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Masonry Design

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.

Properties of concrete masonry units:

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 :

 

Mortar:

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

Grout:

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.

Reinforcement:

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.

Compressive strength of concrete masonry, fm

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

Modulus of elasticity of concrete masonry:

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:

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.