November 19, 2014 by On-Site Magazine
The most recent version of the concrete standard for Canadian construction has been released. The CSA A23.1/2-2014 Concrete Materials and Methods of Concrete Construction/Test Methods and Standard Practices for Concrete is essentially the industry’s bible. As such, it has been proposed for inclusion in the upcoming National Building Code (NBC) for 2015.
Previous versions of the NBC have included the CSA standard, but this newly released version has been updated to reflect changes in the marketplace. The standard covers a broad range of issues for concrete testing, placement, formulation and performance requirements. The intent is to provide a sound technical basis for the long-term performance of concrete structures through the use of proper mix designs and construction practices, including performance verification testing. Actual structural design of concrete structures is covered by a sister standard CSA A23.3
The standard is quite detailed with many changes reflected in the new document. In this column we will highlight a few of the most significant changes.
The introduction of the newly manufactured Portland Limestone cement (PLC) in the 2009 version had imposed a limitation of use in high sulphate-resistant applications or exposure classes. Recent research shows the ability to blend it with supplementary cementing materials to provide the needed sulphate resistance for those exposure classes referenced in the standard, and have been included in the new version.
Exposure classes, which dictate the performance specifications of the concrete application, were added to previous versions of CSA A23. Two new classes have been added. One to address interior concrete floors finished with a steel trowel. The other is for structurally reinforced concrete exposed to manure or silage gases. Rapid Chloride Penetration timeline requirements for C-XL, A-XL, C-1 and A-1 Classes have been extended to 91 days, in order to best address actual performance needs on the project.
In the test method section two new testing methods have been added: scaling resistance of concrete surfaces exposed to deicing chemicals using mass loss and electrical indication of concrete’s ability to resist chloride ion penetration.
Other changes to the standard include in-depth descriptions of stakeholder responsibilities and in the area of testing where additional certified third-party on-site testing results can be used to determine actual performance. It is noted that both field and laboratory testing should be done by the same certified testing company.
One of the major (and more controversial) changes to the standard is in the area of residential exposure classes. The CSA Standing Committee recommended changing the R-1 (residential concrete for footings for walls, columns, fireplaces and chimneys) and R-2 (residential concrete for foundation walls, grade beams, piers, etc.) from a 15 Mpa at a 0.70 water/cement ratio to a 25 Mpa at 0.55 water/cement ratio. It also recommended changing the R-3 class (residential concrete for interior slabs on ground not exposed to freezethaw) from 20 MPA with a water/cement ratio of 0.65 to 25 MPA at a water/cement ratio of 0.55.
These were proposed in the public review in order to more correctly line up with similar structures in the commercial and industrial fields. The Standing Committee on Housing and Small Buildings (SCHSB is the group responsible for Part 9 of the NBC) when presented with the proposed changes requested the rational for the changes. During many information exchanges between the two groups the CSA Committee was unable to convince the SCHSB that the proposed changes would improve the performance of residential basements. The argument is that residential basement failures are not due to concrete mix design issues but rather the result of poor construction practices. The NBC will adopt the new version of the CSA standard but will write an exclusion clause and retain the current residential exposure classes from the 2010 NBC.
As part of the information gathering process used to investigate the potential changes, it was discovered that there is a broad range of concrete supplied in different market places in Canada. Concrete ranges from 32 Mpa in some provinces to 15 Mpa in others. Use of concrete requirements beyond the minimum of the code are not the issue. But what did come to light is that survey responses still indicated problems with basements even at the proposed strengths, which is a strong indication that the CSA solutions might not minimize the reported problems.
This whole issue on the discrepancy between the concrete industry and the building codes brings to the forefront the need for the concrete industry to be more diligent in working with its customers. This ensures when the time comes to change or propose improvements to the standards, both sides can recognize the value and have the necessary tools to assess the rationale and the changes.
Provided by the Canadian Ready-Mixed Concrete Association. Send comments to email@example.com