Expert: Dan Griffin - 5/29/2008

Question
QUESTION: I was in the middle of pouring a 3,250 sqft 5in thk slab this morning when it started raining.  My contractor ran out of tarps before we could get it all covered.  It looks like it is going to continue to rain the rest of the day (they didn't predict it).  What are my options now?  Will the rain weaken the concrete that has been poured?  My concrete contractor said they could come back in a couple of days and pour a 1in layer back over all of it and finish it.  Is there any other information that I should know about it?

ANSWER: Roy, this is sounding like someone gambling and losing on the weather.  The concrete will actually be better and stronger because of the rain.  YOu have washed out the top and the finish is ruined.  What kind of building can stand a 1" cap??  Plumbing, etc won't make.  Bonding a cap to the first pour can be done, the important thing is to remove any and all loose and sandy material to make sure you are bonding to structurally sound concrete.  At the least I would go after it with truly high pressure water blast, other choices include sand blast, etc.  YOu probably did not get the control joints cut in the slab so random cracking is in place and will transfer into the cap slab.  There are several decorative treatments that can be used at 1/4" with good results.  Make sure you use bonding agent either in the mix or applied to the existing or both.

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QUESTION: This is for a single story house.  The lag bolts going through the sill plate will be 6" deep, so they should go through the cap and into the 5" and perimeter beam.  The contractor now has said he plans to pour 2" on top.  I do have some load bearing walls on the interior, do you think the 2" cap will crack under the sill plate?  All of the floor will be covered by wood or tile.  Is the bonding agent something the contractor needs to spray before pour or is it something that the concrete mixing company will need to add to the entire mix?  Thank you again.  Do I need to wait any additional time before framing?  I plan to start framing two days after the cap has been poured.

Answer
You and your builder should contact your local ready mix supplier.  Ask to have there technical outside salesman to come see your problem and seek their advice.  A 2" cap will allow you to use a 3/4 and down design mix with bonding agent, sometimes called chip mix.  Use their expertise and advice, though the finishers will probably not be happy.  Make sure that the cap won't change or create other problems.  If the present slab is sound and you can remove any latence or loose material, you may not need the cap.  A skim coat of Planipatch or similar may be adequate.  Again, seek the advice of local ready mix supplier.  Planipatch is a Mapei product that can be applied per room or over the entire slab.