MASONRY - Materials for Civil and Construction Engineers - page 314

8.2 Mortar

Mortar is a mixture of portland cement, lime, sand, and water. Adding a small
percentage of lime to the cement mortar makes the mortar “fat” or “rich,”
which increases its workability. Mortar can be classified as lime mortar or ce-
ment mortar. Lime mortar is made of lime, sand, and water, whereas cement
(or cement-lime ) mortar is made of lime mortar mixed with portland cement
(Portland Cement Association 1987).
Mortar is used for the following functions:
■ bonding masonry units together
■ serving as a seating material for the units
■ leveling and seating the units
■ providing aesthetic quality of the structure
Lime mortar gains strength slowly with a typical compressive strength of
0.7 MPa to 2.8 MPa (100 psi to 400 psi). Cement mortar is manufactured in four
types: M, S, N, and O. Type M has the lowest amount of hydrated lime, where-
as type O has the highest amount. The compressive strength of mortar is tested
using 50-mm cubes according to ASTM C109. The minimum average com-
pressive strengths of types M, S, N, and O at 28 days are 17.2 MPa, 12.4 MPa,
5.2 MPa, and 2.4 MPa (2500 psi, 1800 psi, 750 psi, and 350 psi) (ASTM C270).
Mortar starts to bind masonry units when it sets. During construction,
bricks and blocks should be rubbed and pressed down in order to force the mor-
tar into the pores of the masonry units to produce maximum adhesion. It should
be noted, however, that mortar is the weakest part of the masonry wall. There-
fore, thin mortar layers generally produce stronger walls than do thick layers.
Unlike concrete, the compressive strength is not the most important prop-
erty of mortar. Since mortar is used as an adhesive and sealant, it is very im-
portant that it forms a complete, strong, and durable bond with the masonry
units and with the rebars that might be used to reinforce masonry walls. The
ability to bond individual units is measured by the tensile bond strength of
mortar (ASTM C952), which is related to the force required to separate the
units. The tensile bond strength affects the shear and flexural strength of ma-
sonry. The tensile bond strength is usually between 0.14 MPa and 0.55 MPa
(20 psi to 80 psi) and is affected by the amount of lime in the mix.
Other properties that affect the performance of mortar are workability,
tensile strength, compressive strength, resistance to freeze and thaw, and water
retentivity. The water retentivity is a measure, according to ASTM C91, of the
rate at which water is lost to the masonry units.

8.3 Grout

Grout is a high-slump concrete consisting of portland cement, sand, fine
gravel, water, and sometimes lime. Grout is used to fill the cores or voids in