Water-Cement Ratio

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HeidelbergCement and Italcementi: Reorganization in the Works

The plans for reorganization keep location and brand the same, but can impact... More

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A Mix Design's Ingredients

What can we tell our contractor-customers who insist on a low water-cement ratio but who then complain the concrete is hard to finish? More

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Eliminating Corrosion

I recently read some papers on the corrosion of concrete in sewer manholes. Apparently, this is not a problem of gasses floating around in the sewer, but an issue of anaerobic bacteria (Thiobacillus) living on the surface of the concrete, creating sulfuric acid, which corrodes the concrete. The prevailing response has been to specify liners. However, there is an antimicrobial agent that does not allow Thiobacillus to live on (or in) the concrete. By eliminating the bacteria, you can eliminate the acid and the corrosion. The other issue is the high sulfate content of sewage. This could be addressed by following Portland Cement Association’s “Design and Control of Concrete Mixtures” requirements for concrete exposed to sulfate-containing solutions, (i.e. low water cement ratio, use of a pozzolan, higher compressive strengths, low alkali cements). By designing for a “moderate” sulfate environment and using an antibacterial agent as an admixture, you could eliminate the need for a liner. What other issues should be addressed in this type of application? More

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Normal Concrete Weight Loss

I am working on providing material for crane counterweights. What is considered a normal weight loss (in pounds per cubic foot) for concrete from its plastic weight to a normal exposure weight after a month or two? My guess would be in the area of a pound or two per cubic foot. More

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More on Limestone Aggregate and Warehouse Floors: A Reader Responds

As a specialist in industrial floor-hardening products, I am responding to a couple of comments in the "Troubleshooting" column from the October 2002 issue. The comments were in reference to specifying limestone as coarse aggregates in concrete floor slabs. In the last paragraph, the column suggested that a larger surface area coarse aggregate would require a lesser amount of portland cement. That may be true, but it doesn’t tell the whole story. I think you should also examine the aggregate shape. More

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Concrete Temperature and SCC

One question recently posed on Aggregate Research Inc.Æs Industry Forum (www.aggregateresearch.com/forum) drew a number of interesting and important answers that may help you this season. The questioner wrote: ôIÆve noticed that a variation in ambient temperature can have a big effect on the workability of self-consolidating concrete [(SCC)]. IÆve found that when my materials are 75û80¦ F, itÆs best for both flow and reduction of bleed. Is anyone else dealing with this?ö More

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What's the Allowable Fly Ash Limit?

What is the highest percentage of fly ash that can be used in ready-mixed concrete? Is durability the most important issue in determining the limit? More

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Do Fibers Reduce Permeability?

A customer has asked us to include fibers in the mix design for a water retention tank. The idea is that the fibers will reduce the permeability of the concrete. Is that true? More

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Flash Set vs. False Set

We were supplying a slab on grade with ordinary 3000-psi concrete when a section of the slab flash set or possibly false set; we are not sure. What is the difference between the two? The contractor asked us if he should retemper the area with water or finish it. What should we have suggested to the contractor? More

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The right way to truck-batch bagged pigment

Recently we supplied an integrally colored pea-gravel mix for a job that used textured vertical formwork. The contractor insisted on adding the dissolvable color bags at the jobsite just prior to placement. We dispatched 8-cubic-yard loads in standard 10-cubic-yard rear-discharge mixers. The driver reported that the contractor instructed him to run the mixer for about 4 to 5 minutes of full-speed drum rotation after throwing in 15 1-pound bags. Our belt placement operator later reported that he had to stop the conveyor several times to remove large pieces of undissolved paper bags. While we haven't heard of any problems on the job from the contractor, this doesn't seem to be the way things should be done. What's the correct procedure to eliminate any bag residue in the concrete? More

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