Shrinkage Cracks

[Contech2011]

Hi All,

We have a situation where early shrinkage cracks occured after 1-2 hours of the concrete was placed. Enclosed are the pics of these cracks. The grade of concrete is 45 Mpa (flow upto 600mm) and was pumped up 16 storeys by placing boom. Ambient temp was 41 deg centigrade max.. concrete temp 30 degrees max...Observations on-site revealed that no immediate protection was in place by burlaps, PVC sheets, no wind barrication. The surface seemed dry but the concrete was wet inside...The contractor is complaining that the concrete did not set within first 2-3 hours and thats the reason he could not make late finishing and early curing, as the masons feet were inserted into the concrete..

Please analyse the reasons and solutions to avoid such cracks..

 

[concretemasonry]

Normal weight or lightweight concrete?

If lightweight, was the aggregate pre-wetted?

At what location was the slump/flow measurement taken?

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[Ron]

hccindia...these are known as "plastic shrinkage cracks". They occur when the surface evaporation exceeds the bleed rate of the concrete. This is caused by hot, dry conditions and/or windy conditions.

There is no reasonable repair for this. Some autogenous healing will occur, but generally the discontinuous plastic shrinkage cracks will interconnect by further drying shrinkage of the concrete, thus creating meandering shrinkage cracks.

Consider wind breaks and surface fogging during the finish operation.

 

[Contech2011]

The concrete is normal weight. Slump measurement was taken ex-truck and then pumped by placing boom.

What shall be the sequence of finishing operations immediately after pouring?

When shall the water be sprinkled? When the concrete did not attain its initial setting (upto 3-4 hrs) how can the workers carry out floating operations?

Is it recommended to immediately cover the surface after initial finishing and then wait for the concrete to set (3-4 hrs) and then start late finishing? During this water has to be sprinkled?

Batching plant uses washed sand, prewetted aggregates.

Please explain in detail..

Thanks

 

[hokie66]

A retarder of some sort must have been used. In the described atmospheric conditions, premature setting is usually the problem, so the concrete plant must have overdone its precautions and has created another problem. What was the retarder and how much? The crack pictured, and probably your others, are plastic cracks as Ron said. Either plastic shrinkage, or possibly plastic settlement in some instances. How thick is the slab? Does the crack go all the way through? Was any attempt made to close the cracks after they opened? Ron didn't mention sprinkling...he said fogging.

 

[Contech2011]

SLab is 300mm thick..A superplasticizer was used Conplast SP430R @ 2% by wt of cement. Cement content is 490 Kgs/cum.
No retarder was used...The crack is still not inspected from the bottom as shutters will be opened after 14 days... We hope it is not thru-n-thru crack.

 

[hokie66]

The crack in your photo, and that is the only one we have to see, could well be a plastic settlement crack over a reinforcing bar. If so, it may not go through. If plastic settlement is the cause, you should be able to see a pattern of sorts on the surface, more or less following the bars. The appearance of the slab indicates that little or no effort was given to finishing, and that makes me guess that not much effort would have gone toward compaction either. Lack of compaction allows the concrete to settle on its own, and plastic settlement cracks result. Too much credence is given to the "self levelling" affects of plasticizers.

Conplast SP430 does retard set, especially when used in the relatively high concentration which you described.

 

[scone38]

Ron is spot on.... "Consider wind breaks and surface fogging during the finish operation" At the very least properly applied evaporation retardant should be used. With those temperatures it should be specified to the contractor he has to be prepared with some method. Some cements are slower setting than others as are changing bleed rates. Speeding up concrete set will help to some degree but its almost impossible to have producers satisfy all the changing variables from day to day. Concrete producers are working with constantly changing materials (cement being #1) and I don't think it would be economically feasible for them to modify all the time without incredible amounts of testing. They have their hands full trending mixes for compressive strength, admixture characteristics, and air content (when used). The best way to address these issues is making sure you have a qualified contractor who is familiar with these situations and is forced to be prepared with proper methods to deal with it. It is by far the most economical way to deal with the situations that arise in the field.

 

[Drumchaser]

It is hard to scale your picture (next time throw a pencil down next to crack for clarity), so I am unclear as to whether they are plastic shrinkage cracks.

Results of the strength test specimens for this placement could be interesting.

The admixture supplier should be able to give major input on the proper and improper characteristics of their product.

 

[msquared48]

"the masons feet were inserted into the concrete..". If that's true, these must be darn wide cracks...

So, where is the mason in the photo?

Mike McCann
MMC Engineering