ASTM C780-08

During preconstruction and construction evaluations, use of these test methods establishes specific and overall performance characteristics for the mortar system.

Preconstruction testing of mortars prebatched by weight provides information for the selection of the individual mortar system best suited for the masonry to be constructed. The recommended tests and their significance are as follows:

Consistency determinations by cone penetration allow gaging the water additions for all mortars included in the preconstruction test series. Even if the mortar consistency as measured at the construction site is at a different penetration value than those measured during the preconstruction tests, the cone preparation test serves to standardize water additions for mortars being considered as alternatives before construction. Additional testing of mortar water content-consistency relationships (Annex A4) will allow relating these two factors to batch-to-batch variations at the construction site.

Consistency retention by cone penetration using disturbed or undisturbed mortar samples provides a means of establishing the early-age setting and stiffening characteristics of the mortars. Because laboratory testing is conducted under static climatic conditions, consistency retention test results reflect the relative performance of the mortar systems under test. The same general relationships are expected to hold during testing at the construction project, except as they are influenced by jobsite weather conditions.

Mortar water-content determinations (Annex A4) allow measurement of the water content of the mortar mixture. Mortars prebatched using moist masonry sand may be mathematically analyzed for mortar water content; however, this test, when used for preconstruction evaluation, establishes the effectiveness of the test method and serves as the control or base for tests performed at the construction site.

Mortar aggregate ratio testing provides a method for determining the ratio of aggregate-to-cementitious materials. The sieving operation employed during this test is incapable of separating an individual cementitious material when more than one such material is used, but can accurately establish the aggregate-to-cementitious materials ratio of the mixture.

Mortar air-content testing is useful in establishing the value of this component of the mortar. This test is of particular importance in evaluating mortars that contain air-entraining portland cement, air-entraining lime, masonry cement or any combination thereof.

Compressive strength testing of molded mortar cylinders and cubes establishes one of the characteristics of hardened mortar. Mortar compressive strength test values are not representative of the actual compressive strength of mortar in the assembly and are not appropriate for use in predicting the compressive strength that would be attained by the mortar in the masonry assembly. The measured compressive strength of a molded mortar specimen is almost always lower than the strength of the same mortar in the wall, primarily as a result of differences in mortar water content and specimen shape. Mortar compressive strength is influenced by mortar water content at the time of set. Because molded mortar specimens are not in contact with absorptive masonry units and are not subjected to other mechanisms of water loss, they have higher water contents than mortar in the wall. Higher water contents almost always result in lower strengths. Specimen size and shape also affect compressive strength. Cylinders and cubes exhibit different strengths even when made from the same mortar mix. Both of these specimen configurations yield lower strengths than what would be attained if a specimen having the same size and configuration of a typical mortar joint could be reliably tested.

Note 3—When cube and cylinder test specimens from like mixtures are to be compared, the cylinder compressive strength is approximately 85 % of the cube compressive strength.

Splitting tensile strength of molded mortar cylinders provides a method for determining the splitting tensile strength developing characteristics of the mortar. The measured strength is dependent upon the mortar water content at the time of set, along with other factors, and reflects the general strength attainable by the mortar in the masonry. The measured value, however, is not representative of the actual strength of the mortar in the masonry.

Testing during the actual construction may employ one or more of the test methods described in 4.2. Repetitive testing using these test methods on consecutive or intermittent batches provides a method for measurement of batch-to-batch variations in the mortar production. Testing during actual construction may be referenced to laboratory testing and used to predict later age mortar characteristics. In addition to the comments in 5.2, the following test meanings may be obtained from construction project testing:

Consistency by cone penetration is used as a quick reference for indicating batch-to-batch variations in mix ingredients and mixing time. Erratic consistency readings indicate poor control during batching and mixing, but they do not indicate if cement, sand, or water additions are improper. Other test methods must be used to isolate and identify the unsatisfactory proportioning or mixing procedure, for example, cement to aggregate, mortar water, or air content tests.

Consistency retention by cone penetration tests establishes the early-age setting and stiffening characteristics of the mortar. These properties are influenced by mix proportions and ingredients, weather conditions, effects of chemical additives, and mixing time.

Individual and repeated evaluations of mortar water content (Annex A4) show the ability of the mixer operator to properly and consistently add water to the mixer.

Individual and repeated tests for mortar aggregate ratio show the ability of the mixer operator to properly and consistently add the cementitious material and sand to the mixer, and will establish batch-to-batch variations in the composition of the mortar.

Individual and repetitive tests for mortar air content show the changes caused by variations in mixing time, mixing efficiency and other factors.

Comparison of compressive strength tests of field batched mortars to preconstruction mortar compression tests, each conducted in accordance with this test method, can be used to identify variations in mortar mix constituents and/or proportions. Variations in compressive strength values typically indicate changes in mix water content, mixing procedures, mix materials, material proportions, and environmental conditions.

Note 4—Variations in the measured compressive strengths of field-sampled mortar and between the measured compressive strengths of construction and pre-construction mortar samples should be expected. Many of these variations result from sampling mortar from the mixer or mortarboard and do not necessarily translate into significant mortar strength variations in the wall. Unit suction will remove water from the mortar in the wall and the curing conditions are different. However, significant variation between measured compression strength values should prompt evaluation of probable causes of this variation. Conducting companion mortar aggregate ratio tests would assist in determining if changes in mix constituents and proportions are the likely cause. (See 5.2.6 for additional information).

Splitting tensile strengths of molded cylinders stored in accordance with the test method reflect the approximate strength of the masonry mortar because lower water-cement ratio mortars are found in the wall than are typically measured for mortars from the mixer.