Concrete Masonry Design
Mechanical properties of concrete masonry:
Topics:
General: The mechanical properties of concrete
masonry wall, pilaster, column, lintel etc. depends on the properties of
concrete masonry units, mortar, grout, reinforcement and how the units
were arranged.
Material: Portland cement, Hydrated lime,
Pozzolans, Normal weight or light weight aggregates.
Dimension of commonly used Concrete masonry units:
|
Nominal
size
|
Actual
size
|
Face
Wall thickness
|
Cross
section (in2/ft)
|
Section
modulus (in3/ft)
|
Moment
of inertia (in4/ft)
|
Radius
of gyration (in)
|
|
8x4x16
|
7-5/8”x3-5/8”x15-5/8”
|
3/4”
|
22.4
|
21.2
|
38.5
|
1.31
|
|
8x8x16
|
7-5/8”x7-5/8”x15-5/8”
|
1-1/4”
|
42.8
|
86.8
|
330.9
|
2.78
|
|
8x12x16
|
7-5/8”x12-5/8”x15-5/8”
|
1-1/2”
|
57
|
273.6
|
1043
|
4.28
|
Type of commonly used hollow core concrete blocks
:
Material: Portland cement/Masonry cement water, lime,
sand, and admixtures
Mixes: Type M, S, N and O. depend on proportion of
mixes. Compressive strength of cues for mortar types are as follows:
|
Mortar
Types
|
Average
compressive strength at 28 days (psi)
|
|
M
|
2500
|
|
S
|
1800
|
|
N
|
750
|
|
O
|
350
|
Ingredients: Portland cement, fine aggregates, coarse
aggregate, Lime.
Mix proportions: depending on its strength, a
commonly use mix is one part of cement with 1/10 of lime, three parts of
fine aggregates, and 2 parts of coarse aggregate with maximum aggregate
size limits by the grout space.
Joint reinforcement: Ladder type or truss type,
usually, 9 or 10-gage wires.
Cell reinforcement: rebars same as concrete
reinforcement.
Bond types: Running bond and stack bond.
The compressive strength of masonry varies with the
type of mortar and the strength of units.
There are two methods to determine the strength of masonry during
construction. One is based on
prism test; the other is based on values specified in the codes.
The values list in International Building Code, 2003, Table
2105.2.2.1.2 are as follows:
|
Net
area compressive strength of concrete masonry units (psi)
|
Net
area compressive strength of masonry (psi)
fm’
|
|
Type M
or S mortar
|
Type N
mortar
|
|
1250
|
1300
|
1000
|
|
1900
|
2150
|
1500
|
|
2800
|
3050
|
2000
|
|
3750
|
4050
|
2500
|
|
4800
|
5250
|
3000
|
Em = 900 fm’
(ACI 530 Section 1.8.2.2.1)
Thermal expanson coefficients: kt =
4.5 x 10-6 in/in/oF
Shrinkage coefficient:
Masonry made of non-moisture controlled concrete
masonry units: km = 0.5s1
Masonry made of moisture controlled concrete masonry
units: km = 0.15s1
Where s1 = 6.5x10-6 in/in.
Masonry control joints are used to allow expansion
and shrinkage of masonry wall, minimize random cracks, and distress.
Spacing of masonry control joints recommended in the commentary of
ACI 530.1 is 25 ft or 3 times of wall height.
It also recommends placing control joints at returns and jambs of
openings. Shear key may be used to control wall movement in the
out-of-plan direction.
|
