Determining the Strength of Solid Burnt Bricks in Historical Structures

Burnt bricks lined with mortar were among the materials used in the construction of historical load-bearing structures. The need to know the strength characteristics of masonry material arises during repairs of historical structures and their subsequent static securing. These can be investigated either by sampling a masonry specimen, or the employment of non-destructive testing methods. With destructive testing that involves removal of a sample from the structure, the structure being tested could be damaged by the removal of a whole brick. When determining the strength by using just a small test sample extracted from the masonry (brick cutouts, or 50 mm diameter drilled cores) the problem is insufficiently accurate conversion rates for the compression strength of a small masonry test specimen, and for overall brick compression strength. For non-destructive testing it is necessary to have a calibration relationship between the parameter from non-destructive testing and strength. Solid burnt bricks sized 303(290) × 145(140) × 70(65) mm are the most widely used building material in brick masonry buildings in the Czech Republic. Both load-bearing and non-bearing structures have been built from these bricks. The load-bearing structures mainly involve extra bearing masonry and ceiling vaults. The article presents findings involving the utilization of non-destructive testing methods – hardness measurement methods and dynamic methods for determining the compression strength or tensile bending strength of solid bricks. Of the hardness measurement methods, the Waitzmann hardness tester (impression method) was used, while the ultrasonic pulse method (direct and semi-direct transmission) was the dynamic method employed. The article also states the required calibration relationships between the non-destructive testing parameter (the β ratio for hardness measurement methods, and ultrasound pulse velocity for the ultrasonic pulse method) and brick strength, and the evaluation carried out on their usage in practice. It further presents a comparison of ultrasonic pulse velocity depending on the technique of transmission). STRENGTH CHARACTERISTICS OF SOLID BRICKS Two documents published at the turn of 19 century have contributed to the development of masonry construction in the present territory of the Czech Republic, namely the Building Regulations [9] of 1787, and the Patent of 1819, licensing subjects to manufacture bricks [5]. The Patent [5] also established the dimensions of solid bricks to 303 × 145 × 70 mm. In 1864, new Building Regulations were issued, which established the dimensions of bricks to 290 × 140 × 65 mm (in some cases 220 × 105 × 50 mm). These Building Regulations specified also the requirements for strength characteristics of solid bricks. Fig. 1 below summarizes brick specifications from 1870 to 1945. Interval: 1870 to 1900 Brick Type Minimum compression strength [MPa] Sintered brick (“tuhovka”) Only quality requirements, such as “very firm” and “perfectly weather-proof” (darker colour, sintered material, sharp edges, highpitched sound when struck), were specified, Class 1 20 Class 2 14 Common brick Class 3 8 1 9th International Conference on NDT of Art, Jerusalem Israel, 25-30 May 2008 For more papers of this publication click: www.ndt.net/search/docs.php3?MainSource=65