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Winter 2001-2002
CODES AND SPECIFICATIONS UNDERSTANDING
AND
WORKING THROUGH THE MAZE
TESTING, INSPECTION AND CONSTRUCTION - PART
III
 This
is Part III in a series of three articles to
understand and locate codes and specification
provisions related to concrete masonry design and
construction. The purpose of these articles is not
so much to provide specific design guidance, but to
direct a design engineer to various provisions,
primarily in the 1998 California Building Code.
Many other codes and standards such as ASTM,
UBC-Standards and Masonry Standards Joint Committee
(MSJC) provisions are also referenced where
appropriate.
Provisions in 1998 California Building Code are
discussed, rather than 1997 UBC Provisions, because
it is a document based on 1997 UBC adopted in
California and also covers State of California
Amendments, which are applicable to public schools,
community colleges, essential services buildings,
and hospitals.
Part I dealt with Material and Product
Standards.
Part II dealt with Structural Design
Provisions.
As discussed in Part I, a concrete masonry
assemblage is a complex system comprising of four
primary materials, concrete masonry units,
mortar, grout and reinforcement.
Unreinforced masonry is not permitted in seismic
zones 3 and 4 as defined in the Uniform Building
Code (1997 edition).
Interaction of each of the materials combined
with variability in material properties and
construction tolerances makes it extremely
important to test materials and assemblages and
inspect construction to assure a high degree of
reliability in quality of the final product.
This article, final in a series of three
articles on "Codes and Specifications" attempts to
summarize the requirements for a reliable final
product.
TESTING
The primary purpose in testing concrete masonry
is to verify the specified compressive strength,
f 'm. Other important areas cover
the compressive strength of grout and mortar and
shear strength of masonry cores.
It should be noted that project specifications
may require only some of the tests; e.g., testing
cores in double shear is required only for projects
under State of California Amendments. In California
Building Code (CBC) 1998 edition, testing is
covered in Section 2105, Quality
Assurance.
CONCRETE MASONRY UNITS (CMU's)
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Testing of concrete masonry units can
be divided into three areas:
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a.
b.
c.
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Dimensions
Moisture content/absorption
Compressive strength (discussed under
verification of f 'm
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a.
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Dimensions
When required, details of measurement
of a unit including calculating equivalent
web thickness and equivalent wall
thickness are covered in UBC Standard
21-4. Detail test procedures are
covered in ASTM C140. Equivalent
wall thickness is useful in calculating
fire resistance of concrete masonry.
Linear drying shrinkage is the
change in linear dimension due to drying
from a saturated condition to an
equilibrium weight and length. ASTM
C426 gives detail procedures to
conduct this test. Although moisture
controlled units (Type I) are no longer
recognized in current ASTM Standards, UBC
Standard 21-4 is based on 1995 edition of
ASTM Standards and still covers moisture
controlled units. This fact should be
recognized. Linear drying shrinkage
calculations are useful for assessing
movement of wall due to shrinkage so that
appropriate control joints can be designed
to accommodate such movement.
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b.
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Moisture Content/Absorption
As discussed above, ASTM does not
require moisture content to be determined
as Type I and Type II designations have
been eliminated.
UBC Standard 21-4 still
specifies Type I and Type II units, since
it is based on an older edition of ASTM.
Procedures to determine moisture content
are detailed in ASTM C140.
Water absorption depends on
density of unit. Maximum allowed
percentage of water absorption is given in
UBC Standard 21-4 Table "B" and in ASTM
C90 Specification.
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c.
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Verification of Compressive
strength, f 'm
1998 California Building Code (CBC)
specifies that verification of f
'm of concrete masonry can
be achieved by one of the
following three methods:
1. Masonry Prism Testing
2. Masonry Prism Test Record
3. Unit Strength Method
Design professionals routinely specify
two or more of the above methods to verify
f 'm, this is not
correct. It neither adds to extra safety
provisions nor to clarity of the
intention. It actually creates added
confusion.
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Masonry Prism Testing is the
preferred method, as it closely
simulates the actual construction,
considering grout, mortar and the masonry
units.
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1.
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Masonry Prism Testing (Section
2105.3.2)
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UBC Standard 21-17 and ASTM C1314 are
applicable documents.
CBC Section 2105.3.2 specifies
requirements as follows:
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a.
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Tests are to be conducted at 28 days
unless relationship between 3 day, 7 day
and 28 day tests has been established for
the project.
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b.
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Prior to start of
construction
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i.
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A set of 5 prisms in conformance with
UBC 21-17 is required.
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ii.
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Materials for prisms are to be the same
as for the project.
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iii.
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Prisms are to be constructed under the
observation of the engineer, special
inspector or an approved agency.
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iv.
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Prisms are to be tested by an approved
agency.
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c.
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During Construction
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i.
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Minimum one set of three prisms. Each
5,000 square feet of wall area requires
one set of three prisms.
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ii.
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When one half allowable stresses are
used in design, no prisms are required,
but a letter from the supplier certifying
that materials used in project are the
same as in the five prisms constructed
prior to construction, is required.
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2.
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Masonry Prism Test Record
(Section 2105.3.3)
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This is an effective method of
verifying f 'm but is
not commonly used for reasons unknown.
This method allows verification of f
'm by prism test
records.
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a.
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Prior to Construction
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i.
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Prisms are to be representative of
project construction.
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ii.
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Test records of at least 30 prisms is
required.
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iii.
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Prisms shall have been constructed
under the observation of an engineer,
special inspector or an approved
agency.
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iv.
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Prisms shall have been tested by an
approved agency.
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v.
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Average compressive strength of the
test record to be
at least 1.33 f 'm.
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b.
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During Construction
Requirements are same as under Masonry
Prism Testing.
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i.
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When one half allowable stresses are
used, field testing during construction is
not required. A letter similar to that
described under "c ii" is required.
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3.
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Unit Strength Method (Section
2105.3.4)
This is perhaps the most commonly used
method to verify f 'm.
It needs to be clearly understood that
this method essentially requires
compliance with three components, concrete
masonry units, mortar and grout.
Concrete Masonry Units
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a.
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Prior to Construction
Three units are to be tested. Tests
should conform to UBC Standard
21-4, which is based upon ASTM C140
Specification.
Compressive strength must comply with
CBC, Table 21-D. In general, for type M or
type S mortar, the required unit strength
varies from 1.25 to 1.6 times f
'm.
For example,
for f 'm = 1500 psi,
unit strength > 1900 psi
f 'm = 2500 psi, unit
strength > 3750 psi
Linear interpolation of values given in
Table 21-D is allowed.
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b.
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During Construction
For each 5,000 square feet of wall
area, three units are to be tested and
should meet the same compressive strength
requirements as noted above.
Please note that the compressive
strength results are based on the average
compressive strength of three units. An
individual unit shall not test below 1700
psi.
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Mortar
Mortar should comply with mortar type
of Table 21-D. For specific mortar type,
the proportions should comply with Table
21-A
State of California Amendments
for schools, hospitals and essential
services buildings, requires mortar to
attain a minimum compressive strength of
1500 psi at 28 days for f
'm of 1500 psi.
They also require proportions of mortar
ingredients to be those required for type
"S" mortar.
For requirements of mortar sampling,
see Section 2105 A 3.4 (2).
Grout
At least one test for each 5,000 square
feet of wall area is required
(minimum one test per project).
Grout minimum compressive strength
should be the same as f
'm, but
not less than 2000 psi.
Selecting proportions and other details
are covered under Section 2103.4.2
and 2103A.4.2 when one half the
allowable stresses method is used, no
testing of the units or grout is
required. A letter certifying that they
meet the requirements of Table 21-D is
needed, however.
When masonry does not meet the
requirements of three methods described
above, acceptance may be given by the
building official if prisms cut from the
masonry meet the following
requirements.
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4.
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Prisms Cut from Constructed
Masonry (Section 2105.3.5)
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a.
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Masonry construction is to be at least
28 days old.
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b.
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A set of three prisms for each 5,000
square feet of wall area shall be cut (at
least one set is required for the
project).
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c.
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Testing shall comply with UBC
Standard 21-17.
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d.
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The average compressive strength of the
prisms shall be equal to or more than f
'm based upon net mortar
bedded area.
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State of California Amendments also
require:
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5.
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Core Testing of Masonry
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The Requirements are:
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a.
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Minimum two cores of 6-inch diameter
for each 5,000 square feet of greater of
wall or floor area.
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b.
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50% of the cores to be tested for f
'm.
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c.
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Core tests should show compressive
strength equal to or greater than assumed
f 'm.
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d.
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50% of the cores are to be tested in
shear.
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e.
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Shear strength shall be minimum
psi.
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For other details on core testing, see
Section 2105A.3.1 and Section
2105.A.6
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GROUT
Number of samples required is covered
under "3" Unit Strength Method.
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Grout is commonly specified with
required compressive strength. Testing is
covered in UBC Standard 21-19,
which is based on ASTM C476
Specification.
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It should be noted, although Table 21-D
footnote 4 allows the grout strength to be
equal to unit strength (e.g., 1900 psi
unit strength for 1500 psi masonry
strength), the minimum required grout
strength is 2000 psi at 28 days
(Section 2103.4.1).
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MORTAR
Mortar is not generally specified by
its property of compressive strength. It
is specified by proportion of
ingredients. However, when mortar is
to be tested for compressive strength,
requirements are given in UBC Standard
21-15 based on ASTM C270
Specification.
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Specimens should be prepared and tested
per UBC Standard 21-16. The
specimens are 2-inch by 4-inch
cylinders.
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Compressive strength must be at least
1500 psi at 28 days
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When mortar is specified to meet a
specific compressive strength, laboratory
testing shall be conducted on 2-inch cubes
and need to meet the requirements of
UBC Standard 21-15.
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Type of mortar - Average compressive
strength
M - 2500 psi
S - 1800 psi
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Maximum air content shall not exceed
12%.
State of California Amendments
require at least one set of mortar
specimens (three cylinders) at the
beginning of masonry work on three
successive days and at one week interval
thereafter.
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INSPECTION
The prime purpose of inspection is to
ensure that workmanship and construction
are in compliance with plans and
specifications and meet the design
intent.
In California Building Code, inspection
is covered under general section "Quality
Assurance" (Section 2105), but we have
broken it into two categories,
testing and inspection for
clarity and simplicity.
Inspection, in specific, is covered in
Section 1701.5 Item 7.
Generally, the following items should
be inspected, noted and reported to the
building official, if required.
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1.
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Preparation and taking of test
specimens
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2.
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Foundation surfaces and
rebars
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3.
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Grout space prior to
grouting
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4.
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Grouting operations
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5.
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Protection of masonry
assemblage
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When one half-allowable stresses are
used in design, special masonry inspection
is not required.
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When strength design is used, special
inspection is required (Section
2107.1.2)
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State of California Amendments
have following specific requirements:
(Section 2105A.7)
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1.
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All structural masonry work is to be
continuously inspected during laying and
grouting by specially approved
inspector.
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2.
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Special inspector is to be approved by
the enforcement agency.
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3.
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Inspector is to make test samples,
perform tests as required, check
materials, details of construction and
construction procedures.
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4.
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Inspector is to furnish a verified
report that, of his personal knowledge,
the work covered by the report complied in
all respects with duly approved plans and
specifications.
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CONSTRUCTION
General construction requirements are
covered in Section 2104.2. Some of
the highlights are given below:
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1.
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Concrete masonry units shall not be
wetted unless approved.
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2.
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After initial mixing of water and dry
material components,
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a. Mortar must be used within 2½
hours.
b. Grout must be used within 1½
hours
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a. Cold Weather Construction
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Details are covered in Section
2104.3. Numerous specific requirements
cannot be covered here, but some of the
important points are noted below:
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1.
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Tops of open unfinished walls are to be
covered with a strong weather resistive
material at the end of each day or
shutdown.
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2.
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Units should not be laid below
20OF.
Sand and mixing water are to be heated to
produce mortar temperature between
40OF and
120OF.
(State of California Amendments do not
allow units to be laid below
40OF, or
grouting when air temperature is below
20OF).
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b. Hot Weather Construction
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This condition is not specifically
covered in CBC provisions. Some of the
critical concerns are; early drying up of
mortar and grout before curing, and lack
of humidity for curing in general.
One of the effective ways to mitigate
these concerns is to use cool water for
mixing.
Shading of masonry materials is also
useful. For more details see
MSJC Specifications.
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c. Placing Masonry Units
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Details are covered in Sections
2104.4 and 2104A.4.
In general, the following should be
observed.
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1.
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Mortar shall be sufficiently
plastic.
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2.
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Initial bed joint thickness
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¼ inch to 1 inch.
Subsequent bed joint thickness
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¼ inch to 5/8 inch.
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d. Reinforcement Placing (Section
2104.5)
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Tolerances in placing reinforcing bars
may have impact on design assumptions as
unit thicknesses are fairly small compared
to allowed tolerances.
For example, for units > 8
inches,
but < 24 inches, tolerance is
+ 1 inch for placement of
reinforcing bars.
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e. Grouted Masonry
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Nearly all structural masonry in
California is grouted. Details are covered
in
Section 2104.6.1.2.
California State Amendments
provide a detailed description of low lift
grouting
(see 2104A.6.1.2.2) and high lift
grouting (2104A.6.1.2.3).
In general,
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1.
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Grout spaces should be clean so that
mortar projections do not exceed ½
inch.
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2.
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Grouting of any section of wall is to
be completed in one day with no
interruptions more than one hour.
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3.
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Between grout pours, a horizontal
construction joint shall be formed
stopping a minimum of 1½ inch below a
mortar joint.
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4.
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Grout shall be mechanically vibrated
for consolidation.
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Grouting limitations related to maximum
height of a pour and required dimensions
of clear grout spaces are provided in
Table 21C of the California
Building Code.
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f. Construction Tolerances
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Questions arise many times, related to
acceptable deviations of constructed
masonry work from those on plans and
specifications. California Building Code
does not address this area. One of the
best resources is MSJC Specifications
(ACI-530.1/ASCE 6/TMS 602). This document
does not exist separately anymore and is a
part of MSJC Building Code Requirements
for Masonry Structures.
This document provides acceptable
tolerances in units, mortar joints,
variations in plumbness, alignment and
locations.
As covered in Part I, it is common to
have minor cracks, minor chipping or minor
imperfections in masonry walls. For
specific acceptance criteria see ASTM
C90-01, and Part I in Spring/Summer
2001 issue of "Masonry Chronicles".
This final article in a series of three
articles on "Codes and Specifications"
completes this particular series. For Part
I, see Spring/Summer 2001 issue and for
Part II, see Fall 2001 issue of "Masonry
Chronicles".
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This issue of "Masonry Chronicles" was
written by Dr. Vilas Mujumdar, Executive Director
of Concrete Masonry Association of California and
Nevada.
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