concrete deterioration

[MikeE55]

The bottom of a cast in place grandstand is deteriorating. The paint is delaminating in several spots because the concrete is extremely soft and crumbling. Not sure if deicing chemicals were ever used on the top surface, which is also painted and looks sound. Anyone have any ideas for the cause and how to investigate? Location is the Southeast - age is approximately 50 years. I haven't determined yet the depth of the softness - looks like it is at least 1" to 2". I will suggest removing unsound material and repairing with a repair product, but I would like to know why it it doing this if it is possible to find out.

 

[mmillerpe]

Freeze-Thaw cycles? You may want to consider petrographic testing to see if that sheds any light on the situation. It can also help in determinine the depth of effective deterioration.

Is is just the deterioration of concrete and delamination of the paint? Are there any deeper issues such as spalling due to corrsion of reinforcing steel. You will need to consider doing a condition study to see what is really going on.

 

[MikeE55]

There are some areas with spalling due to corrosion, but they are pretty limited. There could be some freeze/thaw going on - maybe water soaks in from above and is trapped by the paint film below. The space below is used for storage - not conditioned space so it would be subject to freezing.

 

[csd72]

Take core sample to test for chloride penetration and carbonation.

There are also some insitu compression tests that can be done to estimate residual strength.

I would recommend propping it up before carrying out any works.

 

[JAE]

The water exposure to concrete beams many times results in slow oxidation of the bottom bar reinforcement and bottoms of the stirrups. This results in expansion of the steel as it rusts - usually spalling off the bottoms of the beams.

Some things that might be considered:
Verifying that the reinforcement hasn't lost significant area due to rusting (cleaning off rebar and measuring diameter of "good" steel).

Testing concrete for chloride content (if there is high chlorides you should consider adding zinc sacrificial anodes to the repair).

Removing old "soft" concrete and replacing with patching compounds or with just concrete or with shotcrete repair methods (remove concrete to at least 3/4" around reinforcement to ensure good bond.)

 

[OHIOMatt]

This issue could potentially be Alkli-Silica reation. This is a chemical reaction that takes place in the concrete and requires water, reactive aggregates and an alkaline condition. Given that you state that this takes place at the bottom of the structure, contact with water is more likely. The aggregate will transform into a gel.

The only way to test for it that I am aware of is to have it examined under a micro scope.

If this is the case, simply covering it will not solve your problem.

A call to the state DOT may reveal if reative aggregates are present in the local area.

 

[Ron]

Agree with mmillerpe...you need a petrographic examination of the concrete.

What you've described sounds like advanced carbonation and efflorescence.

Removing 2" of concrete will likely get you to the rebar level. If that's the case, you'll need to consider some supplemental reinforcement, pinning, or carbon fiber overlay.

Get the petrography done. It should tell you a lot. If you are in the Atlanta area, there are several firms in the area that can do petrography.