AASHTO LRFD Crack Control Width 2

[SENK151]

When designing a Bridge RC Deck slab exposed to seawater salt, is it correct to use an exposure factor of 0.75 (Crack Width=0.33mm) for crack control?
AASHTO doesn't mention clear criteria for marine structures. Is there any other standards to reference for this condition?

Thank you in advanced.

 

[DLBridge]

Yes, I would use a Class 2 exposure factor for corrosive environments. If this is for a bridge deck, AASHTO is the appropriate standard to use... Also consider reading your local or state dot design standards. They may have additional requirements specifically tailored for the local conditions.

There are methods of protecting the concrete reinforcement such as low permeability concrete mixes, corrosion inhibiting admixtures, etc. What design life is required for the bridge deck?

 

[STrctPono]

DLBridge, Do you mean to ask what "Service" life is required for the bridge deck?

It is correct to use the Class 2 exposure for this type of condition. Beyond this, AASHTO Bridge Design spec is pretty useless when it comes to practical design of concrete elements exposed to corrosive environments. SHRP2 has some helpful publications. There are a multitude of options available for addressing these issues. Research is ongoing all the time when it comes to bridge decks. Virginia DOT s a pretty active player.

There is a lot of competing thoughts when it comes to epoxy coated vs GFRP vs ChromX vs Stainless Steel rebar...

Methacrylate based coatings...

Calcium Nitrite vs Amine Carboxylate corrosion inhibitors...

Clear cover, W/C ratio, pozzolan admixtures...

There is so much you can do to address the issue and every State has a different take on the matter.

For instance, I live in a heavily atmospherically corrosive environment due to the presence of the ocean but we don't use deicing salts on our bridges. Therefore, our bridge decks almost never experience any sort of corrosion issues... but the windward side face of the railings, bent caps, columns, wingwalls, etc. take a huge beating.

 

[SENK151]

DL Bridges and STrctPono, thank your for your helpful comments. I will check local requirements.
The service life of the concrete deck is 100 years.

Unfortunately AASHTTO doesn't offer much information about requirement for marine environments.

For the deck slab concrete cover of 50 mm is being considered. However, does coastal situation (Concrete Cover:75mm) also apply to deck slab? Deck slab would be protected by waterproof membrane.

 

[DLBridge]

STrctPono - yes, meant to ask service life. Thanks for clarifing.

SENK151 - a 100 year service life is the gold standard for new builds. As STrctPono mentioned, different DOTs have different thoughts on how to achieve that. NYSDOT's bridge design manual has some decent discussion on service life. It should also be mentioned that NYSDOT specifies a HPIC (high performance internal curing) mix...

 

[STrctPono]

SENK151,

AASHTO doesn't really address service life in any meaningful way. They address design life requirements. These are two totally different things. If you are particularly interested in service life then you should look at this SHRP2 document titled: "Design Guide for Bridges for Service Life". I believe it is available for free download on the internet.

Also, if you are actually interested in running a service life check on the bridge you can start with something like Life-365 program. It's based on Fick's 2nd law and predicts service life based on diffusion of chloride ions through concrete cover. The default settings and options are very limited but if you familiarize yourself with the program and the how you can input different values, you can tweak the program to work for a multitude of scenarios. It doesn't consider cracked concrete and only considers corrosion due to chloride ions but I am not aware of a better program. Be aware, it was created by a consortium of chemical admixture companies such as Grace and BASF so take that with a grain of salt.

Either way, justifying a 100-year service life in a marine environment without going to stainless steel will be tough. You can maybe get there with 3" clear cover, 0.35 w/c ratio, 5 gallons of calcium nitrite, and silica fume in the mix with 9100 ChromX bar but it will be tight. Plus, a 0.35 W/C ratio is going to be tough to pour in the field. If it's precast, then no problem.

 

[OSUCivlEng]

STrctPono is correct.

Design Life = How accurate will the loading be sometime in the future, i.e. the return period.
Straight from the definitions in Chapter 1 of AASHTO LRFD:
Design Life - Period of time on which the statistical derivation of transient loads is based: 75 years for these Specifications.

Commentary of Section 1.3.2.1:
The Strength I Limit State in the AASHTO LRFD Design Specifications has been calibrated for a target reliability index of 3.5 with a corresponding probability of exceedance of 2.0E-04 during the 75-year design life of the bridge. This 75-year reliability is equivalent to an annual probability of exceedance of 2. 7E-06 with a corresponding annual target reliability index of 4.6. Similar calibration efforts for the Service Limit States are underway.

The load and resistance factors in the AASHTO LRFD specifications are based on statistical models. 75 years was chosen as the return period and the live load factors were calibrated based on that. Therefore the maximum live load effect the bridge is designed for is the maximum expected load in a 75 year period.

Service Life = How long with it last before it fails.

People often confuse Design Life and Service Life by saying the bridge has a Service Life of 75 years.

 

[BridgeSmith]

The definition we use for Service Life is along the lines of how long we anticipate until it requires major rehabilitation or replacement. We try to avoid ever saying we expect a bridge to fail, since that would imply something catastrophic.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[IDS]

I think its worth mentioning that different standards have different definitions for "design life". For instance the Australian Bridge Code definition is:
"The period adopted in design for which a structure or structural element is required to perform its intended purpose with periodic maintenance and without replacement or major structural repairs"
This is pretty much the same as BridgeSmith's "Service Life".

Also "How accurate will the loading be sometime in the future" is very different from the "return period".

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[STrctPono]

From SHRP2:

"The end of service life for a bridge element, component, or subsystem does not necessarily signify the end of bridge system service life as long as the bridge element, component, or subsystem could be replaced or resume its function with a retrofit."

"The service life of a bridge element, component, or subsystem ends when it is no longer economical or feasible to repair or retrofit it, and replacement is the only remaining option."

"The service life of a bridge system ends when it is not possible to replace or retrofit one or more of its components, elements, or subsystems economically or because of other considerations."

"Service life should be equal to or greater than the design life"

IDS, That is interesting. It sounds like your code wraps AASHTO's definition of Design life and Service life into one. Per the OP's question, if I were to design a bridge in a Marine environment to the minimum requirements of AASHTO LRFD I would be turning over a maintenance nightmare to the DOT as the service life requirements are never really addressed and it would never meet the target 75 years or 100 years. I wonder how the Australian Bridge Code addresses a situation like this that is different than AASHTO.

 

[IDS]

IDS, That is interesting. It sounds like your code wraps AASHTO's definition of Design life and Service life into one.

I wouldn't say it wraps the two definitions into one. What AASHTO calls "Service Life" it calls "Design Life". What AASHTO calls "Design Life" it calls "Load Return Period", although it doesn't actually have a definition for that term in the General or Load sections.

In my opinion using "Design Life" to mean "Load Return Period" is quite confusing.

As far as design for aggressive environments is concerned the Australian Code says essentially "do a proper durability design, using accepted rational procedures".

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[BridgeSmith]

I think the SHRP2 quotes from STrctPono define Service Life, as AASHTO defines it, fairly well.

The Design Life number of years is used in the statistical probabilities for developing Load Factors to meet the target reliability index, so in that respect, I can see how it's similar to the "Load Return Period". However, for the bridge designer, the AASHTO Design Life has more applicability to fatigue cycles than it does probabilities related to maximum loads. It's primarily used to calculate the number of cycles of a particular stress level that a component is expected to experience, to determine if it's likely to develop fatigue cracks within the Design Life.

Rod Smith, P.E., The artist formerly known as HotRod10