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RE: Concrete Slump

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You have received some very good advice from Jay Shilstone.

Concrete slump is a commonly used indicator of workability and for water content. The higher the water contents can result in more shrinkage and therefore more shrinkage cracking.

Increasing the slump for workability and lower w/c ratios are commonly done with water reducing agents such as super plasticizers. My favorite category would be a polycarboxylate super p. Another variable is the method of placement. The mix for a pumped concrete is different for a concrete transported by bucket.

I would suggest a preconstruction meeting. Invite all of the parties to get agreement on a trial mix. And do a test placement and mimic as close as possible the anticipated field conditions. Invitees to the preconstruction meeting as a minimum should include the finisher, pumper, cement supplier, admixture supplier, aggregate supplier (especially if LW is used), concrete supplier, and the concrete mix engineer. I would even suggest that someone pay for an outside concrete mix master such as Jay Shilstone.

I would want to know how as much as possible about how it is to be transported, placed, screeded, finished, and cured.

After everyone is heard, develop the mix, do the test placement, refine the mix and repeat the test placement if required. If the mix is pumped, mimic the extremes of the pump as much as possible. The mix will behave differently when the boom is extended horizontally as opposed to raised for closer placements. (The air entrainment drops with a large vertical drop.)

Another variable of which you will need to be the weather. Temperature, humidity, and wind should be anticipated. Your mix will probably have to be adjusted for the season. But you should be able to work out the basics with your trial placement.

Keep in mind that the light weight aggregates (if used) will add a degree of complexity. Consult with the mix contractor's preferred supplier.

I have had some very successful placements in adverse conditions by using proper planning. I have also witnessed some major screw ups on other people's projects. I like to learn from other people's mistakes. It is cheaper.

Harold Sprague

From: The Best Always <the_best_always(--nospam--at)>
Reply-To: <seaint(--nospam--at)>
To: seaint(--nospam--at)
Subject: Concrete Slump
Date: Wed, 14 Feb 2007 00:58:48 -0800 (PST)


It is my understanding that high concrete slumps would result to excessive cracks. Anyone knows what is the best slump value for a highrise building (say 50-storey building)?

Many thanks.

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