42907 Chap.2 Durability, design life and service life
Durability, design life and service life
Week 1 separates three terms that ordinary usage runs together, and the separation decides what a report is claiming. Durability is qualitative, design life is the quantitative target a code was written to deliver, and service life is the period a structure actually remains fit for purpose.
The concrete structures code targets 50 years and the bridge code 100, which is why the design project runs at a hundred. The chapter also sets out the three routes to a durability design, prescriptive, first principles and performance based, and the four chemical attacks the subject models.
What this chapter covers
- 01
Durability described qualitatively, design life quantitatively, service life as an observed outcome
- 02
The phrase with minor maintenance, and why it limits what a design life may assume
- 03
Design life targets: 50 years for buildings, 100 years for bridges, 150 years for exceptional structures
- 04
Why neither code suits a 150 year target
- 05
The standards family: piling, liquid retaining structures and maritime structures
- 06
The prescriptive route and the four things it explicitly does not do
- 07
The first principles route, and why it needs long term field data
- 08
The performance based route: testing concrete properties against a reference concrete
- 09
Mechanical, chemical and physical causes of deterioration
- 10
The four chemical attacks this subject models, and which two attack the steel rather than the concrete
Choosing a design route, and saying why the other two were not enough
- +1The prescriptive route runs first and is never skipped: classify the surface, read the minimum strength and the required cover. That value is a floor no design may go below.
- +1The first principles route runs next, because the target is long and the binder is not ordinary. The tabulated cover was calibrated for the design life the code was written for and for conventional concretes; a 50 per cent slag blend has a markedly different diffusion coefficient.
- +1Performance testing supports the model rather than replacing it: the diffusion coefficient and the permeable voids of the actual mix are the inputs the model needs, and without them the model runs on tabulated values for a nominally similar concrete.
- +1Specify the greater of the prescriptive and the modelled cover and state the difference. That difference is the margin, which is the one thing a table alone cannot report.
Key terms
- Durability
- The ability of a structure and its members to perform the functions they were designed for, over a specified period, when exposed to their environment. It is described qualitatively, so a sentence that gives a durability in years has used the wrong term.
- Design life
- The period for which a structure or member is intended to remain fit for its purpose, with minor maintenance. It is a quantitative target set by the code that governs the structure type, not a choice made by the designer.
- Service life
- The period for which a structure or member actually remains fit for its purpose. It is an outcome rather than a target, and the gap between it and the design life is what the deterioration models of this subject exist to predict.
- Prescriptive design
- Designing durability by reading a code table: characteristic strength, cover, cement type and water to cement ratio. It is fast and auditable, but it reports a required value without any indication of the margin behind it.
- First principles design
- Designing durability by modelling the ingress process itself, such as the chloride profile against depth and time. It reports a margin, but it needs long term field data and is difficult for new binder types.
- Performance based design
- Designing durability by measuring the concrete's own transport properties and comparing them with a reference concrete whose field performance is known. It tests the concrete rather than the mix design.
Durability, design life and service life FAQ
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.
Why does the subject insist on modelling when a code table already gives a cover?
Because a table returns one number and no margin. It cannot say whether the cover is adequate by ten millimetres or short by ten, and it was calibrated for the design life and the concretes behind the code rather than for the binder and micro environment in front of you. The model supplies the margin, and the signed difference between the two answers is what a discussion section is built from.
Which code applies to my structure?
The structure type selects the code and the code states the design life it was written to deliver. Buildings and general concrete structures fall under the concrete structures code at 50 years; bridges fall under the bridge design code at 100 years. That is why this subject's design project runs at a hundred years throughout.
For an exceptional structure targeting 150 years the lecture is explicit that neither code is suitable.
Does the subject model every cause of deterioration it lists?
No, and that is worth knowing before you write. The taxonomy covers mechanical, chemical and physical causes, from abrasion and impact through freeze thaw and fire. Four chemical attacks are modelled: chloride induced and carbonation induced corrosion of the reinforcement, sulfate and acid attack from aggressive soils, and alkali silica reaction.
The first two attack the steel, so the design variable is cover; the second two attack the concrete, so the design variable is the mix.
Assessment move
Learn the three definitions well enough to correct a sentence that misuses them, because that is exactly how they are tested and it is also how marks are lost in a report. Attach each one to its giveaway word: qualitatively for durability, intended to remain for design life, remains for service life.
Memorise the two design life figures and the code each belongs to, and make a habit of writing the figure at the top of every calculation. For the three routes, practise saying what each cannot do rather than what it does, since the limitations are what decide when to use which.
Finally, be able to place any named deterioration process into the mechanical, chemical or physical family and say whether its design variable is the cover or the mix.
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