concrete cracking

[mh25]

During the final stages of a concrete slab placement the floor began reacting as a clay and showing signs of plastic shrinkage cracking (some of the cracks were so large you could put your hand in). Temp was 90 with winds under 10mph and humidity was approx 50%. Not conditions where you would normally see a reaction such as this due to rapid drying. The contractor is claiming that this was caused by an overdose of retarder (2% was plant added). I have had trouble gathering information to either substantiate or refute this claim.

 

[concretemasonry]

Do you know what the job site concrete temp was when it was unloaded and when the concrete was batched relative to the time when the shrinkage cracks were oberserved (or finishing was difficult)?

Do you have times on the batching and unloading for ALL the loads? Could the loads have been dumped out of sequence? Most good R/M companies have complete time recorders including movement times and distances.

What admixtures were used in addition to the "retarder"?

What was the recommended dose of retarder?

did all the loads have the retarder?

Dick

 

[mh25]

Temp was 80 degrees at the time of unloading vs 90 degrees when the shrinkage cracks were observed.

Trucks were dumped sequentially and in less than an hour after batching.

High range Water Reducer was jobsite added to all loads.

Recommended dose of retarder was 5oz/CWT of cementitious material which comes out to 26oz/Cy.

All loads had the retarder in the mix.

Also, this pour was a total of 180 Cy and the cracking was only observed in the final 30Cy.

 

[hokie66]

The size of those plastic shrinkage cracks must have been something to behold. I have seen may plastic cracks, but never one that a hand will fit in.

The contractor could be right, but what does the concrete supplier say? It also could be something to do with the combination of the retarder and water reducer. Was the water reducer added under the supervision of the concrete supplier?

Another thing to check is if the same cement was used throughout.

 

[shin25]

Sometimes, over dose of plasticizers act as accelerator. You may want to check out how much of this was added at the job site.

 

[mh25]

They were like nothing I have ever seen before!

Supplier blames it on an overdose of water reducer saying that it had to rupture the surface in order to evaporate?

The recomened dose on the mix design was not to exceed 14oz/CWT of cementitious material, which comes out to 74oz/Cy or 740oz/ truck. The trucks carried a 5 gallon jug of water reducer which is less than the recomended amount per truck.

 

[BarryEng]

If you consider the fundamantals of mix design, the mix proportions may be the problem.
For 1 m3 of concrete (with a 4:2:1 mix) you require about 0.93 m3 (theoretically, depending on the shape of the aggregate) of large aggregate. If you fill a 1 m3 box with aggregate, then fill the voids with small agg., fill those voids with cement & then fill the voids in the cement with water, you should have a mix that is not subject to bleeding, plastic settlement cracking & plastic shrinkage cracking.
Bleeding is caused by the agg. & cement settling & forcing water to the top (evidenced by water paths when the formwork is stripped).
Plastic shrinkage cracking is usually caused by hot drying winds blowing over fresh concrete. The cooling effect (chilling effect) is so great, that excessive drying (& shrinkage) takes place on the surface (relative to the rest of the concrete).
In Australia, we generally ensure that the temp. of the concrete at the time of pouring, is < 32 deg C. To achieve this, the ready mix companies shade aggregate stacks, blow air thru the stacks (for cooling) & use ice water for mixing.

None of the above, answers your actual problem, but I think that I would look very closely at the mix design first. The problem now, is that the contractor & concrete supplier will have VERY selective memories - that is what concrete batch mix dockets are for.

I wonder if the contractor used a super plasticiser? I have used this before & problems arose when the mix was made with a 200 mm slump (soup) & a 'flash set' was experienced. Reducing the slump to 180 mm MAX fixed up that problem.

I cannot understand why (suddenly) the last 30 cy was the problem. You may have to question (very closely) the concrete supplier AND the contractor (to see if their stories are the same) as to 'what changed?'

I agree with all of the suggestions of the threads above, until you find an answer.

 

[hokie66]

Look for a fresh batch of red hot cement in the last 30 cy. You said 80 and 90 degrees, that was the concrete temperature, right?