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For reasons entirely unrelated to quality of their work I've lost faith in the company that just poured a concrete slab at the (only) entrance to my property.

I am unaware of the details of the mix but the description on the quote was "dig down 150mm compact hard fill then box up with 150mm boxing lay steel mesh and pour 20mpa 20mm concrete with a broom finish"

I was assured that this would be adequate, even with trucks going on the concrete (this would be a fairly regular occurance)

I live in Auckland, and its late spring, meaning the temperatures vary between about 8c and 18c, and 50-80% humidity at the moment.

Before getting the job done, I discussed timings with the contractor and was advised it would be OK to drive my cars over it 2 days after he finished pouring. After an additional day I could drive a light van over it, and trucks could drive over it 14 days after it was poured. It has now been 2 days since it was poured - but the contractor has not yet come back to cut joints in the concrete. Nonetheless he has again confirmed its OK to drive on it.

I have just taken measurements of the slab, and if I am reading it correctly - I'm not sure I am - it is between 150mm and 170mm high at various points. Somewhat concerning however is the low placement of the bit of mesh that is sticking out at the end as per this picture:

enter image description here

I have the following questions - and would appreciate any insights -

  1. Is it really OK to drive on the concrete after only 2 days (I've done some reading online, and have some doubts ), on the other hand, I'm not sure how long I can delay it being driven over due to obligations to others.

  2. Am I correct in my belief that the concrete will further strengthen by in the order of 25% over the next 12 hours, and is this significant ?

  3. If I drive over concrete which is still in the early stages of curing/setting and it doesn't visibly crack, am I weakening the eventual strength of the concrete ?

  4. If I drive on it prior to joints being cut am I more likely to damage it?
  5. Is the placement of the mesh a concern ?
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    Expansion joints are for thermal expansion. How's the weather been? – Harper Nov 10 '17 at 8:35
  • @haroer -thank you - Rain morning before concrete poured - no rain whole once it has been poured, but overcast weather, and temps of 8 to 17c. Some Rain expected for tomorrow. – davidgo Nov 10 '17 at 9:12
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    Having the wire at the bottom of the slab is worthless. They are supposed to pull it up to the middle as they place the concrete. It will not provide any strength at all at the bottom. – ArchonOSX Nov 10 '17 at 9:53
  • I second the comment regarding the mesh. If it is at the bottom all over you have a huge problem. Even if the mesh was properly pulled up into the center line of the slab thickness it is really a minimal technique. A good driveway slab installation would also have a grid or re-bar installed into it. You should search around the edges of the whole slab to see how much more evidence you can find of "mesh at bottom". If there is more then you need to get the contractor out there to remove this botched job and install the slab properly. – Michael Karas Nov 10 '17 at 12:51
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    Were there instructions to daily or twice daily gently wet the slab to promote curing? – Jim Stewart Nov 10 '17 at 14:03
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Is it really OK to drive on the concrete after only 2 days?
No. Even if control joints were cut, the concrete is at a fraction of its ultimate strength (maybe 15-30%, depending on several factors). You run a high risk of breaking it at other locations.

Will the slab further strengthen by in the order of 25% over the next 12 hours?
Maybe, but that's 25% of the current cure extent, which could still be a minor fraction of the ultimate strength. Cure rates depend on many things, from mix to water content to weather to after-pour care. The cure amount does grow substantially over the first few days, but it's folly to take guesses. Give it at least a week (ideally two) if at all possible.

If I drive over concrete which is still in the early stages of curing/setting and it doesn't visibly crack, am I weakening the eventual strength of the concrete?
Cracks due to vehicle traffic are usually apparent right away, but it is possible that you'll crack it and not know it until dirt starts to accumulate on the line or it opens further. If it doesn't crack, it's probably not weakened in any substantial way.

If I drive on the slab prior to joints being cut am I more likely to damage it?
No, but it is more likely that you'll end up with crooked (uncontrolled) cracks, which are ugly.

Is the placement of the mesh a concern?
Yep. It should've been pulled up into the center third of the slab's thickness as the pour occurred. It's possible that what you see there is an anomaly, but it's not a good sign.

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No, 48 hours is not good. " High Early strength" cement reaches 50% of minimum strength after 7 days ( I think at 70 F about 21 C, longer at lower temperatures) . You can check at the Portland Cement Association site. Cement gets stronger for a long ( year) time. Initial compaction ( like vibration) improves the final strength . It if good to keep the surface damp for a couple days after pouring. I would say rebar is necessary for truck traffic ; mesh is for sidewalks -- but I am not a Civil Eng.

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    A ceramic engineer told me nearly all cement is " high early" , it is the standard. – blacksmith37 Nov 10 '17 at 18:24
  • High early concrete (not cement) is probably common or standard in some industries, like road construction, but not across the board or for residential use cases. It's more expensive. – isherwood Nov 10 '17 at 21:07
  • "High Early" describes the cement not the concrete mix. Although it is the concrete that gets stronger for many months . – blacksmith37 Nov 12 '17 at 3:05
  • I see that Type 3 cement is sometimes referred to as HE. My mistake. In practice, however, it makes more sense as a builder to describe the mix. – isherwood Nov 12 '17 at 15:20
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  1. Nope. I'd wait about a week.
  2. Eh... concrete takes 28 days to fully cure, so I'm gonna say no.
  3. Nope. Won't hurt it a bit.
  4. Probably not. But It does need to have joints cut into it.
  5. Absolutely. Concrete inherently has tons of compression strength, but terrible tensile strength. This is where wire mesh/rebar comes into play. It helps keep the concrete from flexing too much, which can lead to a lot of damage. Wire mesh that is sitting underneath the concrete like that is doing no good at all. Might as well not have paid for it in the first place. Although I will say that the wire may not be like that everywhere, (there are always spots higher and lower). However, having wire mesh visibly sitting underneath the concrete like that screams SHABBY to me, and you should keep that contractor's information, and that picture, in case you start having problems.
  • Hello, and welcome to Stack Exchange. It's unfortunate that the questioner included multiple questions, but it means that you need to be clearer on just what you're answering (people won't cycle back and forth between your answer and the question). So, some more details would help. – Daniel Griscom Nov 16 '17 at 3:02
  • precast.org/2013/10/28-day-myth – isherwood Nov 16 '17 at 12:45

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