swelling spalling concrete in bridge with stalactites.

[qroger]

Hello,
Today I visited a bridge over a railroad. It was 1941 vintage poured in place reinforced concrete. The curb under the poured guardrails was crumbling and the aggregate was coarse. The bridge was on a skew and the deck sections came out as parallelograms. They appeared to have been offset, possibly as if they were expanding in length. One of the poured guardrails was tilting. Under the bridge small stalactites of some mineral, possibly calcium carbonate were hanging under the deck. The aggregate is not limestone or dolomite, but the bridge may have been salted. What is going on and how close to collapse is the bridge?
Thanks,
Roger

 

[JAE]

Well, normally, bridges are (or should be) regularly inspected by the state DOT (if in the USA) and they have very systematized methods of rating the bridges based on levels of deterioration as well as other proceedures in place. I would contact your local DOT and see what they can offer you in terms of advice.

With chloride attack on concrete, you really don't now unless you do some cores and get a lab to analyze the concrete condition.

 

[dirtsqueezer]

I would definitely have to agree. I have a pet theory about the 'stalactites' that I won't go into here. Suffice it to say that the bridge is could be nearing the end of its life, possibly catastrophically. Cracking brings water to reinforcement, which in concrete that old, could be all but absent by now, rendering your bridge, well, questionable at best.

Now just out of curiosity, which direction do the lines of the parallelograms go- up and down, or can you see them flat on the deck? I don't have a licence, but those seem like en-echelon fractures to me.

 

[dirtsqueezer]

One obvious thing I forgot to say. Spalling concrete is a very good sign of rusted reinforcement. As water infiltrates through cracks as you describe, the reinforcement will expand as it corrodes, putting pressure on the concrete and causing spalling from within. It's a pretty well-known phenomenon. Kudos to you for noticing the problem!

 

[Laser28]

I would also like to know where the bridge is - this sounds more like alkali-aggregate reactivity.

 

[jike]

The concrete construction in 1941, most likely, was not air entrained. Is this bridge subject to freeze-thraw cycling?

 

[Ron]

Like Laser28, I would like to know the location. That makes a difference. There are several possibilities to cause your description of distress. These include; alkali-aggregate reactivity, chloride intrusion, freeze-thaw deterioration, and wet/dry cycling.

The skewed joints and parallelogram design sounds like an experiment that happens about every 20 years or so, but it never seems to catch on mainstream (no pun intended!). The regularity of joints in rigid pavements and bridges causes a "sing-song" pattern of vehicle noise that's annoying to drivers. Every now and then someone in a DOT will decide to skew and re-space the joint pattern to break up the noise pattern. It works to some degree but is more costly to implement.

Back to your observations....stalactites from cementitious materials can occur with or without carbonate aggregates. It is a sign of general deterioration of the concrete from wetting/drying cycling. It is comparable to efflorescence on brick from calcium leaching from the mortar. The same happens in this case. Cement's largest constituent is calcium, in one form or another. Calcium easily goes into solution in water, is carried to the surface where it reacts with carbon dioxide, and calcium carbonate is formed. It progressively thickens into the formations you see.

What to do? As JAE suggested, some testing is warranted for chloride intrusion. There are generally two ways to assess chlorides in concrete. One is using a chloride permeability test, commonly used in Canada. It provides you with the potential for chloride intrusion in THAT concrete. The other is to test for soluble chlorides in the concrete from cores or from drill cuttings. This can be done in a gradient fashion to show concentration with depth, but you have to be careful about cross-contamination of the gradient samples. I wrote a procedure to do this about 15 years ago. Post your email and I'll send it to you.

A petrographic examination of a few cores would be a good idea to check the integrity of the concrete. Considering its age, it might not have been air-entrained, so this would be good to know. Further, the integrity of the paste-aggregate bond can be determined from petrography, as well as indications of alkali-aggregate reactivity.

Finally, a load test of the bridge for its rating should be considered. This should be a dynamic load test with real-time monitoring of load and strains to check for the bridge's response to load. With strategic placement of linear deflection devices and strain gages, a good idea can be gained of the load response and that can be compared to a model of the structure to check theoretical vs. actual. This helps you assess the condition of the bridge. Keep in mind that you will likely have to do some "guessing" (educated of course!) as to the original reinforcement or some significant nondestructive mapping to create your model.

Good luck!