Page 112 - Design of Reinforced Masonry Structures

3.6 CHAPTER THREE

According to the Commentary to Specifications for Masonry Structures [3.5], a good
rule of thumb is to specify the weakest mortar that will perform adequately; stronger is not
necessarily better when selecting/specifying mortar for masonry construction. For example,
it is typically not necessary to use Type M mortar for high strength masonry; Type S mor-
tar would provide comparable strength of masonry. The MSJC Code does not distinguish
between the structural properties of masonry constructed from Type M mortar and Type
S mortar. Of the two, masonry cement or cement-lime mortar, either can be used unless
specified otherwise. Masonry cement mortar, generally offering improved convenience,
workability, durability, and uniformity, is used in majority of masonry construction. Codes
may also require the use of Type S cement-lime or mortar cement mortars for structural
masonry in high seismic performance categories.

3.3 GROUT

3.3.1 Functions of Grout
Grout is a mixture of cementitious materials (lime and portland cement) and coarse or fine
aggregate to which sufficient water has been added to achieve required fluidity so that it
can be easily poured in the cells of the masonry blocks or between the wythes of masonry.
When poured, grout should completely fill the voids in the masonry (cells) without segre-
gation of the constituents, and completely encase the reinforcement. The two terms most
associated with grout are grout lift and grout pour. The gout lift refers to an increment of
grout within a total grout pour, whereas the latter refers to the total height of masonry wall
to be grouted prior to erection of additional masonry. A grout pour may consist of one or
more grout pours. Both are discussed further in Chap. 9.
Grout looks like very fluid mortar or concrete, but it is neither. Grouting forms a
key phase in masonry construction; it is the structural backbone of masonry. Reinforced
masonry derives its strength through grouting in the sense that reinforcement would not
bond to the masonry without the grout and therefore would not contribute any strength to
the structure. Grout serves many functions including the following:
1. Grout helps develop bonding between various masonry units to act together as one unit.
2. Grout is often used to structurally bond separate wall elements together. This is most
commonly seen in reinforced construction, where grout is used to bond the steel rein-
forcement to the masonry so that the two elements to act integrally in resisting loads.
3. The grouted cells increase the bearing area for resisting higher compressive loads.
4. The grouted cells increase the stiffness of the walls, and thus increase their resistance
to lateral loads. Masonry cantilever walls are often solidly grouted to increase the walls

weight thereby increasing the resistance to overturning.
5. In two-wythe walls, the collar joint is grouted as one of the requirements for the two
wythes to act integrally.
6. Grout increases volume for fire resistance.

3.3.2 Consistency of Grout

Grout contains high amounts of water to develop sufficient fluidity so that it can be pumped
and poured easily and without segregation of its constituents. Fluidity allows grout to