How long does it take for concrete to cure?

[Cooler]

I know that if you let concrete dry too quickly it compromises the strength.

The county poured a new road surface for a small bridge near my house.  They covered it with plastic sheets and have a water sprinkler system mist over the sheets 24/7.

It's been "drying" for over 2 weeks now.  

Is there a legitimate reason for such a long cure time or are they padding the work to extract extra money from the county?

[TDKPE]

It's actually a chemical reaction rather than a drying process, and keeping the surface from drying can be important for the long term integrity of that surface. Two weeks might be a bit long for water spray, but I don't know what the minimum is, and that would also depend on the mix and the ambient conditions. It's generally accepted that 28 days yields 100% strength, but that's arguable. http://precast.org/2013/10/28-day-myth/

[cndamm]

I was always taught that concrete reached 95% cure in 28 days and the other 5% took 28 years. Then it started to deteriorate. It doesn.t dry it cures, which is why you can pour concrete under water.

[Admiral]

I was always taught that concrete reached 95% cure in 28 days and the other 5% took 28 years. Then it started to deteriorate. It doesn.t dry it cures, which is why you can pour concrete under water.

I read somewhere that the core portions of the Hoover Dam concrete is still curing, 80 years later . . . .

[Imakmst©]

"Theoretically, if kept in a moist environment, concrete will gain strength forever, however, in practical terms, about 90% of its strength is gained in the first 28 days. "

CIVL 1101 - Properties of Concrete
Dept of Civil Engineering, Univ of Memphis

[TDKPE]

Thinking about it, the plastic is to keep the moisture in, but the spray on top of the plastic (not on the concrete itself) may be for cooling purposes.

[Cooler]

Thinking about it, the plastic is to keep the moisture in, but the spray on top of the plastic (not on the concrete itself) may be for cooling purposes.

I assumed that the plastic had micro holes that allowed some moisture through.  There is no way to check though as there is no place to stop.

They are cleverly revising a very narrow bridge that spans a stream.  The span is about 100 yards long.  But it is so narrow that if two full sized pickup trucks were travelling in opposite directions they would likely lose a mirror or scrape the passenger side of the vehicle.

What they have done is built brackets to move the guard rails out of the roadway and attached them to the structure on the sides.  This opens up about 15" in each direction.

But they jack-hammered to the bare structure last year on the north bound side and this year on the south bound side.  I guess I have two more weeks to struggle with this thing (that would be 28 days).

Regards,

Cooler

[Roly]

I assumed that the plastic had micro holes that allowed some moisture through.  There is no way to check though as there is no place to stop.

They are cleverly revising a very narrow bridge that spans a stream.  The span is about 100 yards long.  But it is so narrow that if two full sized pickup trucks were travelling in opposite directions they would likely lose a mirror or scrape the passenger side of the vehicle.

What they have done is built brackets to move the guard rails out of the roadway and attached them to the structure on the sides.  This opens up about 15" in each direction.

But they jack-hammered to the bare structure last year on the north bound side and this year on the south bound side.  I guess I have two more weeks to struggle with this thing (that would be 28 days).

Regards,

Cooler

No micro holes,  concrete curing produces heat.  The water under the plastic is condensation from the moist heat and cooler water.      They also use spray on curing agents which seal the outside to prevent moisture loss.
 We used 28 days for high load structures along with test samples sent to a lab.      For some street repairs we used a high early mix that could be driven on by the next day or before.   
 Don't think they are running up the bill, just doing the work that the county/state requires.   Roly

[Robert Adams]

To add to te info already. If you can't drive on it within generally 24 hours you will never be able to drive on it or it will live a very short life.

[brianwelch]

To add to te info already. If you can't drive on it within generally 24 hours you will never be able to drive on it or it will live a very short life.

Last bridge deck I built was in the mid 90's, in NJ...we had to spray a special white curing compound (more than just a C309 compliant version) then cover the fresh pour with Burlene (poly bonded to burlap, burlap side facing concrete) and keep soaker hoses on the deck for a period of time (don't recall precisely how long).
I am not sure of the current curing procedures in place now, but I am sure they have been adjusted somewhat...