Text: Karl-Johan Loorents, geologist

A number of methods can be used to dimension the thickness of stone slabs, including the EN 1341:2012 Annex A method. This is a simplified calculation method that can be used for a number of different types of natural stone and structures. However, experience from a number of projects suggests that the method frequently produces conservative values.

With this in mind, Minera Skifer carried out a test project with the CBI Swedish Cement and Concrete Research Institute (CBI/SP) in Borås in Sweden in order to compare calculated values with values based on direct testing of a number of fixed dimensions.

The test series was conducted with slabs of Offerdal slate with the following dimensions: 500 x 400 x 25; 500 x 400 x 30; 800 x 400 x 25; and 800 x 400 x 30 mm. For each of the sets of dimensions, 12 slabs were tested, a total of 48.

An electromechanical materials testing machine was used, with addition of the load as described in the standard for this type of test. The load was applied to a 100-mm axle to simulate a statistically widespread point load, as a load effect that simulates a truck wheel or a support from a crane

The slate slabs were mounted on Terra Level Pedestals equipped with deformation transducers.

The test results are shown in the table below.

The “Breaking load” column gives the mean value for breaking point in the respective test series (dimensions). The flexural strength of the slabs is shown as characteristic values (see e.g. EN 12372 Annex A, compared with the lowest expected value). Values for calculated thickness are based on a simplified calculation method in EN 1341, Annex A, with safety factor (Fs) 1, as well as values shown in Table 1 as the calculated dimensioning thickness.

To verify precision between measured and calculated values, the results – dimensions (length, width, thickness), breaking point and flexural strength – from the test for the simplified calculation method were used. This step verifies the actual breaking point for a slab with specific dimensions, of which the value can be compared with the calculated value. The “Difference in thickness” column shows the percentage difference between actual values and calculated values.

The test results show that the simplified calculation method in EN 1341 generates conservative values for slate with the tested dimensions.

The project included a comparison between EN 1341 and EN 12372 to find a general method of calculating the permitted load on a slab mounted on a pedestal system. Prisms were sawn from the test series material and tested as described in EN12372. The result for the flexural strength of these prisms was compared with previously measured values for the slabs (Table). The method comparison shows that the breaking point is overestimated. It is therefore not advisable to use EN12372 to calculate the permissible load for slabs mounted on a pedestal system.

Based on the aforementioned recommendation and experience from construction using slate, the following is a recommendation.

With regard to dimensioning slabs suitable for mounting on pedestal systems, taking due account of empirical knowledge and results from the tests at CBI/SP, the following general rules can be recommended: