Redosing of concrete with High Range Water Reducers.
Posted by Ron from Sydney, OT, AU on June 4, 2007
Hi Bruce,
My name is Ron Maloney. I work for a government department in Australia (Roads and Traffic Authority).My branch delivers new roads and bridges to the state.
The question Is what do you think of redosing of high strength concrete ?.
The concrete has had the water reducer added at the plant and hauled for approx. 1 hour. The slump is reverting, and is to old to retamper. The contractor would like to redose to improve the slump. What do you think?
Re-dosing of admixtures in concrete mixes is fairly common for normal strength mixes, especially when the ambient temperature is high which cause high slump loss.
In your case for high strength mixes, I would say it would be safe to re-dose, if the concrete has not lost its workability completely. I think re-dosing a concrete having a slump of above 2” should not be a problem, but this is only a good guess I can make based on my experience.
If re-dosing is something you think you will anticipate regularly, I would recommend you to do a couple of lab trial mixes, in which you should re-dose the concrete mixes with admixtures when the slump has dropped to different levels and observe if they have any negative influence of re-dosing on strength, air and permeability.
At site, higher the slump before re-dosing, the better it is. Never re-dose if you feel the concrete has started to take initial set..
Also while re-dosing always try to re-dose using only a pure superplasticizer which contains no retarder. This is to avoid excessive retardation. Under no circumstances with high strength concrete would I re-dose more than once.
Goodluck
