Hollow sound beneath garage floor

[drollins]

About 1 year ago I built a new home located in Raleigh, NC. A few months ago I was bouncing a basketball inside my double-car garage and I noticed a hollow sound near the center of the garage along with excess vibrations when the basketball hit the floor as compared to areas around the edge of the garage near the foundation walls. The garage floor has two expansion gooves dividing the garage floor into 4 sections. The expansion grooves have about 1/16" - 1/8" wide cracks running the the full length of the goove. I have also noticed that each of the four sections are slightly concave and we have also noticed the ability to use your fingernail and flake up small pieces of cement from the floor. Underneath the cement we have about 3-4 feet of limestone which was not packed down prior to pouring. I believe this is the cause of the hollow sound. As usual, the cement contractor says everything is good. I have not parked my cars in the garage in fear of making what I believe is a problem worse.

I am wanting to know if I do have a problem and some recommendations on my next step which would show the contractor I know what I'm talking about.

Please feel free to ask me additional questions, I will do my best to answer them.

Thank you,

 

[austim]

Hi, drollins.

My main answer is merely an oldie, but still a goodie. Talk to a structural engineer in your area, and be prepared to pay for some independent advice specific to your garage. Also, if there was an engineer involved in the design of your house, talk to him/her (and don't offer any payment ;-)).

My guess is that you are quite correct - no compaction of 3-4 feet depth of fill = settlement = hollow sound and concave surface. You probably have a void directly under the slab where it is spanning over an area where the settlement is particularly bad.

Proof might be difficult without drilling several holes through the slab. But you might find that your problem can be corrected by grouting under the slab and into the loose limestone, in which case you would need some holes anyway.

Another remote possibilty is that there is a problem with the slab itself.

 

[Ron]

drollins...you mention that you have "expansion joints" crossing near the center of the slab. (These are actually control joints...to control the location of cracks). You also mention that the hollow sound is near the center of the slab. Further you have an apparent concave surface.

Though compaction of the limerock could be an issue, the more likely cause of the hollow sound, coupled with more vibration is that the slab is "curling" at the joints. This is caused by surface shrinkage being greater than subsurface shrinkage of the slab.

If it is pronounced, repeated load from the vehicles will cause a crack to occur about 1 to 2 feet away from the joint and running parallel to the joint in the wheel path, otherwise it isn't much of an issue other than an irritation.

Still a good idea to follow austim's advise and have an engineer look at it. Find one who has investigative experience in concrete problems. If you have a problem finding anyone, post here again...I know a few people in your area who could help you.

Ron

 

[drollins]

Ron and Austim,

Thank you for your sound advice. After looking in the local yellow pages and finding a long list of structural engineering consultants, I would appreciate it if you (Ron) could notify me of someone to contact. What does an evaluation usually cost?

DRollins

 

[Ron]

DRollins...contact Dave Cunningham at (919) 363-9899. He should be able to help you or at the least refer you to someone.

Evaluation cost will depend on time spent, typically at about $100 to $120 per hour for engineering time.