Unplanned outages at cement plants can affect cement supplies for producers.
PCA Unplanned outages at cement plants can affect cement supplies for producers.

{ QUESTION } In our area there is a shortage of portland cement. Why does this happen, and what can I do about it?

{ANSWER } Cement shortages can arise from many causes. In the current economy there is a relatively healthy supply throughout the U.S. Despite this, in localized areas, there are shortages. The reasons are complicated: There can be interruptions in manufacturing, transportation, or distribution. The changes in hours of service as well as individual state requirements for vehicle loading have restricted the supply of trucking. Rail capacity in the Midwest has been affected by the activity in the North Dakota oil fields and there have been unplanned outages at cement plants. So the shortage can occur due to problems other than unexpected demand.

Extending your supply

As for the second part of the question, there are a few things that can be done to extend the cement supply by changing construction methods, materials, mix designs, or some combination of these. One method is to increase the fly ash or slag content in your mixtures. This will extend the amount of cement available by reducing the amount that you are using per project. Be careful that you don’t just take your normal mixture and replace fly ash or slag pound for pound. Fly ash will act like a water reducer, and so the water content can be reduced at a fixed slump. As a result, a lower water-cement ratio can offset some of the prolonged setting that is reported with some fly ash. Using an accelerator (non-chloride based if there is any steel present) can also offset the effect of fly ash on setting.

Slag will have a varying effect based on the grade of the slag and its chemistry. Run a trial batch at the expected placing temperature and adjust mixtures as needed to offset any setting effects.

If you’re using high cement contents for acceleration or for cold weather concreting, you should avoid that practice. ACI 306 recommends adding an additional 100 pounds of cement for heat. In these days of sustainability and with the high cost of cement, the better option is to use accelerators or insulation.

Improve quality

Finally, spend some time tightening up quality control. Reducing lost loads is always a good idea, but it is even more important in times of lean supply. Reducing the variability at the plant (the standard deviation used in mixture design in ACI 301) can allow a reduction in the cement content needed to meet strength requirements. Analyze your top mixtures and see if you can reduce the cement a little and still meet the statistical requirements for strength. In these times, any small amount will help.

This article was contributed by Kevin MacDonald of Beton Consulting Engineers. E-mail kmacdonald@betonconsutingeng.com. Visit www.betonconsultingeng.com.

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