# How is the design life of a reinforced concrete structure calculated?

The specifications for large structural projects typically call for the structure to have a specific design life. This can be 50 years, 100 years, etc.

Accommodating the design life for steel can be as simple as adding additional thickness to account for the expected corrosion over this time period. This calculation would also take into consideration any variations based on coatings or type of steel.

History has shown that unreinforced concrete structures can last hundreds of years. The Romans have some examples of this such as the Pantheon.

The problem with reinforced concrete is that eventually the reinforcing will corrode, expand in size, and cause the concrete to crack. There can also be issues with the aggregates that are used.

How can a designer calculate and, by contract, guarantee the life of a reinforced concrete structure?

• Afaik most buildings of the today are planned with 60 yrs. – peterh Feb 24 '15 at 3:32
• This question seems to be the wrong way around to me. You don't design a structure and then calculate its design life; you determine what design life is required and then design to meet this. Perhaps the question should be "What aspects of a concrete structure's design are affected by its design life?". Also, I have never heard of a designer contractually guaranteeing the life of a structure. – AndyT Feb 24 '15 at 14:10
• @AndyT You are correct that you design a structure to meet a design life. Maybe the question could be asked in a slightly different way, but the end result is the same. How can you guarantee a design life requirement. And yes, I have seen it in contracts. – hazzey Feb 24 '15 at 17:42
• Design life affects design loads which are statistically based. e.g. Wind load is dependent on design life, because the longer the intended life of the structure, the more likely it is to see higher winds. But, the design wind isn't guaranteed not to be exceeded within the design life - it's a statistical expectation, not a hard fact. An engineer therefore can't guarantee that the structure will survive for its intended life. (This could form part of an answer to "What aspects are affected by design life?", but not "How can you guarantee the life?") – AndyT Feb 24 '15 at 23:21
• @hazzey What a great (and complex!) question. I thought you might be interested in some ongoing research wrt developing bridge design criteria for 100 year service life. It's been on my reading list for awhile -- you've finally given me the motivation to dig in! – CableStay Oct 17 '16 at 0:27

Design life can be one of two different things, and they aren't interchangeable.

A reference to '100 years design life' might mean that it's designed for a '1-in-100-years' loading case (wind load, or tidal surge, or whatever). This is solely about a means of quantifying the magnitude of loading. It is actually nothing whatever to do with the durability of the structure, it's about the strength of the structure.

The question asks about a different matter - the durability, and specifically the durability of reinforced concrete. It is quantified normally by reference to past experience in the particular environment telling you what the critical deterioration mechanism is likely to be, and then either reference to a standard solution, or a calculation of the lifetime for that mechanism. The calculations are normally to some degree empirical.

For a 'standard' structure, with a 'normal' exposure condition, 'normal' concrete characteristics, and 'normal' design life requirements, there will be standard solutions in the relevant design code, that probably simply defines the quantity of cover that will satisfy the design life. What constitutes 'normal' will depend upon the jurisdiction of the design code - different blends of cement are available in different parts of the world, and what is 'normal' for a national design code in a wholly-temperate country won't be 'normal' in the tropics or polar regions.

For example, in a structure in the splash zone in the Arabian peninsula, frost attack is not going to be a problem, but physical salt attack (or salt weathering) will be. Frost attack is where water freezing in pores and cracks expands and breaks the concrete. Salt weathering is where salty water is wicked up and evaporates at such a rate that salt crystals grow within the pores and break the concrete.

If a designer strays outside what their local design rules consider 'normal', or if the environment is especially aggressive, or the durability requirements are unusually onerous, then a specific calculation will be required.

The most common failure of reinforced concrete is that metallic reinforcement starts to corrode. Steel in concrete does not corrode because the concrete is very high pH, and steel in a high-pH environment is 'passivated' and does not corrode. However, slowly over time carbon dioxide from the atmosphere diffuses into the concrete and neutralises it. If you know the characteristics of your concrete, you can predict how fast that happens (by reference to empirical experience).

What normally actually triggers the corrosion (at least in marine or other salty environments - eg road salt), however, is chloride attack, where chloride ions diffuse in from the surface. Once the concentration of the chloride ions at the surface of the bar reaches a critical value, corrosion will soon take hold. You can calculate this, if you assume a concentration of chloride at the surface (from empirical data), and know the characteristics of the concrete (either empirical data, or by testing how fast chloride ions diffuse through it, but beware that as the concrete ages, its characteristics change, and you need to allow for that), and know the critical threshold (from empirical data).

There's a handy free program that does this calculation for you called Life-365, and it comes out of an American Concrete Institute committee. It does the chloride diffusion calculation for you, draws graphs and stuff, and if you're in the USA it even has the empirical data you need built-in so you don't need to look up what the local conditions are. (I use the program, but am not associated with it otherwise). The manual to the program has more detailed discussion of the science behind it, but the best thing is you can just play with it and see what effect changing something has on the life.

If you do the calc and you don't get enough life, then either you can put the reinforcement deeper (so it takes longer for the chloride to diffuse to it), or you make the concrete more resistive to chloride diffusing through it, or you use bar that needs a higher threshold value of chloride (stainless, say), or you surface treat the bar, or the concrete, or you put in galvanic or electro-chemical systems, or corrosion inhibitors, or something else. Lots of this stuff comes back to empirical data - they've tested it, and have test data that shows it will prevent corrosion for n years if you put in x amount of whatever.

• You are correct the the question is about long term durability and not load occurrence periods. – hazzey Mar 14 '15 at 13:33
• Interesting! I can understand the theory that you can calculate theoretical durability from your concrete type, reinforcement depth and local conditions. But in terms of actual design, I would always expect it to be the other way around. You start with "I need x years design life" and then codes/standards state what reinforcement depth and concrete mix is acceptable. – AndyT Mar 16 '15 at 13:28
• Yes, but that's the same in pretty much all design - you guess / know / intuit the answer, then check it satisfies the requirements. For example, what depth concrete beam to use? There's no equation that tells you what you require, you make an assumption and check it. For durability, codes give some guidance (along the lines you suggest) but if it really matters or it's an unusually aggressive environment you do a calc such as the one done by life-365. – achrn Mar 16 '15 at 13:48

I can't answer this question in terms of a building structure; however, I can for a reinforced concrete pavement, which may still be some interest for you.

In line with the other answers, a trial pavement design is devised, which is then assessed against the design life. The load departed on the pavement is expressed in terms of a standard axle repetition over the design life. For example, a pavement might be designed to withstand 1x105 standard axle repetitions over a 40-year design life. This is called the Design Repetitions.

A trial pavement is selected, then a fatigue analysis is undertaken which determines the pavement's Allowable Repetitions. The Design Repetitions are then divided by the Allowable Repetitions and if this value, defined as the Cumulative Damage Factor (CDF), is < 1.0, then your pavement will survive the design life.

So, CDF = n / N where n = Design Repetitions, N = Allowable repetitions. Note that you could do a back-calculation to determine the design life if you knew the other terms.

• What "other answers" are you referring to? Do you mean the comments? It's often best to provide an inline link to referenced content, for the sake of clarity. – Air Mar 4 '15 at 0:26