MAX ACCEPTABLE SLUMP FOR CONGESTED COLUMN? 2

[allanex50]

WE ARE USING PLASTICIZER FOR WORKABILITY OF CONCRETE POURING BUT OUR DESIGN MIX SPECIFIED SLUMP NOT TO EXCEEDS 150mm FOR COLUMNS AND BEAMS. HOWEVER, SUCH SLUMP DOES NOT WORK PROPERLY RESULTING ONLY TO HONEYCOMB. CAN WE ADJUST THE SLUMP UP TO 200mm? Please any expert can give me advise on this.Thanks a lot.

 

[JedClampett]

I'm no expert, but I'll give it a shot:
In my opinion, if you're using plasticizer with a 6 inch slump concrete, you're doing all you can from that end. I would try using a pea gravel mix or a superplasticizer rather than making the concrete too soupy.
Your article is in reference to tremie concrete. Is that the situation? It says that the use of Super-P is discouraged. However, it's from 1986 and I think that Super-P is more widely accepted now.
How congested are your columns?

 

[Ron]

A few comments, Allanex50....

First, please don't use all caps for posting. It is akin to "shouting".

Second, if the aggregate size is correct in your mix design for the actual rebar spacing, a 150mm slump should be fine, provided appropriate consolidation measures are being used in the field (vibration). If you are vibrating and still getting voids in the concrete, then it is likely you have the wrong coarse aggregate size for the rebar clearance and you should do as JedClampett noted...use a smaller coarse aggregate. If you get the slump too high for a vertical placement such as this, you will risk segregation of the aggregate.

Third...no one here can give you "permission" to go to a 200mm slump. That will have to come from the structural engineer of record or the architect. Perhaps the design did not appropriately consider constructability, so you might need to work with the design team to get this right. I would not recommend arbitrary nor capricious changes.

 

[allanex50]

Thanks jed & ron,

Further clarifications: max size of c. aggregates is 19mm only, main rebars spacing is ok, but specified tie bars is three set. Pipe of pump crete can hardly reach to lower part of the column because of these tie bars, much less if using tremie. To me 150mm slump is quite ideal for columns and beams but for columns such as our case, honeycomb is a problem.

 

[Ron]

For 19mm CA size, the spacing between any rebar should be no less than 60mm and the cover from any formed surface should also be at least 60mm.

The size of the bar is irrelevant to the CA size, so these rules of thumb apply to all bar sizes.

 

[asixth]

I recently had a situation like yours where I specified a 20mm aggregate, 120mm slump for an 80MPa mix for very congested columns. The concrete supplier came back to me with an alternative 10mm aggregate and a spread test in-lieu of a slump test. I'm not sure what a spread test is but I will need to look into it and whether it is covered in any concrete specifications.

 

[Qshake]

asixth - a spread test is just what it sounds like. Unlike a conventional slump which maintains somewhat of a cone shape as the cone is removed from the concrete, less viscous, Super-P, concrete will make a puddle on the floor and the spread of that puddle is measured.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[dik]

and not pea gravel... you want angular aggregate... I typically spec a 4" slump and let the superplasticiser do its job... proper consolidation is essential. 19mm aggregate (3/4") is common, but for conjested rebar... might consider using 10mm... (3/8").

Dik

 

[DST148]

@allanex50: Since slumps vary for normal concrete(without super-plasticizers) and super-plasicized concrete, I have seen contractors submitting different mix designs for conventionally placed concrete and pumped concrete.
Ref ASTM C94 Standard Specification for Ready-Mixed Concrete - Tolerances in slump - When the project specification for slump are written as a "maximum" or "not to exceed requirement: If specified slump is more than 3"(75mm), then plus tolerance is 0" and minus tolerance is 2 1/2"(65 mm).
You may also refer to thread167-227472 for discussion.

 

[DST148]

@allanex50: I have found another reference, ACI 301-05: Specifications for Structural Concrete, section 4.2.2.2 Slump - When high-range water-reducing admixtures (super plasticizers) conforming to ASTM C 494 is permitted to increase the slump of concrete, concrete shall have a slump of 2 to 4 inches before the admixture is added and a maximum slump of 8 inches at the point of delivery after the admixture is added unless otherwise specified.

 

[DST148]

@Ron: Your second post regarding the relation between the size of the coarse aggregate and the clear spacing between rebars....."for 19mm CA, spacing between any rebar should be no less than 60mm...".
ACI 318-05, section 3.3.2(c) lists nominal maximum size of coarse aggregate shall be not larger than 3/4 the minimum clear spacing between individual reinforcing bars, bundles of bars etc. The commentary even allows size limitations on aggregates to be waived if the workability and methods of consolidation of concrete are such that the concrete can be placed without honeycombs or voids.
The maximum size of coarse aggregate requirement in relation to clear spacing between bars is similar in ACI 301 and PCA Publication - Design and Control of Concrete Mixtures, 14th Edition.
Am I missing requirement given in some other code / standard?

 

[hokie66]

Honeycombing is normally caused by lack of consolidation and/or leaky formwork. Perhaps the OP can share with us how the concrete was compacted, or at least what was attempted. 19 mm is not large for coarse aggregate, so that is not the problem.

By the way, Ron, where did that 60 mm come from? That's a lot more than the norm.

 

[Ron]

hokie66...our code requires aggregate no larger than 3/4 of the spacing between individual rebars. Rebars should be spaced about 1.5d where d is the bar diameter. Those two requirements often lead to difficult placement and voids.

For compressive stress across aggregate boundaries, 3 times the max. nominal CA size is mandated for testing.

Ideally this would be the case for in-place as well, but not practicable either. My 60mm reference was to that as a rule of thumb for cover...not interior rebar spacing.

For interior spacing, I would set max. CA size at 1/2 spacing gap.

 

[hokie66]

So for 19 mm aggregate, the gap could be 38 mm. To comply with the 1.5 db clear, that would allow 25 mm bars, about #8. For larger bars, as would be typical in heavily reinforced columns, the gap would be greater if based on the bar size.

Cover for columns, in accordance with 7.7.1, should be 1.5" minimum, about 38 mm. My standard number is 40 mm. More cover is only used for severe exposure environments.

 

[hokie66]

Another thing to look at in unconsolidated concrete in columns is the elapsed time after adding the plasticizer. The effective time is limited, and with the relatively small amount of concrete required in columns, a truck which stands around for a while won't retain the same properties.

A lot of problems which show up on this site could be avoided by adequate consideration during design. Columns with congested reinforcement are not economical...higher concrete strength and/or larger columns are much better than adding reinforcement. Using "magic concrete" gets too much emphasis.

 

[Hoaokapohaku]

I agree with Hokie about the possibility of the plasticizer losing effectiveness over time. However, many products are dependent upon agitation to maintain workability. In slow column construction, often the drum rotation at discharge is too slow for the platicizer to be effective. Two or three fast rotations of the drum will restore most of the lost slump. This should not be considered retempering because the stiffening of the mix is due to the property of the plastisizer and not due to hydration of the cement.