How to vibrate high slump concrete and achieve good finishes

[mm712275]

I experienced a very difficult situation in vibrating high slump concrete in a vertical cast mould. The size of aggregate is 5mm max. The water/cement ratio is 0.4. Cement/aggregate ratio is about 1:2.5. Superplastizer is MBT glenium SP8S. Slump is about 200mm. The admixture is about 1.5% of the cement weight. The mould is steel made and contain 16 no. of individual mould. So it is a battery mould. Each of the mould is of size 600mm in height. 2.6m long and 85mm in thickness. I found it very difficult to vibrate the concrete so as to achieve good finishes ( F4 ). The surface always contain blow-holes and niddle holes. Some of those blow-holes is more than 5mm in diameters. What is the best method to vibrate the concrete and yet to have good finishes ?

 

[Ron]

It sounds as though you might be overvibrating. Try just using a rubber mallet to help consolidate the mixture. With the slump you have and the application, minimal to no vibration should be needed.

Ron

 

[RiBeneke]

Blow holes on the moulded surface are often the result of the air finding difficulty in migrating to the surface. This can be caused by a release agent that leaves the mould surface too 'oily'
or by a mould surface that is too new and smooth
or by vibration that is too vigorous, as Ron suggested.
I suggest that you try to roughen your mould surface with 100 grit sandpaper. This produces a capillary active surface that helps the air to disperse.
You could also try other release agents.

Richard Beneke

 

[Ginger]

You could employ shutter vibrators to limit the surface blow holes on a complez shaped form. These do tend to be quite powerful so you may only need to use the vibrators in limited bursts. This will require a little practice.

Regards

Andy Machon
Andy@machona.freeserve.co.uk

 

[Gourile]

Hi!
The other fellows are right. As a conclusion let's say:
1) With the slump you described, you won't need much vibration.
2) Avoid inner vibration of the fresh concrete as much as possible, as it may cause bleeding or agregates segregation.
3) Try to use external vibration of the moulds. It helps traped air bubbles near the mould surface to be released. The device and frequency should be chosen by an expert. This method is good for finished surfaces if performed accurately. You will find a refrence in ACI MCP (ACI Manual of Concrete Practice)(Sorry I don't remember the commitie number, but you will easily find it if you have access to that reference.)