TM 5-809-3/NAVFAC DM-2.9/AFM 88-3, Chap. 3
shear-bond of overlapping units depending on the
assumed to change linearly with the compressive
direction of flexure and type of construction as
strength of masonry, Pm, as follows:
Em = 1000 f'm < 3 000,000 psi
depicted in figure 3-4b. Flexure which induces
(eq 3-2)
shear-bond stresses between overlapping units may
be limited by shear bond strength or by flexural
3-8. Efflorescence. Efflorescence is a fine, white,
tensile unit strength.
powdery deposit of water-soluble salts on the sur-
(2) The flexural capacity of reinforced ma-
face of masonry or in the pores of masonry. The
sonry is essentially limited by masonry compressive
most common salts are sulfate and carbonate
strength or by tensile strength of the reinforcement.
compounds of sodium, potassium, calcium, magne-
Compression reinforcement can add to flexural
sium and aluminum, although others exist.
a. Effect. The primary effect or objection is the
strength in beams, particularly if it is confined.
Vertical reinforcement in shear walls is used to
appearance of efflorescence on the surface of ma-
provide tensile strength for in-plane and out-of-
sonry, both clay and concrete. It can be a serious
plane reversible moment. Failure of slender shear
visual defect. However, under certain conditions,
walls in in-plane flexure is characterized by
salts deposited below the surface of a masonry unit
progressive damage to the masonry at the com-
can cause cracking and spalling due to forces
pression face followed by buckling of the un-
generated by salt crystallization. This can further
confined vertical reinforcement. Confinement of
degrade appearance, but has the more serious effect
vertical reinforcement in shear walls retards pro-
of reduced structural properties.
b. Source. The main source of salts is the port-
gressive damage and increases ductility.
(3) Tests of prisms and short walls under
land cement used in mortar and grout. Other
eccentric compression indicate that at failure the
sources can include the masonry units, sand used in
maximum compressive stress, due to combined
mortar and grout, and the water. Hydrated lime
bending and compression, calculated on the as-
used in mortar does not generally contribute to
sumption of linear elastic behavior, exceeds ulti-
efflorescence.
c. Cause. Water-soluble salts are brought to the
mate uniaxial compressive stress, by a factor on the
order of 4/3.
surface of masonry in solutions of water and
deposited there by evaporation. The salts solution
ity of masonry, Em, may be obtained by instru-
may migrate across the surfaces of the units or
menting compression specimens, prisms, in accord-
through the pore structure of the masonry units.
ance with ASTM E 111. Experimental evidence
Therefore, the conditions which lead to efflores-
indicates that moduli obtained from tests of flat-end
cence are:
prisms corresponds well to moduli of full-scale
(1) A source of soluble salts must be present.
walls. Although the true stress-strain relationship of
(2) A source of water to dissolve the salts
masonry is non-linear (basically a parabolic curve),
must be available.
in many applications it is possible that dead load
(3) The water must be in contact with the
stress is sufficient to achieve the initial stiffening
salts for a sufficient time to dissolve them and carry
represented by the lower portion of the curve of
the solution to the masonry unit surface and into
figure 3-5, thus justifying use of the inner portion
the pores of the units.
d. Control. Because the salts must be in solution
which is often approximately linear. The design
methods in this manual assumes Em is linear and is
3-8
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