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Bruce Strickland
National Marketing Manager
Sika Corporation
(800) 933–SIKA
Bruce has over 30 years experience in the admixture industry. In his career, he has held positions in product development, marketing, sales and sales management. Bruce has also spent time in...more»»
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
How many 60 lb bags needed to cover a space of 28″X28″X3″?
Posted by John from Lancaster, CA, UM on June 3, 2007
How many 60 lb bags of premix cement NEEDED to cover a space of
28″ X 28″ X 3″ . Am using a wheel barrel to mix it in however, don’t know when to stop mixing , that is, using the water of course?
Please help me….am new to this, hard breaking back work and sweatty type of work, but it has to be done.
Also pouring cement in between 2″ X 3″ (measure above).
Counted about 15 squares of 28X28X3 and I figure about 75 sacks of premix cement…Am I way off?
Thank you,
John
Generally companies selling premix cement bags mention the amount of mortar or concrete produced by a bag of premix cement for a given quantity of added water. So look for the coverage on the premix bag.
The volume of the space you are trying to cover is 28” x 28” x 3” = 1.36 cubic foot. If the manufacturer claims a 60 lbs bag produces 0.42 cubic foot of concrete for a given water content, you will need 3.29 lbs of premix cement per hole. So, for 15 such holes you will need roughly 48 number of 60 lbs bags. But again, this is for an assumed amount of water, if the amount of water you add change, your yield will be different.
Alternately you can bring 3 or 4 bags to fill one space to get an estimate of how many bags are need to cover the remaining hollow spaces.
Foundations
Posted by dennis from arlington, VA, US on April 16, 2007
When building foundation walls there is a technique where the cinder blocks are dry-stacked and a polymer is applied to the outside for strength and waterproofing. What is this technique called and where can I find more info on this? Thanks
Please contact the National concrete masonry association (NCMA) at www.ncma.org for this information.
thanks
drying troubles
Posted by lara from hendersonville, NC, US on April 14, 2007
Dear Bruce,
i have been working with mortars mixed with lime and different aggregates tocome up bwith my own decorative mortars
lately since I work in dry and hot conditions i have problems with the mixture
it still dries the way Iexpect it to dry
but it has no more strenght
it seems to me that the water that i am adding is evaporating to quickly so it does not allow to make a strong bond
I have understood that lime by itself holds water longer in the mixture and works as a natural plastifier
wood there be any agent available that could help me with this problem still keeping the lime in the mix
I have looked into acrylic polymers but it seems to me that they are reducing the water ratio
I hope you have the answer for my problem
regards
lara
The strength of a mortar mix primarily depends on the water to cement ratio. Also, since you are working in a hot environment, the loss in the workability of the mortar mix will be faster due to the temperature and evaporation losses.
I would recommend you to use either a water reducing and retarding admixture or a high range water reducing and retarding admixture for your purpose. These admixtures will not only enable you to reduce the amount of water needed in the mix, which will improve the strength of the mix but will also help to mix to be workable for a longer time.
concrete in saltwater
Posted by dan from new york, NY, US on April 9, 2007
are there any admixes you would recommend for a concrete pillar(roughly 3ft x 3ft by 2 ft) that will be underwater twice a day –it will be used to support a boat davit. also if salt water was used in the mixing of the concrete aggregate would it be any stronger—thank you
You can use a high range water reducing admixture, to reduce the water content of the concrete mix; this will make the concrete more impermeable. You can also add a acrylic or latex polymer to improve the water proofing property of the concrete.
I would recommend you to use regular tap water instead of sea water.
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