Reinforcing concrete large concrete driveway
I live near the ocean in central Oregon. The weather here is typically between 30 and 80 without any real freeze thaw cycle. The driveway is on an ancient sand dune that is stable. Two concrete contractors have recommended a 4” slab without aggregate or reinforcing using 3500 psi mix. It will be five separate pours for around 3200 sq ft. The separate pours will be physically joined using epoxy-secured rebar.
They say that relief cuts, expansion joints, and cure-and-seal products will prevent any cracking, shifting, settling, etc. This is a very expensive job, so I’m concerned that they are wrong or are trying to keep the cost down. I’d rather save for another year if the job I really want needs to be more expensive.
Any opinions – well, I mean on-topic opinions.
Replies
Concrete ALWAYS cracks as it cures due to shrinkage. The relief cuts are there to create a weakened plane in the concrete and promote cracking along the cuts. I would recommend going with at least welded wire fabric (WWF 6x6:W1.4xW1.4) reinforcing properly positioned on chairs - otherwise the concrete will push the wire to the bottom and it becomes useless.
Any concrete contractor who tells you your driveway won't crack is lying right off the bat. Concrete with rebar and wire cracks so your driveway, without either of those will crack that much quicker. On sand, after it cracks, it will start to shift and sink as water works through the cracks. I can send you pictures of my concrete driveway on sand if you need proof.
I've priced replacement and it is expensive so I'm going with pavers which will look better, are repairable and much less expensive.
Ask them for references.
Ask them for references -- other similar jobs they've done that are several years old.
How do they know?
The driveway is on an ancient sand dune that is stable
Only way I would make a statement like that is after a proctor test was run. A 90 to 95% proctor results would tell me the soil or sand was stable, otherwise assume it is not.
Shrink cracking is not controled by cutting or adding control joints. Shrink cracking is controlled by the cure rate. Concrete produces heat durring the hydration reactions and causes the whole mass to expand slightly. As the reaction goes to completion over the first 48 to 72 hours and the slab starts to cool you see shrink cracks form. Controll joints do help with this if they encompass no more than 100 to 120 sq. ft. areas. but the best method is both sealing and wet curring for as much as seven days depending on local weather conditions.