Masonry Magazine April 1997 Page. 35
Masonry Mortar
Masonry sand provides the basic "framework" for mortar. Sand particles are coated and lubricated by the mortar paste to provide body and flowability required in the plastic mortar and are bonded together as the paste hardens to provide required structural properties. Masonry sand is obtained from natural sand deposits or produced by crushing and grading stone, gravel or other mineral materials. Quality of sand affects both workability characteristics of plastic mortar and properties of hardened mortar such as compressive strength, bond strength, and drying shrinkage. Sand used for masonry construction should be clean and well graded.
ASTM C 144, the Standard Specification for Aggregate for Masonry Mortar, outlines gradation requirements for both natural and manufactured sand. The specification also provides for qualification of sands that do not meet the gradation requirements. Thus, a sand not meeting the gradation requirements of ASTM C 144 may be used in a mortar if that mortar meets the property requirements of ASTM C270, the Standard Specification for Mortar for Unit Masonry. In many regions, most masonry sands used are qualified on this basis.
A homogeneous mixture of portland or blended cement and inorganic plasticizing materials such as hydrated lime or pulverized limestone, together with other agents introduced to enhance mortar properties. The portland or blended cement contributes to compressive and bond strength of mortar. Plasticizing materials and other agents optimize workability, board life and water retention; contribute to improved durability; and reduce drying shrinkage and water absorption of mortar.
ASTM C 91, the Standard Specification for Masonry Cement, defines requirements for three classifications of masonry cement:
Type N - For use in preparation of ASTM C 270 Type N mortar.
Type S-For use in preparation of ASTM C 270 Type S mortar without further addition of cements or hydrated lime.
Type M - For use in preparation of ASTM C 270 Type M mortar without further addition of cements or hydrated lime.
Mortar cement is a relatively new product designed for use in demanding structural masonry applications. Mortar cement, like masonry cement, is a hydraulic cement, primarily used to produce masonry mortar. It is typically used as a prepackaged product that is mixed with sand and water to produce mortar at the job-site, although it is sometimes also used in bulk mortar production systems. However, mortar cement must meet lower maximum air content limits than masonry cement and is the only mortar material or system that has minimum bond strength requirements.
A specification for mortar cement was first developed by the Masonry Industry Code Committee (MICC) and was incorporated into the Uniform Building Code Standards by the International Conference of Building Officials (ICBO) in 1991. Within the UBC, mortar cement (UBC Standard 21-14) is now accepted for use in structural masonry in all seismic regions. In addition, in the design of unreinforced structural masonry, higher allowable flexural tensile stress values apply when mortar cement or portland cement-lime mortars are used than when masonry cement mortars are used. Reflecting increased availability and use of this material, ASTM C1329, the Standard Specification for Mortar Cement, was approved this past May. UBC Standard 21-14 and ASTM C 1329 define requirements for Type N, Type S, and Type M mortar cements, intended to be used to make mortars of the corresponding type designations. Proposals are currently being considered to include mortar cement mortars in ASTM C 270 and the Masonry Standards Joint Committee's Building Code Standards for Masonry Structures (ACI 530/ASCE 5/TMS 402).
Whether present as an integral part of masonry cement or as a separate ingredient added at the mixer with hydrated lime, portland cement acts as the glue which holds
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