A Study of the Ancient Mortars Used in Shapouri Bridge of Khorramabad and the Feasibility of Using them in the Restoration of Bridges through Laboratory Methods

Shapouri Bridge (Tagh-e-Pil-e-Eshkastah) is one of the remnants of Sasanian dynasty which is located in the south of Khorramabad, capital city of Lorestan. The great ancient bridge of Shapouri has undergone many damages due to the passage of time and natural factors so that it has lost its solidarity and integration which makes its restoration inevitable. To choose the appropriate mortar is a significant challenge for the restoration of bridges. Therefore, the main question is to identify the employed mortars in the construction of this bridge and, consequently, to study the feasibility of using the original mortar instead of the modern ones. To do so, first, some mortars of the bridge were removed as samples from different parts such as the foundation, the bridge truss, and the deck. These samples, then, were analyzed using XRD and XRF methods. The results of the analysis indicate that the type of the mortar used in the bridge truss is Gache Nim Pokhte-Nim Kob whereas lime mortar and a compound of lime and ash are used in its foundation and deck overlay respectively. In the next step, the reproduced mortar of Gache Nim Pokhte Nim Kob was analyzed using XRD analysis and mechanical experiments. The results were then compared with the bridge mortar indicating that Gache Nim Pokhte-Nim Kob is an easily stiffened mortar with suitable compressive and tensional resistance. Once exposed to humidity, the compressive and tensional resistance of the mortar increases. Considering these results and the survival of the mortar upon Shapouri Bridge for more than a thousand and four hundred years, it is recommended to use such mortar in the restoration of Shapouri Bridge.