Moisture content requirements in CMUs

Masonry Construction, July-August, 2005 by Ron Willis

Engineers and inspectors sometimes get real picky about the moisture content requirements in concrete masonry units on a project. Here is the best way to deal with the moisture issue.

The first step is to look at ASTM C90, page 74, Fig X1.1., "Selected Climatic Maps of the United States," to determine the mean annual relative humidity percentage for the area. For example, the map (below) indicates that the Dallas/Fort Worth area has a mean annual relative humidity of 60% to 70%.

[FIGURE X1.1 OMITTED]

Then move to ASTM C90, page 72, Table 1., "Moisture Content Requirements for Type I Units." This table (below) shows that item "C--Mean annual relative humidity of 50% to 75%" covers the Dallas/Fort Worth example. The Engineering and Testing Sheet delivered by the block provider in our example indicates that the product being used on the project has a linear drying shrinkage percentage of 0.034. This figure puts it on the middle line of the table, from 0.03 to 0.045. The middle column and the middle line meet at 35% absorption. In other words, you typically could not lay these units with an absorption figure of more than 35%.

The Engineering Testing Sheet showed that the dry unit weight of the CMU on this job is 84.5 lb/cubic foot. The water absorption is 14.9 lb/cubic foot. To determine how many cubic feet are in a single block, multiply the width times the height times the length of a single block, divided by the cubic inches in a cubic foot, times the volume percent solid, or 7.625 x 7.625 x 15.625 / 1728 x 0.55 = 0.289146988 cubic foot/unit.

The water content of a single unit is determined by weight. Tests show that a totally dry block is 84.5 lb/cubic foot. A totally absorbed block is 14.9 1b/cubic foot heavier. We are allowed 35% absorption, therefore, 14.9 times the total cubic foot per unit (0.289146988) would be 4.308 lb x 0.35 (percentage allowed) = 1.5 lb. Consequently, any more than 1.5 lb over a totally dry block weight is too much absorption. This calculation takes a technical approach, but there is another way.

At the beginning of a project, put a block unit into a barrel of water and let it soak overnight. After removing the block from the barrel, weigh it immediately, which is the total saturated weight.

Then take the same block unit and leave it in an oven long enough to ensure that the unit is totally dry, which may take 12 to 24 hours. Remove the block from the oven and weigh it, which is the dry weight. The difference between the two is the total (100%) water content.

Leave the same unit out in the open (on top of the block stack is best), exposed to rain. After an overnight rain, weigh it for water content and divide that figure by the total (100%) water content weight. This approach provides the percentage of water content after a rain. No one will dispute this method.

To avoid any problems with water content, simply keep all material covered and off the ground, on pallets. The material stays dry and void of any water content.

Table 1. Moisture Content Requirements for Type I Units

                            Maximum Moisture Content, % of Total
Total Linear              Absorption (average of 3 units) Humidity
Drying Shrinkable, %      (A) Conditions at Jobsite or Point of Use

                          Humid (B)   Intermediate (C)   Arid (D)

Less than 0.03               45              40             35
0.03 to less than 0.045      40              35             30
0.045 to 0.065, max          35              30             25

(A) -- See Appendix X1 (Fig. X1.1) for map of mean annual relative
humidity.

(B) -- Mean annual relative humidity above 75%.

(C) -- Mean annual relative humidity of 50% to 75%.

(D) -- Mean annual relative humidity less than 50%.

Ron Willis owns Masonry Estimating and Consulting Services (www.meacs.com). He can be reached at 817-822-8595 or ron@meacs.com.