Page 109 - Design of Reinforced Masonry Structures

MATERIALS OF MASONRY CONSTRUCTION 3.3

Types M, S, and N mortars are typically specified in building codes. In Seismic Design
Categories C, D, and E, as well as for empirical design of masonry, mortar types M or S are
typically required. Glass units require Type S or N mortar [3.2, 3.6]. Neither mortar Type
N nor masonry cement are permitted to be used as part of the lateral force resisting system
*
in masonry structures in Seismic Design Category D, E, and F [3.3 (Section 1.17.4.4.2.2)].
Note that the modulus of rupture of masonry using these mortars, regardless of the manner
of laying masonry units (running bond or stack bond, discussed later), is significantly lower than
for Type M or S mortars made from portland cement/lime or mortar cement (see Table A.7).
It should be noted that the modulus of rupture values for Type N mortar made from portland
cement/lime is typically 75 percent of those for Type M or S mortars; the corresponding val-
ues of the modulus of rupture for masonry cement are much lower.
ASTM A270 places requirements on constituents of mortar. A brief description of vari-
ous constituents of mortars and governing requirements, summarized from Refs. 3.9 and
3.10 follows:

1. Portland cement (ASTM C150-52a [3.12]), one of the main constituents of masonry
mortar, is hydraulic (sets and hardens with chemical interaction with water) cement.
Types I, II, and III are permitted according to ASTM C270-02. Air-entrained portland
cements (IA, IIA, and IIIA) may be used as alternates to each of these types.
2. Masonry cement (ASTM C91) is hydraulic cement consisting of a mixture of port-
land cement or blended hydraulic cement and plasticizing materials (such as limestone,
hydrated or hydraulic lime) together with other materials introduced to influence such
properties as setting time, workability, water retention, and durability.
3. Mortar cement is hydraulic cement similar to masonry cement, but mortar cement stan-
dard also includes a minimum bond strength requirement.
4. Blended hydraulic cements (ASTM C595) consists of standard portland cement or air-
entrained portland cement combined through blending with such materials as blast fur-
nace slag or fly ash.
5. Quicklime (ASTM C5) is calcinated (burned-decarbonated) limestone. Its major con-
stituents are calcium oxide (CaO) and magnesium oxide (MgO). Quicklime must be
slaked with water prior to use. The resultant lime putty must be stored and allowed to
hydrate for at least for 24 h before. For this reason, quicklime is rarely used in mortar.
6. Hydrated lime (ASTM C270-05) is a dry powder obtained by treating quicklime with
enough water to satisfy its affinity with water. ASTM C270-05 designates Type N
(normal), Type S (special), and air-entraining (Type NA and Type SA) to hydrated
limes. Slaking of hydrated lime is not required, thus it can be used immediately.
7. Aggregates (ASTM C144) [3.13] for mortar consist of either natural or manufactured
sand. They represent the largest volume and weight constituent of the mortar. Sand acts
as inert filler, providing economy, workability, and reduced shrinkage, while influenc-

ing compressive strength. An increase in sand content increases the setting time of a
masonry mortar, but reduces potential cracking due to the shrinkage of the mortar joint.
The special or standard sand required for certain laboratory tests may produce quite
different test results from sand that is used in the construction mortar.
8. Water for masonry mortar (ASTM C270-05) must be clean and free of deleterious amounts
of acid, alkalis, or organic materials. Potability of water is not in itself a consideration, but
the water obtained from drinking supply sources is considered suitable for use.
9. Admixtures are functionally classified as agents promoting air-entrainment, water reten-
tivity, workability, or accelerated set, among other things. Many substances, such as

*Seismic Design Category is a classification assigned to a structure based on its occupancy category and the
severity of design earthquake ground motion at the site [3.6]. See Chap. 7 for discussion on this topic.