## 5 Finite Element Code
### Basic Characteristics of Finite Element Code
This charter is important as many principles still applied today are first stated. However, at this point, the concept of compatibility is absent, as they approve the judicious use of all resources at the disposal of modern technique and more especially reinforced concrete [5]. In the next decades, the detrimental effect of the use of incompatible materials is obvious on many cultural heritage assets, and thus, in the declaration of Amsterdam in 1975, it is stated that misapplied contemporary technology and ill-considered restoration may be disastrous to old structures [223], as well as that new materials and techniques should be used only after approval by independent scientific institutions [224]. This lays the foundations for a new approach, which is applied today, of preserving monuments with compatible and performing materials, aiming to preserve monuments and historical structures with structural integrity for future generations, however without compromising the values they carry [225]. This approach is not easy to follow, as various considerations must be taken into account. The values of the historical asset must be preserved, thus any restoration material and technique must be compatible with the values of the monument (aesthetic, historical, etc.). The monument must be adequately strengthened to avoid collapse and ensure its preservation for future generations. However, this must be done with the application of compatible restoration materials, which do not jeopardize the longevity of the historical materials of the asset and do not harm them in any way [69]. It is obvious that the above considerations are often in conflict, as some compatible materials may not present the demanded mechanical strength to achieve the required structural reinforcement. For this reason, it is important for researchers today to develop the appropriate tools that can assist in decision making to enable the selection of the optimum restoration material, which can provide adequate strengthening, without jeopardizing compatibility [45]. ### Historical Mortars as a Basis for the Design of Restoration Mortars

In the restoration of masonry cultural heritage assets, restoration mortars play an important role, as they are not only the most vulnerable material of masonry, but they also carry the memory of the civilization that produced them, whether it was a small society in some village of the Mediterranean using local materials, or the Roman empire, with its vast resources and technological advances. Thus, the analysis of historical materials is three-fold: one can learn about the history of the people that designed and produced the historical mortar, as this is a value, the value of memory; one can design new restoration mortars through the reverse engineering approach, ensuring compatibility and continuity; one can reveal the secrets of the great craftsmen of the past, in order to produce not only compatible, but enhanced restoration mortars. ### Historical Mortars

Mortars have been used since antiquity and the raw materials as well as the technology of their production varies according to the use of the mortar in the structure, the raw materials available and the technological state of the era and region. Historical mortars can be separated into categories, the main ones being: (i) typical lime mortars, which is the oldest type of mortar along with gypsum mortars [226]; (ii) gypsum mortars; (iii) hydraulic lime mortars; (iv) pozzolanic mortars, where lime is mixed with pozzolanic additives; and (v) crushed brick-lime mortars [63, 64]. The study of a great number of historical mortars has allowed researchers to establish ranges for the values of their characteristics, assisting in their categorization, as can be seen in [61] where value ranges of the chemical and physico-mechanical characteristics of historical mortars belonging to different types are stated, while PCA has assisted in the correlation of these properties and the categorization of historical mortars [227]. moisture environment [230].