Expert: Bruce E. Johnson - 11/21/2008

Question
QUESTION: I need to pour a 12'x 38'x 4" drive way in preparation for a metal carport.
Q1. wire mesh vs rebar?
Q2. wait time prior to erecting carport?
Q3. sand in foundation need to be covered? (Paper) etc.  we used tar paper 50 + y. ago.
Q4. what quality of concrete?
And any helpful suggestions you could provide.

ANSWER: Hi Paul, 1. I prefer rebar in a thickened edge at the perimeter where your building will be located and wire mesh in the slab.  The same with the driveway, a thickened edge on both sides with one #5 rebar and wire mesh in the slab.  Control joints every twelve feet.
       2.  Three days is usually enough time before beginning your building however in most cases 24 hours should suffice.
       3. 6 mil visqueen should be used in the carport area but is not required in the driveway.  That way if you ever enclose the carport you will have vapor barrier protection in place already.  I never use paper or visqueen on exterior flatwork.
       4. 3000 # concrete or higher with a maximum 5-6" slump.
I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com

---------- FOLLOW-UP ----------

QUESTION: Please explain the installation of the Control joints every twelve feet. (materials/depth etc.)
With and depth of the thickened edge of perimeter?
Thank you so very muce.
Paul

Answer
Hi Paul, a control joint is either a cut joint one inch deep done with a concrete saw as soon as the concrete has been finished and can be walked on.  Preferably within the same day.  Or it can be a tooled joint using a one inch deep joint tool.  Or it can be a cold joint, the slab is formed in twelve foot long sections and every other section is poured and then while those sections have set up enough to pull the forms between sections you pour the remaining sections.  Or you can use pressure treated wood or cedar to create the slab separations and leave the wood in place.  I'm sure you have seen all of the above at one time or another.  Look at any driveway or sidewalk and you will see these joints.   Sidewalks usually have control joints every five feet and a cold joint every twenty feet.  These joints try to encourage the concrete to crack in straight line or controlled joints instead of random cracking.  Concrete will crack. The best we can do is try to control it.  If you use wire mesh in your slab it has become the standard to cut every other wire in the location of your control joints.  A lot of engineers are specifying this now.

As far as the thickened edge goes, an extra 4 inches deeper on the sides for a total of 8" deep and a width of 8" on the driveway and 12" x 16" where the carport will be sitting.  I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com