Expert: Bruce E. Johnson - 7/3/2009

Question
QUESTION: Hello, I was hopping you could answer my question. I am doing a driveway extension. If I drill some holes through the fiber expansion joint and run rebar through it, does the defeat the purpose of putting in an expansion joint to begin with. Please help. Thanks William

ANSWER: Hi William, not necessarily.  You can drill a 5/8" hole for a #4 rebar and then slip the rebar into the slab end without epoxy.  This will allow the rebar to slip inside and move with the expansion and contraction between the two slabs.  I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com

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

QUESTION: Dear  Bruce, Thank you for your answer. I think I understand how control joints work and and why expansion joints should go in between the house foundation and drive way--stem wall and slab--but why should an expansion joint go in the middle of lets say a 50 foot driveway? Shouldn't a control joint every 10 feet take care of that?
One more question: Should I put an expansion joint in between my existing driveway and my new driveway? My existing driveway is 10.5 ft. by 35 ft., and my new extention driveway is going to be 7.5ft. by 35 ft.
Thank you for the help, William

Answer
Hi again William, control joints and expansion joints are different animals. Due to the nature of concrete, it will crack.  The control joint give the concrete a place to do that.  An expansion joint compresses and gives the slabs a place to go when it expands to keep from displacing the concrete or moving laterally.  So you really should use both types of joints when pouring large concrete slabs.  I generally use an expansion joint every 20 feet and a control joint every ten feet on slabs 20 feet wide so in your case an expansion joint every 40 feet is more appropriate and control joints every ten feet.  An expansion joint between the new and old slab is recommended.  You don't necessarily need to connect the two slabs with rebar if you have good compacted base. One common practice nowadays is to cut your wire mesh every other strand along your control joints to help encourage cracking in those areas.  I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com