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NCMA TEK
National Concrete Masonry Association
an information series from the national authority on concrete masonry technology

REINFORCED CONCRETE MASONRY TEK 14-2

Structural (1997)

Keywords: allowable stress, allowable stress design, ASTM Table 1—Standard Material Specifications
standards, construction techniques, flexural strength, grout, grout-
ing, inspection, loadbearing walls, mortar, reinforced concrete Concrete Masonry Units
masonry, shear walls, sizes & shapes of concrete masonry, ASTM C 90 Loadbearing Concrete Masonry Units
strength design, structural properties UBC 21-4 Hollow and Solid Loadbearing Concrete
Masonry Units
INTRODUCTION Mortar
ASTM C 270 Mortar for Unit Masonry
Structural elements constructed of reinforced concrete UBC 21-15 Mortar for Unit Masonry
Grout
masonry effectively resist applied loads through the com-
ASTM C 476 Grout for Masonry
bined tensile strength of reinforcement and the compressive UBC 21-14 Grout for Masonry
strength of masonry. The benefits of incorporating reinforce- Aggregates
ment are improved ductility, structural integrity, and greater ASTM C 144 Aggregate for Masonry Mortar
resistance to flexural and shear stresses. Walls, columns, ASTM C 404 Aggregates for Masonry Grout
pilasters, and beams can be designed to resist dead, live, wind, Reinforcement
seismic, and lateral earth pressure loads using the combined ASTM A 82 Steel Wire, Plain
capabilities of masonry and reinforcement. ASTM A 615 Deformed and Plain Billet-Steel Bars
Reinforced concrete masonry walls are used extensively ASTM A 616 Rail-Steel Deformed and Plain Bars
in most structural applications—warehouses, institutional ASTM A 617 Axle-Steel Deformed and Plain Bars
buildings, retaining walls, shear walls and loadbearing walls ASTM A 706 Low-Alloy Steel Deformed Bars
ASTM A 767 Galvanized Steel Bars
in multistory hotel and apartments. They provide an eco-
ASTM A 775 Epoxy-Coated Reinforcing Steel Bars
nomical system of construction, particularly when high lat- ASTM A 951 Masonry Joint Reinforcement
eral load resistance is required. UBC 21-10 Joint Reinforcement for Masonry

MATERIALS
Mortar—Ingredients for masonry mortar are governed by
Materials used for reinforced masonry—units, mortar, applicable product specifications. Mortar types are generally
grout, and steel reinforcement, are governed by specifications specified to comply with ASTM C 270 (ref. 5). Mortar is
that are referenced in building codes. Applicable specifica- governed by either of two alternative specifications:
tions for these materials are listed in Table 1. 1. the proportion specification prescribes the parts by volume
of each ingredient required to provide a specific mortar type
Units—Reinforced concrete masonry is constructed of hol- 2. the property specification allows approved materials to
low units, solid units, or a combination of both. Single wythe be mixed in controlled percentages as long as the resultant
walls are constructed of hollow units with vertical reinforce- laboratory prepared mortar meets prescribed compressive
ment and grout placed in designated cores of the block. strength, water retention, and air content requirements.
Horizontal reinforcement, such as reinforcing bars grouted Mortar Types M, S, and N are permitted for construction
into bond beams, or joint reinforcement placed in mortar of reinforced concrete masonry. Building codes require the use
joints, is also often used. Multi-wythe walls are built with of Type S or M mortar in Seismic Performance Categories D and
either hollow or solid units with grout and reinforcement in E, and in seismic zones 3 and 4 (refs. 2, 7, respectively).
the space between wythes.
Units must be laid up so that the vertical spaces to be Grout—Ingredients for grout used in masonry construction
grouted provide a continuous, unobstructed opening Some include cementitious materials, aggregates, and hydrated
projects require reinforcement to be in place before masonry lime. ASTM specifications contain requirements for propor-
work is begun. These requirements have resulted in the tions for each of these ingredients. However, it is typical
development of open-end block shapes which are designed to practice to specify compressive strength based on design
be placed around the reinforcement. Some of these shapes are requirements rather than specifying proportions of each
illustrated in Figure 1. ingredient.
TEK 14-2 © 1997 National Concrete Masonry Association
 When grout is placed in a masonry wall, water is Table 2—Tolerances For Placement of Reinforcement
absorbed into the masonry units, reducing the volume of
grout. The effects of grout volume loss may be minimized by Placement of reinforcement
reconsolidation before the grout starts to set. Expansive grout Flexural members .. + 1/2 in. (13 mm) for d < 8 in. (203 mm)
admixtures are sometimes recommended in addition to con- . + 1 in. (25 mm) for d > 8 in. (203 mm) but < 24 in. (610mm)
solidation and reconsolidation to reduce voids in the grout. ................... + 11/4 in. (32 mm) for d > 24 in. (610 mm)
These materials are added at the job site, and cause the grout Walls (vertical bars) .............+ 2 in. (51 mm) along the
to expand slightly after placement, which compensates for length of the wall
volume reduction due to loss of water. Clear spacing between bars and face of unit
Fine grout ........................................... > 1/4 in. (6.4 mm)
Steel Reinforcement—The two principal types of reinforce- Coarse grout ....................................... .> 1/2 in. (13 mm)
ment used in reinforced masonry are deformed steel bars and Minimum cover joint reinforcement
horizontal wire joint reinforcement. Standards for the most Exposed to weather or earth ............... > 5/8 in. (16 mm)
commonly used types of reinforcement are listed in Table 1. Not exposed to weather or earth .......... > 1/2 in. (13 mm)

CONSTRUCTION
grouting the bond beam in one operation. The grout pour
Placement of hollow units for reinforced concrete ma- should then extend a minimum of 1/2 in. (13 mm) above the
sonry construction requires the following considerations: bond beam course.
• Vertical cores to be grouted are constructed so that a con- • Place vertical reinforcement where required, ensuring that
tinuous, unobstructed opening of approved dimensions is cavities containing reinforcement have a continuous unob-
maintained for proper placement of reinforcement and grout. structed cross section complying with Table 3.
• Care should be taken to minimize mortar protrusions into • Place grout of fluid consistency in those cavities which
the spaces to be grouted. contain properly positioned reinforcing bars and all other
• When hollow unit walls are not fully grouted, mortar is cavities required to be grouted.
placed on those cross webs adjacent to the cores to be • Consolidate the grout with a vibrator (grout pours 12 in. (305
grouted, to confine grout to specified locations. mm) or less may be consolidated using a puddling stick).
• Vertical reinforcement is secured in its proper location by • Repeat the operation at the next higher level. Low lift
the use of bar positioners or by tying vertical and horizontal grouting requires no special concrete block shapes or
bars together. special equipment.
• Metal lath, or other suitable material, is used in partially Methods of delivering grout to the wall include hand
grouted masonry below bond beam courses to confine grout bucketing, pumping, or the use of a concrete bucket with a
to specified locations. spout to direct the grout into the cores, whichever is most
Placement of steel reinforcement in its specified location advantageous to the contractor. Complete consolidation of
is critical to the performance of reinforced masonry. The grout is accomplished by vibrating or puddling each lift, while
flexural resistance of reinforced masonry is based on the penetrating into the previous lift.
element's effective depth, d, which is the distance from the A grout lift should not terminate at a mortar bed joint nor
compressive face of the masonry to the centerline location of where horizontal reinforcing bars are placed. A grout key
the tensile reinforcement. between lifts, located at least 1/2 in. (13 mm) below the mortar
Building codes contain allowable tolerances for place- joint, ensures adequate shear transfer. One course may be laid
ment of reinforcement in walls and flexural elements, and above the lift height to obtain proper grout coverage of
tolerances for the distance between vertical bars along the horizontal reinforcing, and the grout poured to a height
length of a wall. A summary of tolerance requirements is approximately 1/2 in. (13 mm) above the bed joint. The final lift
contained in Table 2. is poured to the top of the wall.
In addition to allowable tolerances, codes prescribe
requirements for lap splicing and minimum permissible High Lift Grouting—On larger projects, grouting is often
space between reinforcement and adjacent masonry units for delayed until walls are built to story height or to the full height
fine and coarse grouts to ensure that grout completely sur- of the wall. Grout is then placed into the wall in several
rounds and bonds to the reinforcement. succeeding 5 ft (1.5 m) maximum lifts. This procedure is
referred to as high lift grouting.
Low Lift Grouting—Methods of placing grout in concrete There are several advantages of high lift grouting on
masonry elements are high lift and low lift grouting. The larger projects. Vertical steel can be placed after the wall is
construction sequence of low lift grouting is as follows: erected; its location can be checked by the inspector; and the
• Build the masonry to scaffold height, placing horizontal grout can be transit-mixed and placed by a grout pump or
reinforcement as the wall is laid up. Low lift grouting concrete bucket within a relatively short time. Cleanout
procedures limit the maximum height of masonry to 5 ft openings of sufficient size for removal of mortar droppings
(1.5 m) prior to grouting (ref. 2). When a grout pour and other debris must be provided at the bottom of all vertical
coincides with a bond beam course, an additional course cavities containing reinforcement.
of masonry should be placed above the bond beam to permit Horizontal reinforcing bars are positioned as the wall is
 Table 3—Grout Space Requirements has particular advantages in providing for loads which are
unpredictable, such as seismic loads or hurricane wind loads.
Minimum grouta Strength design of masonry is recognized by the Uniform
Maximum space dimensions Building Code (ref. 7).
Specified grout pour for grouting cells of Reinforced masonry design relies on reinforcement to
grout height, hollow units resist tension, hence the tensile strength of masonry units,
type ft (m) in. x in. (mm x mm) mortar and grout are neglected. By contrast, unreinforced
Fine 1 (0.3) 11/2 x 2 (38 x 51) masonry design considers the tensile strength of masonry in
Fine 5 (1.5) 2 x 3 (51 x 76)b resisting design loads. The advantages of reinforced masonry
Fine 12 (3.7) 2 /2 x 3 (64 x 76)c
1 include significantly higher flexural strength and ductility as
Fine 24 (7.3) 3 x 3 (76 x 76) well as greater reliability. Improved ductility of reinforced
Coarse 1 (0.3) 11/2 x 3 (38 x 76) masonry is also a function of reinforcement, which continues
Coarse 5 (1.5) 21/2 x 3 (64 x 76) to elongate well beyond the design level, allowing deforma-
Coarse 12 (3.7) 3 x 3 (76 x 76) tion beyond design levels without loss of strength. These
Coarse 24 (7.3) 3 x 4 (76 x 102) deformations allow overloads to be redistributed to other
members, thus improving structural performance when ac-
a
Grout space dimension is the clear dimension between any tual loads exceed design load levels.
masonry protrusion and shall be increased by the diameters of Reliability of reinforced masonry is due to the predict-
the horizontal bars within the cross section of the grout space. able tensile strength of steel reinforcement and compressive
b
UBC (ref. 7) requires 11/2 x 2 (38 x 51) strength of masonry, which results in a predictable strength
c
UBC (ref. 7) requires 13/4 x 3 (44 x 76) of reinforced masonry elements.

erected. Vertical bars may be installed prior to laying ma- DESIGN LOADS
sonry or may be inserted from the top of the wall after the
masonry is placed to story height. Vertical bars should be held Allowable stress design is based on service level loads,
in position at intervals not exceeding 200 bar diameters (ref. 7). which are typical load levels expected to occur when the
When design requirements result in a large amount of structure is in use, and members are proportioned using
closely spaced vertical steel reinforcement, or when rein- conservative allowable stresses (see Table 4). Strength
forcement is required to be in place prior to installation of the design of masonry is based on a realistic evaluation of
masonry units, a variation of the vertical steel placement may member strength subjected to factored loads which have a low
be employed. The vertical bars can be secured in their proper probability of being exceeded during the life of the structure.
position at the foundation or base of the wall before units are Minimum design loads for allowable stress design (service
laid up. Instead of threading hollow units down over the loads) and for strength design (factored loads) are included
vertical rods, open-ended units are typically used, enabling in Minimum Design Loads for Buildings and Other Struc-
the mason to lay the block around the steel reinforcement as tures (ref. 3).
the wall is being erected. These units are manufactured with
one or both end webs removed, resulting in an "A" or "H" ALLOWABLE STRESS DESIGN
shape, as illustrated in Figure 1.
Mortar protrusion larger than 1/2 in. (13 mm) must be Allowable stress design principles and assumptions for
removed prior to grouting (ref. 2). All reinforcing, bolts, other reinforced concrete masonry are:
embedded items, and cleanout closures must be securely in • Members are proportioned to satisfy applicable conditions
place before grouting is started. The grouting operation of equilibrium and compatibility of strains within the range of
should be continuously inspected. allowable stresses when subjected to design service loads.
• Strain in the reinforcement, masonry units, mortar, and
STRUCTURAL DESIGN grout is directly proportional to the distance from the
neutral axis. Therefore, plane sections before bending
Engineered reinforced concrete masonry is designed remain plane after bending.
either by the allowable stress design method or by the strength • The tensile strength of masonry units, mortar and grout,
design method. Engineered masonry, in which design loads is neglected.
are determined and masonry members are proportioned to • Reinforced concrete masonry is a homogeneous, isotropic
resist those loads in accordance with engineering principles material. Reinforcement is perfectly bonded to masonry.
of mechanics, is most frequently analyzed by the allowable • Stress is linearly proportional to strain within the working
stress method. This method is considered a conservative stress range.
approach to design; however, it does not predict material
performance and behavior if masonry is stressed beyond Flexure
allowable limits. Flexural compression and tension stresses are deter-
The limit states design method evaluates member capac- mined in accordance with accepted allowable stress design
ity (strength limit state) as well as member deformation under principles. This results in a triangular distribution of com-
service loads (deformation limit state). Limit states design pressive stress from zero at the neutral axis to a maximum at
the extreme compression fiber. Tensile stress in reinforce- • Strain in the reinforcement, masonry units, mortar, and
ment is based on the strain in the steel multiplied by its grout is directly proportional to the distance from the
modulus of elasticity. Strain in reinforcement increases lin- neutral axis. Therefore, plane sections before bending
early in proportion to the distance from the neutral axis to the remain plane after bending.
centroid of reinforcement. Flexural members are propor- • The tensile strength of masonry units, mortar, and grout
tioned such that the maximum calculated tensile and com- is neglected.
pressive stresses are within allowable stress limits. Increased • Reinforced concrete masonry is a homogeneous, isotropic
flexural strength due to compression in reinforcement located material. Reinforcement is perfectly bonded to the masonry.
on the compression side of the neutral axis is typically • Masonry compressive stress distribution and masonry strain
neglected unless it is confined by lateral ties to prevent is assumed to be rectangular and uniformly distributed over an
buckling of the reinforcement. equivalent compression zone, bounded by the compression
face of the masonry, with a depth of 0.85c (see Figure 3).
Axial Compression • The maximum usable strain at the extreme compression
Axial loads acting through the neutral axis of a member fiber of the masonry is limited to 0.003.
are distributed over the net cross-sectional area of masonry.
The compressive resistance of reinforcement is neglected Flexure
unless the reinforcement is confined by lateral ties in accor- Research (ref. 6) has confirmed the accuracy of using the
dance with the provisions for columns to prevent buckling of rectangular stress block model for calculating flexural strength
the reinforcement. Masonry members are proportioned such of masonry. The required moment strength, Mu, is limited to
that the maximum axial compressive stress does not exceed the nominal moment strength, Mn = As fy(d-a/2), multiplied by
the allowable axial compressive stress. The allowable axial the strength reduction factor for flexure, φ = 0.8 (refs. 7, 8).
compressive stress is based on the compressive strength of To ensure ductile behavior, the maximum reinforcement
masonry, a slenderness coefficient, and an allowable stress is limited to 50% of the reinforcement which produces
coefficient.

Combined Axial Compression and Flexure Table 4—Allowable Stressesa for Reinforced
Most loading conditions result in a combination of axial Concrete Masonry
load and flexure acting on the reinforced masonry member.
Superimposing the stresses resulting from axial compression Compression
and flexural compression produces the combined stress. Axial Pa = (0.25 f'mAn + 0.65AstFs)[1-(h/140r)2], where
Members are proportioned such that the maximum combined h/r < 99
stress does not exceed the allowable stress. Pa = (0.25 f'm An + 0.65AstFs)(70r/h)2, where h/r > 99
Flexural ....................................................... Fb = 1/3 f'm b
Shear Shear
Shear acting on flexural members, shear walls, or rein- Where reinforcement is not provided to resist the
forced masonry columns is resisted by the masonry or by entire shear:
reinforcement. Flexural members ................................. Fv = (f'm)0.5,
Where the masonry is designed to resist shear, the shear 50 psi max. (0.3 MPa)
force is distributed over an area equal to the effective width of Shear walls
the member multiplied by the length of wall between the M/Vd < 1 ....................... Fv = 1/3[4-(M/Vd)](f'm)0.5
centroid of tension reinforcement and the location of the [80-45(M/Vd)] psi max.
resultant compressive force. M/Vd > 1 ..... Fv = (f'm)0.5 35 psi max. (0.2 MPa)
The member is proportioned such that the maximum Where reinforcement is provided to resist all the
shear stress is limited to the allowable stress value or, calculated shear:
alternatively, shear reinforcement is provided to resist the Flexural members .......................... Fv = 3.0(f'm)0.5,
entire shear force. The required shear reinforcement is pro- 150 psi max. (1.0 MPa)
vided parallel to the direction of the shear force and distrib- Shear walls
uted over a distance equal to the effective depth of the M/Vd < 1 ....................... Fv = 1/2[4-(M/Vd)](f'm)0.5
member. This reinforcement orientation provides shear resis- [120-45(M/Vd)] psi max.
tance across a potential 45o diagonal tension crack in the M/Vd > 1 ... Fv = 1.5(f'm)0.5, 75 psi max. (0.5 MPa)
masonry. Steel Reinforcement
Tension
STRENGTH DESIGN Grade 40 ............................ Fs = 20,000 psi (138 MPa)
Grade 60 ............................ Fs = 24,000 psi (165 MPa)
Strength design principles and assumptions for rein- Joint reinforcement ........... Fs = 30,000 psi (207 MPa)
forced concrete masonry are: Compression .......... Fs = 0.4 fy, 24,000 psi max (165 MPa)
• The strength of members is based on satisfying the a
applicable conditions of equilibrium and compatibility of refs. 1, 2, 4, 7
b
strains when subjected to factored design loads. UBC (ref. 7) limits Fb to 2,000 psi (13.8 MPa) max.
 Bond Beam Unit "A" Shaped Unit "H" Shaped Unit
Figure 1—Block Shapes For Reinforced Construction

Vertical steel - Hold in position

top and bottom and at intervals of
200 bar diameters.

Metal lath under bond

beam to confine grout

Steel in bond beams is set in

place as wall is laid up.

Cells containing steel are

filled solidly with grout.
Vertical cores should
Floor slab provide a continuous
cavity.

Unless wall is fully grouted, place mortar on

cross webs adjacent to grouted cells to confine
grout to the grout space.
Footing

Figure 2—Reinforced Concrete Masonry Construction

εm
C = 0.85 f'm ab
c a = 0.85c
d
M Neutral axis
d - a/2

εs
T = fs As T
fs = Es εσ (fy max.)

Figure 3—Equivalent Stress Block Model for Strength Design of Reinforced Concrete Masonry
balanced strain conditions, ρbal (ref. 7). Balanced conditions M maximum moment occurring simultaneously with design shear
occur when reinforcement reaches its specified yield strength force, V, at section under consideration, in.-lb (N.m)
at the same time that masonry reaches its maximum usable M n nominal moment strength of a cross section before application
compressive strain of 0.003. This limit on reinforcement of strength reduction factors, in.-lb (N.m)
M u required moment strength at a cross section to resist factored
ensures the steel yields at strength level loads.
loads, in.-lb (N.m)
In addition to complying with flexural strength require- Pa allowable compressive force in reinforced masonry due to axial
ments, members should also be designed to have adequate load, lb (N)
stiffness to limit deflections or any deformations that may Pn nominal axial load strength, lb (N)
adversely affect strength or serviceability of a structure. Pu factored axial load, lb (N)
r radius of gyration, in. (mm)
Other Load Effects V design shear force, lb (N)
Axial compression, shear, and other load effects must be Vu factored shear, lb (N)
checked to ensure design strengths and permissible limits are ε strain
φ strength reduction factor
not exceeded. Code criteria covering these load effects vary,
ρ ratio of reinforcement area to gross masonry area, As/bd
and the designer should reference the applicable code for ρbal reinforcement ratio producing balanced strain conditions
these provisions.
REFERENCES
NOTATIONS: 1. BOCA National Building Code. Country Club Hills, IL: Building
An net cross-sectional area of masonry, in.2 (mm2) Officials and Code Administrators International, Inc. (BOCA),
Ast total area of laterally tied longitudinal reinforcing steel in a 1996.
reinforced masonry column or pilaster, in.2 (mm2) 2. Building Code Requirements for Masonry Structures, ACI 530-
Av cross-sectional area of shear reinforcement, in.2 (mm2) 95/ASCE 5-95/TMS 402-95. Reported by the Masonry Stan-
a depth of equivalent rectangular stress block, in. (mm) dards Joint Committee, 1995.
c distance from extreme compression fiber to neutral axis, in. 3. Minimum Design Loads for Buildings and Other Structures, ASCE
(mm), a/0.85 7-97. American Society of Civil Engineers, 1997.
d distance from extreme compression fiber to centroid of tension 4. Standard Building Code. Birmingham, AL: Southern Building
reinforcement, in. (mm) Code Congress International, Inc. (SBCCI), 1994.
db nominal diameter of reinforcement, in. (mm) 5. Standard Specification for Mortar for Unit Masonry, ASTM C
Es modulus of elasticity of steel, psi (MPa) 270-96a. American Society for Testing and Materials, 1996.
Fa allowable compressive stress due to axial load only, psi (MPa) 6. TCCMAR Research Program (Technical Coordinating Commit-
Fb allowable compressive stress due to flexure only, psi (MPa) tee for Masonry Research).
Fs allowable tensile or compressive stress in reinforcement, psi (MPa) 7. Uniform Building Code. Whittier, CA: International Conference
Fv allowable shear stress in masonry, psi (MPa) of Building Officials (ICBO), 1997.
f'm specified compressive strength of masonry, psi (MPa) 8. 1994 NEHRP Recommended Provisions For the Development of
fy specified yield stress of steel reinforcement, psi (MPa) Seismic Regulations For New Buildings. Building Seismic Safety
h effective height of column, wall, or pilaster, in. (mm) Council, 1994.

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