Commentary on the Flat-Jack Test – Factors Influencing the Measurements

The paper presents the results of several flat-jack tests conducted on large sections of brick walls prepared in a laboratory. The influence of two basic factors was studied – the shape of the flat-jacks and distance between them – on the magnitude of the recorded strains. The three most common flat-jack shapes (semi-circle, rounded-rectangle and rectangle) were used for that purpose. The aforementioned spacing adopted for the purpose of the test was 3, 5 and 7 layers of bricks forming the analyzed brick wall specimen. Both of these factors have a significant impact on the boundary conditions of the test and the recorded results. For the purpose of comparison, brick prisms of a height corresponding to the spacing of the flat-jacks were cut out of the tested walls. The entire walls and then the cut out prisms were subjected to a load with a compressive force in order to estimate the modulus of elasticity E of the brick wall. Comparison of the values obtained shows that the semi-circle flat-jacks widely used in engineering practice might overestimate the rigidity of the wall tested. The reason seems to be the small surface area of this type of flat-jack and insufficient depth of its placement inside the wall bed joint. Only rounded-rectangle and rectangle flat-jacks allowed for estimating results that were more similar to those recorded in the other tests on masonry. Double flat-jacks testing allows for a minor-destructive assessment of the deformation parameters of the outer layer of brick wall. The recorded values were comparable with the results obtained by other authors.