Danish Technological Institute, DK-8000 Aarhus, Denmark. E-mail: PDC@dti.dk
Determination of the characteristic compressive strength in existing masonry (fk) can be relevant in a number of situations, for example when constructions are exposed to additional loads or when, walls are removed. Necessary parameters for determining fk are the compressive strength of the unit fb and the compressive strength of the mortar in the joints fm. From EN 1996-1-1 (3.2)  one gets for general purpose mortar: fk = K × fb0,7 × fm0,3 where K is a parameter related to the group of units. For solid units (group 1) K is equal to 0.55.
While fb is easily found by testing samples of units, fm is more complicated to determine in situ. Taking out small specimens often fails and chemical analysis does not take the hardening process into account and is in general inaccurate.
To overcome these problems the X-drill has been developed. The concept is to measure the maximum torsional moment of the X-drill when “tapped” into a predrilled small hole in a mortar joint. The torsional moment per unit of depth when hammered into the mortar joint is linearly proportional to fm and the method gives a more accurate prediction of fk compared to the standardized method of measuring fm through EN 1015?11 .