Is it ok to pour piers without use of a vibrator 3

[getitrightconcrete]

We are concerned about the work a concrete company has done. We learned halfway through the project that they should have been using a vibrator during the pours. We made them use one for the retaining walls, but they had already poured all the piers. The piers are 16' deep, 16" diameter. The cages are 4 vertical bars arranged in an 8.5" square with stirrups every 6". They did shake the top of the cages as the concrete neared the top, but I don't think it was doing too much. Should I be concerned or would the concrete been likely to flow well enough to prevent large voids?

 

[Ron]

It is not always necessary to vibrate concrete, but it depends on a lot of factors. Assuming the mix was fluid enough to all the cage to be moved around, then vibration was likely not necessary.

Check the slump of the concrete and the type of mix. If the mix was a high strength grout mix with a high slump (over 8 inches or so for this application), then vibration wouldn't likely be necessary.

The purpose of vibration is to consolidate the concrete and reduce the number of large voids you can sometimes get through the combination of mix design factors, the slump conditions at the time of placement, and the geometry of the pier/reinforcement interface.

 

[JAE]

I agree - at the most, you could have them vibrate the top 5 feet or so of the pier, but usually even this is not necessary.

Piers are a very rugged thing and in most cases the concrete is self consolodating in a vertical shaft configuration.

 

[Qshake]

Voids in the piers would not concern me, but if the contractor says he shook/moved the rebar cage near th eend of the pour that concerns me. In fact, even if he didn't shake or move the cage I would be concernd over possible partial excavation collapse that could lead to reduced diameter shafts/piers. The piers at 16" are not very big to begin with so any necking down could be a concern.

Were permanent casings used? Or a tremie method or was the excavation dry enough that the walls of the excavation could be self supporting?

Regards,
Qshake

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

 

[BFPartners]

Vibration serves two main purposes:

1) Consolidates the interface between successive pours.
2) Eliminates concrete hanging on the reinforcing creating voids within the structure.

If the piers were poured in one continuous pour then you will not have irregularities in the concrete. So check the volume.

Check the concrete tix from the trucks. Compare the recorded volume used from the tix, verses the volume from a quantity take-off from the drawings. If the two differ by a significant volume you might have voids within the pier.

 

[Ron]

BFPartners...gross volumes such as you mention are not accurate enough to indicate voids in the concrete. There are other reasons for the volumes to differ, so by that you wouldn't know whether the volumetric difference represented voids or necking or slight depth differences.

 

[Zambo]

There are 2 separate issues here. First whether a vibrator was needed. For this the workability should have been checked. A slump test, thogh not the most appropriate method, with a reading of 200 - 275mm I would expect to be OK.

Secondly there is the posiibility of coallapse of the bore. Either a casing, bentonite or polymer may have been used. Alternatively the geotechnical engineer may have advised that the vertical face would be self supporting. More information is needed before an opinion can be given on this.

Zambo

 

[JAE]

A lot of drilling contractors (from my time in South Texas) used guide bars along the sides of the pier rebar cage to keep the pier steel in line and somewhat equi-distant from the shaft earth sides...so shaking the cage wouldn't be a huge cause for alarm. In fact, most shafts end up being a bit larger than anticipated as the drill bit and "kelley" bar are never perfectly in line - there's always a bit of wobble and warp in the system which causes larger shafts than the actual diameter of the bit.

In fact, the older engineers that I learned under used to only specify 2" of clear distance even though ACI calls for 3". The reason was that the rebar supplier would take the specified shaft diameter, say 18", and set his hooping or ties at 18-2-2 - 14" dia. The shaft would end up being 20 to 24 inches and you'd end up with more than the required 3" clear.

 

[getitrightconcrete]

Thanks everyone for all the insights. The concrete was specified to be 2500psi (if that helps with guessing the slump). The bores did not use a casing and the cages had small concrete standoffs wired to the corners (about every 2') to keep them centered in the bore. So I don't think shaking the cages would have caused any necking and they only shook them when the bore was nearly full. So should I still not worry too much about whether the concrete was consolidated?

 

[Zambo]

depends on the workability (i.e slump test) if test not carried out then you should at least be able to find out from the supplier what the design mix slump was.

Zambo

 

[Ron]

getitrightconcrete....The compressive strength does not indicate the slump. You can achieve almost any slump at almost any compressive strength.

Check the mix design to see what slump was ordered. It was likely not less than that when placed.