Page 115 - Masonry and Concrete

Understanding Masonry
114 CHAPTER FOUR

mixes may be as high as 5000 psi, but need not exceed either the
requirements of the construction or the strength of the units them-
selves. Although compressive strength is less important than bond
strength, simple and reliable testing procedures make it a widely
accepted basis for comparing mortars. Basically, compressive strength
increases with the proportion of cement in the mix and decreases as
the lime content is increased. Air entrainment, sand, or mixing water
beyond normal requirements also reduce compressive strength.
For residential construction, mortar compressive strength is not a
critical design factor because both the mortar and the masonry are
much stronger in compression than is typically needed. Compressive
strength is important in loadbearing construction, but structural fail-
ure due to compressive loading is rare. More critical properties such as
flexural bond strength are usually given higher priority. Masonry mor-
tars generally should not have a higher compressive strength than is
necessary to support the anticipated loads. An unnecessarily strong
mortar with high cement content is brittle and may experience more
cracking than a softer mortar with higher lime content, which is more
flexible and permits greater movement with less cracking.

4.6.2 Cementitious Materials

The most common cementitious ingredients in masonry mortar and
grout are portland cement and lime, but some proprietary masonry
cement mixes contain other chemical or mineral additives in addition
to or instead of some proportion of the basic portland cement and
lime.

Portland Cement: There are five types of portland cement, each with
different physical and chemical characteristics as described in Chap-
ter 2. Not all of the five types are suitable for masonry construction.
Type I is a general-purpose cement and is the most widely used in
masonry construction. Type III is often used in cold weather because it
gains strength rapidly and generates more heat during the hydration
process. This can help keep fresh mortar or grout from freezing and
shorten the time required for protection against low temperatures.
Air-entraining portland cements (designated as Types IA, IIIA, etc.)
are made by adding a foaming agent to produce minute, well-distributed
air bubbles in the hardened concrete or mortar. Increased air content

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