Masonry Magazine February 1993 Page. 21
NCMA-TEK
TEK 1076
An Information series from National Concrete Masonry Association
MASONRY MORTAR TESTING
Keywords: air content, compressive strength, cube strength, cylinder strength, field testing, laboratory testing, masonry cement, mortar cement, mortar testing, quality control, water retention
INTRODUCTION
Masonry mortars are composed of cementitious materials, aggregates, water, and admixtures when specified. Cementitious materials include portland cement, masonry cement, mortar cement, slag cement, blended hydraulic cement, and hydrated lime or lime putty. Aggregates consist of natural sand or manufactured sand. Admixtures may include such materials as coloring pigments, water repellent agents, accelerators, or air-entraining agents. Table 1 indicates the general effect of mortar properties on mortar performance.
ASTM C270, "Standard Specification for Mortar for Unit Masonry," covers requirements for selection, quantity and use of specified ingredients. Two alternative procedures are permitted.
The first method, called Proportion Specification, prescribes a range of proportions by volume of cementitious materials and aggregates for each of 10 classifications of mortar. Physical testing of the mortar is not required to demonstrate compliance with the Proportion Specification. However, proportions are permitted to differ within limits from the ratios listed in the Proportion Specification, if tests are made to demonstrate compliance with a specified minimum compressive strength, minimum water retention, and maximum air content. This is referred to as Property Specification.
Tests required for compliance with the Property Specification are performed on specimens containing materials in proportions required by project specifications. Compressive strength is determined in accordance with ASTM C109, "Compressive Strength of Hydraulic Cement Mortars." Water retention and air content are determined in accordance with ASTM C91, "Specifications for Masonry Cement."
The following sections describe procedures for determining compliance with the specifications in accordance with the various test methods.
TEK 107A © 1992 National Concrete Masonry Association
COMPRESSIVE STRENGTH TESTING
One of the most universally recognized properties of masonry is compressive strength. While this property may not be the most important for masonry mortar, it is often perceived as such because it is generally understood and is relatively easy to determine. However, confusion and misinterpretation sometimes exist in project specification requirements for mortar strength because there are several different compressive strength test methods included in ASTM Standards and model building codes. These methods were established to address specific needs, and they differ from each other in test requirements for obtaining, conditioning, and testing mortar samples and specimens.
Laboratory Testing
As an example, when using the Property Specification, verifying compliance with ASTM C270 requires compressive strength to be conducted in accordance with ASTM C109, "Method for Compressive Strength of Hydraulic Cement Mortar," except that certain modifications regarding storage and conditioning of specimens are prescribed. These modifications are contained in ASTM C91, "Specification for Masonry Cement." Compressive strength testing in accordance with ASTM C270 is conducted on specimens which are proportioned, mixed and conditioned in the testing laboratory. Water content of the mortar sample is regulated by the requirement that the consistency of the mortar produces a flow of 110+5% (ASTM C91). Compressive strength tests as prescribed in Section 5.4.1 of ASTM C270 are made on 2 in. mortar cubes cast in non-absorbent molds and cured in a moist room or moist cabinet meeting the requirements of ASTM C511 until tested.
ASTM test methods emphasize the importance of extreme care in observing the testing procedures used to verify C270 requirements. According to Note 8 of ASTM C109: "Reliable strength results depend upon careful observance of all the specified requirements and procedures. Erratic results at a given test period indicate that some of the requirements and procedures have not been carefully observed, for example, those covering the testing of the specimens as prescribed in 10.6.2 and 10.6.3. Improper centering of specimens resulting in oblique fractures or lateral movement of one of the heads of the testing machine during loading loading will cause lower strength results."