Cracks in concrete 2

[gosai]

I have some crack issue on site. The concrete was poured between June and September 2009. The concrete used was 40 MPa concrete, the slump measured was 40mm and 50mm at the time of pour. The concrete cylinders strength results were ranging between 35 to 60 MPa. The slab poured at 45 angle for channels. The cracks were noticed in slab in July 2010 (see attached photo). I am just wondering what causes slab to cracks so quickly. The slab was poured on granular base.

 

[Ron]

Those are shrinkage cracks. They were likely there long before becoming obvious. They are also going to/from penetration features in the slab, which have caused restraint to the slab (thus the shrinkage cracks). If you do cores through the cracks you might also find thickness anomalies.

 

[rapt]

gosai,

About 70% of the shrinkage occurs in the first 12 months after pouring the concrete. As your time period is about 9-10 months, you would be expecting shrinkage cracks by July.

 

[gosai]

Are the slump and the temperature of concrete (25 C) at the time of pour contributes to shrinkage cracks?

 

[Ron]

gosai...both can contribute; however, 25C is a good temperature for concrete placement. You have to look at temperature differentials at the time of placement. For instance if your concrete temperature is 30C and the air temperature is 15C at time of placement, this can cause immediate shrinkage during the initial setting of the concrete.

If you have a high slump that was caused by a high water-cement ratio, then you will have a lot of shrinkage. If your high slump was a result of admixtures, shrinkage is not usually an issue from the slump indication. A 40-50mm slump is not much, so I wouldn't consider that to be a contributing factor. It is more likely just drying and autogenous shrinkage, coupled with some physical restrain conditions from the pipe and variable thickness.

 

[hokie66]

40 to 50 slump and 25C are both good. I would think the cracking is more likely related to curing or lack thereof. The panels look small in plan, but also thin. You said it was cast on a granular base. Plastic under the slab? Weather conditions? Method and time of curing?

 

[rscassar]

Is that Milton, Brisbane, AUS?

 

[gosai]

Hokie66,

There is no plastic placed between granular base and concrete. As per sub contractor wet curing was carried out for 7 days by placing wet burlap on top.

Kikflip,
It is not Milton, Brisbane, Aus this one is in America

Thanks for all your help