Page 131 - Design of Reinforced Masonry Structures

MATERIALS OF MASONRY CONSTRUCTION 3.25

The thermal strain in the wall is calculated from Eq. (3.10):
∆ L 0 0648
.
ε = = = 225 10 ) in in.
−4
.
.(
/
T
(
)
L ( 24 12)
6
Modulus of elasticity of concrete masonry: E = 900 f ′ = 900(2000) = 1.8(10 ) lb/in. 2
m
m
Thermal stress is given by Eq. (3.11):
6
−4
2
σ = ε E = (2.25 × 10 ) (1.8 × 10 ) = 405 lb/in.
T
T
m
2. Shortening of wall:
Change in temperature, ∆T = 70 – 40 = 30
The change in the length of wall is given by Eq. (3.9):
−6
∆L = k L∆T = (4.5 × 10 )(24 × 12)(30°) = 0.039 in.
t
3.9 INFLUENCE OF MOISTURE ON MASONRY:
SHRINKAGE
Both clay and concrete masonry are affected by presence of moisture because both are
porous materials. Although the response to presence of moisture is volume change in
masonry units, clay and concrete respond differently. Clay masonry expands upon contact
with moisture; the deformation is irreversible (i.e., the material does not return to its origi-
nal size upon drying) [3.47]. Concrete masonry, on the other hand, shrinks because it is a
cement-based material which shrinks due to moisture loss and carbonation. The main cause
of shrinkage, however, is the loss of moisture as concrete dries and hardens.
Shrinkage is a time-dependent volume change phenomenon typical of concrete struc-
tures including those constructed from concrete masonry units. As the term implies, shrink-
age reduces volume of concrete. Although there are many causes of shrinkage, the principal
cause is loss of moisture in concrete. The primary type of shrinkage is called drying shrink-
age or simply shrinkage, which results from evaporation of water in the capillaries (con-
crete is porous) and the adsorbed water (electrically bound water molecules) from the
surface of gel particles. This water is about one molecule thick, or about 1 percent of the
size of gel particles [3.43]. As concrete hardens, this water evaporates slowly and concrete
continues to shrink.
The magnitude of shrinkage strain in concrete depends upon many variables, some of
which are as follows:
1. Relative humidity: Humidity affects rate of evaporation. Shrinkage strain is largest for

relative humidity of 40 percent or less. Concrete in highly humid climates shrinks less
than in arid climates. This is the drying shrinkage, more than 90 percent of which occurs
within the first few weeks after casting.
2. Composition of concrete: Composition of concrete. The hardened cement paste shrinks,
whereas the aggregates do not. Thus, the larger the fraction of hydrated cement paste in
a concrete mix, the greater the shrinkage. Shrinkage is also affected by the water/cement
ratio; high water content reduces the volume of aggregates, which act to restrain shrink-
age [3.30].
3. A secondary form of shrinkage is carbonation shrinkage, which occurs in carbon diox-
ide rich environments such as those found in parking garages. At 50 percent relative