**3. Structural survey of the historic brick bridge**

One of the fundamental preconditions of the structural survey is detailed analyses of the consequences of the interaction between the external environment and degradation processes in time including and taking into account namely the specificities of repeated cyclic temperature and moisture effects. In prominent cultural and historical monuments, these requirements are of utmost significance and necessity. Protection of structures against these effects is a fundamental part of preservative measures avoiding degradation. Mechanical states of stress due to these effects very often exceeded the stresses and deformation caused by force effects. In the case of moisture, attention must also be paid to chemical and microbiological degradation processes strongly dependent on cyclically changing climatic conditions.

Main objectives of the survey:


#### **3.1 Laboratory testing of bricks**

Experimental testing remains the most practised approach towards remedial techniques and information obtained in real buildings can be very useful, especially if the systematic survey was performed.

A total of 14 pieces of original bricks from the 17th century were delivered to the laboratory. The set was marked "N" according to the brand, which most likely means the designation of origin (Nicolsburg—Mikulov). Depending on their appearance and dimensions, the bricks actually correspond to early Baroque bricks—they are lower and wider than usual for bricks from a later period as probably brick N 12 (marked "GN" and other shapes). The bricks, with the exception of brick N 12, had a similar appearance, shard colour and dimensions. Two samples were stored in the archive, 12 pieces of bricks were selected for testing (**Figure 12**).

The samples were divided according to non-destructive tests—ultrasonic and resonant. If the samples were divided into sets only on the basis of a visual impression or even at random, the results could be significantly affected. Based on the long-term experience, the first individual frequency of transverse oscillation was used as crucial for the brick classification. The frequency of oscillation responds very sensitively to both the quality of the material and, in particular, the internal faults in the material, outside invisible.

#### *3.1.1 Brick bulk density test*

The bricks were first dried at 105°C to a constant weight, then they were measured, weighed and the bulk density was calculated in the dried state ρd, u.

$$
\rho\_{d,u} = \frac{m\_{dry,p}}{V\_{g,p}} \Big[ kg \cdot \text{m}^{-3} \right],
$$

*m*dry,*p* dry sample weight [kg] *Vg*,*p* product volume [m3 ]

*A Deep Review on a Historical Brick Bridge in South Moravia; Reconstruction and Assessment DOI: http://dx.doi.org/10.5772/intechopen.102602*

#### **Figure 12.**

*The bricks used for laboratory testing are marked "N" (Nicolsburg). Their appearance and dimensions correspond to early baroque bricks except lower and wider brick N 12 from a later period (marked "GN") [4].*

The bulk density of individual brick samples was relatively uniform, ranging from 1546 to 1641 kg/m3 .

#### *3.1.2 Brick water absorption test*

Furthermore, they were subjected to a water absorption test, and the water absorption of the bricks was calculated from the difference in weights in both boundary moisture states. The weight absorption was also very balanced—from 15.4 to 18.1% (for a different sample N 12).

#### *3.1.3 Brick frequency test*

The bricks were selected for the following sets according to the resonant frequencies as well as the anomalies in the frequency curves:


In addition to the natural oscillation frequencies, the magnitude of the oscillation amplitude and the sharpness of the frequency curve are also monitored. High oscillation frequency, clear and sharp oscillation amplitude (basically high, clear and clean tone, which the brick sounds when knocking) is typical for quality material without defects, lower frequency together with indistinct or double amplitude, on the contrary, indicates defects or discontinuities in the internal structure of the brick. All samples showed very sharp vibration curves, except for samples N 3, where there were signs of internal structure defects [4].
