Combining different concrete mixes in one pour

[Gammyleg]

Hi,

I have come across a few instances where different grades of concrete have been placed during the same pour opearation, eg C40 concrete for the main slab & c60 concrete below where the columns will sit, also similar with waterproof & non waterproof mixes. The latter presumably being most beneficial because it negates the issue of cold joints.
Are there any official documents/standards to give guidance on what is acceptable, particularly in relation to cement contents, blends, setting time and thermal differentials?

Thanks

 

[MotorCity]

I assume you are talking about two different concrete mixes that are combined or mixed together in the same pour (as opposed to pouring and curing mix "A" and then pouring a mix "B" on top of mix "A"). It sounds like you are describing the puddling of stronger concrete around columns for increased slab shear resistance (this is a very common practice).

Since there are an infinite number of possibilities of combining various mixes, I doubt you will find anything more than a rule of thumb or research based on specific combinations. In terms of performance, it depends on how different the mixes are in terms of materials, quantities, etc.

 

[bimr]

"Placing" of concrete has been discussed:

http://www.eng-tips.com/viewthread.cfm?qid=407964

http://www.eng-tips.com/viewthread.cfm?qid=226353

 

[Gammyleg]

Thanks for the replies,

The particular circumstance is a raft slab pour approx 1.2m thick with an high density of rebar in the bottom 500mm with no room for vibrating pokers. The get around has been to specify SF2 self-compacting concrete for the bottom 500mm reverting back to the original F5 flow concrete above. Both are 10mm C50/60 strength mixes

SCC = 162kg/m3 CEM1 + 378kg/m3 GGBS
F5 = 240kg/m3 CEM1 + 240kg/m3 GGBS

Although not unheard of, it is uncommon for UK SCC mixes to contain more than 50% replacement and hence the supplier is unwilling to offer one. The concerns are mainly the thermal differentials between the two mixes and also any marked difference in setting times with potential for cracking.

 

[Ron]

In my opinion, this should not be done. The mixes listed can have significantly different strength gain characteristics and can easily lead to unwanted cracking or delamination.

 

[hokie66]

Agree with Ron. And further, I don't view "self-compacting" concrete as an acceptable solution for placement in overly congested reinforcement. At the bottom of the mat, how will you ever know? Sounds like the bars should have been placed in layers. Good luck.