Reliability of Visual Assessment- Reinforce Concrete Structures 2

[Pierre Vivier]

I have been tasked to carry out a structural condition assessment of old reinforced concrete structures at a very large water works. The purpose of the investigation is to identify any structural integrity concerns and propose preventative maintenance action and/or further investigation where/if required. Most of the structures are 50+ years old, and the client would want to push these structures for a couple more decades.

All the structures are of high importance in the operation of the facility, and thus lengthy shutdowns must be highly motivated. The inspection would thus first consist of only a visual assessment, which, if structural deterioration has been identified, can be used to motivate shutdowns for further condition assessment such as nondestructive and destructive testing.

My questions is; how much do you trust a visual inspection? Do you believe that for reinforced concrete structures of this age, if no signs of distress or deterioration is evident, that a visual inspection can with fair confidence conclude that a structure is not currently at risk, nor requiring further investigation which may lead to major rehabilitation, structural strengthening, or even partial reconstruction?

 

[Ron]

A visual condition assessment can be very beneficial; however, it has its limitations. It depends a lot on the experience of the observer...for example, when is a crack something to be concerned about.

There are several guidelines out to help with a visual assessment, such as "Guidelines for Structural Condition Assessment of
Existing Buildings", SEI/ASCE 11-99 and there are some relatively good US Government publications on the subject under the Uniform Facilities Assessment process.

I would supplement the visual inspection with at least some nondestructive testing/observations. For instance, Ground Penetrating Radar (GPR) is a common tool to help locate rebar. You can then do some limited destructive testing to assess the condition of the rebar, and at the same time obtain a sample of the concrete to do some laboratory testing such as a petrographic examination of the concrete or even a simple depth of carbonation test to assess the potential for lack of protection of near surface rebar.

Just because concrete structures are old does not mean they are not structurally nor functionally viable. Good luck and be careful when your write your report not to put yourself in a liability position by making statements about how long the structures will remain in good condition. Remember, your condition assessment is a snapshot in time....not a guarantee of future performance.

 

[bridgebuster]

Ron brings up a good point about avoiding estimations of remaining service life. It's subjective; you can make a guess based on the rate of deterioration. I've had issues with clients who wanted definitive numbers about the remaining life of various structures.

 

[RickyTickyTavi]

A visual evaluation of structural elements can be good. For reinforced concrete I also will bring a small rock hammer to tap the concrete and listen for delaminated concrete. As far as "risk" goes, provided the loads are within the confines of the original designs and its condition is sound, then it should perform as well as the code of the original construction thinks it will. Some items in an old building are not code-compliant today, but it is important to note that those building are subject to the codes of their original construction unless they have a risk category change or are required by statute to upgrade.


However, your second question regarding future strengthening is different question. The owner is only asking for a visual assessment. I often put into reports language indicating I have not performed calculations regarding the structural adequacy of the building. If the owners asks, "can you perform a visual assessment, and also check this slab for all the pumps we put on it in the 70's?". Then my answer might be, "it appears sound, but the beams are insufficient for the loads applied, you should strengthen, etc."

 

[BridgeSmith]

Other types of non-destructive testing are available as well as the GPR and sounding mentioned above. Half-cell potentials of the rebar can help identify corrosion and impact echo (detailed electronic sensor sounding) can identify the delamination, including depth. We've used to these to assess the condition of bridge decks, in addition to chemical testing of concrete samples. We do carbonation testing occasionally, but more often chloride levels are what concerns us, because of the exposure to deicing salts. It just depends on what the concrete is exposed to.