Bleeding on concrete 5

[concretebaby]

HI
Lean concrete mix poured in a slab, 50 cm thick. Upon casting considerable bleed water appear on the surface (pic attached). The concrete mix tested for fresh properties like slump, air content, bleeding and found to be ok. The slump is about 18 cm and no bleeding observed at all.

But why is bleed water standing on the surface after few hours of casting. No water table beneath and the mix has more than 50% of fine aggregate even though the cement content is less (about 300 Kgs) as it is a lean mix 15 Mega Pascals at 28 days.

Kindly share your views.

 

[JAE]

Concrete normally bleeds - say for slabs-on-grade.
Especially for a "lean" concrete as you state, there is possibly less Portland cement there to receive all the water and, well, it bleeds upwards.

Normal practice is to strike off the concrete and float. Then let the slab stand for a period and let the bleed water rise. Then pull off the water from the surface and finish the final troweling.



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[Ron]

The bleed water showing is excessive. It was likely caused by an error in the moisture content compensation at the time of batching, but as JAE noted, lean mixes are more prone to bleeding. You will likely see some "map cracking" at the surface and you will have lower abrasion resistance and lower general durability of the slab. Lean mixes are not a good idea for exposed slabs.

 

[SlideRuleEra]

The slump is about 18 cm...

(7 inch slump). Too much water in the mix. IMHO, no structural concrete should have slump that high (unless superplasticizer is used). I would expect slump more like 8 cm to 11 cm.

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[hokie66]

15 MPa is not structural grade concrete, so anything goes with the mix design. It is only fit for things like replacing over-excavated areas under footings, etc.