Masonry Magazine February 1993 Page. 22
To facilitate centering test specimens, the compression testing machine is required to have a spherically seated upper bearing block attached at the center of the upper head. The diagonal or diameter of the bearing surface is required to be only slightly greater than the diagonal or diameter of the specimen.
Field Testing
Mortar used in masonry construction exhibits different characteristics than laboratory mixed mortar, mainly because of the absorptive properties of the masonry units. The rate of absorption of water from mortar by adjacent masonry units is influenced by the suction properties of the units as well as temperature and humidity conditions.
ASTM Standards do not include a compressive strength test procedure that accurately accounts for the effects of masonry unit suction and environmental conditions on the mortar strength. However, a test method is available that provides an indication of consistency of field mixed mortars. This method is described in ASTM C780, Method for Preconstruction and Construction Evaluation of Mortars for Masonry. As stated in section 1.3 of the scope of this standard:
"Construction-site testing procedures permit the establishment of conformance to the proportion specifications and quality control of mortar production. Mix-composition measurements permit the rapid assessment of conformance with the proportion specification and quality control, whereas later-age strength testing provides verification that the mortar ingredients are compatible and are performing normally. The test results obtained under this test method are not required to meet the minimum compressive values in accordance with the property specifications in Specification C270."
Although no standard methods exist for evaluating the physical properties of hardened mortar removed from a structure, ASTM C780 does address the compressive strength testing of molded masonry cylinders (2" x 4" or 3" x 6") and cubes (2").
C780 requires that the materials and mixing procedures used for preparing the mortar in the laboratory should be identical to those used in the construction. Once the mortar has been mixed to the appropriate consistency, the mortar specimens are fabricated immediately. The mortar is placed into the cylinder molds in three layers and in the cube molds in two layers of approximately equal volume, with each layer consolidated by the procedures dictated by C780.
Specimens are stored for a period of 24 hours in a protected area at a temperature of 40-90° F before they are placed in a moist room or closet. After 24 hours in the moist environment, the mortar specimens are stripped of their molds and placed back into the moist room or closet until they reach test age.
Prior to testing the specimens in compression, any obvious irregularities in the bearing surfaces of the specimens are corrected by grinding, and the top and bottom of the specimens are capped with gypsum plaster or sulfur mortar to provide uniform parallel bearing surfaces. The specimens are not allowed to dry prior to testing, but rather they are tested in a moist condition. If there is a time delay between capping and testing, the specimens are placed back into the moist room or kept covered with wet burlap; however, capped specimens are not immersed in water - this may result in loss of bond between the specimen and the cap.
Once the appropriate measurements have been taken to determine the cross-sectional area of the specimen, the axis of the specimen is aligned with the center of thrust of the spherically-seated (upper) bearing block. Load is applied to the specimen continuously and without shock. The compressive strength, type of failure, and appearance of the mortar is reported.
Compressive strength of mortar determined in accordance with the provisions of C780 is not the compressive strength of the mortar in masonry. In reality, the water content of mortar decreases significantly when it comes in contact with the surface of the concrete masonry unit, thus increasing the mortar's compressive strength characteristics. Also, a mortar joint has a shape factor that varies significantly from that of the molded cylinder or cube specimens. Shape factors of specimens can greatly affect compressive strength results. Due to this shape factor, the compressive strength results of the mortar cylinders and mortar cubes cannot be compared directly, even if made from the same mortar batch. Because cylinder mortar specimens have a height to thickness ratio of two compared to one for a mortar cube, Note 1 of C780 states"... when cube and cylinder test specimens from like mixtures are to be compared, the cylinder compressive strength may be considered to be equal to 85% of the cube strength." Mortar bed joints, because of their shape and contact with masonry unit surfaces, possess strengths greater than those that would be measured if the same mortar was tested as a cube or cylinder.
Quality control measures during construction of a masonry building often include periodic testing of job-mixed mortars for compressive strength. Such testing is useful in that it provides an indication of how consistently the mortar ingredients are being proportioned during the course of construction. Since mortar strength is related to water-cement ratio at the time of sampling, comparative tests of samples mixed at various stages of construction are meaningful only if water contents of the mixes are known. ASTM C780 includes procedures for determining the water content of mortars mixed at the construction site. These procedures are detailed in Method C780, Annex A5. Preconstruction laboratory test results should be used as the base or index for tests at the contruction site.
Uniform Building Code Standard 24-22, "Field Test Specimens for Mortar," uses another method to obtain mortar specimens for compressive strength field testing. This method prescribes 1/2" to 5/8" thickness of mortar to be spread on the masonry units and allowed to stand for one minute. The mortar is then removed from the units and placed in 2" x 4" cylinders for compressive strength testing.
WATER RETENTION
When the Property Specification of ASTM C270 is specified, tests are required to determine that the minimum water retention of the mortar is at least 75% when tested in