Expert: Roger Zona - 9/26/2007

Question
QUESTION: Hi Roger, I secured chain link fence posts with about 160# of bagged concrete.  Less than three weeks after, I have loose posts still in the crete; shrinkage?  Also, pouring a new walk out front(4" is code); got any suggestions for compacted base material?  Heavy clay soil of NE Illinois.

ANSWER: Warren,
Please excuse my delay in responding.  (My wife and I just celebrated an anniversary)

Typically, bag goods type of concrete tests at 4000 lb strength and is usually quite adequate for fence posts.  You used two bags which also seems adequate.  Did you mix the cement and water before putting it in the hole or did you pour the cement in dry and mix it in the hole?  The first method is the best.  Also, the soil may have absorbed the moisture from the concrete mixture thus not allowing it to set up and harden properly.
Is your sidewalk thickness (4") a nominal dimension?  If so, a 2"x 4" is good for forms.  Also, will the sidewalk be reinforced?  Normally we don't reinforce sidewalk with wire mesh but it is important that you cut in deep control joints every 4 or 5 feet.  The base can be clean structural fill or soil.  That is - no organic material; grass, roots, peat, etc.  after digging out for forms, use a "sheepsfoot" compactor to tamp the surface thoroughly.  Clay is usually a good subgrade.
Hope this was helpful.

Roger

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

QUESTION: Premixed in a wheelbarrow prior to pouring(with a shovel all round). I vibbed it so it wouldn't have occlusions. It Rained three days prior to digging and pouring for the posts.  Local code changes made sidewalks a mandatory 4" compacted base and 4" concrete minimum. I had not planned on meshing the walk but 1" deep expansion joints every 45".
The primary concern was which base would best; pea gravelsand, crushed concrete, crushed limestone, or something else. I'd like this project to not fail due to base settlement.

Answer
Warren,
The best subgrade base would be crushed limestone or "road rock", at least 4" thick and well compacted (95%).  OK, so you either have the forms cut to true 4" or use a 1"x 6" and bury the board 1 1/2".  If available, readymix concrete with fibre aggregate would be a good substitute for reinforcing.

Roger