spalling of concrete cracks

[bigm]

Have recently poured two 15000 square foot slabs to receive post-tensioning. The owner wished to use sealed bare concrete as a finish on one of the slabs. Cracks were expected but spalling of the cracks occurred also. It was not everywhere but at a lot of places. The worst was about 1-1/2" wide in places and about 1/4" deep. The mix had a lot of fines and the aggregate was as follows: gravel-1550lbs; pea gravel 450lbs; sand 1121lbs; cement 470 lbs.
The cylinders broke very well at 7 days (about 3700 psi). The day was hot, very dry and a 25 mph wind was blowing throughout the days of the pours. The contractor used a plasticizer for the second pour. The slump ranged about 5-1/2" for the first pour and about 7-1/2" with the plasticizer pour. Would the cause of the spalling be more attributable to quick drying of the concrete because of the weather conditions or created by a large number of fines in the concrete. The concrete finish appeared to have been overworked as seen by the swirl finish and a darker gray color. Also small aggregate could be seen at the top of the crack. Thanks for your replies.

 

[Rjeffery]

The condition is known as scale(ing). Spalling would be deeper and cover a much larger area. A question that really needs to be answered before a definitive explanation an be had is 'when looking at the chip that lifted off is there a piece of the aggregate on the chip and visable in the depression?' e.g. did the aggregate fracture or did the chip just 'slip' away from the aggregate?

 

[bigm]

Thanks Rjeffery:
The aggregate did not appear to fracture but just
slipped.

 

[dik]

5-1/2&quot; slump is a bit wet and with the heat and wind, there's a good chance that the surface 'dried' before it got a chance to hydrate, approximately 1/4&quot; deep <G>; that's likely the cause of the scaling.

The shear developed between the shrinkage layer and the very weak hydrating layer below can develop microcracking that will cause the surface to exfolliate.

 

[nhyankee2]

dik is correct. The lack of adheasion in this case could be caused by the surface drying rather than curing. I have also seen the surface scale due to working the bleed water back into the surface but the alligator cracking (indicative of drying and shrinkage) would not appear. the general fix for this problem is to remove the week top layer and use a thin set leveling compound. If the surface is to recieve a coating of some type like VCT tile, then gypcrete could be used. In this case, where the client only wants a sealed surface a stronger more abration resistant surface leveler should be used.

The reason Rjeffery asked about the fracture pattern is to rule out silicia reactivity of the aggregate with the cement used in the concrete.

good luck and please keep us posted.