Pattern Concrete Discoloration and Cracking

[Dianium]

What precisely are the causes of the concrete discoloration. I have a seawall cap I am working with, and it appears that cracks have developed along the top and sides at approximately 24" to 30" o/c. I have also noted that there is discoloration of the concrete as well as pattern discoloration in some locations.

My initially thoughts are the lack of control joints caused the cracks and perhaps pouring on top of an existing CMU wall where the cause, but after seeing the discoloration, I think it's something else. I have attached pictures to help visualize what is happening.

 

[TehMightyEngineer]

The first and second picture show crazing cracks; usually caused by the surface of the concrete being allowed to dry too quickly (likely either lack of proper moisture control during curing or a high temperature during/after the pour). Craze cracking is typically just an appearance defect though may cause durability issues down the road. [URL unfurl="true"]http://www.concreteconstruction.net/concrete-surfaces/fine-and-random-craze-cracks.aspx[/url]

The discoloration could be many things but my guess is on just different batches of concrete during the pour. This likely isn't an issue other than appearance.

The larger, straight cracks are plastic shrinkage cracks and would have been fixed using control joints. However, you have what I would say is a minor amount of plastic shrinkage cracking and it appears they're only visible due to the moisture from the adjacent body of water. I'd consider these something to monitor for growth but otherwise wouldn't worry much about them.

Overall it looks like a successful pour.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[TehMightyEngineer]

One minor note, given the age of the pour my guess is craze cracking. However, alkali-silica reaction cracks look a lot like craze cracking but continue to grow over time. Monitor the cracks and if they continue to grow or the cracking gets much worse then contact the contractor ASAP. Craze cracking is much more likely, though.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[Dianium]

Thanks for the input, the client is concerned over durability. The cap is on salt water, so I also thought about the Alkali-silica reaction, and will monitor that. Would you recommend filling the cracks with a crack injection system? Also, what about cement type? I read that type II and V cements should be used in these types of environments and I know the contractor used a type I.

 

[TehMightyEngineer]

Yes, often a type II cement would be used for sea water exposure to resist sulfate attack. However, if this wasn't specified to the contractor then I can't fault them for not using a type II cement. I would think a type V cement would be overkill but it varies by local conditions which is appropriate. I wouldn't reject it just because type I was used.

Those cracks look too small to inject and I wouldn't bother even if they were. The cracking is too minor to be concerned about yet, but down the road they might widen and a epoxy injection might be appropriate. Still, I would suspect this would be no worse than normal.

Is there any reinforcement in the cap or is just plain concrete placed over the masonry blocks?

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[Ron]

Your crack patterns are twofold....consistently spaced transverse cracks and "map" cracking. Both are caused by shrinkage; however, the map cracking is caused by excessive bleed water and a higher w/c ratio at the surface.

Type I cement is fine for this application. The mix design, placement, finishing and curing for this application are more important than the cement type. With excess bleed water, you would see the same indications without regard to the type of cement.

This is not alkali-silica reaction.

 

[Dianium]

Cap is reinforced, but it's just a hair short of the 0.5% of the area. Thanks, for the help.