When specifying CMU, is moisture control important?
Moisture-controlled units limit the amount of moisture contained in a block at the time they arrive on a site so that they are in close equilibrium with the ambient environment. These units are assigned the Type-M designation using the fourth facet in Table 1 in CSA A165.1.
The purpose of this page is to provide insight on how to specify the use of moisture-controlled units. However, in most cases, the moisture content of masonry units does not need to be limited when movement joints are provided in accordance with best practices.
This page is part of a larger series of specification examples meant to highlight cases where vague or poorly written items have lead to issues on masonry jobs across Canada.
Part of our Masonry Specification Series
Offering recommendations accompanied with background explanatory material to explain how these recommendations were formed. Click here to see the full series.
Disclaimer
The information contained here is intended to serve as educational content for designers, specifiers, or contractors. It is not to be relied upon for formal technical advice, as masonry projects may have details and considerations that are unique to a particular project and may be beyond the scope of the content of this page.
Recommendations
Table 1 of CSA A165.1 is used to classify concrete block masonry units using a 4-facet system. The fourth facet can be used for the classification of a moisture-controlled unit by specifying that the block must be a Type-M CMU.
While this page aims to provide guidance on specifying the use of moisture-controlled units, it is generally more effective to ensure correct construction procedures that follow CSA A371, and for designers to clearly specify the regular spacing of movement joints, rather than simply mandating the use of moisture-controlled units.
Decades of practice and experience have demonstrated that the most effective means to mitigate moisture related movements and cracking of concrete masonry walls is through the proper and effective detailing of movement joints. A moisture-controlled unit will never mitigate all forms of movement. Shrinkage due to moisture loss is but one example of many different factors that contribute to wall cracking. A far superior and effective solution is following industry best practices with regularly spaced and well-detailed movement joints.
Discussion
Concrete block masonry units (CMU) expand when they take on moisture and shrink when they lose moisture. If the moisture content in the block is with the same rage as the surrounding environment, then it is in equilibrium and there will be no expected moisture-related movement (there will still be volumetric changes over time due to factors such as carbonation and changes in temperature) . For moisture-controlled units, the intent is to dry the concrete block masonry units to a moisture content that is more closely in equilibrium with the anticipated average service environment for the block, in order to minimise dimensional changes in the concrete blocks due to changes in moisture.
Table 1 of CSA A165.1 is used to classify concrete block masonry units using a 4-facet system. The fourth facet can be used for the classification of a moisture-controlled unit by specifying that the block must be a Type-M CMU.
Through testing of the CMUs, the properties can be determined. The properties of interest are a combination of the total linear shrinkage and the moisture content of the block. These properties will determine whether the block can be designated as a Type-M moisture-controlled CMU. An example of a potentially problematic specification for moisture-controlled blocks that also indicates the maximum linear shrinkage of the block can be seen below :
Example of potentially problematic specification:
2.01 MATERIALS
.1 Standard concrete block units [Type [______]] [______]: to CAN/CSA-A165 Series (CAN/CSA-A165.1).
.1 Classification: [H] [SS] [SF]/[10] [15] [20] [30]/[A] [B] [C] [D] [N]/[M] [O].
1. Linear shrinkage and moisture movement of blocks shall not exceed 0.03%.
Controlling linear shrinkage is not handled by a one-size-fits-all metric
In accordance with the standard, there are multiple combinations of maximum moisture content and total linear shrinkage, depending on the average relative humidity at the point of manufacturing, that are allowed for CMUs to achieve the Type M designation. When testing the CMU block, the average annual relative humidity (%) at the location of manufacture must be known. Linear shrinkage and moisture content of the block shall be determined through testing, and the average of 5 specimens shall be taken. Based on the average linear shrinkage of the blocks, the maximum allowable moisture content of the blocks can be determined using Table 1 in CSA A165.1. The average moisture content of the blocks determined through testing must be below this maximum allowable value. A Type-M moisture-controlled CMU will be shipped by the producer at or below the determined maximum allowable moisture content, whereas a Type-O CMU does not introduce any limit to the moisture content.
Using the moisture content and linear moisture shrinkage characteristics of concrete masonry units, based on the properties of CMUs as they leave the manufacturing facility, can be challenging. An accurate accounting of expected moisture movements would require knowledge of the moisture content of the masonry assemblage, once installed in the building, as well as the expected range of relative humidity expected in the completed structure (for interior applications). Additionally, the impact of thermal movements, carbonation shrinkage, and creep due to applied stresses (if applicable) would need to be accounted for. Such a detailed analysis is seldom undertaken during the design phase, and movement joints are typically specified in accordance with CSA A371 and industry best practices such as those described here regardless of whether Type M or Type O CMUs are specified for a project.
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Part of our Masonry Specification Series
Offering recommendations accompanied with background explanatory material to explain how these recommendations were formed. Click here to see the full series.
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As part of our overall effort to educate the design community and our contractor members, CMDC has compiled a collection of articles meant to address common areas of misunderstanding or confusion that have resulted in issues in past projects.