Page 118 - Design of Reinforced Masonry Structures

3.12 CHAPTER THREE

with a much higher water-cement ratio into what are essentially porous masonry forms. The
initially high water-cement ratio of grout is rapidly reduced as the masonry absorbs water.
Mortar differs from grout in that it often contains hydrated lime, finer aggregates, and only
enough water to provide workability [3.7, 3.11].

3.5 COMPRESSIVE STRENGTH OF MASONRY

3.5.1 Compressive Strength of Masonry versus
Compressive Strength of Masonry Units
Like reinforced concrete, the design of reinforced masonry structures is based on two key
parameters: (1) the compressive strength of masonry (denoted by symbol f ′ ) discussed in
m
this section and (2) yield stress of reinforcing steel (denoted by symbol f ) discussed in the
y
next section. Similar to concrete, masonry is strong in compression and weak in tension.
Accordingly, compressive strength of masonry is used as a basic design parameter in struc-
tural design of masonry, just as compressive strength of concrete is used as a basic design
parameter in concrete design. Steel reinforcement is used in both reinforced masonry and
concrete to resist tensile forces as well as compressive forces.
At the outset, it should be recognized that masonry consists of three separate materi-
als: (1) masonry units, (2) mortar, and (3) grout. However, the parameter whose value
is required/specified for design is the compressive strength of masonry, not that of
masonry units. The specified compressive strength of masonry represents the com-
pressive strength of structural unit made from these three different materials bonded
together, unlike concrete for which the compressive strength of only one material—
concrete—is required. As a rule, the strengths of these three materials measured sepa-
rately are required to be at least equal to or greater than the specified compressive
strength of masonry. Refer to Tables A.4 and A.5 for the strength requirements for clay
and concrete masonry, respectively.

3.5.2 Methods of Evaluating Compressive Strength of Masonry
The Code [3.2] under its quality assurance program requires certification of compliance for
2
the compressive strength of masonry prior to construction and for every 5000 ft of masonry
work during construction. There are two methods for arriving at the value of the compres-
sive strengths of clay and concrete masonry as specified in MSJC-08 Specification Section
1.4A [3.4]: (1) the unit strength method and (2) masonry prism method. Typically, one of
the two methods has to be specified by the engineer or the architect. When the method is not

specified, the Specification [3.4] permits the contractor to select the method of determining
compressive strength of masonry. The unit strength method is less expensive than the prism
test method as it eliminates the costs associated with the making of test prisms and labora-
tory testing; however, it is more conservative than the prism test method. Both methods are
discussed briefly in the following sections.

3.5.2.1 Unit Strength Method This method requires masonry units to be tested prior
to and during construction to ensure their adequate strength. The value of f ′ (specified
m
compressive strength of masonry) is based on the compressive strength of the masonry
units and the type of mortar as listed in Tables A.4 and A.5. Clay and concrete masonry
units should conform to their respective ASTM Specifications (discussed in Chap. 2). Clay
masonry units should conform to the following ASTM specifications as applicable, and be