Standard Test Method for Cold Bonding Strength of Refractory Mortar

Fireclay brick halves are bonded with a 1/16" thick joint. After air and oven drying, the MOR is determined for the brick/joint assembly. Testing of 5 specimens is suggested by ASTM. At least 10 lbs. of mortar should be submitted for analysis.

$495 per test of 5 specimens



The data developed by this test method show the bonding strength and intrinsic strength of different qualities of air-setting mortar. The procedures described in this test method measure the bonding strength and intrinsic strength of air-setting mortars when used with specific brands or lots of refractory brick.

Note that it is possible to obtain results with these methods for brick and mortar combinations which are incompatible for use at high operating temperatures. The user must determine this compatibility. Only brick and mortar combinations known or intended to be compatible for a particular use should be tested.

Properties of the brick, including its strength, apparent porosity, and pore size distribution, can affect the measurement of both the bonding strength of the brick-mortar composite and the intrinsic strength of the mortar. The average modulus of rupture of the test brick, as determined by Test Method C133, should exceed that expected for the mortar being tested. The testing of specific brands or lots of brick and mortar is preferred.

Three modes of failure are available: the break may occur within the brick itself, at the brick-mortar interface, or within the mortar itself. The latter provides a practical measurement of the strength of the mortar itself and is the predominant mode of failure with the four-point flexure test described. The three-point flexure test measures the failure strength of the brick-mortar composite. Failure within the brick itself suggests that either the particular brick contained a serious flaw, or that the mortar joint strength is of the same magnitude or greater than that of the refractory brick. The mode of failure should always be reported in the final report.

A ruggedness test for this test method performed in 1985 found the following variables to be rugged: specimen size (2.5 to 3.0 in.), mortared surface (cut versus uncut), joint thickness (1/16 to 1/8 in.), pulling of rods (fast versus slow), the loading rate (500 to 1000 lbf/min), the method of load application (3- versus 4-point), and the operator.

The cold bonding strengths of refractory mortars obtained by this test method are suitable for use in research and development, quality control, and for establishing and evaluating compliance with specifications between producers and consumers.


1.1 This test method covers the determination of the bonding strength of air-setting refractory mortar by determining the flexural strength (modulus of rupture) at room temperature of oven-dried brick-mortar joints.


Extracted, with permission, from ASTM C198 Standard Test Method for Cold Bonding Strength of Refractory Mortar, copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428. A copy of the standard may be purchased from ASTM International, phone 610-832-9555, e-mail:,


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