Re-Evaluation of the CSA S304 Concrete Masonry Flexural Compressive Strength Reduction Factor
Date
2022-11-29
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0000-0002-8454-6689
Type
Thesis
Degree Level
Masters
Abstract
Results from research conducted between the years 1980 and 2000 have shown that masonry assemblies
are weaker when loaded perpendicular rather than parallel to the head joint. These results likely influenced
the inclusion and maintenance of the χ-factor in Clauses 10.2.6 and 11.2.1.6 of the CSA S304 Design of
Masonry Structures to reduce the flexural compressive strength when the internal compressive stress acts
perpendicular to the head joint. The results from recent research conducted since 2000 have shown that the
χ-factor is conservative and should potentially be removed from future editions of the CSA S304. The
objective of this experimental investigation was therefore to re-evaluate the χ-factor in an attempt to resolve
differences in reported research results and to ensure safe and relevant code provisions.
To the author’s knowledge, the χ-factor had not been established from the results of full scale tests on
reinforced masonry members; rather, it was based exclusively on the results of prism tests. A total of 24
reinforced masonry members were therefore constructed and tested under four-point loading. The 24
members consisted of 12 pairs of flexural specimens where the internal compressive stress was applied
either parallel or perpendicular to the head joint. Each pair was constructed using one of 12 unique
configurations. The 12 configurations were constructed using three block sizes (200, 250, and 300 mm) and
four reinforcement ratios (0.66, 0.99, 1.44, and 1.99).
There was no difference in the flexural capacity and failure mode of the specimens when the internal
compressive stress was applied parallel versus perpendicular to the head joint. Removing the face shells
from the specimens post-failure showed that the grout had consolidated between the webs of adjacent
concrete masonry units (CMUs). The concrete masonry units used in the experimental program consisted
of knockout units cut from standard, frogged-ended units. Based on these results, the χ-factor should be set
to unity when good grout consolidation can be ensured.
It was, however, hypothesized that the unit type influenced the grout consolidation in the space between
the webs of adjacent CMUs and so affected the flexural capacity of the reinforced masonry members. The
masonry assemblage strength of each reinforced masonry member was determined by constructing and
testing a masonry prism alongside each reinforced masonry member using the same type of knockout units
cut from standard units. An additional 12 prisms were then constructed with the web height of standard
CMUs left intact to evaluate the effect of web height on the masonry assemblage strength. Sixteen prisms
were further constructed using square-ended CMUs to evaluate the effect of CMU type on the masonry
assemblage strength. The orientation of compressive stress and the web height was found to significantly
affect the masonry assemblage strength for these additional prisms. Removing the face shells of the prisms
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constructed with square-ended CMUs showed that voids formed in the space between the webs of adjacent
CMUs.
A numerical moment-curvature model was therefore developed and shown to have a reasonable correlation
with the experimental data. Moment-curvature models were then extrapolated to evaluate the effect of voids
caused by poor grout consolidation on the flexural capacity of reinforced masonry members. The voids
were set equal to the web height between adjacent square-ended CMUs. The χ-factor was found to be
linearly correlated to the area of the void intercepting the compressive stress block. It was therefore
recommended that Clauses 10.2.6 and 11.2.1.6 of CSA S304 set χ = 1 when good grout consolidation can
be ensured and set the χ-factor as a linear function of the void area intercepting the compressive stress block
when poor grout consolidation is expected. Notes in Clauses 10.2.6 and 11.2.1.6 of CSA S304 should
therefore also explain that poor grout consolidation should be assumed when coarse aggregate in the grout
mix is used for masonry members constructed with the full web height of CMUs left intact or with square ended knockout CMUs.
Description
Keywords
reinforced concrete block masonry, flexural compressive strength, load orientation, web interruption, effect of voids, χ-factor
Citation
Degree
Master of Science (M.Sc.)
Department
Civil and Geological Engineering
Program
Civil Engineering