Plastic Shrinkage - Why?

I recently had a slab poured, 5600 square feet, which quickly developed what I would call plastic shrinkage in about seven localized areas 3-4 square feet each. The slab is 5.5 inches thick with a 6 mil poly vapor barrier. The concrete was ordered with a slump of 5 and without fiber mesh or any accelerator or retarder. When we began placing the concrete the temperature was about 55 degrees F. The pour was complete in about 2.5 hours and allowed to set for about 2 hours before an attempt was made to finish it. The temperature rose into the upper 70's during the day with a five mph wind. An attempt was made to pan the entire slab but the majority was still too soft. Some areas, not necessarily the first to be poured, were firm enough to be finished to a good hard troweled finish. About 5 hours after the completion of the pour most of it was firm enough to be panned. We began the pour at 7:30 am and were still attempting to finish it at 5:00pm. Only certain areas would produce a good finish. Other areas were rubbery and would quickly open up with shrinkage cracks minutes after finishing. These areas were actually "sticky", when attempted to hand trowel them they would grab the trowel. We sawcut the slab at about 7:00pm and applied, using a squeegee, a curing agent without sealer. In the morning there were about 7 areas of cracking that I would characterize as plastic shrinkage. Most of these areas are accompanied by a slight hump. Some areas of the slab have a sticky residue, possibly from the curing agent. The concrete supplier admitted to adding a water reducer but would not elaborate. The men who finished the concrete felt that some type of additive, most likely a retarder, had been added and not mixed thoroughly.
I'm curious if anyone has seen this before and has any explanation? How can I prevent it in the future? Should I have added an accelerator for these weather conditions?

 

[ishvaaag]

I would focus in the admittedly present water reducer, trying to identify it to check the specifications.

Also, if only some zones were sticky the ability of such product to be properly diluted (was the product in bad conditions, or was it incorporated in the specified way, or mixed enough?).

 

[Ron]

I would agree with your finishers (that's a shock to me...I rarely agree with anything a finisher says!)

Poor distribution of a retarder can cause differential drying, curing, shrinkage and set...all of which can lead to plastic shrinkage of sorts.

The most common cause of plastic shrinkage is when the surface evaporation rate exceeds the bleed rate of the concrete. Using a water reducer decreases the bleed water, which under certain wind/temperature/humidity conditions can exacerbate the plastic shrinkage issues.

Your Ready Mix supplier owes you a print out of exactly what went into that mix. Get that info and try to evaluate these issues.

Ron