabus you revise. The single most useful thing you can build is a working sheet with one row per structural member and one column per decision: micro environment, governing load, exposure class, binder, binder content, water to binder ratio, characteristic strength, curing, first principles cover, prescriptive cover, signed difference and final cover.

Fill it as the semester goes, because every lecture supplies one more column. Drill the two calculations until they are automatic in both directions, the error function solution for chloride and the root of time law for carbonation, paying particular attention to the unit conversions, since both run in metres and seconds and report in millimetres and years.

Learn the exposure tables by using them rather than by memorising them, and always write the sentence that justifies a classification beside the class itself. Watch the three direction pairs this subject reverses relative to intuition: permanently submerged is milder than tidal, blended binders help against chloride and hurt against carbonation, and a permeable soil in groundwater is the harsher soil condition.

Finally, confirm every date and submission mode on the subject site rather than from any study aid, because the subject's own pages do not agree with each other on several of them.

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